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…

SECTION 2: PRECIPITATIONPg. 439

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

AIR MASSES & FRONTSPg. 442

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

SECTION 4: STORMSPg. 450

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 hurricaneThe hurricane gets its energy from the warm, humid

water creating bands of wind and heavy rain.