A Cold Front
A cold front is a battalion of thunderheads rampaging across the country side like an advancing army. It is a fast-moving wedge of cold polar air which pushes the warm, moist tropical air up sharply to form cumulonimbus thunderheads. A cold front brings a sudden wind change, heavy rain, hail, thunder and lightning. After the cumulonimbus clouds of the front have passed you usually get cold sunny weather with puffy little cotton wool clouds.