CLOUDS AND PRECIPITATION and the WATER CYCLE
A cloud is a visible aggregate of tiny water droplets and/or ice crystals suspended
in the atmosphere. Clouds can exist in a variety of shapes and sizes. Some
clouds are accompanied by precipitation: rain, snow, hail, sleet, and even
freezing rain. The purpose of this module is to introduce a number of cloud
classifications, different types of precipitation, and the mechanisms responsible
for producing them which includes the water cycle.
Background:
Water is known to exist in three different states; as a solid, liquid or gas.
Clouds, snow, and rain are all made of some form of water. A cloud is comprised
of tiny water droplets and/or ice crystals. A snowflake is an aggregate of many
ice crystals. Rain is just liquid water.
Water Vapor:
Water existing as a gas is called water vapor. When referring to the amount of
moisture in the air, we are actually referring to the amount of water vapor. If the air is
described as "moist", that means the air contains large amounts of water vapor.
Common sources of moisture for the United States are the warm moist air masses
that flow northward from the Gulf of Mexico. If we say that the air is dry, it means it
has a low amount of water vapor mixed in with it. Water vapor is invisible to the
eye; but its importance can not be undervalued. It is primarily in the vapor phase that
“water” is transported from place to place on a global scale. The atmosphere
transports huge quantities of water (vapor) and energy, redistributing these so as to
maintain a balance of water and energy all over the globe. Again water vapor is the
gas form of water. All matter exists as a solid, liquid, or gas.
At any given temperature, there is a
Temperature
(oC)
Vapor Capacity
(g/m3)
maximum amount of water vapor that
can exist in a volume of air. Once the
-20
1.1
maximum water vapor concentration is
-15
1.6
reached for a particular temperature,
-10
2.3
-5
3.4
0
4.8
to liquid or solid (rain or snow). The
5
6.8
higher the temperature, the more water
10
9.4
vapor a volume of air is capable of
15
12.9
20
17.3
25
23.2
holding water vapor increases, and as it
30
30.5
cools, the capacity decreases.
any additional vapor will change phase
holding. As air warms, the capacity for
Cloud Types:
Clouds are classified into a system that uses Latin words to describe common
the appearance of clouds as seen by an observer on the ground.
High-Level Clouds =
cirrus
Mid-Level Clouds =
cumulus
Low-Level Clouds =
stratus
Cirrus Clouds
“thin and wispy “ The most common form of
high-level clouds are thin and often wispy cirrus
clouds. Typically found at heights greater than
20,000 feet (6,000 meters), cirrus clouds are
composed of ice crystals that originate from the
freezing of water droplets. Cirrus generally
occur in fair weather and point in the direction of air movement at their elevation.
Cumulus Clouds
The bases of mid-level clouds typically appear between
6,500 to 20,000 feet (2,000 to 6,000 meters). Because of
their lower altitudes, they are composed primarily of
water droplets, however, they can also be composed of
ice crystals when temperatures are cold enough.
Altocumulus may appear as parallel bands. The
presence of altocumulus clouds on a warm and humid
summer morning is commonly followed by thunderstorms
later in the day.
Low-level Clouds
Low clouds are composed of mostly water
droplets since their bases generally lie below
6,500 feet (2,000 meters). However, when
temperatures are cold enough, these clouds may
also contain ice particles and snow. Stratus
clouds are dark, low-level clouds accompanied
by light to moderately falling precipitation
Fair Weather Cumulus Clouds
“puffy cotton balls floating in the sky”
Fair weather cumulus have the
appearance of floating cotton and
have a lifetime of 5-40 minutes. Known
for their flat bases and distinct
outlines, fair weather cumulus exhibit
only slight vertical growth, with the
cloud tops designating the limit of the
rising air. Given suitable conditions, however, harmless fair weather cumulus can
later develop into powerful thunderstorms.
Cirrostratus Clouds
Cirrostratus are sheet-like, high-level clouds
composed of ice crystals. Though cirrostratus can
cover the entire sky and be up to several thousand
feet thick, they are relatively transparent, as the sun
or the moon can easily be seen through them.
Sometimes the only indication of their presence is
given by an observed halo around the sun or moon.
Halos result from the refraction of light by the cloud's
ice crystals.
The formation of clouds can lead to precipitation.
When cloud particles become too heavy to
remain suspended in the air, they fall to the
earth. Precipitation occurs in a variety of forms:
hail, rain, freezing rain, sleet or snow.
Saturation When a volume of air filled to its capacity for water vapor, it is said
to be saturated. When air is saturated it cannot hold any more water vapor.
What air is saturated its relative humidity is 100%. When the air is saturated with
water vapor, the result is precipitation. . Precipitation may be composed snow,
rain, drizzle, or hail. Saturation can be achieved in two different ways.
1.
As a volume of air cools in temperature, its capacity to hold water vapor
decreases.
2.
Saturation can also occur if water vapor is added to a volume of air until it is
filled to its capacity.
Rain
Rain develops when growing cloud droplets become too heavy to remain in the
cloud and as a result, fall toward the surface as rain (due to the force of gravity).
Hail:
Hail is a large frozen raindrop produced by intense thunderstorms. As the
snowflakes fall, liquid water freezes onto them forming ice pellets. The pellet
gets larger and larger as more and more droplets become attached to the first
pellet. Upon reaching the bottom of the cloud, these ice pellets are sometimes
brought backup to the top of the cloud, where they continue to attract more rain
drops. As the rain drops (inside the cloud) become stuck to the ice pellets, it
continues to grow and grow. As the ice pellets once again fall through the cloud,
another layer of ice is added and the hail stone grows even larger.
Freezing Rain
Ice storms can be the most devastating form of winter weather. Ice storms are
often the cause of automobile accidents, power outages and personal injury.
Regular rain falls through the sky, except it freezes upon impact with the cold
surfaces of the ground.
Sleet
Sleet is less prevalent than freezing rain and is defined as frozen raindrops that
bounce on impact with the ground or other objects. It is very similar to freezing
rain in that it causes surfaces to become very slick, but its easily visible.
Snow
Snowflakes are simply aggregates of ice crystals that collect to each other as
they fall toward the surface. Since the snowflakes do not pass through a layer
of air warm enough to cause them to melt, they remain in tact and reach the
ground as snow.
THE WATER CYCLE:
adapted from the work of Amber Wozniak: awozniak@nmu.edu
Run and get a glass of water and put it on the table next to you. Take a good long look
at the water. Now -- can you guess how old it is?
The water in your glass may have fallen from the sky as rain just last
week, but the water itself has been around pretty much as long as the
earth has!
When the first fish crawled out of the ocean onto the land,
your glass of water was part of that ocean. When the Brontosaurus
walked through lakes feeding on plants, your glass of water was part of those lakes. When
kings and princesses, knights and squires took a drink from their wells, your glass of water
was part of those wells.
The earth has a limited
amount of water. That
water keeps going around
and around and around and
around in what we call the
"Water Cycle". This cycle is
made up of a few main
parts:




evaporation (and
transpiration)
condensation
precipitation
collection
Evaporation:
Evaporation is when the sun heats up water in rivers or lakes or the
ocean and turns it into vapor or steam. The water vapor then goes into the air.
Transpiration: is the process by which plants lose water out of their leaves.
Transpiration gives the water vapor back up into the air. Water vapor reaches the
atmosphere primarily through evaporation and transpiration
Condensation:
Occurs when water vapor in the air gets cold and changes back
into liquid, forming clouds. You can see the same sort of thing at home... pour a glass of
cold water on a hot day and watch what happens. Water forms on the outside of the
glass. That water didn't somehow leak through the glass! It actually came from the air.
Water vapor in the warm air, turns back into liquid when it touches the cold glass.
Precipitation: Precipitation occurs when so much water has condensed that the air
cannot hold it anymore. The clouds get heavy and water falls back to the earth in the
form of rain, hail, sleet or snow.
Collection: When water falls back to earth as precipitation, it may fall back in the
oceans, lakes or rivers or it may end up on land. When it ends up on land, it will either
soak into the earth or become part of the “ground water” that plants and animals use to
drink or it may run over the soil and collect in the oceans, lakes or rivers where the cycle
starts all over again.
A More Detailed Water Cycle Illustration
The oceans cover more than 70% of Earth's surface. Each year, approximately a
1-meter layer of the ocean surface is evaporated into the atmosphere. An equal
amount is returned to the oceans by the combination of precipitation and runoff
from rivers and streams.
While there is a world-wide balance in the amount of precipitation and
evaporation, a local balance does not exist at most locations on Earth.
Imbalances are reflected in the types of climates observed. Moist climates (such
as tropical rain forests) are found where precipitation exceeds evaporation, while
dry climates (deserts) prevail where evaporation exceeds precipitation.