Six-In-One Weather Station
(630-0868A) About Clouds and Forecasting Faxback Doc. # 46976
CLOUDS AND VISIBILITY
A cloud is simply a big group of water molecules or ice particles in the
form of water vapor that stays clumped together and floats in the air
above the earth. Clouds most often form as the result of air rising from
the earth in one of these ways:
Convection - the heating of the ground creates bubbles of warm
air that rise into the atmosphere.
Topographic Lifting - the lifting of air up the side of a mountain. The
windward side of a mountain (the side that the wind
is blowing against) tends to have more clouds than
the leeward side (the side facing away from the
wind).
Convergence - the meeting of air from several different directions
at the earth's surface near low pressure systems. A
hurricane (over water) is an example of a low
pressure center that causes air to converge at the
earth's surface, resulting in severe thunderstorms
over land.
Frontal Lifting - the lifting of air along a frontal boundary (the
front of a storm, for example). Cold fronts and
warm fronts force air to rise. The most dramatic
cloud formations result from a cold front, because
the slope of the front is steeper than a warm front.
CLASSIFYING CLOUDS
When you look up at clouds in the sky, some clouds seem close enough to
touch, and some seem to be very far away. Some days, the clouds even seem
to form in neat, clean shapes.
Clouds can float from a few feet above the earth (fog) all the way up to
40,000 feet, and higher. There are 10 different classes of clouds. As
the chart in Faxback Doc. # 47312 illustrates, you can tell clouds apart
based on how high they are in the sky (altitude) and their shape or how
they look.
Cirrus - 20,000 to 40,000 feet
Cirrus clouds are made of very fine ice crystals, which is why they have a
somewhat soft, fuzzy appearance.
Cirrostratus - 20,000 to 40,000 feet
Cirrostratus clouds are the classical harbinger (sign) of bad weather.
They form slowly, like a screen, until they finally cover the whole sky.
Cirrocumulus - 20,000 to 40,000 feet
Cirrocumulus clouds usually appear in large patches and are arranged in
attractive looking formations. They most often form ahead of warm fronts,
when a high-level airstream is blowing from the south. Cirrocumulus clouds
are a sign of increasing humidity in the upper air.
Altostratus - 8,000 to 20,000 feet
From its looks, an altostratus cloud might be called the most "boring"
type. Altostratus clouds most often appear gray in color, but can
sometimes appear blue-gray. A growing, thick layer of altostratus clouds
usually means it will rain soon, because they usually form when a storm
front is approaching.
Altocumulus - 8,000 to 20,000 feet
Altocumulus clouds are the most common type of cloud and appear as big
fleecy shapes, and are often confused with cirrocumulus clouds. Jet vapor
trails, which look like long, straight clouds, form only at cirrocumulus
level and help to tell the difference between cirrocumulus and altocumulus
clouds.
Stratocumulus - below 8,000 feet
Stratocumulus clouds are the most widespread type of cloud. Seen at sea
under almost all weather conditions, they usually appear as a dense,
low-hanging cover and usually dissolve quickly. Stratocumulus clouds often
darken in the evening, but they are not a sign of bad weather.
Nimbostratus - below 8,000 feet
Nimbostratus is another classic bad weather cloud. Prolonged rainfall
usually comes with this cloud, which typically forms and appears with a
warm front. Nimbostratus is a layer cloud, extending from just above the
earth's surface to the upper boundary of the troposphere (about 7-10 miles
above the earth).
Cumulus - 8,000 to 45,000 feet (base to top)
There are many different types of cumulus clouds - small, medium, or large
size. Cumulus clouds usually formed at or around the spot in the sky where
you see them, and almost always disappear within an hour. Their formation
means upward movement of the air.
Stratus - below 8,000 feet
Stratus clouds are the lowest-lying type of cloud, and generally appear in
the form of fog or mist. When the weather report refers to a high,
fog-type cloud, it is referring to a Stratus cloud. Stratus clouds have no
structure (shape) and often form over the sea and along the coast.
Cumulonimbus - 10,000 to 60,000 feet (base to top)
Cumulonimbus clouds are thunderstorms. They form when a cumulus cloud
shoots upwards so strongly that its upper region freezes. Cumulonimbus
clouds usually develop a delicate, fuzzy structure, but they also can form
caps or collars that resemble cirrus clouds.
When a cumulonimbus cloud's ragged-looking upper region spreads out like
an anvil, it has moved against a barrier layer preventing any further
upward movement.
FORECASTING THE WEATHER
Every day, meteorologists on TV and the radio give a weather "forecast."
What they are actually doing is predicting what the weather will be for
the next 24 hours, or even the next few days, based on the current weather
conditions. Meteorologists use a variety of different tools, such as
weather maps and models, to explain their forecast.
Weather Map
In Faxback Doc. # 47313 you will find a typical weather map of the United
States that you might find in your local newspaper.
Weather Map Symbols
As you can see, different symbols are used on the map to represent the
various weather conditions like rain, snow, wind, cold fronts, and so on.
The charts in Faxback Doc. # 47314 and in Faxback Doc. # 47315, show the
symbols you might find on a weather map and explain what they mean.
Station Model
Unlike a weather map, a station model shows all of the current weather
conditions at a specific geographic location, such as an airport. For
example, when you fly on an airplane and the pilot announces the weather
conditions of the city you're flying to, he/she is probably getting that
information from a station model.
A typical station model is shown in Faxback Doc. #47316.
MAKING YOUR OWN WEATHER FORECAST
Using the readings from your weather station, you can try to forecast the
weather like a meteorologist. To forecast the weather as accurately as
possible, it is important that you take readings from your weather station
at least twice, if not several times a day.
To help you keep track of your observations, you might want to write them
down so you can refer to them later. Each time you take a new set of
readings, compare them to the last readings and see if you can spot any
patterns or conditions that affect changes in the weather.
Forecasting with the Clouds
As you use your weather station to measure the weather conditions, don't
forget to look around! The type and number of clouds in the sky can tell
you a lot about the weather to come. As you observe the clouds, look them
up on the following table.
Type of Cloud Most Likely Weather
Thickening Cirrus Precipitation within 12-24 hours
Altocumulus Precipitation within 6-12 hours
Altocumulus with "castle-like" Severe storms by afternoon
appearance in the morning
Altostratus Drizzle, fog, and rain (or possibly snow
and ice, if during the winter)
Stratocumulus with breaks Generally dry and cool weather
of sun
Stratocumulus overcast Showers will develop
Nimbostratus overcast Steady precipitation
Cumulus with no vertical Nice weather
development
Cumulus with vertical Showers and storms by afternoon
development
Cumulonimbus Thunderstorms soon
Cumulonimbus with mammatus Severe weather in the area
(curve-shaped) clouds
Forecasting with Wind and Barometer Readings
After you measure the wind speed and direction with your weather station,
and find the current barometric pressure in your local newspaper, you can
look up the intersection of those readings in the chart below.
Wind Sea-Level Barometric Most Likely Weather
Direction Pressure (Inches)
SW to NW 30.10 to 30.20, steady Fair, little temperature change
for one to two days
SW to NW 30.10 to 30.20, rising Fair, followed within two days by
rapidly warmer and rain
SW to NW 30.20 and above, steady Continued fair, no marked
temperature change
SW to NW 30.20 and above, falling Fair with slowly rising
slowly temperatures for two days
S to SE 30.20 and falling rapidly Wind increasing; rain within
12-24 hours
SE to NE 30.10 to 30.20, falling Rain in 12-18 hours
slowly
SE to NE 30.10 to 30.20, falling Increasing wind and rain within
rapidly 12 hours
E to NE 30.10 and above, falling In winter, rain within 24 hours.
slowly In summer, with light winds, rain
may not occur for several days
E to NE 30.10 and above, falling In winter, rain or snow with
rapidly increasing winds. In summer,
rain probable within 12-24 hours.
SE to NE 30.00 or below, falling Rain will continue one or two
slowly days
SE to NE 30.00 or below, falling Rain with high winds followed in
rapidly 36 hours by clearing, cooler
S to SW 30.00 or below, rising Clearing within a few hours and
slowly fair for several days
S to E 29.80 or below, falling Severe storm soon followed within
rapidly 24 hours by clearing and, in
winter, colder
E to N 29.80 or below, falling Severe northeast gale and heavy
rapidly precipitation. In winter, snow
followed by cold wave
To W 29.80 or below, rising Clearing and colder
rapidly
(BR/eb 4/17/98)
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