XC Skies Layers

XC Skies provides many forecast layers within the main XC Map application. The parameters (layers) are described here and grouped within similar sections.

Thermal Parameters

Top of Usable Lift (TOL)

Measured in feet or meters above mean sea level (MSL), this parameter is commonly the most looked at by pilots. The top of the usable lift is essentially the potential height a glider might climb in convectively driven thermals.

This parameter accounts for a glider sink rate of about 250-300 feet per minute (1.2 meters per second), with this consideration already applied to the estimate. 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 often in large cumulus nimbus cloud formations. 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.

Top of Usable Lift Above Ground

This parameter subtracts the ground elevation from the Top of Usable Lift for every 1 square km within the forecast area. This is a useful parameter when considering cross country routes. You may find that one route versus another provides more "clearance" over the ground, showing regions with greater XC potential while providing a little more safety.

Thermal Tops (Thermal Index = 0)

This parameter depicts the absolute thermal tops ignoring glider sink rates and weak vertical velocities. This is commonly where the Thermal Index (TI) is equal to 0, which simply means that the parcel of rising air has reached equilibrium with the surrounding air. This is sometimes also called the boundary layer, however, these two terms depict different processes. The thermal tops can often be much higher that a glider can reach due to a variety of reasons including, strong wind shear, mechanical lift, wave formations, and large scale advection.

The thermal tops parameter also neglects moist adiabatic lapse rates when determining the heights. A dry adiabatic lapse rate is used. When dew point is reached before the lifted parcel of air reaches equilibrium, thermal can continue to ascend considerably higher. This is especially true in moist and warm conditions which often result in towering cumulus or cumulus nimbus.

Thermal Top Uncertainty

Because surface temperatures play a large role with initiating thermal convection, this parameter is used to "nudge" the amount of avialable energy at the moment a parel begins to lift. Specfically, this parameter estimates the top of the lift by warming the surface an additional one (1) degrees C.

This might be a reasonable proxy to the sensitivity of the air mass over a given location instead of inspecting the structure of all temperature profiles in a region. In areas with large uncertainty, it could be expected that the thermal tops could vary in height by some fraction of the uncertainty depth (both higher or lower).

This parameter might also give an indication of over-development potential in moist regions. When a small increase in temperature hints at a much higher top of lift value, the thermodynamic profile of the air parcel could vary considerably. It is recommended in regions of high uncertainty to inspect a sample skewt-t or review the air temperature and moisture profile for additional information.

Thermal Updraft Velocity

This is the average thermal strength or upward (vertical) velocity, usually measured in meters per second or feet per minute. All thermals are different, having different sources and triggers, which implies different localized surface temperatures. This value is a bulk average over a square kilometer and effectively smooths out small areas with considerably stronger thermal cores, or conversly weaker lift at the edges of thermals. Skill level of the pilot and the turning radius of the glider will yield different experiences when comparing this averaged value to true conditions.

It is typical for the greatest vertical velocities to exist at some prime location within any given thermal where equilibrium is the furthest apart. This moment can vary from thermal to thermal and day to day. As a rule, it is fair to apply a factor of 2 to predict these extreme cases in dry air masses, and likely a factor of 1.5 in moister areas. However, the average vertical velocity of a glider climbing within the parcel will tend to be the average or lower.

Subract your glider sink rate from this parameter to arrive at the effective vertical velocity.

Thermal Index (averaged profile)

This parameter takes the average thermal index (TI) value across the entire lifted parcel of air. As a parcel ascends, the thermal index is the temperature difference of the expanding and cooling parcel compared to the environmetal air at the same level. A negative difference in temperature implies the parcel has not yet reached equlibrium. Once the parcel reaches equilibrium, the TI is theoretically 0. The average TI value for any given point is essentially the area bound by the parcel temperature curve and that of the environmental temperature.

CAPE is derived in a similar fashion, but measured in terms of potential energy. The averaged TI is a simpler form that can relate back to the thermal index. This single TI value, however, does not tell the entire story of a thermal's profile, but is useful at a glance on a map.

Buoyancy to Shear Ratio

The buoyancy to shear ratio (BS) is a comparion of thermal vertical velocities to wind shear forces across the profile of a lifting parcel of air. In other words, how will a thermal be affected by changes in wind speed and direction as it rises? The quantification of this parameter is to take a single derived value for both the average buoyancy and average shear through the boundary layer. Plenty of problems arise with this technique because it "smooths" both terms of the ratio. For example, a strong wind gradient might exist near the surface, yet taking the average of the profile based on potential thermal tops, a BS ratio might still indicate good thermal organization. Most pilots eventually experience conditions where there is broken lift everywhere, and frustratingly none of it seems to be usable.

XC Potential

Subject to debate and criticism (especially the authors themselves), this parameter was once removed and reluctantly added back to XC Skies. The idea is to provide a single parameter that considers multiple parameters in a single layer. At the moment this parameter attempts to identify the locations where XC is possible based upon a comparison of 4 different criteria. They are the following:

  • 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.

  • 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.

  • 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. This is effectively a "safe" threshholding of XC potential for pilots getting into flying distance.


In parcel theory CAPE is known as Convective Available Potential Energy and is a relatively straight forward summation of the amount of energy available before a lifting parcel of air reaches equilibrium. CAPE is almost always represented as Joules per kilogram (J/kg). When interpreting CAPE values, it is common to use the value as simple guidance to determine the stability of an airmass. For example, these values are commonly used to designate regions of instability:

  • CAPE below 0: Stable.

  • CAPE = 0 to 1000: Marginally unstable.

  • CAPE = 1000 to 2500: Moderately unstable.

  • CAPE = 2500 to 3500: Very unstable.

  • CAPE above 3500-4000: Extremely unstable.

XC Skies calculates a CAPE value from the surface to equilibrium using the appropirate dry and/or moist adiabatic lapse rate.

Lifted Index

The lifted index is a commonly used value to represent the instability of an airmass by taking the difference of the environmental air temperature at 500 mb from a lifted parcel of air using the dry adiabatic lapse rate to the same level. This value, like CAPE, is a simple expression of the instability of the lower atmosphere in general. It can be useful to quickly identify areas of over development potential. Wikipedia contributors have provided this useful table:

  • LI 6 or Greater, Very Stable Conditions

  • LI Between 1 and 6 : Stable Conditions, Thunderstorms Not Likely

  • LI Between 0 and -2 : Slightly Unstable, Thunderstorms Possible, With Lifting Mechanism (i.e., cold front, daytime heating, ...)

  • LI Between -2 and -6 : Unstable, Thunderstorms Likely, Some Severe With Lifting Mechanism

  • LI Less Than -6: Very Unstable, Severe Thunderstorms Likely With Lifting Mechanism

For safe and comfortable soaring, a typical rule is that an LI value of anything less than -2 is not going to be a very fun day. Typical ranges of 2 through -2 tend to be good fair weather soaring conditions in areas of higher elevations.

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. 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 regimes, such as the U.S. Southwest, and poorly in more humid and lowland areas. As numerical weather predictions get better, and more data is assimilated in near real-time, this parameterization will get better.

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.

Rain & Thunderstorms (precipitation)

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 details of 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.

Precipitable Water - Column

The precipitable water layer represents the amount of moisture within any given column of air from the surface through the top of the atmosphere. The precipitable water is the amount of precpitation that would result if all moisture were extracted from the column. The majority of Moisture is located within the lower half of the atmosphere. This is useful because it provides a quick look into the amount of potential rain and energy stored in any given column of air. In mid latitudes, values greater than roughly one inch (or about 25 mm) indicate good potential for rain if other existing conditions exist.

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

Wind Speed at the Top of Usable Lift

This parameter shows the wind speeds at the top usable lift as a single layer. Whether the top of lift is near the surface, or 1000's of meters above the ground, this layer is effectively the contour of winds that might be expected at the top of climbs and the start of glides.

Surface Wind Gust

This layer shows the surface wind gusts from the selected model output. This variable is not modified by XC Skies.

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.

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.

Mean Sea Level Pressure

This paramter is the model output for mean sea level pressure (MSLP). The MSLP is caculated by taking the surface pressure and adjusting it to mean sea level (1013.25 hPa). MSLP allows for a quick comparison of pressure isobars which show where pressure boundaries exist, such as Low and High systems.