# Dew Point Calculator

> Computes the dew point temperature from air temperature and relative humidity with the Magnus formula (Alduchov & Eskridge coefficients a = 17.625, b = 243.04 °C), plus vapour pressures and a comfort classification.

- Calculator id: `dew-point` · Category: Weather & Environment (`weather`) · Tool name: `calculate_dew_point`
- Canonical page: https://tttkmbb.com/weather/dew-point · This document: https://tttkmbb.com/weather/dew-point.md · JSON definition: https://tttkmbb.com/weather/dew-point.json

## Purpose

Computes the dew point temperature from air temperature and relative humidity with the Magnus formula (Alduchov & Eskridge coefficients a = 17.625, b = 243.04 °C), plus vapour pressures and a comfort classification.

**Use when:** You know temperature and relative humidity and need the dew point, the humidity comfort level, or the temperature at which condensation forms on a surface.

**Do not use when:** You know the dew point and need relative humidity or absolute humidity (use relative-humidity), or the temperature is below −40 °C or above 50 °C where the Magnus fit is not validated.

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `temperature` | number |  | required | Air (dry-bulb) temperature in temperature_unit; the fit is validated for −40 to +50 °C. (min -60, max 140) |
| `temperature_unit` | enum: c \| f |  | optional, default "c" | Unit of the temperature input(s). |
| `relative_humidity_percent` | number | % | required | Relative humidity in percent (70 means 70 %). (min 1, max 100) |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `dew_point_c` | number | °C | Temperature at which the air would be saturated (100 % RH) at constant pressure. |
| `dew_point_f` | number | °F | Dew point in Fahrenheit. |
| `dew_point_depression_c` | number | °C | Air temperature minus dew point; 0 means saturated air (fog or dew likely). |
| `vapor_pressure_hpa` | number | hPa | Partial pressure of water vapour, e = RH/100 × es(T). |
| `saturation_vapor_pressure_hpa` | number | hPa | es(T) over liquid water from the Magnus formula. |
| `comfort` | string |  | NWS comfort bands: below 55 °F dry, 55–65 °F sticky, 65 °F and above oppressive. |

## Formula

`γ = ln(RH/100) + a·T/(b + T); Td = b·γ/(a − γ) with a = 17.625, b = 243.04 °C, T in °C; es(T) = 6.1094·exp(a·T/(b + T)) hPa; e = RH/100 · es(T)`

Magnus-type formula with the Alduchov & Eskridge (1996) coefficients, accurate to about ±0.35 °C for −40 to +50 °C over liquid water. The dew point does not depend on air pressure as long as the vapour content is unchanged.

## Data Sources

- Alduchov OA, Eskridge RE (1996) Improved Magnus form approximation of saturation vapor pressure, J Appl Meteorol 35:601-609 — https://journals.ametsoc.org/view/journals/apme/35/4/1520-0450_1996_035_0601_imfaos_2_0_co_2.xml (peer_reviewed, retrieved 2026-09-24)
- Wikipedia – Dew point — https://en.wikipedia.org/wiki/Dew_point (reference, retrieved 2026-09-24)
- NWS La Crosse – Dew point vs humidity (comfort thresholds) — https://www.weather.gov/arx/why_dewpoint_vs_humidity (government, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/dew-point?temperature=…&relative_humidity_percent=…`
- `POST https://tttkmbb.com/api/v1/calculate/dew-point` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/dew-point · OpenAPI operationId `calculate_dew_point` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "dew-point", "inputs": {…}}`

## Example

- 25 °C, 70 % RH: inputs `{"temperature":25,"temperature_unit":"c","relative_humidity_percent":70}` → `{"dew_point_c":19.1,"dew_point_f":66.5,"dew_point_depression_c":5.9,"vapor_pressure_hpa":22.13,"saturation_vapor_pressure_hpa":31.62,"comfort":"Oppressive: air heavy with moisture, uncomfortable for most people (65 °F / 18 °C and above)"}`
- 86 °F, 40 % RH: inputs `{"temperature":86,"temperature_unit":"f","relative_humidity_percent":40}` → `{"dew_point_c":14.9,"dew_point_f":58.9,"comfort":"Sticky: noticeably humid, muggy evenings (55–65 °F / 13–18 °C)"}`

```
GET https://tttkmbb.com/api/v1/calculate/dew-point?temperature=25&temperature_unit=c&relative_humidity_percent=70
```

## Limitations

You know the dew point and need relative humidity or absolute humidity (use relative-humidity), or the temperature is below −40 °C or above 50 °C where the Magnus fit is not validated. Magnus-type formula with the Alduchov & Eskridge (1996) coefficients, accurate to about ±0.35 °C for −40 to +50 °C over liquid water. The dew point does not depend on air pressure as long as the vapour content is unchanged. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**Dew point or relative humidity: which describes mugginess?**

Dew point. Relative humidity depends on temperature (90 % RH at 5 °C is dry air), whereas a dew point above about 18 °C (65 °F) feels oppressive at any temperature.

**What if the dew point is below 0 °C?**

Below 0 °C the frost point (saturation over ice) would differ slightly; this calculator always uses the over-water Magnus formula, as most meteorological reports do.

## Related

- [Relative Humidity Calculator](https://tttkmbb.com/weather/relative-humidity.md) — Inverse problem: relative and absolute humidity from temperature and dew point.
- [Heat Index Calculator](https://tttkmbb.com/weather/heat-index.md) — Apparent temperature from temperature and humidity.
- [Air Density Calculator](https://tttkmbb.com/weather/air-density.md) — Humidity also lowers air density.