Cloud Formation and Climate
How does water react to heating or cooling? How much energy can it store? It turns out that water is much more efficient in storing heat than any other common substance on Earth. To increase the temperature of fluid water by one degree Centigrade takes one calorie of heat energy for each gram of water. For comparison, to do the same for a rock takes 0.2 calories per gram, and for petroleum, 0.5 calories per gram. To change a gram of ice into water (without increasing the temperature) takes 80 calories. Conversely, when water freezes, it releases this same amount of heat into the environment. To change a gram of water into vapor requires 580 calories! This is what makes evaporation the best cooling mechanism around. Our body takes advantage of this fact when cooling itself by sweating: the evaporation of the sweat extracts heat from the surroundings, lowering the temperature of the skin. The heat expended in the phase change of water to vapor is not lost, but is contained in the vapor in latent form. When the vapor condenses, the heat is freed for warming the surrounding air. (We shall see later how this release of heat during condensation drives hurricanes.)
The phase changes of water combined with its unique heat-related properties are intimately involved in all aspects of climate and weather. Water transfers and stores heat on an immense scale, and thereby evens out the temperature differences between day and night, summer and winter, tropics and polar areas. Consider the temperature differences between day and night: they are largest in the water-starved desert. By contrast, in areas where water is abundant evaporation during the day tends to lower the temperature, and condensation during the night (the familiar dew) tends to raise it. During the day, clouds protect the ground from the heat of the sun, and at night, from the black coldness of space. Seasonal differences are similarly tempered by the great heat reserves of water, hence the mildness of coastal climates. Likewise, geographic differences are moderated by the transfer of heat through water motion and water phase changes. Most of this transfer is by water vapor in the air. Warm winds blowing over warm ocean regions pick up the water, cooling the sea surface. In cold regions, the air becomes unable to hold the vapor. The vapor condenses and gives off its heat of evaporation. If it freezes to snow, it also gives off its heat of fusion, to the surrounding air. Thus, a blizzard brings warmth - a somewhat unexpected conclusion, perhaps, to those who have been surprised by a snowstorm during a hike in the mountains.
The Properties of Clouds
Clouds consist of fine water particles floating in air and which
are dense enough to prevent the direct transmission of light. The particles can
even be frozen, making them ice clouds. Condensation of water occurs when the
moisture in the air exceeds the capability of the air to hold the water vapor
(upon cooling moist air or mixing it with cold air) and when microscopic particles called
aerosols are present
on which the vapor can condense.
Aerosols are any small particle, solid or fluid, suspended in air. Abundance values
typically range from 100 to 10,000 particles per cubic centimeter, for air over
land, with higher values found in cities. Sizes vary greatly, but typically are
near 0.1 micrometer or less. The particles originate from wind-blown sea-salt
or dust, volcanic eruptions, from burning of vegetation, from combustion of coal
and petroleum products, and other natural and anthropogenic processes. Aerosol
particles serve as nuclei for condensation of water droplets and for growth of
ice crystals and also influence the radiation balance directly. In the lower stratosphere,
concentrations are extremely low, and much of the aerosols there consist of droplets
of sulfuric acid.
Clouds come in many different types, depending on particle size and density, temperature
and phase, thickness and elevation, cloud size and dynamics of change. The properties
and the response of different types of clouds is crucial to the improvement of
prediction of how the Earth's climate will respond to global warming. On the whole,
higher temperature will produce greater moisture in the air, which will favor
the formation of clouds in the cool regions of the atmosphere (which may move
upward as the surface warms). Also, the general increase of pollution of the atmosphere
that is associated with increased human activity (burning, agriculture, diesel
engines) should favor the availability of cloud nuclei. Clouds also provide for
"cloud albedo," as they reflect much of the sunlight into space. Clouds also trap
infrared radiation. How the balance between these two effects will shift is uncertain.
From a geological viewpoint (considering the climate patterns on a warm Earth,
before the presence of large glaciers) it seems reasonable to expect that increasing
clouds in the tropics will cool the tropics and increasing clouds in high latitudes
will warm the cold regions.
Examples of some clouds types (clockwise from top left):
cirrus; altocumulus; cumulus; stratocumulus. (From: UCAR;
Advanced Study Program