Weather-balloon soundings

This page covers fetching a weather balloon’s measurements from the University of Wyoming’s archive and flying a rocket through them. The balloon carries a radiosonde, which measures the air on the way up, from the ground to 30 km or more. What it records is a sounding: temperature, pressure, humidity and wind at a column of heights. Hundreds of stations release one at 00 and 12 UTC (universal time, the time at Greenwich) every day. This page is for anyone who wants a flight in the air that was measured near a launch, instead of a forecast (Launch-day weather, NOAA forecasts: GFS and RAP) or the standard atmosphere.

How far to trust it.

• hpr turns the archive’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 three recorded soundings, below.
• A sounding is a measurement, but only at its station and time. The nearest station can be 100 km or more from a launch site (Santa Teresa, below, is about 130 km from Spaceport America), and the balloon goes up hours before or after the flight. Nothing here measures how much that changes a flight.
• Each row’s height is checked against the row before it, which catches a gross error in a pressure or height (57 hPa recorded for 557). More than 10 bad rows in a row refuse the answer, and so does a ground that the rows after it agree is wrong. That can refuse a good ground too, when the first few rows after it are a little off. A wrong wind, humidity or temperature is not caught: on layers under about 100 m thick the check allows any temperature from −150 to 80 °C.
• The archive serves two versions of most soundings, and they can disagree near the ground. In the example below, Calisto is 402 m from the pad at apogee in one and 563 m in the other.
• The tests replay recorded answers; the live connection to the archive is not tested in CI.

Code: hpr_net::wyoming (API reference), written for the second weather increment, M5.2b. It needs the net feature of the hpr crate. The choices are in ADR-120: University of Wyoming soundings.

What hpr asks for

A request names a station and the sounding’s hour. The station is its World Meteorological Organization (WMO) number, such as 72364 for Santa Teresa, New Mexico; the archive’s page has a map of them. WyomingRequest::latest_before(station, launch_time) picks the last 00 or 12 UTC sounding at or before a launch, with times in seconds since 1 January 1970 UTC (Unix time). The balloon goes up about an hour before that hour.

hpr asks for the archive’s comma-separated text, one row per level, and reads these columns. Pressures are in hectopascals (hPa, 100 Pa; sea-level pressure is about 1013 hPa).

The units are in the column names, and hpr refuses a file with any other. The archive serves two versions of most soundings:

The coded message holds the standard pressure levels (850, 700, 500 hPa and so on) and the significant levels between them, where the temperature or wind changes its trend.

The answer goes through hpr-net’s cache (Online data and the cache), and wyoming::fetch(&client, &request, now) takes the time now, in Unix seconds, to judge how fresh a saved copy is. For a day after its hour the archive’s copy of a sounding can still fill in, as a station’s later messages arrive:

Offline mode answers from the disk only, however old the copy. An answer hpr can’t read is never saved, so it can’t replace a good copy. A sounding the archive doesn’t have comes back as an HTTP error (404, or 400 for a BUFR file the station doesn’t send), which the request reports; no test covers that.

The archive states no terms of use. Show the credit “Sounding from the University of Wyoming’s radiosonde archive” wherever you show the sounding. Every answer carries it.

How the answer becomes a sounding

Heights. The file gives geopotential meters, which hpr converts to heights above sea level at the first row’s latitude with the World Meteorological Organization’s formula (WMO-No. 8, its Guide to Instruments and Methods of Observation, eqs. 12.15 and 12.16, as the atmosphere page explains). The balloon drifts as it climbs. Converting at the latitude it reached instead would move a height by about 0.8 m per degree of drift at 10 km, and 2.5 m at 30 km; the example’s balloon drifted 0.06°.

Why geopotential. Air pressure falls with height at a rate set by the air’s temperature and humidity. The hypsometric equation turns that into how thick each layer between two pressures must be, in geopotential meters:

thickness = (R_d T̄_v / g₀) ln(p_bottom / p_top)

Here p_bottom and p_top are the pressures at the layer’s bottom and top, R_d is dry air’s gas constant (287.05 J/(kg·K)), g₀ standard gravity (9.80665 m/s²) and T̄_v the mean of the two rows’ virtual temperatures: the temperature, raised a little for the humidity (WMO-No. 8, eqs. 12.17 and 12.18). In the three recorded soundings, across the 13 layers between the standard levels from 850 to 10 hPa, the recorded thicknesses match that to 0.01% to 0.03% on average; single layers are off by 0.05% to 0.13% on average, either way. Read as meters above sea level instead, the layers would be 0.51% to 0.60% too thin on average. So they are geopotential meters, as the column says. The test holds the average under 0.1% as geopotential meters, and more than 0.4% too thin as meters above sea level.

Each row is checked against the row before it. A row with a pressure, height and temperature fits the row before it when its height above that row is the layer’s thickness, give or take an allowance: 5% of the thickness, plus what rounding the two pressures can move it, plus 30 m. The pressures are rounded to 1 hPa in the coded message at 100 hPa or more, and to 0.1 hPa elsewhere. For example, from the coded message’s row at 557 hPa to the next at 549 hPa:

Rows are kept from a chain. hpr looks for the longest chain of rows from the ground in which each row fits the one before it in the chain. The chain may pass by up to 10 rows at a time, and the rows it passes by are left out. Of two chains equally long, it takes the one whose layers fit more closely: the smaller total of each layer’s miss as a share of its allowance (a miss of 25 m with an allowance of 50 m is a share of 0.5).

Why a chain: every row kept must also lie above the last row kept, so one bad row kept can hide the good ones. A 557 hPa row with a digit lost, 57 hPa at 5,035 m, would be kept, and every good row up to 57 hPa, about 20 km, would lie below it and be dropped. It would have to be 18.6 km above the row before it, at 570 hPa, not 183 m, so it doesn’t fit: the chain passes it by, from 570 hPa to 549 hPa, and it is left out. Two or three bad rows that fit each other are passed by the same way, since a chain through them would have to pass by more good rows. A row with no wind or humidity is left out of the profile but can still be in the chain, so the next row is checked across a thin layer, not a thick one. A row with no temperature can’t be checked: the chain steps over it without counting it, and it is left out as missing a value.

In the three recordings every row fits the one before it, missing by at most 1 m beyond rounding. So the 30 m and the 5% are margin, and the check is for gross errors, not small ones:

• The 30 m leaves room for the coded message’s heights from 500 hPa up, which are rounded to 10 m. A height 30 m off misplaces its level by about as much as a 0.33% to 0.55% pressure error (the air’s scale height, the climb over which pressure falls by a factor of e, is 5.5 to 9 km).
• The 5% is a judgment, not a measurement: no row in the recordings needs any of it. It leaves room for a layer whose inner rows have no temperature, where the mean of its two ends’ temperatures gives its thickness less well.

Then hpr keeps:

• The ground, the first row: the pressure, temperature, humidity and wind at the station when the balloon was released. A first row with a value missing or impossible refuses the answer.
• One row of each run of rows in the chain with the same pressure, the middle one. The BUFR file gives pressures to 0.1 hPa, and high up the balloon climbs tens of meters while the pressure falls that much, so runs of rows share one pressure. The rounded value is the pressure at about the middle of its run. In the example, 1,931 of the BUFR file’s 5,851 rows are the other rows of such runs. The coded message has none.
• Each such row that lies above the last row kept, higher and at a lower pressure. Rows that fall or stay at one height, a balloon coming down, fit but don’t lie above, and are left out however many.

It leaves out:

• A row the chain passes by, as above.
• A row with a value missing or impossible: the last row often has no wind, and a pressure below 0.1 hPa or above 1,200 hPa, a temperature outside −150 to 80 °C, a height outside −1 to 60 km, a wind speed below zero or above 300 m/s, a humidity below zero or a direction beyond 360° is dropped the same way.

The profile lists every row it left out, and why. Two things refuse the answer:

• A ground the rows after it disagree with. No row before the ground checks it, so hpr also looks for chains that start after it, at one of the next 11 rows with a pressure, height and temperature within the bounds. If one of them beats every chain from the ground (it is longer, or as long and fits more closely), the ground is taken as wrong. Bad rows right after a good ground can do the same: rows that miss the ground but fit the rows above them, within the allowance, beat it when the chain through them is longer (it passes by fewer good rows than it holds bad ones, less one) or as long and closer. That happens in narrow bands of error, just past the ground’s allowance: in the BUFR file, whose first layers are about 8 m thick, two rows 31 m high refuse the answer, but not 20 m high (they fit the ground and are kept) or 35 m (they fit nothing but each other, and are passed by); in the winter coded message, three rows 45 m high. So do 11 bad rows that fit each other, however far off. hpr can’t tell these from a bad ground, and refuses the answer. Rows grossly off (850 m, say) fit nothing above them and are passed by, up to 10.
• More than 10 rows after the chain’s end. The chain can pass by only 10 rows at a time, so 11 bad rows in a row end it. hpr takes that to mean the chain’s end is wrong, or all of them are (a block of heights 1 km off). A long run of rows with no temperature can also refuse the answer: the layer across the run is then too thick for its two ends’ temperatures to give. In a BUFR file 10 rows are about 10 s of the climb, some 50 m; in a coded message they can span kilometers.

A refused answer names the lines and is not cached; the other version, or the sounding 12 hours earlier, may be usable instead.

In the profile:

• Humidity above 100%, which radiosondes can report in cloud, is kept in the level as recorded and taken as 100%.
• Between levels the sounding works as for any other: the temperature and humidity are linear, and the pressure is hydrostatic (as the atmosphere page explains). The wind is interpolated by its speed and direction; pass WindInterpolation::Components to interpolate its east and north parts instead, as RocketPy does. Above the top level the standard atmosphere continues, and the air is marked as extrapolated.

An example

crates/hpr/examples/wyoming_sounding.rs fetches the Santa Teresa sounding a launch at 15:30 UTC on 21 June 2025 would have had, in both versions, and flies Calisto (one of the example rockets) from the station without its parachutes. It makes these calls:

1. Client::new(transport, Cache::new(folder), Mode::Online) sets up the fetching. A real program passes hpr_net::Http::new() as the transport (what fetches) and keeps its cache in Cache::platform_dir(). This one never uses the network: a stand-in transport answers with the two versions recorded for the tests.
2. WyomingRequest::latest_before("72364", launch_time) picks the 12 UTC sounding, and wyoming::fetch(&client, &request, now) fetches and reads it. Setting the request’s version to WyomingVersion::Bufr asks for the BUFR file instead.
3. sounding.sounding(WindInterpolation::SpeedDirection) makes the profile, and profile.wind() its wind.
4. hpr_sim::Environment::new(earth, profile, wind) puts both in a flight’s environment.

It prints the ground and five standard levels, with heights in meters above sea level, and each version’s second row. Then it prints the air at the pad and above it next to the standard atmosphere, and where Calisto is at apogee: east and north of the pad, negative for west and south. Each flight starts on its own sounding’s ground. The third flight keeps the BUFR file’s ground row and its rows from 1,438 geopotential meters up (the height of the coded message’s second row), dropping those between. Run it from a copy of the repository with cargo run --example wyoming_sounding -p hpr --features net. It prints:

Santa Teresa, New Mexico (72364), 2025-06-21 12 UTC
Sounding from the University of Wyoming's radiosonde archive
freshness: Fetched
released 58 minutes before 12 UTC
coded message: 227 levels kept, 1 left out
BUFR file: 3920 levels kept, 1931 left out

level (hPa)   height (m)   temperature (°C)   humidity (%)   wind (m/s)   from (°)
        872         1254               28.4             31          5.7        265   the ground
        850         1482               26.6             32         11.8        270
        700         3165               14.6             36          2.6        215
        500         5903               -5.3             63          8.2        180
        250        11002              -40.1              7         13.9        260
        100        16724              -73.1             13         13.9        255

second row       above the ground (m)   wind (m/s)   from (°)
coded message                     186         10.7        269
BUFR file                           8         11.1        264

height above the pad (m)   pressure (hPa)   temperature (°C)   density (kg/m³)
     0 sounding                     872.0               28.4            1.0022
     0 standard                     871.4                6.9            1.0842
  1000 sounding                     778.5               22.3            0.9151
  1000 standard                     770.3                0.4            0.9811
  3000 sounding                     613.5                4.9            0.7664
  3000 standard                     596.5              -12.6            0.7977

Calisto to apogee      apogee (m above the pad)   east of the pad (m)   north (m)
coded message                              2848.1                -401.4       -23.4
BUFR file                                  2822.5                -561.3       -48.0
BUFR above 1,438 m                         2845.9                -419.1       -19.1
standard, calm                             2811.0                  -5.1         0.0

freshness: Fetched means the answer came from the transport, not the cache. That early morning (6 a.m. local) was 21.5 °C warmer at the station than the standard atmosphere, and the air was 7.6% less dense at the ground and 3.9% less dense 3 km up. Calisto climbs 1.3% higher in it than in the standard atmosphere with no wind; the thinner air and the wind both play a part.

At apogee Calisto is about 400 m west of the pad, upwind: the wind blows from the west, 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), as on the Launch-day weather page.

The two versions put Calisto’s apogee 26 m apart, and the BUFR flight 160 m further west. Most of that is the first 186 m of the climb. Both start from the station’s wind at the ground, 5.7 m/s. The coded message’s second row, 186 m up, is 10.7 m/s from the west, so its wind grows steadily over that climb. In the BUFR file the wind is already 11.1 m/s 8 m above the ground, where the rocket is slowest and turns into the wind the most. Without its rows below 1,438 geopotential meters the BUFR flight is 419 m west at apogee and peaks at 2,846 m, within 18 m and 2 m of the coded message’s. Which of the two is nearer the wind a rocket meets is not measured: a balloon’s first seconds of drift are not a steady wind either.

How it is checked

The tests in crates/hpr-net/tests/wyoming.rs use three answers recorded from the archive: Santa Teresa on 21 June 2025 at 12 UTC in both versions, and Salt Lake City on 15 January 2025 at 12 UTC, a winter sounding. No test uses the network.

• The sounding is sampled at every kept row’s height. It gives back the recorded pressure to a relative 1e-12, the temperature to 1e-9 K, the humidity to 1e-15 and the wind to 1e-9 m/s. The test reads the expected values from the recording itself, not through the code under test, and applies the rules above on its own.
• The rows left out are exactly those the rules leave out: the last row of each coded message (no wind), and 1,931 rows of the BUFR file (the other rows of a run). The coded messages keep 227 and 241 rows, the BUFR file 3,920.
• Every BUFR row, kept or not, lies within 0.08 hPa of the profile’s pressure at its height; the worst is 0.071 hPa. Keeping the first row of each run instead would miss by 0.093 hPa.
• The ground’s wind, 5.7 m/s from 265°, is checked against its east and north parts worked out by hand: 5.678 m/s east and 0.497 m/s north.
• An edited file drops a row with a missing temperature; one with a humidity below zero, a temperature of −999 °C, a negative wind speed or pressure, or a direction of 361°; one that fits but lies below the last row kept (and not merely the row before); and a row at the ground’s pressure. The rows left out are listed in line order. A humidity of 103% is kept and taken as 100%, and a wind from 360° reads as from north.
• Every row with a pressure, height and temperature in the three recordings fits the one before it, missing by at most 1 m beyond rounding (0 m in the coded messages, 0.97 m in the BUFR file). The test works out the thickness on its own, with the file’s mixing ratio (grams of water vapour per kilogram of dry air) for the humidity. Unit tests pin the check, and the worked example above, to thicknesses and allowances worked out by hand, 0.01 m either side of the edge.
• Rows the chain passes by, with the rest of the answer kept:
  • A pressure missing a digit (57 hPa at 5 km); the profile is the recording’s without the row.
  • A height raised 850 m, and a height lowered to 4,000 m.
  • Two bad rows in a row, and two blocks of six raised rows with a good row between them.
  • A bad row after which the balloon bursts and falls back.
  • A row whose vapour pressure would exceed the air’s pressure (16 hPa at 30 °C, saturated).
  • A row that only just fits the row before it, alone: 854 hPa 50 m low, 101 hPa 95 m high, and a BUFR row at 112.8 hPa 35 m high. The next row misses it, and a chain through it is not longer and fits less closely. The rows before and after it are kept.
  • A row that misses the good row before it but only just fits the one below that, alone: 808 hPa raised 50 m, and a BUFR row at 244.2 hPa raised 35 m. A chain could skip the good row to reach it, but the chain through the good row is longer, or as long and closer.
  • A BUFR row at 150.6 hPa raised 50 m, whose pressure is rounded to 0.1 hPa. In the coded message, 549 hPa raised 45 m fits within its pressures’ 1 hPa rounding and is kept.
  • Blocks of bad rows that fit each other but not the good rows beside them, which are kept: 820 and 808 hPa raised 50 m; 101, 100 and 99 hPa raised 100 m; BUFR rows at 112.8 and 112.7 hPa raised 50 m; and two BUFR rows at 13.9 hPa lowered 50 m, and two at 10.1 hPa lowered 60 m, each inside a run sharing one pressure.
  • The two or three rows after the ground raised 850 m; the ground and the rest are kept.
  • A row inserted at 870 hPa recorded 65 m above the ground, where the air’s thickness puts it about 20 m up. It misses the ground, but the next row fits the ground, and a chain starting at the inserted row fits less closely: the row is left out and the answer is not refused.
• Rows kept or left out as they fit:
  • With no wind, or no humidity, from 250 to 55 hPa (73 rows), every row after the gap fits and is kept. Checked across the gap instead, the next row would not fit.
  • Rows falling or staying at one height at the end are left out without refusing the answer. When a row that fits and lies above follows them, the rows are left out and the row is kept. Of rows crossing back and forth over the last row kept, the first above it is kept.
  • A BUFR run of three rows at one pressure whose middle is raised 100 m keeps its first row.
  • A row at 0.1 hPa and −150 °C, 55.6 km up, fits the row below it and is kept.
• Answers refused:
  • A block of 10 rows raised 1 km is left out; 11 refuse the answer.
  • In the coded message, a ground at 87.2 hPa (872 with its decimal point misplaced), at 125 m (1,252 m with a digit lost), 8 hPa off, or 52 m or 80 m high; and a BUFR ground 34 m high, whose first layer is 8 m thick. Also a ground at 1,200 hPa, at −1,000 m or at −150 °C: these are within the bounds, but no row above fits them. In the coded message a ground 40 m high, or at 80 °C, fits the first layer (186 m thick) and is kept with every row. With only one row after a bad ground, the ground is kept and the row left out. A row with no temperature right after a bad ground isn’t counted: the refusal names the row after it.
  • Rows after a good ground that beat it: two raised 31 m and four lowered 35 m in the BUFR file; three raised 45 m in the winter coded message and four raised 50 m in the other; and 11 rows raised 1 km. Two BUFR rows raised 20 m fit the ground and are kept, leaving out the three good rows no higher than them. Two BUFR rows raised 35 m, and 10 coded-message rows raised 1 km, are passed by. A chain without the ground may start only at the 11 rows after it: after 11 rows that fit nothing, the answer is refused for the gap.
• A row raised within its allowance is kept, and leaves out the good rows just above it, which now lie below it. Raising one row, or a block of two or three, by 20, 35, 50 or 100 m, or lowering it by 50 or 100 m, refuses no answer. It loses at most 2 other levels of a coded message. In the BUFR file, sampled at every 150th row (to keep the test under a minute) and at the rows where a sweep of every row (run once, not in the tests) found the worst, it loses at most 8 other rows for one row and 10, about 50 m of the climb, for a block. Lowering one row, or changing its pressure by 3%, loses no other.
• Values just past the bounds above are left out. Unit tests keep values at each bound; at the top row, temperatures of −150 and 80 °C and a wind of 300 m/s are kept and make a profile.
• 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. A copy saved while the sounding was young is fetched again once it has settled.
• An answer that isn’t the archive’s text (such as an error page) is not saved: with no earlier copy the request fails, and with one, the earlier copy comes back, marked stale.
• Text with no header, a missing or doubled column, a unit other than the one required, a row with too few or too many fields, a field that isn’t a number, a first row with no date, and a first row with a value missing are refused. A header or a row of a million commas, and more than 100,000 rows, are refused without being read further.
• The heights are geopotential, by the hypsometric check above: the test holds each average under 0.1% as geopotential meters, and more than 0.4% too thin as meters above sea level.

What it leaves out

• How far a station’s sounding is from the air over a launch site is not measured, in distance or in time. A rocket flown hours from the balloon, 100 km away, flies other air.
• Which version is nearer the truth near the ground, where they differ most, is not known.
• A level with any value missing is left out whole, even when its other values are good.
• The check catches a gross error in a row’s pressure or height. It keeps:
  • A wrong wind, or a wrong humidity: the check doesn’t use the wind, and humidity moves the thickness by only a few percent (1.6% for saturated air at 30 °C and sea-level pressure). Only a wind outside 0 to 300 m/s is caught.
  • A wrong temperature that keeps the thickness within the allowance, and a height error within it: 50 m on the example’s layer, more on thicker ones.
  • A row whose pressure and height are both wrong yet fit each other.
  • A wrong ground that fits the row after it (in the coded message, 40 m high, or 80 °C instead of 28.4 °C), or has only one row after it.
  • Bad rows that fit their neighbours: a block of them can be kept, and good rows beside them passed by instead, no more than the block holds, and fewer unless the block fits more closely. In the coded message, 683 and 673 hPa both raised 50 m are kept, and 664 hPa is left out. High in a BUFR file, where rounding the pressure to 0.1 hPa widens the allowance, four rows at 13.9 hPa lowered 50 m are kept, and the two good rows at 14.0 hPa before them are left out.
• A good ground followed by bad rows that fit the rows above them is refused, as if the ground were wrong, when the chain through them wins: two BUFR rows 31 m high do it.
• A row, or a short block, kept a little too high leaves out the good rows just above it: in the tests, at most 2 other levels of a coded message and 10 other rows of a BUFR file (8 for one row).
• Only the archive’s comma-separated text is read, not its other formats, and there is no list of stations to search by place.
• Its terms of use are not stated; only soundings from U.S. stations, which are U.S. government works, are committed as test data (ADR-120).
• The command line fetches them with hpr weather wyoming, but hpr sim doesn’t fly them yet (issue #265), and the Python package doesn’t fetch soundings.