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Weather Data, Part II: Thermal Activity

pterodactal             monarch                    condor

Signs of Thermal Activity, the Structure of Thermals, the Diurnal Thermal Cycle, and Reading the Sky


Signs of Thermal Activity

Signs of thermals include: (1) objects heavier than air moving upward without any apparent expenditure of energy, (2) frequent changes (i.e. over a time scale of seconds to minutes) in the direction and velocity of the wind, (3) dust devils, and (4) particular types of clouds (see below and see next page). Objects moving upward (or remaining suspended at the same level) include thistle down, leaves, soaring birds, soaring butterflies, and gliders. In each case, the energy lifting the object upward is the kinetic energy of the air. One of the easiest ways of locating soaring monarch butterflies is to rely on the presence of soaring birds, such as gulls, hawks, vultures, swallows, or swifts, to mark a thermal and examine the air in their immediate vicinity. If the season, wind, and temperature conditions are favorable and the butterflies are reasonably abundant, they apparently exploit most thermals, including those with birds. Locate the birds, and you locate the butterflies. A wind that changes direction and velocity frequently indicates that thermals are drifting through and past your site. Dust devils are organized cores of plume thermals. Clouds associated with thermals include low to mid level cumulus clouds, and cumulonimbus clouds. Thunderstorms, tornadoes and hurricanes are also associated with strong lift but are best studied from a great distance.

Structure of Thermals

vortex thermalThermals are convection systems that range in diameter from a meter or less near the ground to a thousand meters or more at several thousand meters above the ground. Thunderstorms are giant thermal systems and often have more than one convection cell. Thermals exploited by animals for soaring are considered to occur in two basic types; vortex rings and plumes. The first diagram on the right illustrates a vortex ring thermal, the diagram on the left shows a plume thermal. In each case, straight black arrows indicate wind direction, curved black arrows shows the circulation of air, and red arrows indicate the flight path of a butterfly. A series of crossbars on the red line indicate flapping flight, absence of crossbars indicate gliding, or soaring flight. The diagrams illustrate four important characteristics of thermals: (1) rising air (lift), in the core of the convection system can be surrounded by a larger area of descending air (sink), plume thermal(2) the relative narrowness of the core of rising air requires a soaring butterfly, to circle to remain within the lift and avoid flying out of the thermal into air that is not rising, or even air that is sinking, (3) as the butterfly nears the top of the thermal, the diverging air flow tends to push the migrant out of the core and into descending air, and (4) thermals drift downwind. The diagram on the right also shows that butterflies can fly under a vortex ring without encountering lift or sink. The diagram on the left shows that plume thermals lean downwind because wind velocity increases with altitude. Because to this lean, glider pilots are taught to search for lift upwind of the clouds. Finally, the diagram on the left shows a butterfly gliding down from a higher altitude encountered the thermal and has begun to soar upward. If it links a series of upward soaring climbs with straight glides, always finding another thermal before it reaches the ground, the butterfly would be able to fly for hours without having to flap its wings. low thermal entry The vortex ring and the plume model are both approximations of the actual shapes of thermals and there is no hard line dividing the two. In a sense, all thermals are vortex rings. Even though a a plume thermal appears to consist entirely of a rising column of air entirely sustained by in rushing, warm, surface, air, there is more to the system. A plume thermal is also surrounded by an large area of cooler air gently settling to replace the warm air rushing upward. A plume thermal can be considered a vortex ring attached to the ground. However, the turbulent, chaotic, convection systems do not necessarily follow the rules, and often organize in surprising ways. Near the ground, thermals may lack definite boundaries, blending with local convective circulation between adjacent hot and cool regions (e.g.rock outcrops in fields, fields next to forests, etc.) or they may be organized into clearly defined systems such as dust devils. Monarch butterflies are often able to exploit thermals near the ground. The second diagram on the right indicates how migrating monarch butterflies fly upward to enter a plume thermal from the bottom. Butterflies initially proceeding by cruising flight within 3 meters of the ground will abruptly begin to fly upward, beating their wings more frequently, as they gain altitude. At an altitude of about 15 m, the butterflies begin to soar in circles and continue to gain (or maintain) altitude as they drift downwind. My field data indicate that butterflies engaging in this maneuver manage to soar away at least 95% of the time. It is not know how the butterflies are able to recognize that they are at the base of a thermal.

Diurnal Thermal Cycle

On a typical day under the influence of a high pressure system the morning begins with a clear sky, the ground feels cool to the touch, and there is no wind. The nocturnal inversion is in place with cooler, denser air at the surface. Air temperature increases with altitude up to the top of the inversion layer (altitudes of 100 to 300 m are common), then temperature decreases with altitude.cumulus puffs over mountain As the sun begins to heat the ground, and the ground begins to heat layer of the air immediately above, the inversion starts to become unstable. Around mid morning, Thermal Puffs (i.e. small parcels) of warm, moist, air begin lifting off from the ground. The parcels of warm air are quickly undercut by inflowing cooler air, become detached, and begin rising. Rising puffs cool by expansion, but continue upward as long as they are still warmer than the surrounding air. As long as the inversion remains intact, puffs of warm air cannot rise very far before encountering a layer of air of similar temperature and coming to a halt. As heating intensifies, some of the warmer puffs are able to push past the top layer and into the cooler air above. Once clear of the inversion, the puffs are able to rise much higher before encountering a layer of air of similar temperature. Small, wispy, clouds form when the rising parcels of air are finally cooled to their dew points. Clouds formed by rising puffs lack flat bottoms and are short lived. The small, poorly defined, clouds to the right of the peak in the picture above left are probably thermal puffs. Glider pilots watch for these clouds and consider their appearance an indication to get ready to launch as full scale convection is about to begin. cumulus clouds As the sun rises higher in the sky, accelerating the rate of heating of the layer of air near the ground, thermal puffs become more numerous, larger, and often show obvious vortex ring, or partial ring shapes and the nocturnal inversion becomes increasingly unstable. At some point, usually well past mid morning, the nocturnal inversion abruptly collapses, a normal lapse rate condition is established, and large scale convective activity is triggered. Fully developed plume thermals appear. The collapse of the nocturnal inversion, the mixing caused by thermal puffs, vortex rings, and the generation of plume thermals, finally beings the wind to ground level. The first picture on the right shows typical, fair weather cumulus clouds, often described as Scattered Cumulus. The clouds are the tops of plume thermals and are characterized by a puffy white, piled up appearance, and flat bases. The flat bases mark the level of the dew point. When viewed at close range, the the flat appearing base is usually seen to be concave as the warm (relative to the surroundings), moist, air rushing upward pushes into the cloud. As cool, descending, air replaces the rising warm air, the bases of plume thermals are eventually undercut and the column of warm air lifts from the ground and rises into the base of the cloud. Cut off from a continuous supply of warm, moist, air, the cloud loses its flat bottom and quickly dissipates. cumulus clouds Most medium size cumulus clouds form, grow, and begin to dissipate within about 20 minutes. However, as temperatures near the ground continue to rise, thermal activity and the production of cumulus clouds often increase to the point where the clouds spread and fuse to form a layer of stratocumulus cloud. The second picture on the left either shows cumulus clouds beginning to fuse into a stratocumulus layer. Since the layer is broken, the clouds are often called Broken StratocumulusThe reduction in the amount of light reaching the ground reduces the supply of warm air and slows convective activity. Gaps may then develop in the stratocumulus cloud, allowing more sunlight reach the ground and initiating a new cycle of heating, thermal development, and increase in cloud cover. Weather reports often use the term 'sunny with cloudy periods' to describe this cycling of convective activity and cloud cover. On the other hand, if conditions are such that a stratocumulus layer does not form, then strong thermals will continue to be generated from about mid morning until late afternoon. The second picture on the right shows this condition. Note that once a butterfly is near cloudbase, it should be able to soar for hours as it would encounter thermal after thermal as it makes its way across the sky. As the day progresses, the mixing and heating of the air causes the dew level to rise, development of convective activity usually proceeds in one of two directions, depending upon humidity, temperature, lapse rate, rate of heating, topography, and other factors. Most days in most palces in the temperate zones, thermals become larger and spaced further apart.plume thermals If conditions are right, the thermals may become aligned in rows extending for tens of kilometers, creating long bands of lift with parallel bands of sink on either side. These highly organized convection systems are called cloud streets and, if aligned in the appropriate direction, make it possible to soar in a straight line underneath the clouds for tens of kilometers. However, if the pilot attempts to cross from one cloud street to another, the strong sink is likely to force an out-landing. Of course the butterflies can avoid this by switching from soaring to flapping flight. The other pattern of convective development can be much more exciting. Cumulus clouds can continue to build into towering cumulonimbus, or thunderstorms. Unlike the rather explosive beginning of convection earlier in the day, the decline in thermal activity at the end of the day is a more gradual process.Towards the end of the day, as the setting sun heats the air near the ground less and less, the distance between thermals continues to increase and the velocity of the upward moving air slows and eventually quits. . Eventually, except perhaps for a few thermal puffs, all convective activity stops and the clouds dissipate. This diurnal cycle of thermal development is the reason that the sky is usually clear of clouds in the morning hours and the hour or so before sunset. If is also the reason that migrating monarch butterflies, having apparently spent hours aloft soaring, begin to descend from the sky in the late afternoon or, on particularly good days, just prior to sunset.

As you watch the sky watching for migrating butterflies, take time to study the different types of clouds. Note thermal puffs (vortex thermals), cumulus clouds (plume thermals), stratocumulus, and fractocumulus. If the sky was clear in the morning, but clouded over with stratocumulus by about noon, check to see if the butterflies are still soaring in the weaker lift maintaining the cloud layer by transporting moist air aloft. Periods of overcast skies can provide particularly good viewing conditions for watching soaring butterflies and birds because the light grey background is easier on the eyes than bright whites of clouds or the glare of a cloudless sky. Try to guess whether a particular bit of wispy cloud is just beginning to build into a cumulus cloud, or is the last evaporating remnants of a dead thermal. Whether or not you guessed right will be soon be apparent. Also consider whether there appears to be a sufficient density of flat bottomed clouds (indicating active convection) to provide the butterflies with an abundant sources of lift to soar cross country. With a little practice, you will find that you've become reasonably good at reading the sky and are able to anticipate the breakup of the nocturnal inversion, appreciate the diurnal thermal cycle, recognize cloud streets, and have a good idea where the lift will be found when thermals are working. When you begin to plan a route across the sky, from one source of lift to another, you're already thinking like a glider pilot. It's probably only a matter of time before you start taking lessons. Or perhaps not.