Sun radiation in England

Question

I used to live in Italy, and when it was sunny, well, as expected it was pretty hot. Both under the direct light of the sun through the atmosphere, and in the shade, where the temperature was of course lower. I would formalise this saying that in Italy I experienced a certain $\Delta T_{it}$ between shady and exposed outdoor surfaces. Now I live in England, and I have always noticed that the difference is way bigger. In general, when it's sunny (sure, UK doesn't help such observations) the sun seems to warm more, and in general I experience a bigger $\Delta T_{UK}>\Delta T_{it}$.

I do not know (because I haven't had the opportunity to actually measure the temperature) whether:

• It is only a perception-effect;

• Sun radiation is effectively less filtered, and this results in a higher temperature under its direct light;

• The air is cold and the air ventilation doesn't allow it to warm to a point such that its presence would "mitigate" the difference $\Delta T$

• What is the reason for this? How much is the Latitude relevant?

Answer 1

I suspect this is largely a trick of perception, but it's interesting to see what might affect it.

If we take the intensity of sunlight at the point on the Earth where the Sun is directly overhead to be $I_0$ then the intensity of sunlight at an angle $\theta$ to the north of this point is given by:

$$ I(\theta) = I_0 \cos\theta $$

Rome is at a latitude of 41.9ºN and London is at a latitude of 51.5ºN. At the equinoxes the Sun is directly overhead at the equator, so the ratio of the sunlight intensity, $R_I$, is just:

$$ R_I = \frac{I_{\text{London}}}{I_{\text{Rome}}} = \frac{\cos 51.5}{\cos 41.9} \approx 0.84 \tag{1} $$

At the winter solstice the overhead point is about 23º south of the equator so we have to have to add 23º to both latitudes and the ratio given by equation (1) falls to:

$$ R_I (\text{winter}) = \frac{\cos 51.5 + 23}{\cos 41.9 + 23} \approx 0.63 $$

and in summer we have to subtract 23º from both latitudes and we get:

$$ R_I (\text{summer}) = \frac{\cos 51.5-23}{\cos 41.9-23} \approx 0.93 $$

If we do the experiment in summer then the intensity of the sunlight is similar in both cities so the sunlight feels roughly the same warmth. But a quick Google tells me that the summer temperature (the temperature in the shade) in London is 5º - 10º lower than in Rome. So the sunlight should feel roughly the same but you should feel colder in the shade in London - which is what you describe.

Things are a bit more complicated in winter. The winter sunlight in London is only two thirds as strong as in Rome, but then London gets a lot colder in winter than Rome does. I suspect you'd feel colder in London whether you were in the sun or the shade :-)