Which one of the following statements is correct with reference to normal lapse rate?
- (a)Temperature is highest at ground level and decreases with increasing altitude.
- (b)Temperature is lowest at ground level and increases with increasing altitude.
- (c)Temperature remains stable with increasing altitude.
- (d)Temperature first increases with increasing altitude and gradually starts decreasing.
Correct — A, Temperature is highest at ground level and decreases with increasing altitude. The normal lapse rate is the average fall of air temperature with height in the troposphere, about 6.5 °C for every kilometre climbed — roughly one degree per 165 metres. Sunlight passes through the air with little absorption and heats the ground; the ground then warms the air above it by conduction, convection and long-wave radiation. Air therefore gets its heat from below, and the further you go from that source the colder it becomes.
- (b)Temperature is lowest at ground level and increases with increasing altitude. — That describes a temperature inversion, which is a local and temporary reversal — clear winter nights in valleys, for instance — not the normal lapse rate.
- (c)Temperature remains stable with increasing altitude. — An isothermal layer does occur in the lower stratosphere, but the lapse rate is defined precisely by the change in temperature, so a no-change answer contradicts the term.
- (d)Temperature first increases with increasing altitude and gradually starts decreasing. — This inverts the stratosphere-mesosphere pattern and in any case does not describe the troposphere, which is where the normal lapse rate applies.
The troposphere is the weather layer — about 8 km deep over the poles and 18 km over the equator — and it is heated from the bottom up. Temperature falls steadily through it until the tropopause, where the fall stops. Above that the stratosphere warms with height, because ozone there absorbs ultraviolet radiation directly.
Two words are worth separating. Lapse rate is the rate at which temperature falls with height in still air; adiabatic lapse rate is the cooling a parcel of air undergoes as it rises and expands, about 9.8 °C per km when dry and roughly 5 °C per km once condensation begins. Comparing the two tells a forecaster whether rising air will keep rising, which is the basis of atmospheric stability.
- The normal lapse rate is about 6.5 °C per kilometre in the troposphere.
- The troposphere is roughly 18 km deep at the equator and 8 km at the poles.
- Temperature stops falling at the tropopause and rises again through the stratosphere because of ozone absorbing ultraviolet radiation.
- The dry adiabatic lapse rate is about 9.8 °C per kilometre; the saturated rate is lower because condensation releases latent heat.
- A temperature inversion reverses the normal pattern locally and traps smoke and fog near the ground.
- Confusing the environmental lapse rate with the adiabatic lapse rate.
- Treating an inversion as a violation of the lapse rate rather than a local exception.
A definition item, a numerical value in °C per km, or a layer-by-layer temperature-trend question.
Normally, the temperature decreases with the increase in height from the Earth's surface, because 1. The atmosphere can be heated upwards only from the Earth's surface. 2. There is more moisture in the upper atmosphere. 3. The air is less dense in the upper atmosphere. Select the correct answer using the codes given below:
- (a) 1 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer(c) 1 and 3 only
Prelims asked for the reason rather than the pattern — why temperature normally falls with height, and which of the usual explanations actually holds.
The normal lapse rate of temperature of Earth’s atmosphere drops to 0 °C at the
- (a) upper part of ionosphere.
- (b) upper boundary of the tropopause.
- (c) lower part of mesosphere.
- (d) upper boundary of stratopause.
Answer(b) upper boundary of the tropopause.
The same lapse rate followed upward to the height at which the fall stops.
- practice — not a real PYQ
The normal lapse rate of temperature in the troposphere is approximately
- (a)1 °C per km
- (b)6.5 °C per km
- (c)9.8 °C per km
- (d)16 °C per km
Answer(b) 6.5 °C per km — about one degree for every 165 metres of ascent.
- practice — not a real PYQ
Temperature rises with height in the stratosphere mainly because
- (a)the air there is denser
- (b)ozone absorbs ultraviolet radiation
- (c)water vapour condenses
- (d)it is closer to the Sun
Answer(b) ozone absorbs ultraviolet radiation — the absorbed energy warms the layer from within.