Hottest part of the day ?

Simple question - what is the hottest part of the day ? And why ?

In sun awareness articles you’re warned to avoid exposure between noon and 2pm. There’s the old song about mad dogs and Englishman going out in the midday sun. It makes sense to me if we assume that the hottest part of the day is when the sun’s rays are most concentrated…

But - it’s just not true. We’re having a mini heatwave here in Paris right now, at one o’clock it was ‘only’ 28°C (82 °F) now at almost five it’s gone up to almost 32°C (90°F). We’ve had the same pattern for a good fews days now.

So - is the ‘midday sun’ thing an old wives’ tale ? Does poor air quality play a role ?

Statistically it is early afternoon. The sun’s rays heat up the Earth. At high noon (solar noon, not chronological (I just made up the phrase solar noon so do not think I know too much about what I am talking about)) the rate of heating is highest. All this time we are also loosing heat out to space. Even though once we go past noon the rate of heating slows down, it is still positive. At some time the rate of heat loss is larger than the rate of heat gain, and things start to cool down… in the early afternoon.

Though this holds as an average over time, or any single day with no clouds and dead air, local weather patterns will change it somewhat. If a cold front moves in around 11 AM, you may have the hottest part of the day in the morning.

Oh, but none of that matters as far as sun exposure goes. It doesn’t matter how cold it is, just how much direct sunlight you get. You can get sunburn while skiing.

Feh! Call me when you get to 49ºC and we’ll talk about heat waves.

You are confusing time of maximum sun exposure with maximum air temperature which will lag somewhat. The air mass takes time to heat up in the day and takes time to cool at night as heat radiates out. Cloud cover will have an impact on this and it’s why temperatures can drop more quickly on a clear night. In the worst part of the summer here sometimes it doesn’t cool significantly at night and we often have temperatures over 38ºC after 10:00PM.

The air temperature might not reach its peak at midday, due to hysteresis, but midday could still be the worst time to venture out in terms of sunburn/sunstroke etc, as the sun’s rays will be strongest at that time.

Feh! Call me when you regularly get 80-100% humidity and we’ll talk about heat waves.


Yeah but we’re on the edge of Europe we don’t ‘do’ extremes, nor do we really do A/C :wink: And ShibbOleth ? There’s some weird humidity pockets on the public transport you really wouldn’t wanna go near :eek:

Um, Mangetout , hysteresis ? Physics and I were never good friends but I’ll look it up I promise.

flight - look at you! You invented a new term ! Cool. Don’t worry I’m not into that whole sunbathing thing - I’m slapping on the factor 36 for a trip round the market at the moment.

So to summarise - the highest temperature on any given day follows on from the period of maximum sun exposure, the temperature continues to rise until heat loss overtakes heat gain. I can live with that.

We have clear cloudless skies but it could be possible that a layer of polluted air has the same result - holding the heat in for longer. OK.

Been there, done that, got the t-shirt, which was immediately sopping wet. Also lived in Geneva several years back and suffered through a couple week spell of 33 C plus, in which we had to decide whether to suffer through very aggressive mosquitoes or totally dead heat every night. I chose to work as last as possible.

I believe the correct phrase is “local noon.”

That’s when the sun is the strongest, but my unscientific observations tell me that around 4PM is the hottest.

Oh lighten up people. I know what humidity will do even at lower temperatures. I worked in Langen Germany (just south of Frankfurt) for nearly the whole month of August last year so I know how bad it can get in Europe. Unlike the US air conditioning is almost unheard of. No AC in the office which became an oven with afternoon sun on the windows and none in the hotel, not even ventilation other than an open window.

If it makes you feel any better Phoenix has higher than normal humidity from all the pools and canals and heat island effect and during the thunderstom seaon everyone is miserable.

Basically, the temperature at any point will increase until the amount of heat lost to radiation into space and through convection is greater than the amount gained through sunlight. Obviously, that’s going to depend on local environmental factors and lattitude.

This site has a hypothetical curve that seems to put the peak between 2 and 6pm. In their example, the time is earlier in winter months, probably beacuse the heat gained through the sun is much less when the sun is never very close to directly overhead.

Not in affecting the temperature. But since you brought it up, I’ll mention that (in a typical summer day) ozone pollution peaks even a little later than temperature. That’s because ozone is formed by a reaction between pollutants and sunlight (and goes faster at higher temps), so it’s being produced fastest after local noon, and keeps building up until later in the afternoon, when the natural breakdown rate matches the falling production rate.

So as far as breathing is concerned (if you’re in a major city or other place with ozone issues), the late afternoon feels worse than noon, even if the temperature is lower.