Know When to Call in Sick by Matt Gerdes

Reviewing Meteorological Factors to Avoid Missing the Best XC Days

There are five major factors that will help you decide weather to fly.

1. Surface Winds and Winds Aloft
2. Barometric Pressure
3. The Thermal Index (T.I.)
4. Jet Stream
5. Approaching Fronts

Evaluating these factors will help you get a picture of what it will be like out there, and there are several ways to gather the information needed to paint that picture. One of the easiest and most accurate ways is calling the pre-flight briefer. In most of North America it’s a toll free call to a real live person, who has access to a huge database of weather information. There is a kind of formula for getting the answers you need from the briefer, as you will see in the final paragraphs. The Internet is also a great place to get weather data, anytime anywhere.

Perhaps one of the most important things is knowing your proposed site. Every site has its own peculiarities, and knowing them is beneficial not only to predicting the best days of flying, but to your safety as well. Some sites react poorly to certain wind directions, or need the atmospheric pressure to be within a general usable window to yield the best thermals. Keeping a log book with past weather conditions briefly recorded will always be an asset to assessing the day’s potential.

At Chelan, our home site in WA, we know it’s going to be a good one when the pressure is around 30.05 or 1015mb, winds aloft are light out of the W or WSW, and the Jet Stream is far north of us. By recording weather data in our log-books on the best days, predicting the weather becomes simple.

WINDS ALOFT

Commonly recorded at intervals of 3000ft, winds aloft are one of the most important factors of the day’s weather. High winds aloft can be a drag if an inversion layer breaks mid day and you end up landing out, going backwards. Too much wind almost always decreases the organization of lift, making climbs more difficult. The right amount of wind can create epic conditions for flying xc, so look at the forecast carefully, from the surface to 12,000ft, or 18,000ft if you’re flying a higher altitude sight. Ideally, it will be light on the surface, gradually increasing to less than 25 knots at 12,000ft or the likely ceiling of usable lift.

THE PRESSURE

The pressure is a crucial factor that will determine the day’s character. If the pressure is high, expect thermals to be small and bullet like if there is enough heating, or almost non-existent if there isn’t. Commonly thermals are disorganized and unpredictable when the pressure is too high, with large areas of sink occurring in places that might normally harbor great lift.

If the pressure is too low, watch out- an overly unstable atmosphere can ruin your whole day. Low pressure combined with a high Thermal Index can rapidly lead to over-development and cloudsuck. In general, watch for clouds that grow to be taller than they are wide, as those indicate lift that can be stronger than most pilots enjoy, not to mention dangerous.

Medium to mild-low pressure can help to create the conditions that we live for. Get in the habit of checking and recording the atmospheric pressure in your log book every day you fly. The best days will probably follow a trend that you can use to your advantage in your morning weather analysis. Typically 30.00 to 31.00 inches is good. (Around 1015mb).

THERMAL INDEX

The Thermal index is a tool for determining the strength of lift and potential for over-development. It is a simple formula using puddle temperatures and the dry adiabatic lapse rate to find a measurement that will help you determine the day’s atmospheric stability. It sounds complicated, but isn’t- it’s a quick and helpful tool. I won’t go into it in depth, as that can cause drowsiness.

To get the TI we need to know the puddle temps on the ground, and the altitude of the freezing level. Low puddle temps and a high freezing level will give us a mellow TI. High puddle temps and a low freezing level will indicate a less stable atmosphere, and a high TI.

Puddle temperatures- Look at the days forecasted high- lets say it’s 75 degrees. If your site is in a drier climate, and the terrain is dry grass or dirt and rocks, there is the potential for puddle temps to reach 110F. Sounds high, but try leaving a thermometer on the ground at around noon for about ten minutes, and see what you get.

The Adiabatic lapse rate is the rate at which rising air cools- about 5.5F per 1,000ft. If you predict a puddle temp of 110F at 2,000ft, and the freezing level is at 12,000ft, then there is a temp difference of 78F over 10,000 ft. The Adiabatic lapse rate tells us that there should be a 55F difference. Subtract 55 from 78- you get 23, that’s the TI. For novice pilots a TI measurement of around 10 or 15 is great, and advanced pilots will enjoy anything up to around 35. Above 35 is pretty unstable, and you’d might as well go to work.

METRIC: The Dry Adiabatic Lapse Rate is 10C per 1000m. In General, even the most turbulence hungry pilots should avoid a Metric TI measurement of more than 20.

THE JET STREAM

One pre-flight briefer scoffed at a lowly paraglider pilot asking for the location of the Jet Stream. He sneered, “Lear Jet pilots call in and don’t ask about the Jet Stream!” Indeed, what is happening at 35,000ft seems to be of no concern to us. However, years of careful weather observation by para/hang pilots in the western US has shown that when the Jet Stream is within 100 miles/160km of our site, it directly affects flying weather, and usually in an unpleasant manner. Surface winds can change suddenly and unpredictably, winds aloft can increase quickly throughout the day, and thermals tend to be very disorganized, and sometimes hardly usable.

In the morning the pressure can be perfect and stable, winds aloft may indicate the probability of a nice tail wind, but by the time you get to launch and look at the weather, it will have most likely deteriorated to the point that you affect a miraculous recovery and end up at work by lunch. The Jet Stream being within 100 miles of your site is usually a great reason to not call in sick.

APPROACHING FRONTS

This one is pretty self explanatory. By either looking at a satellite image or asking the briefer, find out if there will be a significant piece of weather arriving in your area by mid- day. Don’t expect it to stay sunny all day, or the briefer to tell you anything you don’t ask about.

It actually only takes about 10 minutes to gather and assess all of this information. You should be looking for light winds aloft- hopefully from almost zero at the surface increasing gradually with altitude, a medium to med-low pressure, a TI of around 20-25, (10-20Metric) the Jet Stream being more than 100 miles away, and no significant fronts on the horizon.

If you haven’t been recording weather conditions on XC days in your log-book, then start doing it if you care about forecasting flight weather. Not all of us can fly whenever we feel the urge, and choosing the best days carefully is how you will get the most out of your flying.

CALLING THE PREFLIGHT BRIEFER

Dial 1 800 WX BRIEF. When you get the briefer on the line, give him your name, and specify that you are a paraglider or a hang glider pilot. Ask for Winds Aloft from the surface through 12,000ft, plus temperatures. The info will be given in a series of numbers- learn what they mean. Ask for the pressure in your area, the location of the Jet Stream, whether or not there are any significant approaching fronts or systems in the vicinity, and finally, ask for a ‘Synoptic Outlook’. That’s weather nerd-speak for “give me the low down on what’s up”. Don’t ask the briefer for the TI, figure it out for yourself.

On the Web. In the US try www.paraglide.com/Weather

THANKS TO DIXON WHITE AND MARTY DEVIETTI FOR 100% OF THIS INFO.