Why do weather fronts get so much attention in pilot training materials? Because understanding the dynamics of frontal weather probably is one of the most important thingss a pilot can learn. Without a solid knowledge of how fronts work, your weather briefings won’t mean much. And because a large proportion of accidents are weather-related, that means you’ll be less safe. What you don’t know can hurt you!
Here’s the bottom line: All fronts create lifting forces, which in turn cause air to rise. Rising air cools, this cooling causes condensation, and it’s this condensation that causes the clouds and precipitation that lowers ceilings, reduces visibility, and can even create thunderstorms. And that’s frontal dynamics in a nutshell. But the three main types of fronts create lifting forces by different means, and wind and temperature profiles aloft have a lot to say in the character of a front.
Cold fronts. Cold fronts occur when a colder air mass runs into warmer air. (I’m saying “colder” and “warmer” because when it comes to fronts, temperature is a relative matter, and one that bears heavily on the nature of a front.) Cold air is dense, so think of it as piling up on the ground as its air mass travels with the prevailing winds aloft. Where does the cold air come from? Typically, from the colder climates to the north or west.
Sooner or later, this cold air will run into warmer air. Because of its density, the colder air more or less barges into the less dense, lighter, and warmer air mass ahead of it. Think of the cold air as a metaphorical snowplow, plowing its way forward and sending the warmer air upward. Voila! There’s your lifting force.
Viewed in cross-section, cold fronts have steep leading edges because of cold air’s comparative density. This means that the warm air lifted ahead of it rises fairly rapidly, and this rapid rise is what causes the cumulus and cumulonimbus (thunderstorm) clouds that are typical of cold fronts. It’s also responsible for the kind of large water droplets that can cause clear icing in cumulus clouds if temperatures are between minus 10 and 0 degrees Celsius. It also means a lot of turbulence and instability in the vicinity of the cold front. Typically, the rising clouds ahead of a cold front can extend as far as 100 to 150 nm.
Speaking of instability, here’s where temperatures aloft figure in the picture, and not just with cold fronts. Imagine a balloon filled with air taken from near the surface, or at a low altitude. Let the air in that balloon represent a parcel of free air, and whether it rises or falls depends to a large extent on the temperature of the air surrounding it, called the ambient air temperature. Ambient air warmer than the balloon/parcel temperature? Then the balloon/parcel will sink; that’s a stable situation. Ambient air colder than the balloon/parcel temperature? Watch out: This represents unstable air. The warmer air parcel will continue to rise in the colder air aloft. This is how thunderstorms are created, with the warmer air getting its boost from temperature alone (as with air mass thunderstorms caused by surface heating), or by riding up the slope of a front, where it is thrust into ever-colder air.
As you may suspect by now, the temperature difference between the air ahead and behind the cold front’s surface position helps to determine the intensity of the frontal passage. There are benign, slow-moving cold fronts that barely have any clouds, and there are violent, fast-moving cold fronts with huge temperature differences that can create dangerous squall lines ahead of the front.
Warm fronts. When warm air overtakes colder air, it rides up and over the colder air beneath it in a process called overrunning. Again, this is a function of the differing air densities. Warm air, being less dense and lighter, can’t penetrate the cold air ahead of it, so it slides over it at a shallow angle. Even so, there is lifting along this frontal surface, and the cirrus clouds that usually give advance notice of a warm front typically appear some 800 to 900 nm before the warm front’s surface position.
Warm frontal lifting is more gradual than a cold front’s, but because a warm front’s cloud masses cover such a large area it’s very likely that low clouds and precipitation can affect areas as large as several states. The high-altitude cirrus clouds pass (at about the 15,000 to 20,000-foot level), followed by successively lower layers of altostratus, altocumulus, stratus, nimbostratus, and cumulonimbus clouds.
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 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 high-speed 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 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.
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