Atmospheric stability is certainly a complex concept! Stability in the atmosphere is usually described in terms of 'lapse rates'. An understanding of latent heat is also imperative to conceptualizing atmospheric stability, therefore this page will focus on both lapse rates and latent heat as precursors to our understanding of atmospheric stability.

Even before we begin here, it is highly recommended that you visit our "Water & Weather" page. There you will learn some of the important properties of water that will greatly enhance your understanding of atmospheric stability here.

Lapse Rates
Lapse rate defines the way in which temperature varies with altitude. Lapse rates come in two
general flavors: The Environmental Lapse Rate (ELR) and The Adiabatic Lapse Rates. The latter
comes in two flavors of its own: The Dry Adiabatic Lapse Rate (DALR) and The Saturated
Adiabatic Lapse Rate (SALR) sometimes called The Wet Adiabatic Lapse Rate (WALR) and sometimes it is also called The Moist Adiabatic Lapse Rate (MALR)! Ugggg! We'll just call it SALR.

The ELR is the actual variation of temperature with height at a certain time and place. It varies from place to place and from time to time so there really is no fixed rate. Meteorologists measure temperatures through a vertical profile by releasing weather balloons with mini-weather stations attached to them called radiosondes. Sometimes, meteorologists drop these mini-weather stations from a plane at high altitude with a parachute attached. These latter type of measuring devices are called dropsondes.

The word 'adiabatic' refers to a thermodynamic process in which heat neither enters nor leaves the system in question. Yet temperatures drop with altitude in the troposphere. Why is that? Considering the fact that no heat leaves the system, how is it possible that air gets cooler with altitude?

The following illustration shows how pressure decreases with increasing altitude. The reason pressure is lower with altitude is due to the fact that there are fewer 'air molecules' at altitude than there are closer to Earth. Air becomes thinner (air molecules become increasingly scarce) as you go up. As a result, air expands. The following illustration opens in separate window. Be sure to close the window to return to this page!
See Illustration

This is a good point to list a couple important laws of physics! These links will open in a new window, so just close the windows when you are ready to return to this page!
The Law of Adiabatic Expansion/Compression
The Law of Hydrostatic Balance

Since pressure decreases with elevation, rising air parcels (see Law of Buoyancy) will expand. There will be less inward pressure relative to outward pressure exerted on that parcel. The process of expansion effectively takes up some internal energy of that parcel; this energy is heat energy, therefore the result is removal of heat translating in lower temperatures. The energy is still there (it is neither created or destroyed). It is simply not detectable by measuring instruments (thermometers)!
Check out The Ideal Gas Law!

Temperature doesn't just suddenly drop. It drops at a rate (-deg/elev) known generically as the Adiabatic Lapse Rate. So what is this rate? It depends on whether the air parcel ascending into the sky is saturated or not. If it is 'dry' (not saturated), it will rise and expand experiencing temperature drop as heat energy is absorbed during the expansion process. If, on the other hand, it is saturated, then it will rise, expand and reach its dew point temperature causing the water vapor in it to condense on condensation nuclei.

During condensation, water vapor is undergoing a phase change from gas to liquid requiring that every gram of water vapor release 539 calories of heat energy (read more about this in our "Weather's Water" section). This heat energy is released thus lowering the rate at which the air parcel cools as it ascends. For example: if air cooled 10 units per 1000 feet (lost 10 units of heat), then condensed and released 3 units of heat, then we would subtract 3 units from the original 10 giving us a slower rate of 7 units per 1000 feet.

Now let's throw in another variable: momentum. The momentum of any object is just its mass multiplied by its velocity, or mv. Momentum expresses the persistence of movement, or inertia, of a body in motion. The pressure exerted by a gas (in this case air of a thermal) is related to the molecular motion of the gas molecules. When a molecule collides with a surface (another molecule) it produces a force on that surface. That force multiplied by the by the time duration of the collision is called the impulse. Impulse is related to momentum (both are properties of mass).

It is likely that the thermal will have momentum on its side as it ascends. So even if it reaches a temperature equal to that of surrounding air, it may still ascend another 300 meters or so just from momentum! In this scenario, the thermal will become slightly cooler than surrounding air, but this makes it denser than surrounding air and it will soon descend. Though, it is likely to have momentum on its side as it descends now! In which case it will simply descend below the level at which its temperature equals the temperature of surrounding air and heat adiabatically causing it to become slightly warmer than surrounding air. Again, it will begin to rise and may pass the point again, each time having less and less momentum. See it here>> click here

Okay, now let's throw another factor into the mix: moisture! We discussed the SALR earlier, and learned that it is lower than the DALR due to release of latent heat as water vapor condenses.

Condensation level is the level at which moisture in rising air (thermals) starts to condense and form droplets of cloud. It is the point at which dew point temperature and air temperature become equal. You should already be aware that dew point temperature is the temperature to which air must be cooled to become saturated with water vapor. Further cooling leads to the formation of cloud droplets or fog in the atmosphere or the deposit of dew on surfaces. If air temperature equals dew point temperature, relative humidity equals 100%.

Condensation level is very nearly the base of any cumulus cloud. You can indirectly see it best when there are large cumulonimbus or towering cumulus clouds in the sky. Their flat bases are very nearly indicative of condensation level! You'll notice there aren't any clouds below this level.

The following animation will illustrate an air parcel's orographic ascent up a mountain slope, reaching condensation level, then proceeding up the slope and descending the other side. This example is commonly used in the classroom because it not only illustrates DALR and SALR, but it also shows condensation level and DPLR as well! It also effectively illustrates the cause of rain shadow desert environments.
See it here >>click here

Having watched the animation at the link above, you are now aware of the lifting condensation level, and how it can be considered to be the boundary delimiting areas where a thermal will rise at DALR and where it will rise at SALR. You have also learned that lifting condensation level is very nearly located at the base of any CB (cumulonimbus) or TCU (towering cumulus) cloud. If you have the free Flash plug-in, then check out the following interactive animation depicting linear relationships between DPLR, SALR, DALR and LCL! It opens in a new window, click here
 

Here in the San Fernando Valley, I generally restrict this equation to times between sunrise and about 10 to 11am when reporting cloud bases to aircraft.

We have covered DALR, SALR, DPLR, LCL and ELR. Now let's look at inversions or as they are sometimes referred to as inversion layers.

However, an inversion is where this cooling stops, and a warming trend with height occurs. The following illustration is adapted from the animation on the previous page you looked at showing lapse rates' relationships with lifting condensation level. The following illustration shows SALR, DALR, DPLR, and LCL, but it also shows a new element: the ELR. It illustrates on one of seemingly infinite amounts of possibilities available in nature!

The inversion in the above illustration begins at 2,000 meters above the ground. From that point upward, the actual air temperature increases with altitude. This increase in temperature with elevation makes the atmosphere above 2,000 meters very stable. This layer, or inversion layer as it is called, acts as a 'lid' restricting the height to which a thermal can rise. The thermal's temperature is relatively cooler than ambient temperatures in the inversion layer, therefore the thermal is technically denser than air in the inversion. As such, the thermal will no longer be displaced by denser air around it and will subsequently cease its ascent.