Download interior structure of the earth

MR. SURRETTE
VAN NUYS HIGH SCHOOL
CHAPTER 6: WEATHER
CLASS NOTES
HUMIDITY
There is always water vapor in the air. A measure of water vapor is humidity. Humidity is the mass of
water contained per volume of air. As air temperature increases, so does the amount of water it can
hold.
RELATIVE HUMIDITY
Weather reports use relative humidity in their forecasts. Relative humidity is the percentage of water
vapor in the air at a given temperature. For example, a relative humidity of 50% means the air is
holding half as much water vapor as it can.
SATURATION
Saturated air holds the maximum amount of water vapor. Saturation occurs when air temperature drops
and water molecules in the air stick together, or condense.
CONDENSATION OF WATER MOLECULES
PRECIPITATION
As a parcel (piece) of air rises in the atmosphere, it expands and cools. If there are larger particles or
ions (electrically-charged particles) in the air, water vapor condenses on these particles and forms a
cloud.
PRECIPITATION
As the size of cloud droplets grow they fall as precipitation in the form of rain, sleet, or snow.
DEW
As air cools below a certain temperature, called the dew point, the air’s ability to hold water vapor
decreases. At this point, water from the air condenses on any available surface. The surface may be a
twig, a blade of grass, or the windshield of a car. This causes early-morning dew.
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MR. SURRETTE
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FROST AND FOG
When the dew point in the air is at or below freezing, frost condenses on any available surface. Fog
forms when a large mass of air cools and reaches its dew point.
ATMOSPHERIC STABILITY
Stable air is air that resists upward movement. When stable air is forced to rise, it spreads out
horizontally. Clouds also spread out into thin horizontal layers when they develop.
TEMPERATURE INVERSIONS
Temperature inversions occur when the upper regions of the atmosphere are warmer than the lower
regions. Warm air in the lower atmosphere cannot rise higher than the hot air above it and becomes
trapped.
SMOG IN LOS ANGELES
HIGH CLOUDS
High clouds form 6,000 meters or more above the ground. The names of high clouds have the prefix
cirro-. For example, cirrus, cirrostratus, and cirrocumulus are types of high clouds. Air at this
elevation is cold and dry, so high clouds are composed of ice crystals.
MIDDLE CLOUDS
Middle clouds form 2,000 to 6,000 meters off the ground. They are denoted by the prefix alto-. For
example, both altostratus and altocumulus are middle clouds. Middle clouds can be composed of
water droplets or ice crystals.
LOW CLOUDS
Low clouds range from the surface up to 2,000 meters. Low clouds are called stratus clouds.
Stratocumulus and nimbostratus clouds are types of low clouds. Low clouds are composed of water
droplets.
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MR. SURRETTE
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CLOUDS WITH VERTICAL DEVELOPMENT
Vertical cloud development is caused by rising air currents. Vertical clouds are the cumulus clouds, the
familiar “cotton ball” shaped clouds. Sometimes cumulus clouds turn dark and release precipitation.
Then they become cumulonimbus clouds.
AIR MASSES
Various air masses cover large portions of the Earth’s surface. Each large portion of the Earth’s surface
imparts unique characteristics to the air mass it touches. There are six general categories of air masses.
They are determined by humidity and temperature.
DISTRIBUTION OF AIR MASSES
TYPES OF AIR MASSES
Continental polar (cP) and continental arctic (cA) air masses produce very cold, dry weather in the
winter, and cool, pleasant weather in the summer. Maritime polar (mP) and maritime arctic (mA) air
masses produce cool and moist weather.
TYPES OF AIR MASSES
Maritime tropical (mT) air masses create warm, moist weather. Finally, continental tropical (cT) air
masses are hot and dry.
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CLASSIFICATION OF AIR MASSES
ATMOSPHERIC LIFTING MECHANISMS
Clouds are excellent indicators of weather. For clouds to form, air must be lifted. The three principal
lifting mechanisms are: convectional lifting, orographic lifting, and frontal lifting.
CONVECTIONAL LIFTING
When hot air rises, the cold air above it sinks. This circulatory motion produces convectional lifting.
OROGRAPHIC LIFTING
An air mass pushed upward over a mountain range undergoes orographic lifting.
OROGRAPHIC LIFTING
The air descending on the back of a mountain (leeward slope) is dry because most of the moisture has
been removed by precipitation on the windward slope (upslope) side of the mountain. The dry, leeward
sides of mountains are called rain shadows.
RAINSHADOW EFFECT
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FRONTAL LIFTING
The contact zone between two different air masses is a front. Sometimes, when two air masses make
contact, one air mass rides over the other. This is called frontal lifting.
COLD FRONTS
When a cold air mass moves into a warm air mass, the contact zone is a cold front. The cold air forces
the warm air upward, where it condenses to form clouds.
COLD FRONT
WARM FRONTS
When warm air overtakes a cold air mass, the contact zone is a warm front. The warmer air rides up
and over the colder, denser air. This creates widespread cloudiness and some precipitation.
WARM FRONT
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THUNDERSTORMS
A thunderstorm begins when humid air rises, cools, and condenses into a single cumulus cloud. The
cloud builds and develops upward. Particles of precipitation grow larger and heavier until they fall as
rain.
THUNDERSTORMS
At the mature stage of a thunderstorm, the rising warm updraft and the sinking cold downdraft form a
storm cell within the cloud.
TORNADOES
Winds slowly rotating over a large area speed up when their radius of rotation decreases. A tornado is a
funnel-shaped cloud that touches the ground from a large cumulonimbus cloud. Winds in a tornado may
reach 500 miles per hour.
HURRICANES
As air moisture and thermal energy increase, surface winds may converge and form strong vertical wind
shears. These wind shears cause rising warm, moist air to tilt inward and spiral. These conditions cause
hurricanes. Hurricanes produce tornadoes, rough waters, and wind speeds approaching 200 miles an
hour.
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