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XC Maps Parameters

Thermal Parameters

Top of Usable Lift

Measured in feet or meters above MSL (mean sea level), this parameter is commonly the most looked at by paraglider and hang glider pilots. The top of the usable lift is how high a glider could potentially thermal as the vertical velocity slows down near the absolute top of thermals. This accounts for a glider sink rate of about 250-300 feet per minute. This parameter does not consider the lapse rate changing due to cloud formation before the top of the usable lift is reached. In very moist parcels of air where condensation (cloud) occurs before the top of usable lift is reached, the lapse rate will change from a dry adiabatic lapse rate (DALR) to a moist adiabatic lapse rate (MALR). Because of the uncertainty of cumulus cloud formation within forecast models, it is not reasonable to attempt finding the true top of usable lift, which can at times be thousands of feet higher, and typically in large cumulus nimbus cloud formations (in which case you would not be flying...hopefully). The "Cumulus Cloud Depth" parameter will give some type of indication as to the potential for deep cumulus cloud to occur, along with looking at the "Precip/Thunder Storms" parameter. The Top of Usable Lift is not always "The minus 3" Thermal Index equivalent. In fact, the -3 thermal index often poorly defines where the top of the usable lift will be. Read more about this issue here

Top of Usable Lift Above Ground
This parameter is the "Top of Usable Lift" minus the ground elevation for each given 1km area. This is a useful parameter when considering cross country routes across terrain. You may find that one route over another in a downwind direction gives you more "clearance" over the ground, thus increasing the potential to fly further and providing a little more safety.

Thermal Tops (Thermal Idx=0)
This parameter is where the absolute thermal top is likely to be in dry air. Again, this does not account for moist adiabatic lapse rates and so can be grossly understated in moist areas where large cloud formation occurs and over-development ensues. This is commonly where the Thermal Index (TI) is equal to 0, which simply means that the parcel of rising air has reached the same temperature of the surrounding air. Where the TI
equals zero is often not the true top of lift depending upon the depth of the boundary layer. With a deep boundary layer, the velocity of some thermal updrafts reach the boundary layer (TI=0) and still have enough momentum to continue upwards even more. "Punching through" the inversion and broken or elongated haze domes are good evidence of this phenomena.

Thermal Height Variability
This parameter tells you what the top of the lift would be if the surface was just 2.2 more degree C (4 F) warmer. This characterizes the sensitivity of the air mass over a given location. Where you find areas of uncertainty, you can assume that the Top of Lift and Top of Usable Lift are likely to be slightly different in either direction--higher or lower. This parameter can give you an indication of over-development as well. When a small increase in temperature gives a top of lift value thousands of feet higher, you might expect to see large cloud formation even if the air mass is relatively dry near the surface.

Thermal Updraft Velocity
This is the average thermal strength or upward velocity, usually measured in meters per second or feet per minute. All thermals are different, having different sources and triggers, which also implies different localized surface temperatures. Because hang gliders and paragliders can turn in such small areas, we experience the very essence and micro characteristics of each and every thermal. A core will be stronger with turbulence near its edges, yet the entire parcel of air may be rising, and quite possibly have multiple cores.  This parameter is simply the maximum updraft average of the entire thermal. There will certainly be weaker and stronger updraft velocities for any given location and time.

XC Potential
This parameter attempts to identify the locations where XC is possible based upon a comparison of 4 different criteria. They are the following:
  1. Red. Buoyancy/Shear is greater than 6, average wind speed within the boundary layer (ground through thermal tops) is 10 MPH or less, thermal velocities are greater than 490 feet per minute (2.5 M/S), and the top of the usable lift is greater than 2,000 ft. above the ground.
  2. Orange. Buoyancy/Shear is greater than 4, average wind speed within the boundary layer (ground through thermal tops) is 14 MPH or less, thermal velocities are greater than 490 feet per minute (2.5 M/S), and the top of the usable lift is greater than 2,000 ft. above the ground.
  3. Blue. Buoyancy/Shear is greater than 4, average wind speed within the boundary layer (ground through thermal tops) is 18 MPH or less, thermal velocities are greater than 490 feet per minute (2.5 M/S), and the top of the usable lift is greater than 2,000 ft. above the ground.
The attempt to characterize good XC areas is of course subject to interpretation. One pilot's concept of good XC conditions is not necessarily the criteria for another's. Nevertheless, this parameter gives a quick review of regions where wind speed is low enough to not break apart thermals, and where that wind is not too strong. For those who are after huge XC distance, this parameter will likely not identify those regions due to a search criteria of relatively light winds aloft areas.

Cloud & Weather Parameters

Cumulus Cloud Base
Finding cloud base is tricky due to deriving realistic dew point values from the model output. In addition, as thermals rise and mix with the surrounding air, the entrainment process will change the moisture characteristics of the rising air mass, making cloud base even more of a moving target to predict. With that said, this parameter tries to identify where cloudbase is likely to be when the surface moisture is lifted adiabatically to condensation (dew point). The parameter has validated well in dry areas across the planet, such as the U.S. Southwest, and poorly in more humid and lowland areas. We are working on providing better parameterization with cloud base, so use the current results skeptically.
 
Cumulus Cloud Depth
This parameter represents how deep cumulus clouds might be if cloud base exists below the absolute Top of Lift. When dew point is reached and clouds form, if the thermal is still rising, the lapse rate will change from a dry adiabatic lapse rate (DALR) to a moist adiabatic lapse rate (MALR). We do not attempt to apply a MALR to the rising parcel and assume where the Thermal Index reaches 0 is the top of lift. Where clouds do form well below the top of the boundary layer, if the general air mass is moist, over-development is likely to occur.

Sky Cloud Cover
This parameter comes directly from the forecast model output and is interpolated across the area it represents. The exact edges are likely to be slightly inaccurate but the gerenal location and cloud cover percentage represents what the models are predicting. This parameter includes all levels of clouds for any given column. Forecast models are known to poorly predict cloud cover, so the usefulness of this parameter is in question.

Sky Cloud Cover (without high)
The same as Sky Cloud Cover above, only without "High Clouds" represented in percentage of cover. High clouds can often be very thin and sometimes do not impact soaring conditions enough to even note them. Of course high clouds can also be very thick, having a high optical depth.

Sky Cloud Cover (convective)
This parameter describes the cloud cover percentages for given areas due to convective events. This parameter comes directly from the forecast model and likely only represents very large scale convection events, such as towering clouds as a result of severe over-development or thunderstorms.
 
Precip/Thunder Storms
This parameter utilizes several model variables to characterize thunderstorm potential. Most forecasts break the probability of thunderstorms into select categories. We have chosen the following:
  • T-Storms Likely: There is a great potential for thunderstorms in the area, and they will most certainly develop throughout that zone.
  • T-Storms Possible: Thunderstorms are possible within a given area, but may not fully materialize into mature storms or will only effect a smaller portion of the noted area.
  • Scattered T-Storms: Thunderstorms may develop within the area but will be widely scattered across the region. Storms will typically form over higher elevation and mountain tops, or very sparsely across flatlands. This type of forecast can be quite good for soaring as it indicates plenty of regional instability and enough moisture to likely form clouds.
 Predicting thunderstorms and precipitation is a challenging task for forecasters. The parameters used are broken into thresholds for each category and come directly from the model output.

Wind & Turbulence Parameters

Wind Speeds at Levels
For every 2000 foot interval we provide the winds aloft as an additional map layer. These winds are available to a 5km resolution grid, which should be plenty accurate for any given point. We have also provided the 10m surface winds. These surface winds will likely always be wrong for mountainous areas or near large bodies of water. The coarseness of the forecast models does not resolve the details of surface winds as they relate to the influences of terrain and localized modifiers. These surface winds will likely be much more accurate over large flatland areas, such as the plains of the Midwest U.S. We are skeptical to even present this information, but if it helps to characterize some regions it may be worth having available. Additionally, we have provided a single wind layer that shows the winds at the Top of Usable Lift. This will give you an indication of what the winds are expected to be when you top out lift, which is useful information for deciding what direction to maintain for cross country flights.

Top of Usable Lift Wind Speed
This is the wind speed that can be expected at the top of usable lift. These winds speeds come directly from the model output and represent the contoured levels of the Top of Usable Lift parameter.
 
Thermal Top Turbulence
A modified Richardson Number is used to characterize the potential for turbulence caused by wind shear near the boundary layer top. Specifically, we analyze each 1km profile slightly above and below the top of the lift and compare the wind vector values across that profile slice. A smaller number in our case  represents low turbulence, where a higher number (truncated at 10) represents a very turbulent field. A true Richardson Number is actually the other way around.

Surface Parameters

Surface Temperatures
Temperatures from the model output are generated to a 1km grid based upon the influences of elevation and their relative temperature profiles above the surface. Validating our temperature scheme across the U.S. for 5000 points observed within 15 minutes of our forecast time shows us that 80%-90% of those sample points are accurate to within 2 degrees C over a wide range of weather systems. To be expected, complex (mountainous) terrain accounts for the majority of temperature discrepancies. We are uncertain how well the surface temperatures represent other parts of the world because we do not have simple means to validate at this point. We will be implementing a solution soon for increased validation.

Surface Heat Flux
This parameter indicates the amount of available radiation which is used to invoke the thermal process on the ground. Heat flux values consider the sky cloud cover, soil moisture, and solar radiation. The interaction of the sun, ground, and everything in-between comprises the Energy Budget of the Earth.  The flux parameters are critical in providing clues to the Heat Transfer mechanisms for thermals to form at the surface and rise upwards.

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