paragliding training center
Central Texas Region
by Had Robinson with much assistance from Tom Bird and our weathermen at the National Weather Service – thank you.
It depends on the weather, the time of day, the skill of the pilot, his equipment, and whether he is flying HG, PG, or PPG. Why stay out of the air? Read this report and you decide.
Generally, HG pilots can fly anytime of day if the winds/weather are safe. Thermal strength is generally not an issue as HG handles negative G's and turbulence much better.
With all forms of paragliding, it is not so simple. Firstly, the weather/winds have to be safe. That is, they must be within the pilot's comfort margin and skill level. His flying equipment must be appropriate for the conditions. Secondly, the pilot must consider the strength of the thermals at the time he is in the air.
Soaring above the desert near Santa Teresa, NM.
We had one of those special days for our region! Winds were light, the
atmosphere moderately unstable, and there was moisture in the air! Air
was going up everywhere and it was easy to thermal to cloud base (around
10,000' MSL). Thermals were big, fat, powerful, and smooth. This
kind of air doesn't occur more than 20 times a year here. When it
does, drop everything and fly!
A powerful, towering cumulonimbus cloud forming over
the Franklin Mountains in El Paso, TX. Ordinarily, moist air is heated
near the ground, rises, and condenses completely into a cloud at the
altitude known as cloud base. Cloud base is the altitude where the dew
point and temperature of the air are the same. However, here the
conditions are just right and there is plenty of warm moist air that is
still condensing into water droplets at cloud base, releasing more heat, and
gaining even more velocity to penetrate the colder air above (the warmer a
gas is, the more buoyant it is). It will keep moving up in a vicious
cycle, condensing, releasing heat, and going higher until there is no more
condensable moisture in the air mass. Wind velocity inside these
monsters can exceed 100 mph and the cloud can top out at over 60,000' high.
When the rain droplets/hail get heavy enough to overcome the upward wind
velocity, we have precipitation that reaches the ground. Pilots have
been sucked into these clouds and trapped – with often unhappy results.
Even heavy aircraft stay away from them.
Surface winds should not be more than 17 mph for foot launched PG, much less for PPG, and even less flying a trike/quad. These are only estimates – it's up to the pilot to decide what conditions he is comfortable in. Always be sure to check the winds aloft before launching as we need to know what's going on above us. In particular, winds aloft may be 180 degrees different than at the surface and this will cause turbulence above launch and problems in trying to bench up from our mountain sites. Generally, our best air is from the WSW at the surface and the direction is within 30 degrees or so for the next 10,000' AGL of altitude. If barometric pressure is dropping or low, we can expect more buoyant air which is more fun to fly in.
If there is 50% cloud cover, for example, thermals will be suppressed and we can enjoy calmer air all day long. However, the clouds must not be towering nor threatening rain, virga, or hail. Sunny clear weather pretty much guarantees the presence of thermals and turbulent air so we have to be careful what time of day we launch. Inversions in the atmosphere almost guarantee that thermals will not rise above the top of the inversion. As the day continues, the air near the ground will be turbulent everywhere as the hotter shallow air mixes with the air aloft. In the fall and winter, the heating of the sun is much less and thermals and turbulence will be weaker. It is often possible to fly all day during this season. We like thermals but ones that are not too strong.
Cirrus clouds over El Paso – these clouds were
formed hundreds of miles southwest of us and were subsequently subjected to
mixing from high winds aloft and were shredded.
we like modestly strong thermals that average around 600'/min. When the air is stronger than this, we risk a collapse of our glider because of turbulence in the vicinity of the occasional thermal which may be much stronger than average. Low barometric pressure encourages thermals to be more organized and smooth, rise more easily, and gain more speed. There is less turbulence near thermals during low barometric pressure conditions. If the lapse rate (the normal cooling of the atmosphere with altitude) is also "steep", it will guarantee that the thermals will ascend to great altitudes in the troposphere and a pilot will be able to also get very high. Inversions (where the air becomes warmer with altitude) can stop the rise of thermals like a brick wall and, consequently, can limit the height which a pilot can also rise. Some thermals are so strong that they can penetrate an inversion. If a pilot is lucky enough to find such a strong thermal, he can go right up with it and leave other pilots thousands of feet below.
With high barometric pressure (fairly normal for the southwest), thermals tend to be sharp edged with great turbulence between areas of ascending and descending air. This vertical shear often causes collapses in a paraglider as the pilot moves through them. They usually spontaneously recover but are not an enjoyable event when they occur. While the average strength can be 600'/min, for example, an occasional "boomer" can drift through which can be much, much stronger than the average (1,200'/min, for example). There are tools, like XC Skies, that can help us estimate the average strength of the thermals for the day. If we fly too early or too late, we will not find lift. If we fly in the middle of the day during late spring, summer, and early fall, we may find thermal strength to be excessive. What is excessive for one pilot may be too little for another – this is what flying skill and knowledge is all about. If we can find ridge lift (when air hits a vertical surface, like a cliff) we prefer to fly either very early or very late in the day as we do not need the presence of thermals to stay up. However, it is never wise to fly in thermals and ridge lift at the same time as the ridge lift tends to concentrate or merge thermals.
we usually prefer calm air which means launching at dawn or in an inversion. Typically here in the desert, the air does not get excessively thermic during the late spring, summer, and early fall until about 10AM, daylight time. This can vary, however, but pilots can tell when the air starts to get active and they can then choose to land. Things start calming down around 5:00PM. The air is always the calmest at dawn. Modest thermic activity will make the air turbulent but it is not dangerous. If you never want to experience a collapse, fly at these times. If you fly at dawn and are also within an inversion, you will experience the calmest air possible. (See below for info on flying in an inversion.)
The time of all weather related info is often given in "Z" (zulu), "UTC" (universal time coordinated) For example, it is (6) hours later than MDT and (7) hours later than MST. Here is an easy to read chart published by the NWS of U.S. local times converted to UTC time. GMT time can be different than UTC because the former time zone observes daylight savings time.
Altitude and air pressure are often expressed in millibars. Here is a millibars to feet conversion table. Here is a millibars to inches of Hg. conversion table (barometer readings). Go here for an explanation of weather symbols and how to do a surface weather analysis.
There are at least (4) micro-climates in south central New Mexico and west Texas. Throughout the year we typically have a daily inversion that forms in the Rio Grande valley on the west side of El Paso. An inversion means that we have a still body of air to fly in that can be easily 1,000' thick or more. These inversions are very resistant to disturbances from layers of air that are moving above them. East of the Franklin Mountains, the inversions are not as strong and are of much less significance for us ultralight pilots. The mesa west of the Rio Grande valley rim is still another climate zone. As we move away from the valley the inversion gets thinner (and thus weaker) than in the valley. There is a daily inversion out in the desert but it is even weaker and thinner than in the mesa on the west rim of the valley. It is often less than 50' in thickness. The raw atmospheric soundings will give you a good idea of how thick the inversion is (if any) for your particular region.
Two fronts (bodies of air) colliding near Ft. Davis,
TX. Warm moist air from the Gulf of Mexico (lower layer) meets cooler
dry air from Rockies. This sort of event is hazardous for soaring
pilots if they are flying in the vicinity. The shredded clouds
indicate high winds at that altitude. It would not be fun to be flying
an ultralight anywhere near this sort of weather.
Unfortunately, weather forecasts do not take into account the presence of inversions. Who really cares – except us ultralight pilots who fly in them? This is why the forecasts may say winds are "such and such" but you step outside and it is calm. Late in the afternoon, even while the sun is still up, the earth rapidly cools off by radiating heat into space. This is what forms the inversion – a layer of air near the ground which is much cooler than the air above it. We learn in physics class that liquids or gases of different temperatures do not tend to mix. As the body of air cools near the ground, it becomes more stable and becomes isolated from the air masses above it. The inversion typically lasts until mid morning the next day and is about 1,300' thick (from the ground at about 3,700' MSL to over 5,000' MSL). It can be easy to see an inversion because the air within it traps pollution. This is why there is usually a thick smoky haze over the valley and also over Juarez every morning. Where the haze/smoke ends is the top of the inversion. Inversions are useful events for us because they stop strong winds aloft from reaching down through the inversion and affecting our flying.
What shape is the inversion? My experience flying in inversions suggest that they have a flat shape but curve down and thin out at the edges. The eastern edge of our daily inversion is the Franklin mountains and the western edge is a mile or two west of the Doña Ana County airport. If you get west of the valley and up on the rim, you can see the haze which usually ends just below the pass at Transmountain – which is just over 5,000' MSL. The area east of the Franklin Mountains rarely experiences a significant inversion.
Below is a typical inversion over the Rio
Grande valley (not visible, far in the distance, and just in front of the
mountains). The inversion is particularly visible because of the
immense amount of smoke generated by the city of Juarez, Mexico which mixes
with the much cleaner air found in El Paso. The inversion is isolated
from the air above it because it is so much cooler. The inversion
appears weaker towards the left of the photo but that is because of the
presence of less pollution from Juarez. The depth of the inversion is
typically 1,000' to 1,500' in thickness from the base of the valley.
Today it is about 1,400' as we can see the top of it which almost reaches
Why do we care about this daily inversion? It means that winds aloft can be some speed and direction but the air near the ground within the inversion is calm, particularly in the Rio Grande valley. If you study the soundings and forecasts carefully, in particular upstream (up wind) of where you would like to fly, you can predict when it is safe to fly even when winds aloft are strong – and potentially unsuitable or unsafe to fly in.
When winds aloft are relative light or moderate, the inversion is a nuisance, especially if we are interested in thermalling away that day. This is because the inversion traps everything in it, including thermals. Thermals will go up in an inversion and, when they reach the top of it, will abruptly stop. My experience in ascending within a thermal that formed in an inversion is that there is sudden turbulent air when the thermal reaches the top of the inversion. Imagine squirting a garden hose at a brick wall. It goes along fine until it hits the brick and then it goes everywhere. It is just the same. If you are not expecting this turbulence it can be frightening. The best thing is to just get away from the thermal as fast as possible. If you are very lucky, the thermal might just be strong enough to punch a hole through the top of the inversion – something I have yet to experience.
Line clouds over the Franklin Mountains in El
Paso, TX. This event was caused by warm moist air coming from the east
moving west and being pushed up and over the mountains. As it went up,
the temperature dropped below the dew point and clouds formed. As it
continued on, the air mass dropped down on the other side and the clouds
Winds aloft are usually from the west or southwest. If winds near the surface are moderate or strong from the east, for example, you can expect gusting when flying. If the Jet stream is overhead, gusting can be particularly dangerous if there is no inversion that can protect us. For example, I once hiked all the way to the top of Mt. Riley expecting to enjoy the good flying conditions that day but the Jet was overhead. I spent about 30 minutes at launch just studying the air. It was coming in at a steady 10-14 mph from the southeast. And then – wham – a 35+ mph gust from the Jet hit the summit of Mt. Riley. I packed up my glider and hiked back down.... It was a simple and easy decision.
Generally, if there is ridge or trough moving overhead (gusting forecast) do not plan on flying that day, especially if the Jet is overhead which will increase the turbulence of the air below. You will often encounter mild to moderate turbulence flying through an inversion layer into winds aloft or back into the inversion, especially if the wind direction of each layer is different. If you have no idea what is going on in the atmosphere, you might be frightened at encountering turbulence "out of nowhere." That is why we should not be ignorant of the weather – it really is not rocket science. Suffice it to say, your personal safety depends on knowing what is going on.
Cumulonimbus forming over Dry Canyon in Alamogordo.
The dark bottoms of these clouds indicate that they are towering and could
easily suck up a pilot! We all happily watched this while at the DC
Light winds from the north are common in the early AM in the valley. These winds are a result of the daily inversion which forms for hundreds of miles in the Rio Grande river valley. Imagine that the valley contains a huge lake of cooler air and what does it do? It flow downhill, just like water. In fact this effect can be quite strong in some areas, particularly near the lakes about 100 miles north of us. Winds there are hardly light and sometimes reach 15-20 mph. Like here, they suddenly end around 10AM as the inversion dissipates. If you drive north on I-25 you will see warning signs about gusty winds posted next to the deep canyons that the interstate highway traverses. At the end of the day, sudden cooling of the air in the mountains can unleash a torrent of air that races down these canyons towards the Rio Grande valley. To a lesser extent, we experience this katabatic flow when we fly late in the day in the Franklins. Often, the winds near the base of these mountains will reverse – going from east to west. This is why it is good to have windsocks in the LZ and pay attention to them.
Going to the sod farm on the west rim of the valley, the early morning winds may be different still. They may be from the west as the cold night air continues its flow, draining towards the valley from the surrounding desert mesa and the Potrillo Mountains. Think of it all as a river of air flowing from higher to lower elevations until the sun's heat dissolves the inversion allowing the winds above to mix down to the ground. Practically, it means that we often must do two or more setups at the sod farm – one early and then one around 9AM. If there is a wind change forecasted for higher up in the atmosphere, we may have to work around that, too.