RASP Demystified – A Clear Guide for Paragliding Students
- FlyWithJordan
- 4 days ago
- 7 min read
RASP (Regional Atmospheric Soaring Prediction) is one of the most powerful free forecasting tools available to paraglider and hang glider pilots. Many pilots either avoid it because it looks complicated, or they only use one or two maps and miss most of its potential. This guide is written for students and newer cross-country pilots to help you understand what RASP actually is, why it’s useful, and how to start using it confidently.
Four types of pilots and RASP
Never heard of it.
Terrified of it – looks too technical.
Use it a little (usually just the “thermal strength” map).
Expert users who get almost everything out of it.
If you are in groups 2 or 3, this is for you.
What is RASP?
RASP is a high-resolution weather forecast specially made for soaring flight (gliders, paragliders, hang gliders).
It takes a normal global weather model and runs it again at much finer detail over a smaller area, then adds extra calculations for the exact things free-flight pilots care about: thermals, cloudbase, overdevelopment risk, wind in the boundary layer, sea-breeze fronts, convergence, etc.
Where do I find it?
Just Google “RASP Southern California” (or “RASP” + your region). Most countries that have good paragliding/hang gliding areas now have their own version.
How does RASP actually work? (simple version)
A big global weather model (GFS) predicts the weather for the whole planet on a fairly coarse grid.
RASP takes that global forecast and zooms in on a smaller region with a much finer grid (typically 4–5 km boxes instead of 50 km).
It then adds special soaring calculations that normal weather models don’t do.
The results are turned into colorful maps (called BLIPMAPs) and uploaded to a website for pilots to use.
New forecasts usually appear early in the morning for “today” and “tomorrow”.
Why is RASP so much better than normal weather apps for paragliding?
Much higher resolution → it “sees” hills, valleys and coastlines properly.
Predicts things that matter to us:
– How strong thermals will be
– How high the boundary layer (flyable air) will be
– Where and when cumulus clouds will form
– Risk of overdevelopment / thunderstorms
– Sea-breeze fronts and convergence lines
– Wind strength and direction inside the thermal layer
Getting started – the main screen
When you open a RASP site you normally see:
A map of your region on the left.
Lots of menu boxes on the right.
Clicking anywhere on the map moves time forward 30 minutes or 1 hour.
Ctrl + click on a spot gives the exact number at that point.
Shift + click opens a “sounding” (vertical slice of the atmosphere) for that location.
The most useful parameters for paraglider pilots
(These are the ones students should learn first)
Thermal Updraft Velocity (often called wstar)
Average thermal strength in meters per second (or feet per minute).
Rough guide:
< 2 m/s → weak, probably local soaring only
2–3 m/s → decent
3–4 m/s → strong
4 m/s → very strong (can be rowdy lower down)
Important: this number does NOT subtract your glider’s sink rate and ignores cloudsuck, so real climb rates will usually be higher than shown.
Boundary Layer Height (BL Height or Hbl)
The average top of blue thermals. Cloudbase is usually a few hundred meters below this on blue days, or right at the top when cumulus are working well.
Cu Potential or Cu Cloudbase where CuPotential > 0
Shows where cumulus clouds are likely and what their base will be. Very useful!
Overdevelopment (OD) Potential or OD Cloudbase where ODPotential > 0
Shows risk of the sky filling in or thunderstorms developing. If big areas go dark red/purple later in the day → be careful.
Surface Wind and BL Average Wind
Surface wind = wind on take-off and landing.
BL Average Wind = the wind you will experience while thermalling. Use this one for planning routes.
BL Max Up/Down Motion (sometimes called wblmaxmin)
Great for spotting convergence lines and sea-breeze fronts (long thin lines of strong lift).
Simple daily routine for students
Every flying morning:
Look at Thermal Updraft Velocity for the whole day – when and where is it strongest?
Check BL Height – is it high enough to get away from your local hill?
Look at Cu Cloudbase – will there be workable clouds or a blue day?
Check OD Potential later in the day – will it spread or stay clear?
Look at BL Average Wind – is the drift OK for getting back or doing a triangle?
Important things to remember
RASP is a forecast model, not a crystal ball. It can be wrong, especially with clouds and exact timing.
Compare areas and times rather than believing absolute numbers.
Wind high up can be very different from surface wind – always check the sounding or BL Average Wind.
On sea-breeze days the forecast can change quickly – keep refreshing the page.
The very high resolution “wave” or “1–2 km” models are great when they exist, but the normal 4–5 km ones are already excellent for thermal flying.
Final student tip
Start simple: every day you plan to fly, open RASP and look at just three maps:
Thermal Updraft Velocity
BL Height / Cu Cloudbase
BL Average Wind
After a couple of weeks you will start to “see” what the maps are telling you. Then gradually add the other parameters.
RASP is not magic and it’s not scary – it’s just the best free tool we have for planning good paragliding days and staying ahead of the weather. The more you use it, the better your flying decisions become.
What is “Puddle Temperature” in paragliding forecasts?
Puddle Temperature (sometimes called T-puddle, Surface Potential Temperature, or BL Top Temperature) is the temperature that a strong, undiluted thermal (a “puddle” of hot surface air) would reach when it rises all the way to the top of the boundary layer.It is one of the most useful single numbers for experienced thermal pilots.
Simple explanation
Imagine you scoop up the hottest air right next to the ground on a sunny slope (the air that has been baking on dark rock or dry ploughed fields).
If that bubble of air rises in a straight line without mixing with colder surrounding air, it keeps almost all of its heat.
When it finally stops rising at the top of the convective layer, its temperature at that height is the puddle temperature.
Why pilots love it
On a blue day (no cumulus), the puddle temperature is almost exactly the temperature at the top of the strongest thermals.
→ Trigger temperature on the ground ≈ puddle temperature + 8–12 °C (depending on height of the day).
If surface temperature is still 5 °C below that trigger, you know strong climbs have not started yet.
On a cumulus day, cloudbase forms where rising air parcels cool to their dew point.
The difference between puddle temperature and the forecasted temperature at cloudbase level tells you instantly whether the day will be under-developed (big gap = high cloudbase, blue holes) or over-developed (puddle temp close to or warmer than cloudbase temp = risk of spreading out or CBs).
How is it actually calculated in RASP / BLIPMAPs?
The model finds the maximum potential temperature in the lowest few hundred meters above the surface anywhere in the grid box (usually the hottest, driest ground – dark fields, south-facing rock, etc.).
It then lifts that parcel dry-adiabatically (≈ 9.8 °C per 1000 m, or 3 °C per 1000 ft) all the way to the top of the forecasted boundary layer.
The temperature it has when it arrives at the top is the puddle temperature (sometimes labelled wstar-related T or Max. Surface Parcel Temperature at BL Top).
In RASP you usually see it as one of these parameters:
BL Top Temperature or Puddle Temperature
Max. Thermal Top Temperature
T at BL Top from surface parcel
Rule-of-thumb uses for paraglider pilots
Blue day trigger on the ground ≈ Puddle Temp + 10 °C (rough average; adjust for your site height).
If current ground temperature is still well below that → wait; strong thermals haven’t started.
If puddle temp is much colder than the temperature forecast at cloudbase height → high cloudbase, probably blue gaps.
If puddle temp is close to or warmer than the temperature at cloudbase height → easy cloudbase, possible overdevelopment later.
Once you get used to checking puddle temperature every morning, you will very quickly know whether you need sunscreen and patience (blue, late start) or whether you should rig fast before it overdevelops. It’s one of the single most powerful numbers on the entire RASP page!
Description Links:
RASP FORECAST MODEL DESCRIPTION:
BLIPMAPs predict thermal soaring conditions resulting from surface heating of the Boundary Layer (BL), the scientific term for the turbulent atmospheric region mixed by surface-based thermals (so thermal tops occur at the top of the BL). The RASP BLIPMAP program uses numerical weather model predictions to provide parameters suited to the needs of soaring pilots and presents them in graphical format. Relative differences, both in location and in time, are expected to be more reliable indicators of soaring differences than are the precise numerical values.
A sequence of forecasts, all for the same validation time given on the regional BLIPMAP webpage link, is produced during the day as new observational data becomes available, each updated forecast having a shorter forecast period between the latest observation and validation times. [Model initialization (observation analysis) time + Forecast Period = Validation (forecast) time] At the top of each forecast plot is the name of the parameter, the validation date and time, and the forecast period. The regional RASP pages give forecasts for the "Current" day and also provide the previous forecast for each time (which can be for either the current day or the preceding day). Archived forecasts from previous days (for a single time only) can be viewed using the RASP Archive Viewer.
The parameters are averages over grid squares as forecast by the WRF (Weather Research and Forecasting) model. Several model grids are forecast for each location, progressing from coarsest to finest resolution with the area covered decreasing for a finer resolution grid. Because model solutions from different grids must be hammered together at their boundaries, due to inevitable mis-matching between the grids, forecasts near the grid boundary are less accurate than those nearer the center of the domain - a dashed white line indicate the frame outside of which forecasts are strongly affected by boundary condition errors.
The parameter values are represented by color hues which increase in "warmness" as the value increases in magnitude. A screen magnifying tool, such as the freeware Super Magnify for Windows machines or Xzoom for X11/Linux/Unix machines, helps when discrimination between adjacent contours is difficult; alternatively, many browsers are capable of increasing/decreasing the size of an image, and Firefox users can install the Image Zoom plug-in extension to add that capability.
Geographic outlines are depicted on each BLIPMAP in white. The model topography is plotted as black contours, to assist in location identification but also to emphasize the smoothed nature of the model topography. The BLIPMAP does not predict thermal lift created by small-scale terrain features which are not resolved by the model topography, which often give localized updrafts significantly stronger than those over the surrounding smoother terrain.
