Effect of location on temperature

[h2005__uk__]

Interesting replies

My sensor's about 1.5m above the ground, 2m away from the trunk of a fairly large Hawthorn tree and 1m from the trunk of a small plum tree. Air flows quite freely around the sensor.

Most of the tree vegetation starts a few metres off the ground though - what actually provides shade for my sensor is some ivy that's grown on the trees.

[chrcoluk]

This is because we're measuring the temperature of the air.

If you try to measure the temperature in the sun you actually measure the temperature of the instrument you're using to do the measuring, not that of the air.

As all objects have differing albedos ( the ability to reflect solar radiation) then any thermometer exposed to the sun will not give an indication of the temperature you actually feel as human skin has a different capacity to absorb solar radiation than a thermometer.

There are special Black bulb in vacuo thermometers ( a matt black thermometer in a matt black shield ) for trying to measure the

temperature in the sun but even these are only comparable with each other and tests have shown that there can be significant variation in the readings of two such thermometers exposed side by side.

 

ok but I still see it as misleading, a human will be in the sun, even if they have more capacity to absorb heat they will ultimately feel hotter than a temp measured in the shade.

 

does air itself absorb heat, get hotter in sun? if it gets hotter then the air measurement in the shade is not accurate for sure

 

I read on another site somewhere they do it in the air as it helps all the calculations they have to do for weather predictions, so I took from that it is better for people who want to predict weather patterns but less useful for average joe bloggs who will be out in the sunlight.

 

the sigificant variance is explained simple, try standing in the sunlight mid july and tell me you dont feel significantly hotter.

Edited August 6, 2014 by chrcoluk

[Terminal Moraine]

ok but I still see it as misleading, a human will be in the sun, even if they have more capacity to absorb heat they will ultimately feel hotter than a temp measured in the shade.

 

does air itself absorb heat, get hotter in sun? if it gets hotter then the air measurement in the shade is not accurate for sure

 

I read on another site somewhere they do it in the air as it helps all the calculations they have to do for weather predictions, so I took from that it is better for people who want to predict weather patterns but less useful for average joe bloggs who will be out in the sunlight.

 

the sigificant variance is explained simple, try standing in the sunlight mid july and tell me you dont feel significantly hotter.

As the differential in air temperature between different parts of the globe is the main driver of all the weather on earth then it makes sense to measure the temperature of the air rather than that of the ground, a car roof or your skin in the sunshine.

 

The air absorbs only a small amount of direct radiation from the sun and heats up due to secondary radiation from the ground, or other objects, which are much more effective at absorbing solar radiation; it is this effect which results in the heat island effect of large cities as the radiation absorbed by the buildings is given back to the air as heat.

 

The temperature you actually feel as you stand in the sunshine is a very subjective one and bears little relation to the actual air temperature. It is dependant not only on the strength of the sun but also on humidity and wind speed.

For instance you might stand in the sunshine in Antarctica on a calm day with an air temperature of -20c and feel warmer than standing in the sunshine in England in March with a fresh breeze, a humid air mass and an air temperature of +5c. It would be nonsensical to say that, based on what you have felt, Antarctica is therefore warmer than England in March, hence we measure the temperature of the air in the shade; it's a level playing field the world over with every reading comparable with every other reading, subject to the accuracy of the thermometers or sensors used.

[chrcoluk]

which is why these "feel" temperatures are what weather reporters need to report, not basic shade air temperature.

 

The guy on the street cares about how hot he feels not all the science.

 

e.g. here is rarely wind not even a breeze as leics is the most central city in the country furthest from the coast, wind is very rare here.  I live at the bottom of a dip in the land as well.  Even in winter its usually light wind at the most.  Heavy wind is extremely rare maybe 2-3 days a year.

Edited August 7, 2014 by chrcoluk

[h2005__uk__]

Didn't think it worth setting up a new thread for this question, but I've spotted some unofficial weather stations show spikes in the afternoon, presumably where the sun's hitting the instruments. Does this happen with 'official' stations too? Or is a 'spike' (perhaps up/down by a degree or so depending on changing cloud cover) indicate that too much sun is hitting the sensor?

[hillbilly]

Didn't think it worth setting up a new thread for this question, but I've spotted some unofficial weather stations show spikes in the afternoon, presumably where the sun's hitting the instruments. Does this happen with 'official' stations too? Or is a 'spike' (perhaps up/down by a degree or so depending on changing cloud cover) indicate that too much sun is hitting the sensor?

There should be no sun hitting the sensor!!

[Polar Maritime]

which is why these "feel" temperatures are what weather reporters need to report, not basic shade air temperature.

 

The guy on the street cares about how hot he feels not all the science.

 

e.g. here is rarely wind not even a breeze as leics is the most central city in the country furthest from the coast, wind is very rare here.  I live at the bottom of a dip in the land as well.  Even in winter its usually light wind at the most.  Heavy wind is extremely rare maybe 2-3 days a year.

 

The temperature is just that, No matter who or what animal feels it. I called round to see my Mother today, And she felt cold!

 

Yes Hilly, Sensor should always be in the shade. 

Edited August 31, 2014 by Polar Maritime

[Interitus]

Didn't think it worth setting up a new thread for this question, but I've spotted some unofficial weather stations show spikes in the afternoon, presumably where the sun's hitting the instruments. Does this happen with 'official' stations too? Or is a 'spike' (perhaps up/down by a degree or so depending on changing cloud cover) indicate that too much sun is hitting the sensor?

 

As mentioned direct sun should not hit the sensor, but usually it should be sensitive enough to register temperature changes of the ground surface in response to changing cloud cover. As a slight variation to this, an interesting bit of investigation showed that it may be possible to detect the effects of aircraft jet exhaust on temperature by comparing data from Heathrow and Northolt around the time of the disruption due to the Icelandic volcanic ash cloud - https://tallbloke.wordpress.com/2012/05/12/uhi-at-heathrow-met-office-site-3/comment-page-1/