There is no such thing as bad weather in Scotland, only inappropriate clothing. - Billy Connelly

Colliding air masses are known as fronts. The approaching front, usually from the west in the northern hemisphere collides with the outgoing front, forcing it to move eastward. The faster the incomming front approaches the more violent the collision between the two fronts, resulting in stormier weather.

There are three types of fronts

Warm Fronts A warm air mass arrives and rises slowly above the cold air ahead and cools to its dew point.

Signs - Low barometric pressure, highhumidity, low cloud ceiling.
Result - Fairly calm winds, at the fronts leading edge; steady rain for days.

Cold Fronts Fast-Moving unstable cold air pushes under the warm air ahead, forcing it up quickly and cooling it.

Signs - High barometric pressure, high cloud ceiling, good visibility unless precipitation is present.
Result - Fair weatherthat can change quickly; strong winds, generally from the north or west; and severe but brief thunderstorms or snow squalls.

Occluded Fronts A collision of three air masses. A fast-moving cold front over-takes a warm front, lifting the warm air mass. The incomming cold front then collides with the departing cold air mass. If the incomming cold front is warmer than the departing one, called a warm occluded front (WOF). The new cold front climbs over the existing one while trapping the warm front high in the middle. If the incomming front is colder than the departing one, a cold occluded front (COF). It wedges under it.

Signs - Wind direction changes, usually so it blows from the north-northwest; falling, then rising barometric pressure.
Result - Storms possible; light to heavy rain folowed by dry weather after the front exits. With WOFs cold temps get milder; with COFs, cold temps get colder.