The temperature scale used by every weather service in South Asia. Water freezes at 0°C and boils at 100°C, and the thresholds that trigger heat warnings are all set in it.
MeasurementWhat is Celsius?
Celsius is the temperature scale used for weather by almost every country on Earth, including India, Pakistan, Bangladesh, Nepal and Sri Lanka. It places 0°C at the freezing point of water and 100°C at its boiling point under standard sea-level pressure, and divides the gap into a hundred equal degrees.
Every forecast, warning and station report from the India Meteorological Department, the Pakistan Meteorological Department and the Bangladesh Meteorological Department is issued in Celsius, and so is every number on Mausam Online. The thresholds inside official warnings are defined in Celsius too, so reading the scale fluently is not a conversion exercise — it is how you know whether a bulletin applies to you.
The scale is named after the Swedish astronomer Anders Celsius, who proposed it in 1742. His original version ran the other way round, with 0 for boiling and 100 for freezing; it was inverted to the familiar form shortly afterwards. The unit was formally named the “degree Celsius” in 1948, replacing the older term “centigrade”, which was ambiguous because it also named a unit of angle.
How the scale is defined
Modern metrology no longer defines Celsius from water at all. The SI base unit of temperature is the kelvin, and the degree Celsius is defined from it:
temperature in °C = temperature in kelvin − 273.15
A degree Celsius and a kelvin are exactly the same size, so a difference of 5°C and a difference of 5 K are identical. The only difference is where zero sits. Absolute zero — the point at which there is no thermal energy left to remove — is −273.15°C, or 0 K. Nothing can be colder.
Since the 2019 revision of the SI, the kelvin is fixed through the Boltzmann constant rather than through any physical substance. The practical consequence for weather is nil: 0°C is still, to any precision a thermometer on a roof in Patna can resolve, the freezing point of water.
One caveat worth knowing in the mountains. The 100°C boiling point holds at standard sea-level pressure, and pressure falls with altitude — so water boils below 100°C in Shimla, Gangtok and Skardu, lower still on the high passes. Hence the pressure cooker in every Himalayan kitchen: rice and dal will not cook properly in water that cannot get hot enough.
Converting between Celsius, Fahrenheit and Kelvin
Fahrenheit survives mainly in the United States and a few small territories, so conversion comes up mostly when reading American sources or older colonial-era records.
- Celsius to Fahrenheit: °F = (°C × 9/5) + 32
- Fahrenheit to Celsius: °C = (°F − 32) × 5/9
- Celsius to Kelvin: K = °C + 273.15
For a quick mental estimate, double the Celsius figure and add 30. That turns 40°C into 110°F, against a true value of 104°F — close enough to know what kind of day is being described, and wrong enough that you should use the real formula for anything that matters.
The two scales cross at −40, where −40°C and −40°F are the same temperature. It is the one figure worth memorising.
Celsius readings that matter in South Asia
A Celsius number only means something against a local reference. These are the ones the region actually uses.
- 0°C — freezing. In South Asia this is a mountain and northern-plain figure: Himachal Pradesh, Uttarakhand, Kashmir, Gilgit-Baltistan, Ladakh, Sikkim and, on the coldest winter nights, parts of Punjab, Haryana and western Uttar Pradesh where frost damages standing crops.
- 30°C — the IMD’s heatwave threshold for hilly regions. In the plains it is an ordinary day; in a hill station it is a genuine anomaly.
- 37°C — normal human core body temperature, and roughly the point at which the air stops helping you cool and starts loading heat onto you.
- 37°C — also the IMD’s heatwave threshold for coastal areas, which is lower than the plains figure for a reason: coastal humidity makes a lower temperature more dangerous.
- 40°C — the IMD’s heatwave threshold for the plains, and the number most often quoted in April and May bulletins for Rajasthan, Madhya Pradesh, Vidarbha, Punjab, Sindh and the Indo-Gangetic belt.
- 45°C and above — extreme heat by any standard, and the IMD’s heatwave threshold on actual maximum temperature alone; its severe-heatwave threshold is 47°C.
The declaration rules also work on departure from normal, not just absolute values: once the terrain threshold is reached, a heatwave can be declared when the maximum runs about 4.5°C or more above the local normal, and a severe heatwave at a larger departure. This matters because 38°C is unremarkable in Nagpur in May and alarming in Srinagar in any month.
Why the same Celsius reading feels different
A thermometer measures air temperature in the shade. It does not measure what the air does to a body, and in South Asia the gap between the two is often the whole story.
Humidity. At 38°C with dry desert air, sweat evaporates and the body copes. At 33°C in the Bengal or Sindh delta with very moist air, sweat barely evaporates and the body sheds heat poorly. The lower Celsius reading is the more dangerous condition — which is why forecasters pair temperature with the heat index and wet-bulb temperature.
Wind. Moving air speeds evaporation and carries heat away. The same 42°C with a breeze and in dead calm are different experiences, and a hot dry wind can make things worse rather than better by adding heat faster than evaporation removes it.
Sun and surface. The shade reading says nothing about standing on asphalt in direct sun, and the official city figure is routinely cooler than a dense, unshaded neighbourhood inside the same city.
Night minimums. A high maximum is survivable if the night cools. Heat deaths climb when the minimum stays high for several nights running and the body never recovers — exactly what an urban heat island produces during a humid heatwave.
Frequently asked questions
Why does South Asia use Celsius rather than Fahrenheit? Celsius is the international standard for meteorology, and India, Pakistan and Bangladesh all report and warn in it. Fahrenheit is now used routinely only in the United States and a few small territories.
How do I convert Celsius to Fahrenheit quickly? Multiply by 9/5 and add 32 for the exact figure. For a rough estimate, double and add 30 — good enough to picture the day, not good enough for a medical or engineering decision.
What Celsius temperature counts as a heatwave in India? The IMD uses different thresholds by terrain: about 40°C in the plains, 37°C in coastal areas and 30°C in hilly regions, and the maximum must also run roughly 4.5°C or more above the local normal, unless it reaches 45°C or more. Severe heatwave criteria sit higher still.
Is a degree Celsius the same as a kelvin? As an interval, yes — they are exactly the same size, so a rise of 3°C is a rise of 3 K. They differ only in where zero is placed: 0 K is absolute zero, which is −273.15°C.
Where can I see current Celsius readings for my city? Every Mausam Online city page shows the current temperature, the day’s maximum and minimum and the feels-like value in Celsius. Try Delhi, Karachi, Dhaka, Nagpur, Srinagar and Thiruvananthapuram.