Clouds and Convection

This brings us to trying to understand what it means to have a stable or unstable air mass situation. An absolutely stable atmosphere is when the environmental lapse rate, or the actual lapse rate is less than the moist adiabatic lapse rate. The air parcel that is rising is cooling faster than the environment even during saturation. The air mass is prevented from continually rising.

An absolutely unstable atmosphere is when the environmental lapse rate or the rate of temperature reduction with elevation is greater than the dry adiabatic lapse rate. The air may be saturated or unsaturated but it cools at a slower rate than the environment, so always be warmer and will always continue to rise.

Finally, there is the conditionally unstable environment, where the environmental lapse rate is somewhere between the wet and dry adiabatic lapse rates. Exactly what happens depends on how saturated the air is with moisture. If the moisture levels are high, it will cool closer to the rate we see in the wet adiabatic lapse rate, and you will get continual rise of the air parcel.

You can often plot the rate at which an air parcel cools with elevation, noting that at some point the air parcel will become completely saturated. What happens then? This is when and where clouds form. The term Lifting Condensation Level or LCL is this level at which a dry adiabatically lifted air parcel is saturated. You can follow the temperatures as the altitude rises in the air parcel to where it becomes warmer than its environment while it is rising along the moist adiabatic lapse rate line. At the appointed time it becomes warmer than its environment, you would call this the Level of Free Convection or LFC. Further above this, the air parcel will suddenly become cooler than its environment. This is called the equilibrium level or EL.

CLOUDS AND CONVECTION

Now that you have some idea of how air moves, rises, and falls, you should be able to better understand exactly how clouds form in more detail. Clouds as you know are collections of suspended water or ice particles somewhere in the atmosphere. While these suspended particles do have weight and do fall from the sky, while in a cloud their mass is so small they are in effect suspended.

There is a diagram called the Clausius–Clapeyron diagram. What this shows you is the saturation level of air different temperatures. It is not a linear relationship. Figure 17 shows you the exponential relationship between pressure and temperature when it comes to air saturation:

Figure 17

This Diagram shows you different ways that clouds can form in the atmosphere. As you can see, you can plot the saturation point at which clouds will form just by looking at the temperature. Knowing what you know about air parcels, you can determine 3 possible ways a cloud might form. For example:

• You can add moisture to the air parcel. You do this every time you exhale on a very cold day. You can also see it from cooling towers, contrails, and sea smoke.

Contrails you see all the time are simply condensation or vapor trails from aircraft.

• You can also remove heat from an air mass. At night time, especially on clear nights over land areas, you will get radiation fog from cooling just above the ground surface. Evection fog comes from air that moved over oceans, cooling the air as it flows, forming fog. These fog types involve cloud formation due to heat removal.

• Cooling by means of adiabatic expansion is essentially what we've been talking about as the 3rd option. In this case, you have upward movement of an air parcel that cools to the point of condensation. You can also see it along aircraft wing tips or near tornadoes when vortices form.

You already know that clouds cannot form without a place for the water droplets to collect. These places are called cloud condensation nuclei or CCN; these are of course the aerosols found everywhere in the atmosphere. You can imagine that places having carbon dust from fires, volcanic activity, or even sea salt crystals above the ocean are those with a lot of these condensation nuclei in them.

Have you ever heard of cold clouds? Clouds with this name are made from ice crystals. These ice crystals are caused by special nuclei called ice nuclei. Dust in the atmosphere often is a nucleus for ice to form. Meteorologists use silver iodide to seed clouds in this way.

You can define a cloud by the water phase inside it. Warm clouds have only water droplets in them made from ice crystals. Cold clouds often look fuzzy around the edges and are almost always of a higher altitude. There are also mixed phase clouds that of course have both water and ice crystals as part of the cloud. High clouds such as tower clouds are often mixed phase clouds with both ice and water in them at different altitudes.