Weather

Weather is the condition of the Earth’s atmosphere at a particular time and place. Earth’s atmosphere is the thin layer of gases that surrounds the planet. Made up mainly of molecules of nitrogen, oxygen, carbon dioxide, and water vapor. Some other gases as well as solids are also found in the atmosphere.

Nitrogen makes up 78% of our atmosphere. Oxygen is the second most abundant gas in the atmosphere. Carbon dioxide is found in small amounts. Plants use this to make food. Water vapor is water in the form of a gas. Particles of dust, smoke, and chemicals are also present in the atmosphere.

Layers of the Atmosphere (heading toward space) 1. Troposphere This is the layer where weather occurs 2. Stratosphere Contains the ozone layer a thick blanket of gas that blocks most of the deadly UV radiation from the sun. without this layer life on Earth wouldn’t exist Mesosphere -the middle layer of the atmosphere Thermosphere -the edge of the atmosphere.

Wind A horizontal movement of air from an area of high pressure to an area of low pressure is called wind. ALL WINDS ARE CAUSED BY DIFFERENCES IN AIR PRESSURE! This is due to unequal heating of the atmosphere Air circulates by convection currents

Remember… Convection currents are a method of heat transfer. Cold air sinks and warm air rises.

Winds are described by their direction and speed. Direction is determined by a wind vane, which points in the direction from which the wind is coming. Wind speed is measured with an anemometer.

How Clouds Form Clouds form when water vapor in the air becomes liquid water or ice crystals. Condensation causes this to happen. Condensation = water vapor in the form of a gas turns into a liquid by losing heat energy.

Types of Clouds There are 3 main types of clouds. Each type is classified by altitude (height in the atmosphere). Each type is also associated with a different type of weather.

Cumulus Look like fluffy round piles of cotton They can grow in size and height Usually indicate fair or clear weather Low level clouds in the atmosphere.

Stratus Form in flat layers “strato” means “spread out” Usually cover all or most of the sky Low-level clouds in the atmosphere

Cirrus Wispy, feathery clouds Form only at high levels in the atmosphere where temperatures are very low Made of ice crystals

Clouds that form at or near the ground are called FOG.

Air Masses Air masses are huge bodies of air that have a similar temperature, humidity, and air pressure throughout it. Wind belts called the Prevailing Westerlies generally push air masses in the continental U.S. from west to east.

Air masses from the North are Polar (P). Air masses from the South are Tropical (T). Air masses from the Oceans are Maritime (m). Air masses from the land are Continental (c).

An air mass coming down from Canada would be cold and dry. An air mass coming up from the Caribbean would be warm and wet.

Fronts As huge masses of air move across the land and the oceans, they bump into each other. Just like oil and water, THEY DON’T MIX! The area where the air masses meet is called a Front. When air masses meet at a front, the collision of air particles often causes storms and changeable weather.

There are 4 types of fronts: 1. Cold Front= when a rapidly moving cold air mass runs into a slowly moving warm air mass, the cold air slides under the lighter warm air, pushing it upward. Cold fronts move quickly, so they can cause abrupt weather changes, including violent thunderstorms.

2. Warm Front At a warm front, a faster moving warm air mass collides with a slowly moving cold air mass. Because the cold air is more dense, the warm air moves over the cold air. Warm fronts move more slowly than cold fronts, so the weather may be rainy or foggy for several days.

Compare & Contrast Venn Diagram for Warm and Cold Fronts

3. Stationary Front When a cold and warm air mass meet but neither one has enough force to move the other. The front is “stalled” Rain, snow, fog, or clouds can exist for many days as a result.

4. Occluded Front When a warm air mass is caught between 2 cooler air masses. The cooler air moves under the warm air and pushes it upward. The temperature near the ground becomes cooler. The warm air mass is cut off, or occluded from the ground.

http://www.classzone.com/books/earth_science/terc/content/visualizations/es2002/es2002page01.cfm?chapter_no=visualization

Cyclones (low pressure) “L” on a weather map A swirling center of low pressure air. Winds spin counterclockwise in the Northern Hemisphere. Low pressure is usually associated with storms and precipitation.

Anticyclones (high pressure) “H” on a weather map A swirling center of high pressure air. These move clockwise in the Northern Hemisphere. High pressure is generally associated with dry, clear weather.

By keeping track of cyclones and anticyclones, meterologists can predict the weather. Thunderstorms = a usually brief, heavy storm that consists of rain, strong winds, lightning, and thunder. Lightning = an electric discharge that takes place between 2 oppositely charged surfaces. Thunder = the sound caused by the rapid expansion of air along an electrical strike.

A Tornado is a rotating column of air with very high winds and a funnel-shaped cloud that touches the ground. Leaves a narrow path of destruction

A Hurricane is a severe storm that develops over tropical oceans that carries high winds and rain. Leaves a large path of destruction

Severe Weather Clips http://www.sciencekids.co.nz/videos/weather.html http://weather.weatherbug.com/forecasts/now/brockport-ny-14420