Refer to figures.
Cold fronts have an exacerbating effect on thunderstorm formation, because of the way they force warm air to rise and condense. The cold, very dense air of a cold front acts like a fast moving plow against the warm, moist air in front of it, lifting it with great intensity.

Ahead of a warm front:

• Clouds - Increasing to 8/8, base lowering, Ci, Cs, As, Ns.

At the warm front:

• Clouds - 8/8, base very low, Ns, St.
• If the air is unstable => there can be thunderstorms around the warm front.

In the warm sector:

• Clouds - 6/8 to 8/8, some large breaks may occur, base low, St, Sc.

At the cold front:

• Clouds - 6/8 to 8/8, base low but rising, Cu, CB, sometimes Ns.
• Thunderstorms are associated with Cumulonimbus clouds.

Behind the cold front:

• Clouds - 6/8, base lifting, Cu, Cb.