A front can be described as the interface between two or more air masses. Fronts are regions of developing 'weather' and should be envisioned to have both horizontal and vertical motions associated with them.

Frontal Systems are Mid-latitude phenomena. They result from the wave patterns that mix together air masses in the continual attempt to equalize temperatures across the globe. While restricted to the Mid-latitude cell, they form, mature and dissipate over several days as they go about mixing different air masses. They are ever recurring and become more intense during winter when contrasts in temperature (between north and south latitudes) are greater. Occasionally, during the spring and fall when temperature contrasts between the north and south can be extreme, severe weather events often produce record-breaking events. Temperature contrasts, reflecting the 'energy' of the atmosphere, are significant measures of forewarning the likelihood of severe weather events. 

Cold Fronts

A cold front is a Warm-Cold air boundary where colder air replaces warmer air as it moves and rotates in its wave-like motion. Since the colder air is denser (heavier) than the warmer air, it has the effect of lifting the warmer air and forcing it to mix and condense. This creates 'weather' in the form of rain and thunderstorms.

Wintertime cold fronts often bring bitterly cold air (cA), while summertime passages of cold fronts may change in only a few degrees of cooler temperatures, and even somewhat less humid conditions.

Implications for Storm Spotters

Barometric pressure will fall during the approach of a cold front.

Winds ahead of the front tend to be S/SW while winds behind the front are W/NW.
(weather stations rely on this wind switch as an indication of the front’s passage)

Passage of a cold front is marked by a drop in temperature and moisture along with a rise in barometric pressure.

Warm Fronts

A Warm Front is the interface between a warm and cool (or even cold) air. Warm Fronts are generally slower moving and slope ahead of its ground position. The 'weather' associated with Warm Fronts usually precedes the actual ground-level temperature change as the warm air is mixing ahead of the actual front's position. Warm Fronts can produce extended periods of rainy and drizzly weather (ahead of its passage), with sudden clearing occurring as the boundary front passes.

Warm fronts typically form to the east of low pressure centers, where southerly winds drive warm air northward. The warm air will ride over the pool of cold air causing an extended layer of clouds well in advance of its ground position. Summertime warm front passages can lead to long duration's of light to moderate rain or drizzle, while in winter it produces snow or freezing rain and sleet (sometimes all three!). The clouds and precipitation associated with the warm front can stretch for hundreds of miles over the cold air, thus a slow-moving warm front can mean hours, if not days, of dreary, wet weather before the warm air finally arrives.

Implications for Storm Spotters

Like the cold front, barometric pressure will fall during the approach of a warm front.

Winds ahead of the warm front to be S/SE while winds behind the front are S/SW.

Passage of a warm front is marked by a steady rise in temperature and moisture along with a slight rise then fall in barometric pressure.

Stationary Fronts

The stationary front signifies the beginning of a frontal mix and wave cycle. The air lying quietly for a day or two, with no upper air wave system in place to start the mixing process, is what is marked as a 'stationary' boundary. Weather along stationary boundaries are varied, sometimes with light rain and drizzle, but more often just quiet cloudy, or even sunny conditions prevailing. There is little likelihood of severe weather events under these conditions. However, once upper air waves start developing, stationary frontal conditions can suddenly change and rain storms move in with surprising rapidity.

Occluded Fronts

Toward the end of a wave cycle when the mixing of cold and warm air masses is well underway, there is little distinction of temperature/humidity within the 'mixed' air mass and a situation of occlusion occurs. It is at this stage that occluded fronts are identified and drawn at the apex of the warm and cold front intersection. Cold fronts generally move faster than warm fronts and push along and into the warm front boundary. This is another way on conceptualizing occlusion and indicates a termination of the mixing process and a weakening of the weather system.

The Life Cycle

Frontal systems are associated with mixing processes within the Mid-latitude Ferrel cell. The use of 'fronts' is to distinguish regions of 'different' air in an attempt to follow its cycle. Each front type is connected within the cycle and has somewhat distinguishing weather associated with it. To better understand the life cycle and interplay of fronts within such a Mid-latitude system it is worth seeing this in animation. This whole process and cycle is referred to as Cyclogenesis (the generation of Cyclones or Lows). The cycle lasts anywhere from 3 to 7 days.

Stage 1 - The Start - storm system begins as a weak disturbance somewhere along the stationary front where cold air from polar regions resides alongside warmer air to the south.

Stage 2 & 3 - A protrusion of cold air moving southwards forces an uplift of the warm less-dense air. The dynamics of pressure identifies that motions are taking place around a center of ever-deepening lower pressure.

Stage 4 & 5 - Distinguishing boundaries of temperature and wind directions show circular rotations and the cold and warm fronts follow this passage. Pressure continues to drop at the center of the rotation. 

Stage 6 & 7 - With mixing continuing between the contrasts of air masses, the Low is maturing quickly. The faster moving cold front is overtaking the slower moving warm front maximizing the interchange of energies and of weather phenomena. 

Stage 8 to 10 - When mixing and equalization achieved with the system now occluded, it has reached the final stages of the cycle. Within a short time, perhaps a day or two, the system begins to dissipate, the winds calm, and the cycle begins anew.

The Dryline

While not a frontal system, the dryline boundary occasionally plays a role in the development of severe weather in the central US. Drylines (sometimes referred to as the dew point line) are characterized by a significant contrast in moisture levels across air masses. They are typically found as a north-south moisture gap across the central plains and are readily distinguished in Water Vapor imagery. These are more predominant during the spring and early summer months. When favorable upper level winds and surface fronts occur with Drylines present, the impact for significant thunderstorms with large hail, damaging winds, and deadly tornadoes becomes much more likely.

Author's Bio: 

Athena is not just a High Priestess of Wicca and a spiritual adviser, she is also an avid weather geek and is a storm spotter and storm chaser.