Occluded front: When a cold front catches up to a warm front. Types of Fronts Stationary front A front that is not moving. Cold front is a leading edge of colder air that is replacing warmer air. Warm front is a leading edge of warmer air that is replacing cooler air. Occluded front: When a cold front catches up to a warm front. Dry Line Separates a moist air mass from a dry air mass. Fronts -- the boundaries between air masses A front is defined as the transition zone between two air masses of different density. Fronts extend not only in the horizontal direction, but in the vertical as well. Therefore, when referring to the frontal surface (or frontal zone), we referring to both the horizontal and vertical components of the front.

A. Cold Front is a transition zone from warm air to cold air. A cold front is defined as the transition zone where a cold air mass is replacing a warmer air mass. Cold fronts generally move from northwest to southeast. The air behind a cold front is noticeably colder and drier than the air ahead of it. When a cold front passes through, temperatures can drop more than 15 degrees within the first hour.

The station east of the front reported a temperature of 55 degrees Fahrenheit while a short distance behind the front, the temperature decreased to 38 degrees. An abrupt temperature change over a short distance is a good indicator that a front is located somewhere in between.

B. Warm Front. A transition zone from cold air to warm air. A warm front is defined as the transition zone where a warm air mass is replacing a cold air mass. Warm fronts generally move from southwest to northeast and the air behind a warm front is warmer and more moist than the air ahead of it. When a warm front passes through, the air becomes noticeably warmer and more humid than it was before.

Diagram of Warm and Cold Fronts

Looking at a Cold Front in more detail

Another view of a Cold Front

Looking at details of a Warm Front

Another view of the Warm Front

C. Stationary Front a front that is not moving. When a warm or cold front stops moving, it becomes a stationary front. Once this boundary resumes its forward motion, it once again becomes a warm front or cold front. A stationary front is represented by alternating blue and red lines with blue triangles pointing towards the warmer air and red semicircles pointing towards the colder air.

D. Occluded Front - when a cold front overtakes a warm front. A developing cyclone typically has a preceding warm front (the leading edge of a warm moist air mass) and a faster moving cold front (the leading edge of a colder drier air mass wrapping around the storm). North of the warm front is a mass of cooler air that was in place before the storm even entered the region.

As the storm intensifies, the cold front rotates around the storm and catches the warm front. This forms an occluded front, which is the boundary that separates the new cold air mass (to the west) from the older cool air mass already in place north of the warm front. Symbolically, an occluded front is represented by a solid line with alternating triangles and circles pointing the direction the front is moving.

Formation of an Occluded Front

Two Types of Occluded Fronts Two types of Occluded Fronts are recognized, as explained in the diagram:

Special situation of an Occluded Front Warm air trailing advancing cold air is caught within a zone in which a second Cold Front is following the trailing edge of the first Cold Front. The warm air is squeezed upwards forming mostly cumulonimbus clouds that can produce strong thunderstorms: Special situation of an Occluded Front

Dry Line is a moisture boundary. A dry line is a boundary that separates a moist air mass from a dry air mass. Also called a "Dew Point Front", sharp changes in dew point temperature can be observed across a dry line. Dry lines are most commonly found just east of the Rocky Mountains, separating a warm moist air mass to the east from a hot dry air mass to the west.

Yellow dashes mark the Dry Line Actual Weather Map Yellow dashes mark the Dry Line

On the previous weather map ; Dew points east (ahead) of the dry line shown above range from the upper 50's to low 70's with winds from the southeast. West of the dry line, dew points were in the 20's and 30's, a decrease of nearly 50 degrees. Air temperatures ahead of the dry line were generally in the 70's and 80's while behind the dry line, temperatures ranged from the mid 80's to mid 90's. Drier air behind dry lines lifts the moist air ahead of it, triggering the development of thunderstorms along and ahead of the dry line (similar to cold fronts). It is not uncommon for tornadic super cells to develop along a dry line.