NOW
The station reading with humidity, dew point, wind and the send index. Today, the next twelve hours, sunrise, daylight, sunset.
Android · free · from the Red River Gorge
The climber's weather.
“A gamechanger app.”
— The Snyde, 2026
Every weather app tells you it will be 72 and sunny. None of them tells you the rock will be greasy. SendTemps adds the temperature to the humidity, gives you one number for every hour and every day, and warns you when the wall is set to coke-bottle.
- 1permission, and it's internet
- 16days out, NWS over Open-Meteo
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40°F
Mainly clear
Humidity 76%, dew 33°
Wind NW 8 mph
SEND 116 - ACCEPTABLE
40° CRISP · 76% TOO HUMID
Now: KJKL, 12 min ago
SWEATY ROCK NOW
Coke bottling likely: the dew point reaches shaded rock still cold from the night, and it sweats.
TODAY
Sunny
55 / 34
85
Best to send at 3 PM
NEXT 12 HOURS - NWS
YOUR WALLNot set - which way does it face?
›ALERTSNone in force
›DISCUSSIONThe forecasters' own reasoning, from NWS
›The number
The send index = temperature in F + humidity in %.
That's the whole formula. It's crude, it works, and it sorts a week of forecast into go and don't faster than any dew-point table. Every hour and every day in SendTemps carries its number, in the colour of the band it lands in.
55°F
+
30%
=
85 PRIME
PRIME
under 90
Cold enough, dry enough. Friction is free and the crux goes.
GOOD
90 – 109
A real day out. You will not blame the conditions.
ACCEPTABLE
110 – 129
Working a project, warming up, or you just drove four hours.
GREASE
130 and up
Hands off. Go bouldering in a cave or go to the gym.
Your bands, not ours
Those cut-offs are the defaults. BANDS lets you move every one of them — the send index, temperature on its own, humidity on its own, the two dew-point rules, and the guesses behind your wall's rock temperature — and the whole app re-colours to your numbers. Sea-level granite and desert sandstone don't want the same thresholds.
Temperature and humidity keep bands of their own, and every hour shows them in their own colours beside the number. An hour that is simply too cold or too hot takes that colour on its number too, and is never picked as the best to send while a better one is there.
Coke bottling
The rock sweats before you do.
Rock holds the night's cold for hours after the air warms up. When the dew point climbs to meet that stored cold, water condenses on the wall — the same way it beads on a bottle out of the fridge. The forecast says clear and 50. The holds are wet until one in the afternoon.
SendTemps follows the rock the sun doesn't reach: as cold as the night got, then warming a few hours behind the air. Every hour whose dew point reaches it is flagged before you load the car — so the morning you would have burned at the base is the morning you sleep in, and an afternoon the rock has warmed through is not called wet. Set your wall, and it checks the rock's own worked-out temperature instead.
An app from the Red, where the humidity is not a rounding error and coke bottling is not an edge case — it is most of the season.
SWEATY ROCK UNTIL 1 PM
Coke bottling likely: the dew point reaches shaded rock still cold from the night, and it sweats.
DAMP AIR NOW
The air is near its dew point, so nothing dries.
The three rules
- SWEATY ROCK
- The hour's dew point is at or above the rock: rock in the shade, as cold as the night got, then warming after the air, two thirds of the way in 3 hours — or, with your wall set, the rock's own temperature. The rock is that cold; water goes onto it.
- FROSTY ROCK
- The same, on rock at or below 32°F. Below freezing the water comes as frost instead of sweat, so the holds frost over.
- DAMP AIR
- The hour is within 4°F of its own dew point, a little wider under trees in leaf. Nothing on the wall is going to dry. Above freezing only: below it there is nothing liquid on the rock to dry.
Both numbers are yours to move in BANDS, as ROCK LAG and DAMP AIR. Flagged hours carry a teal stripe right through NOW, HOURLY and 16-DAY, blue for frost, so you can see the sweat lift across the morning instead of finding out at the base.
Your wall
The rock, not the air.
A north face at 9 AM and a south face at noon are different days on the same forecast. Tell SendTemps which way your wall faces, how it leans and what grows in front of it, and it works out the sun on that face and the rock's own temperature, hour by hour.
With a wall set, the number judges the rock. A cold winter day on a sunny face can come up in range, and a sunny face stops being called wet on a morning when only the shaded one is. Leave it at NONE and nothing changes.
It's arithmetic, so it shows its working. The SUN page puts these equations on the screen with every number in them, and says which were fetched, which were worked out, and which are guesses. It knows nothing of the ridge across the canyon that shades your wall at ten.
The working
Where the sun strikes the face — A its aspect, T its tilt: 70° slab, 90° vertical, 110° steep
cos θ = cos(elev) · cos(sunAz − A) · sin T
+ sin(elev) · cos T
The light on the face
POA = DNI · max(0, cos θ) · τ_direct
+ DHI · (1 + cos T)/2 · τ_diffuse
+ GHI · albedo · (1 − cos T)/2
Where the rock is heading, and where it is
T_eq = T_air + a · POA / (1 + b · wind)
T_rock = T_before
+ (T_eq − T_before) · (1 − e^(−1/τ))
Wet
SWEATY ROCK when dew point ≥ T_rock
Fetched: DNI, DHI and GHI from Open-Meteo, the mean of the hour before each stamp, and the air, wind and dew point. Worked out: the sun's elevation and azimuth, half an hour before the stamp, and the face's angle and views. Guessed, and yours to move: the trees' τ, a = 0.040°F per W/m², b = 0.15 per mph and τ = 3 hours — until someone measures real rock.
16-DAY · SLADE, KY
BEST TO SEND TODAYsend 85, 3 PM
A November day at the Red. These seven are the National Weather Service's; days 8–16 carry on from Open-Meteo, drawn paler. Each bar runs through the temperature bands' colours, and a teal stripe marks a day that coke-bottles. Your own week will look nothing like this one.
Sixteen days
Answer the group text before you open the app.
One row a day: rain chance, the high-low bar on a shared scale, the numbers, and the day's best send index in its colour. Today is the day, the rain arrives Monday, Tuesday is a write-off at 133, and it is clean again by Thursday. You can read that in two seconds from the car.
BEST TO SEND picks the day, and on HOURLY it picks the hour — over a 12, 24 or 48 hour or 6 day window, with LOW, HIGH and the coke-bottling call for whatever window you're looking at. Tap or drag across the chart to read any single hour.
What's in it
Every page, one tap apart.
HOURLY
TEMP, HUMID, WIND, RAIN or SEND over 12H, 24H, 48H or 6D. Drag to read an hour; LOW, HIGH and BEST TO SEND for the window.
16-DAY
A row a day with the best day to send. NWS for the first week, Open-Meteo for the rest, marked so you know which is which.
YOUR WALL & SUN
Which way your wall faces, how it leans, the trees in front. Then the sun on it and the rock's temperature, worked out under the equations.
RADAR
The nearest NWS radar's last hour, renewed for ten minutes while it's open, then on RELOAD. Pinch to zoom. The last loop is kept for no signal.
ALERTS
NWS watches, warnings and advisories in force — on top of NOW in the severity's colour, in full on the page, and on the widget.
DISCUSSION
The Area Forecast Discussion from your city's NWS office. The forecasters' own reasoning, section by section, in plain sight.
AIR + SPACE
US air quality this hour and its peak, PM2.5, PM10, ozone, UV index, pollen, NOAA's space weather scales, and the moon, drawn.
ON THIS DAY
This hour 10, 20, 30 and 40 years ago, red where it was warmer and green where it was cooler, against the 1991–2020 normal.
CLIMATE
How the last ten days actually ran against the normal — the average departure, then a row a day with rain, high, low and a bar.
WIDGET
The city, the reading, its send index, the coke-bottling warning, today's best hour, and the next twelve hours as bars on the home screen.
CITIES & BANDS
Your crags, found by search and kept on the phone. Every cut-off adjustable — the send index, temperature, humidity, and the two dew-point rules behind the sweat warning.
CRAG MAP
Every crag in the app on a map the phone draws itself, with the towns, roads, rivers and the lie of the land. Nothing downloads to draw it, so it works at the base. A pin off? MOVE PIN sends the fix.
OFFLINE
The last forecast for each city is saved and shown with the time it was taken. The canyon has no bars. The app still opens.
Privacy
It asks for one permission, and that permission is internet.
No location. No account. No sign-in, no email, no keys, no analytics, and no log of what you looked up. Your cities, your days, your walls and your bands live on your phone and are excluded from backups and device transfers.
HTTPS only, three times over: the network config refuses cleartext, the client refuses a non-https URL, and a redirect can never drop to http. There's no WebView anywhere in it.
- Read the full privacy policy →
- No location permission
- No account, no email
- No analytics, no trackers
- No ads, ever
- No notifications to allow
- Nothing leaves the phone but the forecast request
Where the numbers come from
Public data, credited.
National Weather ServiceForecasts, station readings, radar, alerts and area forecast discussions. US, public domain.
Open-MeteoForecasts and sunlight worldwide, city search, and ERA5 reanalysis for days no station holds. CC BY 4.0.
NOAA NCEIHourly station records, daily summaries and the 1991–2020 normals behind ON THIS DAY and CLIMATE.
CAMS & NOAA SWPCAir quality and pollen, and the space weather scales, the Kp index and the G scale ahead.
Two of the least predictable things in the world are the weather and climbing. This app is a guide, not a verdict: we have watched a GREASE afternoon turn PRIME after a passing storm, and a PRIME morning turn into lightning by three. Use your own judgment first, let this help, and never let a forecast keep you home on one of your days.
SendTemps is not a safety device and no substitute for official warnings. Forecasts are other people's models and observations, passed through arithmetic; the rock, the sky and the decision are yours.
Get SendTemps
Android 8 and up. Free, and it stays free. If a number looks wrong to you, it probably is, and I want to hear about it.
Got it already, and a number looks wrong? jeff@sendtemps.org