Fix the mistake: Weather Phenomena and Changes in Weather

High and Low Pressure Systems and Atmospheric Circulation

Differences in atmospheric pressure create wind and are the fundamental cause of weather changes. Understanding the structures of high-pressure systems and low-pressure systems is the starting point for studying meteorology.

Basic Knowledge

A region with higher atmospheric pressure than its surroundings is called a high-pressure system, while a region with lower pressure is called a low-pressure system.
In a high-pressure system, descending air currents form at the center, and near the ground, winds blow outward in a clockwise direction (Northern Hemisphere). Descending air makes clouds less likely to form, so clear weather is common.
In a low-pressure system, rising air currents form at the center, and near the ground, winds blow inward in a counterclockwise direction (Northern Hemisphere). Rising air makes clouds more likely to form, so cloudy or rainy weather is common.
Wind moves from areas of high pressure toward areas of low pressure. However, because of the Coriolis force caused by Earth's rotation, wind curves to the right of its direction of travel in the Northern Hemisphere. Therefore, winds near the ground cross isobars at an angle.

Further Study (Background and Significance)

The fundamental cause of the formation of high- and low-pressure systems is uneven heating by solar energy. The equatorial region warms under strong sunlight, producing rising air currents, while polar regions cool, producing descending air currents. This difference drives the general circulation of the atmosphere and transports heat from low to high latitudes.
Weather near Japan generally changes from west to east along with the westerlies. By examining a weather map and reading how closely spaced the isobars are (= the pressure gradient), we can predict wind strength. The narrower the spacing between isobars, the steeper the pressure difference and the stronger the wind.
The Siberian High is a high-pressure system over the continent that develops in winter and brings northwesterly monsoon winds to Japan. In contrast, the summer Pacific High sends warm, humid air from the southeast, creating the hot, humid weather of the rainy season and summer.

Points to Note (Common Confusions)

① Do not confuse the directions of circulation around high- and low-pressure systems. In the Northern Hemisphere, high pressure is clockwise and low pressure is counterclockwise. ② The Coriolis force is reversed in the Southern Hemisphere (deflection to the left). ③ Whether pressure is high or low is determined not by its absolute value but by its relative difference from the surroundings. ④ “High pressure = clear weather” is a general tendency, not an absolute rule (thin clouds may also appear).

Practice

1. In which direction does the surface wind near the center of a low-pressure system rotate in the Northern Hemisphere?
2. Which is dominant at the center of a high-pressure system, rising air or descending air?
3. State the main reason that weather in Japan changes from west to east.