Chapter 18: Water, Clouds,
and Precipitation
Water in the Atmosphere
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The amount of water vapor in the air can vary
from 0-4% by volume depending on location.
However, its importance can't be overstated.
Water is responsible for clouds, rainfall, snow,
and for moderating Earth's temperatures.
Water Vapor in the Air
Changes of State in Water
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Earth is unique in that water can exist in three
different states at normal temperatures.
When water is a solid (ice) and it melts into a
liquid, it absorbs heat while it is melting.
The amount of heat that is required to change
a state of water is called latent heat, because
while the water is changing a state, it's
temperature does not change.
Changes of State in Water
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When liquid water changes into a gas by
evaporation, it requires latent heat to change
the liquid into a gas.
So, to clarify, if you set out a piece of ice at
room temperature, the ice and the melt water
that is produced by the ice would all stay at 0
Celsius until all of the ice melted.
If you boiled water in a pan, the water would
not get any hotter than 100 Celsius until all of
the water evaporated completely.
Latent Heat and Water
Other Important Changes of State in Water
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If a gas changes into a liquid, this process is
referred to as condensation.
If a solid turns directly into a gas without
changing into a liquid first, this process is
referred to as sublimation.
If a gas turns directly into a solid without
changing into a liquid first, this process is
referred to as deposition.
Changes in State of Water
Changes in State of Water
Humidity vs. Relative Humidity
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The word humidity is a very general term and
can have many different meanings.
Sometimes we may say “the humidity is high”,
or “it's really humid outside today”.
But this is a general term that really has no
quantitative value associated with it.
Meteorologists use relative humidity to
measure how much water vapor is in the air.
Relative Humidity
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An important key concept that is crucial to
understanding relative humidity and other
important meteorological processes is the fact
that warm air has the capacity to hold more
water vapor than cold air.
If a parcel of air contains the maximum
amount of water it can hold at a specific
temperature, we say it is saturated.
Warm Air holds more moisture than Cold Air
Relative Humidity
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Relative Humidity is a ratio or percentage of
how much water vapor is in the air compared
to how much water vapor could be in the air to
make the air completely saturated.
There are basically two different ways to
change the relative humidity of air.
The first way is, you could simply add or
remove moisture from the air.
Relative Humidity
Relative Humidity and Dew Point
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The second way you can change the relative
humidity of a parcel of air is to change the
temperature of the air.
If you change the temperature of air, you change the
capacity of the air to hold moisture.
For example, If the temperature of air is lowered, the
relative humidity increases.
Once air temperature drops to a temperature where
the air becomes saturated, this temperature is
referred to as the dew point temperature.
Dew Point
Dew Point and Fog
Relative Humidity and Dew Point
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So for example, if a parcel of air at 20 Celsius
had a relative humidity of 50%, and the air
temperature dropped to 10 Celsius, the
relative humidity would become 100%.
So in this example, 10 Celsius would be the
dew point temperature.
Relative humidity can be measured with a
hygrometer or a sling psychrometer.
Hygrometer and Sling
Psychrometer
Adiabatic Temperature Changes
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Have you ever pumped up a bike tire and felt the
pump or tire get warmer. Or let air out of an air
compressor and felt the cold air being released.
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This is because the increase in pressure causes
the molecules in the air to collide more frequently,
creating thermal energy or heat.
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As altitude increases, air pressure decreases, and
temperature decreases.
Adiabatic Temperature Changes
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These changes in air temperature caused by
changes in altitude and pressure are called
adiabatic temperature changes.
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When air is unsaturated, it cools about 10
Celsius every 1km increase in elevation.
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This is called the dry adiabatic rate.
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When air is saturated, it cools about 5-9
Celsius, every 1km increase in elevation.
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This is called the wet adiabatic rate.
Adiabatic Temperature Changes
The Formation of Clouds
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When air rises and cools, it eventually
reaches its dew point.
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When air reaches its dew point,
condensation occurs, and this forms clouds.
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There are four different mechanisms that
cause air to rise and form clouds.
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They are orographic lifting, frontal
wedging, convergence, and localized
convective lifting.
Orographic Lifting
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Orographic lifting occurs when air rises as
it ascends over mountains, forming clouds.
Orographic Lifting
Frontal Wedging
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Frontal wedging occurs when a parcel of
warm air encounters a parcel of cold air.
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When this occurs, the warm air ascends over
the cold air, creating a wedge-shaped front.
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As the ascending warm air rises, it cools to
its dew point and forms clouds.
Frontal Wedging
Convergence
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When air masses converge and collide,
like in the case of areas of low pressure,
then the resulting air masses rise.
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This mechanism of rising air and cloud
formation is called convergence.
Convergence
Localized Convective Lifting
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Because the surface of the ground is not
equal, some places heat up more quickly
than other places.
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For example an area with bare rock is more
likely to heat up than an area with vegetation.
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In places that heat up more quickly, a rising
pocket of hot air results causing localized
convective lifting.
Localized Convective Lifting
Stability of Air
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When meteorologists refer to stability of air,
what they are referring to is how rapidly air is
rising or falling.
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Hot air is more buoyant than cold air. This
causes hot air to rise.
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If air has very little difference in temperature
with height, the density will be close to the
same and the air will be stable.
Stability of Air
Stability of Air
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Unstable air that rises quickly, forms clouds.
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Air that gets colder gradually as elevation
increases is usually somewhat stable.
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Air is usually most stable air when it gets
warmer with increasing elevation.
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When this happens it is referred to as a
temperature inversion.
Temperature Inversion
Condensation of Water Vapor
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When air becomes saturated with water
vapor, condensation occurs.
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When condensation occurs, there has to be
some surface for the water to condense on.
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In the air, the water usually condenses on
small particles called condensation nuclei.
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Condensation nuclei may include dust,
smoke, volcanic ash, pollen, ions etc.
Types of Clouds
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Cirrus – the name means “a curl of hair”.
Clouds are high, white, and thin. They have
a delicate wispy or feathery appearance.
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Cumulus – the name means “a pile”. Clouds
look like rounded cloud masses. They have
flat bases with the appearance of rising
domes or towers. Frequently described as
having a cauliflower appearance.
Cirrus Clouds
Cumulus Clouds
Types of Clouds
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Stratus – name means “a layer”. Clouds
appear as sheets or layers that cover most or
all of the sky. No distinct separation.
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Types of clouds can also be subdivided by
their elevation above the ground.
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High clouds – Different types of cirrus
clouds are found at these elevations. Usually
associated with good weather.
Stratus Clouds
Types of Clouds
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Middle Clouds – Clouds that are found
about 2-6 km (about 1-3 miles) above the
ground are classified as middle clouds.
These clouds are usually named with the
prefix alto- before them.
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Low Clouds – Clouds below 2 km (1 mile).
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Nimbus is a Latin word that is used to
describe a rainy cloud.
Types of Clouds
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By joining the different names of clouds we
can classify many different types of clouds.
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For example, for high clouds we can have
cirrus, cirrostratus, and cirrocumulus.
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For middle clouds we have examples like
altocumulus and altostratus.
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For low clouds we have examples like
stratocumulus, stratus, nimbostratus,
cumulus and cumulonimbus.
Types of Clouds
Types of Clouds
Types of Clouds
Fog, Hail, and Sleet
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Fog is a cloud that forms close to the ground.
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One way fog can form is when a parcel of air
reaches its dew point.
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Another way that fog can form is when cool air
moves over warmer water and evaporation from
the water forms fog.
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Hail and sleet form when liquid water falls from a
warm pocket of air and then goes through a layer
of air at freezing temperatures.
Fog
Hail and Sleet