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WEATHER
Air Masses and Weather
• An air mass is a large body of air in the lower
troposphere that has similar characteristics
throughout.
– Can be several kilometers in diameter and several
kilometers high.
– The temperature and humidity for the air mass depends
on where it originates and when it moves it takes the
characteristics from its origin with it.
– As air masses travel, there characteristics can change
based where it has traveled to and how long it sits in the
new location.
– Earth’s topography can also contribute to changes in the
temperature and humidity of an air mass as it travels.
Air Masses and Weather
• There are many different types of air masses.
Meteorologists classify air masses based on
where they originate.
– Temperature of an air mass depends on whether
the air mass originates in the arctic, polar, or
tropical region.
– The humidity depends on whether the air mass
comes from land (continental) or ocean (maritime).
Air Masses and Weather
• There are 5 types of air masses:
– Continental Arctic (cA): originate in the arctic regions,
• Extremely cold and since cold air is incapable of holding
moisture it is also very dry.
• Though they may warm slightly as they move southward they
still cause extreme cold waves in regions they enter.
– Continental Polar (cP): originate over land areas of Alaska
and Canada.
• These air masses are somewhat warmer than cA there is only a
slight difference between temperature and humidity of these
two types of air masses.
• Though this air is cold and dry it will create precipitation in
some regions.
• One such region is when it passes over the Great Lakes in the
fall, the warm water warms the air slightly that it can hold water
vapor and then deposits the moisture downwind, creating the
lake-effect snows.
Air Masses and Weather
– Maritime Polar (mP): originates over the ocean at high
latitudes
• These air masses are both cold and damp.
• It is not as cold as cP air due to the contrast in temperatures
between land(colder) and oceans (warmer).
• When mP air cools to its dew point, fog, clouds, or precipitation
results.
• mP air coming off of the Pacific causes a wet winter climate in the
Pacific Northwest.
• mP air coming off of the North Atlantic brings heavy snowstorms,
called nor’esterns to the East Coast in the winter and cool clear
weather in the summer.
– Maritime Tropical (mT): originates over a warm tropical
ocean, acquires both warmth and moisture.
• In the summer mT air rotates clockwise around the high pressure
over the Atlantic Ocean bringing heat and humidity to the
Midwestern and Eastern United States
• Due to the large amounts of moisture, thunderstorms often
develop during the heat of the day.
Air Masses and Weather
– Continental Tropical (cT: originates over deserts
• This air mass is hot and dry.
• Many times originates as a maritime air mass but becomes dry
as it passes over mountains.
• In the summertime cT air produces tremendous heat waves in
much of the United States.
• mT air usually as temperatures
no higher that 100oC, cT air
can exceed 100oC
• Since cT air is dry it does not
bring clouds or thunderstorms
to cool the air
• If the dry air remains in
agricultural regions too long it
can cause serious damage to
crops.
Fronts and Lows
• Weather in the mid-latitudes is always changing. The
weather changes are due to the movement of lowpressure systems and their associated frontal systems.
• Front is the
boundary between
two opposing air
masses. Air masses
of different types
do not easily mix.
Fronts and Lows
– A front can range from 200 meters to 200 kilometer in width, can be
as high as 5 kilometers and as long as 2000 kilometers.
– Fronts are most common at mid-latitudes where northward moving
tropical air masses and southward moving polar air masses often
meet.
– Air masses on either side of the front may differ in temperature, in
humidity, or in both.
– Fronts can have steep slopes (ranging from 1/50 to 1/300).
– Fronts usually bring precipitation.
• Weather associated with a particular front depends on the types of
air masses involved and the speed at which the front is moving.
• Fronts are usually classified according to the temperature of the
advancing front.
• There are four basic kinds of fronts:
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Cold
Warm
Occluded
Stationary
Fronts and Lows
• Cold Front: the boundary between an advancing cold air mass and
the warmer mass it is displacing.
– Cold air is denser than warm air, so the cold air slides underneath
the warm air and forces it upward.
– Cold fronts have steep slopes.
– Weather a cold front brings depends on mostly on the type of air
mass it is displacing.
– When cP displaces mT air, thunderstorms often form.
– When cP displaces cT air, there will very little to no precipitation.
– A passing front may cause no greater change than a shift in winds.
– In the summer some cold fronts cause a change in humidity but
little change in temperature.
– In the winter a cold front maybe marked by rain or snow.
– Because a cold front has a steep slope the precipitation associated
covers a narrow band of ground.
– Cold fronts move quickly so precipitation usually ends shortly after
the front passes.
Fronts and Lows
• Warm Front: the boundary between an advancing warm air mass
and the colder air mass it is displacing.
– The warm air rises above the denser cold air mass, the cold air mass
retreats slowly.
– The slope of a warm front is more gradual than that of a cold front.
– Weather changes associated with a warm front are less dramatic.
– Signs of an approaching warm front are high cirrus clouds which may warn
of approaching precipitation more than a day before it arrives.
– Cirrus clouds are followed by cirrostratus and lower and thicker stratiform
clouds. These clouds form in the warm stable air sliding up the frontal
surface.
– The clouds may stretch 1500 kilometers ahead of the place where the
warm front touches the ground.
– The lower thicker stratus clouds can also most screen out the sun and the
moon.
– Finally heavy nimbostratus clouds arrive, and steady rain or snow begins.
– The area of precipitation can stretch hundreds of kilometers ahead of
where the warm front touches the ground.
– Precipitation can last for a day or more.
– Thunderstorms occasionally form but are not typical of a warm front.
Fronts and Lows
• Occluded front: when a cold front “catches up” to a warm
front. This happens because cold fronts typically moves
twice as fast as warm fronts.
• Stationary front: is a front that is not moving forward. This
occurs because the approaching warm front is not strong
enough to force the cold front to continue its forward
motion.
– If a stationary front remains stationary for too long, flooding
can occur.
Weather Associated with Pressure Systems
• Weather you see from a low-pressure system will
depend on where the center of the low passes in
relationship to your location.
– If the low-pressure systems center passes to the
north of you, a warm front may move over your
location, followed by a cold front.
– If the low-pressure systems center passes to the
south of your location, no fronts by pass over you,
but you may experience steady rain or snow.
– Low-pressure systems bring precipitation to areas
that they pass over or nearby.
Winds Associated with Pressure Systems
• Winds blow outward from a high-pressure system in a
clockwise direction in the Northern Hemisphere, and
in a counterclockwise direction in the Southern
Hemisphere.
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There is little to no wind in the center.
Air here takes on the temperature and humidity of the area.
Thus highs are where air masses generally form
A high represents one air mass
• Winds blow inward in a low-pressure system in a
counterclockwise direction in the Northern
Hemisphere, and in a clockwise direction in the
Southern Hemisphere.
– There is little to no wind in the center.
– A low is surrounded by two or more air masses
Thunderstorms and Tornadoes
• Thunderstorms: storms with lightning, thunder, rain and
sometimes hail
– The cumulonimbus clouds form in warm, moist unstable air.
– Storms can attain heights up to 20 kilometers in the
atmosphere.
– Weather can consist of torrential rains, damaging winds,
lightning, thunder, hail, and tornadoes.
– Can occur at any hour, they often occur during the afternoon
because of surface warming throughout the day causes air to
become unstable.
– Consist of one or more convection cells, that can exist for an
hour or less.
– Formation of new convection cells can extend the life of a
thunderstorm.
– Some thunderstorms may cover a entire state and last for as
long as a day.
Thunderstorms and Tornadoes
• Thunderstorms (continued)
– Most form along a large scale low-pressure system
frontal boundary which forces air to rise and can start
and stop for days.
– Often occur in lines hundreds of kilometers along the
frontal surfaces or ahead of the front in squall lines.
– Very large single-celled thunderstorms with particularly
strong updrafts are called super cells.
• Lightning is a discharge of electricity from a
thundercloud which can occur from
– Cloud to ground
– Cloud to cloud
– Between clouds
Thunderstorms and Tornadoes
• Tornadoes is a violent rotating column of air that usually
touches the ground
– Destructive and often unpredictable so they are difficult to
study.
– Predicting the path is also difficult.
– By studying the tornadoes from a distance scientists have
discovered that for a severe thunderstorm to produce a
tornado a there must be a rotating updraft called a
mesocyclone.
– Mesocyclone is an updraft that occurs when low-altitude
winds are blowing at a different speed and in a different
direction than the winds higher up.
– A mesocyclone may become visible at the base of a the storm
and may lower to form a wall cloud.
– Only about a third (1/3) of the mesocyclones produce a
tornado and some tornados form without a visible
mesocyclone or wall cloud.
Thunderstorms and Tornadoes
– Often appear as a vortex or funnel-shaped cloud of flying
debris. Some are rope shaped and others are wedge
shaped with more than one vortex.
– Funnel-shape due to air pressure at the center being very
low and air is sucked into the funnel expands and cools;
water vapor in the air condenses, forming a funnelshaped cloud. If the air is drier or the air pressure inside
the tornado is higher, it may consist of dust and debris
creating a loud roaring sound.
– They appear at the back edge of a thunderstorm and
travel with the storm.
– Tornadoes can travel in any direction and often move
erratically.
– Tornadoes can occur anywhere in the United States and
at any time, however, they occur most often in Tornado
Alley in the spring and summer.
Thunderstorms and Tornadoes
– Tornado Alley extends from Texas northward to North
Dakota where conditions are more favorable to their
formation.
• The warming of the surface in the area in the spring and
summer along with wind conditions in the region can be ideal
for the formation of mesocyclones.
• Warm, moist air may blow in from the south near the ground
while cool, dry air may blow in from the southwest or west
higher up in the atmosphere, setting conditions for tornadoes
to form.
– Tornados are classified using the Fujita scale (F-scale)
which indicates wind speed range and effects.
Storm or Tornado
Watches and Warnings
• Storm or Tornado Watch: If conditions are
favorable for a severe storm or tornado to
form a WATCH is issued.
• Storm or Tornado Warning: If a severe storm
or tornado has been seen and is approaching
a WARNING is issued.
Hurricanes
• Hurricanes are large storms that can last for a day or
more and affect an area hundreds of kilometers wide.
• Hurricane is a large rotating storm of tropical origin
that has sustained winds of at least 119 km/h.
– The air pressure in the center is extremely low and gets it
energy from the heat of surface ocean water.
– The winds and rains are comparatively mild in the outer
edges and they increase in intensity the closer to the eye.
– The strongest winds and rain occur in the eye wall.
– Inside the eye winds are mild and there is usually little to
no rain.
Hurricanes
• Formation of Hurricanes
– There must be a supply of warm, moist air for extended
periods of time.
– Hurricanes are born over warm water, generally between 5o
and 20o of latitude (north or south of the equator).
– Because hurricanes rely on the transfer of heat from the
ocean, they form only when surface ocean waters are
sufficiently warm, which is usually June through November.
– Hurricanes weaken as soon as they make land fall or move
over cold ocean water.
– Hurricanes are steered by global wind patterns.
– Hurricanes that occur in the Atlantic Ocean north of the
equator initially move to the west/northwest and eventually
north /northeast, however, there paths can vary considerably
and erratic changes can take forecasters by surprise.
Hurricanes
• Hurricanes can cause tremendous amount of
damage along the coast and inland.
– Damage can include winds, inland flooding, large waves,
and storm surge.
– Storm surge results from the strong winds of the eye wall
which blows water into a broad dome. The dome of
water can raise the sea level several meters higher than
it would be otherwise.
– When a storm surge and high tides coincide extremely
dangerous sea levels can swamp low-lying areas. Storm
surge is what cause the massive destruction and deaths
in New Orleans from Hurricane Katrina.
Hurricanes
• Hurricanes are classified using the Saffir-Simpson Hurricane
Scale.
– All hurricanes start as tropical depression then become tropical
storms and eventually can become a hurricane.
– Once a hurricane it can change categories throughout its
lifecycle.
– A hurricane can change categories many times during its
lifecycle depending on wind speeds.
• Hurricane Watches and Warnings
– Watches are issued to
communities that could see
the hurricane arrive in the
next 24-36 hours.
– Warnings are issued to
communities that could see
the hurricane in less than
24 hours.
Winter Storms
• Snowstorm: involves steady snow fall
• Blizzard: a winter storm characterized by high
winds, low temperatures, and falling or
blowing snow. Following three criteria must
be met for a winter storm to be classified as a
blizzard:
– Winds must exceed 56 kilometers per hour.
– Temperatures must be -7oC or lower.
– Falling and/or blowing snow must reduce visibility.
Forecasting Weather
• Meteorologist use large amounts of data for
forecasting the weather.
• Source of these data come from:
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Satellite images
Radiosondes: instruments attached to balloons
Weather stations
Weather radar
Airplanes
Ships
• Meteorologist create weather maps of the data
collected from weather stations.
• There is a huge amount of data that needs to be
on the compact map, meteorologist developed a
station model to compact the data onto the map.
Forecasting Weather
• Station model: includes information on the
temperature, dew point, weather conditions, wind
speed and direction, barometric pressure, and
cloud cover.
• This is a
standard model
so that a
meteorologist
in any country
of the world
can read the
weather map.
Forecasting Weather
• Surface maps give meteorologists the “big picture”
• Surface maps are created by computers.
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First the station models are shown
Then the sea-level air pressures
Then the computer draws isobars every four millibars
Identifies highs and lows
• Forecasters draw in the fronts.
– They use three types of data from the station model to
find the fronts: temperature, wind direction and dew
point.
– They apply the following rules in locating fronts:
• Wind direction changes behind fronts
• Temperatures changes sharply across fronts
• Dew point changes sharply across fronts.
Forecasting Weather
• Modern-day weather forecasts are based on computer
weather models.
• Weather models are large computer programs containing
mathematical equations designed to simulate atmospheric
processes. Many different models are used to make
forecasts.
• Some models predict weather forecasts 2 days in advance
while others predict 10 days in advance.
• All models start with the current weather data, such as
temperature, humidity, and wind at various levels of the
atmosphere.
• The computers transfer the data into their mathematical
equations and predict the future. Predictions include
temperatures, winds, sea-level pressure, precipitation, and
configurations of the Jet Stream.
Forecasting Weather
• Although models are useful tools in the forecasting
process, subtle factors not included in the models
can have a large impact on the weather.
• These factors include such things as urbanization.
Urbanization is the fact that cities are warmer due to
buildings and other man-made materials that absorb
and hold heat that vegetation does not.
– Urbanization can cause heat waves that the computer
models cannot forecast
– Urbanization can cause a cold rain in the city while
surrounding suburbs receive several inches of snow.
• Forecasters use their knowledge of the local area
weather patterns to modify the predictions they
receive from the computer models.