What factors will determine whether or not a rising air parcel will rise far enough and cool enough to become saturated? Why are there so many different types of clouds, and why do some produce precpitation while others do not? To answer these questions, we have to consider what makes the parcel rise in the first place (the method of "lifting") and we also have to consider what type of environment this parcel is rising through. We characterize the environment in terms of its static stability.
One of the most common means of lifting air is referred to as buoyant lifting. When an air parcel at the surface of the earth becomes warmer than the surrounding air, it will become less dense than the surrounding air (can you show this using the ideal gas law?). The hotter, lower density air will tend to rise, and so we refer to it as "buoyant". This air will continue to rise as long as it remains warmer than its environment. [Review the discussion of convection in Chapter 3, p. 91 of Danielson]. A good example of this process is the formation of towering cumulus clouds over Tucson nearly every afternoon during July and August. The heating of the surface by the sun reaches a maximum in early afternoon, and this heating triggers rising motion at the surface (i.e. convection).
Its important to understand the distinction between the parcel and the environment. The parcel is a specific group of air molecules that does not mix with the surrounding air (i.e. the environment). Changes in the temperature of air within the parcel are only due to the expansion/compression of the parcel, or due to latent heat released by water vapor condensing in the parcel. The parcel doesn't "feel" the surrounding air temperature (remember the definition of adiabatic). As a parcel rises, it will cool at the dry adiabatic rate if unsaturated; it will cool at the moist adiabatic rate if saturated.
To know if a rising parcel will remain buoyant and continue to rise means we need to know if the parcel will remain warmer than its environment. The parcel cools as it rises. The environment's temperature also cools as you go up. The key question is whether the parcel will cool faster, or slower, than the environment. This means we compare the parcel's lapse rate (or rate of cooling as you go up) with the environmental lapse rate.
Static stability - is how we determine if a parcel will remain buoyant or not. If we take an imaginary parcel of air at some level in the atmosphere that is initially at rest (i.e. static) and give it a slight upward push, it can either keep rising (unstable case), stay where it is (neutral case) or sink back down (stable case). By comparing the environmental lapse rate to the dry adiabatic lapse rate and the moist adiabatic lapse rate, we can determine if a parcel is warmer than (buoyant, unstable case), colder than (not buoyant, stable case), or equal to the surrounding air temperature.
ATMOSPHERIC STABILITY (Continued)
We know that an unsaturated parcel will rise, expand, and cool at a rate of 9.8 C per kilometer. We also know that if the parcel becomes saturated, then the latent heat released by condensing water vapor will make the parcel cool at a different, lesser rate - 4 to 7 C per kilometer. These two rates, the dry and moist adiabatic lapse rates, set limits on the temperature of a rising air parcel. We can use these limits to divide atmospheric stability into three classes. Each class is determined by comparing the environment's lapse rate (which we measure) with these theoretical limits.
- Absolutely unstable : if the environmental lapse rate is greater than 9.8 C per kilometer (i.e. greater than the dry adiabatic rate), then any rising parcel, saturated or not, will be warmer than it's envirnoment. The parcel will be buoyant in this case, and so the atmosphere is characterized as absolutely unstable.
- Conditionally unstable : if the environmental lapse rate lies in the range between 4 C per kilometer and 9.8 C per kilometer, then the atmosphere is characterizeds as conditionally unstable. A rising parcel could become buoyant if at some point it becomes saturated. Whether it becomes saturated depends on the surface temperature and humidity.
- Absolutely stable : if the environmental lapse rate is less than 4 C per kilometer, then any rising air parcel will be colder than the environment, and will sink back down. The atmosphere is characterized as absolutely stable because no matter if the parcel is saturated or not, it cannot become buoyant.
CHANGES IN STABILITY
Typically, the temperature of the atmosphere decreases as you go up, and it decreases at a rate somewhere between 4 C per kilometer and 9.8 C per kilometer, i.e. the atmosphere is conditionally unstable much of the time. But this can change depending on how the temperature of the atmosphere changes at different levels. You've seen in class how heating the surface throughout the day can make the atmosphere more unstable - that is, more likely for buoyant air parcels to rise, saturate, and produce clouds.
Since warmer air is less dense than cold air, it will become buoyant and rise. If warm air lies above cold air, you can see that rising motion will be inhibited (any rising parcel will be colder than the warm overlying air). This situation is referred to as an inversion. Any atmospheric change that reinforces this pattern will increase the satbility of the atmosphere. Surface cooling, upper level heating, or a combination of the two will reinforce the stable "warm over cold" pattern.
On the other hand, cold air lying over warm air is unstable, since the warm buoyant air below will want to rise, while the cold, heavy air above will want to sink. Any changes in the atmosphere that reinforce this "cold over warm" pattern will decrease the stability of the atmosphere - or destabilize the atmosphere. Examples of changes that destabilize the atmosphere are warming at the surface, cooling aloft, or a combination of the two. Study Figure 6.18 in Danielson to see how changes in the environmental lapse rate alter the stability of the atmosphere.
In addition to changes in temperature due to surface heating or transport (i.e. advection) air warmer or cooler air, another way to destabilize the atmosphere is through lifting. This occurs when air flows over a mountain or other forms of topography (we call this orographic lifting), or lifting can occur along frontal boundaries or in regions of converging air at the center of a low pressure system. As a layer of air is forcibly lifted, the top of the layer will cool more rapidly than the bottom of the layer. This sets up a "cooler air over warmer air" situation, which we know to be unstable.
WHY ARE THERE DIFFERENT TYPES OF CLOUDS?
For a cloud to form, you need to have sufficient moisture in the air and you need to have rising motion or some other means of cooling air to its dew point temperature so that condensation takes place. The type of cloud that forms will depend on the atmosphere's stability (which is determined by the environmental lapse rate) and on the method of cooling - which we can call the cloud formation process. Four of the most common cloud formation processes are:
- Buoyant lifting : whereby warm air rises, expands, and cools adiabatically
- Forced lifting : air rises as it flows over topography, as it over-runs a warm front, or as it is driven upward by the passage of a cold front. ( Figure 6.23 shows the different clouds produced by warm and cold fronts).
- Surface cooling : air next to the surface can be cooled through contact with the cold surface leading to the formation of radiation fog and advection fog. Rising motion is not necessary - these fogs tend to form in stable environments.
- Mixing : two different air parcels - both close to saturation - but having very different temperatures can mix together. When they do, the new temperature and dew point temperature will be an average of the two initial values. Because the saturation mixing ratio increases rapidly with temperature, it is possible that the new mixture can be saturated, even though the two parcels were initally unsaturated. This explains how contrails behind jets form, as well as the steam coming from a tea kettle.
If these formation processes take place in a stable environment, clouds can form, but they will lack vertical development (i.e. stratus-type clouds). If, on the other hand, they take place in an unstable environment, then clouds with pronounced vertical development (i.e. cumulus clouds) can form, often accompanied by precipitation. See Figure 6.3 in Danielson for a summary of all the different cloud types and their formation processes.