Thermals & LTA

1. Thermals & LTA Don Day, Jr. DayWeather, Inc. Cheyenne, WY

3. Ballooning - Thermals • Understand the development of thermals • Thermal structure • Thermal behavior • Thermal lifespan

4. Ballooning - Thermals • Identifying situations of high and low thermal risk • Understanding thermal development through addressing stability, time of day/year & location • Combine knowledge of stability, time of day/year and location to make well informed flying decisions

5. Boundary Layer • The boundary layer is defined as that part of the atmosphere that directly feels the effect of the earth's surface.  Its depth can range from just feet to several thousand feet depending on the local meteorology.  • Turbulence is generated in the boundary layer as the wind blows over the earth's surface and by thermals, such as those rising from land as it is heated by the sun, but also thermals associated with clouds. 

7. Thermals ther·mal ˈTHərməl/ adjective adjective: thermal 1. an upward current of warm air, used by gliders, balloons, and birds to gain height. 2. a thermal garment, esp. underwear.

8. Thermals • Thermals • Columns of warm air that rise from the ground when heated by the sun

9. Thermals • Thermals are a product of instability • The height of thermals depends on the depth of the unstable layer • A thermal’s strength depends on the degree of instability

10. Thermal Characteristics • When triggered, hot air rises • If surrounding air is lighter, thermal continues rising • Thermal stops when it reaches temperature of surroundings • Thermal strength depends on difference of temperature between it and surrounding air • The colder the air aloft the more unstable the atmosphere with higher chance for thermals

11. Making Thermals • Conditions for thermals • Sun heats the ground • Little cloud cover • Dry soil • Pools of warm air can form • Light winds or shelter • Thermal triggers • Mechanical disturbance (man-made or natural) • Hot air near the ground has buoyancy • Air above is lighter (cooler or drier)

12. Thermals

13. Albedo • Albedo is the fraction of Sun’s radiation reflected from a surface. An ideal white body has an albedo of 100% and an ideal black body, 0%. Visually we can estimate the albedo of an object’s surface from its tone or color. This method suggests that albedo becomes higher as an object gets lighter in shade.

14. Albedo

15. Soil Moisture - Important • Wet ground favors thermals less than dry ground since wet ground heats more slowly. • Some flat areas with wet soil such as swamps and tidewater areas have reputations for being poor thermal soaring areas. 

16. Terrestrial factors affecting albedo • Soil color certainly affects reflectivity, with lighter colors having greater albedo than dark colors, and hence exhibit higher albedo. Soil texture is also a factor that affects albedo.

17. Terrestrial factors affecting albedo • Vegetative factors • At the most simplistic level, dark coloration provides the greatest absorbtion and hence the lowest albedo. However, leaf shape is quite important, with leaf shapes that are planar providing a higher reflectivity; this effect explains why conifer forests tend to have lower albedo than broadleaf forests.

18. Terrestrial factors affecting albedo • Topographic factors - Overt slope differences • For example, areas of steep slope can be expected to produce lower effective albedo, simply because the angle of reflection forces incoming radiation to endure a subsequent path that is subject to further absorption by secondary incidence and also due to a longer path length of travel for reflected electromagnetic waves.

22. Thermals • Thermals are in motion • Always changing • Impacted by the wind • Moving • Impacted by terrain • Can converge with other thermals and phenomenon

23. Thermals can “eat” inversions

24. Know your Indexes • K – Index • Lifted Index

25. Know your Indexes • K – Index • The K-Index is a measure of thunderstorm potential in meteorology. According to the National Weather Service, the index harnesses measurements such as "vertical temperature lapse rate, moisture content of the lower atmosphere, and the vertical extent of the moist layer."

26. Know your Indexes • Lifted Index – IMPORTANT! • A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability - the more negative, the more unstable the air is.

27. Lifted Index • Know the Lifted Index Number • Positive GOOD, zero or below BAD

29. Sun angle

30. Sun & Thermals • Take into account • Latitude • Time of year • Albedo – soils, terrain, etc.

31. Mountainous Terrain

32. Thermal Forecasting • Estimate solar heating of ground • Cloud cover • Time of year/day • Estimate lapse rate and dew point of air • Actual and forecast soundings • Strength of thermals • Boundary layer depth (top of thermals)

39. Dr. Jack’s Blip Maps • Developed for gliding/soaring • http://www.drjack.info/ • Select RAP & NAM BLIPMAPs • Also BLIPSPOTs