METR 104:
Our Dynamic Weather

(Lecture w/Lab)
Questions about Clouds Dr. Dave Dempsey
Dept. of Geosciences
SFSU, Fall 2012

1. What are clouds?

Clouds consist of aggregates (groups or collections) of many, many tiny ice crystals or droplets of liquid water, suspended in the air. Individually these cloud particles are usually too small to see, but collectively they scatter enough light from the sun (or moon) for us to see them as amorphous....well, clouds!


A cumulonimbus cloud (photo by Bill Schmitz)


2. How do clouds form?

The recipe for making a cloud is:

  1. Ingredients Needed

    1. water vapor (This is regular H2O in the form of an invisible, odorless gas, which is what liquid water becomes when it evaporates. There's always at least some in the air.)

    2. aerosols (These are tiny, solid particles of dust, smoke, ash, salt, pollen, etc. suspended in the air—there's usually lots of these around, even when we can't see them)

  2. Instructions

    1. Cool the air until some of the water vapor begins to condense onto some of the aerosols, forming lots of tiny droplets of liquid water.

      (If it gets cold enough in the cloud, the droplets can freeze into tiny ice crystals.)

An altocumulus cloud (photo by Jay Shafer)


3. How can air cool enough to form clouds?

Mostly, air cools enough to form clouds when air rises by some means or another.

Why? Well, first, atmospheric pressure decreases with increasing altitude. Hence, as air rises the pressure on it decreases. What happens to air when the pressure on it decreases? It expands. As a result of expanding, it cools. (Air coming out of a pressurized car or bike tire cools by expansion in the same way.) If air rises far enough, it will cool enough for some of the water vapor in it to condense to form tiny droplets of liquid water—that is, a cloud.

All clouds that produce rain, snow, hail, and other forms of precipitation form this way.

Stratus cloud
Stratus cloud

There are several other ways for air to cool enough for a cloud to form.

As the most prominent example, if air comes into contact with the earth's surface and the surface is sufficiently colder than the air, then heat will flow out of the air and into the earth's surface. This occurs by a process called conduction. (The same thing happens to your skin when you touch something colder than your skin.)

If air cools enough this way for a cloud to form, then the cloud will of course be next to the earth's surface. This kind of cloud is simply what we call fog.

Other ways of cooling air enough to make clouds can produce only relatively minor clouds such as contrails (behind high-flying jet airplanes); the clouds that form in your exhaled breath, in air emerging from smokestacks and car exhaust pipes on a cold day; clouds that form in your bathroom when you take a hot shower; clouds that form on the surface of a lake on a cold day, etc.

Fog
Fog
(photo by Beth Conant)

4. How can air rise?

There are several ways in which air might rise (and thereby encounter lower pressure, expand, and cool enough by expansion for a cloud to form). These ways include:

       (A) When air becomes warmer than immediately surrounding air, it becomes less dense and rises.

       (B) When winds "collide" or converge near earth's surface, air is forced upward. This can happen, for example:

    1. in a low-pressure area (for example, in thunderstorms, hurricanes, and midlatitude cyclones)
    2. along cold fronts and warm fronts (in midlatitude cyclones)

       (C) When winds encounter mountains, the mountains force the air upward.

Illustrations and more details:

       (A) If air becomes warmer than the air immediately next to it, then it will become less dense than the surrounding air. Air that is less dense than the surrounding air will "float" upwards. (Whence the expression, "warm air rises.")

For example, that comes into contact with a warmer underlying surface, such as the sun-heated ground or a warmer body of water, will gain heat by conduction. Some patches of this air will become warmer than neighboring patches (because the heating is seldom uniform), and the warmer, less dense patches will rise. They might rise far enough, and hence cool enough by expansion under lower pressure, for clouds to form.

(Right: Fair-weather cumulus clouds forming above sun-heated ground.)
cumulus cloud

       (B) When winds near the earth's surface "collide" or converge, the air must go somewhere. The only place it can go is up.

Where do winds tend to converge? Two common situations include:

    1. Where the atmospheric pressure near the earth's surface is relatively low. Higher pressure in surrounding areas tends to push air into the lower-pressure area, where the air converges and rises (diagram, right). Not coincidentally, storms (such as thunderstorms, hurricanes, and midlatitude cyclones) all have relatively low-pressure centers near the earth's surface, so they are characterized by convergence of winds near the earth's surface, rising air, and clouds.
Convergence of winds
    1. Where a large "mass" of relatively cold air pushes against a large mass of relatively warmer (and hence less dense) air, or vice-versa, the warmer, less dense air will be shoved upward.

      The boundary between such air masses is what we call a "front". Fronts are features of a type of storm called midlatitude (or extratropical) cyclones.

      A cold front (symbolized by the blue triangles in the animation below) is the leading edge of a mass of cold air pushing into a mass of warm air.cold front

      A warm front (symbolized by the red half-circles in the animation below) is the leading edge of a mass of warm air pushing against a mass of colder air.warm front

      Both kinds of fronts are situations where winds converge and air rises.


      (C) When winds encounter a mountain or mountain range, air is forced up the slope. Much of the mountainous western U.S., where the prevailing winds are from the west, provides an example. Air coming from the Pacific Ocean is often forced up the western sides of the Casade Mts. of Washington, Oregon, and northern California and the Sierra Nevada Mts. of California. Hence, clouds often form and precipitation falls there.

In contrast, when the air subsequently descends on the eastern (lee) side of these mountains, the increasing pressure on the descending air compresses the air and it warms up. Any clouds present in the air will evaporate. This creates a "rain shadow" on the downwind side, which in turn creates the Great Basin Desert of eastern Oregon, Nevada and Utah.

orographic cloud

5. How do clouds produce precipitation?

This question is beyond the scope of this modest exercise! However, your own experience tells you that not all clouds produce precipitation (rain, snow, hail, etc.).

To form precipitation, many, many tiny cloud particles (a million is a typical number) somehow have to collide and coalesce to form pieces of ice or water drops that are big enough to fall out of the cloud and reach the earth's surface before they evaporate.

The process isn't simple, but as a general rule, only clouds that are sufficiently deep can produce precipitation. If they are sufficiently deep, then their upper parts will be cold enough for some of the water droplets to freeze. It's the coexistence of ice crystals and liquid water droplets in the cloud that leads to precipitation.

Clouds that are deep enough for precipitation to form in them generally form only in storms of one type or another—as thunderstorms, in tropical storms and hurricanes, and in midlatitude cyclones.

rainshaft
A rain shaft
beneath a thunderstorm
hurricane
Hurricane Kate
(Atlantic Ocean)
midlatitude cyclone
A midlatitude cyclone
over California


6. How can we see global patterns of precipitation-producing clouds and storms?

Because precipitation-producing clouds are generally deep, their tops tend to be relatively high up in the atmosphere and they are therefore relatively cold. Such deep, cold-topped clouds show up particularly well on infrared satellite images. Hence, infrared satellite images are particularly useful for spotting precipitation-producing storms.

Infrared satellite image
A recent composite, global, infrared weather-satellite image. [The "Z" attached to the observation hour stands for "Zulu", which is a code name for zero, which refers to the longitude (0°) of Greenwich, England. The standard time in Greenwich is referred to as "universal time coordinates", or UTC. On weather satellite images and weather maps, the UTC time at which the data were recorded is usually given. Pacific standard time (PST) is eight hours behind UTC, so 12Z is 4 am PST.]

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