What's the science behind low humidity and fire danger?

Low humidity often accompanies strong downslope winds which increase fire danger. This is because downslope winds increase pressure and temperature but lose moisture when referencing the Ideal Gas Law. The dry air wicks away what little moisture is left in fuels, which in turn increase their capacity to burn.

Posted: Oct 26, 2020 6:22 PM
Updated: Oct 27, 2020 8:46 AM

CHICO, Calif. - Low humidity often accompanies strong downslope winds which increase fire danger. This is because downslope winds increase pressure and temperature but lose moisture when referencing the Ideal Gas Law. The dry air wicks away what little moisture is left in fuels, which in turn increases their capacity to burn.

Winds, dry fuels, and warm temperatures can all contribute to fire danger but dry air plays a big role in fire danger.

Most of what we see when it comes to fire danger is strong downslope winds. This air is coming from high in elevation (mountains) and dropping to a lower elevation (valley).

Sinking air loses moisture and warms. But how?

Air is traveling from an area of low pressure (very high in elevation) to an area of higher pressure (lower in elevation). This means as air sinks down the mountains into the valley, the pressure will continue to increase.
Based on the Ideal Gas Law, Pressure (P) is equal to Density (D) times the Gas Constant (R) times Temperature (T). The equation is written like P=DRT. So pressure is proportional to density, and temperature. This means that if pressure changes, temperature or density must also change, so as to not violate the Ideal Gas Law. With air pressure increasing as the air travels downslope, the temperature is the variable on the other side of the equation that will increase as a result.

Warm air contributes to fire danger, but so does dry air. Downslope winds, the air that sinks, becomes drier because of the air warming.

Let's say we have an imaginary air parcel in the mountains and we bring it into the valley. The temperature would increase based on the principles that we discussed. But what won't change? The dewpoint temperature.

The dewpoint temperature is similar to the relative humidity. It is another way of measuring moisture in the air and is measured in degrees just like the air temperature. Whenever the air temperature equals the dewpoint temperature, the air is 100% saturated. This means you'll begin to see fog, clouds or rain begin to form, especially if the dewpoint temperature is greater than the air temperature. If the dewpoint temperature is less than the air temperature, the air is not completely saturated. The lower the dewpoint temperature becomes or the higher the air temperature becomes, the drier the air will be.

Now, why did I just describe dewpoint temperature to you? Whenever air descends down the mountains, its temperature increases because of pressure increasing. Moisture isn't being changed by the airdropping, however, so the dewpoint temperature will remain the same. This means the difference between the air temperature and dewpoint temperature will grow, meaning as air descends it will become much drier.

Drier air contributes to fire danger by wicking away moisture in already record dry vegetation. Air is always exchanging moisture with its environment. If it is very humid outside, if there is frost, dew, fog, or even rain, that moisture will get transferred from the air into the vegetation. If the air is bone dry, the air will take away what little moisture the vegetation has, making fuels like a tinder box, ready to burn.

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Chico
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Hi: 62° Lo: 38°
Feels Like: 50°
Oroville
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Clear
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A cold front has moved over northern California on this day before Thanksgiving, and it brought light rain and snow showers to the higher terrain, gusty wind and cooler air for all. Thanksgiving will still be breezy, but also mild and pleasant.
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