Seeking Nirvana #3
Had a brilliant flight the other day! Climbing to a little over 13,000'msl and gliding under perfect flat-bottomed cu, nice easy glides over the Painted Desert. We knew the day would be good, and it was - 50 miles worth.
Remember everything about days you liked, or didn't like, and go look at the variables in the weather maps and make some notes. It would be best if you did your studies prior to flying so that you can see if your predictions were correct. We can either just "hope" we get a good happy flight or we can stack the odds in our favor. Although sites are different, they do have many similarities.
One very common issue in evaluating each day is the strength of the inversion. A clear night, usually found in high pressure conditions, can result in cooler air pooling near the ground, particularly in mountainous or desert areas. Cloud cover, which is generally associated with less stable lower pressure conditions, doesn't usually build much of an inversion as the air under the clouds stays warmer. Within an inverted atmosphere the air actually gets warmer as you ascend some hundreds or thousands of feet, and then generally cools from that point further upward, although there may be more inversions layers at even higher altitudes. There are a few things to consider with inversions.
An inversion can mask the fact that there are strong upper level winds. It's hard to believe, but different temperature zones of air can stratify the atmosphere. You can compare this to descending in a deep pool of water and suddenly finding a different temperature. Different temperature layers of air don't mix very well. Noting that one layer of clouds flows one direction and another layer of clouds is flowing a different direction helps show atmospheric layering. For this reason balloonists can launch and float one direction at one altitude and then, occasionally, float a different direction by ascending or descending into a different strata of air. You might note an inversion simply by looking across the horizon and noticing that the air looks dense with pollution, dust or smoke. If reliable forecasts aren't available, take a helium balloon and track its ascent. Simply hiking/driving to the top of some sites isn't enough of the story because the inversions level may be too far overhead.
If it's blowing hard above the inversion it will most likely, eventually, blow hard on the surface. When the puddle of cool night air warms a mixing will probably occur resulting in the upper level winds sweeping the ground. You can research upper level winds by locating the "Winds Aloft Forecast". Our website www.paraglide.com has easy access to this chart. The W.A.F. lists forecasted winds for the next 6, 12 or 24 hours. You'll note in the attached chart that there are abbreviations for cities within different regions. You will need to select your own region and then determine the abbreviation for your city by going through the NWS (National Weather Service) website and selecting the list of city abbreviations. You may also call 1-800-wx-brief and press #1, and ask the briefer what the winds are doing at 3,000'and 6000' agl (above ground level) for your area. The upper level winds are given initially in increments of 3,000' agl. For example, if your area is at 1,500ft above sea level you'll get information Starting with 3,000ft above seal level and then 6,000ft.
As a side note, we are supposed to check in with the briefer prior to flying to confirm that there aren't any notams (notice to airman) which might have shut down our airspace. This requirement was put in place following the 9/11 disaster. Keep in mind that you need to call from a local phone in order to get the local station. If you use your cell phone you will get the station near your cell "home".
If you discover that the winds in the next "layer" overhead are predicted to blow hard, more than 12 knots (knots are 15% stronger than mph), you may want to launch and land early in the day or wait until the evening. Imagine launching in what you anticipate as mellow air only to find that the thermals are extremely bumpy and it becomes very windy. Without knowing the upper level winds you might make a morning launch on an east facing slope, a slope that is generating a little up slope wind because it faces the sun, but be in real danger. This type of thing is extreme in mountain flying sites like Boulder or Aspen, Colorado, Northern Arizona, Northern New Mexico, Utah and the deserts of California. You might be flying when the inversion breaks up and a strong upper level wind blows through. This can have a number of negative consequences. It can put you in the "lee" or "rotor" side of the mountain, it can make landing at your target area difficult, and there are often some unruly thermals mixed into this wind. On high pressure days air tends to flow more horizontally than vertically, so valley winds can get really strong. Sites that are at or near sea level, or that are affected by a marine airmass often have less inversion related issues. Most of the mountain flying sites in Europe and the Eastern U.S are also generally unaffected by inversion issues.
In the Northern Arizona high desert a strong inversion, which usually relates to high pressure, can result in flows that makes surface winds pretty strong. Our local briefers refer to this as a surface jet. This seems to be more pronounced when a low is within 100 miles. You might not have found any strong upper level winds on the NWS, so this can be confusing. Wait out these winds and you might find that they'll completely shut off as the inversion melts away later in the morning. This particular breeze can be good soaring at some locations, but when it shuts off you might look for a strong upslope wind to take over and some strong thermal releases. In a short while the surface winds often pick up again and usually switch from flowing downslope to flowing upslope. Keep in mind that cold air sinks and warm air rises.
Even if the upper level winds are fairly light you might get bounced around as you descend or ascend through the inversion line. This might occur as you are descending from a high mountain launch that is above the inversion. You might also find, that as you ascend in thermals, that you experience some rough air as you bump into the inversion layer. You might track a thermal up to the inversion level and then feel as though the thermal disappeared. Widen your search pattern downwind as it's possible a stronger part of the core made its way through the inversion. Watch smoke and you may notice it changes heading due to winds or an inversion and might even flatten out at different altitudes. As smoke creeps along horizontally it might find a place in the inversion where it can continue rising, this may be what thermals experience. Once you find that thermal you may have to core tightly as it may pass through a small opening in the inversion. If you have launched above an inversion and have done well climbing, be cautious about taking on a glide that might cost you so much altitude that you'll end up under the inversion and have trouble climbing back up. When flying on high pressure days try and fly from large mountain sites and get high and stay high.
Inversions indicate high pressure and thermals rising within high pressure are often very sharp edged and can make your flying less enjoyable. This happens because the air is essentially "heavy" it is referred to as a generally descending airmass. Not all dust devils are dragons, but as the pressure increases and the thermal index gets stronger, they can be glider tossing. We'll discuss the thermal index more in the future. Until then, try and pick your favorite conditions every time you fly.