Flight Training - June 2012 - 36

COLD 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
36 / FLIGHTTRAINING.AOPA.ORG

Cold air

STATIONARY FRONT

Cold air

Warm air

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.


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Flight Training - June 2012

Table of Contents for the Digital Edition of Flight Training - June 2012

Flight Training - June 2012
President’s Perspective
Right Seat
Letters
Tucked in for the Night
How It Works
Success Story Training Products
News AOPA Action
Since You Asked
ASI News
Final Exam
Flying Carpet
The Hunters
5 Weather Technologies That Changed Aviation
Front Fundamentals
Technique
Weather
Accident Report
Flight Lesson
Living the Dream, in 18 Months
Career Advisor
Tech Talk
Hot or Not
A 'Minimalist' Flight Training Syllabus
Advertiser Index
Debrief
Flight Training - June 2012 - Flight Training - June 2012
Flight Training - June 2012 - Cover2
Flight Training - June 2012 - 1
Flight Training - June 2012 - 2
Flight Training - June 2012 - 3
Flight Training - June 2012 - President’s Perspective
Flight Training - June 2012 - 5
Flight Training - June 2012 - Right Seat
Flight Training - June 2012 - Letters
Flight Training - June 2012 - 8
Flight Training - June 2012 - 9
Flight Training - June 2012 - Tucked in for the Night
Flight Training - June 2012 - 11
Flight Training - June 2012 - How It Works
Flight Training - June 2012 - Success Story Training Products
Flight Training - June 2012 - News AOPA Action
Flight Training - June 2012 - 15
Flight Training - June 2012 - Since You Asked
Flight Training - June 2012 - 17
Flight Training - June 2012 - ASI News
Flight Training - June 2012 - Final Exam
Flight Training - June 2012 - 20
Flight Training - June 2012 - 21
Flight Training - June 2012 - Flying Carpet
Flight Training - June 2012 - 23
Flight Training - June 2012 - The Hunters
Flight Training - June 2012 - 25
Flight Training - June 2012 - 26
Flight Training - June 2012 - 27
Flight Training - June 2012 - 28
Flight Training - June 2012 - 29
Flight Training - June 2012 - 5 Weather Technologies That Changed Aviation
Flight Training - June 2012 - 31
Flight Training - June 2012 - 32
Flight Training - June 2012 - 33
Flight Training - June 2012 - Front Fundamentals
Flight Training - June 2012 - 35
Flight Training - June 2012 - 36
Flight Training - June 2012 - 37
Flight Training - June 2012 - Technique
Flight Training - June 2012 - 39
Flight Training - June 2012 - Weather
Flight Training - June 2012 - 41
Flight Training - June 2012 - 42
Flight Training - June 2012 - Accident Report
Flight Training - June 2012 - Flight Lesson
Flight Training - June 2012 - Living the Dream, in 18 Months
Flight Training - June 2012 - Career Advisor
Flight Training - June 2012 - 47
Flight Training - June 2012 - Tech Talk
Flight Training - June 2012 - Hot or Not
Flight Training - June 2012 - A 'Minimalist' Flight Training Syllabus
Flight Training - June 2012 - 51
Flight Training - June 2012 - Advertiser Index
Flight Training - June 2012 - 53
Flight Training - June 2012 - 54
Flight Training - June 2012 - 55
Flight Training - June 2012 - Debrief
Flight Training - June 2012 - Cover3
Flight Training - June 2012 - Cover4
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