Custom cross-sections with windy

The windy team recently released Route Planner, a tool to generate cross-sections over one or several legs of a flight. It’s seriously useful!

Give it a try with a desktop browser (support on mobile is still limited):

  • Set time and the ECMWF model, and zoom the map so all legs of the planned flight will fit
  • right-click on the start point and select “Distance and planning” in the dropdown
  • click one or several waypoints of your flight
  • at the bottom, select “Load route forecast for”: “VFR flt”
  • make sure the dropdown near the bottom shows “Directions north up”. See here why.
VFR Route forecast for Walchsee-Udine, Friday Aug 16, 8:00 loc

Displaying trajectories for the masses

How would you distribute windy trajectories during weather briefings at balloon events? You did save the result as GPX file. And now what?

Paper: meh. Display screenshot on projector? confusing spaghetti. Post a KML or GPX file on a whatsapp group? hardly anybody could open such files on their mobiles and pads.

Pretty easy with GPSVisualizer:

  • upload the GPX file so it is Internet-accessible and note its URL (e.g. as a WordPress media file – here’s an example GPX file for you to play with)
  • goto GPSVisualizer.com and paste the URL of the GPX file into the “Send a file to The Atlas” field:
GPSVisualizer Atlas page
  • hit the “map it” button – you should see something like this:

That’s thew view you’d want to distribute – people can pan and zoom around, and select/deselect heights and models in the trajectory set.

To create a URL for this view:

  • Click the blue “Link to this view” field at the right bottom:
Generating a public URL with GPSVisualizer Atlas

And here is the URL which you can post online – displaying the above map.

Edit: There is currently a minor bug in the GPSvisualizer web service – uploaded tracks vanish after a certain time, and the message ‘File … contains no usable data’ is displayed.

To work around this issue, add ?cache=0 to the Atlas link like so.

Trajectory forecast vs track flown: a case study

During the recent Sagrantino Cup event I created daily trajectory forecasts with the windy.com traj plugin for the morning flights and asked pilots to share actual GPS tracks so I could compare actual versus forecast. Here is one example:

ECMWF forecast versus actual track – click to enlarge

The KML file for this screenshot is available here, and it is instructive to play with it – have a look at it yourself in Google Earth.

  • Most of the flight was between 800m and 1000m altitude, so I selected the 2000ft (amber), 2500ft (red) and 3000ft (blue) trajectories.
  • The ECMWF model had the best match, so only ECMWF forecasts are shown.
  • The flight started on the valley floor, within an inversion and a slight northerly flow.
  • The match is excellent – especially with the 2500ft trajectory, and the descent phases around 11km and 15km into the flight show the northeasterly flow forecast by the amber line.
  • As to be expected, the track is shorter than the forecast – slightly more than half the distance.
  • my gut feeling after comparing several forecasts to tracks: ECMWF wind speed forecasts are a tad on the high side.

Here is a second example:

  • trajectory colors: red: 2500ft, blue: 3000ft, purple: 5000ft
  • most of the flight was 1000-1200m altitude
  • the track’s match with the 3000ft trajectories (all models) again is very good!

Creating wind trajectories with windy.com

Update: head over here for the current version of the trajectory function – the text below documents plugin versions below 0.5.1.

windy.com is a phenomenal resource for weather forecasting. It’s less of a “will it rain at X by Y” type app but rather a data viewer for many different layers of information – you will have to draw your own conclusions. It also sports a variety of forecast models (among them the $$$ ECMWF) – for free. And it supports user-written plugins for specific purposes, which has unleashed a lot of creativity by weather-affine geeks.

For balloonists, the traj plugin is very useful – it can create tracks to answer the question “If I start at X and fly level in altitude Y at time Z, which track would I fly?”. And it can answer this according to several weather models – in Europe that’d be ECMWF, GFS and ICON-EU.

Here’s a short how-to for generating trajectories:

  • The traj plugin currently only works on desktop browsers – not (yet) on mobiles
  • The plugin must be explicitly loaded after pointing your browser to https://www.windy.com/ : click on the top left “Hamburger” (three horizontal bars) icon. In the dropdown, click “Install Windy plugin” and scroll down until you find the “Trajectory” plugin – then click on “Open”.
  • two windows will appear – one with explanations and one for parameters.
  • Select altitudes and duration in the parameter window.
  • Set a start time using the time selector at the bottom of the screen.
  • Select a model (ICON, ECMWF or GFS) in the right bottom model selector.
  • Choose a start location – either by clicking on the map window (the “picker”) or choosing a favorite by clicking the heart icon in the parameter window which will present a dropdown of your favorite locations
  • click “Start” – which changes the label on this button to “Stop”
  • the progress can be seen on-screen as the curves are drawn
  • the black bullets denote hourly intervals
  • Click on the lines, or the bullets for detail information (time, speed, model, model generation time etc)
  • when the plugin is done, you can download the trajectories as GPX files by clicking “Save”
Trajectory tutorial video (slightly out of date)

Advanced features include:

  • simulation of ascent and descent – if set to zero, just level flight will be simulated
  • backwards trajectories – those answer the question “If I want to go to location X, where do I need to start?”
  • duration of the trajectory: up to 240 hours – useful for gas balloon flights

For a discussion of the features and for reporting errors, see the Windy community website topic on the traj plugin.

For the curious: an example GPX file generated by the traj plugin

For geeks: here is the plugin code.