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7.4 Relative Humidity

Relative humidity is the percent of water vapor in the air compared to what would be present if the air were saturated. Fully saturated air is fog. Relative humidity is always expressed as a percentage.

Relative humidity can be determined by measuring the dry bulb and wet bulb temperatures in the field. Using these measured values, the dew point and relative humidity can be determined with the use of tables. These psychrometric tables are valid for specific elevation ranges because relative humidity and dewpoint change with atmospheric pressure, which varies with elevation.

Relative humidity also changes with temperature. Dewpoint, however, remains nearly the same as long as the moisture content of the air is not changing. This fact can be used to help estimate the minimum relative humidity for the afternoon, using the predicted high temperature for the day and the observed dewpoint. On a typical sunny day, temperatures in the lower atmosphere will decrease about 5.5 degrees F for every 1,000 feet in elevation. As temperatures approach the dewpoint, the same amount of water vapor will result in a higher relative humidity.

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 tables relate dry bulb, wet bulb, dewpoint, and relative humidity.

Click on the graphic below to view a multimedia version of the psychrometric table lesson (includes audio), or read the text version of the steps involved below:

psychrometric table

1) Locate the wet bulb temperature among the numbers along the top row of the table. These are the wet bulb temperatures and are shown in green.
2) Locate the dry bulb temperature among the numbers in the far left column. These are dry bulb temperatures and are shown in black.
3) For a specific dry bulb temperature and wet bulb temperature, find the intersection of the appropriate row and column. The values at that point are the dewpoint temperature and the relative humidity. Within each box, the dewpoint is the top number (in blue), given in degrees Fahrenheit. The relative humidity is the bottom number (in red), given as a percent.

For example, for a dry bulb temperature of 40°F and a wet bulb temperature of 35°F, the dewpoint is 29° F and the relative humidity is 65%.

Test your knowledge by completing the following exercises. For each exercise, use your mouse to drag the magnifying square to find the appropriate relative humidity and dewpoint and type your answers into the spaces provided.

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NEW! S-130, Firefighter Training (Blended) Now Available

Date: June 10, 2025
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The S-130, Firefighter Training (Blended), equips new wildland firefighters with essential skills through a mix of online learning and hands-on training. The course features a required online component and an instructor-led exercise, reinforcing online concepts with practical performance-based training.

The course builds competencies in wildland fire operations, safety, and equipment use, applicable across various operational environments. Both the online component and instructor-led field exercise must be completed to receive a certificate of completion. Students are required to finish the prerequisite S-190, Introduction to Wildland Fire Behavior, before enrolling in S-130, Firefighter Training (Blended).

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EMC Memo 25-001: Interim Rapid Extraction Module (REMS) Training and Qualification Requirements

Date: June 3, 2025
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Emergency Medical Committee

The Emergency Medical Committee has issued interim guidance for Rapid Extraction Module Support (REMS) teams. Until further notice, Type I and II REMS teams must designate a team leader qualified at Firefighter Type 1 Squad Boss (FFT1) or higher. This temporary change replaces the Single Resource Boss (SRB) requirement, which is currently not feasible due to administrative barriers.

This memorandum does not include any other changes to the NWCG Standards for Rapid Extraction Module Support, PMS 552. This interim change takes effect immediately and will remain in place until further notice.

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EMC Memo 25-001: Interim Rapid Extraction Module (REMS) Training and Qualification Requirements

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Equipment Bulletin 25-002: Chaps, Chain Saw, M-2020, Nonconformities Affecting Use, Appearance, and Serviceability

Date: June 2, 2025
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The Equipment Technology Committee issued Equipment Bulletin: 25-002 to address a manufacturing nonconformity affecting Forest Service specification, 6170-4K Chain Saw Chaps. The issue applies to chaps manufactured in 2024 and 2025 and distributed through FedMall. These chaps may have incorrectly bound edges that expose inner protective layers.

Independent purchasers should inspect all chain saw chaps received from FedMall beginning in 2024, prior to use. Review the full Equipment Bulletin: 25-002 for inspection criteria and recommended actions.

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2024 Wildland Fire Emergency Medical Service Awards

Date: May 22, 2025
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The NWCG Emergency Medical Committee (EMC) is proud to announce the recipients of the 2024 Wildland Fire Emergency Medical Service Awards. Each year, EMC recognizes individuals and groups who have demonstrated exceptional actions or accomplishments that go above and beyond their normal mission or job duties.

Congratulations to all the awardees and nominees. Through leadership and initiative, they have made significant contributions to the safety and well-being of the wildland fire community. These honors are well deserved.

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