Atmosphere and Weather
Main Idea: The atmosphere’s air pressure changes.
 Gravity causes air molecules to be pulled toward the Earth’s surface.
 Air pressure is the force of molecules pushing on an area. Air pressure
pushes in all directions.
Main Idea: The atmosphere has wind patterns.
 Since the Earth’s surface heats unevenly, air pressure can differ from
place to place at the same altitude.
 The pressure differences will cause air to move horizontally as wind.
 These winds can travel in steady patterns called global winds.
 The earth’s rotation affects wind direction.
 The Coriolis Effect is the influence of the earth’s rotation on objects
that move over earth. In the Northern Hemisphere winds are
deflected to the right, while the southern hemisphere winds are
directed to the left.
 Uneven heating can also cause jet streams. These are usually in the
troposphere. These winds also move west to east around the globe.
Examples: polar jet stream and subtropical jet stream
Global winds are caused by the uneven heating of Earth’s curved surface. The
Coriolis Effect prevents winds from flowing directly from high pressure over the
poles to low pressure over the equator. Three belts of winds, trade winds,
westerlies, and easterlies are divided by hemispheres. These winds belts are
separated by calm regions of high or low pressure: doldrums, horse latitudes, and
subpolar lows, and polar highs.
Doldrums
What, Where, and How:
 Do you ever get the doldrums (feel sluggish)? The word “doldrums” can be
used to describe a person who doesn’t feel like doing anything (someone
who isn’t active). Just like people, winds can also be inactive and called
“doldrums”. Areas on the Earth which are known for having little or no
winds are called the doldrums. The doldrums are located at the equator.
Why do the doldrums occur?
 Remember how we learned that air near the equator expands and rises?
The rising air is a vertical movement. Because there is no horizontal
movement of air, the winds at the equator are very weak and the doldrums
are created.
Characteristics:
 The doldrums are an area of low pressure. Rising warm, humid air
condenses and forms clouds at the doldrums. Trade winds also converge
(come together) here.
Horse Latitudes
What, Where, and How:
 Just like the doldrums, the horse latitudes are also an area with weak
winds. The horse latitudes are located at 30-degrees latitude (remember:
both North and South latitudes). Long ago, when sailors would travel at
this latitude their ships would get stuck for weeks because there was no
wind to power their sails. The region was called the “horse latitudes,”
because sailors would throw their horses overboard or eat them after
running out of food and water for them!!!
Why do the horse latitudes occur?
 Just like at the equator, there is no horizontal movement of air at 30degrees latitude. Since the movement of air is strictly vertical, there is very
little wind.
Characteristics:
 Air sinks at 30-degrees latitude and causes high pressure. Many of the
world’s deserts are located at this latitude.
Wind Belts
There are 3 major global wind belts: the trade winds, the prevailing westerlies,
and the polar easterlies. Be able to identify their location on a map.
Trade Winds:
 This is how the trade winds are formed:
1. Air sinks at 30-degrees latitude.
2. The air blows toward the equator.
3. The wind is deflected (curved) due to the Coriolis effect.
 These events produce the trade winds which occur between 30-degrees
North latitude and the equator AND 30-degrees South latitude and the
equator. So, there are trade winds in each hemisphere.
 The winds blow from the east to the west, but are curved a little bit due to
the Coriolis effect.
Westerlies:
 This is how the westerlies are formed:
1. The air that sinks at 30-degrees latitude also blows towards the poles.
2. The wind blows from west to east and is deflected by the Coriolis effect.
 The westerlies occur between 30-degrees and 60-degrees latitude in each
hemisphere (north and south). The westerlies blow from the west to the
east but are curved a bit.
Polar Easterlies:
 This is how the polar easterlies are formed:
1. At the poles, cold air sinks.
2. The air flows back towards the equator.
3. The air is deflected by the Coriolis effect.
 The polar easterlies are located between 60-degrees and 90-degrees
latitude in both the northern and southern hemispheres. They blow from
the east to the west but are slightly curved due to the Coriolis effect.
Main Idea: Weather changes as air masses move.
 Air masses are a large volume of air in which temperature and
humidity are nearly the same at different locations at the same
altitude. Air masses can be as follows: continental (dry), or maritime
(moist) / polar(cold) or tropical (warm)
 As global winds move air masses, they may change air masses
characteristics slowly.
 A front is the boundary between air masses. Three fronts are cold,
warm, and a stationary.
 Cold fronts-warm air is pushed by steeply by cold air, Brief heavy
storms may form.
 Warm fronts-warm air rises gently over cold air; This front brings
steady rain or snow.
 High pressure systems bring fair weather.
 A Low-pressure system develops stormy weather.
Main Idea: Low-pressure systems can become storms.
 Hurricanes form over warm ocean water.
 Hurricanes begin as a tropical depression or low-pressure system
over warm tropical water.
 Tropical storms have winds at 65 km/h. Tropical storms are classified
as hurricanes when they reach 74 mph. Hurricanes reach wind can
reach speeds to 120 km/h.
 Hurricanes lose power over cooler water or land.
 Winter storms from when two air masses collide, making a strong
low-pressure system. Blizzards are blinding snowstorms with winds
at 56km/h.
Main Idea: Vertical air motion can cause severe storms.
 Thunderstorms are formed when warm, humid air is forced upward. A
cumulonimbus cloud forms as condensation forms and energy is released.
 The up and down air motion of a thunderstorm and cause a tornado. A
tornado is a violently rotating column of air between a cloud and the
ground.
Main Idea: Weather forecasters use advanced technologies.
 A meteorologist is a scientist that studies the weather.
 Know your map symbols.
 Isobars are lines that connect places that have the same air pressure.
(Students have a sample map outlining the isobars.)
 Meteorologists use computer models to analyze data and forecast weather.
 Isotherms connect temperature readings.(Students have a sample map.)
Wind Belts: You do not need to know the cells.
This is the temperature Map that we completed in class. Have
your student explain how we drew the isotherms.
Surface Temperature
Map Solution
This the Pressure Map that we completed in class.
Surface Pressure Map
Solution
Map Symbols