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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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2025-2026 Wildland Fire Leadership Campaign: "Leadership Levels" Now Available

Date: October 15, 2025
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Leadership Committee

NWCG is excited to announce the release of the Leadership Committee’s 2025–2026 Leadership Campaign, themed “Leadership Levels.” This annual initiative offers all students of leadership and wildland fire management an opportunity to engage with essential leadership skills and knowledge needed to lead effectively in dynamic environments.

The campaign is structured into four quarters and may be completed at any time between October 1, 2025 -December 31, 2026. Leadership levels will be released quarterly.

Quarter One materials are now available on the Wildland Fire Leadership Development Program website and focus on Leadership Level 1, Follower (Self-Leadership).

References:

2025/2026 NWCG Leadership Campaign - Leadership Levels

Wildland Fire Leadership Development Program

Leadership Committee

Updated! L-280, Followership to Leadership (Instructor-led) Course

Date: October 3, 2024
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Leadership Committee

NWCG is pleased to announce the updated L-280, Followership to Leadership (Instructor-led) course is now available. L-280 is intended for operational personnel at the Firefighter Type 1 level, and for individuals pursuing Leadership Level 2, New Leader. 

This instructor-led course is a primer on leadership and the second formal course in the Wildland Fire Leadership Development Program. It helps emerging leaders build the confidence, skills, and mindset needed to lead effectively in high-risk environments. 

For more information about leadership development and available courses, visit the Leadership Committee Web Portal.

References:

L-280, Followership to Leadership (Instructor-led)

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Equipment Advisory 25-01: Pre-2006 Fire Shelter Degradation and Discontinuation of Use

Date: Sept 26, 2025
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Equipment Technology Committee 

The Equipment Technology Committee (ETC) has released Equipment Advisory 25-01: Pre-2006 Fire Shelter Degradation and Discontinuation of Use. This advisory recommends all pre-2006 fire shelters should be removed from service by January 1, 2026.

This advisory recommends firefighters and fire managers to:

  • Inspect fire shelters immediately
  • Replace and destroy any pre-2006 shelters (identified by white or pink insert label)
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For further details, please refer to the complete advisory. 

References:

Equipment Advisory 25-01: Pre-2006 Fire Shelter Degradation and Discontinuation of Use

NWCG Fire Inspection Flowchart, PMS 411-1

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NEW! S-219, Firing Operations (Blended) Now Available

Date: Sept 11, 2025
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Fuels Management Committee 

The S-219, Firing Operations (Blended) course is now available on the Wildland Fire Learning Portal. Developed through the Incident Performance and Training Modernization (IPTM) effort, this training supports individuals working towards Firing Boss, Single Resource (FIRB) incident qualifications.

This blended course combines online and instructor-led training to provide students with the knowledge and skills required to perform the duties of the FIRB, as described in the NWCG Incident Position Standards for Firing Boss, Single Resource, PMS 350-105.

Any changes to qualification pathways will take effect with the next update of the NWCG Standards for Wildland Fire Position Qualifications, PMS 310-1, scheduled for January 2026.

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S-219, Firing Operations (Blended)

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