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Summer simmer index calculator

At 90 °F and 60% relative humidity, the summer simmer index lands in the discomfort band. It folds heat and humidity into one thermometer-like number for warm weather only, and refuses readings below 70 °F. Free in your browser.

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Worked example

If you enter

Creativity: 90
Relative humidity: 60

You get

Temperature c: 32.2222222222
Humidity factor: 0.22
Temperature excess: 32
Apparent temperature: 82.96

The summer simmer index is a warm-weather scale of perceived warmth that folds dry-bulb air temperature and relative humidity into one thermometer-like number. John W. Pepi published the index so summer comfort could be read without crossing a two-way heat chart: hotter air and wetter air both raise the score, while dry air pulls it back toward the dry-bulb reading. This summer simmer index calculator applies the classic Pepi identity, SSI equals 1.98 times the Steadman indoor apparent temperature minus 56.83, with air temperature in degrees Fahrenheit and relative humidity in percent.

What the summer simmer index measures and how it is defined

The summer simmer index is a rescaled indoor apparent temperature designed for warm, humid weather rather than for winter wind or for the National Weather Service outdoor heat index. Start from the Steadman indoor apparent temperature AT equals T minus the quantity 0.55 minus 0.0055 times relative humidity, all multiplied by T minus 58, with T in degrees Fahrenheit and relative humidity in percent.

That inner term is a linear humidity correction: at one hundred percent humidity the subtracted factor vanishes and apparent temperature equals air temperature; at lower humidity the factor grows and the index falls below the dry-bulb reading because sweat can evaporate. Pepi then stretches that apparent temperature onto a summer comfort scale with the affine map SSI equals 1.98 times AT minus 56.83. The stretch is not cosmetic.

It places everyday midsummer readings on a thermometer-like axis whose published bands run from slightly cool through comfortable, slightly warm, increasing discomfort, discomfort, increased discomfort, and on into dangerously uncomfortable and extremely dangerous territory. Use it when you want the Pepi summer simmer number itself, when a lesson must show how humidity moves perceived warmth on a dedicated summer scale, or when a script should fail closed below seventy degrees Fahrenheit instead of silently inventing a winter reading the index was never meant to cover.

What to enter and which readings are refused

Relative humidity must lie between zero and one hundred inclusive. Negative humidity is unphysical and is rejected; values above one hundred percent are rejected as well. Temperature must be at or above seventy degrees Fahrenheit.

Values below that warm-condition floor fail with a message that temperature is below seventy degrees Fahrenheit, because the summer simmer index is defined only for warm conditions. Example: ninety degrees Fahrenheit at sixty percent relative humidity yields an SSI in the discomfort band.

What you can do with it

Classroom comparison with the NWS heat index

Compute the Pepi summer simmer index next to a Rothfusz heat index so students see two different humidity-aware summer scales, units, and validity floors.

Outdoor-event and public-health briefings

Turn station temperature and humidity into an SSI comfort band for heat-awareness notes when the briefing should use the simmer scale rather than the NWS chart.

Warm-space HVAC and greenhouse checks

Estimate how a humidity swing changes perceived warmth in an already warm room without standing up a full psychrometric inversion.

FAQ

What formula does this summer simmer index calculator use?

The inner parenthesis is the Steadman indoor apparent temperature.

Why does the calculator reject temperature below 70 °F?

The summer simmer index is defined only for warm conditions.

Is this the same as a heat index calculator?

No. A National Weather Service heat index uses the Rothfusz polynomial (and a simple rule below eighty degrees).

What units should I use for temperature and humidity?

Temperature is dry-bulb air temperature in degrees Fahrenheit, accepted only at or above 70 °F. Relative humidity is a percent from 0 to 100. The constants 58, 1.98, and 56.83 are calibrated for the Fahrenheit form.

What do the summer simmer index bands mean?

They run from slightly cool through comfortable, slightly warm, increasing discomfort and discomfort, up to dangerously uncomfortable and extremely dangerous. For example, 90 °F at 60% humidity falls in the discomfort band.

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/summer-simmer-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/summer-simmer-index \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"temperature":90,"relative_humidity":60}'

const res = await fetch("https://api.kit.forhosting.com/earth/summer-simmer-index", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "temperature": 90,
    "relative_humidity": 60
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/earth/summer-simmer-index",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "temperature": 90,
    "relative_humidity": 60
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/earth/summer-simmer-index", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"temperature":90,"relative_humidity":60}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"temperature":90,"relative_humidity":60}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/earth/summer-simmer-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

{
  "temperature": 90,
  "relative_humidity": 60
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "earth.summer_simmer_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

Temperature below seventy degrees Fahrenheit is rejected with invalid_input, because the index is defined only for warm conditions and a cooler input would look precise while sitting outside the published domain.

Input contract, response shape, and error behaviour

POST a JSON object with two required finite numbers. temperature is the dry-bulb air temperature T in degrees Fahrenheit; aliases t, temp, temp_f, temperature_f, and air_temperature are accepted. relative_humidity is RH in percent; aliases rh, humidity, rh_percent, and relative_humidity_percent are accepted. Each field may arrive as a JSON number or as a numeric string; missing, blank, or non-finite values fail with invalid_input naming the field. On success the body echoes the rounded temperature and relative_humidity, reports temperature_c as the Celsius conversion of the same dry-bulb reading, exposes humidity_factor as 0.55 minus 0.0055 times RH, temperature_excess as T minus 58, and apparent_temperature as the inner Steadman AT. The primary result is summer_simmer_index. category and category_label name the published comfort band, threshold restates seventy, and unit is fahrenheit. formula and equation reprint SSI equals 1.98 times AT minus 56.83 with the substituted numbers, method is pepi_summer_simmer_index, and steps walks through the factor, the excess, the apparent temperature, and the final affine map for teaching UIs. Failed validation is not billed; successful analytic calls use the published $0.002 base price with no model tokens and no outbound network.

Where a summer simmer index API earns its keep

Meteorology instructors need a deterministic oracle that reproduces the textbook Pepi identity so lab write-ups can compare the summer simmer index with a National Weather Service heat index without spreadsheet drift, and so a cool-air input is marked wrong instead of quietly returning a number that looks like a winter comfort score. HVAC and greenhouse notebooks sometimes want the simmer reading when the space is already warm and the question is how much extra perceived warmth a humidity swing will add. Research and grading pipelines that batch thousands of (T, RH) pairs need fixed twelve-decimal rounding and a hard seventy-degree floor they can assert in tests. Public-health and outdoor-event briefings still quote SSI bands when they want a humidity-aware summer scale that is not the Rothfusz chart; a tiny parse API lets a lesson page or a briefing script apply the same arithmetic the instructor just wrote on the board. Keep temperature in Fahrenheit at or above seventy degrees, keep relative humidity between zero and one hundred percent, pin summer_simmer_index and category in tests, and you get a reproducible warm-weather comfort index for courses, docs, and edge workers without standing up a psychrometric library of your own.

Technical questions

What does the summer simmer index calculator cost?

The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input, including temperature below seventy degrees Fahrenheit, is not billed.

Read the full KIT documentation →

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