Download Air Masses and Weather

Earth / Environmental Science
Ch. 20 – WEATHER PATTERNS & SEVERE STORMS
AIR MASSES
•
•
•
In middle latitudes, summer heat waves and winter cold spells
are familiar experiences
Summer waves - high temperature and high humidity, (few days
it has stormy weather & relatively cool water)
Winter cold spell (or waves) - characterized by period of frigid
temperature under clear skies, usually followed by cloudy, snowy
and relatively warm weather.
Air Masses and Weather
•
•
Immense body of air that is characterized by similar temperature
and moisture at a given latitude
It can be 1600km (or more) across and several kilometers thick
Movement of Air Mass
•
•
Through out air mass is going southward, it becomes warmer.
(See Figure 2, p.559)
As it moves the characteristic of an air mass change and so does
the weather in the area which the air mass moves.
Classifying Air Masses
•
•
•
Air masses that influence the weather in North America are mP,
cP, mT, cT (see Figure 3, p.560)
Polar (P) air mass forms at high latitude (toward Earth’s poles)
and its character is cold
Tropical (T) air mass forms at low latitude and its character is
warm
•
•
•
Continental (c) air mass forms at land and its character is dry
Maritime (m) air mass forms at water and its character is humid
(P & T indicates its temperature; c & m indicates the surface
where it was formed)
Weather in North America
•
•
•
Influenced by cP & mT air masses
cP is from North (e.g., Canada, Alaska, Arctic area).
mT is from warm water (e.g., Gulf of Mexico, Caribbean Sea, and
the adjacent Atlantic Ocean)
Continental Polar Air Mass (cP)
•
•
•
Uniformly cold and dry winter and cool and dry in summer
It brings clear skies and cold temperatures (associated with cold
waves)
The difference in temperature between Great Lakes and adjacent
land area (especially when cP air mass pushes southward across
the lake) produces heavy snow
Maritime Tropical Air Mass (mT)
•
•
•
Warm and loaded with moisture; usually stable.
mT air is source of much of the precipitation received in eastern
2/3 of U.S.
In summer, it invades the central and eastern U.S. and brings
high temperature and oppressive humidity
Maritime Polar Air Mass (mP)
•
•
•
•
When cP (cold, dry, stable) is crossing North Pacific, it changes
and becomes mP (mild, humid, unstable) (see Figure 7, p.562)
It is often accompanied by low clouds, showers
When it advances inland against the western mountains, uplift of
the air produces heavy precipitation on windward slope of
mountain
In winter, at northeastern U.S., nor’easter (northeaster) is
formed
Continental Tropical Air mass (cT)
•
•
•
It least influences the weather of North America
Only occasionally do cT air masses affect the weather outside
their source region
It causes extremely hot, drought-like conditions in Great Plain &
it results in mild weather in Great Lake region (Indian summer)
FRONTS
Formation of Fronts
•
•
•
It forms between two contrasting air masses. When two air masses
meet, they form a front (which is a boundary that separates two air
masses)
When air masses on both sides are moving and going same, it acts
like a barrier
However, in most cases the distribution of pressure across front
causes one air mass to move faster than the other.
Type of Fronts - Warm Fronts
•
•
•
•
It forms when warm air moves into an area formerly covered by
cooler air
Its movement is slow so, its slope is very gradual (see Figure 10,
p.565) and it cools to produce clouds and frequently precipitation
(at large area for extended period)
Clouds change from Nimbostratus (Ns)
Altostratus (As)
Cirrostratus (Cs)
Cirrus (Ci)
A gradual increase in temperature occurs with the passage of a
warm front.
Type of Fronts - Cold Fronts
•
It is formed when cold, dense air moves into a region occupied
by warmer air. (See Figure 11 p.566)
•
When the cold front moves, it becomes more rapidly & steeper
than warm front
Cold front can lead to heavy downpours, gusty winds and
towering clouds often can be seen in distance
After cold front passes, weather clears soon and temperatures
drop and wind shifts
•
•
Type of Fronts - Stationary Fronts
•
The flow of air on either side of front is neither toward the cold
•
air mass nor toward the warm air mass
In such cases, the surface position of the front doesn’t move,
and stationary front forms.
Type of Fronts - Occluded Fronts
•
•
•
When an active cold front overtakes a warm front, an occluded
front forms (see Figure 12, p.567)
It develops as the advancing cold air wedges the warm front
upward.
Most precipitation is associated with the warm air being forced
upward
Middle Latitude Cyclones
•
•
•
•
•
Middle-latitude cyclones are large centers of low pressure that
generally travel from west to east and cause stormy weather
Most of it has a cold front, extending from the central area
Its airflow is in a counterclockwise direction and in toward the
center of the low pressure
See the Figure 14A, p.569 (First, front is formed; second, over
time it takes wave shape; third, air flow and pressure change
resulting in a counterclockwise flow of air
And cold front closes in on the warm front and cold front lift
produces an occluded front and finally it weakens).
The Role of Airflow Aloft
•
•
•
These rotating surface wind systems are actually generated by
upper-level flow
More often than not, air high up in the atmosphere fuels a
middle-latitude cyclone
The surface air that feeds cyclone generally originates as air
flowing out of an anticyclone (see Figure 15, p.570)
SEVERE STORMS
Thunderstorms
•
•
A thunderstorm is a storm that generates lighting and thunder. It
frequently produces gusty winds, heavy rain, and hail
Produced by a single cumulonimbus cloud
Occurrence of Thunderstorms
•
•
•
•
2000 thunderstorms in progress on Earth at any one time.
More that 16 million occur annually around the world
At tropics (where it is warm, and there is plenty of moisture
instability) about 45,000 thunderstorms occur.
United States - 100,000 thunderstorms each year (frequently
Florida, Gulf Coast)
Development of Thunderstorms
•
•
•
Thunderstorms form when warm, humid air rises in unstable
environment.
Thunderstorms develop in this order:
o First, cumulus stage (which involves strong updraft (or
upward movements of air) and supply warm and moist air).
o Second, mature stage (which point in the development of
thunderstorm) at this point, thunderstorm is most active
(Gusty wind, lightning, heavy precipitation, hail are
produced).
o Third, dissipating stage - at this time downdraft (or
downward movement of air) throughout the cloud.
During this final stage, the air which was cooling by precipitation
and high above colder air makes storm die down (see Figure 17
A,B and C)
Tornadoes
•
These are violent wind storms that take the form of a rotating
column of air called a vortex.
•
The vortex extends downward from a cumulonimbus cloud.
Occurrence and Development of Tornadoes
•
•
•
•
In U.S. in April through June, the frequency of storms is greatest
December and January is less frequent.
Most tornadoes form in association with thunderstorms
Important process in the formation of many tornadoes is the
development of a mesocyclone - which is a vertical cylinder of
rotating air that develops in the updraft of a thunderstorm (it
causes wind lower in the atmosphere to roll). (see Figure 18
p573)
Tornado intensity
•
•
•
•
Pressures outside of the storm are higher than inside storm
(10% more)
This difference causes air to rush into tornado from all directions
As the air streams inward, it spirals upward around the core.
Eventually, the air merges with the airflow of the cumulonimbus
cloud that formed the storm. Because of the tremendous amount
of pressure change associated with a strong tornado, maximum
wind can sometimes approach 480 kilometers per hour.
Fujita scale – used to estimate tornado intensity (See Table 1
p.574)
Hurricanes
•
Whirling tropical cyclones that produce winds of at least 119km/h.
Occurrence of Hurricanes
•
•
Most hurricanes form 5-20 degrees North & South latitude.
(especially North Pacific is greatest)
Coastal region of southern and eastern U.S. experience fewer
than five hurricanes per year.
Development of Hurricanes
•
•
•
•
Hurricanes develop most often in the late summer when water
temperatures are warm enough to provide the necessary heat
and moisture to the air
It begins as a tropical disturbance (depression)
An inward rush of warm, moist surface air moves toward the
core of storm The air then turns upward and rises in the eye wall
(which is a doughnut-shaped wall that surrounds the core)
The eye of the hurricane (which is the center point) is where
precipitation ceases and winds subside. This is the warmest part
of the storm.
Hurricane Intensity
•
•
•
Using Saffir-Simpson Hurricane Scale (See Table 2 p.577)
When hurricane eye moves onto land, storm surge (which is
65~80km wide dome of water) is created.
When a hurricane moves over the cool ocean, it is weakened
because the cool ocean cannot supply adequate heat and
moisture.