Description of weather overlays

  • Moderator

    ECMWF definitions:

    I think windy uses "Averaged total lightning flash density in the last 3 hours (litota3)"
    with unit conversions.

    P.S. as the model's grid is 9x9 km, I propose to keep the original ECMWF density
    (units of flashes per 100 sq. km per hour).
    Windy converted values down to 1 sq. km,
    but this is a "very small" area for lightning forecast
    and maybe it is prone to larger errors.

  • Sailor Moderator

    Thanks for this information. However, I still can’t understand how the thunderstorm maps can change every 3 hours step if the parameter used is « per day » (which is the case of the 4 parameters described in the ECMWF document). These parameters are probably useful for statistical comparisons, but not for forecast.
    And it’s what is explained in this document :


    So the ECMWF provide probably data to Windy in [ flashes / 100km2 / hour].... to be confirmed

  • Moderator

    I don't know how windy computes the lightnings parameters provided on the picker.
    Maybe is "flashes / 100km2 / hour", multiplied by 100 (to provide flashes per sq. km),
    multiplied by 3 (to provide flashes in 3 hr timesteps).
    Windy's developers know!

    I have also to mention that flashes per sq. km per day, is a UNIT, not a parameter.
    Think about "rain rate" (or intensity):
    in a stormy day we may have (for a small time period) a "rain rate" of 500 mm/hr
    but the total rainfall may be only 50 mm /24 hrs.

  • Sailor Moderator

    I understand perfectly what does mean a rate of rain, or flashes density, per hour, or per day, or per 3 hours per day...
    What bother me is the discrepancy between map displays and units supposed to be used.
    In the screenshots below the weather picker (for the same place) shows:
    At 17h, 0.04 l/km2
    At 20h, 2.22 l/km2
    At 23h, 0 l/km2


    Showing such a variation, I can’t imagine that these values are expressed per day.

    And as you said, Windy’s Developers must know which units are really used.

  • Moderator

    This is a new product and we don't know much about it.
    Personaly, I use it as an "light-moderate-heavy" indication
    rather than arithmetic values.

  • @gkikas-lgpz
    I have trouble using
    I was able to just click on carbon monoxide and see but that ceases as the choice for CO is no longer there. What happened or what am i doing wrong?

  • Moderator

    Press "more layers"

    and then activate "CO concentration".

  • @eand said in Description of weather overlays:


    Could you explain difference between:

    • wawes
    • swell 1
    • swell 2
    • swell 3

  • Sailor Moderator


    As you probably know the shape of waves, in a given place, is a mix of different kind of waves:

    • The windsea (also called wind waves) is produced by the local wind.

    • The main swell is produced by a strong wind far away.

    • Secondary swells are produced also by winds far away but in other places.

    All of them have different heights, come from different directions and show different periodes (which is the time between 2 wave crests)
    Swell 1 is the main swell, swell 2 and 3 are secondary swells (the swell classification 1, 2, 3 depends on their respective heights)
    In this example below, Wind gives the direction of windsea. Swell 1 has a height of 0.6m and a period of 9 seconds.


    The best surf conditions correspond to a strong swell with weak windsea and secondary swells.

    A cross sea thrown up by a strong wind and a strong swell whose directions make an angle close to 90º gives hard conditions for navigation.

  • Also it would be appropriate to post a legend up on the main page about what each of the layer icons represent. In this list of descriptions there are no corresponding icons. The names of the layers did not appear in the version of Firefox I was using.

  • @gkikas-lgpz I am just working on it. Coming soon both in this post and on >> "about these data" section ("clock" icon).

  • Hi ,
    I am trying to get data related to weather overlays()wind,wave height,current,current direction) in my application. My input parameters would be lat,long and datetime. Can anyone please advise how to implement using windy api

  • Is there a description of Wind Accumulation? Seems like it would follow this description from above

    "Rain accumulation: Total rain accumulation in the next hours or days."

    But I am seeing it change for the path of Hurricane Florence southward along the South Carolina and Georgia coasts, which matches the spaghetti model's adjustments I've seen as well.

    Is Wind Accumulation an indicating factor in those models I guess? Also is there a plan to add those as a layer to the Windy interface?

    Just added the app and started recommending to friends - very much enjoying it so far!

  • Sailor Moderator

    I’ve just discovered the Wind accumulation today, thank to your question !
    I don’t know if I’m right, but for me this layer is the wind accumulation on 10 days (?) for the model you have selected, nothing to do with «Spaghetti models » (that I prefer calling Multimodels forecast). It is probably not either the Ensemble Model, but just the standard Deterministic Model that is selected i.e. ECMWF, GFS or NAM. But this is to be confirmed.

  • Moderator

    What is "wind accumulation" ?
    I can not find any definition here

    I've (also) just discovered this parameter and I see that is not changing
    for the whole forecast period.

    It reffers to the max wind for the forecast period?
    or to mean direction and wind speed for the forecast period?

    We can accumulate rain or snow, but ... wind? Is not stacked !

  • Administrator

    @gkikas-lgpz It is maximum intensity projection - means that maximal value of the forecast period is taken for each point.

  • Sailor Moderator

    Thanks, but strange that there has been no announcement and explanation for this new layer.
    It’s a way to predict the track of a strong and localised storm that’s OK. Is « Wind accumulation » the best name for this layer? But I’ve not another name to propose.

  • Administrator

    @idefix37 It was released quickly because of the current hurricanes and typhoons. Probably some adjustments will be made and we will also add the description.

  • Sailor Moderator

    Sorry the announcement was in the next post !

  • Sailor Moderator

    Today there are 2 storm tracks in the North Atlantic:
    One is the track of Helene. The second one, it seems impossible to identify what makes it
    (Note that the tracks are black on my screen because I changed the wind colour settings: strong winds are black for me)


    On this track there is e.g. 154km/h at this point: N 57º16’ W16º36’.
    If the time slider is moved over the 10 days there are only 2 maximum at the same point: 95km/h on Wednesday and 104km/h on Sunday.
    So, how is it possible to have 154km/h and what is the explanation of this narrow track when there is no localised storm?

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