Temperature & radiation

B. 1.25 m

In India the thermometers sit in a Stevenson screen with the bulbs 1.25 m above the ground - high enough to avoid the extreme ground temperature, shaded from direct sun and rain, and ventilated by louvres.

B. Inversion

Temperature rising with height is an inversion - a negative lapse rate. It is the most stable arrangement the atmosphere can have.

C. About half to one hour after sunrise

The ground keeps losing heat until the incoming sunshine becomes stronger than the outgoing radiation, which happens a little after sunrise. So the coldest time is about 30 minutes to an hour after dawn - also when radiation fog is often thickest.

B. Short waves

The hotter the body, the shorter the wavelength of its strongest radiation. The sun (about 6000 K) sends short-wave radiation; the much cooler earth sends back long-wave (infrared) radiation.

B. Inversely proportional to the absolute temperature

Wien: wavelength of peak emission = constant / T (absolute). Double the temperature and the peak wavelength halves. That is why the sun radiates short waves and the earth long waves.

C. Warm

Cloud absorbs the long-wave radiation escaping from the ground and sends much of it back down. The surface cannot cool as much, so cloudy nights stay warmer than clear ones - and fog and frost are less likely.

B. Calm

With calm air the heating by day and cooling by night stay concentrated in a thin layer near the ground, so the range is large. A strong wind mixes the heat through a deep layer and flattens the range. Clear skies and dry land add to it; cloud, sea and wind reduce it.

B. 80%

A fresh snow surface reflects about 80% of the incoming solar radiation: its albedo is very high, which is why snow-covered ground stays cold by day.

C. T⁴

Stefan's law: the energy a black body radiates is proportional to the fourth power of its absolute temperature. Double the temperature and the radiation rises sixteen-fold.

C. Overcast

Cloud blocks the sun by day and traps the earth's radiation by night. An overcast day therefore shows the smallest difference between the maximum and minimum temperatures.

C. Latent heat (evaporation and condensation)

Water evaporated at the surface carries its latent heat up with it and releases it when it condenses into cloud. This latent-heat route accounts for about 77% of the non-radiative heat flow from surface to air; sensible heat carries the rest.

B. Partially

Water vapour lets short-wave sunlight through but absorbs much of the earth's long-wave radiation, except in a few "window" wavelengths. That partial absorption is the greenhouse effect - and why humid nights cool less than dry ones.

C. 1400 hrs

The sun is highest at noon, but the ground keeps gaining more heat than it loses for another 2-3 hours. The maximum therefore comes in the early afternoon, around 1400 local time.

A. Lapse rate

A lapse rate is how fast temperature falls as you climb. If temperature stays constant with height the layer is isothermal; if it rises, that is an inversion.

B. Decreases with height

The troposphere is heated from below by the earth, so normally it is warmest at the surface and cools as you go up - on average about 2 °C per 1000 ft.

A. More intense

Every body radiates according to its temperature, so a warmer surface sends out more long-wave radiation at night. That is why the hottest days can be followed by rapid cooling under a clear sky.

B. +7 °C

Descending 6000 ft at 2 °C per 1000 ft warms the air by 12 °C: -5 + 12 = +7 °C. Going down, add; going up, subtract.

A. The difference between the maximum and minimum temperature in 24 hours

"Diurnal" means daily. The diurnal range is the day's maximum minus the night's minimum. The summer-winter difference is the annual range.

A. Insolation

Insolation (incoming solar radiation) is the solar energy received per unit area of the earth's surface. It depends on the sun's elevation, the length of the day and the transparency of the atmosphere.

C. Reflecting power of a surface

Albedo is the reflecting power of a surface - the fraction of incoming solar radiation it reflects. Fresh snow has a high albedo; dark soil and the sea have a low one.

A. Equator

The correct answer is: Equator.

B. +5 °C

ISA at FL100 is 15 - 20 = -5 °C. Add the 10 °C deviation: -5 + 10 = +5 °C.

A. Hot bodies

The hotter a body, the more energy it radiates - the total rises with the fourth power of its absolute temperature - so intense radiation comes from hot bodies such as the sun.

C. −56.5°C

The ISA temperature falls at 2 °C per 1000 ft from +15 °C at sea level to -56.5 °C at the tropopause (11 km), and then stays at -56.5 °C up to 20 km. So -56.5 °C is the lowest ISA temperature below 20 km.

A. Lapse rate

The fall of temperature with height is the lapse rate. A layer where temperature rises with height is an inversion, and one where it stays constant is isothermal.

A. The tilt of the earth axis as it orbits the sun

The earth axis is tilted about 23.5 ° and keeps pointing the same way all year. So each hemisphere is tilted towards the sun for part of the orbit - longer days and a higher sun, giving summer - and away from it for the other part. The earth is actually closest to the sun in January, which shows distance is not the cause.

B. Fall, rise, remain about the same

The ground loses heat by radiation, so the temperature falls. No moisture is added or removed, so the dew point - which depends on the moisture content - stays about the same. As the air temperature falls towards that unchanged dew point the relative humidity rises, which is why radiation fog forms in the early morning.

B. -18 °C

The ISA lapse rate is 2 °C per 1000 ft. FL140 is 3000 ft higher than FL110, so the temperature falls by 3 x 2 = 6 °C, and -12 - 6 = -18 °C. Climbing always makes it colder, which rules out -6 and -9 before any arithmetic.