Transcript Air Masses, Fronts

Air Masses, Fronts
and Weather Systems
Again, Let’s Recall the
5+ Basic Elements of the Atmosphere
-- Also called Elements of Weather and Climate
Solar Energy ▪ Temperature ▪ Pressure ▪ Wind ▪ Precipitation
In this segment, we’ll discuss the last of these elements:
+ Air Masses (and Fronts)
 Movements of Air Masses and Fronts are vital to
our understanding and prediction of Weather
Systems
 Weather Forecasting requires that we understand:
 Cyclones (L=Lows) and Anticyclones (H=Highs)
 Air Masses and Fronts
What is an Air Mass?
• A large body of air with similar temperature and/or humidity
• Occupies thousands of square miles of the Earth’s surface,
and can be several miles high
• Born in a source region where they acquire their
temperature and moisture characteristics
Source Regions
• Stable source regions are generally in the subtropics or near
to the poles
• They are often regions of low relief and calm wind
that prevent turbulent mixing and allow the air to take on the
conditions of the surface over which it forms
• Areas dominated by high pressure serve as good source
regions; enables the air to move outward
Air Masses are Classified According to:
1. Surface of source region [Identified by lower case letters, c and m]
with associated Moisture characteristics
 Continental (c)  Forms over land  dry
 Maritime (m)  Forms over ocean  moist
2. Latitudinal extent of source region [Cap letters, E, T, P and A]
with associated Temperature characteristics
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
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Equatorial (E)  Very low latitudes  warm
Tropical (T)  Low latitudes  warm
Polar (P)  High latitudes  cold
Cold Arctic (A)  Poles  very cold
Global Distribution of Air Masses
North America is affected by 5 types of air masses –
Temperature and moisture characteristics of the air masses –
Air Masses are thus identified by letter codes,
indicating their moisture and temperature
characteristics, and their sources regions –
Continental Arctic (cA), typically extremely cold and dry.
Continental Antarctic (cAA), also exceedingly cold but drier
than its arctic counterpart as its source region is a continent.
Continental Polar (cP), considered cold and dry but warmer
than the arctic air mass to its north.
Continental Tropical (cT), originates in the great desert
areas of Earth (e.g., Sahara, Arabian, and Australian).
-- extremely low humidity is due to the lack of available water.
Maritime Polar (mP), cool and moist; brings mild weather to
coastal locations.
Maritime Tropical (mT), warm and moist; responsible for much
of the precipitation east of the Rocky Mountains.
Maritime Equatorial (mE), form near the equator; very warm
and humid; convection and convergence of these in the ITCZ
responsible for heavy rainfalls in the equatorial rainforests.
 The North American region is affected by the first five
types of air mass (cA, cP, cT, mP, mT -- See Map)
 Note: often a lower case “k” or “w” may be added to
air mass names to
indicate if they are colder (k) or warmer (w) than the surface below;
e.g., mTk or mTw – may suggest stability or instability in the air mass.
What is a Front?
• Fronts are boundaries that
separate air masses
• Where air masses meet and
do not mix.
4 Types of Fronts:
Cold Front
Warm Front
Stationary Front
Occluded Front
 Note: The direction the teeth or the bumps face
is the direction the front is moving
Cold Front
When a fast moving cold air mass runs into a
slow moving warm air mass and the thicker cold
air slides under the lighter warmer air.
COLD FRONT
 Cold air displaces warm air upward
 Much steeper than warm fronts
 Advances faster than a warm front
 Violent weather – Cumulonimbus
clouds, thunderstorms, tornadoes,
hail, short, sharp showers
Warm Front
When a moving warm air mass collides with a
slowly moving cold air mass and the warm air
moves over the cold air.
WARM FRONT
 Warm air replaces cold
 Gentle slope (1:200)
 Covers a wide area with its weather
 Stratus clouds get lower as front
approaches – Drizzle or steady rain
Stationary Front
When a cold air mass and warm air mass meet,
but neither air mass has enough force to move
the other air mass.
STATIONARY FRONT
 Cold and warm air masses
next to each other
 Neither air mass advances
 Areas under them experience clouds,
drizzle and rain (even possible
thunderstorms) for several days
Occluded Front
When a warm air mass is caught between two
cooler air masses and the thicker cold air
masses move underneath the thinner warm air
mass and push it upward.
OCCLUDED FRONT
 Faster-moving cold front
overtakes a warm front –
usually occurs in the latter stages
of a mid-lattitude cyclone
 Lifts all the warm air off the ground
 Clouds and rain possible
Which types of Fronts and Weather
Systems can you find on this map?
Map Interpretation