Skip to main content

Firefighter Math: 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.

NWCG Latest Announcements

2026 Updates to the NWCG Standards for Wildland Fire Position Qualifications, PMS 310-1

Date: January 23, 2026
Questions? 
Please contact: Incident and Position Standards Committee

The Incident and Position Standards Committee has updated the NWCG Standards for Wildland Fire Position Qualifications, PMS 310-1.

These updates address changes to incident position names and requirements for qualification, training, and experience. Updates related to the implementation of Complex Incident Management (CIM) and Incident Performance Training Modernization (IPTM) are also included.

The NWCG Wildland Fire Position Qualification Flowchart, PMS 308 has also been updated to reflect the recent changes.

References:

NWCG Standards for Wildland Fire Position Qualifications, PMS 310-1

NWCG Wildland Fire Position Qualification Flowchart, PMS 308

IPSC Memo 26-01: January 2026 Updates to the NWCG Standards for Wildland Fire Positions Qualifications, PMS 310-1

Incident Business Committee Memo 26-01: Updated Emergency Equipment Shift Ticket, OF 297

Date: January 22, 2026
Questions? 
Please contact: Incident Business Committee

The Incident Business Committee (IBC) has released Memorandum 26-01: OF 297 and 2026 Fire Use. This memo informs the wildland fire community that the updated Emergency Equipment Shift Ticket, OF 297 is available for download and can be ordered through the Great Basin Cache.

Beginning January 15, 2026 the previous version of the OF 297 form has been discontinued.

For further details, please contact your IBC representative.

References:

Incident Business Committee Memorandum 26-01

Great Basin Cache

NEW! S-390, Introduction to Wildland Fire Behavior Calculations (Blended) Available Now

Date: December 22, 2025
Questions about RT-130? 
Please contact: Fire Behavior Subcommittee

NWCG is excited to announce that the new S-390, Introduction to Wildland Fire Behavior Calculations (Blended) training is now available on the Wildland Fire Learning Portal.

This third course in the series combines online and instructor-led training components aimed at individuals who are involved in planning, managing, and executing wildland fire and prescribed burn operations; who require a thorough understanding of fire behavior calculations to enhance effectiveness and safety. This includes students who require the knowledge and skill necessary to perform the duties of a Type 3 Incident Commander (ICT3), Division/Group Supervisor (DIVS), or Prescribed Fire Burn Boss Type 2 (RXB2).

Students are required to be qualified as any Single Resource Boss position and complete the prerequisite S-290, Intermediate Wildland Fire behavior (Blended) course, before enrolling in S-390.

References:

S-390, Introduction to Wildland Fire Behavior Calculations (Blended)

Wildland Fire Learning Portal

NEW! S-320, Introduction to Incident Management Teams (Blended) Available Now

Date: December 18, 2025
Questions about RT-130? 
Please contact: Incident and Position Standards Committee

NWCG is excited to announce that S-320, Introduction to Incident Management Teams (Blended) is now available on the Wildland Fire Learning Portal!

This blended course combines online learning with instructor-led training, designed for individuals seeking to build leadership skills and gain experience in incident management.

Students are required to complete the prerequisite trainings ICS-100, Introduction to Incident Command System (ICS), ICS-200, Basic ICS for Initial Response, and ICS-700, An Introduction to the National Incident Management System (NIMS) prior to attending S-320.

References:

S-320, Introduction to Incident Management Teams (Blended)

Wildland Fire Learning Portal