Download File topic 7

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Topic 7
Meteorology (Weather)
Weather: a description of the atmospheric variables (temperature, pressure, wind, and moisture) at a
certain place and time.
 Temperature
- measured with a thermometer
- depends mostly on intensity and duration of insolation
- generally, as latitude increases, temperature decreases
 Air /Atmospheric / Barometric Pressure
- air/atmospheric/barometric pressure: the force of the air pushing on the Earth
- measured with a barometer
- lines connecting points of equal pressure on a weather map are called isobars
- warmer air is less dense so warmer air molecules rise……rising air is not pushing on the Earth, therefore
the pressure of the air is “low” [inverse relationship:  T,  P]
Warm air = Low pressure
Cool air = High pressure
- if moisture enters the air, water vapor molecules replace heavier air molecules (O 2, N2) making the air
lighter so it rises……rising air is not pushing on the Earth, therefore the pressure of the air is “low”
[inverse relationship:  moisture,  P]
So…….“HIGH” = cooler, drier air……………….“LOW” = warmer, moister air
 Humidity (Moisture)
- measured with a hygrometer or psychrometer
- absolute humidity: the actual amount of water vapor in the air. But, the dampness we feel really
depends on how close the air is to being saturated with moisture…….this is relative humidity.
- warm air has more energy and therefore greater ability to evaporate water, so relative humidity
depends on air temperature. As the temperature drops, less evaporation occurs and more condensation
occurs. When it reaches a temperature where more condensation is occurring than evaporation (=NET
condensation), it is saturated, RH = 100%, and the condensed droplets will combine to form a cloud.
- RH can be changed by:
1. adding or removing moisture
2. changing temperature
- dew point: the temperature where NET condensation occurs (RH = 100%)
If dew point  0C, water vapor condenses to water droplets (on the ground, this makes dew)
If dew point  or  0C, water vapor sublimates directly to ice crystals (frost on the ground)
- condensation nuclei: dust, salt, pollen, smoke, etc. that act as surfaces or centers for water vapor to
condense on
- a cloud is a bunch of water droplets or ice crystals that aren’t heavy enough to fall yet
- clouds that form at ground level are called fog
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 Precipitation
Precipitation is any form of water that falls from the atmosphere and reaches the ground
 Rain - drops of water
 Snow -- falling ice crystals (dew point and air temperature at surface  0C)
 Sleet -- raindrops fall through a layer of colder air ( 0C) and freeze into pellets of ice
 Freezing rain -- raindrops freeze when they hit the cold ( 0C) ground, forming a layer of ice
(sometimes referred to as “black ice”)
 Hail -- forms only in tall clouds; starts as an ice crystal way up high and then picks up layers of water as
it drops, then freezes over as it rises again, and continues the process many times
 Wind
- measured with an anemometer
Wind is the horizontal movement of air across the Earth
- created by differences in air pressure
- wind always blows from an area of high pressure to an area of low pressure
- winds are named according to where they come from (EX/ a north wind is coming from the north)
The jet stream is an area of high speed winds in the upper atmosphere caused by huge pressure
differences. They strongly influence weather patterns.
Meteorologists use station models {see RT} to plot the weather conditions at any given moment on a map.
 Air Masses
- air mass: a huge body of air that forms over a region
- the characteristics of an air mass depend on the source region if forms over:
If it forms over:
Land

dry

continental (c)
Water

moist

maritime (m)
High latitudes

cold

Polar (P)
Low latitudes

warm

Tropical (T)
Very high latitudes

frigid

Arctic (A)
EX/ an air mass that forms over the Gulf of Mexico would be warm and moist (mT)
- Low pressure air masses are called “Lows” or “cyclones”
- High pressure air masses are called “Highs” or “anticyclones”
In the northern hemisphere,
Winds blow into a low, counterclockwise
Winds blow out of a high, clockwise
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 Fronts
A front is a place (boundary) where two air masses meet. A front forms when one air mass moves into an
area already occupied by another air mass.
 A cold front forms when a cold air mass moves into a warmer air mass.
- cold air usually moves faster
- cold dense air moves forward close to the ground, forcing the warm less dense air to rise rapidly
- warm air rises, cools to its dew point, forms a towering dark thundercloud (cumulonimbus) with heavy but
brief precipitation
- after the front passes, pressure rises and temperatures are cooler
 A warm front forms when a warm air mass moves into a cooler air mass.
- cold dense air stays close to the ground, forcing the slow warm air to gradually rise
- warm air rises slowly, cools to its dew point, forms a sheet of low, thick, clouds (nimbostratus) with
light, continuous precipitation
- after the front passes, pressure rises and temperatures are warmer
 A stationary front forms when a warm air mass and a cool air mass meet but don’t move much or at least
don’t collide into each other. If there’s any precipitation, it is similar to a warm front.
 An occluded front forms when a cold front catches up to a warm front
- the cold air pushes the warm air above it and completely lifts if off the ground
- the weather is a mixture of a cold front and warm front
 Storms
Thunderstorm
- most common storm
- forms when warm moist air rises fast, cools to the dew point, forms a tall cumulonimbus cloud
- condensation releases heat which further warms the air, causing it to keep rising, cooling, and
condensing (this creates strong upward air currents)
- heavy precipitation causes strong downward air currents
- upward and downward air motion causes electrical charges to build up until a huge spark is discharged
(lightning)
- the electrical current in the lightning stroke heats the air in its path quickly, causing the air to expand
violently which produces sound waves (thunder)
Tornado / Twister
- small violent storm that lasts only a few minutes
- spawned by thunderstorms
- frequently occur in “Tornado Alley” (southern Midwest) from March to July
- form best with a tremendous cold front, flat land, and huge low pressure in jet stream
- extremely low pressure in center (vortex) and usually less than 100 m wide
- winds outside vortex can be 800 km/hr
- storm usually travels northeasterly, about 50 km/hr
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Hurricane
- storm begins as a tight Low over a warm ocean
- does not form with fronts -- all warm air
- hurricane season is May to November
- warm moist air rises around the eye (center), pulling more air in and causing winds to increase
- air rises quickly, cools, forms many thunderstorms
- condensation releases heat, warms the air and creates a warm core in center
- pressure differences intensify, winds increase, more moisture gets pulled in, more condensation, more
heat…..the whole process gains energy
- Effects of hurricane:
- heavy rains
- tremendous winds
- can spawn many tornadoes
- storm surge (sudden rise in sea level)
- hurricane classification:
Wind speed
 39 mi/hr = a tropical depression
39 – 73 mi/hr = a tropical storm (gets named)
 74 mi/hr = a hurricane
- as winds intensify and pressure decreases, the hurricane gets classified as Category I – V
- hurricanes die out when they lose their source of energy…..either by traveling over land or cold water
 Storm Safety
Regardless of the type of storm (or natural disaster of any sort), the two most important things to keep
in mind when preparing for safety are:
1. Have an emergency kit ready and available with nonperishable food, water, flashlight, radio with
batteries, blankets, first aid materials, and spare cash.
2. Have an emergency evacuation plan: know how to exit the house safely, where family members are
going to meet, and have a planned route to escape the neighborhood or city if necessary.