How do you read a psychrometric chart?

Plot the air as a single point — dry-bulb temperature along the horizontal axis and relative humidity on the curved lines reading upward — then read off the wet-bulb and dew point that the chart ties to that point. Lay the comfort zone (roughly 23–29 °C, 30–65 % RH) on the same chart, and the point's position relative to that box tells you which passive strategy the climate needs.

Why does a desert cooler work in Jodhpur but not in Kolkata?

An evaporative cooler works by adding moisture to the air, which only cools if the air is dry enough to accept it. Jodhpur's dry air has large dew-point head-room, so evaporation drops the temperature several degrees. Kolkata's air is already near saturation, so the cooler adds humidity without cooling — it just makes the room feel clammier.

What is dew-point head-room and why does it matter?

It is the gap between the air's dry-bulb temperature and its dew point — the room evaporation has to work in. A large gap (more than about 15 °C) means evaporative cooling is viable; a small gap (under about 5 °C) means the air is too close to saturation and evaporative cooling is futile. The gap, not the temperature, predicts whether water-based cooling will help.

Lesson 1.2

Climate-Responsive Design/Module 1 · The Tools of Climate Analysis

Lesson 1.2 · The Tools of Climate Analysis

Reading a Psychrometric Chart

32 °C in Jodhpur and 32 °C in Kolkata are different worlds — and one chart knows the difference.

32 min Interactive lessonFree · open lesson

The hook

Two cities, one thermometer, opposite worlds

Two cities both read 32 °C. In Jodhpur the air is bone-dry — sweat flashes off your skin and the shade feels almost cool. In Kolkata the air is near-saturated — sweat just sits there, your shirt clings, and the same 32 feels punishing. The thermometer is honest and useless at once: it reports how hot but says nothing about how wet. The missing variable is moisture, and the psychrometric chart is the one instrument that puts both numbers on a single picture.

Same 32 on the thermometer, opposite worlds. Always plot the moisture before you trust the temperature.

Dry-bulb, RH, wet-bulb, dew point

Moist air needs four numbers to pin it down, and a psychrometric chart carries all four at once.

Dry-bulb is plain air temperature — what an ordinary thermometer reads, the horizontal axis of the chart.

Relative humidity (RH) is how full of moisture the air is, as a percentage of the most it could hold at that temperature — the family of curved lines sweeping up the chart.

Wet-bulb is the lowest temperature evaporation alone can reach; it is the ceiling on every evaporative-cooling trick.

Dew point is the temperature at which the air starts to condense — the trigger for mould, sweating pipes and corrosion.

The four are not independent: fix any two and the chart hands you the rest. That is why a single plotted point holds a whole climate.

The four properties of moist air, all located by a single point on the psychrometric chart.

Plot the point, read the cure

Plot a city as one point — dry-bulb across, humidity up — and lay the comfort zone on the same chart (roughly 23–29 °C and 30–65 % RH). Now the point's *position relative to the box* tells you the direction of the miss, and the direction of the miss is the direction of the cure.

Jodhpur lands to the right of comfort, hot and dry — you must drag it *left*, and dry air will happily take up water, so evaporative cooling is the move. Kolkata lands in the upper-right, hot and nearly saturated — dragging it left is hopeless because the air won't accept more moisture, so the only way in is *down*, via air movement.

This is the quiet power of the chart: one image converts a vague complaint about weather into a named strategy.

Jodhpur and Kolkata against the comfort box: each city's miss points to its cure.

When the wet-bulb passes about 32 °C, sweat stops working and the body cannot cool itself — the real physics behind every "feels like" warning.

The worked example

Three altitudes on the same idea

Read the band that fits you — or all three.

HomeownerWhat to ask for, in plain language

A desert cooler (an evaporative "air cooler") is brilliant in dry Jodhpur or Delhi, where it can shave several degrees off the heat. Run the identical machine in humid Kolkata or Mumbai and it just makes the room clammier — it is adding the one thing that air already has too much of. Match the cooler to your climate, not to the advertisement: dry city, evaporative cooler; humid city, a fan and a through-breeze first.

ProfessionalHow to put it in the brief

Make plotting the design-month afternoon on a real psychrometric chart a standard step of your climate analysis. The vector from that point to the comfort box names the lead passive strategy before you draw a wall — and one image will justify the whole envelope concept to a client far better than a paragraph. This reading is formalised in Lesson 1.4, where the same plot is overlaid with the Givoni strategy zones.

StudentThe numbers, derived

The relationships behind the chart: RH = (p_v / p_sat) · 100, where saturation pressure follows the Magnus relation p_sat = 0.6108 · exp(17.27·T / (T + 237.3)) kPa. Jodhpur at 32 °C / 20 %: p_sat ≈ 4.76 kPa, so p_v ≈ 0.95 kPa, giving a dew point ≈ 6 °C — about 26 °C of head-room, so evaporation has plenty of room to work. Kolkata at 32 °C / 85 %: p_v ≈ 4.05 kPa, dew point ≈ 29 °C — only ~3 °C of head-room, so evaporation is choked. Same dry-bulb, opposite problem, and the dew-point gap is the number that explains why.

Misconception check

“High temperature is what makes a climate hard to design for.”

Only half the story. Dry heat is tractable: thermal mass, evaporative cooling and night flushing all work in Jodhpur or Bikaner because the air will take up moisture and the night actually cools. Humid heat defeats those easy moves — the saturated air won't accept evaporation, the nights barely fall, and the body's own sweat-cooling stalls. A 38 °C dry afternoon is often kinder to design for than a 33 °C saturated one. It is the moisture, not the mercury, that decides how hard a climate is.

Try it

Run the method yourself

Run the chart-reading method once, on your own city, before the next lesson.

1Find your city's hottest-month afternoon dry-bulb temperature and its relative humidity.
2Place the point against the comfort zone: to the right (hot-dry), upper-right (hot-humid), or to the left (cool).
3Name the lead strategy from the direction of the miss — drag left with water, or drag down with air movement.
4Estimate the dew-point head-room: more than ~15 °C and evaporative cooling is viable; under ~5 °C and it is futile.

↳ Use the worksheet below to record your answers.

Take it with you

Plot Your Climate (PDF)A printable worksheet for this lesson's Try It.

Take this with you

Weather is two numbers

Weather, for a designer, is really only two numbers: how hot, and how wet. The psychrometric chart holds both at once, and a single plotted city reveals not just its discomfort but the direction of its cure. Every later strategy in this course — ventilation, mass, evaporative cooling, night flushing — is, at bottom, an attempt to drag a climate's point into the comfort box from whichever side it sits on.

Related concepts in the glossary

Psychrometric chart Dry-bulb temperature Wet-bulb temperature Dew point Saturation vapour pressure

Recap

Air takes four numbers — dry-bulb, relative humidity, wet-bulb and dew point — and the psychrometric chart carries all four at once. Plot a city against the comfort box and the direction of the miss is the direction of the cure: hot-dry drags left with evaporative cooling, hot-humid drags down with air movement. The dew-point head-room decides whether evaporation can work at all.

Carry forward →

The chart tells you *how* a climate misses comfort. The next lesson asks *how much*, and *for how long* — because 2 °C too warm for two months is a different problem from 8 °C too warm for six. The degree-day turns that into a single number you can design against.