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The thermal forecast contains the most dense atmospheric data we ever put in a diagram. The carefully assembled graphs include detailed information about surface conditions, stability indices, lapse rate, humidity, clouds and winds. It is separated into 4 parts:

Surface conditions

All diagrams show hourly data for Ashburn for 3 days. Yellow areas indicate daylight.

  • 2m temperature and 2m dewpoint: Equivalent with measurements 2 meters above ground. The 2m dewpoint temperature indicates how much water is in the air at ground level, from where potential thermals would start. A larger difference between temperature and dewpoint means less humidity and thus a higher cloud base. Fahrenheit is not yet supported.
  • Precipitation: Total precipitation (rain, convective and snow) in millimeter rain gauge. As it takes a lot of heat to evaporate water, wet ground heats slower and is thus less favorable for thermals than dry ground. Also, thermals start earlier in dry conditions, when no rainfall happened before.
  • 10m wind and 80m wind: Wind speeds in 10 and 80 meters above ground in kilometers per hour. Thermals develop under calm conditions or with light, variable wind. However, with 10 to 20 km/h wind, the thermals tend to be better organized. Stronger winds generally means also more wind above, which could produce wind shear, destroying the thermals. Look at the wind shear diagram.

Stability indices

Indices are scaled to fit 4 sections: poor, ok, good and excellent. Not all indices are reliable in all weather conditions or geographical regions. In dry climates CAPE and Lifted Index underestimate soaring conditions, because of the low moisture in the atmosphere.

  • Soaring conditions daily summary (ThrHGT): For each day the maximum heights of dry thermals as well as the maximum expected soaring height for a glider plane is given. Heights are in meters above ground level (not above sea level). A value of 0m indicates that dry thermals do not support a glider plane. Furthermore, the thermal index for 700, 800, 850 and 900 hPa (mb) is forecast.
  • Updraft velocity / lift (m/s): An estimate of the maximum strength of thermals solely determined by surface conditions (heat, moisture and solar radiation). Uplift caused by wind is not considered (Mountain waves, convergence etc). Minimum 1.5 m/s, good 2 m/s, excellent >2.5 m/s.
  • K-Index: A measure of stability considering temperature and humidity between 700 and 500 hPa. Be aware the K-Index values can change significantly during summer over short periods of time due to temperature and moisture advection.  In the winter, when temperatures are very cold, the moisture terms are very small. So, even the K-Index is fairly large, it does not mean that conditions are favorable for thunderstorms because of the lack of moisture.
    K-IndexSoaring conditions
    below -10no or weak thermals
    -10 to 5dry thermals or 1/8 cumulus with moderate thermals
    5 to 15good soaring conditions
    15 to 20good soaring conditions with occasional showers
    20 to 30excellent soaring conditions, but increasing probability of showers and thunderstorms
    above 30more than 60 percent probability of thunderstorms
  • Lifted index (LI): Another measure of instability (negative values) or stability (positive values). Be aware that strongly negative values indicate excellent soaring conditions, but severe thunderstorms are likely and could be very dangerous.
    Lifted IndexSoaring conditions
    6 or greaterVery stable conditions
    2 to 6Stable conditions. Thunderstorms are not likely
    0 to 2Showers probable. Isolated thunderstorms possible
    0 to -3Slightly Unstable. Thunderstorms possible, With Lifting Mechanism (i.e., cold front, daytime heating, ...)
    -6 to -6Unstable, Thunderstorms Likely, Some Severe With Lifting Mechanism
    below -6Very Unstable, Severe Thunderstorms Likely With Lifting Mechanism
  • CAPE (J/kg): Convective Available Potential Energy is a measure of the atmospheric stability affecting deep convective cloud formation above the boundary layer. Higher values indicates larger updraft velocities and greater potential for thunderstorm development. Values around or greater than 1000 suggest the possibility of severe weather should convective activity develop.

Lapse Rate / Humidity / Clouds

This graph shows an atmospheric profile over time. It provides an overview of the thermodynamic stability and clouds. The bottom of the diagram corresponds to the forecast model ground level, which might differ significantly from the actual location height in complex terrain. All color scales are fixed to compare forecasts at different locations and times.

  • Lapse rate is measured in kelvin per 100m height difference. Inversions (very stable conditions) have positive values and are colored in yellow to red. The boundary between green and blue corresponds to the standard atmospheric conditions. Darker blues indicate conditions favourable for updrafts. Purple areas indicate dry unstable conditions which can only exist close the ground or for very short times in the atmosphere. This would make even stones fly. Surface instability up to 200 meters is generally not shown. Important note: Lapse rate is an average caused by the mixing of up and down drafts. Actual updrafts can have much lower lapse rates.
  • Relative humidity (thin colored lines): Convective clouds develop more likely in moist air.
  • Convective clouds (astrix area): when convective clouds start developing, thermal soaring is at its best and finding thermals is greatly simplified. Thermals are beneath growing cumulus clouds. The convective cloud base is indicated as thick black line. Towering cumulus and cumulonimbus clouds have very strong updrafts and can thus become very dangerous.
  • Cloud cover (hatched areas): Unless a hatched area is also marked with astrix (convective clouds), these clouds are no good for updrafts and also due to shading strongly reduce any potential development of updrafts.
  • PBL height (thick white line): Planetary Boundary Layer height describes the average height a surface air parcel could move upwards. Buoyancy and wind (mechanical mixing) affect this height. Mixing due to convective clouds is not considered.

Horizontal Wind / Temperature / Vertical Wind Shear

Upper air wind conditions for the next days are displayed here. Strong wind shears are dangerous and should be avoided. Already weak wind shears disrupt thermals. The color scales are also fixed.

  • Wind speed (colored background): Purple and dark blue represent calm winds. White numbers indicate the actual speed in kilometers per hour. Wind arrows show the horizontal wind direction and not up- or downdrafts. A downwards pointing arrow shows a north wind heading towards south.
  • Temperature lines (thin colored lines): Small colored numbers show the temperature profile over time. Freezing level height (0°C) is marked as thick white line.
  • Wind shear (thick colored lines): Wind shear can strongly disorganize thermals. Stronger larger thermals are more resistant to wind shear than smaller ones. Generally a shear of 2 km/h /100 m distorts thermals so that they are difficult to use. Especially for high values actual wind shears might be much stronger than indicated. The forecast values represent hourly mean values not resolving gust winds.