HumidityLens — Outdoor Humidity Patterns for Indian Cities

HumidityLens uses outdoor relative humidity at 2 metres, derived from temperature and dewpoint in the ERA5-Land reanalysis dataset, accessed via Open-Meteo's historical weather archive. For each city, it aggregates the last 5 complete instances of each calendar month — for example, August means August 2021 to 2025 — to produce a stable, climate-scale picture rather than a single year's weather noise.

Relative humidity (RH) is the amount of moisture in the air as a percentage of the maximum it could hold at that temperature. Warm air holds more water vapour than cold air, so RH can be high on a cool, damp morning and low on a hot, dry afternoon even when the absolute moisture content is identical. This is why HumidityLens reports hour-by-hour patterns: a daily average hides the swings that matter for mould growth, dust mites, and human comfort. To measure the actual RH inside your own room, see our guide on how to measure humidity in a room.

The 40 to 60% RH band follows the range identified in peer-reviewed building-health research (Sterling et al., 1985, ASHRAE Transactions) as minimising the combined risks of biological growth at high RH — mould, dust mites, and bacteria — and irritation effects at low RH — dry airways, skin, and eyes. Hours outside this band are flagged as either "dry" (below 40%) or "high humidity" (above 60%).

A month is classed "high humidity" if more than half its hours sit above 60% RH, and "dry" if more than half sit below 40%. The "most humid" and "driest" labels mark the single month (or months, on an exact tie) with the highest such share.

Outdoor RH sets the baseline for what comes through your windows and walls. Ventilating during high-humidity hours brings moisture indoors; ventilating during dry hours removes it. HumidityLens's dial shows you exactly which hours of the day are safest for opening windows, and which months call for active humidity control — a dehumidifier in sustained high-RH periods, or a humidifier in genuinely dry ones.

This is outdoor climate, not your indoor air. Ventilation, cooking, air-conditioning, and building materials all affect indoor RH independently. But knowing the outdoor pattern tells you when ventilation will help and when it will make things worse. For practical steps to bring high indoor humidity back down, see our guide on how to reduce humidity in a room.

ERA5-Land is a reanalysis dataset: real observations blended with a physics-based weather model on a roughly 9 km grid. It is not a direct station reading. Local conditions can differ from the nearest grid point, especially in coastal, hilly, or otherwise mixed terrain. The data is accessed via the Open-Meteo Archive API, which provides free access to ERA5-Land historical weather variables.

Based on outdoor relative humidity (at 2 metres, derived from temperature and dewpoint) from the last 5 complete instances of each calendar month — for example, August means August 2021 to 2025 — fetched from Open-Meteo's historical weather archive using the ERA5-Land reanalysis. All hours are local time for the selected location.

The "Right now outside" reading comes from a separate Open-Meteo current-conditions endpoint, fetched automatically when the page loads and whenever you change city. It is today's live value, not part of the 5-year averages, and the page simply omits it if unavailable.

The 40 to 60% comfort band follows the range identified in peer-reviewed building-health research (Sterling et al., 1985, ASHRAE Transactions) as minimising the combined risks of biological growth at high RH and irritation effects at low RH.

Night and day comparisons use each date's real sunrise and sunset for this location, not fixed clock hours — night is any hour outside sunrise to sunset, and the afternoon comparison window is within 3 hours of solar noon (the midpoint of that day's sunrise and sunset).

This is outdoor climate, not your indoor air. Ventilation, cooking, air-conditioning, and building materials all affect indoor RH independently.

A month is classed "high humidity" or "dry" if more than half its hours sit above 60% or below 40% RH respectively. "Most humid" and "driest" mark the single month (or months, on an exact tie) with the highest such share. A location's overall character is whichever band holds the largest share of all hours across the full window.