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Convective currents, also known as thermals, are localized vertical air movements. Thermals form when the ground is much warmer than the surrounding air and that air is somehow vertically perturbed. The warm air congeals into bubbles which rise until they have cooled to ambient air temperatures. Thermals are a few hundred to a few thousand feet wide and have vertical speeds of a few hundred to a few thousand feet per minute. They can develop whenever the ground is warmer than the overlying air, and are particularly prevalent in desert areas in the warmer months, and after the passage of a cold front at any time of year.
Thermals involve both ascending and descending air; for every rising current, there is a compensating downward current. Different surfaces radiate different amounts of heat. Asphalt, plowed ground, rocks, sand, and barren land give off a large amount of heat; water, trees, and other areas of vegetation tend to absorb and retain heat. As a result, updrafts are likely to occur over pavement or barren places, and downdrafts often occur over water or expansive areas of vegetation like groups of trees. These convective currents cause the bumpy, turbulent air sometimes experienced when flying at lower altitudes during warmer weather. The air is most turbulent between 10 AM and 5 PM, when solar heating of the ground is at its strongest. Unexpected bouncing around occurs when you cross the boundary between up- and downdrafts. Hitting an updraft on final approach can induce lift and cause overshooting; hitting a downdraft can cause undershooting and hard landings, as it feels like the air’s support suddenly drops out from beneath your wing.
Thermal convection also brings about an extreme turbulence phenomenon which is a great hazard for skydivers – dust devils. Dust devils are vertically stretched thermal columns which have rotation induced by the movement of air around obstacles on the ground or wind shear layers above the ground. Dust devil formation is favored under clear skies with light wind conditions over very hot surfaces, such as over barren desert areas in summer and in the early afternoon. Once formed, dust devils have no preferred direction of rotation. They move with the speed and direction of the average wind in the layer that they occupy, which is why dust devils seem to “lean,” and why tall dust devil plumes may make abrupt changes in direction once they reach a certain altitude. Dust devils are typically 5-100 feet in diameter, with larger diameters when the ground wind speed is higher. Dust devils have average internal wind speeds up to 34 mph, have average lifetimes of 4 minutes or less, and may reach several thousand feet into the air, though dust is typically only lifted 100-300 feet AGL.Dust devils form with both greater frequency and greater intensity when surfaces are heterogeneous (when there are lots of surfaces with different thermal characteristics in close proximity). While dust devils which pick up debris in desert environments are easy to see, they can be impossible to see if they form or pass over areas like asphalt or grass where there is no debris to pick up. When the wind is out of the west at SDAZ, it is common for dust devils to form over the tarmac and whip east down the wind line into the main landing area. Often the only indication that a taxiway dust devil has just crossed into the main is the unexpected gusting of the wind and the sudden, wild fluctuations of the windsock and tetrahedron –which may be very hard to see when you’re under canopy. Flying through a dust devil can collapse your parachute, leading to injury or death. If you are under canopy and see a dust devil, steer cross-wind to avoid it if at all possible without causing a collision or making a low turn. If you do end up in a dust devil, experts recommend keeping your parachute in full flight for maximum pressurization, using minimal inputs to keep the wing over your head, and preparing for a hard landing (PLF). Your best strategy is to remain vigilant during conditions which are prime for dust devils, and if conditions start deteriorating, stay on the ground.
Dust devil formation typically has an abrupt onset between 10:30 and 11 AM, with a peak around 1 PM, after which dust devil frequency tapers off slowly. Why? Earlier in the morning, a temperature inversion exists between the surface and a few hundred meters above it due to overnight heat loss. After sunrise, the surface is warmed by solar radiation, and thermals rise until they become colder than their environment. The thermodynamic efficiency of these early-morning thermals is very small, because they are very shallow. But they will continue to form and rise until the surface temperature is large enough to produce thermals that can break through the inversion layer – which typically happens around 11 AM. At this point, the depth of the thermals can increase to a few thousand meters, which produces an abrupt increase in the thermodynamic efficiency of both thermals and dust devils. The peak in the dust devil occurrence around 1 PM is due to a peak in the surface heat input concurrent with the presence of thermodynamically efficient convective plumes.
DID YOU KNOW???
The MATADOR study, done in the Eloy desert in 2002, discovered strong oscillations in the surface heat flux and the intensity of boundary layer convection with a timescale of about a half hour. The researchers hypothesized that this is due to the interaction between atmospheric convection, airborne dust, and solar radiation. Intense boundary layer convection produces increases in the concentration of atmospheric dust via “convective plumes and vortices” (thermals and dust devils) – which contribute about 35% of the global budget of atmospheric mineral dust! Then, airborne dust absorbs and scatters solar radiation, producing a decrease in surface temperature and stabilizing the atmosphere. This, in turn, produces decreases in the intensity of atmospheric convection and dust flux. The half-hour timescale is of the order of the convective timescale – the amount of time it takes boundary layer convection to disperse the dust plume and intensify again. So if you’ve noticed that the majority of dust devils tend to come in clusters with calm periods between them, you’ve noticed this effect. This doesn’t mean that you aren’t going to encounter a dust devil if you time which loads you’re on to correspond with this cycle. The plane may end up in a holding pattern for some reason, and dust devils can still form off-cycle!
Further your knowledge:
Further your knowledge MATADOR study in Eloy, 2002: http://onlinelibrary.wiley.com/doi/10.1029/2003JE002219/full
NASA on 2005 Eloy dust devil study: http://www.nasa.gov/vision/universe/solarsystem/2005_dust_devil.html
Dust devil thermodynamical theory: https://www.researchgate.net/publication/249609878_A_Simple_Thermodynamical_Theory_for_Dust_Devils
Field Measurements of Dust Devils (includes excellent chart of observations of characteristics): https://core.ac.uk/download/pdf/78386312.pd