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Weather
 Weather can change rapidly.
 The weather at a location depends on:
 air temperature
 air pressure
 fog
 humidity
 cloud cover
 precipitation
 wind speed and direction
Climate
 Climate is the average of a region’s weather over time.
 The climate for a particular place is steady, and
changes only very slowly.
 Determined by:
 angle of the Sun
 likelihood of cloud cover
 air pressure
Humidity
 Relative humidity is the percentage of water vapor a
certain volume of air is holding relative to the
maximum amount it can contain.
 Since warm air can hold more water vapor than cool air,
raising or lowering temperature can change air's relative
humidity.
 The temperature at which air becomes saturated with
water is called the air's dew point.
Clouds
 Clouds have a big influence on weather:
 by preventing solar radiation from reaching the ground.
 by absorbing warmth that is re-emitted from the ground.
 as the source of precipitation.
 Water vapor is not visible unless it condenses to become a
cloud.
 Water vapor condenses around particles:
 Dust
 Smoke
 Salt crystal
 Billions of these water droplets together make a cloud.
Cloud Classification
 The most common classification used today divides
clouds based on their altitude.
Height Prefixes
Shape Prefixes
Cirro:
Cirrus:
High clouds bases starting
above 6000 m.
Latin meaning “hair”; Wispy,
stringy clouds.
Alto:
Cumulus:
Middle clouds with bases
between 2000-6000m.
Latin meaning “pile or heap”;
Puffy, lumpy looking clouds.
Strato:
Stratus:
Low clouds below 2000m
Latin meaning “layer”;
Featureless sheets of clouds
Nimbus:
Latin meaning “cloud”; Low,
gray rain clouds
High Clouds
 Cirro
 Form from ice crystals where the air is extremely cold
and can hold little water vapor.
 Cirrus, cirrostratus, and cirrocumulus
Middle Clouds
 Alto
 Made of water droplets, ice crystals or both, depending
on the air temperatures.
 altocumulus and altostratus clouds
Low Clouds
 Strato
 Nearly all water droplets.
 Stratus, stratocumulus, and nimbostratus
Vertical Clouds
 Clouds with the prefix 'cumulo-' grow vertically
instead of horizontally.
 Have their bases at low altitude and their tops at high
or middle altitude.
 Clouds grow vertically when strong air currents are
rising upward.
Fog
 A cloud located at or near the ground .
 When humid air near the ground cools below its dew
point, fog is formed.
 Science on the SPOT: The Science of Fog
Precipitation
 Can form in place.
 Dew forms when moist air cools below its dew point
on a cold surface, such as a flower.
 Frost is dew that forms when the air temperature is
below freezing.
Precipitation
 The most common precipitation comes from clouds.
 Rain & Snow
 Rain falls from clouds when the temperature is fairly warm.
 If temperatures are cold, the droplet will hit the ground as a
snowflake.
Precipitation
 Less common types are sleet and hail.
 Sleet is rain that becomes ice as it hits a layer of
freezing air near the ground.
 Hail forms in cumulonimbus clouds with strong
updrafts.
 An ice particle travels until it finally becomes too heavy
and it drops.
Air Masses
 Large body of air that takes on the characteristics of
the area over which it forms.
 Where the air mass forms is its source region.
 Surface of Source Regions:
 Land: Continental (c); Dry
 Water: Maritime (m); Humid/Wet
 Temperature of Source Regions:
 Warm: Tropical (T)
 Cold: Polar (P)
 VERY Cold: Arctic (A)
 Arctic: Very cold, dry
 Maritime Polar: (mP) Cold, humid
 Continental Polar: (cP) Cold, dry
 Maritime Tropical: (mT) Warm, humid
 Continental Tropical: (cT) Warm, dry
Air Mass Movement
 Air masses are slowly pushed along by high-level
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winds.
When an air mass moves over a new region, it shares
its temperature and humidity with that region.
Storms arise if the air mass and the region it moves
over have different characteristics.
Cold air masses tend to flow toward the equator.
Warm air masses tend to flow toward the poles.
This is one of the many processes that act towards
balancing out the planet’s temperatures.
Fronts
 Where two air masses meet.
 The air masses have different densities:
 One is lifted over the other, creating a low pressure zone.
 The greater the temperature difference between the
two air masses, the stronger the winds will be.
Cold Fronts
 Cold dense air displaces warm air, and pushes the
warm air up along a steep front.
 This front moves rapidly.
 Cumulus and cirrus clouds can form.
 Strong cold fronts may lead to:
 Thunderstorms
 Squall lines
 Tornadoes
 High winds
 Snow showers
Seasonal Cold Fronts
 Spring and summer:
 The air is unstable so thunderstorms or tornadoes may form.
 Spring:
 If the temperature gradient is high, strong winds blow.
 Autumn:
 Strong rains fall over a large area.
 Winter:
 The cold air mass is likely to have formed in the frigid arctic
so there are frigid temperatures and heavy snows.
 After the front passes, temperatures usually drop and skies
clear.
Warm Fronts
 Advancing warm air slides over cold air.
 Atmosphere is relatively stable:
 The less dense, warm air moves over the colder, denser
air.
 Warm fronts are characterized by less severe weather
but they last longer.
 Extensive cloudiness (cirrus & stratus)
and precipitation (steady light rain or
snow in front of them)
Warm Fronts
 Warm fronts move slower than cold fronts
 After the front passes, skies clear and usually
temperatures increase.
Stationary Front
 When two air masses meet and neither is strong enough
to replace the other.
 The boundary stays where it is, and this is called a
stationary front.
 Can bring days of rain, drizzle, and fog.
 Winds usually blow parallel to the front, but in opposite
directions.
 Can last several days,
then will likely break apart.
Occluded Front
 Forms when a cold front is moving faster than a
warm front; the cold front catches up to and takes
over, the warm front.
 Precipitation is common on both sides of the front.