Air Pressure, Local Winds & Global Wind Belts: Sixth Grade Science
Wind is moving air generated by horizontal atmospheric pressure differences, organized into local breezes and planetary wind belts deflected by the Coriolis effect.
The Physical Nature of Air Pressure
Although air feels weightless and invisible, air is physical matter that possesses mass:
- A column of air one square meter in cross-section extending from sea level to the top of the atmosphere weighs approximately 10,000 kilograms (over 22,000 pounds)!
- Air pressure is the force exerted by the weight of this overlying air column pressing down on a unit of surface area.
- At sea level, standard atmospheric pressure is 101.3 kilopascals (101,300 Pa), equivalent to 14.7 pounds per square inch (psi) or 29.92 inches of mercury on a mercury barometer.
- Air pressure decreases steadily with increasing altitude because as you climb higher up a mountain, there is less total mass of air stacked above you.
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| AIR PRESSURE AND TEMPERATURE |
| |
| COLD, DENSE AIR (High Pressure System) |
| - Molecules moving slowly, packed closely together |
| - Heavy air SINKS toward ground, pushing outward |
| - Associated with CLEAR SKIES, sunny weather, and dry conditions |
| |
| WARM, LIGHT AIR (Low Pressure System) |
| - Molecules moving fast, expanding and spreading apart |
| - Light air RISES upward into the sky, pulling inward |
| - Associated with CLOUDY SKIES, rain, storms, and precipitation |
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How Pressure Differences Create Wind
Wind is simply air in horizontal motion. Wind is generated by the unequal heating of Earth's atmosphere by solar radiation, which creates localized and regional pressure gradients:
- Air always flows horizontally from areas of HIGHER pressure to areas of LOWER pressure.
- Pressure Gradient Force: The difference in pressure between two regions divided by the distance between them. The steeper the pressure gradient (the closer the isobars on a weather map), the faster the wind blows.
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| PRESSURE GRADIENT AND WIND |
| |
| [ HIGH PRESSURE ZONE (1,024 mb) ] ===> [ LOW PRESSURE ZONE (992 mb) ] |
| (Cold, dense, sinking air) (Warm, light, rising air) |
| |
| Air rushes horizontally from HIGH to LOW: WIND! |
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Local Daily Wind Systems: Sea and Land Breezes
Along ocean coastlines, differences in the specific heat capacities of water and dry land generate daily reversing wind systems:
- Sea Breeze (Daytime): During the day, dry land heats up much faster than ocean water because land has a low specific heat capacity. The air over the land warms, expands, and rises, creating an area of localized low pressure. The cooler, denser air over the ocean (high pressure) rushes inland toward the beach as a refreshing, cool sea breeze.
- Land Breeze (Nighttime): At night, the land cools down very rapidly, while the ocean retains its heat due to water's high specific heat capacity. The air over the ocean is now warmer and rises (low pressure), while the cold, dense air over the land sinks (high pressure) and blows offshore out to sea as a land breeze.
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| SEA BREEZE VERSUS LAND BREEZE |
| |
| DAYTIME SEA BREEZE: |
| [ Warm Land ] <============================= [ Cool Ocean ] |
| (Warm air rises; Low Pressure) (Cool air sinks; High P)|
| |
| NIGHTTIME LAND BREEZE: |
| [ Cool Land ] =============================> [ Warm Ocean ] |
| (Cold air sinks; High Pressure) (Warm air rises; Low P) |
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Global Wind Belts and the Coriolis Effect
On a planetary scale, the equator receives direct, intense solar radiation year-round, while the poles receive slanted, indirect sunlight. If Earth did not rotate, air would simply rise at the equator and flow in two giant loops directly to the poles. However, Earth's 24-hour rotation deflects moving winds via the Coriolis effect (to the right in the Northern Hemisphere, to the left in the Southern Hemisphere), breaking planetary atmospheric circulation into three distinct circulation cells in each hemisphere:
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| EARTH'S THREE GLOBAL WIND BELTS |
| |
| 90° N: North Pole (Polar High Pressure) |
| \ |
| v POLAR EASTERLIES (Cold winds blow from NE to SW) |
| 60° N: Polar Front (Subpolar Low Pressure) |
| ^ |
| / PREVAILING WESTERLIES (Blow from SW to NE; Steers US storms)|
| 30° N: Horse Latitudes (Subtropical High Pressure; Deserts) |
| \ |
| v TRADE WINDS (Blow reliably from NE to SW; sailed by ships) |
| 0° N: Doldrums / ITCZ (Equatorial Low Pressure; calm, rainy belt) |
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The Three Global Wind Systems
- Trade Winds: Steady, reliable winds blowing from the subtropical high-pressure zones (30 degrees latitude) toward the equatorial low-pressure zone. In the Northern Hemisphere, they blow from the northeast (Northeast Trade Winds); early merchant sailing ships relied on them to cross oceans.
- Prevailing Westerlies: Blowing from southwest to northeast across temperate middle latitudes (30 to 60 degrees). The Prevailing Westerlies steer the vast majority of weather systems and storms across the continental United States from west to east.
- Polar Easterlies: Cold, dry winds blowing from the freezing polar highs westward toward the subpolar low-pressure belts at 60 degrees.
Exercises and Solutions
Exercise 1: Tracing Coastal Breeze Conversions. A family rents a beachfront cottage on the Atlantic coast on a sunny summer day. At 2:00 PM, a strong, refreshing breeze blows from the ocean directly onto the sand, cooling the sunbathers. At 3:00 AM that night, a gentle breeze blows in the opposite direction—from the coastal forest out over the dark ocean waves. (a) Identify the daytime and nighttime wind systems; (b) Explain why the wind reversed direction based on specific heat capacities and pressure differences.
Solution 1: (a) The daytime wind is a sea breeze; the nighttime wind is a land breeze. (b) Dry land has a low specific heat capacity (it heats and cools rapidly), while ocean water has a very high specific heat capacity (it heats and cools very slowly). During the afternoon, solar radiation heats the beach sand much faster than the ocean; warm air over the land expands and rises, creating a localized low-pressure zone. The cooler, denser air over the ocean forms high pressure, rushing inland toward the land as a cool sea breeze. At night, without solar radiation, the land radiates heat rapidly and becomes significantly colder than the ocean. The air over the warm ocean is now warmer and rises (low pressure), while cold, dense air over the land sinks (high pressure) and blows offshore out to sea as a land breeze.
Exercise 2: Evaluating the Coriolis Effect on Transcontinental Aviation. A commercial jetliner departs from Miami, Florida, flying due North toward Montreal, Canada. If the flight computer navigates in a straight line without making continuous directional heading adjustments, explain why the airplane will miss Montreal and drift far out over the Atlantic Ocean to the East.
Solution 2: The airplane will drift to the east because of Earth's rotation and the Coriolis effect. Earth rotates from west to east. At the equator and low latitudes (like Miami), Earth's surface travels eastward at roughly 1,600 km/h. At higher northern latitudes (like Montreal), the rotational circumference is smaller, so Earth's surface moves eastward at only roughly 1,100 km/h. When the airplane takes off from Miami, it carries Miami's faster eastward rotational momentum with it. As it flies north over ground that is rotating more slowly eastward, the airplane outruns the ground beneath it, causing its flight path to curve to the right (eastward) relative to Earth's surface. Navigators must continuously steer slightly to the west of their target to compensate for the Coriolis deflection and land safely in Montreal.