Launch-day weather

This page covers fetching the weather over a launch site at launch time from Open-Meteo, a free weather service, and flying a rocket through it. The weather arrives as a sounding: temperature, pressure, humidity and wind at a column of heights, from the ground up to about 24 km. It is for anyone who wants a flight in a day’s forecast instead of the standard atmosphere with one wind.

How far to trust it. hpr turns Open-Meteo’s answer into a profile that gives back the pressure, temperature, humidity and wind of every level it keeps, to rounding error. That is checked on two recorded answers, below. How good the forecast is depends on the weather model behind it, and nothing here measures that: no flight has been flown in Open-Meteo’s weather and compared with its log, and no forecast has been compared with a weather balloon. The wind on the launch rail is the model’s wind 10 m above the ground. The tests replay recorded answers; the live, encrypted (HTTPS) connection to Open-Meteo was checked once by hand, not in CI.

Code: hpr_net::open_meteo (API reference), written for the first weather increment, M5.2a. It needs the net feature of the hpr crate. Weather from a file you download is on ERA5 weather files, the air a weather balloon measured is on Weather-balloon soundings, and NOAA’s own GFS and RAP forecasts are on NOAA forecasts: GFS and RAP. The choices are in ADR-119: Open-Meteo’s pressure levels as a sounding.

What hpr asks for

Open-Meteo has two services with winds above the ground (its documentation). Both give the output of numerical weather models, hour by hour, on 19 pressure levels: heights named by the air pressure there, in hectopascals (hPa; sea-level pressure is about 1013 hPa). They run from 1000 hPa, about sea level, to 30 hPa, about 24 km up.

hpr asks for the two whole hours around the launch time, in UTC (universal time, the time at Greenwich). On each level it asks for the temperature, relative humidity, wind speed and direction, and the level’s geopotential height. At the ground it asks for the pressure, the temperature and humidity 2 m up, and the wind 10 m up. The request names the units, and hpr refuses an answer in any other. Wind directions are where the wind blows from, clockwise from north, as weather services give them.

The answer goes through hpr-net’s cache (Online data and the cache), so a second request within the “stays fresh” time above is answered from the disk, and offline mode answers from the disk only. An answer hpr can’t read is never saved, so it can’t replace a good copy. hpr writes the site’s latitude and longitude to 5 decimal places, about 1 m on the ground and far finer than any model’s grid. So a site given to 8 decimals or fewer, which your program keeps in radians and turns back into degrees (that can change its last digits), still finds its saved answer. Open-Meteo’s data is licensed CC BY 4.0: show the credit “Weather data by Open-Meteo.com (CC BY 4.0)” wherever you show the weather. Every answer carries it. The free service is for non-commercial use, under 10,000 calls a day (Open-Meteo’s terms). A self-hosted Open-Meteo server can stand in for Open-Meteo’s own: set the request’s endpoint.

How the answer becomes a sounding

• The ground is at the answer’s elevation, with the ground pressure, the 2 m temperature and humidity, and the 10 m wind. Putting the 10 m wind at the ground makes it the wind on the launch rail, rather than jumping to the lowest level’s wind above it (Loft lesson L6: a forecast profile that stepped at its lowest level).
• Levels below the ground are left out. The weather models report all 19 levels everywhere, inventing values beneath high ground. A level is kept only when its pressure is below the ground pressure and its height is above the ground. A level with a missing value at either hour is left out too, and so is one whose relative humidity is outside 0 to 100%. The profile lists every level it left out, and why.
• Heights. The models give a level’s height in geopotential meters, which hpr converts to heights above sea level at the site’s latitude with the World Meteorological Organization’s formula (WMO-No. 8 eq. 12.16, as the atmosphere page explains). Open-Meteo’s documentation calls the value an altitude above sea level, so hpr checks which it is on the two recorded answers. Air pressure falls with height at a rate set by the air’s temperature and humidity (the hypsometric equation), which fixes how thick each layer between two levels must be, in geopotential meters. From 500 to 30 hPa, the recorded layers match that to within 0.03% to 0.13% on average. Read as meters above sea level instead, they would be 0.58% to 0.68% too thin. So they are geopotential meters.
• Between the two hours every value is linear in time. The wind is interpolated by its east and north parts, so a wind that turns through the hour takes the shorter way round.
• Between levels the sounding works as for any other: the temperature and humidity are linear, the pressure is hydrostatic, and the wind is interpolated by its speed and direction (or by its east and north parts, as RocketPy does, if you ask). Above the top level the standard atmosphere continues, and the air is marked as extrapolated.
• Humidity is taken as relative to liquid water. A model that reports it relative to ice at cold levels changes the density there by very little: the two differ by at most 27 Pa of water vapour (near −12 °C), which moves the density by under 0.03% at 400 hPa. Higher up the air is colder and the gap smaller: about 6 Pa at −40 °C, 0.08% even at 30 hPa.

An example

crates/hpr/examples/open_meteo_weather.rs asks for the weather over Spaceport America at 15:30 UTC (9:30 local) on 21 June 2025. It makes five calls:

1. Client::new(transport, Cache::new(folder), Mode::Online) sets up the fetching. The transport is what fetches (Online data and the cache). A real program passes hpr_net::Http::new() and keeps its cache in Cache::platform_dir(). This one never uses the network: a stand-in transport answers with the historical-forecast answer recorded for the tests, which holds 15:00 and 16:00.
2. OpenMeteoRequest::new(latitude, longitude, time, OpenMeteoApi::HistoricalForecast) says what to ask for, and open_meteo::fetch(&client, &request, now) fetches it and reads it at the launch time.
3. profile.sounding(WindInterpolation::SpeedDirection) makes the sounding, and sounding.wind() its wind.
4. hpr_sim::Environment::new(earth, sounding, wind) puts both in a flight’s environment.

The program prints the ground and the levels below 6 km, then the air at the pad and above it next to the standard atmosphere. Last, it flies RocketPy’s Calisto (one of the example rockets) in both, without its parachutes. Run it from a copy of the repository with cargo run --example open_meteo_weather -p hpr --features net. It prints:

Open-Meteo over 32.99° N, 106.97° W at 2025-06-21 15:30 UTC
Weather data by Open-Meteo.com (CC BY 4.0)
freshness: Fetched

level (hPa)   height (m)   temperature (°C)   humidity (%)   wind (m/s)   from (°)
      859.5         1400               29.6             18          3.3        162   the ground
        850         1482               28.2             16          2.9        189
        800         2014               23.4             16          2.8        225
        700         3161               14.5             28          6.8        236
        600         4439                3.3             60          9.8        219
        500         5895               -7.0             70          9.9        200
Below the ground, left out: 1000, 975, 950, 925, 900 hPa

height above the pad (m)   pressure (hPa)   temperature (°C)   density (kg/m³)
     0 Open-Meteo                   859.5               29.6            0.9858
     0 standard                     856.0                5.9            1.0687
  1000 Open-Meteo                   765.2               20.4            0.9058
  1000 standard                     756.3               -0.6            0.9667
  3000 Open-Meteo                   602.9                3.6            0.7565
  3000 standard                     585.2              -13.6            0.7854

Calisto to apogee      apogee (m above the pad)   east of the pad (m)   north (m)
Open-Meteo                                 2880.9                  15.7      -155.6
standard, calm                             2821.3                  -5.1         0.0

freshness: Fetched means the answer came from the transport, not the cache; a second call within the hour would say Cached. The ground is at 1,400 m, so the five levels from 1000 to 900 hPa are left out. That June morning was 24 °C warmer at the pad than the standard atmosphere, and the air was 8% less dense there and 4% less dense 3 km up. Calisto climbs 2.1% higher in it. At apogee it is 156 m south of the pad (and 16 m east), upwind: the wind blows from the south-southeast at the ground, turning to the southwest by 600 m above the pad (2,014 m above sea level), and a rocket just off the rail, still slow, turns into the wind (weathercocking) and flies that way. The calm flight’s 5.1 m west is Earth’s rotation: a climbing rocket is pushed west (the Coriolis effect), and with the rotation turned off it drifts 0.006 m.

How it is checked

The tests in crates/hpr-net/tests/open_meteo.rs use two answers recorded from Open-Meteo for Spaceport America: one from the historical-forecast service (21 June 2025, 15:00 and 16:00 UTC) and one from the forecast service (2 October 2026, 18:00 and 19:00 UTC). No test uses the network.

• At each recorded hour, the sounding is sampled at every kept level’s height. It gives back the recorded pressure to a relative 1e-12, the temperature to 1e-9 K and the wind to 1e-9 m/s, and holds the recorded humidity. The expected values are read from the recording by the test itself, not by the code under test. The ground is checked the same way.
• Both answers leave out exactly the five levels below the 1,400 m ground, and keep 14. Each half of the rule (the pressure, the height) leaves a level out on its own, in an edited answer.
• At 10 and 30 minutes past the hour, every value is the weighted average of the two hours, and the wind the weighted average of its parts. A wind from 350° then 10° is from due north at half past, and a direction never reads 360°.
• A humidity of 100% at both hours stays 100% at any time between; above 100%, a level is left out and the ground refused.
• The request’s address is the one recorded, so a replayed answer fills the cache. A second request is answered from the cache, and offline mode answers without the network. tests/http.rs does the same over HTTP, from a test server on the machine’s own address.
• An answer with an hour’s values missing is not saved: with no earlier copy the request fails, and with one, the earlier copy comes back, marked stale, and stays saved. A saved answer that can’t be read at a later launch time in the same hour is fetched again online, and is the error offline.
• Text that is not JSON, Open-Meteo’s error answer, a unit other than the one asked for, a missing field, hours outside 1970 to 9999, and a time outside the answer’s hours are refused.
• The heights are geopotential, by the hypsometric check above; the test holds each average to the ranges quoted there.

What it leaves out

• Nothing checks a forecast against the weather that came. Weather-balloon soundings, the measured air, can be fetched too (Weather-balloon soundings), but no forecast has been compared with one. NOAA’s Global Forecast System and Rapid Refresh (GFS and RAP) can be fetched by name, or read from a whole GFS file you download (NOAA forecasts: GFS and RAP).
• The 2 m temperature and humidity and the 10 m wind are placed at the ground itself, so the whole launch rail sees the 10 m wind. A real wind is weaker close to the ground; how much that changes a rocket’s turn into the wind off the rail is not measured.
• A relative humidity above 100% at the ground refuses the answer rather than trimming it.
• When Open-Meteo refuses a request (a place or date it doesn’t cover), the error names the HTTP status but not Open-Meteo’s reason, because the HTTP transport drops the body of a failed answer.
• Only one place and one launch time per request, and Open-Meteo’s own choice of weather model unless you name one.
• The command line fetches it with hpr weather open-meteo, but hpr sim doesn’t fly it yet (issue #265), and the Python package doesn’t fetch weather.