By Lori and Nune

* Chp 11.2 Water Patterns

*The Changing Weather

*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.

Weather Fronts*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.

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.

*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.

Day and Night Cycles

*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.

Seasons

*Cycles That Affect Weather

*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

* 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.

Drought * 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.

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.

*Severe Weather

*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.