Flight Training - June 2012 - 37

There are three types of bad news in all this. One is that the rain or other precipitation from a warm front’s clouds fall into the colder air below. This subjects the falling droplets to additional cooling, which can cause widespread precipitation fog and low ceilings and visibilities. In the colder months of the year, this cooling can cause vast areas of snow. Add in the warm front’s slow speed across the ground and you have the makings
OCCLUDED FRONT

slows so much that it loses its original nature. They are rather like a tug of war between colder and warmer air masses, and have the characteristics of both. That’s why they’re shown on surface analysis charts by a frontal boundary with alternating cold and warm-front symbology. The weather in stationary fronts is likely to reflect the more dominant front in the combination, with much depending on the degree of saturation and the stability of the air masses involved—so, yes, thunderstorms are possible. Stratus clouds and steady light rain or drizzle is usually the rule, and as with warm fronts, stationary fronts tend to cover large areas with IMC. Surface winds are very light, or calm, and blow parallel to the frontal boundary.
OCCLUDED FRONTS. Now here’s a mess. Occluded fronts happen when cold fronts catch up to, and then overtake, the warm fronts ahead of them. The end product is two fronts in one: a front at the surface, and one aloft. Again, widespread cloudiness, precipitation, and thunderstorms are possible in this unstable brew. As with warm and stationary fronts, thunderstorms may be embedded within the cloud mass. Embedded thunderstorms are bad news in large cloud fields because they are hidden from view by clouds, fog, precipitation—or all three—and therefore, difficult to visually avoid by keeping your distance. Sure, datalink Nexrad imagery and onboard weather radar can help you steer clear of any precipitation echoes by the prescribed minimum of 20 nm, but that doesn’t help if you don’t have this equipment—and it sure won’t help if a storm cell suddenly pops up around you. There are two types of occluded fronts: a warm front occlusion, and a cold front occlusion. With cold front occlusions, advancing cold air pushes underneath a cool, yet warmer air mass—the same way a run-of-the-mill cold front undercuts a warmer air mass ahead. In a warm-front occlusion, warmer air overruns colder air—the way a warm front rides over colder air. Either way, a sector of warmer air is lifted between the two air masses. THUNDERSTORMS. By now you’ve noticed common threads among the weather produced by frontal activity: thunderstorms and icing. Now that we’re in the warm seasons in the northern hemisphere, let’s go to brutal basics about what’s necessary to create thunderstorms. There are three elements that must be in place for them to happen. One is moisture. This is typically provided by southerly flows of air ahead of cold fronts, especially east of the Rocky Mountains. Another is unstable air. Remember the warm parcel of air rising ever faster in the cold ambient air in our balloon example? That’s the central idea. Warm air rises, all right, but only if the air around and above it is colder. The last ingredient is a lifting force to kick off the process. This could be thermals rising off a parking lot, flows over high terrain that send air upward, upper-air dynamics that draw air from lower altitudes—or the fronts we’ve just reviewed.
Editor at Large Thomas A. Horne is a meteorologist who has been writing about weather since 1982.
JUNE 2012 FLIGHT TRAINING / 37

Warmer air Cooler air

WARM FRONT

Warm air

of a couple days’ worth of dreary skies and instrument meteorological conditions. Thunderstorms are also a possibility. Unstable, saturated air gets a lift by rising along the frontal slope and creating convective clouds. This is called elevated convection because the lifting is initiated at altitude in a process that’s often enhanced by highspeed cores of jet stream winds aloft. Oh, and let’s not forget icing in the winter months. Huge swaths of subfreezing clouds and rain mean that diversions and escapes to ice-free conditions may take more time and distance—and the whole time you’re trying to escape, more ice can accumulate. Winter warm fronts may sound all balmy and tropical, but they are among the most dangerous systems out there.
STATIONARY FRONTS. As the name implies, these fronts don’t move much. They happen when either a cold or warm front

Flight Training - June 2012

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