[ HOME ][ Reading Room ]

OBJECTIVE FORECAST METHODS TO EVALUATE
THERMAL CHARACTERISTICS FOR SOARING FLIGHT

PART TWO

INTRODUCTION

In last month/s installment, we examined the origin, data sources, data issues, calculations, and preliminary verification of the Thermal Index (TI). The TI is computed to forecast the likelihood and quality of thermals to be exploited by soaring pilots as they continue their relentless, holy quest for pegged fun meters. In this month/s issue of Cloudbuster, we will examine the two key values of the TI, 'Top of Lift' (TOL) and -3 TI height, along with the calculations for the average expected Lift Rate (LR) and Temperature of First Usable Lift (TFUL).

REVIEW OF TI AND THERMAL FUNDAMENTALS

Recall from last month/s article, that the TI is a quantitative index of thermal strength that/s been developed and refined by sailplane pilots. It/s the arithmetic difference between the environmental air temperature and the temperature of a rising surface air parcel, or thermal. When computing the TI, the surface parcel's starting point is usually taken as the expected maximum afternoon temperature. The environmental vertical temperature profile is taken from numerical weather prediction model data, or from the morning upper air balloon flight from a nearby rawinsonde station.

Thermals are a byproduct of strong differential heating of the Earth/s surface by Ol' Sol. This heating causes some air parcels to become less dense than other, nearby parcels. When the flying area is in under the influence of HIGH pressure at the surface, subsiding air from aloft acts to dislodge these buoyant surface parcels, allowing them to freely ascend without any additional input of energy. As the unsaturated parcels rise, they expand and subsequently cool at the dry adiabatic lapse rate (DALR) of ~5.5°F/1K' (9.8°C/km).

HEIGHT OF THE -3 TI

Success in hang gliding is often measured by how much altitude was gained during a flight, and/or by how many upper surface inspections were made. Steep lapse rates approaching the DALR and a deep mixed layer are essential ingredients for this kind of 'sky out' success.

Sailplane pilots have observed that for a given maximum afternoon temperature, the altitude they will most likely reach will be the height of the -3 TI. The height of the -3 TI is where the arithmetic difference between the environmental temperature and the parcel temperature is -3. When cumulus are present, the -3 TI height is usually very close to cloudbase. Needless to say, the higher the -3 TI, the better the soaring day.

The synoptic weather conditions that favor a high -3 TI day occur when surface temperatures are very warm and temperatures aloft are cold. These days typically occur in the late spring when the increasing angle of the Sun/s rays warm the atmosphere/s lower levels, yet the upper atmosphere is still cold from the winter. This 'cold over warm' combination produces steep DALRs and deep mixed layers up to ~10K' in the eastern United States. To a lesser extent, the same type of conditions are present in the autumn, although due to the shorter length of day, the surface does not usually get as warm, nor is the upper air as cold because summer/s heat is still present.

TOP OF LIFT

A thermal will continue to rise beyond cloudbase until it reaches a point where its temperature is equal to that of the environment. At this point, the TI = 0, which is referred to as the 'Top of the Lift' (TOL). Since the temperature difference between the environment and the parcel is now zero, any further ascent by the parcel would place it where it would be colder, or more dense than the surrounding air. By comparison, the parcel would now heavier and it would sink.

Since TOL is the point where the thermal stops rising, on a cumulus day you can observe that point as the top of the dry, convective cloud. The difference between the -3 TI height and the TOL height is usually the vertical extent of the cumulus. A large calculated difference in height between these two measures might be a good indicator a cloud/s 'suck' potential, since it/s the taller, fair weather cumulus that tend to have the more vigorous and long lived updrafts.

During a thermal/s ascent, it will have acquired some momentum, but its magnitude is minimal and can be ignored. This momentum can be observed in the cumulus cloud top where small cloud 'arms' appear to curl over before evaporating. This cloud 'arm' feature is called a Kelvin-Helmholtz stability wave and the direction of the curl will tell you something about the wind direction at that altitude. Where this type of momentum can be a significant factor is in thunderstorm updrafts, where it will often produce an overshooting top as the top of the cumulonimbus breaches the lower reaches of the stratosphere. Thunderstorms with overshooting tops frequently produce violent weather in the form or high wind, large hail, and/or tornados.

LIFT RATES

The basic premise for computing the Lift Rate (LR) is, the higher the thermal rises, the stronger the updraft. There are sailplane pilots who have collected enough data over the years to empirically derive a linear regression equation that estimates the average LR based on the TOL height. There are regional variations of the basic equation, but their results are within ~50 '/" of each other. To calculate the expected LR in the East, we compute the arithmetic difference between the TOL and a constant (3000'), multiply that quantity by 0.0485, and then add 200. [LR = 200 + 0.0485*(TOL - 3000)]

A rough LR calculation can be easily made using a forecast for the maximum afternoon temperature (T) and minimum afternoon dew point temperature (Td). Once you have these values, find the absolute, arithmetic difference between T and Td, multiply that quantity by 222.22 (or divide it by 4.5) to derive the TOL estimate. From there, it/s all plug and chug into the remainder of the equation described in the preceding paragraph.

The forecast temperature and dew point information for this calculation can be found on the WRHGC Soaring Weather page.

in the row marked MOS. These data bulletins are produced every 12 hours with three hourly forecast projections for wind, temperature, and dew point valid through 60 hours.

Spring 2000 pilots reports of observed climb rates seem to suggest that the maximum, coreable LR is about three times the calculated LR, although blanket use of this multiplier has not always verified. Pilots are encouraged to relay their LR flight data to the forecaster for use in the ongoing refinement of soaring forecasting techniques.

TRIGGER TEMPERATURE

Some days the lift turns on early, some days it turns on late. XC pilots might benefit from knowing about the early turn on days, whereas late night revelers might benefit from knowing about the late turn on days.

The trigger temperature, or 'Temperature of First Useable Lift' (TFUL) is the surface air temperature that produces the first good thermal. This calculation, as currently reported, is somewhat arbitrary in that it requires a preselected altitude for TOL. The Weekend Soaring Forecast uses 4K' as the TOL for purposes of calculating the TFUL, which, based on observations at Manquin this spring, may be a bit too low. By matching the TFUL to the three hour temperature forecast from the MOS bulletin, the trigger time can be estimated, as well.

There are other, albeit real time, methods that seek to determine when to launch so that the likelihood of staying aloft is high. Once sailplane pilots are on the field, they will sometimes be able to estimate when its time to launch by observing when the dew dries, then adding 30 minutes. This technique probably has less applicability to foot launched hang pilots because of the difference in launch locations, i.e., mountain v. valley, however aerotow pilots may find it useful. Another reliable technique is to wait 30 - 60 minutes after the first shards of cumulus appear over, or in the vicinity of the launch site. Both 'delay' methods inherently allow time for the planetary boundary layer to begin cycling thermal plumes frequently enough to increase the pilot/s probability of finding a good one, thus avoiding in indignity of breaking down after a sled ride, while everyone else is fading fast into the deep blue sky as little specks.

SUMMARY

Even though it/s often been said, 'If you don/t go, you don/t know,' there are times when having a little soaring specific weather forecast information available can get you heading for the hills toward a boo-wah flying day or bagging it so you can stay home and earn some much needed family points.

The wind velocity and soaring indices contained in the Weekend Soaring Outlook and Forecast are intended to relay the latest weather forecast information so WRHGC pilots can make informed decisions about flying conditions, avoid fruitless hours of hang driving, and in some cases not squander their hard-to-earn, easy-to-burn family points. 'Apres vol libre' pilot reports are greatly appreciated and contribute to improved forecasts, which in the long run, benefits everyone.