Learn
What the
numbers mean.
Every reading on this site is judged against a published limit. These pages are where the limits come from, who set them, how the measurement is made, and what it still cannot tell you.
Start here
Four questions that come up more than any others. Each one ends at the live reading for the same subject.
The library
One page per concept. Air and the river have the most, because those are the two readings this organisation has taken longest.
Air
The index, the two particle sizes behind it, and the argument about where they come from.
- Why do two air apps
show different numbers?Four reasons two honest sources give you a different AQI for the same city at the same minute. - What does the CPCB AQI actually measure?Eight pollutants, sub-indices, a minimum-three rule — and what the published city figure is an average of.
- Why is Delhi's air so bad?Geography, season and a regional airshed — and the part of the answer nobody has settled.
- What is Delhi's AQI?One number from eight pollutants, and why it is a worst case rather than an average.
- What is PM10?The coarse fraction — dust, grinding, construction — and why it behaves differently from PM2.5.
- What is PM2.5?Particles small enough to cross from lung into blood, and the standard India sets for them.
- What is source apportionment?How a share of the pollution gets assigned to a source, and why two studies of one city disagree.
Yamuna & water
Two measurements decide whether a river is alive, a third whether it can be touched, a fourth is the water underneath — and a fifth is how it compares.
- What is faecal coliform?The measurement that decides whether water is safe to touch, and the one Delhi fails by a factor of six thousand.
- What is groundwater extraction?The other water, and the only one of Delhi's two that is still being spent faster than it returns.
- How polluted is the Yamuna,
compared with other rivers?A national ranking of 47 rivers — and the coverage hole that stops it seeing the worst stretch of this one. - What is BOD?The oxygen a sample will consume in five days — the standard measure of organic pollution.
- What is dissolved oxygen?The single measurement that decides whether anything can live in the water.
- Why is the Yamuna so polluted in Delhi?Twenty-two kilometres, most of the flow diverted, and the drains that replace it.
Heat
A heatwave is a defined thing in India, and the definition is neither the temperature a body feels nor the night that follows it.
- What is apparent temperature?Why 38°C on the coast can be more dangerous than 44°C inland — and why no Indian criterion counts it.
- Heatwave or extreme heat?One is a declared meteorological event. The other is what actually kills people.
- How does India define a heatwave?The exact IMD rule, gate by gate, including the part that catches the coast out.
- What is a heatwave?In India it is a defined meteorological event, and the definition is not simply "very hot".
- Why warm nights matter moreThe number that is actually rising in India's heat record, and it is not the afternoon.
Forests
Why India's own report says the forest is growing while the satellites say it is going.
- Forest cover or tree cover?Two official classes with two different definitions — and neither of them means "forest".
- Is India losing forest?India's own report says the forest grew. The satellites say it went. Both are right.
- What does tree-cover loss mean?A satellite measurement of canopy disappearing — which is not the same as deforestation.
Rain
IMD classifies a day, not a storm — which is the whole difficulty.
Fire
The number everyone quotes is a count of detections, and which satellite you ask changes it.
Reading environmental data
How environmental numbers are made, where their instruments stop, and what an official count is actually counting.
- What is a climate normal?The reference every “above normal” is measured from — and changing it changes the answer without changing anything real.
- How India counts
environmental deathsOne annual table, one cause per death — and the largest killer in it is the one nobody writes about. - How to read environmental dataSeven questions that catch most of the ways a true number becomes a false claim.
- Measured or modelled?The first question to ask of any environmental number, and the one most often skipped.
- What is a reporting floor?When an instrument returns its own lowest value, that is not a measurement of nothing.
The readings these explain
Each page above hands you back to one of these. This is the other direction.
How these pages are made
No figure on any page in this section is typed by hand.
Every number in a Learn page is an address into the same committed dataset the live page reads. A standard revised by CPCB is revised in one file, and both pages change together. A figure that will not resolve stops the build rather than reaching you.
Where two credible sources disagree, both are printed and the disagreement is named. Where there is no published limit — rainfall, fire detections — the page says there is none instead of inventing a benchmark to fail.
Every page states what its own number cannot tell you. That list is not a disclaimer. It is the part most often left out of the number when it is quoted.
Reuse freely — CC BY 4.0. Each source keeps its own terms, which is why every page names the ones it used.
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