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Weather
Section 1: Water in the Atmosphere
Basics
• The Water Cycle is the movement of water
between the atmosphere and earth’s surface
• Evaporation: Process where liquid water turns
to a vapor
• Condensation is when water vapor changes
back to a liquid (#6)
• Dew Point: Is the temp where condensation
happens (#7)
Humidity
• The measure of the water vapor in the air
– Air can hold various amounts at different
temperatures
• Relative Humidity: is a percentage…it tells how
much is in the air in relation to what it can
actually hold.
• Psychrometers: are instruments used to measure
relative humidity
– Measures based on the temperature change in the
wet bulb of the psychrometer.
Clouds
• Cirrus Clouds:
– Soft, feathery
– Form at high levels in low temperatures
– Made of ice crystals
• Cumulous Clouds:
– Fluffy like cotton balls
– Lower to the ground but can extend higher
– Can be combined with the suffix –Nimbus to
indicate a rain cloud
• Stratus Clouds:
– Form in flat layers and cover most of the sky
– Can produce rain or snow (nimbostratus)
Other Clouds
• The Prefix “—Alto” may be added to a cloud if
it is one of the three main types, but higher
than usual
– Examples: altocumulus, altostratus
• Fog is formed when hot, humid days lead to
cooler nights, causing vapor to condense
lower to the ground.
– Fog tends to form over areas with more water…
Pg. 439
SECTION 2: PRECIPITATION
Precipitation
• Any form of water that leaves clouds and
comes back to Earth’s surface
• Does Precipitation always fall from clouds?
• No
• Remember: precipitation occurs when ice or
liquid is heavy enough to fall
• Types of Precipitation: Rain, Hail, Snow, Sleet,
Freezing Rain
Rain
• Precipitation that falls as drops of water
• Small drops are called Drizzle or mist
– These usually come from stratus clouds
Hail
• Round pellets of ice (larger than 5mm---about
the size of a pea) called hailstones
• Updrafts of wind carry the ice pellet through
the cloud many times, forming many layers of
ice
• Hail tends to have rings when cut in half (like
an onion)
Snow
• Water in the clouds is changed strait to ice
crystals—ie. Snowflakes
• Powder happens when snow falls in cool, dry
air
• Clumpy snow happens in moist, humid areas.
Sleet
• Raindrops freeze after leaving the cloud
• Sleet is usually smaller than 5mm (the size of a
pea)
Freezing Rain
• Rain that freezes when it touches a cold
surface
• Causes ice to build up and coat things at the
surface level
Pg. 442
AIR MASSES & FRONTS
Air Masses
• A huge body of air that has it’s own
temperature, humidity, and air pressure.
• Can spread over millions of kilometers and be
up to 10 kilometers deep
• Characterized by temperature and humidity
• 4 kinds(North America): Tropical, Polar,
Maritime, and Continental.
Tropical
• Warm air masses that form in the tropics
• Have low air pressure
Polar
• Cold air masses that form in polar regions
• High air pressure
Maritime
• Form over oceans
• Can become very humid
Continental
• Air masses that form over land
• Drier than maritime because they don’t have
the evaporation process.
Combining Air Masses
• Types of air mass can be combined to imply
area of Earth and the part of the Earth it forms
over
• Example:
– Maritime Tropical---Forms over tropical oceans
– Maritime Polar---Form over polar ocean areas
– Continental Tropical---Form over tropical Land
– Continental Polar---form over polar land areas
Air Mass Movement
• Moving air masses interact with other masses
causing the weather to change
• Prevailing Westerlies: Major wind belts of US
– Push Air masses west to east
• Jet Streams: Bands of high speed winds 10
kilometers from the surface of Earth
• Fronts: Huge masses of air that move across
oceans and collide
• Don’t’ mix well due to different densities
• Area of the collision develops a “Front”
• 4 types---type of front depends on the
characteristics of the air masses and how they
are moving.
Fronts
• Cold:
– Cold air (dense) sinks…Warm air (less dense) rises
– Cold air mass slides under warm air mass, pushing
the warm air up
– As it rises, it expands and cools, making vapor
change rapidly to water or ice crystals…forming
clouds
– Can move quickly and cause
thunderstorms…brings colder, drier air and lower
temperatures.
• Warm:
– Fast air mass overtakes a colder air mass
– Warm air moves over the cold air
– Rain or snow forms if the air is humid…clouds if
the air is dry
– Warm fronts move slow, lasting several days….
– After---weather is warm and humid
• Stationary:
– Warm and cold meet, but neither move the other
– Water vapor can condense along the front causing
rain, snow, fog, or clouds for days.
• Occluded:
– Warm air mass trapped between two cold air
masses
– Cold is denser, so they go under the warm and
push it upward
– Temperature near the ground becomes cooler
– As warm air cools, weather becomes rainy.
• Read pg. 448…
• Complete the double bubble map for cyclones
and anticyclones
• Complete 2c pg. 449
Pg. 450
SECTION 4: STORMS
Storm
• A violent disturbance in the atmosphere
• 4 types: Thunderstorms, Tornadoes, Snow
Storms, Hurricanes
Thunderstorms
• A small storm that often has heavy rain and
frequent thunder/lightning
• Form in Cumulonimbus clouds (thunderheads)
• Thunderstorms form on hot, humid
afternoons when warm air is forced upward
along a cold front (warm air rises rapidly)
• Lighting is a sudden spark, or electric
discharge that happens when charges jump.
Tornadoes
• A rapidly whirling, funnel-shaped cloud that
reaches down from a storm cloud to touch
Earth’s surface
• Over a lake or ocean, it is called a waterspout.
• Formation:
– Commonly—in thick cumulonimbus clouds
– When thunderstorms are likely
– Warm air mass and cold air mass meet going
opposite directions
Snow Storms
• Occurs mostly in Northern US and at higher
elevations
• Large amount of precipitation is snow
• Heavy snow can block roads
• Can be extremely dangerous if wind picks up,
blowing the snow and hindering visibility
Hurricanes
• A tropical cyclone that has winds of 119
kilometers per hour or higher
• Begins over warm ocean water as a low
pressure area or tropical disturbance.
• Grows into a tropica storm, then a hurricane
The hurricane gets its energy from the warm,
humid water creating bands of wind and
heavy rain.