In general, the air pressure in winter is _ _ _ _ in summer _ _ _ in sunny days and _ _ _ _ in rainy days (choose "greater than", "less than" or "equal to"), and explain the reasons.

The low temperature in winter is due to the high air density.

As for rainy days, look at this explanation:

On the same horizontal plane, if the air pressure distribution is uneven, the air will flow from the high pressure area to the low pressure area. Therefore, the air pressure in a certain area is high, and the air in this area flows out in the horizontal direction. The air above the high pressure area will drop. Because the atmospheric pressure increases with the decrease of altitude, when the air at high altitude falls, the pressure is increased, the volume is decreased, the temperature is increased, and the condensate in the air evaporates and dissipates. Therefore, the high-pressure central area 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 in the surrounding area will flow into the area horizontally, causing the air in the area to rise, the rising air will expand due to the pressure drop, the temperature will drop, and the water vapor in the air will condense. Therefore, it often rains in the central area of low pressure.