2-24. Cold air masses consist of unstable internal conditions as the air at the earth’s surface attempts to warm and rise through the cooler air. Warm air masses are more stable as the air cooled by contact with the ground sinks and warm air above tends to stay there or rise. These changes cause strong, gusty winds in cold air masses, and weaker, steadier winds in warm air masses. Cumulus clouds usually mean cold air masses. As a cold air mass moves a warm air mass, these clouds may change to cumulonimbus clouds and produce thunderstorms. Warm air masses usually mean stratus clouds and extended drizzle.