Why is clear weather a low pressure?

Generally speaking, areas with high air pressure on the ground tend to have sunny days, while areas with low air pressure on the ground tend to have rainy days. The high pressure and low pressure mentioned here are relative and do not refer to the absolute value of atmospheric pressure. If the air pressure in a certain area is higher than the air pressure in the surrounding areas, it is called a high pressure area; if the air pressure in a certain area is lower than the air pressure in the surrounding areas, it is called a low pressure area.

On the same horizontal plane, if the air pressure distribution is uneven, air will flow from high pressure areas to low pressure areas. Therefore, the air pressure in a certain area is high, and the air in that area flows horizontally to surrounding areas. The air above the high pressure area is about to fall. Since atmospheric pressure increases with decreasing height, when the air at high altitudes descends, the pressure it experiences increases, its volume decreases, its temperature increases, and the condensation in the air evaporates and dissipates.

Therefore, the central area of ​​high pressure is not conducive to the formation of clouds and rain, and it is often sunny. If the air pressure in a certain area is low, the air from surrounding areas will flow into the area in the horizontal direction, causing the air in the area to rise. The rising air will expand due to the reduction in pressure, the temperature will decrease, and the water vapor in the air will condense. , therefore, it is often cloudy and rainy in the center of low pressure.

The relationship between atmospheric pressure and altitude is:

As the altitude increases, the atmospheric pressure decreases; within the range of 3000M, for every 12M increase, the atmospheric pressure decreases by 1mmHg, which is about 133Pa.

The magnitude of air pressure is related to altitude, atmospheric temperature, atmospheric density, etc., and generally decreases exponentially as altitude increases. There are diurnal and annual variations in air pressure. During the year, the air pressure is higher in winter than in summer. During the day, the air pressure has a maximum value and a minimum value, which appear at 9 to 10 o'clock and 15 to 16 o'clock respectively. There is also a second highest value and a second low value, which appears at 21 to 22 o'clock and 3 to 4 o'clock respectively. .

Reference for the above content? Baidu Encyclopedia-Low Pressure Weather