Chapter 3

 3.1  Air Mass – large volume of air in which temperature and humidity are nearly the same in different locations at the same altitude  Front – is a boundary between air masses  High Pressure system – is formed when air moves all the way around a high pressure center.  Low pressure system – is a large weather system that surrounds a center of low pressure

 The weather changes when a new air mass moves into your area  An air mass is a large volume of air in which temps and humidity are nearly the same in different locations at the same altitude  Can cover thousands of square kilometer

 There are different characteristics of air masses  Characteristics depend on region where is located  Characteristics of air masses are classified into categories according to the characteristics of the regions

 First word of an air mass category name tells whether the air mass formed over dry land or water  Describes the moisture of the air mass  Continental – formed over land. Air is dry because moisture is pulled to dry land below it  Maritime – formed over water. Air becomes moist as water is pulled from the water below

 The second category tells whether air mass formed close to the equator. Describes air mass temp  Tropical – Form near the equator. Air becomes warm as it gains energy from warm land or water  Polar – Form far from equator. Air becomes cool as t loses energy to cold land or water  Combination of words gives the characteristics. Ex: Maritime tropical – moist and warm Continental polar mass – dry and cold

 Air masses can travel away from originated regions  Move with global pattern of winds  In US, generally move west to east  Can move along with jet stream in more complex and changing patterns  When moving to new region, takes characteristics with it  While moving over earths surface, surface characteristics begin to change the air mass  Can take weeks or days to change entire air mass  If air mass moves quickly, it may not change much

Fronts and Weather  Different types of fronts produce different patterns of weather  Cold Front – moves into regions quickly. Mass of cold and dense air moves forward and warmer air is pushed upward.  Produce tall cumulonimbus clouds and precipitation.  Brief heavy storms likely. Cool and clear air after

 Warm fronts – move more slowly than cold fronts. Warm air moves gradually up and over cold dense air.  Moisture in warm air condenses along front producing clouds.  When approaching, high cirrus clods, then high stratus clouds, and then lower and lower stratus clouds appear

 Stationary Fronts – occur when air masses first meet or when a cold or warm front stops moving  For a while, boundary between masses stays in same location  Each one can move sideways or upward  Upward may produced clouds that cover sky for days at a time  When movement begins, will become a warm or cold front

High Pressure system  Letter H on a map  Air sinks slowly down  Near the ground, spreads out toward areas of lower pressure  In Northern Hemisphere, Coriolis effect makes air turn clockwise as it moves outward  Formed when air moves all the way around a high pressure center  Large and change slowly  Air mass may form if in one location for a while and can be warm or cold  Generally brings clear skies and calm air  Sinking air evaporates clouds

Low Pressure System  Begins as air moves around and inward toward the lowest pressure and then up to higher altitudes  Upward motion lowers the air pressure, moves faster  Rising air produces stormy weather  In Northern Hemisphere, LPS circles counterclockwise  Can develop wherever there is a center of low pressure  Often happens along a boundary of warm and cold air masses  Larger systems such as LPS generally move with global winds, west to east

Section 3.2 Low pressure Systems can become storms  Tropical Storms – is a LPS that starts near the equator and has winds that blow at 65 kilometers/hour or more  Hurricane – a tropical LPS with winds blowing at speeds of 120 kilometers/hour or more(74 mph)  Storm surge – huge masses of ocean water pushed into a costal area by a hurricane  Blizzard – blinding snowstorms with winds of at least 56 kilometers per hour (35mph) and low temps usually below 7ºC (20ºF)

Hurricanes form over warm water  At the equator, warm water provides energy to turn LPS into violent storms  Energy makes warm air rise faster  Clouds and strong winds develop  Wind blow from different directions into the LPS  The Coriolis effect then bends their paths into a spiral  Winds blow faster and faster around the LPS center and this becomes the storm center  Hurricanes are called typhoons or cyclones when formed over Indian ocean or western pacific ocean

Formation of Hurricanes  In US, Hurricanes occur most often between August and October  Energy from warm water is necessary for LPS center to build into a hurricane  Waters are warm enough only near the end of summer  Hurricanes and TS generally move westward with trade winds  Near land, they move north, south or back east  As long as water is warm, storm will grow  Cool waters and moving over land will decrease winds which cause the storm to shrink

 Center of the hurricane is a small area of clear weather  Called the eye  Air moves downward  Air moves quickly around and upward along the eye  Forms a tall ring of cumulonimbus clouds  Ring produces heavy rains and winds

Effects of Hurricanes  Lift cars, uproot trees and tear off building roofs  Can produce tornadoes  Flooding  Storm surges in coastal areas

Storm surges  Sea levels rise several meters  Back up rivers  Flooding shores  Can be deadly and destructive  Large waves add to destruction  Damage can take weeks or months to clean up

National Hurricane Center  Helps people know when to prepare for hurricanes  Puts out TS or hurricane watch within a 36 hour period  Helps people evacuate or moved away to safety areas  Issues warnings within a 24 hour or less period  Warning stays in effect until danger has passed

Winter Storms Produce Snow and Ice  Not caused by LPS  Occur when two air masses collide  National Weather Service will alert within 48 hours of storm  Warning means conditions are already present or will affect an area shortly

Blizzards  Storms where wind blows snow into so it is difficult to see  Occur in many parts of the northern and central US  Can knock down trees and powerlines  Without heat, buildings can become cold, water pipes freeze and roofs can cave in

Lake-Effect Snowstorms  Some of the heaviest snowfall is in the Great Lakes area  Cold air rom NW gains moisture and warmth as it passes over the great lakes  Over cold land, it cools and releases moisture as snow.  Can cover areas downward of GL with clouds and snow even when the rest of the region is clear

Ice Storms  Rain freezes as it is touching the ground and other surfaces  Ice covered roads are slippery and dangerous  Branches or even whole trees will begin to break  Can shut down entire cities

Section 3.3 Vertical air motion can cause Severe Storms  Thunderstorm – a storm with lightning and thunder  Tornado – a violently rotating column of air stretching from a cloud to the ground

Thunderstorms for from rising moist air