Flying The Summer Low
A wimpy winter evolved into a wet, cold spring above the Mason-Dixon line, with plenty of low-pressure systems to entertain instrument pilots.
This article originally appeared in the October 2002 issue of IFR Refresher and is reprinted here by permission.
You couldn't buy a warm day in central Canada during April and May, and that cold pool of arctic air drifted south over the northern plains and pushed deeply into the Ohio River Valley, where it faced off against frequent streams of Gulf moisture energized with Texas heat.
One early morning in mid-May, a flight departing Greensboro, N.C., bound for Beckley, W.Va., a 116-nm, 50-minute flight, would have launched into 14-knot southwesterly winds and 60-degree temps, fought 30-mph headwinds at 6,000 feet, crossed the cold front and arrived at Beckley with the wind huffing out of the northwest at 25 knots and temperatures in the 40s.
The boundary between the contrasting air masses had produced a wave that spun into a vigorous low-pressure system.
A Low Is Born
The contrast between air masses -- cold and warm, dry and wet -- in the middle latitudes is the principal reason that we all got instrument ratings. The boundaries between contrasting air masses, or fronts, give birth to low-pressure systems.
Lows are born of instability in the free air. You can create a model of how a low is born by placing a pencil between the palms of your hands and holding your hands in front of you as if you were praying. Your left hand represents the northerly flow of cold air, and your right hand represents warm air flowing from the south.
Move your left palm downward and your right up. The pencil rotates counterclockwise -- it moves cyclonically, just as the air does when forced along a boundary between northerly and southerly flows.
The air between your palms is low pressure. The shearing that takes place creates a wave that rolls counterclockwise. In the atmosphere, this cyclone begins to suck air into the center, where it rises, pulling warm air in from the south and spreading it to the north above the cold air, which in turn is sinking to the southwest.
A classic low may be depicted with a cold front trailing to the southwest and a warm front or stationary front on the northeast or eastern side of the low. As this cyclone develops and processes the convergence of warm air and cold air, it begins to move to the east or northeast, riding on the mid-level winds at the 500 mb level, or about 18,000 feet.
As a low gets bigger -- or deeper, as meteorologists say -- the more air it sucks into the swirl and the more intense the weather is likely to be.
Deep low pressure produces intense wind, for example, because the gradient between the low's center and adjacent higher pressure air is very steep. On a chart, the isobars -- lines of equal pressure -- are very close together, denoting that air is moving down a steep gradient into the hungry low.
On the flight from Greensboro to Beckley the morning of May 18, for example, the pressure was 29.71 at KGSO, 29.68 at Martinsville, along the front, and at KBKW, west of the front, the pressure was 29.91 -- steep gradients that produced gusty surface winds and 30-knot winds at 6,000.
As the whirling cyclone sucks air into its center and lifts it high into the midlevels of the atmosphere, any available moisture gets pulled in as well, and there's likely to be plenty of water vapor in the southern sectors of the storm.
Although the most abundant moisture will be found near the low-pressure center, the southeast corner usually is the best source of water vapor for the storm. Look at surface dew points in the southeast sector of a low-pressure system for a preflight clue to available moisture.
The weather is likely to be hot and hazy in the warm sector, with increasingly gusty southerlies and irritating chop with moments of moderate turbulence. Sic Sacs may have been invented for flight through the warm sector of a cyclone.
Farther from the center, breaks in the broken layer may at first seem a sign of improving conditions, but such cloud-free holes only allow more solar heating, which in turn will produce even more instability and prepare an already ripe environment for big storms when the cold front arrives and forces the air to rise.
Flying in the warm sector of a storm is flight through the ammunition dump of a cyclone, and the closer that you fly to the approaching cold front, the greater the chance that the instability will produce storms.
Preflight The Route
The huge comma cloud that is produced by these extra-tropical cyclones can cover several states, and taking a look at a satellite picture isn't a bad way to start a preflight briefing for a flight across the front.
Look next at a current surface analysis chart. High temperatures (80s) and high dew points (60s) in the warm sector portend big and possibly severe storms as the daytime heating intensifies.
Take a hard look at the AIRMETs and convective SIGMETs for the routing because though they are routine and plentiful, they will offer up quick clues about nasty stuff -- icing in particular, and turbulence and IFR, especially in mountainous terrain.
If the systems were producing during the night, it's likely that there will be convective SIGMETs in effect. The lines and cells that produced storms across your routing during the night may have created an outflow boundary that will help to force new convection for your flight.
PIREPs, of course, offer clues about the recent history of an air mass and will help you to paint a picture of the flying environment. Be sure to check the report time and the type of aircraft that made the report.
A Cessna 172 may consider continuous light chop such an irritant that it's reported as moderate turbulence. A Falcon may call out a couple of good bumps at 6,000 on the way to FL410, climbing at 4,000 fpm, while you may be planning an hour's flight at 6,000, in what may turn out to be continuous light chop or even moderate.
Check out the TAFs and area forecasts for your routing with the idea that you are trying to verify forecasts by crosschecking with current METARs. A forecast for overcast in the warm sector that turns out to be a scattered layer will dramatically alter the day's events by allowing the sun to heat the surface.
Look for ground truth, as the weather forecasters call it. Read a forecast and then confirm it: Look out the window, check station reports, and ask the briefer for PIREPs.
Big, dynamic systems are very well forecast, but your microscopic course through the maelstrom will be affected by many smaller features. A tornado, for example, is considered to be a micro event within the larger dynamics of a supercell, which is a smallish event within the extra-tropical cyclone.
You can lose a lot of paint flying through micro systems, but you can increase your confidence in your routing by first looking at the big picture and then the smaller features along your routing, always searching for ground truth in forecasts and flying with an ear on the freqs, an eye on the sky in front of you and with your brain tuned to the significance of the warnings.
Finally, the best tactic is to fly thunderstorm country visually. Get on top in the smooth air early, until the broken cumulus towers reach 10,000, then descend in visual conditions to the bases and suffer the bumps but remain visual.
There will be increasing IMC the closer that you get to the center of the low. Layers will close up, bases will go down, tops may be unreachable and you'll need oxygen.
The summer weather map is littered with boundaries between contrasting air masses. Some boundaries will dissolve as the air dries out or daytime heating is turned off by high overcast. Some boundaries will produce fronts that will give birth to low-pressure systems, and some of those lows will become deep, extra-tropical cyclones that will spawn storms that will make the six o'clock news.
The pilot's job is to develop a sense of the disturbance by gathering forecasts and reports that paint a picture of the weather. En route, the pilot watches for evidence that the picture remains intact and that the forecasts verify.