The temperature of air changes as it moves vertically through the atmosphere. These vertical movements and resulting temperature shifts are important for understanding daily weather phenomena. The interaction between moving air and atmospheric pressure dictates whether air warms or cools, influencing everything from clear skies to cloud formation.
Understanding Air Pressure and Movement
Atmospheric pressure is the force exerted by the weight of the air above a given point. This pressure naturally decreases with increasing altitude because there is less air overhead.
To understand air movement, meteorologists imagine a discrete volume of air, known as an air parcel, moving independently within the atmosphere. As an air parcel ascends, it encounters lower atmospheric pressure. Conversely, a sinking air parcel moves into regions of higher surrounding air pressure. These pressure differences cause the air parcel to expand or compress, directly impacting its temperature.
The Principle of Adiabatic Change
Air temperature changes with vertical movement due to adiabatic change. This process occurs when an air parcel expands or compresses without exchanging heat with its environment.
When an air parcel rises, it expands due to lower pressure. This expansion causes air molecules to spread, using internal energy and decreasing the parcel’s temperature.
Conversely, when an air parcel sinks, it is compressed by higher atmospheric pressure. This compression forces molecules closer, increasing their kinetic energy and raising the parcel’s temperature. For unsaturated air, this cooling or warming occurs at a consistent rate, known as the dry adiabatic lapse rate, approximately 9.8 degrees Celsius per kilometer or 5.5 degrees Fahrenheit per 1,000 feet.
Rising Air: Cooling and Cloud Formation
As an air parcel rises, it cools adiabatically at the dry adiabatic lapse rate. If this rising air contains moisture, its temperature will eventually drop to the dew point. The dew point is the temperature at which the air becomes saturated with water vapor, leading to condensation. At this point, water vapor transforms into liquid water droplets or ice crystals, forming clouds.
Further ascent of the saturated air parcel leads to continued cooling, but at a slower rate called the moist adiabatic lapse rate, which is roughly 6 degrees Celsius per kilometer or 3.3 degrees Fahrenheit per 1,000 feet. This slower cooling occurs because the condensation process releases latent heat into the air parcel, partially offsetting the cooling due to expansion. This phenomenon is observable in the formation of cumulus clouds on sunny days or when air is forced upwards over mountain ranges, creating orographic clouds and precipitation.
Sinking Air: Warming and Clear Skies
When an air parcel descends, it undergoes adiabatic warming as it is compressed by increasing atmospheric pressure. This warming occurs at the dry adiabatic lapse rate, approximately 9.8 degrees Celsius per kilometer. As the air warms, its capacity to hold water vapor increases, causing any existing moisture within the parcel to evaporate. This process typically leads to clear, cloud-free skies and stable atmospheric conditions.
Sinking air is a characteristic feature of high-pressure systems, which are often associated with fair weather. A notable example of this warming effect is seen in foehn or chinook winds, which occur when moist air rises over one side of a mountain range, loses its moisture as precipitation, and then descends on the leeward side, warming significantly as it sinks. These winds can cause dramatic temperature increases, sometimes raising temperatures by 20 degrees Celsius in a few hours or even 49 degrees Fahrenheit in two minutes. Another consequence of sinking air is the formation of temperature inversions, where a layer of warmer air sits above cooler air, suppressing vertical mixing and potentially trapping pollutants near the surface.