Weather Patterns

Air exerts pressure Air molecules are constantly moving and bouncing off of surfaces Each time an air molecules bounces off an object, it pushes, or exerts a force on that object Air pressure: the force of air molecules pushing on an area The greater the amount of air molecules, the greater the air pressure

Air pressure is related to altitude and density Air pressure and density are higher at sea level because a greater amount of air pushes down. Air pressure and density are lower at a high altitude because less air pushes down. Therefore, the air at sea level is denser than at higher altitudes.

Air pressure and air motion Air moves from areas of higher pressure toward areas of lower pressure. You can measure air pressure with a barometer. A Barometer’s chamber contracts when air pressure increases. It expands when air pressure decreases.

Uneven heating causes air to move Weather is the condition of the Earth’s atmosphere at a particular time and place. Wind is air that moves horizontally (or parallel) to the ground. Differences in air pressure cause winds.

How winds form 1. Sunlight strongly heats an area of ground. The ground heats the air. The warm air rises, and an area of low pressure forms. 2. Sunlight heats an area of ground less strongly . The cooler, dense air sinks slowly, and an area of high pressure forms. Winds form from convection currents Air moves as wind across the surface, from higher toward lower pressure.

Global Winds Global winds travel thousands of kilometers and lasts for weeks. Uneven heating between the equator and the north and south poles cause global winds.

Coriolis Effect The Earth’s rotation changes the direction of winds and other objects moving over Earth. This is called the Coriolis Effect. Global winds curve as Earth turns beneath them. Winds in the Northern Hemisphere curve to the right. Winds in the Southern Hemisphere curve to the left. Global wind belts-The 3 routes in which global winds travel the Earth Rotation of the Earth is West to East

Areas of calm air separate the global wind belts Calm Regions Doldrums: Warm air rises at these low pressure zones near equator & spreads out toward the poles. Clouds & heavy rain. Fuels tropical storms. Horse Latitudes: Warm air travels away from equator then cools and sinks at these high pressure zones located about 30 degrees North and South of the equator. Clear and dry weather. Wind Belts Trade Winds: blow from the East, moving from horse latitudes toward the equator Westerlies: blow from the West, moving from horse latitudes toward the poles Easterlies: blow from the East, moving from the polar regions towards the mid-latitudes.

Jet streams flow near the top of the troposphere Usually flow from west to east for thousands of kilometers Speeds greater than 200 km/hr or 124 miles per hour Form because of uneven heated Earth’s surface Each hemisphere has two jet streams Helps make weather predictions Effect air-travel time

Sea Breezes Warmer air rises over land during the day and cooler air blows in from water.

Land Breezes Cooler air blows out from the land and warmer air rises over water at night

Local Winds Mountain slopes heat up and cool down faster than the valleys below them, so that winds flow up the slopes during the day and down at night. Bodies of water heat up and cool down slower than land, so that winds flow inland during the day and toward the water during the night Winds that change direction with the seasons are called Monsoons winter monsoons originate over land and are cool and dry summer monsoons originate over water and are moist and bring heavy rains

Water in the atmosphere changes Evaporation: liquid water becomes a gas or vapor Condensation: vapor becomes a liquid, forms clouds Precipitation: liquid water flaws to surface in any form (rain, sleet, snow)

Humidity Humidity is the amount of water vapor in the air, varies place to place and from time to time Saturation: condition in which the rates of evaporation and condensation are equal

Relative Humidity Compares the amount of water vapor in air with the maximum amount of water vapor that can be present at that temperature Example: air with 50% relative humidity has half the amount of water needed for saturation Dew point is the temperature at which air with a given amount of water vapor will reach saturation

Cloud types Cirrus (curl of hair) appear feathery or wispy and are high altitude cirrocumulus and cirrostratus Cumulus (heap or pile) are very tall and are medium altitude Stratus (spread out) form in flat layers and are low altitude Fog is close to ground, smooth appearance Nimbo-produce precipitation Alto-medium altitude

Precipitation Precipitation is formed when water vapor droplets combine with one another. The droplets must become larger and heavier in order to fall. Scientists use a rain gauge to measure amount Snow is measured using a ruler

Types of Precipitation Rain and drizzle form from water droplets or ice crystals that melt as they fall to the ground Freezing rain is rain that freezes when it hits the ground or other surfaces Sleet is rain that freezes into ice pellets while falling through cold air Snow forms ice crystals that merge in clouds Hail forms when ice pellets re-circulate in the clouds until they get too heavy and fall to ground

Pollution and Precipitation Sulfur dioxide and Nitrogen oxides enter the air as pollution and combine with water vapor to form acids. The acids mix with cloud droplets or ice crystals and fall as precipitation Effects plants, animals and water .