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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.

NWCG Latest Announcements

IBC Memo 24-01: OF 297 Signature Order and 2024 Fire Season Use IBC Memo 24-02: Summary of Changes for SIIBM, PMS 902

Date: July 15, 2024
Contact: Incident Business Committee 

The NWCG Incident Business Committee (IBC) has recently released two memorandums. The first provides direction on the use of the Emergency Equipment Shift Ticket, OF 297 for the 2024 fire season. Due to delays in hard copy printing, both the 2024 revision and the older version of the Emergency Equipment Shift Ticket, OF 297 are acceptable for use during the 2024 fire season.

The second memorandum documents the updates of the newly revised NWCG Standards for Interagency Incident Business Management (SIIBM), PMS 902. The summary of changes attached to the memo covers the major updates and process changes from the 2022 version.

References:

IBC Memorandum 24-01: OF 297 Signature Order and 2024 Fire Season Use

IBC Memorandum 24-02: Summary of Changes for the NWCG Standards of Interagency Incident Business Management, PMS 902

IBC Memorandum 24-02 Attachment: Summary of Changes

NWCG Standards for Rapid Extraction Module Support, PMS 552

Date: July 10, 2024
Contact: Incident Medical Unit Subcommittee 

A new publication from the Incident Medical Unit Subcommittee is now available. The NWCG Standards for Rapid Extraction Module Support, PMS 552 will be used as a guide and as an opportunity to begin to build out Rapid Extraction Module Support (REMS) modules for the remainder of Fire Year 2024. These standards will be fully implemented as the minimum standard starting in January 2025.

NWCG Standards for Rapid Extraction Module Support outlines the roles, duties, qualifications, and equipment pertinent to REMS. A REMS team, strategically stationed at wildland fires, plays a pivotal role in prioritizing swift access and medical treatment to injured or ill firefighters for safe and efficient egress off the fireline. This ensures their rapid transport to definitive medical care in cases of emergency during firefighting operations, highlighting the invaluable contribution of the REMS team to firefighter safety and well-being.

References:

NWCG Standards for Rapid Extraction Module Support, PMS 552

National Wildland Firefighter Day

Date: July 2, 2024
Contact: National Interagency Fire Center 

The National Wildfire Coordinating Group (NWCG) would like to recognize July 2, 2024, as National Wildland Firefighter Day (NWFFD). Established in 2022, NWFFD honors the dedication of wildland firefighters and support personnel. This day falls within the 2024 Week of Remembrance (June 30 - July 6), providing an opportunity to renew our commitment to wildland firefighter safety while remembering those who have fallen in the line of duty.

NWCG continues to provide leadership to enable interoperable wildland fire operations among federal, state, local, Tribal, and territorial partners. NWCG standards and training establish common practices to contribute to safe, effective, and coordinated national interagency wildland fire operations.

References:

NWCG.gov

Week of Remembrance

National Wildland Firefighter Day

2024 Week of Remembrance

Date: June 27, 2024
Contact: 6 Minutes for Safety Subcommittee 

As we approach the 2024 Week of Remembrance (WOR), June 30 to July 6, we dedicate this time to thoughtfully reviewing and recognizing the events of the 2018 Mendocino Complex. As such, this year’s theme of “Learning From the Mendocino Complex” embodies a longstanding hallmark of WOR, honoring through learning.

Throughout the week, our energy will be directed toward fostering generative conversations in briefing rooms and at tailgates.

References:

6 Minutes for Safety - 2024 Week of Remembrance

Wildland Fire Lessons Learned Center