Air over warmer areas is less dense than that over cold areas. This causes it to rise and it is this which causes differences in air pressure. The pressure differences at the surface causes a wind to blow from the region of higher to pressure to that of lower pressure. The greater the pressure difference, the stronger the wind. As air moves across the earth, it picks up water.
Warm air picks up moisture more readily, so air masses in low pressure areas are more humid.
When humid air rises, pressure on it decreases, it expands and cools. When the temperature falls to the level where the air becomes saturated, water begins to condense in droplets or ice crystals. These then form into clouds, from which rain or snow may fall. The air, now dried, will rise to high pressure areas, bringing fine weather with it. The meteorologist studies the pattern of temperature, pressure and humidity in the skies.