The lapse rate refers to the average rate at which ?

Step 1: Analyze the question The question asks what the lapse rate measures. The key is to recall what "lapse" means in climatology.

Step 2: Explain the concept In climatology and meteorology, the lapse rate is the rate at which air temperature changes with an increase in altitude in the troposphere. "Lapse" means to fall or decline, so it describes the fall in temperature as you go higher.

• Normal (environmental) lapse rate: about 6.5°C per 1,000 m (or roughly 1°C per 150 m) of ascent.
• Temperature falls with height because the atmosphere is heated mainly from below. The ground absorbs solar radiation and warms the air above it. The higher you go, the further you are from this heat source, and the thinner the air, so less heat is retained.

Step 3: Check each option

• A. Radiant energy converted to heat: This describes absorption of insolation by the earth's surface, not the lapse rate. ❌
• B. Air pressure decreases with altitude: This is true as a fact, but it is not the lapse rate. ❌
• C. Air pressure increases with altitude: This is false. ❌
• D. Temperature decreases with altitude: This is the definition of the lapse rate. ✅

Step 4: Answer The lapse rate is the average rate at which temperature decreases with altitude.

💡 Exam tip: Link "lapse" with "loss": lapse = temperature loss with height. Pressure also drops with height, but the term for that is simply "pressure gradient with altitude," not lapse rate. Also remember the figure 6.5°C per km, because WASSCE and JAMB often ask for it.