RASP Windgram Reading — soaring forecasts highlights on one diagram.   wg

Winds Stability Lift Clouds and Moisture Temperature Altitude Summary
A windgram is a forecast for a single spot on the map over the course of the day
and shows several things about what is expected to happen.
The vertical axis is the altitude above the ground, while the horizontal axis is time of day.

Wind-Barbs show wind speed and direction. Color shows the lapse rate, or rate of change of temperature with altitude. The numbers at the top show the expected upward velocity of thermals at the time where time is on the bottom axis. The little paragliders show how high a 225fpm sink rate glider can expect to get, while the clouds show where any cumulus clouds would form if there is enough moisture.. The temperature is shown on contour lines with freezing level represented as snowflakes.
All these things are superimposed on the height by time graph.
Details below:
windbarbs and their meanings
In this legend, all wind barbs are for wind from the West.
show wind speed and direction.
Speed is in knots. The green color is given to wind barbs that represent less than 9 knots, so single barbs that range from 7.5-12.5 knots are green up to 9 knots,the low end of the range and white above 9 knots, in the high end.
Wind barbs diagram.

The feathers are where the wind is coming from the skinny end is where it is going.

         The wind IS the arrow.

Conversion factors Enter known speed on
right. Click convert.
10 mph → 8.7knots →16 kph → 4.5 mps
10knots → 11.5 mph → 18.5 kph → 5.14 mps
10 kph → 6.2 mph → 5.4knots → 2.78 mps
1 Meters/second (mps) → 197 feet/minute (fpm)
10 Meters/second (mps) → 36kph → 22.8mph

miles (mph)
knots (kt)
kilometers (km/h)
meters/sec (m/s)
(ft/sec) (ft/min)
lapse rate colors

Stability indicated by Background Colors:

We don't need completely unstable air to fly!
  • Backround colors represent the "local" lapse rate at each time and altitude in C°/1000ft.
         For those familiar with soundings, the color represents the slope of the red temperature line.
  • The lapse rate is change in temp / change in altitude as we go up.

    lapse rate colors

    Most foot-launch soaring happens in the range where windgrams are pink or orange.
    Although we do often fly when the lowest levels are red.

    (higher altitude temp) - (lower altitude temp)
         (thousands of feet between them).

    If it's warmer than it should be up high, stable air happens.
    If it's lots colder than it should be up high and moist enough, thunderstorms happen.
    Most flyable days are in between the two extremes.

    The Standard lapse rate is -2.0 C° per 1000 feet —. — -3.6F° per 1000 feet, considered the rule of thumb for marginally stable air.

    In terms of a foot-launch site, if the launch is 2000 feet above the LZ, the standard lapse rate would mean that the air temperature at launch is 4.0C° cooler than at the LZ. If it is cooler than that, you can expect thermals. If it is more than 6C° cooler than the LZ at launch, you can expect strong thermals. During the course of a day, the warm air at the base mixes with the cooler air above. While the sun is out, the surface air keeps getting enough hotter that it still rises through the mixed air. At sites near water, the cooling effect of the water will change the way this happens, decreasing the thermals when the air is warmer than the water, and increasing them when the air is significantly cooler than the water.

    . Which resulted in these flights:.

    September 10, 2011 Compared to a day with a strongly inverted CAP at the same site (Sept 10, 2011) that was reported to have zero wind at launch and no lift.
    To view the windgram for any day in the past few years that you flew a NW site, click here.

    The atmosphere is
    Absolutely Unstable

    The atmosphere is
    Conditionally Unstable
    Thermal soaring can happen

    The Atmosphere is
    Absolutely Stable
    Thermal soaring will not normally happen. (see discussion of the CAP below)

    The Atmosphere
    is Inverted
    Thermal soaring will not happen.
    When the lapse is more negative than the near constant dry adiabatic lapse rate
    -9.8 C°/km.
    -3 C°/1000ft)
    -5.5 F°/1000ft
    When the lapse rate is between
     -9.8 C/km and   -4 C°/km  

    or between
    -3 C°/1000ft  and -1.2 C°/1000ft

    or between
    -5.5 F°/1000' and -2.16 F°/1000'
    When the lapse rate
    is still negative but a
    smaller magnitude than
    -4 C°/km
    -1.2 C°/1000ft
    -2.16 F°/1000ft
    When the lapse rate
    is positive, that is
    when the higher level
    air is warmer than the
    lower air.

    We don't need completely unstable air to fly!
        Note that when the weather forecasters talk about unstable air, they are indicating that it is unstable enough for rain to be likely. Overall stability will depend on moisture content.   More moisture → less cooling as the air rises → increasing instability.
        To think about thermal soaring, remember that the higher the lapse rate the smaller the heating or mechanical trigger required to get air to start rising. It cools as it rises and expands to lower pressure. Once rising, it will rise until it reaches either the level where its temperature matches the surrounding air, or where the temperature has dropped to the dew point and clouds form. Once a cloud forms, the condensation process generates additional heating allowing the clouds to billow up further. These are our favorite cumulus clouds aka Cums, Cus, or Qs and indicate the top of a column of rising air.
        Layers of air where the lapse rate becomes stable are called caps. When there is no cap, the billowing clouds become self-sustaining and can become rain or thunderstorm clouds. Windgrams do an excellent job of showing where the cap is at any given time of day. In the summer, the cap will typically rise during the hotest part of the day and then descend again later. It is rare to ride a thermal as high as the cap, but it is possible for strong thermals to "break through" a cap and soaring can continue to much higher altitudes. There is often considerable turbulence associated with breaking through a cap. Moist thermals that are forming clouds that manage to break through the cap often form rain and thunder clouds.
    see JEFF HABY Thermodynamics Q&A
    TJ Olney


    So, just picture yourself standing on launch looking straight up each time column. Because of your incredible visualization ability, you can see the windbarbs in the air floating above your head. You can also see the changing colors that tell you how quickly the temperature is changing overhead. You will stop climbing before you get to the white, gray or blue layers. The little clouds are just like the little clouds you see at launch. The paraglider is the best pilot you know at the limit of available thermal lift. The snowflakes tell you where your water bottle will start to freeze, and the cross-hatches are the major clouds as a ceiling over your head, not the puffy type, but the widespread type. All the way at the top of the column, you see a sign that tells you the vertical speed limit for thermals that hour. Floating between you and the top number are other numbers that tell you the air temperature in F° at the altitude where they appear. They are connected by lines that help you know whether the temperatures are rising, falling, or steady.
    The annotated windgram from Alan Crouse in Southern California might help as well.
        I developed the windgrams based on two common forms of display often called meteograms or time-height plots. None of the examples that I could find displayed the things that were most relevant to foot-launch soaring or to soaring in general. Dr. Jack Glendenning's RASP system output all the relevant variables and his Blipspots™ are a numerical snapshot of a point in time, but most people seem to respond better to a graphical presentation than to a table of numbers. Hence, I developed the Windgram. They are now in use by RASP operators in many parts of the world. I hope you find them useful. TJ Olney
    See the MM5 tutorial at Cascade Paragliding Club
    or details about Skew-t diagrams.
    A summary of the different ways we try to measure instability.
    top of page Winds Stability Lift Clouds and Moisture Temperature Altitude Summary

    Using Windgrams?