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Hot Air

Air is all around us, but it can be hard to see.  Find out more about why hot air rises, how temperature changes air pressure, and what makes the wind blow.

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Hot Air

hot air molecules

Air is made up of molecules that are constantly in motion. As air warms up, the molecules start to vibrate and bump into each other, increasing the space around each molecule. Because each molecule uses more space for motion, the air expands and becomes less dense (lighter). In other words, the same number of air molecules occupy a larger space or the same sized space with increased air pressure.

cold air molecules

 

The opposite effect happens when air cools. As the temperature drops, molecules move more slowly, taking up less room. The amount of space the air takes up shrinks, or reduces the air pressure.

Why does Hot Air Rise?

Warm air rises, and when it rises it becomes cooler. That information is key to understanding a lot of meteorology (science of weather).

Rising air experiences a drop in temperature, even though no heat is lost to the outside. The drop in temperature is a result of the decrease in atmospheric pressure at higher altitudes. If the pressure of the surrounding air is reduced, then the rising air parcel will expand. The molecules of air are doing work as they expand. This will affect the parcel's temperature (which is the average kinetic energy of the molecules in the air parcel).

One of the results of the Laws of Thermodynamics is that there is an inverse relationship between the volume of an air parcel and its temperature. During either expansion or compression, the total amount of energy in the parcel remains the same (none is added or lost). The energy can either be used to do the work of expansion, or to maintain the temperature of the parcel, but it can't be used for both.

If the total amount of heat in a parcel of air is held constant (no heat is added or released), then when the parcel expands, its temperature drops. When the parcel is compressed, its temperature rises. In the atmosphere, if the parcel of air were forced to descend, it would warm up again without taking heat from the outside. This is called adiabatic heating and cooling, and the term adiabatic implies a change in temperature of the parcel of air without gain or loss of heat from outside the air parcel.

Adiabatic processes are very important in the atmosphere, and adiabatic cooling of rising air is the dominant cause of cloud formation.

 

What makes the wind blow?

When warm air rises, cold air moves in replacing the warm air. This movement of air is what we call wind.

 

Key Points

  • when air rises, its temperature decreases
  • when air subsides, its temperature increases
  • when the temperature of a parcel of air decreases, its relative humidity increases
  • when the temperature of a parcel of air increases, its relative humidity decreases
  • the normal environmental lapse rate applies to still air
  • the dry adiabatic lapse rate applies to rising air, when the relative humidity is below 100%
  • the dry adiabatic lapse rate also applies to air that is subsiding, if there is no moisture present, and no evaporation is taking place

Websites with information on Air

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