The Causes of Weather
The Causes of Weather
 Meteorology is the study of atmospheric phenomena.
The Causes of Weather
Weather and Climate
 Weather is the current state of the atmosphere, including
short-term variations that affect our lives.
• Climate describes the average weather over a long period
of time and is usually averaged over the course of 30
years or more.
• Meteorology, weather, and climate are related.
The Causes of Weather
A Question of Balance
The Causes of Weather
Air Masses
 An air mass is a large body of air that takes on the
characteristics of the area over which it forms.
Air Masses
Classifying Air Masses
The Causes of Weather
• warm and dry continental tropical (cT)
• warm and humid maritime tropical
(mT)
– Air masses are classified
• cold and dry continental polar (cP)
according to their
source regions.
• cold and humid maritime polar (mP)
• arctic (A)
The Causes of Weather
Air Masses
Classifying Air Masses
– Each of the major air
masses that affects weather
in the United States has a
similar temperature and
moisture content as
the area over which
it formed.
The Causes of Weather
Air Masses
Air Mass Modification
Weather Systems
 Coriolis Effect- Deflects moving particles such as air to the
right above the equator; caused by Earth’s rotation and
combines with the heat imbalance founded on Earth to create
the trade winds, polar easterlies, and prevailing westerlies.
 Colder air to warmer areas and warmer air to
colder areas.
Wind Systems
 Trade Winds (Hadley Cell)-30 degrees North and South
latitude, air sinks, warms, and moves to equator in a westerly
direction.
 Prevailing Westerlies- flows between 30 and 60 degrees North
and South latitude, in a circulation pattern opposite that of the
Trade Winds. Surface winds move towards the poles in an easterly
direction. Wind blows from the West. Responsible for the
movement of our weather in the U.S.
 Polar Easterlies-lies between 60 degrees latitude and the poles.
Flows from the northeast to the southwest in the Northern
Hemisphere. Opposite in Southern Hemisphere. Bring COLD air.
WIND SYSTEMS
High Pressure System
 Generally associated with fair, sunny weather
 As high pressure is an area of sinking air, and air tends to dry out as
it sinks, leaving sunny skies
High Pressure Animation
 http://www.juicygeography.co.uk/downloads/flash/highpressu
re.swf
Low Pressure System
 Generally cloudy/rainy areas -- where strong areas of low pressure
bring our stormiest weather
 This is because it's an area of rising air, and as air rises, it condenses
into clouds and rain
 Air moves from higher pressure to lower pressure, so if you have a
high and a low nearby, it can be windy as air rushes between the two
Low Pressure Animation
 http://www.juicygeography.co.uk/downloads/flash/lowpressur
e.swf
Fronts
 When two air masses that have different properties
meet, they do not mix easily. A boundary forms
between the two air masses. This boundary is called a
front. There are four types of fronts:
 cold fronts, warm fronts, occluded fronts and stationary
fronts.
 The weather at a front is usually unsettled and stormy.
Cold Front
 A cold front forms when a
mass of cold air meets and
pushes under a mass of warm
air. The cold air mass forces
its way underneath the warm
air mass and pushes it
upward. Violent storms are
associated with a cold front.
Fair, cool weather usually
follows.
Warm Front
 A warm front forms when
a mass of warm air
overtakes a cold air mass
and moves over it. Rain
and showers usually
accompany a warm front.
Hot, humid weather
usually follows.
Occluded Front
 A cold front travels faster than a
warm front. When a cold front
overtakes a warm front, an
occluded front forms. As the warm
air is pushed upward, the cold air
meets cool air. An occluded front
may also occur when cool air
overtakes a cold front and warm air
is pushed upward. An occluded
front produces less extreme
weather than a cold or warm front.
Stationary Front
 When a warm air mass
meets a cold air mass and
no movement occurs, a
stationary front forms.
Rain may fall in an area for
many days when a
stationary front is in place.