In the northern hemisphere, the jet stream acts as a boundary between northern latitude polar air, and mid latitude warm air. The jet stream can be described as a river of air moving West to East somewhere around 30,000 ft. It is in a constant state of fluctuation in speed, latitude, and altitude. During the summer months the jet stream tends to migrate north and reside mostly in Canada or along the border between Canada and the United States.
A dip in the jet stream can be an indicator of an approaching cold front and the possibility for unstable weather or high winds. As the jet stream moves south into your area, you will notice the upper level air temperatures start to drop. This can decrease the overall stability of the region. If the jet stream is significantly north of your area, you don’t need to be concerned with it. If it is over your region or south of you, keep in mind that the weather conditions will likely be unstable and may change rapidly.
Isobars are lines on a weather map that connect areas of equal barometric pressure. They are read very much the same way you would read the altitude lines on a TOPO map. Lines that are spaced far apart or “loose” indicate a shallow pressure gradient from one area to another with light or no wind. Wind generally tends to move along isobar lines from high pressure zones to low pressure zones. Lines that are very close together or “compressed” indicate a steep pressure gradient and may indicate higher winds as the pressure tries to equalize. If there are more than 2 isobars in your state (about the size of Washington) they are compressed.
The surface pressure is related to the isobars and high vs. low pressure systems. The barometric pressure will have an effect on the stability of the day. High pressure days are generally more stable and usually sunny but may also result in sharper thermal edges. Low pressure days are less stable and may have cumulous clouds and tend to have smoother edged thermals. High pressure systems descend in a clockwise rotation. Low pressure systems ascend counter-clockwise. The barometric pressure trend will help you determine whether a weather system is arriving, leaving, or sitting over you.
The barometric pressure for a day is not a “deal breaker” for flying. It is merely another data point which will help you determine what the day will be like. The standardized pressure is 29.92. If you note that it is at 30.10 you can expect sharper thermals.
Anabatic & Catabatic Flow
At some point you may have stood on top of a hill and felt a breeze lightly gusting up a hill. Sometimes strong, sometimes completely still. This is caused by the sun heating the ground, which heats the air above it. That air tends to flow up hill and is called anabatic flow.
As the heating decreases toward the end of the day, so will the anabatic flow. Once the sun is sufficiently low and no longer provides enough heating, the upper colder air will begin to flow down hill. This is called catabatic flow – sometimes spelled “katabatic flow”. This flow will start in shaded areas first so you want to watch these areas for a change in wind direction.
Gathering Information + Forecasting
To start assessing the weather, you will need to acquire the relevant information. One of the best places to find the current and forecasted data is the internet. There are hundreds of weather focused websites that cover everything from the jet stream, isobars, winds aloft, surface winds etc. There are even pages that focus their weather forecasts specifically for glider pilots, giving you information that you can’t get anywhere else. The problem with the internet is that there is so much information it is hard to know exactly what to get and where to find it. Your instructor or local pilots will be able to tell you what websites to check for the area you plan to be flying in.
Aside from the internet you may also use Flight Weather Service. This is a public service designed for small plane pilots, but is suitable for us as well. You will talk to a flight briefer who has access to a great deal of information and can help you generate a concise report in a few minutes. They may also tell you if there are any airspace restrictions or advisories such as closures due to fire, or military operations. The number (1-800-WX-BRIEF) is toll free anywhere in the United States and your call will be routed to the nearest flight service station. The flight briefers are accustomed to talking to professional pilots, so when you call it’s a good idea to have written down exactly what you want to ask. You will also need to give them your last name as your pilot identification.
Weather band radio (NOAA) is good for getting updates on localized surface conditions and active advisories. This is a good source for current information, but it is not very useful for forecasting. A 2-meter radio is usually capable of picking up the NOAA station. Local instructors should have the frequency for your station. The NOAA forecast discussion on the local forecast page online can be a bit technical and take some getting used to, but is another great resource for finding quality local and regional weather forecasting.
Local weather stations will have general weather information and forecasts, but you will want more detailed and current data. The Weather Channel is usually of little use – “It will be sunny and warm in California” is not detailed enough.
Some of the most commonly used websites for wind and weather information are:
- www.xcskies.com – This site is a paid site specifically for pilots. The information can be overwhelming, so look at it closely with someone who has a subscription to decide if it is something you are willing to spend money on.
Now that smartphones have become so commonplace, there are more and more apps coming out every day that provide wind readings and forecasts, site specific weather info, frontal maps, storm info, and the list goes on. Most will offer real time data from local weather stations – these are usually your best indication of the current conditions. Some apps to look at include:
- Windy – there are two apps with this name and both are good.
- Storm (by Wunderground)
- NOAA Weather
- The Weather Channel
As your skills progress so will your ability to recognize and predict weather patterns and conditions. The more information you acquire the easier it will be to develop a model of how the weather and topography in your area interact with each other. You will be able to develop a profile for a good flying day and a bad flying day.
If you are traveling to an unfamiliar site that you don’t have a detailed profile for, talk with the local pilots about appropriate weather conditions. The locals always know best and some sites can present tricky weather problems that you would only be aware of if you had experience at the site. Trust the locals, even if you see conditions that may look appropriate. They very well may know something you don’t.