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OVERVIEW

The condition of the atmosphere can be described by the appearance of the sky, direction of the wind, and humidity
(“feel” of the air) (Figure 23.1). This is what you do when you step outside and decide to carry an umbrella or not.
You know that the probability of rain or other weather phenomena can be predicted by observing certain patterns in
atmospheric conditions.
Weather proverbs are statements relating to atmospheric conditions by the appearance of the sky, wind direction,
and humidity. They usually are meaningful as forecasting tools, as long as they were not transported from the part of
the world where they were developed. Here are some such proverbs:
Mare’s tails and mackerel scales make tall ships carry low sails.
A rainbow in the morning gives you fair warning.

A ring around the Sun or Moon means rain or snow coming soon.
If the salt is sticky and gains weight, it will rain before too late.
Can you figure out the atmospheric conditions described by the proverbs? Assuming that you live in the
mid-latitudes, where the wind normally moves from west to east, can you determine what the proverbs mean about
weather predictions? Return to these proverbs after you finish the chapter to confirm your skill at forecasting the
weather from observations and weather proverbs.

23.1 CLOUDS AND PRECIPITATION if the atmosphere did not cycle water vapor back over the land,
there would eventually be no water on the land. Both streams
Water cycles continuously into and out of the atmosphere and precipitation are part of a never-ending series of events
through the processes of evaporation, condensation, and pre- involving the ocean and lands of Earth. The series of events is
cipitation. When water evaporates, individual water molecules called the hydrologic cycle. Overall, the hydrologic cycle can be
leave the liquid and enter the atmosphere as the gas called water considered to have four main events: (1) evaporation of water
vapor. While in the liquid state, water molecules are held to- from the ocean, (2) transport of water vapor through the atmo-
gether by attractive molecular forces. Water molecules have a sphere, (3) condensation and precipitation of water on the lands,
wide range of kinetic energies, and occasionally the more ener- and (4) the return of water to the ocean by rivers and streams
getic ones are able to overcome the attractive forces, breaking (Figure 23.2). This definition of the hydrologic cycle involves
away. When they do escape, water vapor molecules carry the la- only the ocean and the lands, but water vapor also evaporates
tent heat of vaporization with them, as discussed in chapter 4. from the land and may condense and precipitate back to the
Because water vapor molecules take energy with them, evapo- land without ever returning to the ocean. This can be considered
ration is a cooling process. If incoming solar radiation did not a small subcycle within the overall hydrologic cycle. The ocean-
supply energy, Earth’s surface and the ocean would soon become land exchange is the major cycle, and many small subcycles also
cooler and cooler from the continuous evaporation that takes exist. The section on cloud-forming processes is about the part
place. The Sun supplies the energy that maintains surface tem- of the hy drologic cycle that returns water to Earth’s surface.
peratures, which allows the ongoing evaporation of water. Thus, The cloud-forming condensation processes will be considered
it is the Sun that supplies the energy required to evaporate water. first, followed by a discussion of the processes that result in
Water vapor in the atmosphere does not remain for more precipitation falling from the clouds.
than several weeks, but during this time, it is transported by
the winds of Earth. Eventually, the air becomes cooled, and the
relative humidity increases to 100 percent. The water vapor in CLOUD-FORMING PROCESSES
the saturated air now condenses to form the tiny droplets of Clouds form when a mass of air above the surface is cooled to
clouds. The water returns to the surface as precipitation that its dew point temperature. In general, the mass of air is cooled
falls from the clouds. Each year, on average, about 97 cm (about because something has given it an upward push, moving it to
38 in) of water evaporates from Earth’s oceans, but only 90 cm higher levels in the atmosphere. There are three major causes of
(about 35 in) is returned by precipitation. The deficit is made upward air movement: (1) convection resulting from differential
up, on average, by the return of 7 cm (about 3 in) per year by heating, (2) mountain ranges that serve as barriers to moving air
streams flowing from the continents into the oceans. If rivers and masses, and (3) the meeting of moving air masses with different
streams did not cycle water back to the ocean, it would be low- densities, for example, a cold, dense mass of air meeting a warm,
ered each year by a depth of 7 cm (about 3 in). On the other hand, less dense mass of air.

566 CHAPTER 23 Weather and Climate 23-2