Foggy fires
Q: Does fire burn in fog?
A:
 Yes, fire burns in fog. The first example that comes to mind are peat fires smouldering through the centuries-even
heavy rain won't put them out. The lowland areas of Indonesia has peat that has accumulated for 7,000 or more years. If
the peat is exposed, it dries quickly and then can ignite easily. Once burning sometimes the fire goes deep into the earth
and is almost impossible to put out. Fog and rain come and go and the fire burns on.
Figure Caption: [NOAA] Fog is a cloud near the ground.
The peat fires show that fire can burn in a fog, but why? You need fuel and oxygen to make a fire. You can certainly get
the fuel so getting sufficient oxygen to the fuel is the first question.
When it's foggy, water vapor in the air condenses out to form tiny liquid water droplets that remain suspended in the air.
That's fog. When water vapor condenses, it does reduce the oxygen in the air but not by much. Here's why:
Consider a parcel of the foggy air. How does it differ from dry air? In particular, how much less oxygen does it have?
Dry air contains about 20 percent oxygen.
When a parcel of dry air becomes foggy air, probably nothing changes about its pressure or temperature. The only thing
that changes is the air becomes more humid, which means that more water-vapor molecules form. Aha! Under these
conditions of constant pressure and temperature, the ideal gas law tells us that the number of gas molecules in our air
parcel will stay the same. So a gain of water-vapor molecules must mean a loss of dry-air molecules (i.e., oxygen,
among others) in order to keep the total number the same. Thus, we do end up with fewer oxygen molecules. How
much fewer is the critical question.
To answer that, we need to know how much water-vapor is in the foggy air because the water-vapor gain equals the loss of dry-air molecules. Saturated specific
humidity is an extremely useful concept that meteorologists use all the time. It tells us how much of the air is water vapor.
"For air at the Earth's surface and between freezing and 30 degrees C (86 degrees Fahrenheit), the saturated specific humidity will vary between 0.5% and
2.5%," says Neal Dorst of the Atlantic Oceanographic and Meteorological Laboratory.
Bingo! We've got it. That is the dry-air loss: between 0.5% and 2.5%.
Dry air is made up of oxygen and other dry-air molecules
(such as, nitrogen and carbon dioxide) so this percentage is an upper bound on the amount of oxygen
that the air parcel lost as the air became foggy. That's not much loss compared to the 20 % oxygen content in dry air and surely not enough to smother a fire.
The fire can burn-it has enough oxygen.
The next question is will the water droplets in the foggy air wet the burning fuel and extinguish the fire. The water droplets are only about 0.01 millimeter in
diameter (slightly larger than red blood cells). A dense fog contains about 1200 visible drops per cubic centimeter of empty space, which is barely enough water
to wet an object's surface. The fog droplets will collect on the wood but the fire will burn fine, once lit, if the fire is reasonably big and hot. Its heat will
evaporate the water suspended in the air as it burns and will dry out new wood if it's not too soggy.
So, fire burns in fog if the fire is hot enough.
(Answered by April Holladay, science correspondent, May 9, 2001)
Further Surfing:
USATODAY.com: Understanding fog
|
