Dew point

Topic: Water in the Atmosphere: Humidity, Condensation, Clouds and Precipitation · NCERT: Class 11, Ch 10 "Water in the Atmosphere"

Meaning

Dew point is the temperature to which air must be cooled to become saturated, with no change in its pressure or its moisture. At saturation the relative humidity (RH) is 100 per cent. The WMO defines it as "the temperature at which moist air is saturated with water vapour" [3].

It matters because it marks where condensation begins. Dew, fog and clouds all form only after air cools to its dew point. The gap between the air temperature and the dew point also shows how humid or dry the air feels.

Explanation

How it works: cause → process → result

• Cause: the air cools.
• Air cools at night, when the ground loses heat.

• Air also cools as it rises, because temperature drops with height in the troposphere.

• Process: it reaches saturation.

• Cooler air can hold less water vapour.
• The amount of vapour stays the same, so RH keeps rising as the air cools.

• At the dew point, RH reaches 100 per cent and the air is saturated (it holds the most vapour it can at that temperature).

• Result: condensation.

• If the air cools below the dew point, it cannot hold all its vapour.
• The extra vapour condenses (turns from gas into liquid droplets) and forms dew, fog or cloud.

Frost point: when the dew point is below 0 °C

• If the dew point is below 0 °C, the vapour turns straight into ice without first becoming liquid.
• This temperature is called the frost point [3].
• So on a very cold night, the grass gets frost instead of dew.

What the dew point tells us about humidity

• The dew point tracks the real amount of moisture in the air.
• More vapour in the air means the air becomes saturated at a higher temperature, so the dew point is higher.

• Dry air must be cooled much more before it saturates, so its dew point is low.

• The gap between air temperature and dew point:

• Small gap: the air is nearly saturated and feels muggy, as in Kolkata or Chennai before the monsoon.
• Large gap: the air is dry, as in Jaisalmer on a May afternoon.

• No gap: RH is 100 per cent, and dew, mist or fog can form.

• Dew point vs RH during the day:

• In the afternoon the air warms and RH falls, but the vapour stays the same. The air now sits far above its dew point.
• At night the air cools towards its dew point. That is why mornings bring dew and mist.

How it is measured

• Wet-and-dry bulb hygrometer (psychrometer): this uses two thermometers. One bulb is wrapped in a muslin wick wetted with distilled water [3].
• Water evaporates from the wet muslin and cools the wet bulb.

• Hygrometric tables (or a psychrometer formula) turn the two readings into RH and dew point [3].

• Warmer air saturates later. Under the Clausius-Clapeyron relation, the most vapour air can hold rises by about 7 per cent for each 1 °C of warming [5]. So warm air needs much more vapour before it reaches its dew point.

Where it occurs

• Humid coasts of India, where air temperature stays close to the dew point:
• Kolkata (Bengal delta) and Chennai before the monsoon feel muggy because the air is nearly saturated.

• Kochi (Kerala, Malabar coast, about 10° N, on the Arabian Sea): in humid air, wet clothes dry slowly and sweat stays on the skin.

• Dry interiors, where air temperature is far above the dew point:

• Jaisalmer on a May afternoon, and Jaipur (Rajasthan, on the edge of the Thar Desert).
• Delhi for much of the year.

• The Sahara has little water to supply vapour, so the dew point stays low.

• Oceans vs continents: RH is higher over oceans than over continents, so the air over the sea sits closer to its dew point.

• With height: air cools as it rises and reaches its dew point. So clouds, rain and storms mostly form in the lowest few kilometres of the atmosphere (the troposphere).
• Weather stations: psychrometers and electronic sensors give humidity and dew-point data for forecasts, agriculture, hydrology and aviation [3].

Don't confuse with

• Relative humidity (RH): RH is a percentage and changes with temperature even when no vapour is added. The dew point is a temperature, and it changes only when the amount of moisture changes.
• Frost point: this is the saturation temperature when it lies below 0 °C. There, vapour turns straight into ice, not dew [3].
• Wet-bulb temperature: this is the reading of the thermometer with the wet muslin, cooled by evaporation. It is used with the dry-bulb reading to work out the dew point. It is not the dew point itself [3].
• Dew: dew is the water droplets that form on surfaces. The dew point is the temperature at which saturation, and so condensation, begins.

Prelims Hooks

• At the dew point the air is saturated, so RH = 100 per cent. Cooling below it causes condensation into dew, fog or cloud.
• Dew point below 0 °C → frost point. Vapour turns directly into ice [3].
• Small gap between air temperature and dew point means humid, muggy air (pre-monsoon Kolkata and Chennai). Large gap means dry air (Jaisalmer in May).
• Trap: cooling air raises its RH but does not change its dew point, because no vapour is added or removed.
• A wet-and-dry bulb psychrometer plus hygrometric tables gives both RH and dew point [3].
• Clausius-Clapeyron relation: about 7 per cent more vapour per 1 °C of warming, so capacity roughly doubles per 10 °C [5].

Mains Points

• How clouds and rain begin (GS-I): air cools at night or as it rises. It reaches its dew point, and its vapour condenses into dew, fog or cloud. This explains morning dew and mist, and why most clouds and rain form in the lower troposphere.
• Humid heat as a hazard (GS-III, disasters): when air temperature is close to the dew point, sweat cannot evaporate, so the body cannot cool itself. Coastal cities such as Chennai, Mumbai and Kolkata face dangerous heat even at moderate temperatures. IMD's Heat Index (temperature plus RH, based on Steadman 1979) and WBGT support heat-action plans and work-hour rules [2].
• Climate change link (GS-III, environment): warmer air holds about 7 per cent more vapour per 1 °C before it saturates [5]. IPCC AR6 (2021) expects heavy rain to intensify at about this rate (high confidence) [4]. For India this means more cloudbursts in the Himalaya and more urban floods in Mumbai and Chennai.

Related concepts

• Humidity
• Absolute humidity
• Specific humidity
• Relative humidity
• Saturated air
• Hygrometer

Read more

• Humidity: how much water vapour the air holds