a large body of air that has the same temperature and humidity throughout classified according to where they originate during the time the air mass is over an area, it determines the weather of that area

Classified according to two characteristics: temperature and humidity. The maritime tropical, continental tropical, maritime polar, and continental polar influence the weather in North America

maritime tropical humid air mass (m)  Originates over the ocean Form over the Gulf of Mexico and Atlantic Ocean and move into the southeastern U.S. Form over the Pacific and affect the west coast In summer they bring hot, humid weather and in winter heavy rain or snow

Originates in warm air  warm air mass (T) (Tropical)

Maritime Polar  Cool, humid air masses form over the icy cold North Pacific and North Atlantic Oceans Affect west coast bringing fog, rain, and cool temperatures

continental tropical  dry air mass (c) Originates over the land

continental Polar  cool air mass (P) Originates in cold air Form over central and northern Canada and Alaska In winter continental polar masses bring clear, cold, dry air to much of North America Storms occur with the CP collides with the MT

Maritime Tropical Atlantic (mT) moves northward across eastern US brings mild, cloudy winters and hot humid summers with hurricanes and thunderstorms

How air masses move In the United States, air masses are moved by: prevailing westerlies-pushes air masses west to east jet stream-bands of high speed winds 10km above earth’s surface that carry air masses

fronts: boundary between two air masses having different temperatures and humidity

warm air overtakes cool air warm air rises brings hot, humid days and precipitation over a large area

cold air overtakes warm air because it is more dense cold air forces warm air up Since warm air holds more moisture than cool air, once the air reaches the dew point heavy rain or snow will form. brings strong storms (squall) with clear, cooler days once it passes

two cold masses sandwich a warm mass warm air rises between them brings strong winds and heavy rains

does not move These can bring many days of clouds and precipitation until it moves.

Cyclones and Anticyclones Occur when boundaries between fronts get distorted by physical features or jet streams and air begins to swirl. Cyclone- (L) swirling center of low air pressure These play a large part of the weather in the United States and winds spin counter clock wise Cyclones and decreased air pressure are associated with clouds, wind, and precipitation Anticlone- (H) high pressure centers of dry air, winds spin clockwise, these cause dry, clear weather

3.2 Storms A violent disturbance in the atmosphere

small-area storms formed by the strong upward movement of warm, moist, unstable air always accompanied by lightning, thunder, and usually rain

formed from cumulonimbus clouds known as thunderheads Form on hot humid afternoons or when air is forced upward along a cold front. Warm, humid air rises rapidly, the air cools forming dense thunderheads

Lightning: a discharge of electricity from a thundercloud to the ground, or cloud to cloud, or ground to cloud

temperature inside lightning flash can reach 30,000°C at this temperature, air expands explosively-sudden expansion makes thunder!

Thunderstorm damage Lightning can cause fires, shatter tree trunks Shock people, cause burns, and heart failure Areal flooding

Thunderstorm safety Remain indoors away from phones, electrical appliances, and plumbing fixtures which can conduct electricity. Avoid metal objects and bodies of water. If you are stuck outside find a low area away from trees, poles, and fences and crouch down.

a narrow, funnel-shaped column of spiral winds that extends downward from the cloud base and touches the ground

strongest winds between 360 and 500 km/hr funnel less than 500 m across always travel with a parent thunderstorm at speeds ranging from 40 to 65 km/hr

funnel is a mixture of clouds and dust pressure gets lower in center air flows toward the funnel and cools to dew point- drops form lowering of condensation level due to low pressure causes cloud to funnel Extremely low pressure-when it touches ground, acts like a giant vacuum

Tornado Alley 800 tornadoes occur in the US each year Tornadoes can occur everywhere in the country but are most likely to occur in tornado alley

Waterspout: tornados over the water- weaker than tornados Usually occur in the spring and summer, and most likely in the late afternoon.

Tornadoes are ranked on the Fujita scale by the amount of the danger they cause. The scale goes from F0 to F5

Tropical depression: wind speeds up to 38 mph; some circular rotation at surface

Tropical storm: wind speeds from mph; can be named, shows drop in pressure, distinct rotation

an intense tropical low- pressure area with sustained winds starting at 75 mph

How Hurricanes Form Form in the Atlantic, Pacific, and Indian Ocean. Forms in the Atlantic north of the equator in August, September, or October It begins over warm ocean water as a low-pressure area, or a tropical disturbance It draws energy for the warm, humid air at the ocean’s surface Winds spiral inward toward the area of lowest pressure at the center. The lower the air pressure at the center of a storm the faster the winds blow toward the center

Movement of Air in a Hurricane

How Hurricanes Form Winds are strongest in a narrow band around the center of the storm. At the center is a ring of clouds called the eyewall. Inside the eyewall is the “eye” which is characterized by calm air and possible clear skies After the eye passes the storm resumes in the opposite direction

There are sometimes gaps in between these bands where no rain is found. In fact, if one were to travel between the outer edge of the hurricane to its center, one would normally progress from light rain to dry back to slightly more intense rain again over and over with each period of rainfall being more intense and lasting longer until reaching the eye. Upon exiting the eye and moving towards the edge of the hurricane, one would see the same events as they did going in, but in opposite order.

A schematic of this banding feature can be seen in the diagram above. The thunderstorms are now organized into regions of rising and sinking air. Most of the air is rising, but there is a small amount found in between the thunderstorms that is sinking.

eye: central area of sinking air; 15 to 20 km wide

Winds most violent just outside the eye

How Hurricanes Move Hurricanes that form in the Atlantic are steered by easterly trade winds toward the Caribbean islands and the south eastern United States Once overland they gradually loose strength.

storm surge: currents formed when hurricanes pile water up along the shore and blow it inland most damaging part of a hurricane because it can wash away beaches, destroy buildings and erode coastlines.

3.3 Predicting Weather Meteorologists- scientists who study the causes of weather and try to predict it Meteorologists interpret weather information from: Satellites-orbit the earth in the exosphere and take images of earth’s surface, clouds, storms, and snow cover.

Meteorologists interpret weather information from: commercial aircraft weather balloons- stay in the troposphere and lower stratosphere. They measure temperature, air pressure, and humidity

Computer forecasts- instruments gather large amounts of data weather stations around the world Radar: electronic device that transmits radio waves in the form of a beam

Data is collected and put into a central computer at the National Weather Service data includes: wind speed and direction, temperature, pressure, humidity, clouds, precipitation Makes a computer model (copy of the atmosphere in computer

Maps are made and forecasts are reported to local stations across the country Weather forecasts are issued by the Weather Service at 10 am, 4 pm, 9pm, and 4 am. Forecasts are updated more often during severe weather.

watch: threat of storm conditions within hours warning: due to strike within 24 hours

Reading Weather Maps Isobars – lines joining places on the map that have the same air pressure Isotherms – lines joining places on the map that have the same temperature

Weather Map Symbols

Newspaper Weather Maps