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codechris

What is temperature

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While we can speak about temperature is the rising of mercury etc etc, I'll start by telling something I recently learnt;

 

As far as I know, and I've asked other English people, in England when temperatures are quoted (TV, internet, newspapers etc) they are the max predicted or currently recorded temperature at that moment. I never thought about it any more then the fact that was the max temperature for that day.

 

However, I've been in France for 6 months now and learnt that when weather is predicted or currently reported they speak of temperature in the shade and so NOT the maximum temperature. This surprised me as I'd never heard of this before and I was sure we didn't do this in England.

 

But it's led me to question when I am reading temperatures from ASN or ATIS or whatever source for flying, what temperature are they quoting. I've always presumed it was the temperature, however now I'm questioning what temperature this is. For example, is this the temperature taken in to account any wind-chill factor? Does it take wind in to the temperature or not. Is that temperature in the shade or in direct sunlight...

 

Any help would be appreciated :)

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Wind is measured at 10m AGL, and the temperature sensors using in SYNOP, METAR, etc... at 1,5m AGL. No "shilling effects" because the sensor is in an enclosure.

 

Wet bulb thermometer temperature ( dew point ) is measured the same way.

 

In cold countries, like Germany, they also give you true and apparent temperature, because -10ºC in a strong wind can cut like if you were in Siberia :-)

 

I'm a very happy guy!  I live in Portugal!!!!

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Temperature is always measured in the shade for reporting.  Wind chill is nothing more than a "feels like" temperature. The actual temperature is still as measured as the air temp. not what it feels like temp. The fact that heat travels from hot to cold and the rate at which it is transferred is dependent on the temp. difference, the specific heat of the medium being measured. Heat will be transferred more quickly in water than air for example, because of the specific heat of water versus air.

Wind chill is the result of the convection from the wind removing the heat more rapidly than if there were no wind. It can be measured many ways. Time it take for human flesh to freeze, which means time. Heat index, is what the "feel like" temp. is. The more humid it is the warmer the body feels because the sweat from the body cannot evaporate as fast as if the humidity were lower. Water removes heat as it evaporates.

To put it very simply, if you were to leave your car out overnight in -20F, with high winds, the car will only measure a temp of -20F. It simply will take less time to cool to the -20F because of the wind removing the heat more quickly.

Bob G.

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Sorry for the delay, I had to move country.

 

So there are two responses here;

 

 

 


In cold countries, like Germany, they also give you true and apparent temperature, because -10ºC in a strong wind can cut like if you were in Siberia :-)

 

and

 

 

 


Temperature is always measured in the shade for reporting.

 

 

One thing I thought of was that in hotter countries, such as Italy, Spain, France, temperatures are may given as shaded as they are naturally hotter. But colder countries, such as England, we report on true, or predicted true, because we are colder. However, I had no basis for thinking that other then I've never heard, nor has any English person I spoke to, the concept of shaded temperature. So I'm left now, especially after the two responses, weather it's just us the English that do not realise that temperature is actually shaded, or if we just report it differently in England.

 

I still can't find an answer to that. And the reason why I feel this is important is because a lot of weather in England comes from the Met Office, a very well respected meteorology centre, and I presume they give information to UK pilots and airports regarding temperature. However, if the basis that in England we report on true temperature, what information are pilots being given. Or, if they are reporting on shaded temperature, then is it just our local weather in newspapers, TV etc that is reporting on the true temperature

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Meteorological temperatures worldwide are measured in the shade, the "True" temperature isn't a temperature at all, but the Met persons way of telling someone that if the temperature is -10c, and the wind is 20kts, to a human being it will feel a lot colder than -10c because we will be losing a lot of body heat if exposed to the wind, it's also known as the wind chill factor.

There is only one temperature useful to aviation, and that's the air temperature - which is the temperature as measured in the shade.

The box the thermoneter is placed in is called a stevenson screen, you can see information about it on Wiki here:

 

https://en.wikipedia.org/wiki/Stevenson_screen

 

Eugene

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Thanks Eugene. Exactly. You can Google it and it gives the same answer.

Bob G.

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I had searched but couldn't find anything conclusive, I guess I was using the wrong search terms.

 

In the weather, if there is a wind chill factor it is stated so that part is a known. However, I don't think I am alone, or the small group of people I asked, that our understanding on temperature is max, not shaded. I find it surprising that this is the case. I only know because I watched the weather in another country

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Directly from the Met Office

 

http://www.metoffice.gov.uk/media/pdf/4/k/Chapter2.pdf

 

Page 23

 

Exposure of thermometers. No matter how accurate a thermometer may be, it can do no more than indicate its own temperature. It is therefore essential that the thermometer is in correct contact with the medium whose temperature it is to measure, in order that it may 'share' its true temperature, and that it is protected from any extraneous source of heat. When measuring the air temperature, particular problems arise on this score, in that the thermometer must be shielded from the heat radiated by the sun, the sea and from the ship itself, yet at the same time the air, which is itself transparent to such radiated heat, must be allowed to flow freely past the thermometer. Even when not in use no normal meteorological thermometer, for whatever purpose it is supplied, must ever be exposed to full sunlight for more than a moment or two, and when stored should be kept in its packaging. A thermometer screen is used to shield the thermometer from external radiation, yet allowing an adequate flow of air.

 

England doesn't use any different system to any other country.Temperature is always measured in the shade.

You can now inform your small group of people and everyone will have learned something ! 

 

Radiated (direct) heat can easily massively exceed the ambient (shade) temperature and many aircraft - and in particular those built with composites, can have a temperature limit above which it is not safe to fly. But this isn't the ambient temperature, its the radiant. Some GA have temperature sensors built into the wingspar or airframe expressly for this purpose. 

 

Hopefully this is useful.

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