Air conditioning controls the temperature, moisture, cleanliness, and movement of indoor air. It cools the air when the weather is hot. It warms the air when the weather is cold. Comfort depends partly on humidity, and air conditioning removes moisture from the air or adds it as needed. Removing dirt and dust from air makes the air more healthful. By controlling air movement, air conditioning brings fresh air into a room and pushes out stale air. In all these ways, air conditioning provides air that makes people comfortable at work, at play, and while sleeping.