Strong Thermals, Dust Devils, Flying in the Lee
As you advance as a pilot, the conditions you chose to fly in will likely become more challenging, particularly if you aspire to do thermal and/or cross country flying. This type of flying takes a lot of practice, and there are clinics offered to help you progress. This section focuses on conditions and situations to be aware of as you begin flying in more demanding conditions.
Flying With Clouds
Clouds can provide us with valuable information; they are great indicators of wind speeds and directions as well as visual clues about thermal activity, strength and other atmospheric conditions. Cumulus clouds are generated by warm air rising and bringing moisture up to its condensation or “dew point.” Cloud development will vary greatly depending on the humidity. In dry areas, you rarely have clouds unless a moist air mass moves in. In humid areas, there can be abundant cloud development, but their presence won’t necessarily translate to abundant lift. Here are some generalities to follow when looking at clouds. Clouds generally start out as wispy areas and grow until the edges are crisp, then become wispy as it deteriorates. If the clouds are wider than they are tall, the lift is likely moderate. If the clouds become taller than they are wide (Marge Simpson hair-doo), they indicates stronger or extreme lift.
If clouds are growing tall early in the day, there is a good change they will over-develop and become nimbus (rain or thunderstorms). If clouds become nimbus and produce rain or virga (Variable Intensity Rain Gradient Aloft), they can generate a gust front or micro-burst. These can cause winds well over 30 mph with severe turbulence. With excessive lift, you may also experience “cloud suck”, where the cloud has started drawing air in to feed itself.
Anytime you are nearing the bottom of a cloud you should take precautions to avoid being drawn into it. In most airspace categories (see FAR 103), you are to keep 500 ft of clearance below clouds. As you get near this altitude, it is wise to start widening your turn radius to find the edge of lift, this will help you judge when you should leave. If you are in 1000 fpm of lift and are within 500 ft of the clouds base, you are likely to enter the cloud after one or two more turns. Also keep in mind that if the cloud is building as you are coming up to it, it can quickly develop and encompass you. Not only is it illegal for ultra-light aircraft to fly in clouds, because visibility is low, it’s easy to get vertigo and become disoriented making it difficult to control the glider. It may also be turbulent at the base of a cloud or inside it.
The lapse rate is used to quantify the stability of the atmosphere in relation to thermal activity. The air is warmed by heat radiating off the ground. Air becomes less dense, and therefore cooler, as you go up in altitude. This means the air is normally warmer at the surface and cools with a gain in altitude, the rate it does this at is known as the Standard Lapse Rate (SLR) and averages 3.6°F/1000 ft. This is an average and a change in conditions, such as a cold front moving in, will increase the standard lapse rate and the rate of cooling. As a parcel of warmed air (thermal) rises through the atmosphere, it cools at 5.5°F/1000 ft, known as the Dry Adiabatic Lapse Rate (DALR. Less than 50%humidity). As long as our parcel of air is warmer and less dense than the surrounding air, it will continue to rise. This happens when the surrounding air is cooling at a rate faster than 5.5°F/1000 ft.
There are many ways to calculate the “stability” of the day. The methods often vary by region and some are more descriptive for certain areas.
One popular method is the Thermal Index, in which you compare the lapse rate of the atmosphere with the Dry Adiabatic Lapse Rate and create an instability scale. The information you will need for a given day is:
- An upper level temperature at a given altitude. It is often easiest to use the freezing level altitude; temperatures are given in Celsius.
- The altitude and expected temperature where you will be flying (surface).For the temperature, we prefer to use what is known as ‘puddle’ temperature.
This is an estimated temperature of the air mass near the surface which will be- come the rising parcels. One way to estimate this is to get the mean temperature between the actual surface of the ground and the ambient air temperature, since heat collects on surface and radiates up, diffusing along the way.
The freezing level, or 32°F is at 12,000ft
The average surface altitude is 3000ft, with a ground temp of 110°F and an ambient of 90°F, giving us a puddle temp of ~100°F.
There is a difference of 9000ft between our 12,000ft temp and our 3000ft temp. That difference is multiplied by the DALR:
12000ft – 3000ft = 9000ft * 5.5°F/1000ft = 49.5°
So there should be a temperature increase of 49.5°F from 12000 to 3000 ft to be stable.
32°F + 49.5°F = 82°F (rounded)
So 82° is the stable temperature for 3000ft. However, we expect the temperatures to reach 100° at the hottest part of the day.
82°F – 100°F = -18, this is the Thermal Index (T.I.) number.
Translating this number into usable information is difficult; there are so many factors and variances between locations. The best thing is to get the T.I. number along with all the other weather information, and if you go flying you can compare the number to the conditions you see on launch and experience in the air. To give you a very generalized scale, 0 to -10 is mild, -10 to -20 is moderate, and -20 to -30 is high, and over this can be extreme. This information is an extremely valuable tool for modeling the stability of the day and deciding if, or at what times of the day, the conditions will be appropriate for your level of experience.
In many regions, it is simpler to get the temperature at some altitude above you, and the predicted surface temperature. A difference of 10°F or more for every 3,000 ft should be a soar-able and relatively smooth day. 12.5°F or more should be very thermic and 15°F or more will be extremely strong.
Thermals generate turbulence as they push upwards and disrupt the normal airflow. This creates eddies along the thermal boundaries. The intensity of the turbulence will depend on the vertical velocity of the thermals, the volume of air rising, wind, humidity, altitude and atmospheric pressure. If wind is mixed with thermals, you have a mixture of horizontal and vertically moving air, which will be more turbulent. In strong wind, if the thermals are weak, the wind can blow them apart. If the thermals are strong, the wind will move around them, creating an area of rotor on the leeward side (see The Lee and Rotor). Generally dry, hot, high altitude areas (deserts) heat up rapidly and produce the strongest thermals and associated turbulence. Humid areas tend to moderate thermal activity, reducing the level of turbulence.
Atmospheric pressure can play a big role in thermal activity and thermal turbulence. With a low pressure, the atmosphere is generally rising, and it is easier for the thermals to rise with it, creating less friction and turbulence at the boundaries. However, if the pressure is too low, thermal activity can become extreme and lead to over-development or thunderstorms, with severe turbulence. An average pressure (29.92) is generally better for thermal conditions. With moderate high pressure, the thermals can build in volume and the friction turbulence could be moderate. If the pressure is very high, there is more atmosphere pressing down on the thermals, making it harder for them to rise, so they build in intensity. When the thermals do force their way upwards, it is in smaller areas with more distinct boundaries, which can feel ‘sharp-edged’. The exact relationship between pressure systems and thermal development is unclear but experienced pilots will attest to its relationship.
The presence of thermals is a consideration when on your landing approach. The wind directions can switch suddenly, if a thermal releases near you, drafting air into it. If you’re low, you will be landing downwind or cross-wind. You may also be lifted or dropped unexpectedly, if you encounter lift or sink. You may have experienced mild versions of this during your training, but it may be much more intense in strong thermic conditions. The greatest concern is encountering turbulence sufficient enough to cause a deflation, surging, or other issues at low altitudes. For these reasons, as a newer pilot, you will want to be very thoughtful about when you chose to fly and land. If you start flying the first thermals of the day and stay up for an extended period of time, you will likely be landing in stronger conditions, keep this in mind and maybe plan to land early. It may be better to fly late afternoon, when the conditions are declining and the landing is more likely to be smooth. If the landing area is green and/or near water, the chances of encountering turbulence is less than in a hot dry field, something to consider when making your decisions.
The USHPA recommends that P2 pilots not fly in thermic conditions exceeding 200 fpm until they understand the risks involved. You will not be required to practice stalls, spins, or in-air reserve deployments before flying in thermic conditions.
When there is a steep lapse rate near the ground, thermals will release aggressively, and the air rushing in to fill the area below it can create a strong rotation called a ‘Dust Devil’. The presence of Dust Devils can indicate strong or extremely unstable conditions. They can also be present when there are opposing forces, such as wind pushing against the anabatic (thermal) flow direction. Dust devils are extremely turbulent and should be avoided. If there are dust devils occurring at a launch, and you decide to fly, exercise great caution and maintain terrain clearance. These are not appropriate conditions to be scratching for lift at low altitudes or close to the terrain. If you are flying a site with light, powdery dust, the dust devils can be used to locate strong thermals. If you choose to utilize these strong thermals, you should only enter them above 1000 ft off the ground, at 500 ft above the top of the visible dust, and opposite their direction of turn, so you enter with a headwind. Many good thermal and cross country sites will have dust devils. Often they are not visible if there are no light materials to be lofted.
The Lee and Rotor
The lee (short for leeward) is the backside, opposite the windward side of an object. The term lee-side and “rotor zone” may be used interchangeably. Both terms refer to areas behind an object which is obstructing the wind flow. As the speed of the wind and roughness of the obstruction increases, so does the amount and intensity of the turbulence. If the wind speed is relatively low and/or the terrain is smooth or rounded, the level of turbulence will be relatively low. Conversely, if the winds are high and/or the terrain is rough, such as a sharp ridge or mountain, the level of turbulence in the lee will be high. You may hear pilots talk about flying in the lee, but you should understand the serious risk involved before attempting it yourself. Thermals in the lee side are protected from the wind and have a chance to build, but when they do release, they are pushing up into an air mass that is moving horizontally, which may create severe turbulence and shear. Also, if you fly into this area looking for lee-side thermals, you are flying into an area of mechanical turbulence.
Rotor is the reason it’s imperative for you to know the direction of the winds around you, as well as the wind at the surface. Knowing the direction and speed of the forecasted winds will keep you from launching into lee-side conditions, or flying into rotor at some point during your flight. When flying in the mountains, be especially cautious, around ridges and valleys, the opportunities for encountering rotor are greatly increased. It is easy to get wind forecasts from weather sources and/or by watching the drift of the clouds, so there is no excuse for being unaware.
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