Air temp, Terrestrial temp different, Why?

[Earthling#44-9a]

Gday. I was just speaking to the local post office worker here who does the weather recordings and they were saying how last night we had a air temp of -0.9c and a ground temp of -4.9c. I enquired as to why the difference in temps and they couldnt help me, so hopefully you can?
Google search told me nought.
I would have thought the ground is cooled by the air temp, so the ground would be higher in temp then the air...but not so. Is this because the ground radiates heat depending on several factors which could bring it below air temp? Such as cloud cover, air temp, wind force, humidity....cant think of any else. But how do these or others combine to do that? If you have links that you can use to explain all this that will be fine..or if you want to type it thats good also. I have a feeling this will either be a dead simple answer or very complicated.
Look forward to your reply.

Oh...I did contact 'National Climate Centre-Climate Information Services' as to why and they said:

Air (screen) temperature is usually measured with a mercury in glass thermometer which is housed inside an enclosure # to protect the thermometer from damage, heating from the sun and cooling from the rain. The thermometer for measuring air temperature is similar to conventional mercury thermometers.

#Professional stations have thermometers placed inside shelters, about 1.2m above ground level. These shelters can be expensive to construct because of the time involved. However they are not beyond the skills of a competent carpenter. The shelter should periodically be painted with a white semi-gloss paint both inside and outside.

Terrestrial minimum temperature is usually measured using the minimum temperature thermometer described above. The thermometer is mounted on "Y" supports immediately above the surface of the ground, slightly tilted toward the bulb end. In grassed areas the grass height is kept between 25mm and 50mm high, with the thermometer bulb in contact with the tips of the grass blades


This explains how they measure the temps but not why they are different....

Knowledge?

[Check twice!]

My logic would be that air temperature fluctuates quicker than a dense object. Water as well as ground may be colder than the air, the sun heats the air quicker than a mass.

This is why some ares (like myself) can experience a cool breeze in the summer. The water in our area has a large influience on temperature.

I know this as clear as mud. I think you get the idea though!

Have a great day
John

[BobL]

It has to do with the radiation absorption and re-radiation properties of matter.

During the day the atmosphere reflects and absorbs some of the solar radiation it receives and rest passes onto the earth's surface which also absorbs and reflects (eg water, vegetation and snow and ice reflect quite a bit). The effect of all this is that dark, dry, rough and unvegetated ground absorbs a lot of heat and heat up quickly - that is why deserts are hot.

During the night the reverse occurs. The ground is the major heat reservoir and re-radiates heat. Dark, dry, rough and unvegetated ground re-radiates its heat very quickly into the atmosphere and cools rapidly - this is why deserts are cold at night. The atmosphere is a slower re-radiator and does not cool down as quickly as the ground. Near the earth's surface, the closer you are to the ground the colder it is.

While water reflects more heat than soil, weight for weight it can store a lot of heat, which is why the oceans don't freeze overnight and coastal zones generally have milder climates. Water in or on the soil also mitigates ground temperature so if it rains overnight this reduces the re-radiation of heat and is why rainy days are generally not as cold as dry days at the same time of the year.

So this is why air temps and ground temps will differ.

[Earthling#44-9a]

It has to do with the radiation absorption and re-radiation properties of matter.

During the day the atmosphere reflects and absorbs some of the solar radiation it receives and rest passes onto the earth's surface which also absorbs and reflects (eg water, vegetation and snow and ice reflect quite a bit). The effect of all this is that dark, dry, rough and unvegetated ground absorbs a lot of heat and heat up quickly - that is why deserts are hot.

During the night the reverse occurs. The ground is the major heat reservoir and re-radiates heat. Dark, dry, rough and unvegetated ground re-radiates its heat very quickly into the atmosphere and cools rapidly - this is why deserts are cold at night. The atmosphere is a slower re-radiator and does not cool down as quickly as the ground. Near the earth's surface, the closer you are to the ground the colder it is.

While water reflects more heat than soil, weight for weight it can store a lot of heat, which is why the oceans don't freeze overnight and coastal zones generally have milder climates. Water in or on the soil also mitigates ground temperature so if it rains overnight this reduces the re-radiation of heat and is why rainy days are generally not as cold as dry days at the same time of the year.

So this is why air temps and ground temps will differ.

Thanks Check and Bob.
Bob, when the earth radiates the heat at night and the closer you are to the surface the colder the air gets, what about underground temperatures? Does it get colder in the top 1m then the ground below? Im guessing the soil below 1m deep would be a fairly consistent temperature depending on the seasons yes? The deeper you go it would even be a more consistent temperature year round yes?
This is all interesting stuff for some unknown reason.

[Check twice!]

BobL
One last comment, in our area our land temperature can be frozen up to 1 meter deep and below 0*C and air temperature at times above 3 - 5*C during winter.

We can also have freezing temps (air) but our ground just crips on the top.

Our water can be -2*C but not freeze, this I thought to be partly because of the density of the water (salt).

Then we have the chinook winds in Alberta, that raise air Temp but not ground.

I understand what you say but I wonder if the earth always listens to science. You answered a question but opened others in my mind.

Would air patterns have anything to do with the fluctions as well as what you explained?

This is like asking the question does hot or cold water freeze faster, interesting!

Curious mind.
Thanks
John

[BobL]

Thanks Check and Bob.
Bob, when the earth radiates the heat at night and the closer you are to the surface the colder the air gets, what about underground temperatures? Does it get colder in the top 1m then the ground below? Im guessing the soil below 1m deep would be a fairly consistent temperature depending on the seasons yes? The deeper you go it would even be a more consistent temperature year round yes?
This is all interesting stuff for some unknown reason.

Underground - as you go deeper it approaches the average temperature for that area for the whole year. If you go deeper still it gets warmer - the earth is after all radioactive and this continues to make enough heat to melt rocks and make lava etc. The miners in some of the deep underground mines in south africa for example have to work in sweltering conditions.

[BobL]

BobL
One last comment, in our area our land temperature can be frozen up to 1 meter deep and below 0*C and air temperature at times above 3 - 5*C during winter.

Don't forget your solar radiation input during winter at your high latitudes is very low - that's primarily why things cool off over the season, and you are right the air temp is very much dependent on where the winds are coming from.

We can also have freezing temps (air) but our ground just crips on the top.

That's because the ground has a large thermal mass and takes a long time (weeks) to cool down.

Our water can be -2*C but not freeze, this I thought to be partly because of the density of the water (salt).

If you are talking about sea water that's the same everywhere.

Then we have the chinook winds in Alberta, that raise air Temp but not ground.

Where do these come from

I understand what you say but I wonder if the earth always listens to science. You answered a question but opened others in my mind.

Err . . . the earth is what it is, so if we can't explain something we have to look at our current understanding of science and accommodate it!

Would air patterns have anything to do with the fluctions as well as what you explained?

Correct!

[AlexS]

Bob, your explanations are comprehensive & spot on. Do you have a professional interest in these things?

[BobL]

Bob, your explanations are comprehensive & spot on.

Thanks Alex,

Do you have a professional interest in these things?

Not specifically in meteorology, but physicists dabble in all sorts of stuff and some spend a lot of time communicating the often incomprehensible to the general public so indirectly yes

[Earthling#44-9a]

Totally agree with you Alex. Bob certainly does have a way of making it all very comprehensible.
Thanks again Bob.

[Check twice!]

I thank you for taking the time to explain, I did have part but you have put the pieces together for me.

Thanks for your time.
John

Have a great day!
This is just one of the values of science and these forums.

[joe greiner]

Underground - as you go deeper it approaches the average temperature for that area for the whole year. If you go deeper still it gets warmer - the earth is after all radioactive and this continues to make enough heat to melt rocks and make lava etc. The miners in some of the deep underground mines in south africa for example have to work in sweltering conditions.

Spot on. Google [alaska "whiplash curve"] for some insight (ignore the artistry hits). As for professional interest, a short course in cold regions engineering is required for Alaskan registration by reciprocity with one's home registration.

Joe