Chapter 16 - Section 2 Study Guide Weather Patterns

Air Masses  An air mass is a large body of air that has properties similar to the part of the Earth’s surface over which it develops.  An air mass that develops over the land is dry, and one that develops over the water is moist.  An air mass that develops over the tropics is warmer than one that develops over northern regions.  Air masses can cover thousands of square kilometers.  When you observe a change in the weather from one day to the next, it is due to the movement of air masses.

Highs and Lows  Atmospheric pressure varies over the Earth’s surface – there are low pressure systems and high pressure systems.  Winds blow from areas of high pressure to areas of low pressure.  As winds blow into a low pressure area in the northern hemisphere, the Earth’s rotation causes the winds to swirl in a counterclockwise direction. These areas are called cyclones and are associated with stormy weather.  Winds blow away from the center of high pressure. Earth’s rotation causes these winds to spin clockwise in the northern hemisphere. These areas are called anti-cyclones and are associated with fair weather.  Low pressure systems are regions of rising air. Clouds form when air rises and cools, so areas of low pressure usually have cloudy weather.  High pressure systems are areas of sinking air, which makes it difficult for air to rise and clouds to form.

Strange News N.D. Gust Flips Trampoline, and Girl on It From Associated Press May 09, :10 PM EDT TRENTON, N.D. - Swirling wind swept a trampoline into the air and over a fence as a 4- year-old girl was jumping on it, knocking her unconscious and breaking her arm and pelvis, witnesses told police. Grace Hove was hospitalized after Sunday's freak accident with the broken bones as well as a dislocated jaw and bruises to a lung and kidney, her mother said." She's expected to make a full recovery," Rae Hove said Monday. "Thank God. It could have been worse.“ Witnesses, including two adults, reported the trampoline was lifted as high as 25 feet, Sheriff's Capt. Bob Stancel said." One man saw the whirlwind, then he saw the trampoline fly up into his view," Stancel said. "He said it was as high as the trees." The trampoline landed partly on a highway with the girl pinned underneath, he said. The swirling wind might have been a "dust devil," a localized, spinning pocket of air, said Jim Assid, a National Weather Service technician. Such meteorological oddities can occur when air heated by the ground rises rapidly through the cool air above it, Assid said. Meteorologists say the diameters of dust devils can range from to nine feet to greater than 900 feet. The average height is about 600 feet above the ground but some have been observed as high as 3,000 feet, not 13 miles. Eds: An earlier version of this story misstated the dimensions of dust devils. Meteorologists say the diameters of dust devils can range from to nine feet to greater than 900 feet, not just up to 10 feet as the story reported. The average height is about 600 feet above the ground but some have been observed as high as 3,000 feet, not 13 miles.

Fronts  A boundary between two air masses of different density, moisture, or temperature is called a front.  Cloudiness, precipitation, and storms sometimes occur at frontal boundaries.  There are 4 main types of fronts: 1. cold 2. warm 3. occluded 4. Stationary

Cold and Warm Fronts  A cold front occurs when colder air advances toward warm air. o The cold air wedges under the warm air. o As the warm air is lifted, it cools, condenses, and forms clouds. o When the temperature difference between cold and warm air is large, thunderstorms and tornadoes may form.  Warm fronts form when lighter, warmer air advances over heavier, colder air. o Warm air slides over colder air and forms a boundary with a gentle slope. o This can lead to hours or days of wet weather.

Draw the weather map symbols for: Cold FrontWarm Front

Occluded and Stationary Fronts  An occluded front involves three air masses: colder air, cool air, and warm air. o An occluded front forms when a cold air mass moves toward cool air with warm air in between the two. o The colder air forces the warm air upward,closing off the warm air from the surface.  A stationary front occurs when a boundary between air masses stops advancing. o Stationary fronts may remain in place for several days and produce light wind and precipitation.

Draw the weather map symbols for: Occluded FrontStationary Front

Thunderstorms  Thunderstorms occur in warm moist air and along fronts. o Warm moist air is forced upward where it cools and condensation forms a cumulonimbus cloud. o Within the cloud, small droplets collide to form larger ones, and the droplets fall towards the Earth’s surface. o Raindrops cool the air and this cool, dense air spreads over Earth’s surface. o Strong winds form from the mixing of dense, rain cooled air and updrafts of warmer air o Hail may form as ice crystals fall to warmer layers and are lifted in colder layers by the strong updrafts in the cloud.

 The movement of air within the cloud can cause different parts of a cloud to become oppositely charged. o When current flows between regions of opposite electrical charge, lightening flashes.  Thunder results from the rapid heating and cooling of air around a bolt of lightening, which can reach five times the temperature of the surface of the Sun. o This extreme heat causes air to expand, then cool and quickly contract. o The rapid movement of molecules forms sound waves heard as thunder.

Tornados  In severe thunderstorms, winds at different heights blow in different directions and at different speeds. This air is called wind shear. o Wind shear creates a rotating column parallel to the ground. o The updraft in a thunderstorm can tilt the column upward into the cloud, creating a funnel cloud. When the funnel comes into contact with the Earth’s surface, it is called a tornado.

Hurricanes  A hurricane is a large, swirling low pressure system that forms over the warm Atlantic Ocean. These storms turn heat energy from the ocean into wind.  A storm must have winds of at least 74 mph to be called a hurricane.  These storms are called typhoons in the Pacific Ocean and cyclones in the Indian Ocean.  Hurricanes are similar to low pressure systems on land, but they are much stronger. o Low pressure develops near the equator. In the northern hemisphere the winds around the low pressure begin to spin counterclockwise.

 The hurricanes that hit North America begin as a low pressure system west of Africa. o These storms travel west, gaining in strength from the heat and moisture of the warm ocean water.  When a hurricane strikes land, high winds, tornadoes, heavy rains, high waves, and flooding cause major damage.  The storm loses power over land because it is cut off from its energy supply of warm, moist air.

Blizzards  A winter storm is classified as a blizzard if the winds are 56 km/h, the temperature is low, the visability is less than 400m in falling or blowing snow, and if these conditions persist for 3 hours or more. Severe Weather Safety  A severe weather watch is issued if conditions are favorable for severe thunderstorms, tornadoes, floods, blizzards, or hurricanes. You should monitor weather conditions.  A severe weather warning is issued if severe weather conditions already exist. In these conditions you should take immediate action to ensure safety.

United Streaming Hurricanes and Tornadoes