Final answer:
On a cloudless day at noon, the radiant heat flux received by a square meter of the Earth's surface from the sun is approximately 1,000 W/m² after accounting for about 29.3% of sunlight being reflected back into space.
Step-by-step explanation:
If you're outside on a cloudless day at noon, the radiant heat flux that a square meter of the Earth's surface is receiving from the sun varies depending on atmospheric conditions, but in a perfect scenario without any atmospheric absorption or scattering, it would be approximately 1,000 W/m². This number comes from the fact that the sun delivers 1,360 W/m² to the top of the Earth's atmosphere, known as the solar constant. However, about 29.3% of this is immediately reflected back into space, leaving approximately 960 W/m² that actually reaches and is absorbed by the Earth's surface. On a cloudless day, we could expect this value to be a bit higher because there are no clouds to reflect additional sunlight.