A day's temperature

Why is the coldest moment just after sunrise?

A day's temperature

Play the day, then change the surface, the sky and the wind. Watch the range shrink or grow.

• Inland plains (Delhi)
• Land has a low specific heat, so it heats fast by day and cools fast at night. The diurnal range can average up to about 20 °C.
• Thar desert
• Dry sand and clear skies give the biggest range of all: scorching afternoons and chilly nights.
• Open sea
• Water's high specific heat, mixing and evaporation keep the sea's day–night change under 1 °C.
• Coast, sea breeze
• At a coastal station the range depends on the wind direction. A wind off the sea keeps it small; a wind off the land makes it almost as large as inland.
• Clear sky
• Sunshine reaches the ground by day and heat escapes freely at night: the largest range.
• Thin high cloud
• Even a thin sheet of cirrostratus cuts off a fair part of the sunshine.
• Low overcast
• Low cloud blocks the sun by day and sends heat back down by night. The lower the cloud, the less the night cooling: the smallest range.
• Calm
• With no wind the day's heat and the night's chill stay in the lowest air: the biggest range.
• Light wind
• A little mixing spreads the heat through a deeper layer.
• Strong wind
• Strong wind mixes the surface air through the friction layer (up to about 1 km), so the range at the ground is small.

Memory hook: Min ½–1 h after dawn. Max 2–3 h after noon.

Takeaway: The ground turns first and swings widest; the screen air follows later. Calm, clear and dry land gives the largest day–night range; sea, low cloud or strong wind the smallest.

Why does the desert swing 20 °C and the sea less than 1 °C?

What sets the daily range

Four places, one day each.

1. Land Land has a low specific heat and heats only a thin top layer: in clear, calm weather the daily range can average 20 °C.
2. Sea The sea mixes its heat through a deep layer and has a high specific heat: its surface temperature changes less than 1 °C from day to night. At the coast it depends on the wind: off the land the range is almost as large as inland; off the sea it is small, and the sea breeze cools strongly.
3. Wind Calm air gives the largest range. In strong wind the surface air mixes with the air above, spreading the day's heat and the night's cold through the friction layer (about 1 km), so the range is small.
4. Cloud Under low cloud the range at the ground is small: less sunshine gets in by day, less heat escapes by night. The lower the cloud, the more it reduces the night cooling. Least daily range: an overcast day. Most: a dry desert.

Takeaway: Biggest daily range: over land in clear, calm weather. Smallest: over the sea, in strong winds (heat mixed through the friction layer, about 1 km) and under low cloud. At the coast it depends on the wind direction.

Why is a cloudy night warmer than a clear one?

Cloudy nights are warmer

Two nights side by side.

1. A clear night The ground radiates its heat as long waves, and on a clear night most of it escapes to space. The ground and the air on it cool strongly: frost and fog form.
2. A cloudy night Cloud absorbs most of the outgoing radiation, and its underside radiates heat back towards the ground. So the night stays warmer. By day the same cloud cuts out sunshine: a thin sheet of cirrostratus removes a fair part of it; thick low cloud or deep fog can block it almost completely. Clear, calm night: coldest. Low cloud: smallest daily range.

Takeaway: By day cloud reflects sunshine and keeps the ground cooler; by night it absorbs the Earth's outgoing radiation and sends part of it back down. So cloudy nights are warmer, and the lower and thicker the cloud, the stronger the effect.