It's getting dark

Water vapor in the air satisfies the equation of state. When rising, the air pressure decreases, which is easy to cause water vapor to condense and form water droplets. More water droplets will form clouds, and more clouds will be cloudy.

When the air sinks, the temperature of the air mass rises, which is easy to cause water droplets to evaporate and form water vapor. The higher the temperature, the greater the amount of water vapor that can be contained in the air, so the higher the temperature of the same amount of water vapor, the smaller the humidity.

So in the high-pressure area, the air sinks, the water drops evaporate, and the cloud amount decreases, so the weather is fine, while the temperature rises and the humidity drops, so the weather is dry.