Fix the mistake: Weather and Changes in Weather

Atmospheric Pressure and Wind

Earth is surrounded by a sea of air. Air itself has weight, and the force with which it presses down on us from above is atmospheric pressure. Differences in air pressure produce wind.

Basic Knowledge

The unit of atmospheric pressure is the hectopascal (hPa), and the average pressure near sea level is about 1013 hPa. As altitude increases, the air becomes thinner, so pressure decreases; at about 5500m, it becomes half that at the surface (about 500 hPa).
Air flows from areas of high pressure (high-pressure systems) to areas of low pressure (low-pressure systems), producing wind. Actual wind is deflected by the Coriolis force caused by Earth's rotation, turning to the right in the Northern Hemisphere. As a result, air flows clockwise outward from the center of a high-pressure system and counterclockwise inward toward a low-pressure system.
Air rises at the center of a low-pressure system, making clouds likely, while descending air at the center of a high-pressure system brings clear weather.

Important Terms
Atmospheric pressure（The force with which the atmosphere presses on Earth's surface. About 1013 hPa at sea level）
Hectopascal (hPa)（A unit of atmospheric pressure. 1 hPa = 100 Pa）
High-pressure system（An area with higher pressure than its surroundings. Clear weather due to descending air）
Low-pressure system（An area with lower pressure than its surroundings. Clouds and rain due to rising air）
Isobar（A line connecting places with the same pressure. The closer the spacing, the stronger the wind）
Coriolis force（An apparent force caused by Earth's rotation. Deflects motion to the right in the Northern Hemisphere）

Further Study (Background and Significance)

Atmospheric pressure was established through Torricelli's mercury-column experiment and Pascal's research in the 17th century. One atmosphere supports a mercury column of 760mm (=1013 hPa).
The spacing of isobars indicates wind speed. The closer the spacing (the greater the pressure gradient), the stronger the wind. Around the center of a typhoon, closely spaced isobars form concentric circles and strong winds blow.
The unit of atmospheric pressure has changed over time. It changed from “mb (millibar)” to “hPa” in 1992, but the numerical values remained the same (1 mb = 1 hPa).
Pain in the ears while climbing mountains and plastic bottles expanding or collapsing are familiar examples of changes in atmospheric pressure.

Key Points
  • 1 atmosphere ≒ 1013 hPa = 760mm of mercury
  • Atmospheric pressure decreases with altitude
  • High-pressure system = descending air → clear weather
  • Low-pressure system = rising air → cloudy or rainy weather
  • Northern Hemisphere: air flows clockwise outward from a high-pressure system
  • Northern Hemisphere: air flows counterclockwise inward toward a low-pressure system
  • Close isobar spacing = strong wind

Points to Note (Easily Confused)

① Remember: high-pressure system = clear weather and low-pressure system = rain. ② Wind direction is opposite in the Northern Hemisphere (deflected right) and Southern Hemisphere (deflected left). ③ The spacing of isobars indicates wind speed; do not confuse them with isotherms (lines connecting places with the same temperature). ④ 1 hPa = 100 Pa, and 1 atmosphere = 1013 hPa = 1013×100 Pa.

Practice

  1. Approximately how many hPa is one atmosphere?
  2. Is there rising or descending air at the center of a low-pressure system?
  3. In which direction does wind flow outward from the center of a high-pressure system in the Northern Hemisphere?

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