rodbailey

Macro Weather
by Dixon White ( http://eagleparagliding.com )

This is the 2nd article in a series of columns that will review step by step concepts concerning weather and flying pointers. Get together with your local instructor and club to discuss these topics in greater detail. Be sure and expand your library of books and videos. This column will recommend certain books and videos, realize that there may be some ideas that are arguable. Practice the weather concepts daily, even when you aren't going flying. Begin to identify the trends that make for the best coastal flying, thermal conditions or exhaust heat sessions. Give your chums a call who flew on days you couldn't and see how close you can get to predicting the conditions. Be thoughtful about going to new areas and how powerful some atmospheric influences may be in contrast to your home sites. There are some sites that become unruly by 9am in August yet others that can be flown all day. Remember - Practice is the Mother of Skill!!

Approaching "lows" and "highs" have powerful effects on the stability of the atmosphere and the wind intensity and direction. Be watchful of your barometer, information sources and the sky for evidence of a low. The low is basically a "puddle" of cool air descending from the poles into a warmer area which is the "high". Weather maps indicate low pressure zones very clearly with an arced line with dangling triangles. The fine gray lines surrounding the "lows" and "highs" (isobars) indicate how steeply the pressure is dropping. Tightly spaced isobars, let's say every 100 miles, generally indicate a high probability of regional wind flow. So, a weather map showing the jet stream over your area, a "low" and tightly spaced isobars isn't promising. Study the weather maps for a couple of days and you'll quickly notice how systems generally move and be able to anticipate the flying conditions.

If a "low" is approaching over night or early in the morning, you may notice earlier thermal activity. This is because the decreasing pressure and lower upper atmosphere temperatures allows thermals to release easier, particularly with direct sunlight. In the scenario of a "low" approaching late in the day, where you've had heating throughout the day, you may notice a thickening cirrus layer of clouds and that your barometer is on the decline. With this late day "low" you may notice more demanding and erratic thermals and strong windy conditions on the ground. The arrival of midday to late afternoon "low" can be of concern to pilots in some areas of the country because conditions can become very intense. In general, the approach of a low will bring winds from the Southwest.

Pilots looking for soft and easy conditions will find some "lows" where the flying is just fine. When a "low" is slow moving, without compressed isobars, there may not be much regional wind flow. If there isn't much solar heating of the ground the air may be very pleasant, but be aware that even a few minutes of sunlight can start the thermals releasing. Be cautious with a "low" that a storm cell can develop and may create strong lift, sink or gust fronts. A thick mid level (stratus) layer of clouds may keep down the solar heating, but it can hide a towering cu nim (cumulous nimbus - raincloud).

As the "high" builds you may notice winds on the surface, and at altitude, more from the Northeast. The jet stream will most likely be far away, to the North is best, and the isobars will spread apart to over 300 miles between each gray line on the weather map. With the increase in pressure and warming of the upper atmosphere you should notice thermals taking much longer to develop and with ever increasing pressure tighter and more sharply edged thermals once they do release. Many pilots fly during "high" pressure systems as the conditions tend to be more predictable. You'll find anabatic flow up East facing slopes in the mornings that can be very user friendly, to a point. Be aware that thermals are ever building and that a heated area (puddle) reaching just the right temperature will suddenly release its power. If you choose to fly as trigger temperature is reached you may need refined glider management skills in pitch and roll control, and be aware that landing zone conditions can be very unpredictable as thermals lift off and change the localized wind flow directions. Keep in mind that thermals will develop and release earlier in the morning in the Summer than in the Winter as a result of more or less sun exposure.