A cold<span> front </span>occurs when a cold air mass advances into a region occupied by a warm air mass<span>. If the boundary between the </span>cold<span> and </span>warm air masses<span> doesn't move, it is called a stationary front. This is due to the (usually) higher humidity in the </span>air<span> of </span>warm<span> fronts compared to that of </span>cold<span> fronts.
