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Frost risk index calculator

See whether tonight could freeze: an evening at 6 °C with a 1 °C dew point and 10 % cloud is expected to bottom out at 1.42 °C, a frost risk index of 32.25, moderate. Clouds and dry air change the answer. Free in your browser.

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Worked example

If you enter

Evening temperature: 6
Dew point: 1
Cloud cover: 10

You get

Result: 32.25
Temperature unit: Celsius
Evening temperature c: 6
Dew point c: 1

A frost risk index calculator estimates how likely a radiation frost is after sunset from three evening observations: air temperature, dew point, and cloud cover. On a clear, dry night the ground and the lowest air lose heat as longwave radiation to space; clouds return much of that energy, so the same evening reading can finish as a hard freeze or as a safe night depending on the sky. This tool applies an Ångström-style radiative cooling heuristic: it first forms a clear-sky overnight drop from a 3.5 °C base plus a dry-air term of 0.30 °C per degree of dew-point depression, multiplies that drop by one minus 0.84 times the cloud fraction, and subtracts the result from the evening temperature to estimate the overnight minimum. That minimum is mapped onto a 0–100 frost risk index with published bands from none through severe.

What the frost risk index is and how radiative cooling is estimated

Radiation frost is the overnight drop of the lowest air and the radiating surface below freezing under a clear sky, not the arrival of a cold air mass. After sunset the ground emits longwave radiation; on a cloudless night much of that energy escapes to space and the shelter-height temperature falls several degrees by dawn. Water vapour in the air also emits downward longwave radiation, so a large dew-point depression (dry air) allows more net cooling than a moist evening at the same dry-bulb temperature.

Clouds act as a greenhouse lid: they absorb the outgoing longwave flux and reradiate a large fraction of it back, which is why an overcast night with a chilly evening often stays above the frost threshold while a clear night with the same evening temperature does not. This calculator encodes that story as a compact heuristic. Dew-point depression δ is evening temperature minus dew point, both converted to Celsius.

Cloud fraction n is cloud cover divided by one hundred. The Ångström (1918) cloud factor used in many net-longwave formulae is one minus 0.84 times n, so a clear sky keeps the full cooling and a complete overcast leaves only sixteen percent of it. Clear-sky overnight drop is 3.5 °C plus 0.30 times δ: a typical mid-latitude 2-m fall from a near-sunset observation to dawn, plus extra cooling when the air is dry.

The actual drop is that clear-sky value times the cloud factor. Estimated overnight minimum Tmin is evening temperature minus that drop. The frost risk index is a clipped linear map of Tmin: four degrees Celsius and warmer scores zero, minus four degrees and colder scores one hundred, and the interval between those anchors is 100 times (4 minus Tmin) over 8.

Bands are none at zero, low below 25, moderate below 50, high below 75, and severe at 75 and above. Frost is flagged as likely when Tmin is below zero. A dew point above the evening temperature would imply relative humidity greater than one hundred percent; the model rejects that reading instead of inventing a negative depression.

What you can do with it

Orchard and vineyard evening frost watch

Convert a near-sunset shelter temperature, dew point, and sky cover into a 0–100 frost risk index and an estimated dawn minimum before deciding whether to start wind machines or heaters.

Classroom and lab checks of radiative cooling

Students enter evening temperature, dew point and cloud cover from a textbook problem and check that the Ångström-modulated drop and the clipped 0–100 index match the published arithmetic.

FAQ

How is the frost risk index calculated from evening temperature, dew point, and clouds?

Dew-point depression is evening temperature minus dew point in °C. Clear-sky overnight drop is 3.5 + 0.30 times that depression. The drop is multiplied by the Ångström factor 1 − 0.84 times cloud fraction. Tmin is evening temperature minus the drop. The index is 0 at Tmin ≥ 4 °C, 100 at Tmin ≤ −4 °C, and linear between those anchors.

What happens when the dew point exceeds the evening temperature?

A dew point above the evening dry-bulb would mean relative humidity above 100 percent, and this radiative frost model is not defined for a negative dew-point depression. Failed validation is not billed.

Is this the same as a dew-point or vapor-pressure-deficit calculator?

No. A dew-point calculator turns temperature and humidity into Td. A VPD calculator returns the saturation–actual vapor-pressure gap. This tool starts from an evening temperature, a dew point and cloud cover and returns an overnight Tmin and a 0–100 frost risk index.

What does the frost risk index calculator cost?

The browser tool is free and runs locally.

How much do clouds reduce overnight cooling?

A great deal. The clear-sky drop is multiplied by one minus 0.84 times the cloud fraction, so a clear sky keeps all of it and a full overcast leaves only sixteen percent. That is why cloudy nights often escape frost.

Related tools

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/earth/frost-risk-index

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/earth/frost-risk-index \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"evening_temperature":6,"dew_point":1,"cloud_cover":10}'

const res = await fetch("https://api.kit.forhosting.com/earth/frost-risk-index", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "evening_temperature": 6,
    "dew_point": 1,
    "cloud_cover": 10
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/earth/frost-risk-index",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "evening_temperature": 6,
    "dew_point": 1,
    "cloud_cover": 10
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/earth/frost-risk-index", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"evening_temperature":6,"dew_point":1,"cloud_cover":10}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"evening_temperature":6,"dew_point":1,"cloud_cover":10}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/earth/frost-risk-index", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "evening_temperature": 6,
  "dew_point": 1,
  "cloud_cover": 10
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "earth.frost_risk_index",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Technical notes

If the dew point exceeds the evening temperature the air would already be supersaturated and the request fails with invalid_input.

How to call the API and what the response contains

Cloud cover must lie between zero and one hundred inclusive and is a percent of sky covered, not oktas and not a 0–1 fraction unless you scale it yourself. Send a JSON body with three required finite numbers: evening_temperature (aliases evening_temp, temp, t, temperature, and evening), dew_point (aliases dewpoint, td, dew_point_temperature, and dew), and cloud_cover (aliases clouds, cloud, cloud_fraction, n, and cloud_cover_percent). An optional temperature_unit field accepts the closed set celsius, fahrenheit, or kelvin and defaults to celsius, which is the native unit of the cooling coefficients. Fahrenheit and Kelvin values are converted to Celsius before the drop is evaluated; the echoed evening_temperature_c and dew_point_c fields always report those converted temperatures so a client can audit the conversion without reimplementing it. A dew point that exceeds the evening temperature fails with invalid_input and is never charged, because a supersaturated evening reading would make dew-point depression negative and the published heuristic is not defined for that case. Missing fields, non-finite numbers, temperatures at or below absolute zero, a cloud cover outside the published band, and an unknown temperature_unit are also rejected by name. On success the body echoes the submitted evening_temperature, dew_point, cloud_cover, and the resolved temperature_unit; it then publishes evening_temperature_c, dew_point_c, dew_depression_c, cloud_fraction, cloud_factor, cooling_clear_c, cooling_c, t_min_c, frost_risk_index, risk_band, frost_likely, result as an alias of the index, method set to angstrom_radiative_cooling, plus formula and equation strings filled with the rounded numbers. Pricing is a flat $0.002 per successful request with unit item; failed validation does not consume prepaid balance.

Orchard use, limits, and searches this page is not answering

Growers, vineyard managers, and municipal road desks use a frost risk index calculator in the evening when a forecast already gave them a dry-bulb, a dew point, and a cloud fraction and they need a single 0–100 number plus an estimated dawn minimum before they decide whether to start wind machines, light heaters, or brine a bridge deck. Classroom and exam-prep generators need a deterministic oracle for the Ångström-modulated drop so a grader can mark both the clear-sky limit (cloud factor one) and the rejection of a dew point above the evening temperature. Prefer this capability when you already have an evening temperature, a dew point and a cloud cover and only need the estimated minimum and the 0–100 index. Enter the near-sunset shelter temperature, the matching dew point, and the percent of sky covered; read frost_risk_index, risk_band, t_min_c, and frost_likely. Keep cloud cover inside the published zero-to-one-hundred band, keep both temperatures above absolute zero, and prefer JSON numbers over pre-formatted strings when you control the client. This page targets people searching for a frost risk index calculator, overnight frost from evening temperature and dew point, or a radiative cooling frost heuristic online. It is not a dew-point solver from temperature and humidity, not a vapor-pressure-deficit tool, not a freezing-rain ice-accretion model for conductors, and not a numerical weather forecast. Round-trip checks are easy: form δ as Te minus Td in Celsius, form n as cloud cover over one hundred, compute (3.5 plus 0.30 δ) times (1 minus 0.84 n), subtract from Te, map Tmin through the published 4-to-minus-4 clip, and confirm frost_risk_index matches within the same eight-decimal rounding.

Integration use cases

Highway and bridge-deck frost briefing

Turn an evening road-weather observation into a deterministic frost_likely flag and t_min_c so a night desk can decide whether to brine a deck without standing up a full NWP suite.

Technical questions

What does cloud_cover mean in this calculator?

Percent of sky covered, from 0 (clear, full radiative cooling) to 100 (overcast, Ångström factor 0.16). It is not oktas. Values outside 0–100 are rejected by name.

Read the full KIT documentation →

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