Answer to "cloud layers" question

CLOUD LAYERS -- John Wahman of Glendale, AZ has a decent understanding about how clouds form as air rises. What he doesn't understand is how there can be layers of clouds above and below other layers. When on an airplane (e.g., Arizona to Florida, probably over New Mexico or west Texas), John has seen as many as three distinct layers at different altitudes.

John wants to know how this can be. -- 1/4/00

John has made a valid observation. While many photographs in textbooks and in other places try to show a particular cloud type, pictures showing multiple cloud layers are often left out. The reason, as John knows, is that it can be confusing.

To answer this question, let's consider the following.

temperature in the atmosphere varies in a fairly uniform manner. There may be sharp changes sometimes, but the temperature will vary usually from say 80 to 75 in a more or less linear fashion over the course of minutes or hours or from one location to place nearby.
moisture in the atmosphere is often much more discontinuous. For example, it may rain on one side of a street and not on the other. There may a layer of clouds that suddenly stops in both horizontal and vertical dimensions.
we tend to see the variation in moisture as it relates to clouds as being horizontal. That is, a cloud layer is present and then it isn't.
think about how the cloud looks vertically. From the ground up, the sky may be cloud-free. Suddenly, there is a cloud (i.e., the cloud starts). Just as suddenly, the cloud ends on top, there may or may not be other clouds above it.
sometimes in the western plains states, a dry line can be found. A dry line is like a weather front, only it provides a moisture discontinuity. Dew points, a measure of atmospheric moisture, can vary by 40 to 50 degrees Fahrenheit or more across a dry line. On January 3, 2000, a strong cold front (acting much like a dry line) moved across Texas and Oklahoma. Check out the dew point variation across the region in this map from Plymouth State College.

Once you can see how vertical breaks or discontinuities can be present, then it is time to understand how they can occur.

Moisture tends to get into the atmosphere from the ground up. Evaporation from ground-based objects (plants, humans, animals, water bodies) adds moisture to the air. Solar heating helps to lift this moisture-laden air. As the air rises, it cools by expansion (internal energy is used up to perform work) and by being in contact with colder air. If the air is moist enough and the lifting strong enough, condensation can occur. Clouds most often formed from vertically rising air are cumulus-type. These are the puffy-looking clouds.

Weather fronts, hurricanes, and even wind flowing over mountains can produce similar uplifting effects. Depending upon the stability of the air, the strength of uplift, and other factors, cumulus and/or stratus (more layered type clouds) may form.

Take a thunderstorm cloud, for example. As it develops, cumulus grows into cumulonimbus (the anvil-looking cloud or thunderstorm). In the process, the bottom of the cloud may eventually rain itself out. As the cloud dies, it often transforms into "debris" consisting of cirrus (high altitude ice clouds) and altocumulus. Hence two cloud types can be created from one. If cumulus and cumulonimbus are nearby, four different cloud types can be in your instantaneous field of view.

Sometimes, cumulus try to become thunderstorms, but they reach a stable layer in the atmosphere which inhibits their vertical growth. The cumulus may then spread out into stratocumulus or altocumulus, with cumulus remaining beneath them. This mixed cloud sky is often found near coastlines where the water is relatively cool. Locations near the New England and Pacific Northwest coasts often see this cloud combination.

Mixed cloud types are also common along warm fronts. Here air is forced to rise in a more gradual manner that results in low, middle and high clouds in various groupings.

Falling precipitation can also act to create clouds through the processes of evaporating raindrops (sublimating snowflakes). This process not only cools the air, but helps to saturate it. This frequently occurs with thunderstorms and along warm fronts. When I grew up, I remember watching the sky and seeing a layer of wind-blown stratus form beneath a layer of altostratus. Within an hour or two, the precipitation (which was evaporating or sublimating while falling...and which created the stratus clouds), now fell through a layer of saturated sky (filled with stratus clouds) and reached the ground.

Finally, just watch some weather satellite image sequences on the evening news or weather reports. On these, you can often see clouds at different levels moving in different directions. That means that clouds formed in different places might both move into the same area.

So, to John Wahman and others, keep looking up and down (depending upon where you are cloud watching). I'm sure you will start to see even more combinations than I have described here.

Take a look back at the Question