Wendy Windblows Everything for Paragliders and Hang Gliders
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Technical Information on Wendy Windblows

What is Wendy?

The Wendy network of talking weather stations was conceived with the idea that users could, purely by telephone, get an accurate idea of the ACTUAL weather conditions, RIGHT NOW, at a flying site before deciding to go there.

Not a Forecast

Note that this is NOT a forecast. It is a "Now-cast", or even an "After-cast". It is what you would get if you had a convenient friend (with a mobile phone) standing at the site, and telling you what the weather's doing.


The stations were originally set up with Hang Gliders and Paragliders in mind, as I, her inventor, have been a Hang Glider pilot for 20+ years, and got fed up with driving 60 or so miles to fly, only to find that, when I got to the site, the weather was totally different to the forecast!

Typical Hilltop Wendy Station



The stations are supported by Subscription from the users. Details are at subscribe details page

Web Data

Now, we need to get the station data back from the actual station site to both our public lines, and our website here.

So, our main computer here in the office rings each station periodically, and uplifts the data digitally. This occurs from 4am - 10pm. (No one wants the data outside those hours).. The interval varies with time of day and traffic, but is currently about every half hour. We hope to reduce this interval soon.

The data thus gathered is then placed on our website data pages.

Thus, the data on the web may be half an hour or so out-of-date. Or not, depending on what time you ring in the uplift cycle. You'll be told the time the data was taken on the website, so you can tell how recent it is.

Phone Data

The Data is also placed on our central phone server, where you can access it by phone if you are a member. More information about the phone service is at: Phone Service Details Page

Data By Text/Email

You can also get the data by text (and by email). Details are Data by Text and Email Page

General Technical Notes on All Stations

These technical notes are aimed principally at Hang Glider and Paraglider pilots, who make up the bulk of the users of the Inland stations. However, the information will be useful to all users, not matter what their interest.

Differences between Met Office Forecasts & Wendy Data

Web data is updated every 30 mins or so - so when you ring a Wendy station, the information is pretty recent. Anywhere from 1 minute to 230 minutes old, depending on when you accessed it.

The station data is also, as I've said, from hilltop sites - Met Office sensors are usually in the flatlands, at military airfields, and civilian airports. The weather in the hills (particularly in plateaus, like the Peak District) is often radically different from the flatland data. The Wendy station at Shining Tor, near Buxton, is only a few miles from Manchester Airport, but 1700 ft above Sea Level. The difference between the Wendy data and the Airport readings are often amazing.

Data Collection and Processing

The wind data is gathered in a way which maximises the meaningfulness from a pilots standpoint. Firstly, pilots are not interested in short-term gusts, so much, as in "thermal-induced" alterations in wind strength.

Therefore, the pulses from the anemometer are integrated over a 30-second period. This is long enough to average out short gusts, but long enough to detect the increased wind caused by a thermal passing through the site.

As a thermal approaches from the front of the hill, it causes a drop in windspeed for a minute or two, and then, as it passes over, the "suck" of the thermal is ADDED to the windspeed, causing an increase in wind lasting maybe 1-5 minutes.

It is this alteration that the users are keen to detect, so the sample period of 30 seconds is ideally suited to sort out thermal-induced changes from simple turbulent gusts.

The station keeps 60 such 30-second "snapshots" in its memory at any one time - that is, 30 minutes worth. Each time it takes a new snapshot of conditions, it over-writes the data received 30 mins & 30 seconds ago, and looks back over the last 30 minutes to find the lowest, highest, and average windspeed.

Thus, what you are getting is a 30-minute "rolling window" that advances through the day in 30-second jumps. Why 30 minutes? Because the average cumulus cloud (of interest to pilots!) takes 20 minutes-odd to form, pass over the site, and decay. Keeping data for 30 minutes enables this "cloud cycle" to be more than covered.

Remember, cumulus clouds have lift under them - they suck upwards. At least, most of 'em do. Therefore, when one is in front of the hill, it causes a drop in windspeed, as the suction is subtracted from the general wind.

Sucking Cloud Upwind of the Site

When the cloud passes over the back of the site, the suction is added to the windspeed.Therefore, if you take the min windspeed away from the max windspeed in the last 30 minutes, and divide by 2, you get the amount of "suck" generated by the clouds locally.

Sucking Cloud Downwind of the Site

It also gives you a good idea how turbulent conditions are: A day where the windspeed is varying from 14-18mph, average 16mph is pretty smooth.

The same average speed of 16mph, but with the wind varying from 5-30mph is a very different proposition, if you're planning to fly in it!

So, the station gives you the max, min, and average windspeed, and average direction in the last 30 minutes.  It then gives you the average speed and direction 30, 60, and 90 minutes ago. This is useful for forward projection. Is the wind getting up, dropping off, backing, veering? The history thus revealed is very useful for tracking the passage of fronts in real-time.

Front Tracking in Real Time

A wind that has switched from WSW to NW, and suddenly increased, in the last 30 minutes, say, with a corresponding drop in temperature, some rainfall perhaps, and a change in sky conditions, surely indicates that a cold front has just passed through.

Similarly, as wind that is backing from W to SSW and steadily increasing, with a darkening sky, but mild temperatures, indicates an approaching warm front.

Sky State & Cloud Conditions

This brings us to one of the sneakiest things about the Wendy stations:    their assessment of cloud conditions.

Each station contains an IR photometer, which measures the intensity of Infrared light in the water-absorption band. Once the sun is, say, 10 degrees above the horizon, the IR light is really only attenuated (cut down) by water between the sun and the station. That is, cloud.

Cloud States and IR light Levels

More Cloud States

Now, the same system of 30-second "snapshots" is used. The station takes a snapshot of the IR sky every 30 seconds. 60 readings (30 mins worth) are kept in memory, and the station reads out the max, min, and average light percentage when asked. What does this tell you?

Well, if there has been no cloud between the sun and station in that half-hour, then all snapshots will read 100%. So, the station will say:

Daylight varying between 100% and 100%, average 100%

Pretty obvious, really - a blue sky.

Now, the opposite - dark thick stratus, post warm front, probably raining. Each of the 60 snapshots will be between probably 8% and 15% light value. Therefore, in the last half hour there have been no gaps, the sky is stable cloud. You will therefore hear:

Daylight varying between 8% and 15%, average 11%

What about a nice post-cold-front day? Well, when the sun is out, the station snapshot will be 100%. When moderate cu covers the sun, it will drop to maybe 50%.

If the station has taken 30 snapshots at 50%, and 30 at 100%, the average will be 75%. So, the report will be:

Daylight varying between 50% and 100%, average 75%.

If it took nearly all cloud snapshots, and only a few of sun, then the average would be near the low figure:

Daylight Varying Between 50% to 100%, average 55%

If most of the snapshots were sunny, then the average would be near the top:

Daylight Varying Between 50% to 100%, average 92%


And, remember, the DEPTH of cloud is given by how low the figures drop - so, these three numbers max, min, and average daylight (IR) give you a complete picture of the sky cloud state, both in thickness and oktas.

The thing to remember is, just as the difference between max and min windspeed gives you the turbulence, the difference between max and min daylight gives you a good idea of the instability.

Imagine the sky immediately after a strong cold front has passed through - as cumis develop and overdevelop, with brilliant blue sky between, turning into black thunderheads, perhaps. The lowest light figure will drop and drop, whilst the top figure stays at 100%, until the overdevelopment spreads enough to cut out the sun altogether for 30 minutes. Then it will rapidly drop to a low figure, as old data is overwritten.

In the most recent version of the stations, an algorithm converts these IR light values to direct Sky States, and reads it out to you. You hear directly, a report such as:

The Sky is Mainly Sunny, with some Scattered Moderate Cloud

We hope to convert all the lines to this method shortly. Of course, you can't work this system unless the sun is at LEAST 15 degrees or so above the horizon. Therefore, the skystate conversion algorithm only operates between certain times of day, depending on the time of year, when the sun should be up. Outside that time window, the report reverts to the raw daylight readings, as above.

Subscribers using the telephone stations can opt for the raw daylight percentages instead of the pre-figured skystate by entering a "0" digit immediately after the station says:

This is (station name) at (time)

You have about 1.5 seconds to enter the control digit. This will cause the report to revert to the more informative raw daylight percentages, if that is what you prefer. Other commands for phone readouts are given on the Subscribers Help Page

Anticipated Height of Cloudbase

The height of cloudbase is given in feet, calculated on the formula developed by that gliding met guru, Tom Bradbury. Lowest temperature last night taken away from highest temperature today, times 400.

It works pretty accurately for most of the year, as the formula is really highest temperature minus dewpoint x 400. However, for most of the flying year (March-October) the lowest temperature last night IS the dewpoint, as the release of latent heat by dew formation puts a floor on the temperature fall.

The system breaks down in winter sometimes, for obvious reasons, but generally, it is pretty reliable. Thinking about it, if there is a cloud blanket and high humidity, then temperature at night doesn't fall very much. Equally, in the morning, the cloud blanket prevents temperature rise due to solar heating, and evaporation of surface water absorbs a lot of heat that would otherwise cause a temperature rise. Therefore small temperature difference from night to day = low cloudbase.

In high pressure times, with clear skies, though, night temperature falls quickly, due to radiation. In the morning, clear skies and dry conditions allow the temperature to rise rapidly. Large temperature difference = high cloudbase (if there is enough moisture to form cloud at all!)

If there is blue sky, the station will still estimate a base height, but obviously, the daylight sensor will tell you that there really isn't any cloud around!

Rainfall Measurement

The rain gauge is a tipping-bucket type, graduated to 1/100 of an inch (0.25mm). The only snag with it is that in winter, it fills with ice and snow, and then, on a lovely blue day, warm sun causes the snow to melt - it runs through the gauge, which reads lots of rain! Not having sufficient electrical power available on most remote sites to heat the gauge above freezing point, there is really no cure for this one.

Rain Gauge and Solar Panel
Inner workings of Rain Gauge

The station records each tip of the bucket, and gives out the total rain since midnight, and the time of the last rainfall. After all, if you're looking for thermal activity, then knowing when it last rained is vital, as wet ground doesn't give off much thermal activity at all!  If it last rained at 2:30am, then maybe no problem. If it last rained 2 minutes ago, then you might as well stay home.....

Speed Readout in Knots/MPH and Beaufort Force

The Hilltop stations read windspeed out in MPH, currently. They have the capability to do knots, km/hour, metres/sec, or Beaufort, but as most footlaunch pilots think in mph, that's what the stations give out at present.

Hang and Paragliders represent about 80-90% of users of these Hilltop stations, although there is a growing number of other users, such as GA pilots, Balloonists, Rock climbers, Walkers, etc, etc. However, with the exception of GA guys, who are Knot-orientated, most of these other users understand MPH better, so we'll stick with that.

Coastal stations seem to be primarily used by Sailors, Windsurfers, etc, and they tend to think in Knots and/or Beaufort Force. Therefore these coastal stations give out the windspeeds in both of these, rather than in MPH.

Getting Data from Remote Sites

Obviously, it is not always possible to have mains power and phone line on top of remote hills in the middle of nowhere. Therefore, the Wendy Windblows stations can be split in two. The wind and other sensors can be installed on the hilltop (to get the best data) and powered by a solar panel. The data is then periodically sent by a low-power UHF radio telemetry link down to civilisation nearby, where there IS power and a phone line. The other half of Wendy, ie. the computer and phone interface, are installed there.

Radio-Linked Station with Solar Power

A good example of this is the Bradwell (Camphill) station near Sheffield. The wind sensors are on the Derby and Lancs Gliding Club airfield at Great Hucklow, but the data is transmitted by radio to the Woodbine Cafe, in Hope, near Castleton, where the main computer that runs the service for Bradwell is situated, as is the phone line into the station.

A display screen in the cafe shows the current weather, and Paraglider and Hang Glider pilots can often be seen in the cafe, slurping coffee, and keeping an eye on the weather data till things improve!

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Telespeed Weather Systems Ltd,135 Aughton Rd,Aughton,Sheffield,S26 3XD,UK0114-287-8936 E-mail:office@wendywindblows.com