1. Weather
and how it affects us…

2. Can you find any storms on this image?
Weather
Weather refers to the state of the atmosphere at a specific time and place. It describes conditions such as air pressure, wind, temperature, and moisture in the air.
Can you find any storms on this image?

3. Weather Factors Air temperature
Air temperature is caused by molecules that make up the air. Air is a combination of 3 gases: oxygen, nitrogen, and CO². If the air molecules are moving quickly then the air feels warm. If the molecules are moving slowly, the air feels cold.
Thermometers are used to measure temperature. They can measure in °C (Celsius) and °F (Fahrenheit).
Vostok, Antarctica reached a chilly 129°F below zero in 1983!
El Azizia in Libya reached a scorching 139°F in 1922!

4. Weather Factors, cont. Wind
Air moving in a specific direction is called wind.
As the sun heats air in the atmosphere, the air expands and becomes less dense. This air has low atmospheric pressure. Cooler air is heavier and tends to sink thus having high pressure. Wind results when air moves from areas of high pressure to areas of low pressure.
A barometer measures atmospheric pressure.
The breeze you often feel on the beach is cool air (high pressure) from the ocean rushing toward the warmer air (low pressure) on the beach.

5. Weather Factors, cont. Humidity and Dew Point
Evaporation is a stage of the water cycle that results when heat causes water on the ground to become a gas (water vapor) and rise into the atmosphere.
Humidity is the amount of water vapor in the air. If the temperature is cooler, there is less water vapor in the air. If the temperature is warmer, there will be more water vapor in the air.
Relative humidity is the percent of water vapor that air is holding. If the air is holding all of the water vapor it can, the relative humidity is 100%.
Dew point is the point at which air is saturated with water vapor and condensation forms. Water vapor condenses (becomes a liquid) when the air is cooler.

6. Weather Factors, cont. Clouds
Clouds form when warm air is forced upward, and it expands and cools. As the air cools, the amount of water vapor needed for condensation to occur decreases. When the relative humidity reaches 100%, water vapor starts to condense and collect around small particles of dust or salt in the air. These water droplets form clouds.
Cumulus clouds are puffy, white clouds with flat bases that can tower to great heights. They can indicate fair weather or storms.

7. Weather Factors, cont. Precipitation
Precipitation is water falling from clouds. Precipitation occurs when water droplets combine and are large enough to fall back to Earth.
The 4 main types of precipitation are:
rain, sleet, snow, and hail
Hailstones can be as small as a few cm across or larger than a softball!

8. Weather Patterns Air Masses
An air mass is a large body of air that has properties that are similar to the area it forms over. The United States is affected by 6 major air masses throughout the year.

9. Weather Patterns, cont. Air Masses, cont.
When 2 air masses having different densities, moisture, or temperature touch, the boundary is called a front.
Clouds, precipitation, and storms can form at frontal boundaries.
The 4 types of fronts include: warm, cold, stationary, and occluded.

10. Weather Patterns, cont. Fronts, cont.
On a weather map, a cold front will be illustrated by a blue line with triangles. A warm front will have a red line with semicircles.
An approaching cold front will always push warm air up creating a warm front!

11. Weather Patterns, cont. Occluded and Stationary Fronts
An occluded front involves 3 different air masses of different temperatures and is shown on maps as purple lines with triangles and semicircles
A stationary front occurs when then boundary between air masses stops advancing. It is shown on maps with an alternating red and blue line that has red semicircles pointing toward the cold air and blue triangles pointing toward the warm air.

12. Severe Weather Thunderstorms
Thunderstorms include heavy rainfall, lightning, thunder, and possibly hail.
Thunderstorms occur in warm, moist air masses and along fronts.
The temperature inside a bolt of lightning can reach 50,000 °F…hotter than the Sun’s surface!

13. Severe Weather, cont. Tornadoes
Some of the most severe thunderstorms produce tornadoes.
A tornado is a violent, whirling wind that moves in a narrow path over land.
Tornadoes occur during thunderstorms when wind at different heights blows at different directions and speeds. These varying speeds can create a rotating column that can become a funnel cloud.
When a funnel cloud touches the ground, it is called a tornado.

14. Severe Weather, cont. Hurricanes
The most powerful type of storm is the hurricane. It is a large, swirling low pressure system that forms over the warm Atlantic Ocean. It must have wind speeds of at least 119 mph to be considered a tornado.
A hurricane develops its strength using the warmth and moisture of the Atlantic Ocean. When it hits land, the lack of moisture causes the storm to lose energy and it disappears.
Hurricane Katrina in 2005