How fast air moves past a fixed point, measured by anemometer ten metres above open ground and reported in km/h, m/s or knots alongside a separate gust figure.
BasicsWhat is wind speed?
Wind is air flowing from higher pressure to lower pressure, bent by the rotation of the Earth and slowed by friction with the ground. Wind speed is simply how fast that air is moving horizontally past a fixed point.
It is one of the few weather variables that is both a comfort factor and a hazard factor at once. A 15 km/h breeze in Chennai in May is the difference between a tolerable afternoon and an unbearable one; the same breeze in Delhi in January is the difference between air you can breathe and air you cannot. At five times that speed it is what takes the roof off.
Wind speed is always reported alongside wind direction, because in South Asia the direction tells you where the air has been. A southwesterly in June has crossed the Arabian Sea and is saturated. A northwesterly in May has crossed the Thar and is bone dry and burning. Same speed, opposite consequences.
How wind speed is measured
The international standard, set by the World Meteorological Organization, is to measure wind at 10 metres above the ground over open, level terrain, far enough from buildings and trees that their wake does not contaminate the reading. That standard exists because wind speed changes sharply with height and surface roughness: inside a dense city the wind can be a small fraction of what the airport station a few kilometres away is recording.
Two instruments dominate. The cup anemometer — three or four cups on a spindle, rotation rate proportional to wind speed — is mechanically simple, robust and still the workhorse of most observing networks. The sonic anemometer times ultrasonic pulses between fixed transducers, has no moving parts, responds instantly and measures the vertical component too.
The reported figure is an average over a fixed interval, not an instantaneous value. Different agencies use different averaging windows — one minute, three minutes and ten minutes are all in use — which is why the same tropical cyclone can be quoted with noticeably different wind speeds by different services. None is wrong; they answer slightly different questions.
Units, scales and converting between them
Four units are in common circulation, which causes endless confusion:
- km/h — the default in India, Pakistan and Bangladesh, and what Mausam Online displays.
- m/s — the scientific standard. One m/s is 3.6 km/h.
- knots — nautical miles per hour, used in aviation and marine forecasts. One knot is 1.852 km/h.
- mph — miles per hour, seen in British and American sources.
Above the raw numbers sits the Beaufort scale, from Force 0 (calm, smoke rises vertically) to Force 12 (hurricane force, from about 118 km/h). It was built to let a sailor estimate wind from what the sea and the rigging were doing, and it survives because those visual cues remain useful.
For tropical systems the IMD applies its own intensity ladder, based on sustained wind speed: Depression, Deep Depression, Cyclonic Storm, Severe Cyclonic Storm, Very Severe Cyclonic Storm, Extremely Severe Cyclonic Storm and, at the top, Super Cyclonic Storm. An upgrade from one rung to the next means the sustained wind has crossed a threshold.
Wind speed across South Asia
The monsoon flow. The southwest monsoon is carried by a strong, persistent low-level jet that crosses the Arabian Sea from the Somali coast and slams into the Western Ghats. It is one of the most reliable large-scale wind features on Earth, and its strength in a given week is a direct control on how much rain the west coast receives.
The wind corridors of the peninsula. Gaps in the Western Ghats — most importantly the Palakkad Gap — funnel that monsoon flow eastward into interior Tamil Nadu, producing months of strong, steady wind. This is why India’s densest concentration of wind farms sits in the southern Tamil Nadu gap country, and why coastal Kutch and Saurashtra is the other major wind power region.
The sea breeze. Every coastal city runs on a daily sea-breeze cycle: land heats faster than water, air rises over the land, and cooler marine air flows in from mid-morning. In Mumbai, Chennai and Karachi this breeze is the main mechanism capping the afternoon temperature, and when it fails heat becomes deadly — a weakened sea breeze is part of why the 2015 heatwave was so severe in Karachi.
The loo. From April to June a hot, dry westerly called the loo blows across the plains of Rajasthan, Punjab, Haryana, Delhi, Uttar Pradesh and Bihar. It is fast enough to feel like a fan and hot enough that the fan makes things worse: once air temperature exceeds skin temperature, wind adds heat rather than removing it.
Winter calm and pollution. In November to January the northern plains from Lahore through Delhi to Dhaka frequently go calm under a temperature inversion. Light winds mean nothing disperses, and PM2.5 from vehicles, industry, crop residue burning and household fuel accumulates. In that season, the wind speed forecast is effectively an air quality forecast.
Why wind speed matters day to day
- Heat and comfort. Wind accelerates evaporation from skin, which is how sweating cools you. In dry heat this works well, and evaporative desert coolers work with it. In humid coastal heat the air is already near saturation, and wind buys you much less.
- Air quality. Dispersal of PM2.5, PM10 and ozone depends almost entirely on how fast the air is moving and how deep the mixing layer is.
- Farming. Wind speed drives crop water demand, decides whether spraying is feasible without drift, and determines whether standing paddy or wheat lodges before harvest.
- Transport and energy. Crosswind limits govern aircraft landings and high winds suspend delta ferry services; wind turbines generate nothing below their cut-in speed and shut down above their cut-out speed.
- Everyday life. Kite flying around Makar Sankranti in Gujarat and Rajasthan lives or dies on wind speed, as does drying laundry, chillies, grain and fish.
Frequently asked questions
What is a normal wind speed? Most inhabited places spend the majority of the year in the light-to-moderate band, a gentle breeze you notice on the skin and in leaves. Coastal and gap-wind locations run consistently higher, and sheltered inland valleys consistently lower.
Why is the wind speed in the forecast higher than it feels outside? Because the standard measurement is taken at 10 metres over open ground. At street level in a built-up area, buildings, walls and trees strip a lot of energy out of the flow. Between tall buildings the opposite can happen and the wind locally accelerates.
What is the difference between wind speed and wind gust? Wind speed is an average over an interval; a gust is the short peak within it, conventionally the strongest three-second burst. Damage tracks the gust, while comfort, evaporation and pollution dispersal track the sustained speed.
Does wind always make it feel cooler? Not in extreme dry heat. Once air temperature rises above skin temperature, moving air transfers heat toward you as well as carrying sweat away, and the net cooling benefit shrinks or reverses. That is the essential unpleasantness of the loo.
Where can I check wind speed for my city? Mausam Online shows hourly wind speed, direction and gusts on every city page. Useful examples across the region include Chennai, Karachi, Delhi, Lahore, Ahmedabad and Coimbatore.