Question

Following statements are made with respect to the mass curve of rainfall:
(I) Slope at a point on the curve can be positive.
(II) Slope at a point on the curve can be zero.
Which one of the following options is CORRECT?

• A. Both statements (I) and (II) are TRUE.
• B. Both statements (I) and (II) are FALSE.
• C. Statement (I) is TRUE and statement (II) is FALSE.
• D. Statement (I) is FALSE and statement (II) is TRUE.

Hint:

The slope of a rainfall mass curve equals the instantaneous rainfall intensity, which is zero in dry spells and positive while raining, but never negative.

Answer 1

The Correct Option is A

A mass curve of rainfall plots the running cumulative rainfall against time, and this question checks whether we understand what its slope physically means at different points in time.

1. Statement (I), slope can be positive: whenever rain is actually falling, the cumulative total keeps increasing with time, so the curve rises and its slope, equal to the instantaneous rainfall intensity, is positive at that moment. Since real storms have periods of active rainfall, this statement holds.
2. Statement (II), slope can be zero: between storms, or during any dry spell, no additional rainfall is added to the running total, so the cumulative value stays constant and the curve is flat over that stretch. A flat curve has zero slope, so this statement holds too.

Since a mass curve is built by continuously adding up rainfall over time, it can never fall; a negative slope would mean negative rainfall, which is not physically possible. That leaves exactly two behaviors for the slope: positive while it is raining, and zero while it is dry, precisely what statements (I) and (II) describe.

Let's summarize:

• Slope of the mass curve = rainfall intensity at that instant.
• Intensity is either positive or zero, never negative, so the curve is always non-decreasing.

Both statements are TRUE, so the correct option is (A).