Skip to main content

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

ETC/RMC Safety Bulletin: 25-001 New Guidance on Laundering Wildland Fire Clothing to Reduce Contamination

Date: Aug 8, 2025
Questions? Please contact:
Equipment Technology Committee
 Risk Management Committee

The Equipment Technology Committee (ETC) and the Risk Management Committee (RMC) have issued Safety Bulletin 25-001: Laundering to Decontaminate Wildland Fire Clothing. Recent research revealed that wildland fire flame-resistant pants and shirts can be contaminated with chemicals from combustion byproducts, including carcinogens, and that common laundering practices can effectively remove these harmful contaminants from wildland firefighter clothing more effectively than previously understood. It is recommended to decontaminate wildland fire clothing as frequently as possible. 

Frequently Asked Questions about this new information and how to implement recommendations can be found on the NWCG Alerts page. Read the complete ETC/RMC Safety Bulletin: 25-001 to learn more. 

References:

NWCG Alerts

ETC Safety Bulletin: 25-001

Frequently Asked Questions (FAQ) on Laundering to Decontaminate Wildland Fire Clothing

Equipment Technology Committee

Risk Management Committee

FAQ Now Available for Archiving Type 1 and Type 2 Incident Positions

Date: Aug 6, 2025
Questions? Please contact:
Incident and Position Standards Committee

To support the transition to Complex Incident Management (CIM), NWCG will archive all Type 1 and Type 2 Command and General Staff (C&G) position qualifications in January 2026, as outlined in NWCG Executive Board Memo 25-002. To assist with this transition, a new Frequently Asked Questions (FAQ) document is now available.

This resource answers common questions about the status of Type 1 and Type 2 qualifications, impacts to incident qualification management, training requirements, and resource ordering considerations. Review the full FAQ to learn more.

For additional information on CIM and the transition of C&G positions, see NWCG Executive Board Memos 23-005, 24-005, and 25-002, as well as the Incident Workforce and Development Group webpage.

References:

NWCG Type 1 & Type 2 Position Archiving FAQ

NWCG Executive Board Correspondence

Incident Workforce Development Group

Incident and Position Standards Committee

Incident Position Standards and the Next Generation Position Task Books Now Available for UASD, UASM, UASL and UASP

Date: July 31, 2025
Questions?  Please contact:
Interagency Fire Unmanned Aircraft Systems Subcommittee
 

NWCG is excited to announce that Incident Position Standards and the Next Generation Position Task Books are now available for all four Unmanned Aircraft Systems positions:

  • Unmanned Aircraft Systems, Data Specialist (UASD)
  • Unmanned Aircraft Systems, Manager (UASM)
  • Unmanned Aircraft Systems, Module Leader (UASL)
  • Unmanned Aircraft Systems Pilot (UASP)

The Performance Support Packages for these positions were developed as part of the Incident Performance and Training Modernization effort. These resources support trainees, qualified personnel, and evaluators in their respective roles.

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.

References:

NWCG Unmanned Aircraft Systems, Data Specialist Position Page

NWCG Unmanned Aircraft Systems, Manager Position Page

NWCG Unmanned Aircraft Systems, Module Leader Position Page

NWCG Unmanned Aircraft Systems Pilot Position Page

Incident Position Standards and the Next Generation Position Task Books Now Available for RADO and INCM

Date: July 30, 2025
Questions?  Please contact:
Incident Logistics Subcommittee
 

NWCG is excited to announce that Incident Position Standards and the Next Generation Position Task Books are now available for Radio Operator (RADO) and Incident Communications Center Manager (INCM).

The Performance Support Packages for these positions were developed as part of the Incident Performance and Training Modernization effort. These resources support trainees, qualified personnel, and evaluators in their respective roles.

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.

References:

NWCG Radio Operator Position Page

NWCG Incident Communications Center Manager Position Page