Weather: Elements, Condensation and Dew Formation

GC Leong treats weather and climate as two different scales of the same subject: weather is the day-to-day state of the atmosphere at one place, described through a fixed set of elements, rainfall, pressure, temperature, humidity, wind and sunshine, while climate is the long-run average of those same elements over decades. This chapter is about the elements themselves, and UPSC has already tested the single most counter-intuitive fact in it: a clear night, not a cloudy one, is what produces dew.

Why a clear sky, not a cloudy one, makes dew

Dew forms when the air right at the ground surface cools below its dew point, the temperature at which it can no longer hold all its water vapour, so the surplus condenses onto grass, leaves and other exposed surfaces. The ground cools that much only by radiating its heat away into space through the night, a process called radiative (or terrestrial) cooling, and the size of that cooling depends entirely on what stands between the ground and open sky.

A clear night lets the ground radiate heat away with almost nothing to intercept it, so the surface and the thin layer of air just above it can cool sharply, often enough to reach the dew point before sunrise. Cloud cover changes this completely: clouds absorb the infrared radiation the ground emits and radiate a good part of it straight back down, which is why the US National Weather Service's own training material describes clouds as acting like "space heaters" for the surface below them rather than an insulating blanket that only blocks outgoing heat. An overcast sky therefore keeps the ground measurably warmer overnight than a clear sky would, which is precisely why dewdrops typically fail to form on a heavily overcast night: the ground and the air just above it never cool enough to cross the dew point. The same mechanism explains frost, since frost is simply dew that condenses directly as ice when the dew point itself is below freezing, and a clear, calm night is the classic condition forecasters watch for a frost warning.

Wind matters here too, though the direct UPSC question has so far tested only the cloud-cover angle. Light or calm wind allows the coldest air to pool right at the surface where radiative cooling is happening; a stronger wind mixes that cold surface layer with warmer air above it, which is why calm, clear nights, not merely clear ones, are the classic dew and frost conditions.

The other weather elements, briefly

Rainfall, temperature and pressure are each covered in depth by their own chapters (temperature and insolation, and pressure and planetary winds, both feed into GC Leong's Climate chapter rather than this one), so this chapter's own distinct contribution is the vocabulary of condensation: dew, frost, fog and cloud are all the same underlying process, water vapour condensing once air cools past its dew point, differing only in where and how that condensation happens. Fog is essentially a cloud that forms at ground level rather than aloft, through the same radiative cooling that produces dew, but strongly enough, or over air already close enough to saturation, that visible water droplets suspend in the air itself rather than condensing only onto solid surfaces.

Quick revision points

• Dew forms on clear, calm nights, not cloudy ones: clouds absorb and re-radiate the ground's outgoing infrared heat back downward, keeping the surface warmer than a clear sky would.
• The mechanism is called radiative (terrestrial) cooling: the ground loses heat to space fastest when nothing (cloud cover) intercepts that outgoing radiation.
• Frost is the same process as dew, except the dew point itself sits below freezing, so water vapour condenses directly as ice.
• Wind disrupts dew and frost formation by mixing the cold surface air layer with warmer air above; calm air lets the coldest layer stay pooled at the ground.
• Dew, frost and fog are the same condensation process in different settings: fog is essentially ground-level cloud, formed the same way dew is, but saturating the air itself rather than only condensing on surfaces.