Air Masses classified by temperature and humidity. Tropical: Warm, form in tropics, low pressure; Polar: cold, form near poles, high pressure; Maritime: form over oceans, humid and Continental: form over land, dry. Cold air “undercuts” warm air. If the warmer air mass contains enough moisture, possible storms, clouds, rain followed by clear, cool weather. Warm air “over-rides” cool air. Front moves slowly. If the warmer air mass contains enough moisture, clouds and rain may occur for several days followed by warmer, more humid weather.