S-130 Weather Observation 3 — Questions and Answers
Question 1: What is the relative humidity level generally considered critically low, significantly increasing fire danger in wildland firefighting?
- Below 50%
- Below 35%
- Below 25% (Correct answer)
- Below 10%
Correct answer: Below 25%
Relative humidity below 25% is typically considered critically low, causing rapid drying of fine fuels and sharply increasing fire danger and spread rates.
Question 2: A firefighter notes the wind direction has shifted 180 degrees and speed has increased. This is MOST likely associated with:
- A sea breeze developing
- Passage of a frontal system (Correct answer)
- An inversion breaking in the morning
- A normal nocturnal drainage wind
Correct answer: Passage of a frontal system
A sudden 180-degree wind shift combined with increasing speed is a classic indicator of frontal passage, which can cause extreme fire behavior.
Question 3: What does the term 'fine fuel moisture content' refer to in fire weather observation?
- The moisture level in large logs and heavy fuels
- The amount of water in the air above a fire
- The moisture percentage in 1-hour timelag fuels like grass and dead needles (Correct answer)
- The relative humidity measured at the fire's edge
Correct answer: The moisture percentage in 1-hour timelag fuels like grass and dead needles
Fine fuel moisture refers to the moisture content of small-diameter, fast-drying fuels such as grasses and dead needles that respond quickly to atmospheric changes.
Question 4: Which cloud type is most associated with stable atmospheric conditions and generally lower fire danger from wind?
- Cumulonimbus
- Cumulus
- Stratus (Correct answer)
- Altocumulus castellanus
Correct answer: Stratus
Stratus clouds form in stable, layered air with little vertical mixing, indicating calm conditions less likely to produce erratic fire-driving winds.
Question 5: How does increasing elevation generally affect temperature and relative humidity in mountainous fire environments?
- Temperature increases and humidity decreases
- Both temperature and humidity increase
- Temperature decreases and humidity generally increases (Correct answer)
- Humidity decreases sharply while temperature stays constant
Correct answer: Temperature decreases and humidity generally increases
As elevation increases, air temperature typically decreases at the environmental lapse rate while relative humidity tends to increase, affecting fuel drying rates.
Question 6: What is the purpose of using a Kestrel weather meter or similar handheld device on a wildland fire?
- To calculate the rate of spread using fire behavior models only
- To take on-site measurements of wind speed, temperature, humidity, and dew point (Correct answer)
- To communicate weather observations to aircraft
- To detect toxic gases in smoke
Correct answer: To take on-site measurements of wind speed, temperature, humidity, and dew point
Handheld weather meters allow firefighters to take real-time, on-site measurements of critical weather variables to assess current and anticipated fire behavior.
Question 7: What is the dew point temperature and why is it important in fire weather?
- The temperature at which fuel ignites; lower dew points increase ignition risk
- The temperature at which air becomes saturated and moisture condenses; low dew points mean dry air (Correct answer)
- The overnight low temperature that fuels recover moisture to; used to plan morning attacks
- The temperature differential between ridge and valley that drives slope winds
Correct answer: The temperature at which air becomes saturated and moisture condenses; low dew points mean dry air
Dew point is the temperature at which air reaches 100% relative humidity; a large spread between air temperature and dew point indicates very dry air and high fire danger.
S-130 Weather Observation 3
What is the relative humidity level generally considered critically low, significantly increasing fire danger in wildland firefighting?