Chp 11.2 Water Patterns

1. Chp 11.2 Water Patterns By Lori and Nune

2. Air masses • An air mass is a body of air that has consistent features, such as temperature and relative humidity. • Air masses get their characteristics from the surface over which they developed. • Examples of it are that if an air mass that forms over a warm dry area will have warm dry conditions. The Changing Weather

3. Weather Fronts Highs and Lows When warm air rises, it creates a decrease in pressure close to earths surface. Therefore an area of low pressure is created. Areas of low pressure are associated with cloudy, stormy weather. • The boundary between two air masses of different density, moisture, and temperature is called a front. • As air masses move from one location to another, they eventually run into each other. • A cold front occurs when a colder air mass moves toward warmer air. • A warm front forms when lighter, warmer air moves over heavier, colder air.

4. Weather Maps • Information on weather factors, high and low- pressure systems, and weather fronts is usually represented in maps. • They also provide useful information on the atmospheric conditions over areas of interest. • The maps contain information about pressure systems and weather fronts for the western United States.

5. Day and Night Cycles Seasons The regular change in temperature and length of day that result from the tilt of Earth’s axis are seasons. Earth revolves around the sun, as a result of Earth’s tilt on its axis, the amount of solar radiation reaching different areas of Earth changes as Earth completes its revolution around the sun • Air goes through a daily cycle of warming and cooling. • As the sun rises in the morning, sunlight warms the ground. • As the sun lowers in the afternoon, its energy is spread over a larger area. Cycles That Affect Weather

6. El Nino and La Nina can affect weather worldwide. • During an El Nino, warmer surface water in areas of the eastern equatorial Pacific ocean leads to more water vapor being in the air above the water • This can result in increased rainfall across the southeastern U.S and the South American countries of Peru and Ecuador. • Heavy rainfall can lead to flooding and landslides. • On the other side of the globe, severe droughts can occur in Australia, Indonesia, and southeast Africa. • Droughts can lead to forest fires as well. • A La Nina event occurs when sea surface temperatures in the eastern equatorial Pacific are colder than normal. • This can produce colder-than-normal and warmer-than-normal winter temperatures in the northwestern U.S.

7. Drought Flood Sometimes when severe weather occurs, it leads to flooding. Floods can occur when water enters an area faster than it can be taken away by rivers, absorbed by the ground, or contained in lakes. Floods are common natural disasters and can occur in many locations. Example in 1993 an enormous flood took place in the mid-west region of the U.S. This flood was on of the most damaging floods in the United states. • Drought • A period of time when precipitation is much lower than normal or absent. • Droughts can last months or years. • Droughts can bring about several hazardous conditions for ecosystems and human populations. • If a long drought occurs where precipitation is low there can be major decreases in water supply. • California experienced significant droughts from 1987 to the beginning of 1993. • These droughts lasted a relatively long time and resulted in a decrease in the water reservoirs. Severe Weather

8. Flash flood • A flood that takes place suddenly. • Flash floods are the most dangerous type of flood. • With deaths at over 200 people a per year, flash floods are number-one reason for weather-related deaths in the U.S • The damaging effects of flash floods is increasing due to human activities. • This is because of the construction of buildings, parking lots, and other structures decreases the amount of vegetation and soil that can potentially absorb runoffs of water.