Question

The effective average solar flux incident to a level surface at top of the atmosphere is \(\underline{\hspace{1cm}}\).

• A. 124 W/m\(^2\)
• B. 242 W/m\(^2\)
• C. 89 W/m\(^2\)
• D. 342 W/m\(^2\)

Hint:

A key number in climate science is the globally averaged solar radiation at the top of the atmosphere. It is always the solar constant (\(\approx 1361\) W/m\(^2\)) divided by 4, which gives about 340 W/m\(^2\).

Answer 1

The Correct Option is D

Step 1: Define the Solar Constant.
The solar constant is the total solar radiation energy received from the sun per unit of time per unit of area on a theoretical surface perpendicular to the sun's rays and at a distance of one astronomical unit (AU). Its value is approximately 1361 W/m\(^2\).

Step 2: Understand the "effective average solar flux".
This term refers to the solar constant averaged over the entire spherical surface of the Earth. The Earth intercepts sunlight over its cross-sectional area (\(\pi R^2\)), but this energy is distributed over the planet's full surface area (\(4\pi R^2\)).

Step 3: Calculate the average flux. \[ \text{Average Flux} = \frac{\text{Solar Constant} \times \text{Cross-sectional Area}}{\text{Total Surface Area}} = \frac{1361 \times \pi R^2}{4\pi R^2} = \frac{1361}{4} \] \[ \text{Average Flux} \approx 340.25 \text{ W/m}^2 \]

Step 4: Compare with the options.
The calculated value is approximately 340 W/m\(^2\). The closest option is 342 W/m\(^2\), which is a commonly rounded and used value in climatology.