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WEATHER
Weather is important in our daily lives. On sunny days, people tend to feel good and do
more things. Rain and clouds, on the other hand, often limit our activities and make people feel
gloomy. Weather includes the daily conditions of the Earth's atmosphere. The earth is surrounded
by a blanket of air that extends to about 1,000 km above its surface. The molecules of gas are
trapped by the pull of the earth's gravity. The composition of the gas mixture in our atmosphere has
evolved through time to its present values: nitrogen-79%; oxygen- 20%; water vapor, carbon
dioxide, and argon gases constituting the other 1%.
Most of the gases were ejected from the earth's crust during long periods of volcanic activity.
The atmosphere is separated into distinct layers based on the temperature changes that occur
from one layer to the next. The layer of the atmosphere closest to the Earth is the troposphere.
This is where we live and where most weather occurs. As you go higher into the troposphere, the
temperature drops. The sun's rays heat the surface of the earth, not the air directly. Therefore, air
closest to the ground is the warmest. The thickness of this layer varies from 17.6 km at the equator
to 6.4 km at the poles.
The stratosphere lies above the troposphere. Air in the stratosphere is thinner than in the
troposphere. It contains very little moisture and dust. As a result, practically no weather phenomena
exist. Ozone is found in this layer about 15 - 50 km. high. It is a form of gas that absorbs most of
the harmful ultraviolet rays from the sun. A significant reduction in ozone would cause an
unhealthy increase in radiation. The stratosphere contains broad, fast-flowing "rivers" of air
circulating around the world. These are called jet streams. The jet streams can change
weather patterns in the troposphere.
Above the ozone layer of the stratosphere, temperature begins to drop once more. This is
the beginning of the mesosphere. Temperatures reach -75 C. to about 80 km above the earth. Then
temperatures begin to rise again in the top layer of the atmosphere, called the thermosphere. The
gases continue to thin out to an altitude of about 600 km. The temperature may reach 20000C due to
solar radiation absorption of gases. When solar energy is absorbed directly by air molecules, the
atoms gain or lose electrons and become charged particles called ions. Many gas molecules between
the altitudes of 80 - 400 km in the mesosphere and the thermosphere have electrically charged
particles. This part of the atmosphere is called the ionosphere. The ionosphere is important in
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cummunications. It can reflect many types of radio waves allowing them to bounce around the
world.
For our purposes, however, we will focus on the troposphere. The troposphere is where we
live and where weather occurs. The main factors influencing and causing weather in the
troposphere include: heat energy, air pressure, winds, and moisture.
HEAT ENERGY, AIR PRESSURE AND WIND
Radiant energy absorbed by the Earth is spread through the atmosphere by conduction,
convection, and radiation. The absorption and transfer of heat energy, however, are not equal.
Several factors affect the amount of radiation that is absorbed by the earth at different places.
Because the earth is a sphere, the sun's rays strike different places at different angles. Near the
equator the sun passes almost directly overhead. North and south of the equator, the surface of the
sphere curves away from the sun. As a result, these locations receive less solar energy. Other factors
include the tilt of the earth's axis, its day and night periods, and its path around the sun.
Unequal absorption of radiation causes unequal heating of the earth's surface. Because the
atmosphere is heated by the earth's surface, it too is heated unequally. Air near the equator is heated
more than air near the poles. Heated air expands. Thus warm air at the equator is less dense than
cold air at the poles. The density of the air determines the force with which it presses down
on the earth's surface. This force is measured as air pressure. Cold air presses down on the earth
with a greater pressure than warm air. Cold air is said to have a high pressure. Warm air is said
to have a low pressure.
Increased air pressure results in large masses of air in the upper atmosphere pressing down
on the layers of air below. This prevents warm, moist air from rising and clouds not forming, or fair
weather. While low air pressure allows warm, moist air to rise and clouds to form and bring
cloudy, rainy weather.
Let's recap. The unequal heating of the Earth, because of its shape and tilt, results in the
unequal heating of the atmosphere that is responsible for the motions and movements of the air
in the atmosphere. Air like most other substances, expands when heated and contracts when
cooled. Heat travels by convection, meaning warm air rises. The faster molecules move, the hotter
the air. As the molecules heat and move faster, they are moving apart. Because there is more space
between the molecules, the air is less dense than the surrounding matter and the hot air floats
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upward. Air moves from an area of high pressure (cooler, denser air) to an area of low
pressure (warm, less dense air) producing local and global winds. Local winds blow from any
direction and cover short distances while global winds blow from a specific direction and cover
longer distances. Wind is said to move horizontally from areas of high pressure to areas of low
pressure. Think of wind moving in a complete circle. As cool air moves into an area of low
pressure, the warm air is lifted; as the warm air rises, it is cooled and sinks back toward the ground
where it is warmed again and thus forming a convection cell. One more time, convection cells are
circular currents of air that result when hot air rises into the upper atmosphere, cools and
contracts, sinks down near the Earth's surface, heats up and expands, and then rises again.
The rotation of Earth causes these air masses to move in the form of wind. Examples of
convection cells include: mountain breeze, valley breeze; sea breeze, land breeze; and
monsoons. Additionally, the movement or circulation of air between the equator and the poles is
affected by the Coriolis effect (bending of the paths of moving objects as a result of the earth's
rotation) and causes wind belts (trade winds, westerlies, polar easterlies).
WATER
Water is not strange to us. It covers more than 70% of the earth's surface. The science of
weather forecasting deals with water not only in the liquid state, but the invisible (vapor) and the
solid (ice) states as well.
A cloud occurs when the invisible water vapor in the air becomes visible water
droplets or ice crystals. The water vapor becomes visible by cooling. That is what happens on a
cold wintry afternoon when you see your breath. Warm air leaving your mouth cools and forms
visible droplets. The same process occurs when water is boiling on a stove. Warm air rising above
the boiling water cools to form droplets commonly called steam. Fog, a cloud in touch with the
ground, occurs when water droplets form on particles near the earth's surface. There are many
ways to make air rise and cool to form clouds. For instance, mountains force air upwards. The air
that is forced to rise over the mountain cools and forms clouds. The second example is an
approaching cold air mass which lifts the air ahead of it. A third process is heat from the sun. Air
heated by the sun rises, cools, and forms clouds. Heat from the sun can provide enough lifting to
produce a thunderstorm.
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As water droplets form, they remain suspended in the atmosphere by hanging onto tiny particles
also suspended in the air such as dust, pollution, ash from volcanoes or even salt particles from the
sea. Because these particles are tiny and light-weight, they remain suspended in the air. Clouds come
in many different shapes. There are four major types. Cumulus are white, puffy, fair weather clouds
common on a warm summer afternoon. Cumulus clouds form when air, heated by the sun, rises
and cools like bubbles rising in an aquarium. If conditions are right, a cumulus cloud can grow into
the cumulonimbus, a towering storm. The third type is stratus, a grey sheetlike cloud layer that
blankets the sky. Finally, one easily identifies cirrus as thin feather-like clouds made of ice crystals
high in the cold atmosphere. Sunlight reflecting through cirrus ice crystals can form what we see as
a ring around the sun or moon.
Without the collection of precise information at all levels of the atmosphere around the
globe, accurate local weather forecasts could not be made. Before an accurate forecast can be made,
weather data reported by a network of stations must be plotted. Meteorologists use symbols to
represent information because there is little room on the map for all of the data reported from the
weather network. A collection of universal weather symbols is used by meteorologist around the
world and the area around each weather station on the map is reserved for a special part of the
weather report. Any meteorologist can look at any weather map and instantly tell what kind of
weather a station is observing. A small circle located at the center of the station city represents the
sky. For example, if the circle is empty, the sky is clear. If the sky is partly cloudy, the circle is half
filled. If the sky is overcast with clouds, the circle is filled in. At the National Meteorological Center
near Washington D.C., the observations are plotted on weather maps. The maps are then
transmitted to a network worldwide so weather forecasts can be made at local offices. Many
television stations receive their weather maps from NMC.
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TEMPERATURE
A thermometer actually measures the average kinetic energy of the various gas
molecules that make up the air around it - let's call them "air molecules."
As you can see in the graphic above, air molecules in colder air move slowly compared to those in
warmer air. The kinetic energy of an air molecule is directly proportional to the velocity of the
molecule. This means that colder air has less kinetic energy than warmer air.
When air molecules collide with a thermometer, kinetic energy is transferred from the
air molecules to the glass and then to the mercury molecules inside the thermometer. As the
mercury molecules begin moving faster they move farther apart, pushing the mercury up in
the thermometer.
In colder air, the energy from the air molecules colliding with the thermometer transferring
to the mercury molecules is less than the energy from warmer air. As a result, the mercury molecules
move slower in the colder air and the mercury inside the thermometer does not expand as far up the
tube as it does in the warmer air.
AIR PRESSURE
The air's pressure is caused by the weight of the air pressing down on the Earth, the
ocean and on the air below. Earth's gravity, of course, causes the downward force that we know as
"weight." Since the pressure depends on the amount of air above the point where you're measuring
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the pressure, the pressure falls as you go higher. The air's pressure is related to its density, which is
related to the air's temperature and height above the Earth's surface.
The air's pressure changes with the weather. Air pressure, in fact, is one of the important
factors that determine what the weather is like. Air pressure is also called barometric pressure
because instruments called barometers are used to measure it.
The U.S. National Weather Service reports air pressure at the surface in inches of mercury
while air pressure aloft is reported in millibars, also known as hectopascals (hPa). Scientists,
however, generally use pressures in hectopascals. Since the measurement is in the metric system,
1,000 millibars equal one bar. A bar is a force of 100,000 Newtons acting on a square meter, which
is too large a unit to be a convenient measure of Earth's air pressure. Inches of mercury and
centimeters of mercury measure how high the pressure pushes the mercury in a barometer.
The use of direct pressure measurements goes back to the late 19th century when the great
Norwegian meteorologist Vilhelm Bjerknes, a leader in making meteorology a mathematical
science, urged weather services to use direct pressure measurements because they can be used in the
formulas that describe the weather, unlike measures of the height of the mercury in a barometer.
HUMIDITY
Humidity is the amount of water vapor in the air and can be described in different ways.
The term that you'll hear most often to describe the humidity is "relative humidity." Another
common measurement of humidity is the "dew point." Unless you are a weather forecaster, or a
scientist or engineer involved in work involving the amount of water vapor in the air, you can easily
go through life without understanding "relative humidity." On the other hand, it is nice to know
what the person on television is talking about when he says "dew point."
Humidity is a key player in the weather. The U.S. Geological Survey estimates that the Earth
has about 326 million cubic miles of water. This includes all of the water in the oceans,
underground and locked up as ice. Only about 3,100 cubic miles of this water is in the air,
mostly as water vapor, but also as clouds or precipitation, at any one time. While this is a small
share of Earth's water, our planet would be very different without it. If Earth's air didn't contain
as much humidity as it does, our weather would be like that of Mars: No clouds (except
dust), no rain, sleet or snow, no thunder and lighting, no fog. And, without all of this water in
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all of its forms, Earth's life, if there were any at all, would be as hard to find as life on Mars is to
find.
Humidity can be less than 100% when it's raining. Humidity is a measure of the amount
of water vapor in the air, not the total amount of vapor and liquid. For clouds to form, and
rain to start, the air does have to reach 100% relative humidity, but only where the clouds are
forming or where the rain is coming from. This normally happens when the air rises and cools.
Often, rain will be falling from clouds where the humidity is 100% into air with a lower humidity.
Some water from the rain evaporates into the air it's falling through, increasing the humidity, but
usually not enough to bring the humidity up to 100%.
Definitions of humidity-related terms
Condensation: The phase change of a gas to a liquid. In the atmosphere, the change of water vapor
to liquid water.
Dewpoint: the temperature air would have to be cooled to in order for saturation to occur. The
dewpoint temperature assumes there is no change in air pressure or moisture content of the air.
Freezing: The phase change of liquid water into ice.
Evaporation: The phase change of liquid water into water vapor.
Melting: The phase change of ice into liquid water.
Relative humidity: The amount of water vapor actually in the air divided by the amount of water
vapor the air can hold. Relative humidity is expressed as a percentage and can be computed in a
variety of ways. One way is to divide the actual vapor pressure by the saturation vapor pressure and
then multiply by 100 to convert to a percent.
Saturation of air: The condition under which the amount of water vapor in the air is the maximum
possible at the existing temperature and pressure. Condensation or sublimation will begin if the
temperature falls or water vapor is added to the air.
Sublimation: In U.S. meteorology, the phase change of water vapor in the air directly into ice or the
chance of ice directly into water vapor.
RAIN
The most common rain gauge used today by official forecasters and airports was invented
over 100 years ago. It consists of a large cylinder with a funnel and a smaller measuring tube inside
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of it. The official rain gauge has a 50 centimeter high cylinder with a 20 centimeter in diameter
funnel that collects water into a measuring tube that has exactly one-tenth the cross sectional area of
the top of the funnel. The reason for the smaller measuring tube is so that more precise rainfall
measurements can be made due to the exaggeration of the height of water in the tube. For example,
one-tenth of an inch of rainfall would actually fill an inch of the measuring tube. A special measuring
stick inserted into the measuring tube takes into account the vertical scale exaggeration. This
exaggeration allows meteorologists to make very precise measurements to one-hundredth of an inch.
The standard rain gauge can measure up to two inches of rain. If rainfall exceeds two inches, water
overflows into the cylinder surrounding the measuring tube. The observer takes the water in the
cylinder and very carefully pours it into the measuring tube after emptying the tube. The observer
then adds the measurement from the water in the cylinder to two inches in order to obtain the final
rainfall amount.
WIND
Wind measurements have always been one of the more difficult observations for forecasters
to take. One of the main reasons is how quickly wind speed and direction can vary over short
distances, especially in cities and other areas with a lot of obstructions. For example, a wind vane
and anemometer duo placed in an alley in New York City may measure a northerly wind at 20 mph,
but if go 20 feet down the street, the wind may be southerly at 5 mph. Buildings and other
obstructions create eddies which make it difficult to determine the true direction and speed of the
prevailing wind. Wind directions are always reported as the direction winds are coming from. In
other words, a northerly wind pushes air from the north to the south.
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