Water in the Atmosphere
Reading Focus
Key Concepts
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What is humidity and how is it measured?
How do clouds form?
What are the three main types of clouds?
Key Terms
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water cycle
evaporation
humidity
relative humidity
psychrometer
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condensation
dew point
cirrus
cumulus
stratus
During a rainstorm, the air feels moist. On a clear, cloudless day, the air may feel dry. As the sun
heats the land and oceans, the amount of water in the atmosphere changes. Water is always moving
between the atmosphere and Earth’s surface.
The movement of water between the atmosphere and Earth’s surface is called the water cycle. As
you can see in Figure 13, water vapor enters the air by evaporation from the oceans and other bodies
of water. Evaporation is the process by which water molecules in liquid water escape into the air as
water vapor. Water vapor is also added to the air by living things. Water enters the roots of plants,
rises to the leaves, and is released as water vapor.
As part of the water cycle, some of the water vapor in the atmosphere condenses to form clouds.
Rain and snow fall from the clouds toward the surface. The water then runs off the surface or moves
through the ground, back into the lakes, streams, and eventually the oceans.
Humidity
How is the quantity of water vapor in the atmosphere measured? Humidity is a measure of the
amount of water vapor in the air. Air’s ability to hold water vapor depends on its temperature. Warm
air can hold more water vapor than cool air.
Relative Humidity
Weather reports usually refer to the water vapor in the air as relative humidity. Relative humidity is
the percentage of water vapor that is actually in the air compared to the maximum amount of water
vapor the air can hold at a particular temperature. For example, at 10°C, 1 cubic meter of air can hold
at most 8 grams of water vapor. If there actually were 8 grams of water vapor in the air, then the
relative humidity of the air would be 100 percent. Air with a relative humidity of 100 percent is said to
be saturated. If the air had 4 grams of water vapor, the relative humidity would be half, or 50 percent.
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Measuring Relative Humidity
Relative humidity can be measured with an instrument called a psychrometer. A psychrometer
(sy KRAHM uh tur) has two thermometers, a wet-bulb thermometer and a dry-bulb thermometer, as
shown in Figure 14. The bulb of the wet-bulb thermometer has a cloth covering that is moistened with
water. When the psychrometer is “slung”, or spun by its handle, air blows over both thermometers.
Because the wet-bulb thermometer is cooled by evaporation, its reading drops below that of the drybulb thermometer.
FIGURE 14Sling Psychrometer A sling psychrometer is used to measure
relative humidity.
If the relative humidity is high, the water on the wet bulb evaporates slowly,
and the wet-bulb temperature does not change much. If the relative
humidity is low, the water on the wet bulb evaporates rapidly, and the wetbulb temperature drops. The relative humidity can be found by comparing
the temperatures of the wet-bulb and dry-bulb thermometers.
How Clouds Form
When you look at a cloud, you are seeing
millions of tiny water droplets or ice crystals.
Clouds form when water vapor in the air
condenses to form liquid water or ice
crystals. Molecules of water vapor in the air
become liquid water in the process of
condensation. How does water in the
atmosphere condense? Two conditions are
required for condensation: cooling of the air and
the presence of particles in the air.
FIGURE 15Cloud FormationClouds form when
warm, moist air rises and cools. Water vapor
condenses onto tiny particles in the air.
The Role of Cooling
As you have learned, cold air holds less water vapor than warm air. As
air cools, the amount of water vapor it can hold decreases. The water
vapor condenses into tiny droplets of water or ice crystals.
The temperature at which condensation begins is called the dew
point. If the dew point is above freezing, the water vapor forms water
droplets. If the dew point is below freezing, the water vapor may
change directly into ice crystals.
FIGURE 16Condensation Water vapor condensed on this insect to form
dew. Predicting What would happen if the surface were below freezing?
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The Role of Particles
But something else besides a change in temperature is needed for cloud formation. For water vapor
to condense, tiny particles must be present so the water has a surface on which to condense. In
cloud formation, most of these particles are salt crystals, dust from soil, and smoke. Water vapor also
condenses onto solid surfaces, such as blades of grass or window panes. Liquid water that
condenses from the air onto a cooler surface is called dew. Ice that has been deposited on a surface
that is below freezing is called frost.
Composition of the Atmosphere
The atmosphere is made up of a mixture of atoms and molecules of different kinds. An atom is the
smallest unit of a chemical element that can exist by itself. Molecules are made up of two or more
atoms. Earth’s atmosphere is made up of nitrogen, oxygen, carbon dioxide, water vapor, and
many other gases, as well as particles of liquids and solids.
Nitrogen
As you can see in Figure 3,
nitrogen is the most abundant
gas in the atmosphere. It
makes up a little more than
three fourths of the air we
breathe. Each nitrogen
molecule consists of two
nitrogen atoms.
Oxygen
Even though oxygen is the second most abundant gas in the atmosphere, it makes up less than one
fourth of the volume. Plants and animals take oxygen directly from the air and use it to release energy
from their food.
Oxygen is also involved in many other important processes. Any fuel you can think of, from the
gasoline in a car to the candles on a birthday cake, uses oxygen as it burns. Without oxygen, a fire
will go out. Burning uses oxygen rapidly. During other processes, oxygen is used slowly. For
example, steel in cars and other objects reacts slowly with oxygen to form iron oxide, or rust.
Most oxygen molecules have two oxygen atoms. Ozone is a form of oxygen that has three oxygen
atoms in each molecule instead of the usual two. Have you ever noticed a pungent smell in the air
after a thunderstorm? This is the odor of ozone, which forms when lightning interacts with oxygen in
the air.
Carbon Dioxide
Each molecule of carbon dioxide has one atom of carbon and two atoms of oxygen. Carbon dioxide is
essential to life. Plants must have carbon dioxide to produce food. When the cells of plants and
animals break down food to produce energy, they give off carbon dioxide as a waste product.
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When fuels such as coal and gasoline are burned, they release carbon dioxide. Burning these fuels
increases the amount of carbon dioxide in the atmosphere.
Other Gases
Oxygen and nitrogen together make up 99 percent of dry air. Argon and carbon dioxide make up
most of the other one percent. The remaining gases are called trace gases because only small
amounts of them are present.
Water Vapor
So far, we have discussed the composition of dry air. In reality, air is not dry because it contains
water vapor. Water vapor is water in the form of a gas. Water vapor is invisible. It is not the same
thing as steam, which is made up of tiny droplets of liquid water. Each water molecule contains two
atoms of hydrogen and one atom of oxygen.
The amount of water vapor in the air varies greatly from place to place and from time to time. Water
vapor plays an important role in Earth’s weather. Clouds form when water vapor condenses out of the
air to form tiny droplets of liquid water or crystals of ice. If these droplets or crystals become heavy
enough, they can fall as rain or snow.
Particles
Pure air contains only gases. But pure air exists only in laboratories. In the real world, air also
contains tiny solid and liquid particles of dust, smoke, salt, and other chemicals. You can see some of
these particles in the air around you, but most of them are too small to see.
Properties of Air
It may seem to you that air has no mass. But in fact, air consists of atoms and molecules, which have
mass. So air must have mass. Because air has mass, it also has other properties, including
density and pressure.
Density
The amount of mass in a given volume of air is its density. You can calculate the density of a
substance by dividing its mass by its volume. If there are more molecules in a given volume, the
density is greater. If there are fewer molecules, the density is less.
Pressure
The force pushing on an area or surface is known as pressure. The weight of the atmosphere exerts
a force on surfaces. Air pressure is the result of the weight of a column of air pushing down on an
area. The column of air extends upward through the entire atmosphere. There is a column of air
above you all the time. The weight of the air in the atmosphere causes air pressure.
The atmosphere is heavy. The weight of the column of air above your desk is about the same as the
weight of a large schoolbus. So why doesn’t air pressure crush your desk? The reason is that the
molecules in air push in all directions—down, up, and sideways. The air pushing down on top of your
desk is balanced by the air pushing up on the bottom of your desk.
Air pressure can change from day to day. A denser substance has more mass per unit volume than a
less dense one. So denser air exerts more pressure than less dense air.
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