Weather
What Weather is and Why it is Important
Weather is the short-term daily characteristics of the atmosphere in a
given region. Human activities are governed by the weather – what
we wear, what we will do. Since weather is so important for humans,
many components or elements of the atmosphere are monitored by
many weather stations around the globe.
The Elements of Weather
The elements of weather include air temperature, pressure, humidity,
precipitation, visibility, cloud cover and wind. Numerous weather
stations around the globe monitor these elements on a continuous
basis.
Instruments Used to Measure Air Elements
A barometer measures air pressure, a thermocouple or thermometer
measure air temperature, an anemometer measures air speed, a wind
vane measure wind direction and wet-and-dry bulb thermometers can
measure humidity (water content of air by percent).
Winds and Air Masses
Weather is transported from place to place by winds and winds depend on
air masses and pressure differences. An air mass is a three-dimensional
parcel of air that has uniform or homogenous temperature and humidity
characteristics. Air masses are named according to where they originate.
North America has 7 major air masses.
Characteristics of the Continental Air Masses
Arctic air is cold and
dry. Continental polar
air is cool and dry.
Maritime polar air is
cool and wet.
Maritime tropical is
warm and wet.
Continental tropical is
warm and dry. These
air masses have their
characteristics
because of the water
or land that they form
over.
Fronts
A front is the boundary where two different air masses meet. The
polar front is the boundary where cold, dry polar air meets warmer,
humid sub-tropical air. The frontal wave theory of cyclonic
development helps explain the weather in mid and high latitude
regions of the world (United States and Canada).
Symbols for Fronts
A line designates a front. Blue icesickles indicate an advancing cold front in the
direction of the points. Red semicircles indicate an advancing warm front in the
direction that the round points to. A line with alternate red semicircles and blue
icesickles pointing in opposite directions indicates stationary warm and cold air
masses. Semicircles and icesickles on the same side of a line indicate an occluded
front of warm air lifted high above cold air masses which is moving in the direction
of the points/semicircles.
The Overtaking of a Moving Cold Front to Warm Air
When a cold front overtakes warm air, it forces the warm air upward because
warm air is less dense. As the warm air rises it cools and forms towering
storm clouds called Cumulo-nimbus clouds that rise high and produce heavy
rains below them (in the cold air region). Fast winds in the upper
atmosphere blow the upper cumulo-nimbus ahead of the cold front to make
cirrostratus and cirrus clouds. Cirrostratus and cirrus clouds foretell a coming
cold front.
Cold Front Symbols
A front is the boundary where two air masses meet. The symbol for a
cold front is blue icesickles and the direction of the front’s movement
is in the direction the blue triangles point.
Fronts Help Explain Weather Changes
Since World War I, the Frontal Wave Theory of Cyclonic Development has
been used to explain how weather changes as warm and cold air masses
bump up against each other. When warm air meets cold air, it rises because
it is less dense. As the warm air rises, two things result – 1) the warm air
cools and releases water as precipitation and 2) a region of lower pressure is
created as the air rises. Thus rain and low pressure are associated weather
conditions.
Cyclone Development
When air rises in the Northern hemisphere it also generates a circular
surface wind movement in a counterclockwise direction because of
the coriolis effect and wind shear, the effect where surface winds
speed up as they rise due to less friction.
Anticyclone Development
Falling air creates a high pressure region and this generates circular
surface winds that rotate in a clockwise direction in the Northern
Hemisphere.
Southern Hemisphere Cyclones and Anticyclones
In the Southern Hemisphere, the direction of cyclones and
anticyclones is reversed due to the coriolis effect and wind shear.
How Surface High and Low Pressure Regions Interact
Winds flow in spirals from high pressure regions to the nearest low
pressure region. For high pressure regions, note the increasing
pressure in hectopascals from outside to inside and the reverse for low
pressure regions.
When Cold Air Overtakes Warm Air
When a cold air mass overtakes a warmer air mass, it lifts the less dense warm air
over itself which causes towering cumulnimbus clouds to form. These clouds
produce thunder showers that falls behind the front into the cold air mass. The tops
of the towering cumulonimbus clouds push into upper atmosphere layers which
have fast-moving winds. These winds push the tops of the Cb clouds ahead of the
front to form first cirrostratus then cirrus clouds ahead of the front. The presence of
CS and Ci clouds predicts the coming of rain (to be caused by the coming cold front).
When Warm Air Overtakes Cold Air
When a warm air mass overtakes cold air, it rises over the cold air forming
an incline. As the warm air rises it forms nimbostratus and the altostratus
clouds which produce steady rains. Eventually as they reach upper levels
with fast winds they form cirrus clouds. Seeing cirrus clouds followed by
raining alto stratus and nimbostratus is a prediction that a warm front is
advancing with fair weather ahead.
Types of Clouds and Rain Patterns Associated With Fronts
Stationary Front
A stationary front is the boundary between a cold and a warm front
that are not moving or moving sideways past each other. The symbol
for this is a line with alternating warm and cold markers in opposite
directions.
An Occluded Front
When a cold front overtakes a warm front, it may push the warm air up over
its cold air and the cooler air the warm front was moving over. An occluded
front is the boundary of cold air with cool air over top of which lies a mass of
warm air that rains into the cold air mass. The symbol for an occluded front
is a line with triangles and semicircles pointing in the same direction and
indicating the general motion of the boundary.
Forming an Occluded Front
When a cold front overtakes a warm front, an occluded front is
formed.
Cloud Formation
As water evaporated from the earth rises in warmed air, it cools (adiabatic cooling)
as it rises. As the rising air cools, its relative humidity rises since cooler air can hold
less water (relative humidity = actual water in air/total water the air could hold at a
given temperature). When the air’s relative humidity reaches 100% (the dew point),
water vapour (gas) begins to condense on microscopic dust or salt particles to form
droplets of liquid water or ice if at a high altitude. These droplets are seen as clouds.
All weather clouds form in the stratosphere, the lowest region of the atmosphere.
Cumulus Clouds
Cumulus clouds are found in fair weather . They do not produce rain.
They are located in the lower regions of the stratosphere. They have a
“cottony or fluffy” appearance. They are composed of liquid water
droplets.
Cumulus from an airplane 
Cirrus Clouds
Cirrus clouds are whispy, high-altitude clouds. They are composed of
ice crystals and do not form precipitation.
Cumulonimbus Clouds
Cumulonimbus clouds are thunder storm clouds that produce much rain (any
cloud with the word, nimbo or nimbus is a rain cloud). These towering
clouds reach from the lower to the upper statosphere. They produce
thunder and lightning and have a typical anvil shape at the top of the cloud.
These clouds signal the approach of a cold front. They are composed of ice
crystals at the top and water droplets at the bottom.
Nimbostratus Clouds
These rain clouds are composed of ice crystals at the top and water
droplets at the bottom. These are lower to mid altitude clouds in the
stratosphere. They are associated with an approaching warm front.
Comparison of Cloud Types
Weather and Cloud Types
Three Causes of Rainfall
Orographic rainfall is caused by warm air being forced up to higher altitudes
by mountain ranges. Convectional rainfall occurs when warmed, moist air
rises then cools and condenses into water vapour. The base of these clouds
is where the condensation point/dew point occurs. As rain falls, heat is
released (vapour condensing to liquid), and this extra energy makes the
storm more active with lightning and thunder. Continental (Ex: Prarie)
storms have this kind of rainfall. Cyclonic or frontal rainfall occurs when cold
and warm fronts interact with each other.
Jet Streams
Jet streams are narrow, meandering streams of upper air at altitudes
between 9-12 km (at the border of the troposphere and stratosphere)
moving up to 450 km/h. In the northern hemisphere there are two jet
streams, the polar and sub-tropic jet streams which move seasonally
south (in winter) and north (in summer).
Jet Streams And Cyclones
Jet streams movement often triggers the development of low pressure cells
or cyclones. Regions under a jet stream will likely experience storms and
changeable weather. Regions north of the polar jet stream will experience
colder, drier air while regions south of this jet stream will experience
warmer, moister air. Meandering jet streams create Rossby waves that
generate cyclones and anticyclones.
Jet Airplanes and Jet Streams
Jet airplanes make use of jet streams when travelling in easterly directions
since the plane’s speed (900 km/h) is increased by the jet stream’s speed.
For a 10 h flight travelling at 900 km/h, if the plane gets in a slower jet
stream of 125 km/h, the flight time reduces to 8.8 h due to the plane’s
increased speed (1025 km/h). For a fastest jet stream of 450 km/h, the flight
time is reduced to 6.7 h. When travelling west, jetliners avoid jet streams.
Weather Station Notation
Weather stations provide weather information by using special
notation.
Weather Station Cloud Cover
Weather Station Cloud Types
Weather Station Wind Speeds and Direction
One knot is 1.852 km/h or 1.151 mi/h
Reading Weather Maps
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Hurricanes or Typhoons
A hurricane is a circular cyclone storm covering from hundreds to
thousands of miles across. Hurricanes are one of the most destructive
natural hazards on earth for people.
Hurricanes Powered by Warm Waters
Hurricanes develop over warm waters at least 27 C at a distance of 8-15 O
latitude either north or south of the equator. Air in these locations becomes
saturated with evaporated water and low pressure regions develop which
move westwards. As condensation occurs in the low pressure region, much
energy is released which causes the low pressure region to expand and gain
in strength. Hurricanes rotate counterclockwise in the N. hemisphere.
Hurricanes Strengthen Over Warm Waters
As long as hurricanes remain over warm waters, energy from warm moist air
condensing will add energy and the hurricane will build and become more
destructive. As hurricanes move northward over cooler waters or over land ,
they lose energy and dissipate. In the N. hemisphere, water is warmest from
June – Nov. which is hurricane season there (Dec –Mar. in hurricane season
in S. hemisphere).
Southern Hemisphere Typhoons (Hurricanes)
Since hurricanes develop around low pressure areas, in the Southern
Hemisphere Typhoons rotate clockwise, moving west and south.
Hurricane Features
A typical hurricane lasts from 6-12 days and develops wind speeds from 117251 km/h with rainfall amounts of 200-300 mm/h (up to 1000 mm/24h)
which often produces flooding. The eye of a hurricane, its centre, is
prominent in strong hurricanes. It is a circular 30-65 km diameter centre of
the hurricane in which the pressure is about 15% lower than the rest of the
hurricane. The eye of strong hurricanes typically has light winds and clear
skies. Around its edges it has tall vertical clouds.
Tornadoes
Tornadoes form in thunderstorms along cold fronts, often initiated by
a jet stream or strong upper atmosphere winds. A tornado has a
central extreme low pressure area (only 100 mb – 10 kpa) around
which rising unstable air swirls. Tornadoes form when warm moist air
is forced over cold air and into strong upper winds.
Tornado Features
Besides their extreme low pressure centres tornadoes have winds that
travel from 160-483 km/h. They have widths from 76m – 3.2 km and
touch down for some metres, kilometres and even for more than 100
km.
Regions at Risk for Tornadoes
Oklahoma, Kansas and Nebraska are states which are at high risk for
tornadoes and are termed tornado alley.
Damage from Tornadoes
High wind speeds and updrafts displace objects and drive objects into
each other. Very high and low pressures that rapidly change can
explode buildings. Severe rain, hail and lightning often accompany
tornadoes.
More Tornado Damage
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Atmosphere Problem: Global Warming
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Atmospheric Problem: Ozone Layer Depletion
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Atmosphere Problem: Acid Rain
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