A front is any boundary existing between differing masses of air; that is, masses that have different overall temperatures and humidity. Fronts come in four types: warm, cold, stationary, and occluded. A warm front is a front that is advancing over a cooler mass of air, while a cold front is just the opposite. Stationary fronts, as one might guess, represent boundaries between warm and cold air that are in relative equilibrium, although they can still move back and forth by as much as several hundred miles and yet be considered “stationary.”