The fraction of the sky hidden by cloud, counted in oktas or shown as a percentage — the number behind every sun, sun-and-cloud or grey-cloud icon in a forecast.
MeasurementWhat is Cloud Cover?
Cloud cover is the proportion of the sky that cloud hides from an observer on the ground. It is the quantity that decides whether a Delhi afternoon in May is punishing or merely hot, whether a Kerala morning in July sees the sun at all, and how cold a Punjab night in January gets. Almost every weather icon you have ever looked at is a picture of a cloud-cover value.
Meteorologists count it in oktas — eighths of the sky dome:
- 0 oktas — clear sky, no cloud at all
- 1 to 2 oktas — a few clouds
- 3 to 4 oktas — scattered cloud, the usual basis for “partly cloudy”
- 5 to 7 oktas — broken cloud
- 8 oktas — overcast, complete cover with no break
Consumer forecasts convert this to a percentage, which is why an app may say 40 per cent cloud cover where an aviation report says three oktas. The WMO weather codes this site uses follow the same ladder: code 0 is clear sky, 1 is mainly clear, 2 is partly cloudy and 3 is overcast.
One important caveat: cloud cover says nothing about thickness. A thin sheet of high cirrus can register as eight oktas while barely dimming the sun, and a low, dense monsoon deck at the same eight oktas can make midday feel like dusk. That is why serious forecasts split cloud into layers.
How cloud cover is measured
By eye. At manned observatories, an observer estimates what fraction of the visible sky is covered. It sounds crude but it remains the reference method, and it has one advantage no instrument shares: a human sees the whole sky at once.
By ceilometer. Automatic stations point a pulsed light source straight up and time the return from the cloud base. This gives an accurate cloud base height but samples only the narrow column overhead, so sky cover has to be inferred from how often cloud passes over a period of minutes — and the instrument can be fooled by a broken sky that happens to be clear directly above the sensor.
From space. Geostationary satellites — India’s INSAT series among them — image the cloud field continuously from above. This is the only practical way to see cloud cover over the Arabian Sea, the Bay of Bengal, the Himalaya or any of the large areas without ground observers, and it is how forecasters track a monsoon cloud shield as it moves inland.
From models. Numerical weather prediction models compute cloud fraction in each layer of the atmosphere and output total cloud cover as a percentage, usually alongside low, mid and high cloud separately. Low cloud governs how grey the day feels; high cloud governs how hazy the sun looks. Cloud is one of the harder things for a model to get exactly right, which is why forecast sky conditions are less reliable than forecast temperature.
Cloud cover across South Asia
The subcontinent’s cloud climatology is essentially the monsoon’s shadow.
- The windward Western Ghats — the coastal strip from Kerala through coastal Karnataka to the Konkan sits under near-continuous cloud for long stretches of June to August, as moist southwesterly air is forced up the escarpment.
- The rain shadow — a short distance east of the same ridge, interior Maharashtra and Karnataka see far less cloud even at the height of the monsoon. The contrast across a hundred kilometres is one of the sharpest in the region.
- The northeast — Assam, Meghalaya, Arunachal Pradesh and the hills around the Brahmaputra valley are among the cloudiest places in South Asia during the monsoon.
- The Indo-Gangetic plain — largely clear from October to April, apart from the winter low-cloud and smog season, and cloudy in bursts during the monsoon rather than continuously.
- The arid northwest — western Rajasthan, Kutch and the Sindh plains have the fewest cloudy days in the region across the year.
- Break-monsoon spells — during a break in the monsoon, cloud cover collapses over central India while the Himalayan foothills stay cloudy and wet, which is why a break brings heat to the plains and flooding to the hills at the same time.
- The northeast monsoon — from October to December, cloud cover shifts to Tamil Nadu, coastal Andhra Pradesh and Sri Lanka while most of the subcontinent dries out.
What cloud cover does to temperature and power
Cloud works in both directions, and which effect dominates depends on the time of day.
By day it cools. Cloud reflects and absorbs incoming sunlight, so a heavily clouded afternoon is cooler than a clear one. This is the mechanism behind the sharp drop in daytime maxima when the monsoon arrives.
By night it warms. The ground loses heat after dark by radiating it to space. Cloud intercepts that outgoing radiation and returns much of it, so a clouded night cools far more slowly than a clear one. Three practical consequences follow: frost in the northern plains and hills requires clear, calm nights; the lowest winter minima come on cloudless nights; and humid, cloudy nights during a heatwave deny the body the overnight recovery it needs, which is part of why humid heat is more dangerous than dry heat at the same temperature.
For solar power it is the whole story. Output tracks cloud cover almost directly. A pattern of scattered cloud creates rapid ramps in generation as individual clouds pass over a plant, which grid operators have to balance minute by minute; a solid overcast deck simply removes most of the resource for the day.
Cloud cover also matters for aviation, where the base height of a broken or overcast layer sets the ceiling for approach and landing, and for agriculture, where consecutive sunless days slow the drying of harvested grain, chillies and pulses.
Frequently asked questions
What does 40 per cent cloud cover mean in practice? Roughly three oktas — scattered cloud with plenty of blue sky between. You would expect a day of alternating sun and shade rather than a grey one, and solar output well above half its clear-sky value.
Why does my forecast show high cloud cover but no rain? Because coverage and precipitation are separate things. A sky can be fully covered by thin high cirrus, or by a dry winter stratus layer over the plains, without producing anything. Read the rain probability alongside the sky condition rather than inferring one from the other.
Why do the icon and the cloud-cover number sometimes disagree? The icon is chosen from a weather code that summarises the whole day or a whole hour, while the cloud-cover figure is an instantaneous percentage. A day that is clear in the morning and overcast by evening may average to a partly cloudy icon that matches neither half.
Does cloud cover affect the UV index? Yes, but less than people expect. Thin and broken cloud lets most ultraviolet through, and reflection off cloud edges can briefly raise ground-level UV above the clear-sky value. Only thick, continuous cloud reduces it substantially.
Where can I check cloud cover for my city? Every Mausam Online city page shows hourly cloud cover with the temperature and rain outlook. Try Pune, Bangalore, Chennai, Shillong, Islamabad or Sylhet.