Pressure differences result in temperature differences and produce moving air. Heating air makes it rise, and cooling air makes it sink. Air moving from a high to low pressure creates moisture, which condenses into clouds or fog. When moisture-laden air is cooled, its ability to hold water is greatly reduced, resulting in condensation, rain, snow or fog. In summary, relative humidity is the comparison of how much water vapor air holds relative to how much it could hold.
Cold air is more dense than warm air.
It descends down to the surface. Cold air collects in sheltered inlets creating a cold anchorage on cold, windless nights. On a global scale, intense solar heating near the equator forces air to rise, creating a low pressure ring around the globe. As air within the ring rises, it cools and water vapor condenses, forming clouds. Dry air then sinks forming a high pressure found at 30 degrees latitude.