Introduction to weather in the southwestby Had Robinson with assistance from Tom Bird of the National Weather Service
When is it safe to fly?
(SIX EASY STEPS THAT WILL HELP YOU DECIDE IF IT IS SAFE TO FLY)
It depends on the weather, the time of day, the skill of the pilot, his equipment, and whether he is flying a hang glider (HG), a paraglider (PG), or a powered paraglider (PPG). Why stay out of the air? Read this report and you decide. The Ultimate Weather Education site (lots of ads) and COMET MetEd (a true online school) are good places to start your weather education. If you have children and want to help them begin their education about weather, you can check out WeatherWizKids.
If you are interested in becoming a storm spotter, click this link NOAA Storm Spotter or, for more general information on weather awareness, this one. The National Weather Service needs trained people on the ground who can give details of storms (hail size, wind damage, flooding, etc.) that are not available by any other means.
With all forms of paragliding, it is not so simple. Firstly, the weather conditions 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 (for our region). This kind of air doesn't occur more than 10 times a year here. When it does, drop everything and fly!
(6) Steps that can help you fly safely in the southwest
- PG pilots can generally fly safely anytime during the day from late
October until early March.
- From late March to early October, pilots have to be more
concerned about thermal intensity and accompanying shear turbulence
during the heat of the day. Why can thermals be so intense here
(crazy)? It has to do with the night and day temperature
difference which, in the desert, is often extreme. Six inches off the ground in July the temperature of the air can be 140F. At night, the temperature can be 70F. That is a
difference of 70 degrees! This difference powers the intense thermals we experience in this region. Generally, a pilot may safely fly the
periods Sunrise + 3 hours; Sunset – 2 1/2 hours. For example,
on June 15th sunrise is around 6:15AM and sunset is around 8:15PM.
Therefore, a pilot might choose not to fly after 9:15AM nor launch before 5:45PM.
More experienced pilots might add an hour or so to the times given here.
These times are roughly for the hottest part of the season and can be
modified at the other times.
- If it is overcast or there is substantial cloud cover, thermal
strength will be much less, even negligible. The "5-5 Rule" (no
gusts greater than 5 mph in less than 5 seconds) is an excellent rule. Strong thermals out in front of launch will
cause blocking of incoming air and pilots can know how
strong the thermals are. The more laminar the air is at launch,
the weaker are the thermals you will encounter in the air.
- If things like
clouds are present, DO NOT LAUNCH. A good rule concerning virga is: if you can see it, stay at launch or get to the ground ASAP. Most of the time virga has no effect on us but why
take chances? There is a difference between virga and ordinary precipitation. Also, take a look at Dixon Whites article on desert air.
- If the
or high velocity winds are high in the atmosphere, thermals can cause this dangerous air to mix with the air below and cause turbulence,
especially if there are pressure waves moving
through the atmosphere.
It is best not to launch, especially if you are in the
mountains. Our experience flying in the southwest has
proven that the Jet can cause serious turbulence. If there is a substantial inversion (you will have to know how to read the balloon soundings) between you and the Jet, you will most likely be OK and
Jet turbulence will be isolated
above the air mass you are flying in. When in doubt, call your local National Weather Service office. It is the job of NWS meteorologists to help the public with questions about
the weather that could endanger lives or property.
Meteorologist, Tom Bird, gives this good advice about the Jet:
The Jet is more of a concern when: 1) most days in the warm season when there is no inversion (or it breaks) and we get deep mixing...which causes strong winds aloft to surface. 2) in the cool season when an upper-low is passing causing instability to transfer the momentum aloft downward toward the surface.
- If you get past the above (5), then do the following:
a.) El Paso National Weather Service Read the 7-day forecast. It is good to know if changes are in store, like a sudden appearance of a front which can mean turbulent air.
b.) NWS hourly graphical forecast Look at the wind speed, direction, and temperature trends over the day. Stay OUT OF THE AIR if there is gusting forecasted when you want to fly. If gusting is forecast, it generally means some disturbance (trough or ridge of pressure) is passing overhead. If winds are more than about 12, you can expect substantial and dangerous turbulence on the lee side of thermals. High winds, in general, will cause weak thermals to break up = turbulence.
c.) NWS Raw Atmospheric Soundings These are the real-time balloon soundings and not a forecast of what's going on in the atmosphere. Here is the explanation of how to use this critical information. If you see high winds (>25 mph) anywhere less than 18,000' MSL (510 mb) you might want to stay on the ground because of turbulence caused by mixing of the atmospheric layers. If there is an inversion between you and the high winds, you are less likely to experience mixing. However, everyone who has flown in these parts for 10+ years will tell you that there are still completely inexplicable atmospheric events that can be hazardous.
d.) Meso West Region (for our region, we customized this link) This is one of our most helpful sites in seeing what may be ahead in the short term and knowing the weather conditions of dozens of stations in our area. To see what is going on in our region: Set "Network" to "All Networks", Overlays 1 & 2 to "Current Wind Speed" and "Current Wind Gust", respectively. Set the Radius tab to 150 miles. Click "Refresh Map". Zoom in to see what is going on in your particular area of interest. To see the trend over 24 hours, click on the desired weather station and then the "wind" tab to see how the speed and gusts are trending. Click on the "Vector Wind" tab to see the trend in wind direction. Station Santa Teresa NWS R (our local NWS station) feeds data to the Internet every minute. For example, if you wanted to know if things are calming down and you have a current strong east wind at your location, go to the Guadalupe Pass (KGD) station and see if things are getting stronger or weaker. This station is very sensitive to any wind changes and you will see the trend easily here. However, it has no history so you have to have been watching it a while. For south central New Mexico and west El Paso always check the NWS Santa Teresa Station, as well. Note: the data at some sites may be an hour old nor do some have a history of reported conditions.
e.) OPTIONAL Study WindyTV It is particularly helpful with its forecasts. Take a look at the three different models (NAM, GFS, and ECMWF). If they are different, it basically means that each of them could be right, depending on local conditions. When conditions are light and variable, wind direction and speed can be all over the place. Another site widely used by meteorologists is Pivotal Weather where you can choose among various models and see a national map of where the high and low pressure systems are and where they may be moving. These systems are what drive the winds we experience near the surface.
f.) OPTIONAL Subscribe to XC Skies and check what the thermals will be doing and, especially, the wind speed and direction at the altitude of where you plan to launch, fly, and land. The buoyancy to shear ratio is very helpful for determining the level of turbulence you can expect. If you do not have a computer, you can listen to the National Weather Service broadcasts. These frequencies can be accessed by many radios. The information is really minimal but better than nothing. Better, pilots may call WXBrief via a cell phone and speak with a trained weather analyst. It is free and the analysts are always helpful.
- If everything looks good to this point, go fly. Always
remember that the forecasts can be wrong. The only SURE data we
have are the balloon soundings and our own observations. For $40
(as of 2014), a pilot can go by Party World and pick up a helium
cylinder, some balloons, and be your own weatherman (sort of). It is amazing
what we have observed sending up our balloons over the years. Read on for some general information on what constitutes "safe
conditions". Remember that you must make the decision to fly or
not – it is solely your responsibility. If in doubt, STAY ON THE
- Too many PG pilots (especially PPG pilots) are careless when it comes to assessing weather conditions. "Whatever...it looks OK at the moment...."
An atomic bomb of energy – 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. The air is gaining even more upward velocity and is able 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.
Wind Speed & Direction
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. If these winds are strong, they may mix (cause turbulence) above launch and problems in trying to bench up from our mountain sites. Is there an inversion aloft which can isolate the strong air above? The balloon soundings will let you know. 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 because of the accompanying turbulence which is near them. This turbulence can easily collapse a paraglider.
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.
Conditions favorable for PG (free flying)
We like modestly strong thermals that average around 800'/min (the pilot would be going up at 600'/min because his glider is sinking at about 200'/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 800'/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.
Conditions favorable for PPG (powered paragliding)
PPGer's 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.)
Zulu time – what is it?
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.
Rio Grande Valley microclimates
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. If the inversion is higher up in the atmosphere, it can be a capping inversion. Such inversions radically affect the formation of storms.
Two fronts (bodies of air) colliding near Ft. Davis, TX. Warm moist air from the Gulf of Mexico meets cooler dry air from the Rockies. The cool dry air is heavier and "slides" underneath the warm moist air, lifting it. It cools and condenses into clouds/fog/rain. 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.
Below is a Radar Scope image of wind direction and velocity from the NWS weather station in Santa Teresa, NM. The black "dot" is the radar location. The winds coming to the radar are green, the winds going away are red. The intensity of the colors indicates the speed of the air. The white lines with dashes across them indicate the location of major storm cells. The white square at the end of the white line is the current location. The white dashes indicate the movement and direction of the cell over a period of 15 minutes. The purple in the image is scatter of one kind or another and can be ignored.
It is obvious why ultralights stay on the ground when storms are near. Notice the ABRUPT change in wind direction near the storms just west of the radar. The surface winds were strong that evening from the southeast, in the low 20's. The power of the storms changed the direction and speed 180 degrees in mere yards.
The Daily Inversion in the Valley
Unfortunately, weather forecasts usually do not take into account the presence of inversions, especially if they are near the earth's surface. 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 near 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 Transmountain Pass.
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 were re-absorbed.
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.
Guidelines if you wish to safely fly within an inversion (either HG, PG, or PPG)
- Always check the
NWS El Paso Forecast before you do anything. It will let you
know what the trend is for the day and, especially, if there is any
gusting. Gusting can happen anytime during the forecast period so
if the forecast includes gusting that is over 18 or 20 mph, it is
probably wise to stay on the ground.
- If you are flying in west El Paso, go to the
NWS Santa Teresa Station and see what the surface winds are doing.
This station has very accurate equipment and gives minute by minute
conditions. The latest
atmospheric sounding data
will tell you what winds aloft are doing. This sounding
data is only helpful 100' or more above the surface as the lower
readings are not accurate. It is worth checking the
Deming branch of the NWS if the winds are westerly as they are
upwind of us.
- You should not fly in the morning if winds aloft (above 6,000'
MSL for our region) are strong (>35 mph) or are getting stronger.
You will have to check the forecast to see if some disturbance is on its
way that might cause the winds aloft to increase and quickly blow out
the inversion. You can tell when the inversion is getting ready to
dissipate as the air becomes "twitchy." If you sense anything
other than dead air when in the inversion, it is time to land.
The evening is the safest time to fly because the inversion gets thicker/stronger which increases its depth and more safely isolates the pilot from the winds aloft. Usually the inversions here end at around 5,000’ MSL (the same height as the pass at Transmountain) so stay well below this altitude if winds aloft are strong. Remember that as you go west and get near the west rim of the Rio Grande Valley, the inversion gets thinner. That is the top of the inversion is lower. This gives us about 1,300’ of air to fly/play in if we launch from somewhere in the valley. If we launch at turf farm #1, this value can be from under 100' so you would have to fly east to enter the inversion. If you live in another region, the skew-t graph of the diurnal balloon soundings will tell you how thick the daily inversion is. Always check the forecast for the evening – there may be a high level trough coming through which will prevent the inversion from forming because of the presence of high winds. The downside of evening flying in the high desert is that the inversion can be 20' in thickness and a pilot has no idea what the air is doing just above. This pilot once launched just at dusk going into a light wind. But just 25' off the ground, the air above was going the opposite direction and he literally fell out of the air. There was damage to the equipment but, thankfully, no injuries. Had he sent up a balloon, it would have showed the quirky, unpredictable winds that occur in and near an inversion.
- WARNING: Be certain that
higher winds aloft (>30+ mph at 6-8k' MSL) are not forecast at any time! This pilot once had the misfortune of not checking this
important bit of info and launched in the late afternoon inversion. A short time later it was blown out by howling winds above and he wound up
FLYING BACKWARDS for a few miles. How did the pilot save his skin?
He allowed himself to be blown over the valley and then quickly
descended. The inversion was still holding there and he easily landed. He was never in any real danger but he could have been dragged badly had
the inversion blown out everywhere. The good news is – from our
experience – that winds aloft above 6,000’ MSL need to be well over 30
mph or so for the inversion not to hold. Always keep your head and
fly your glider. Pilots have been hurt because of panic.
Here is a short video of the inversion suddenly lifting in the morning (if will help if you know how to read the NWS radar images -- source here is Radar Scope). The winds went from calm to dangerous in less than 2 minutes! It was a tidal wave of air that hit the surface (and then backed off a bit).
- Check the morning balloon sounding to determine how thick the
inversion is and what the winds aloft are.
- Do NOT fly higher than the top of the inversion minus a few
hundred feet unless you know that winds aloft are not strong.
Ordinarily, stay below 4,500' MSL if you are over the Valley. However, it is great fun flying out of the inversion into the
winds aloft and moving rapidly across the earth. Pilots can experience enormous speeds over the ground (>55mph) doing this.
- Come down by 10 AM (MST) or 9 AM (MDT). These times can
be increased an hour or so if there is cloud cover or other
circumstances that mitigate the dissipation of the inversion. On rare occasions, the inversion can last throughout the day.
- If you have doubts or don't understand the weather well enough, stay on the ground. Do not rely on others for making your decision whether to fly or not. You are responsible for your own safety! Do NOT spend the rest of your life like these pilots!
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 launch. Notice the dust being kicked up out on the flats by high winds.
Daily Wind Patterns
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 and in the Rocky Mountains in Colorado. Imagine that the Valley contains a huge lake of cooler air. What does it do? It flows 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 turf 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 pilots must pay careful attention to wind direction before launching.