The percentage of moisture in the air relative to the maximum it can hold at that temperature. 100% means saturated — fog, rain or dew are likely.
measurementWhat is Relative Humidity?
Relative humidity (RH) is the percentage of water vapour in the air, expressed as a fraction of the maximum amount the air could hold at the current temperature. RH ranges from 0% (completely dry — almost never seen in nature) to 100% (saturated — fog, dew or rain begin to form).
The standard formula:
RH = (actual vapour pressure / saturation vapour pressure) × 100%In simpler terms: if the air is holding half as much water as it possibly could at its current temperature, RH = 50%.
RH is the most commonly reported humidity measurement because it is intuitive and easy to measure with a hygrometer. But it has a confusing property that catches many people out: RH changes when temperature changes, even if the actual moisture content doesn’t. This makes RH a poor stand-alone indicator of “how humid it really is.”
Why RH is temperature-dependent
Warm air can hold more water vapour than cold air. The saturation vapour pressure roughly doubles for every 10°C increase in air temperature. Consequences:
- The same parcel of air can read 100% RH at 20°C and only 50% RH at 30°C without losing or gaining any moisture — just by warming up.
- That is why morning fog “burns off” as the sun rises: the moisture is still there, but the warming air can suddenly hold much more, so RH drops below 100% and droplets evaporate.
- That is also why RH is naturally higher at night and lower in the afternoon.
For comparing days or comparing cities, dew point is a more honest comfort metric because it doesn’t fluctuate with temperature.
RH in South Asia
South Asia is one of the most humid populated regions on Earth.
Typical seasonal RH:
| Season | Delhi | Mumbai | Chennai | Kolkata | Dhaka | Karachi |
|---|---|---|---|---|---|---|
| Winter (Jan) | 65% | 70% | 75% | 75% | 75% | 60% |
| Pre-monsoon (May) | 30% | 70% | 75% | 65% | 70% | 65% |
| Monsoon (Jul–Aug) | 80% | 85% | 80% | 85% | 90% | 75% |
| Post-monsoon (Oct) | 70% | 75% | 78% | 75% | 80% | 65% |
Coastal cities stay above 70% RH all year. Inland cities like Delhi swing between very dry (30%) in May and very humid (80%) during monsoon.
RH and comfort
The body’s primary cooling mechanism is sweating. The cooler effect comes from sweat evaporating, not just sitting on skin. RH directly affects evaporation:
| RH at 30°C | How sweat behaves |
|---|---|
| 30% | Sweat evaporates quickly — efficient cooling |
| 60% | Sweat evaporates moderately — some cooling |
| 80% | Sweat barely evaporates — minimal cooling |
| 95% | Sweat does not evaporate — no cooling |
This is why 30°C in dry Rajasthan feels manageable, while 30°C in coastal Mumbai during monsoon feels suffocating.
The heat index and feels-like temperature formulas combine air temperature with RH to give a more honest comfort metric. South Asian coastal cities routinely have a feels-like temperature 5–10°C higher than the thermometer reading.
RH and weather
RH provides clues about coming weather:
- Rising RH + cooling air → dew, fog or rain likely tonight.
- Falling RH + warming air → clearing skies, dry afternoon.
- Very low RH (below 25%) → high fire risk, dry skin, static shocks.
- Very high RH (above 90%) → mould risk indoors, electronics issues, dehumidifier needed.
For air conditioner sizing, engineers use the highest expected dew point (not just temperature) to design adequate dehumidification capacity.
How RH is measured
Several instruments measure RH:
- Hygrometer — direct RH measurement; modern electronic hygrometers use capacitive or resistive sensors.
- Psychrometer (wet/dry-bulb thermometer) — traditional method; difference between wet-bulb and dry-bulb temperatures gives RH via lookup table.
- Sling psychrometer — handheld version still used by meteorologists.
- Dew-point hygrometer — chilled-mirror method, most accurate.
Open-Meteo and other weather APIs report RH at 2 metres above ground, the standard meteorological reference height.
Frequently asked questions
What is a comfortable indoor RH? 30–50% is generally considered comfortable for humans. Below 30% causes dry eyes, skin irritation and respiratory issues. Above 60% promotes mould, mites and bacteria. South Asian air-conditioning typically targets ~50% RH.
Why does Mumbai feel worse than Delhi in summer? Mumbai’s coastal humidity is much higher. At equal air temperatures (e.g., 35°C), Mumbai’s RH might be 75% while Delhi is 25%. The dew point in Mumbai is 27°C vs 12°C in Delhi — the same air carries about three times more moisture in Mumbai, making evaporative cooling much less effective.
Can RH exceed 100%? Briefly, yes — it’s called supersaturation. Air can hold slightly more moisture than the equilibrium value if there are no nuclei for condensation. Cloud physicists use this concept. In practice, once nuclei (dust, pollen, soot) are present, excess vapour immediately condenses to droplets, capping RH at 100%.
Does RH affect electronics and food? High RH (above 60%) corrodes electronics, promotes mould on food and accelerates rust. Many South Asian households need dehumidifiers during monsoon to protect cameras, musical instruments and stored food.
Where can I see live RH for my city? Mausam Online shows live RH, temperature and feels-like on every city page. See Mumbai, Kolkata, Delhi, Chennai, Dhaka, Karachi.