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1.0 Introduction

Smoldering fire under high humidity conditionsThis module explains how temperature and relative humidity are related as part of the Intermediate Wildland Fire Behavior course.

Upon completion of this unit, you should be able to:

  • Describe the relationship between dry bulb temperature, wet bulb temperature, dewpoint temperature, and relative humidity.

  • Describe typical day and night (diurnal) variations in air temperature and relative humidity.

  • Determine relative humidity, dewpoint, and wet bulb temperatures using a psychrometric table.

  • Describe the effects of topography, vegetation, clouds, and wind on air temperature and relative humidity.

  • Describe the temperature and relative humidity characteristics of continental and maritime air masses.

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2.0 Temperature

Temperature is defined as the degree of hotness or coldness of a substance.

The primary driver of the Earth’s temperature is solar radiation.

On a smaller, more local scale, heat from a large fire can affect the temperature by warming the atmosphere around the fire.

Air temperature is generally measured using a ‘dry-bulb’ thermometer calibrated to either the Fahrenheit or Celsius/centigrade scale. In this course, temperature will be given in degrees Fahrenheit.

Temperature can vary with time, horizontal distance, aspect and elevation. Variations of temperature over time can be caused by changes in seasons, diurnal, or (day-to-night) changes and weather systems moving through an area.

Dry bulb temperature

Dry bulb temperature represents the air temperature

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2.1 Changes

The magnitude of seasonal and diurnal temperature changes can be large or small, depending on:

  • Latitude

  • Elevation

  • Topography

  • Proximity to the moderating influences of nearby oceans or large lakes.

Annual Mean Temperature Range

Mean Daily Temperature Range

The example shown displays the Annual Mean Daily Temperature Range, or the diurnal range. There is a general decrease in diurnal temperature range as you move toward higher latitudes. The range increases at higher elevations, and the moderating effect of large bodies of water is seen along the coasts of the oceans and the Great Lakes.

Fronts separate air masses

Weather chart illustrates positions of contrasting air masses separated by fronts.  (view animation)

Abrupt changes in temperature can occur as weather systems bring colder or warmer air into a region. Fronts, indicated by the red and blue symbols on a weather chart, separate air masses of different characteristics. An approaching front can cause big changes in fire behavior, and should alert you to potentially dangerous conditions.

2.2 Effects on Fuels

In the wildland fire environment, high temperatures and direct sunlight can preheat fuels and bring them closer to their ignition point. In contrast, lower temperatures have the opposite effect.

Dry surface fuels in a forested area

Direct sun and high temperatures preheat fuels

Firefighters must monitor temperatures and especially temperature trends to evaluate the potential effects on fuels.

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3.0 Atmospheric Moisture

Water vapor in the atmosphere stores an immense amount of energy in the form of heat. This energy can be released during phase changes such as the conversion of vapor to liquid during condensation, and the conversion of liquid to ice during freezing. The warmer air created when heat is released by these processes can contribute to convection, or stronger upward motion in the atmosphere.

Total precipitable water shows the amount of water vapor in the air

Total precipitable water shows the amount of water vapor in the air.  (view animation)

Moisture in the atmosphere is continually changing its physical state by:

  • Condensing into liquid
  • Freezing into ice
  • Melting into liquid
  • Evaporating into vapor
  • Condensing back to liquid

Water is continually transforming into different physical states or phases

Water is continually transforming into different physical states or phases.  (view animation)

All of these changes are related to temperature and involve an exchange of heat. Evaporation and melting require heat, and that heat is taken from the atmosphere, causing it to cool. That same amount of heat is released to the atmosphere when the opposite changes occur, meaning condensation and freezing.

Fires in Southern California in 2007

Southern California Fires in 2007 grew and spread under conditions of high temperatures and low relative humidities.

The amount of moisture in the atmosphere affects whether fuel moisture increases or decreases.

Relative humidity is used in fire weather to describe moisture conditions in the atmosphere because of how it affects fuel moisture. This term expresses how near the air is to saturation. Lower relative humidity conditions allow fine fuels to dry more quickly, or if they are already dry, to remain so. Higher relative humidity conditions inhibit fine fuel drying or moisten fuels. Drier fuels burn more readily, whereas fuels that become too moist will not carry fire as well.

Temperature and relative humidity conditions can act together to help promote large fire growth. High temperatures can preheat fuels and low relative humidity conditions can dry fuels to increase the probability of ignition and support fire spread.

Fuels in the southeastern United States burn despite fog and 90% RH levels

Fuels in the southeastern United States burn despite fog and 90% RH levels.

Relative humidity thresholds for critical fire behavior will vary from one part of the country to the next and from one fuel type to the next.

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3.1 Moisture Terms

In this course, wet bulb temperature, dewpoint temperature, and relative humidity will be used to describe atmospheric moisture. These terms are examined in great detail because each represents a different measurement of the amount of moisture in the air.

Terms used to describe atmospheric moisture

Terms used to describe atmospheric moisture

Dry Bulb - The degree of hotness or coldness of the air.
Wet Bulb - The lowest temperature to which the air can be cooled by evaporation.
Dewpoint - The temperature to which air must be cooled to reach saturation without adding water vapor. Also, the temperature at which dew forms.
Relative Humidity - The ratio of the amount of moisture (water vapor) in the air, to the maximum amount of moisture that air could contain if it were saturated.

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3.1.1 Dry Bulb

The dry bulb temperature describes the air temperature measured by a dry bulb thermometer. Temperature is defined as the degree of hotness or coldness of a substance.

Terms used to describe atmospheric moisture

Terms used to describe atmospheric moisture

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3.1.2 Wet Bulb

The wet bulb temperature is defined as the lowest temperature to which the air can be cooled by evaporation. This is the same type of cooling you feel when you come out of a swimming pool as heat from your skin is used to evaporate water off of it.

The wet bulb temperature lowers as air flows over the water-soaded wick

The wet bulb temperature lowers as air flows over the water-soaded wick  (view animation)

The wet bulb temperature is read from a thermometer that has a water-soaked wick covering the bulb. As air flows over the wick, it cools as water evaporates from it. The amount of moisture in the air surrounding the wick affects the amount of cooling the thermometer measures.

The drier the air, the more evaporative cooling can take place, resulting in a lower wet bulb temperature, and therefore a greater difference between the dry bulb and wet bulb temperatures.

We can measure the dry bulb and the wet bulb at the same time using a psychrometer. This instrument has both the wet bulb thermometer and dry bulb thermometer mounted together.

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3.1.3 Dewpoint

The dewpoint is the temperature to which air must be cooled to reach saturation without adding water vapor. Condensation is observed when the air becomes saturated. For instance, water drops form on a glass of ice water as the air surrounding the glass is cooled to the dewpoint.

Terms used to describe atmospheric moisture

Terms used to describe atmospheric moisture

Dewpoint is one of the most reliable methods for measuring the amount of atmospheric moisture.

  • Unlike relative humidity, dewpoint does not change as the dry bulb temperature changes unless the amount of moisture in the air increases or decreases.

The dewpoint temperature can be found on a psychrometric chart using the dry and wet bulb temperatures. This chart will be introduced later in this module.

Do not confuse the wet bulb and dewpoint temperatures. Remember that the wet bulb temperature is found by evaporating moisture from the wet wick around the thermometer. It will always be warmer than the dewpoint temperature when the relative humidity is less than 100%, and the same as the dewpoint for a relative humidity of 100%.

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3.1.4 Relative Humidity

Relative humidity is the percent of water vapor, in the air, compared to what would be present if it were saturated. For example, a relative humidity value of 45% means the air has 45% of water vapor it would have when saturated. In other words, relative humidity expresses how close you are to saturation, similar to a gas gauge in a car. The gauge does not tell you how many gallons of gas are in the tank; just how close or far it is from being full.

             amount of moisture in air
  RH =  ----------------------------------  X 100
               amount in saturated air

Firefighter in forest with small fire near Lava Creek

Like dewpoint, relative humidity can also be found on psychrometric charts using the dry and wet bulb temperatures.

Relative humidity is always expressed as a percentage, and a value of 100% means the air is saturated.

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3.2 Review Questions

Question 1

In humid, but unsaturated air, which temperature will be the highest? Choose the best answer.

  1. Dry Bulb Temperature
  2. Wet Bulb Temperature
  3. Dewpoint Temperature

Feedback: The correct answer is a, Dry Bulb Temperature. As long as the air is unsaturated, meaning the relative humidity is below 100%, the dry bulb temperature will be higher than the wet bulb temperature and the dewpoint.

Question 2

When the relative humidity equals 100%, the air is saturated, and which of the following are equal? Choose all that apply.

  1. Dry Bulb Temperature
  2. Wet Bulb Temperature
  3. Dewpoint Temperature

Feedback: The correct answers are a, b, and c.  When the air is saturated, it has the maximum amount of moisture possible. Therefore, the dewpoint and the dry bulb, or air temperature, are equal. If you measured the wet bulb temperature under these conditions, it would also be equal to the dry bulb temperature because the air is saturated.

Question 3

Can the dewpoint be higher than the dry bulb temperature? Choose the best answer.

  1. Yes
  2. No

Feedback: The correct answer is b, No. Once the dry bulb temperature has cooled to the dewpoint temperature, saturation is reached. The only way for the temperature to continue to decrease, is for the dewpoint temperature to decrease, which can occur if moisture is taken out of the air through precipitation or dew, or if drier air moves into the area, or mixes down to the surface from higher in the atmosphere.

Question 4

In which situation is there more moisture in the air? Choose the best answer.

  1. Dewpoint of 20°F
  2. Dewpoint of 65°F

Feedback: The correct answer is b. Dewpoint is one of the best measurements of the amount of moisture in the air. The higher the dewpoint, the more water vapor is in the air. So, the correct answer is Dewpoint of 65°F.

Question 5

Why is relative humidity as a measure of moisture in the atmosphere of particular interest in fire weather? Choose all that apply.

  1. Relative humidity tells us how close the air is to saturation.
  2. The amount of moisture that fuels can absorb from or release to the air depends largely on relative humidity.
  3. Firefighters can usually feel small changes in relative humidity.

Feedback: The correct answers are a and b. Relative humidity tells us how close the air is to saturation, which has a direct effect on fuels. Light fuels, such as grass, gain and lose moisture quickly with changes in relative humidity. Heavy fuels respond to humidity changes much more slowly.

Changes in fuel moistures affect a fire’s level of activity.

Regular monitoring of relative humidity throughout the day helps firefighters judge likely trends in fire activity and assess the chances for dangerous fire behavior.

Question 6

Relative humidity thresholds for extreme fire behavior vary over time and space and are different for different fuel types. Choose the best answer.

  1. True
  2. False

Feedback: The correct answer is a, True. Critical thresholds are determined by location based both on typical conditions and fuel types for the area. For example, fuels in the southeast part of the United States typically burn at considerably higher relative humidity conditions than fuels in the western U.S.

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4.0 Temperature and Humidity Relationships

The previous section covered atmospheric moisture and the different ways to measure it, including wet bulb temperature, dewpoint, and relative humidity.

This section focuses on how temperature, dewpoint, and relative humidity are related.

Let’s start by looking at a situation where the amount of moisture in the air does not change over time, meaning the dewpoint temperature remains constant.

Relative humidity varies with temperature

(view animation)  
The blue spheres represent water vapor molecules in three samples of air with the same dewpoint but different temperatures. Note the decrease in relative humidity as the temperature increases. The speed of the molecules in the samples increases as the temperature increases.

This situation allows us to isolate the relationship between temperature and relative humidity. For the same dewpoint:

  • If temperature increases, relative humidity decreases.
  • If temperature decreases, relative humidity increases.

A more general statement is that temperature and relative humidity have an inverse relationship for a given dewpoint.

As temperature increases, the amount of water vapor in the air required to reach saturation, or 100% relative humidity, also increases. Remember that when the relative humidity reaches 100%, the temperature and dewpoint are equal.

Question 1

You are working a fire on a warm afternoon, when the temperature is 85°F, and the dewpoint is 45°F, resulting in an RH of 25%. As the sun goes down, the temperature drops to 65°F while the dewpoint remains the same. What would you expect to happen to the relative humidity? Choose the best answer.

  1. Increase
  2. Decrease
  3. Remain the same

RH - Temp Values

Feedback: The correct answer is a, Increase. Because temperature and relative humidity have an inverse relationship, a decrease in temperature will result in an increase in relative humidity. In this case, the relative humidity would increase to 48% when the temperature drops to 65 °F. Relative humidity has a direct effect on fuel moisture, with an increase in relative humidity resulting in less active fire behavior. You have probably observed that greater progress in controlling a fire is often made overnight when temperatures are cooler and relative humidity is higher.

Keep in mind that if the dewpoint were also to increase, the increase in relative humidity would be even greater.

Question 2

Later that same night, the temperature has continued to cool from 65 to 45°F, while the dewpoint has remained constant. What has happened to the relative humidity? Choose the best answer.

  1. Continued to increase
  2. Began to decrease
  3. Remained the same

RH - Temp values

Feedback: The correct answer is a, Continued to increase. During the night, the temperature has cooled, and the relative humidity has continued to increase. In fact, cooling continued until the temperature reached the dewpoint temperature. At this point, the air became saturated and the relative humidity increased to 100%.

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4.1 Dirunal Relationships

Diurnal relationships between temperature and relative humidity can be observed on a hygrothermograph, which shows a trace of temperature and humidity values measured over a period of time.

24-Hour Trace of Temperature and Relative Humidity

Hygrothermograph trace of temperature and relative humidity over a 24-hr period

As the temperature increases after sunrise, the relative humidity decreases, reaching its lowest point when the maximum temperature is realized, assuming the dewpoint has not changed.

The relative humidity normally reaches its highest value early in the morning when the temperature cools to its minimum. This assumes the dewpoint has not changed overnight.

In this example, the direct correlation between temperature and relative humidity can be observed because the dewpoint remained relatively constant. In general, the diurnal change in temperature is more pronounced than changes in dewpoint.

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4.1.1 Temperature Changes

A sudden weather change, such as an increase in cloud cover, development of thunderstorm outflow, passage of a cold front, or a downslope wind event, can result in abrupt changes in temperature and dewpoint, and therefore, relative humidity.

Thunderstorm with rain shaft extending from the base

Thunderstorm outflow can increase relative humidity by adding moisture and cooling the air

Question 1

Review the following table that shows the dry bulb, dewpoint and resulting relative humidity. Notice that the dewpoint does not change, but the dry bulb temperature does. Which of the following statements are true? Choose all that apply.

  1. Each 20°F increase in air temperature, decreases the relative humidity by about half.
  2. The total amount of moisture in the air has not changed.
  3. The relative humidity steadily increases and the temperature warms.
  4. When the dry bulb and dewpoint are equal, the relative humidity is 100%.
  5. When the temperature reaches 90°F, the air is critically dry.

Table for quiz

Feedback: The correct answers are a, b, and d.  This example shows that in general, for a constant dewpoint, each 20°F increase in air temperature decreases the relative humidity by about half. Also, since the dewpoint does not change, the amount of moisture in the air does not change, although the relative humidity does, resulting in changes in fuel moisture.

When the dry bulb temperature cools to the dewpoint, the relative humidity reaches 100% and the air is considered saturated.

Critical values of relative humidity are defined for each location. A value of 25% may be a critical value for Florida, but not California. You should be familiar with the critical values for your location.

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4.1.2 Dewpoint Changes

In reality, dewpoint temperatures often fluctuate. Fluctuations in dewpoint may be a result of several factors.

Question 1

What is the resulting dewpoint due to the following? Chose whether you would expect an increase or a decrease in dewpoint.

  1. The passing of showers and thunderstorms. [Increased Dewpoint/Decreased Dewpoint]
  2. A breakup or dissipation of an inversion. [Increased Dewpoint/Decreased Dewpoint]
  3. Wind flow off a body of water, such as a lake or ocean. [Increased Dewpoint/Decreased Dewpoint]
  4. Solar heating resulting in strong vertical mixing of the lower atmosphere. [Increased Dewpoint/Decreased Dewpoint]
  5. Evaporation of surface water or the melting of snow and ice. [Increased Dewpoint/Decreased Dewpoint]

Feedback: Mechanisms that increase the amount of moisture in the air, such as showers moving over the area, or wind moving over a lake or ocean, or evaporation into the air, will cause the dewpoint to rise.

Thunderstorm with rain shaft extending from the base

Showers can increase moisture in the lower portions of the atmosphere

Wind moving from an ocean area can bring increased relative humidity

Wind moving over a lake or ocean can collect moisture through evaporation from the surface

Early morning lake fog

Evaporation from the surface causes the dewpoint to rise

Group torching occurring after inversion breaks

Mixing associated with the erosion of an inversion can greatly increase fire behavior

Conversely, mixing of drier air higher in the atmosphere to the surface due to solar heating or the breakup of an inversion will cause the dewpoint to decrease.

A sharp rise and/or fall in dewpoint may also indicate that a more significant change in weather is occurring. A common and dangerous situation along the California Coastal Range occurs when very dry air from high levels in the atmosphere descends as offshore flow develops on the Coastal Range AT NIGHT. A drop in relative humidity from typical overnight values of 70% to values in the teens has occurred many times along the ridgetops after midnight. This type of event once resulted in 50 shelter deployments along the California Coastal Range and has been documented on many other California Coastal Range fires.

Remember that relative humidity is affected by both changes in the dewpoint, or the amount of atmospheric moisture AND the dry bulb temperature.

When looking at each influence separately

  • A rise in dewpoint indicates that atmospheric moisture is increasing and the relative humidity could increase.
  • A steady fall in dewpoint indicates that atmospheric moisture is decreasing and the relative humidity could decrease.
  • A rise in temperature would result in a fall in relative humidity.
  • A fall in temperature would result in a rise in relative humidity.

However, both temperature and dewpoint can fluctuate, so be aware of the combined effects on the resulting relative humidity.

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5.0 Calculating Relative Humidity

In this section, measurements of dry bulb and wet bulb temperatures are used to determine relative humidity with a psychrometric table.

Psychrometric table

Psychrometric table related dry bulb, wet bulb, dewpoint, and relative humidity

A sling psychrometer is a reliable and accurate instrument used to measure dry bulb and wet bulb temperatures. The psychrometer is spun to allow air to flow over the bulbs of the thermometers. The temperatures obtained by ventilating both bulbs at the same time provide the most valid comparison.

Sling psychrometer is used for measuring dry bulb and wet bulb temperatures to determine relative humidity

A sling psychrometer consists of wet bulb and dry bulb thermometers mounted together and attached to a leash that is used to spin the psychrometer

After the dry bulb and wet bulb temperatures are determined, a psychrometric table is used to find the corresponding dewpoint temperature and relative humidity for the measured conditions.

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5.1 Psychrometric Table Tutorial

Psychrometric tables are provided in belt weather kits. You must use the chart for the elevation at which you are taking the observation because relative humidity and dewpoint change with atmospheric pressure, which varies with elevation.

Psychrometric table

Psychrometric table related dry bulb, wet bulb, dewpoint, and relative humidity

  • The numbers located on the top row of the psychrometric table are the wet bulb temperatures.
  • The numbers located in the far left column of the table are the dry bulb temperatures.
  • The dewpoint temperatures and relative humidity values are read by finding the intersection of the wet bulb temperature column and the dry bulb temperature row that correspond to the measured values.
  • Within each box, the dewpoint is the top number and relative humidity is the bottom number.

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5.2 Exercises

Use the psychrometric table below to answer Question 1 or click here for larger version (in a separate window).

Psychrometric table

Question 1

While working the Cascade Complex in Idaho overnight in August 2007, you take a measurement with a sling psychrometer along an 8000 ft ridgeline. You find a dry bulb temperature of 45°F and a wet bulb temperature of 39°F. What are the dewpoint and relative humidity?

Feedback: Correct answers: Dewpoint=33°F, Relative Humidity=62%

 

Use the psychrometric table below to answer Question 2 or click here for larger version (in a separate window).

Psychrometric table

Question 2

The next day, near noon, you take another observation from the same location. While the dewpoint only slightly increased to 34°F, the temperature has warmed to 65°F. What is the current relative humidity?

Feedback: Correct answer: Relative Humidity=31%. From the table, matching the 65°F air temperature with the 34°F dewpoint temperature indicates a relative humidity of 31%. Note that the temperature increased by 20°F with only a slight increase in dewpoint, and the relative humidity dropped by half.

 

Use the psychrometric table below to answer Questions 3 and 4 or click here for larger version (in a separate window).

Psychrometric table

Question 3

Later in the afternoon, winds pick up and become gusty, and dry air mixes down from higher levels of the atmosphere. You take another observation from the same location and find that the wet bulb temperature is 54°F while afternoon heating has brought the dry bulb temperature to 86°F. What has happened to the relative humidity?

Feedback: Correct answer: Relative Humidity=12%. Note that the dry air that mixed down lowered the dewpoint from 34°F to 24°F, but the wet bulb temperature actually increased from 48°F to 54°F because of the increase in temperature.

Question 4

At a higher elevation (8500 ft), the temperature is slightly lower at 80°F and the wet bulb now reads 52°F. How would this affect the relative humidity? Choose the best answer.

  1. The RH would be slightly higher
  2. The RH would be slightly lower

Feedback: The correct answer is a. The relative humidity would be higher at the higher elevation, which has a lower temperature. In this example, the dewpoint was only slightly lower.

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6.0 Local Effects

Downslope windsThis section explores local effects of topography, vegetation, cloud cover, and wind on temperature and relative humidity.

 

 

 

 

 

 

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6.1 Topography

Fuel and weather conditions in complex terrain are influenced by both the slope aspect and elevation.

Effects of aspect

Slope angle and aspect help determine fuel and weather conditions in complex terrain

The intensity of sunlight in complex terrain varies significantly from slope to slope, depending on slope angle and aspect. The amount of sunlight on a slope affects local temperature and relative humidity, which in turn influence the type and amount of vegetation. These conditions are important for firefighters to evaluate when assessing potential fire behavior.

Question 1

In the northern hemisphere, south facing slopes have greater exposure to sunlight. How does sun exposure affect local temperature and relative humidity on each aspect? Choose all that apply.

  1. South facing slopes are typically much warmer, with lower relative humidity and lower soil moisture content.
  2. North facing slopes typically experience the coolest temperatures and highest relative humidity.
  3. East facing slopes warm earlier in the day and thus reach their warmest temperature and lowest humidity later than west facing slopes.
  4. West facing slopes warm later in the day and thus reach their warmest temperature and lowest humidity later in the day than east facing slopes.

Feedback: Answers a, b, and d are correct.

Relative humidity variations with aspect

Heating of the ground and the air near the surface follows the sun. South slopes are warmest and have the lowest humidity, while North slopes are coolest and have the highest humidity. As the sun rises, east slopes are first to warm while west slopes are the last to be exposed to sunlight.

When fire moves onto a slope with a different aspect, changes in temperature, relative humidity, and resultant fuel moisture content, affect the fire’s intensity and rate of spread.

Near ridge tops, mixing of air by winds results in more uniform conditions on each of the aspects.

Be aware of the aspect of the slope where you are working and the influence on temperature and relative humidity.

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6.1.1 Elevation

Elevation significantly affects temperature and relative humidity, which in turn influence fire activity.

Temperature typically decreases with an increase in elevation. Therefore, daytime mixing of the atmosphere near the ground will yield a uniform amount of moisture, and the decrease in temperature with elevation will yield an increase in relative humidity.

The increase in relative humidity with elevation increases the moisture content of dead fuels at higher elevations.

Relative humidity variations with aspect

However, when colder air drains and pools into valleys at night, cooler temperatures and higher relative humidities occur at these lower elevations.

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6.2 Vegetation

Local variations of vegetationVegetation has insulating effects and moderates temperature and relative humidity changes near the ground by intercepting incoming sunlight during the day and outgoing radiation at night. Below a forest canopy, winds are lighter, and mixing of air is reduced.

In addition, because of different processes going on in green foliage, including respiration, photosynthesis, and evapotranspiration, vegetated areas tend to be cooler than those areas composed mainly of bare ground or surface organic litter.

Effects of low or sparse vegetation coverage on the temperature of the underlying surface are much less than areas that have tall, dense vegetation.

Question 1

Apply what you know about how temperature and relative humidity are affected by dense vegetation to create true statements. Choose the term within the pair of options in [brackets] that best completes these statements.

During the day, the [warmest / coolest] temperatures and [lowest / highest] relative humidity are found around the tree tops. The [warmest / coolest] temperatures and [lowest / highest] relative humidity are found beneath the canopy in the shade.

Feedback: During the day, the warmest temperatures and lowest relative humidity are found around the tree tops. The coolest temperatures and highest relative humidity are found beneath the canopy in the shade. The moderating effect of vegetation on the conditions near the surface results in cooler temperatures and higher relative humidity than would otherwise be observed. The warmest temperatures and lowest relative humidity conditions instead occur near the top of the vegetation.

Daytime variation in relative humidity around trees

Question 2

Choose the term within the pair of options in [brackets] that best completes these statements.

The opposite is true at night. The [(warmest / coolest] temperatures and [lowest / highest] relative humidity are found near tree top. The [warmest / coolest] temperatures and [lowest / highest]) relative humidity are found beneath the canopy.

Feedback: The opposite is true at night. The coolest temperatures and highest relative humidity are found near tree top. The warmest temperatures and lowest relative humidity are found beneath the canopy. At night, the moderating influence of vegetation has the opposite effect as seen during the day. Conditions near the surface are warmer with lower relative humidity. The coolest temperatures and highest relative humidity conditions instead occur near the top of the vegetation.

Nighttime variation in relative humidity around trees

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6.3 Cloud Cover

Downslope winds

Cloud cover affects temperature and relative humidity by reflecting incoming sunlight during the day, and intercepting outgoing long-wave, terrestrial radiation at night.

 

 

 

Relative humidity variations due to clouds

  (view animation)

Question 1

Choose the term within the pair of options in [brackets] that best completes these statements. In this case we assume the amount of moisture in the air is not varying, so that changes in relative humidity are due to changes in temperature.

During the day, cloud cover reflects and absorbs incoming solar radiation, [decreasing / increasing] the amount of sunlight reaching the ground resulting in [cooler / warmer] conditions with [lower / higher] relative humidity.

Feedback: During the day, cloud cover reflects and absorbs incoming solar radiation, decreasing the amount of sunlight reaching the ground resulting in cooler conditions with higher relative humidity. Cloud cover can lower daytime temperatures by several degrees and increase relative humidity, thus decreasing fire activity.

Question 2

Choose the term within the pair of options in [brackets] that best completes these statements.

At night, heat lost at the surface is [inhibited / enhanced] by cloud cover resulting in [cooler / warmer] conditions with [lower / higher] relative humidity.

Feedback: At night, heat lost at the surface is inhibited by cloud cover resulting in warmer conditions with lower relative humidity. Higher nighttime temperatures and lower relative humidity result under cloud cover at night. Thus, fires may be more active than normally anticipated overnight.

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6.4 Wind

Wind has a moderating effect on air temperature and relative humidity as it mixes air near the ground with air aloft.

Wind effects on temperature and relative humidity near the ground

Wind effects on temperature and relative humidity near the ground

Question 1

Choose the term within the pair of options in [brackets] that best completes these statements. In this case we assume the amount of moisture in the air is not varying, so that changes in relative humidity are due to changes in temperature:

During the day, wind tends to [lower / raise] air temperature and [lower / raise] relative humidity near the ground.

Feedback: During the day, wind tends to lower air temperature and raise relative humidity near the ground. During the day, as the sun heats the ground, wind effectively mixes cooler air aloft down to the ground keeping surface air temperatures lower than on calm or light wind days. With a uniform amount of moisture in the well mixed atmosphere, the relative humidity will rise as the air is cooled. However, keep in mind that air mixed from higher levels in the atmosphere may be drier, which may counteract the cooling effects and actually lower relative humidity.

Question 2

Choose the term within the pair of options in [brackets] that best completes these statements.

At night, wind tends to [lower / raise] air temperature and [lower / raise] relative humidity near the ground.

Feedback: At night, wind tends to raise air temperature and lower relative humidity near the ground. On a clear, calm night, extensive radiational cooling of the ground results in cooler temperatures and higher relative humidities. Wind disrupts and minimizes radiational cooling at the surface and effectively mixes warmer, drier air aloft with air near the ground, resulting in warmer temperatures and lower relative humidity than on calm or light wind nights.

As with the effects of wind on daytime temperatures, the effects of nighttime wind also can depend on the source of the air. For example, nighttime down-canyon winds off a glacier or snowfield could bring a cooling effect at night rather than a warming effect.

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6.4.1 Other Effects of Wind

Downslope winds

Local winds can cause changes in temperature and relative humidity beyond just mixing air aloft to the ground. For example:

  • Wind caused by air being forced down a mountain side will cause a warming of the air which will lower the relative humidity

Humidity change of air flowing over a lake

  • Winds coming off a large body of water may dramatically increase relative humidity and lower the temperature

Thunderstorm outflow

  • Outflow winds from rain showers or thunderstorms can suddenly cool and moisten the air.

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7.0 Air Masses

An air mass is a large body of air with “homogeneous” or similar temperature and moisture characteristics. This section presents a classification scheme for air masses and the typical characteristics of each type of air mass.

North American terrain

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7.1 Classification

Air masses are identified by their locations of origin.

Air mass source regions of North America

 Air mass source regions for North America

To describe the moisture characteristics of the air mass, the term maritime is used for air masses originating over water and continental is used for those originating over land. Temperature characteristics are categorized by tropical, polar or arctic locations.

Tropical air masses form in high pressure areas in warm, tropical regions. When tropical air masses form over oceans, they are warm and moist, while tropical air masses that form over land, are hot and dry.

Polar air masses form in high pressure areas in the polar and sub polar regions. A polar air mass that forms over water is cool and moist, while a polar air mass that forms over land is cold and dry.

Arctic air masses form near the poles and are very cold and dry.

Question 1

Match the Air Mass Characteristic (left) to the corresponding Air Mass Type (right).

Air Mass Characteristic Air Mass Type
1. Cold and moist a. Maritime Tropical (mT)
2. Warm and moist b. Continental Polar (cP)
3. Extremely cold and dry c. Continental Tropical (cT)
4. Cold and dry d. Maritime Polar (mP)
5. Warm and dry e. Continental Arctic (cA)

Feedback: The correct matches are 1=d, 2=a, 3=e, 4=b, 5=c

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7.2 Effects on Fire Activity

Air mass source regions of North America

 Air mass source regions for North America

Depending on its characteristics, a changing air mass can either increase or decrease fire activity.

Large fire growth, which is sometimes referred to as a blowup, is typically associated with warm, dry, continental air mass conditions that are often seen as a trough or cold front approaches.

Conversely, decreasing fire activity is typically associated with a cooler and more moist maritime polar air mass, such as found behind cold fronts moving onto the West Coast from the Pacific Ocean.

Fronts separate air masses of different characteristics. Knowing the typical characteristics of the air mass behind a front can help you anticipate changes in temperature, relative humidity and potential fire behavior. Also, knowledge of air mass characteristics can help you predict conditions at locations across the country.

Question 1

Match the Source Region (left) to the type of Air Mass (right). Some Air Masses match more than one Source Region.

Source Region Air Mass Type
1. Central America
    (Mexico)
 a. Continental Arctic (cA), air originating in the arctic regions
2. Central Atlantic  b. Continental Polar (cP),a ir originating from over large, often cold and
     dry continental land areas
3. Central Pacific  c. Continental Tropical (cT), air originating from over large, often warm
     and dry continental land areas
4. Gulf of Alaska  d. Maritime Tropical (mT), air originating from over warm oceanic regions
5. Gulf of California  e. Maritime Polar (mP), air originating from over cold oceanic regions
6. Gulf of Mexico  
7. North Atlantic  
8. North Pacific  
9. Northern portions
    of North America
    (Canada)
 
10. Southwest United
      States
 
11. Northern Alaska  

Feedback: The correct matches are 1=c, 2=d, 3=d, 4=e, 5=d, 6=d, 7=e, 8=e, 9=b, 10=c, 11=a

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8.0 Summary

Relative humidity varies with temperature

(view animation)
The blue spheres represent water vapor molecules in three samples of air with the same dewpoint but different temperatures. Note the decrease in relative humidity as the temperature increases. The speed of the molecules in the samples increases as the temperature increases.

This module focused on the relationship between temperature and relative humidity.

An inverse relationship exists between temperature and relative humidity. When the amount of moisture in the atmosphere does not change,

  • If temperature increases, relative humidity decreases.
  • If temperature decreases, relative humidity increases.

Terms used to describe atmospheric moisture

Terms used to describe atmospheric moisture

Several terms are used to describe the temperature and moisture characteristics of the atmosphere including:

  • Dry bulb temperature
  • Wet bulb temperature
  • Dewpoint temperature
  • Relative humidity

When the relative humidity is 100%, the atmosphere is saturated and the dry bulb, wet bulb and dewpoint are equal.

24-Hour Trace of Temperature and Relative Humidity

Hygrothermograph trace of temperature and relative humidity over a 24-hr period

Diurnal variations in temperature and relative humidity show some basic patterns:

  • The highest values of relative humidity are typically seen in the early morning when the lowest temperature occurs.
  • The lowest values of relative humidity are typically seen in the late afternoon when the highest temperature occurs.

Remember, a sudden weather change can result in abrupt changes in temperature and dewpoint, and therefore, relative humidity.

In general, for a constant dewpoint, each 20°F increase in air temperature decreases the relative humidity by about half. However, this rule of thumb does not replace the need to take actual measurements to determine relative humidity.

Psychrometric table

Psychrometric table related dry bulb, wet bulb, dewpoint, and relative humidity

A psychrometric table can be used to determine relative humidity, dewpoint, and wet bulb temperatures. Be sure to use the appropriate table for the elevation of the observation site.

Local variations of vegetation

Local effects on air temperature and relative humidity are caused by characteristics of the site such as topography and vegetation, and current weather conditions such as clouds and wind.

Air mass source regions of North America

 Air mass source regions for North America

Temperature and relative humidity usually depend on the type of air mass. Air masses are classified based on their origin, using maritime and continental to describe the moisture characteristics and tropical, polar and arctic to describe temperature characteristics. Changes in air masses bring very different weather conditions to a region.

A good understanding of how temperature and relative humidity are related is necessary to evaluate effects on potential fire behavior.

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