If pressure decreases by 10 millibars for every 10 metres of ascent in the lower atmosphere, by how much would pressure change on ascending 200 metres from sea level?

Introduction / Context:
This question is a simple numerical application of the idea that atmospheric pressure decreases with height, using a hypothetical rule of thumb. In real climatology and meteorology, the rate of decrease varies with temperature and height, but for basic examination questions you are often given a fixed rate and asked to calculate the change in pressure over a specified vertical distance. This tests your ability to apply proportional reasoning rather than memorize complex formulas.

Given Data / Assumptions:

• The question states that in the lower atmosphere, pressure decreases by 10 millibars for every 10 metres of ascent.
• We assume this rate is constant over the small height range considered (200 metres).
• We start from sea-level pressure, so the change is measured relative to that reference.
• We are asked for the change in pressure after ascending 200 metres, not for the final absolute pressure value.

Concept / Approach:
If a quantity changes at a constant rate with respect to another variable, we can scale that rate up or down proportionally. Here, the rate of change is given as 10 millibars per 10 metres of height. This is equivalent to 1 millibar per metre. To find the total pressure change over 200 metres, we multiply the rate per metre by the number of metres ascended. The result will tell us how many millibars lower the pressure becomes compared to sea level. The correct option will therefore be the one that matches this scaled value and indicates a decrease, not an increase.

Step-by-Step Solution:
1. Interpret the given rate: a decrease of 10 millibars per 10 metres means 10 / 10 = 1 millibar decrease per metre. 2. The total vertical distance ascended is 200 metres. 3. Multiply the rate per metre by the total height: pressure change = 1 millibar per metre * 200 metres. 4. This gives a total change of 200 millibars. 5. Because the rate describes a decrease with height, the pressure at 200 metres is 200 millibars lower than the sea-level value.

Verification / Alternative check:
Even though the numerical rate given in the question is unrealistically large compared with real atmospheric conditions, in the context of an exam you must treat it as an assumed constant. A quick consistency check is to compare the options: values like 20 or 100 millibars would correspond to much smaller vertical changes under the stated rule of 10 millibars per 10 metres. Only a 200 millibar decrease correctly matches a 200 metre ascent at the specified rate. Therefore, the option describing a 200 millibar lower pressure is the only one that fits the given assumption.

Why Other Options Are Wrong:
Option B (100 millibars lower) would be correct only if the rate were 5 millibars per 10 metres, which is not what the question states. Option C (20 millibars lower) is too small and would correspond to a rate of 1 millibar per 10 metres, rather than 10 millibars per 10 metres. Option D (higher than the sea-level value) contradicts the given information that pressure decreases with height and therefore cannot be correct.

Common Pitfalls:
Students sometimes confuse the units in such questions, mixing up "per metre" and "per 10 metres" or forgetting to scale the rate properly over the full distance. Another common error is to allow real-world intuition to override the conditions of the problem, rejecting a large numerical answer because it seems unrealistic. In exam settings, always apply the rate exactly as given unless you are specifically asked to comment on realism. Carefully tracking the units and remembering that a decrease is required will help you avoid sign mistakes and incorrect scaling.

Final Answer:
If pressure decreases by 10 millibars per 10 metres in the lower atmosphere, then on ascending 200 metres the pressure would be 200 millibars lower than the sea-level value according to the stated rule of thumb.