NWCG S-190 Unit 3: Temperature and Moisture Relationships

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S-190 Unit 3: Temperature and Moisture Relationships

Summary:

Weather is the most variable and, at times, the most difficult to predict component of the fire

environment. Temperature and moisture are two weather components that are closely monitored by

firefighters because they have a direct impact on fuels and potential fire behavior.

Incident Position Description (IPD) Alignment:

This unit aligns with the following FFT2 IPD specific duties

(https://www.nwcg.gov/positions/fft2/position-ipd):

• Apply the knowledge of fuels, terrain, weather, and fire behavior to decisions and actions.

Objectives:

Students will be able to:

• Describe dry bulb temperature, wet bulb temperature, dew point, and relative humidity.

• Describe how temperature and relative humidity can influence wildland fire behavior.

• Determine relative humidity and dew point by using a Psychometric Table and given inputs.

Unit at a Glance:

Topic Method Duration

Unit Introduction Presentation 5 Minutes

Introduction to Fire Weather Presentation 5 Minutes

Measures of Atmospheric Moisture Presentation 5 Minutes

Gathering Temperature and Moisture Observations Presentation 35 Minutes

Factors That Impact Temperature and Relative Humidity Presentation 10 Minutes

Total Unit Duration 60 Minutes

Materials:

• Belt Weather Kit (NFES 001154) and handheld weather measurement devices.

• Incident Response Pocket Guide (IRPG), PMS 461, https://www.nwcg.gov/publications/461.

• NWCG Standards for Fire Weather Stations, PMS 426-3, https://www.nwcg.gov/publications/426-3.

• Psychrometric Table, https://www.nwcg.gov/publications/pms437/weather/temp-rh-dp-tables#TOC-

Elevation-6101-8500ft.

• NWCG Glossary of Wildland Fire, PMS 205, https://www.nwcg.gov/glossary/a-z.

• Notebooks for participants.

• Ability to display images and video on large screen.

• White board or easel access for group breakout.

S-190 Unit 3: Temperature and Moisture Relationships

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Slide 1

S-190 Unit 3: Temperature and Moisture Relationships

S-

190 Unit

3: Temperature

and Moisture Relationships

Unit 3: Temperature and Moisture Relationships

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Slide 2

 Review unit objectives.

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Slide 3

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Introduction to Fire Weather

 Play Video

Title Introduction to Fire Weather

Summary A introduction to fire weather, its causes, impacts, and effects on the fire environment.

Time (01:24)

Audio

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Measures of Atmospheric Moisture

 Play Video

Title Temperature and Moisture Relationships

Summary A brief introduction to the relationship between temperature and moisture in the fire

environment.

Time (00:47)

Audio

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Temperature Scales:°F and °C

Fahrenheit (°

F) is the unit of

choice for dry bulb

temperature readings in

wildland fire.

The temperature of the air measured in the shade,

4 to 8 feet above the ground.

Dry Bulb

Temperature

• Dry bulb temperature is the air temperature in our day-to-day lives. When it’s 86 °F outside, it refers

to the dry bulb temperature.

• Air temperature readings are taken 4 to 8 feet above the ground, either by manual observations, or by

automated weather stations.

• Fahrenheit and Celsius are the most common temperature scales used in the world.

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Wet Bulb Temperature

The lowest temperature to which air can be cooled

by evaporating water.

• Evaporation is a cooling process (the opposite of condensation) which results in a decrease in wet

bulb temperature.

• Wet bulb temperature is a good indicator of atmospheric moisture but not a direct measurement. For

example, little or no drop in wet bulb temperature indicates a moist air mass because little or no

evaporation has taken place. On the other hand, a wet bulb decrease of 10° to 15° from the starting

point would indicate a much drier air mass.

• It is important not to confuse wet bulb temperature with dew point temperature.

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Dew Point Temperature

The temperature to which air must be cooled to

reach saturation

. One of the most reliable methods

for measuring atmospheric

moisture.

• For example, if the dry bulb temperature is 80 °F and the dew point temperature is 50 °F, the dry

bulb temperature must decrease by 30 °F (down to 50 °F) for the air to become saturated.

• Dew point temperature is one of the most reliable methods for measuring atmospheric moisture.

• Dew point temperatures may change little from day to day.

• Dew point temperature can be determined with a sling psychrometer with the aid of Psychrometric

Tables or Relative Humidity tables in the IRPG.

• A handheld electronic weather meter can also be used for providing dew point temperature.

However, the handheld device must meet specifications in the NWCG Standards for Fire Weather

Stations, PMS 426-3, https://www.nwcg.gov/publications/426-3.

• Dew point may impact a region with consistent dew point values (more predictable) day to day.

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Relative Humidity (RH)

The ratio of the amount of moisture in the air to

the maximum amount of moisture that air would

contain if it were saturated.

• Relative humidity is expressed as a percentage and can range from 1% (very dry) to 100% (very

moist).

• Moisture in the air, whether in the form of water vapor, cloud droplets, or precipitation, is the

primary weather element that affects fuel moisture content.

• The amount of moisture that fuels can absorb from or release to the air depends largely on relative

humidity.

• Light fuels, such as grass, gain and lose moisture quickly with changes in relative humidity. Heavy

fuels respond to humidity changes at a slower rate.

• Relative humidity is not a direct measurement of atmospheric moisture but a measure of how much

moisture currently exists.

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Temperature and relative humidity have an inverse relationship;

when

one goes up, the other goes

down.

Temperature and Relative Humidity

• There can be a large fluctuation of temperature and relative humidity based on time and location.

However, the majority of large fires occur when air temperature is high and relative humidity is low.

• The graph depicted is an example of a reading from a hygrothermograph, which displays

temperature, and relative humidity.

• Maximum temperature for the day typically corresponds to the lowest relative humidity reading for

the day (usually occurs mid-afternoon, but dependent on time of year and aspect).

• Minimum temperature for the day typically corresponds to the highest relative humidity reading for

the day (usually occurs just after sunrise, but also dependent on time of year and aspect).

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Temperature,

RH, and Fuels

• Changes in temperature and relative humidity have similar impacts on fuels from one part the

country to the next. However, breakpoints for when fuels ignite and burn differ based on climate

zone.

• Critical relative humidity values in Florida are typically 30% to 35%, while in the western U.S.; they

are typically 15% or lower.

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Gathering Temperature and Moisture Observations

Automated Weather Stations

Remote

Fixed

• It is very important to routinely monitor temperature and relative humidity trends.

• Fixed and Remote Automated Weather Stations provide hourly observations to a local database via

satellite. The observations provide weather data on temperature, humidity, precipitation, wind

speed, and solar radiation.

• The observations are monitored by meteorologists and dispatch centers and are used in planned

ignitions, wildfires, and on other incidents, and projects to relay current weather information

representative of an area of interest.

• Fixed Automated Weather Stations are located in permanent locations throughout the country.

• Remote Automated Weather Station (RAWS or Fire RAWS) are portable units set up in

temporary locations to represent a small geographic area, such as a specific fire or incident.

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Gathering Temperature and Moisture Observations

The Belt Weather Kit

Pre-Video Discussion

• A Belt Weather Kit is a belt-mounted case with pockets fitted for anemometer, compass, sling

psychrometer, slide rule, water bottle, pencils, and book of weather report forms.

• Belt Weather Kits are used to take weather observations to provide on-site conditions to the fire

weather forecaster or Fire Behavior Analyst (FBAN). Observations include air temperature, wind

speed and direction, and relative humidity.

• The Belt Weather Kit is one of the most common methods of obtaining weather observations in the

field.

• Use of a Belt Weather Kit is often referred to as slinging or spinning weather, due to the use of the

sling psycrometer.

 Reference the needed weather observations from Spot Weather Forecast in the Incident Response

Pocket Guide (IRPG), PMS 461, https://www.nwcg.gov/publications/461.

 Play Video

Title Belt Weather Kit Tutorial

Summary A tutorial of the Belt Weather Kit and how to accurately sling and record weather.

Time (07:11)

Audio

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Gathering Temperature and Moisture Observations

Handheld Fire Weather Meters

Pre-Video Discussion

• A handheld fire weather meter is a electronic device that measures temperature, humidity, wind

speed, and possibly other atmospheric variables, depending on brand, and model.

• Handheld fire weather meters are used to take weather observations to provide on-site conditions to

the fire weather forecaster or Fire Behavior Analyst (FBAN). Observations include air temperature,

wind speed and direction, and relative humidity.

• There are several manufacturers of these meters. A handheld weather meter is often referred to as a

Kestrel, a common brand name.

• Regardless of make or model, all handheld weather devices must meet the specified NWCG

performance standards, as outlined in the NWCG Standards for Fire Weather Stations, PMS 426-3,

https://www.nwcg.gov/publications/426-3.

 Play Video

Title Digital Weather Meters

Summary An introduction to the abilities, guidelines, and uses of digital weather meters.

Time (05:26)

Audio

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Gathering Temperature and Moisture Observations

Psychrometric Tables

https://www.nwcg.gov/publications/pms437/weather/temp-rh-dp-

tables#TOC-Elevation-6101-8500ft

 Provide students with a printed copy of the psychrometric table valid for elevations between 6,101

and 8,500 feet, wet bulb temperatures 50 °F to 85 °F, and dry bulb temperatures 91°F to 109 °F,

https://www.nwcg.gov/publications/pms437/weather/temp-rh-dp-tables#TOC-Elevation-6101-

8500ft.

• Psychrometric tables are used to calculate dew point and relative humidity based on the observations

obtained in the field by either a Belt Weather Kit or handheld fire weather meter.

 Provide students with familiarization on how to locate and read the following elements of table:

o Location of dry bulb temperature.

o Location of wet bulb temperature.

o Location of dew point.

o Location of relative humidity.

 The tables are a mandatory component of the Belt Weather Kit. All kits should be inspected prior to

use in the field, to ensure the tables are available and intact.

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Psychrometric

Table: Elevations

between 6,101-8,500 feet

43 °F

19%

Wet-Bulb Temperature: 61

RH 15%

or Less

RH 16%

30%

RH 31%

60%

RH 61%

100%

Dry Bulb Temperature: 91

Exercise:

 Instruct students to reference the Psychrometric Table handout and follow along with the slide

example.

 Instruct students to locate dry bulb temperature of 91 °F.

 Instruct students to locate wet bulb temperature or 61 °F.

 Instruct students to determine dew point and relative humidity.

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Knowledge Check

Utilize the provided Psychrometric Table, Elevation 6,101-

8,500 feet, to solve the following problem:

Determine the relative humidity and dew point temperature

given the following information:

Dry Bulb Temperature =

95 °F

Wet Bulb Temperature =

56 °F

Relative Humidity = ?

Dew Point Temperature = ?

 Instruct students to determine the dew point and relative humidity based on the dry bulb and wet

bulb readings provided.

 Use next slide for answer validation.

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Answer Validation

22 °F

Wet-Bulb

Temperatures: 56 °F

RH 15%

or Less

RH 16%

30%

RH 31%

60%

RH 61%

100%

Dry Bulb Temperatures: 95 °F

Answer Validation

• Dew point temperatures = 22 °F

• Relative humidity = 7 %

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Knowledge Check

Find Relative Humidity: 1,400-4,999’ Elevation in the IRPG to

solve the following problem:

Determine the relative

humidity given the following:

Elevation = 1,535’

Dry Bulb Temperature =

°F

Wet Bulb Temperature = 60 °F

 Reference Relative Humidity: 1,400-4,999’ Elevation in the Incident Response Pocket Guide

(IRPG), PMS 461, https://www.nwcg.gov/publications/461.

 Instruct students that in order to determine the wet bulb depression, subtract the wet bulb

temperature from the dry bulb temperature.

 Dry bulb of 76 °F minus wet bulb of 60 °F = wet bulb depression of 16 °.

 Using the correct table, relative humidity = 40%.

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Factors That Impact Temperature and Relative Humidity:

• Elevation

• Topography

• Cloud Cover

• Wind

• Proximity to bodies

of water

Note to Instructor

Each factor will be discussed separately on slides 20-24.

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Elevation

Elevation can impact variations in temperature and relative humidity.

• Warmer temperatures and lower RH values are found in the lower elevations (lower valleys and

lower foothill regions).

• Cooler temperatures and higher RH values are found in the higher elevations (upper third of slopes

and ridgetops).

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Topography

• Topography can influence variations in temperature and relative humidity.

• Aspect is a feature of topography, which determines how much direct sunlight, and solar heating is

received.

• South and west-facing slopes receive more incoming solar radiation, especially during the afternoon,

and have hotter temperatures, and lower RH values.

• North-facing slopes typically experience less solar radiation and have cooler temperatures and higher

RH values.

• Different aspects represent different climate zones in terms of weather, fuel type, fuel moisture, and

overall potential fire behavior.

• An understanding of aspect and time of day provides the firefighter knowledge of possible changes

in fire behavior.

Question: If you are responding to a fire on a south aspect at 1000, what changes in temperature,

and RH can you anticipate to have occurred by 1500?

Answer: The south aspect would have received an increase in solar radiation between 1000 and

1500. Therefore, the temperatures have increased, and the RH has dropped, leading to drier

fuels.

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Cloud Cover

Lower Temperatures

Higher RH

Higher Temperatures

Lower RH

• Cloud cover affects temperature and relative humidity by reflecting incoming sunlight during the

day and intercepting outgoing long-wave, terrestrial radiation at night.

• During the day, cloud cover keeps temperatures cooler and RH higher.

• At night-cloud cover keeps temperatures warmer and RH lower (clouds act as a blanket at night).

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Wind

Day:

• Lowers air temperature

• Raises humidity

Night:

• Keeps air temperatures

warmer

• Lowers humidity

• Wind can influence variations in temperature and relative humidity.

• During the day, wind tends to disrupt surface heating by increasing mixing in the lower atmosphere.

Wind mixes cooler air above the surface with air near the surface, which keeps temperatures cooler,

and RH a little higher during the day.

• At night, wind tends to keep temperatures warmer and RH lower by disrupting radiative surface

cooling. Wind helps prevent warm air from radiating away from the surface.

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Proximity to Bodies of Water

Proximity to bodies of water can impact variations in temperature

and RH.

• An increase in dew point and RH is likely to occur near a large body of water.

• Air over the Gulf of Mexico can move northward into northern latitudes during the winter months,

increasing temperature over the land mass.

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Knowledge Check

Stronger winds at night will:

A. Keep temperatures warmer

and RH low.

B. Lower air temperature

and raise RH.

C. Keep temperatures cooler

and RH higher.

 Instructor can choose to have questions on the next three slides answered by individuals or in

groups.

Question: Stronger winds at night will:

Answer: A. Keep temperatures warmer and RH low.

Answer Validation

Winds at night keep the air mixed near the surface and disrupt radiant cooling (reduces the amount of

heat that escapes through radiant cooling by mixing the air near the surface). Without winds or under

calm conditions, radiant cooling processes are more effective, allowing heat to escape the surface

(surface cooling).

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Knowledge Check

South

-facing slopes are typically:

A. Cooler and drier than

north-facing slopes.

B. Cooler and more moist than

north-facing slopes

C. Warmer and drier than

north facing slopes

D. Warmer and more moist

than north-facing slopes.

Question: South-facing slopes are typically:

Answer: C. Warmer and drier than north-facing slopes.

Answer Validation

1. South-facing slopes receive more incoming solar radiation than north-facing slopes. High sun angle

in the afternoon allows for south-facing slopes to receive the longest period of incoming solar radiation

during the hottest and driest time of the day (which promotes hotter temperatures and lower RH).

2. Different aspects represent different climate zones. If applicable, the instructor should reiterate

differing fuel types from north aspect vs. south aspect.

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Question: Which of the following correctly explains cloud cover at night?

Answer: B. Keeps surface temperatures warmer than would otherwise be expected.

Answer Validation

Clouds at night act as a blanket, disrupting radiant cooling, thus keeping temperatures warmer, and RH

lower.

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 Review unit objectives.