Air vs grass temperature

[Gordon-Loomberah]

My VP2 is mounted the standard 1.5m above the ground, but I am surprised how warm it can be at that height when there is frost on the grass. There was a fair bit of cloud around last evening, so I wasn't really expecting a frost, and the VP2 said it got down to 4.2C at 6:54am, about 25 mins before the sun rose over the ridge about 1km E of here, the cloud had cleared away after midnight. There was a reasonable covering of frost on the ground this morning, and it was all melted within an hour of sunrise. I live on an area of ground which slopes down to the east, so it warms up quite quickly in the mornings, and on calm clear nights it is significantly colder down the front nearer the bottom of the valley, with heavier frosts.

I'm wondering what others' experiences are with the temperate differential between standard recording height and grass level. Frost to 4.2C is about the largest difference I have noticed, but can it be greater?

[Gina]

I too have noticed that the air temperature at 1.5m can be a lot higher than on the ground. I plan later on when I've got myself sorted out better to put a temperature sensor at ground level as well as at the standard 1.5m. It will be a "1-wire" digital sensor accurate to half a degree C and will be part of my 1-wire weather station. These sensors are very cheap at under £2 and just simply connect directly to the 1-wire network. I have just bought a pack of 10 together with some other 1-wire bits from Sheepwalk Electronics, a British company who are cheaper than anywhere else I can find and who give excellent service. I recommend them as a source of 1-wire items and information/links. Here's the link :- http://sheepwalkelectronics.co.uk/index.shtml

[yv1hx]

Gina,

Are you using a DS18B20 One-Wire digital temp sensor?

I have 4 of these for my Thermometer project, they are housed in a small aluminum cylinder like this http://www.adafruit.com/products/381 for direct burial.

[BillW]

Hi Gordy!,
That's a very interesting question and an experiment waiting to be done.
Tonight I shall put a max/ min on the ground and another on the fence. The forecast for tonight is one of those very rare nights when it's supposed to be clear here!
That was a really shallow but nice and steep temperature inversion.
One of the strongest memories of my time in Coona were the giant frozen spiderwebs I'd pass walking into town to catch the bus. This never failed to impress me! It reminded me of when I was at uni a few years before. One of the experiments investigated just this sort of thing by making a temp profile using a 10m mast with sensors on it. It was done during the day but very close to the ground (less than a meter) there could be some really strong temp gradients.
I'll let you know how it goes!.
Cheers,
Bill.

[Gordon-Loomberah]

Hey Bill, I'll be interested to see the results. Within a few days after my first posting I saw minima of 4.6 and then 5.2C with a frost, but other days have been around 2-3C and not had frosts, at least near the VP2, although there may have been down the front in the valley. I think it's really critical that there is no mixing due to wind, in order to get the greatest temperature differential in the lowest 1.5m of atmosphere.

cheers

[BillW]

Hi,
Mixing, or lack off, is critical as is the humidity.
Anyway from last night, it was a cool night. The wind was calm and the lower thermometer read 8.5 C and the upper thermometer read 7 C, a difference of 1.5 C at 7.10am this morning.
I'll need to calculate the humidity values but that's in the usual ball park for this type of weather. I'd love to try in a genuinely open rural location.
I hope this settled spells continues for another couple of nights to catch the Persieds. Caught 60 on the video system last night.
Cheers,
Bill.

[BillW]

Measurement number 2!
Last night was another nice nights. It certainly felt warmer than the previous night and with a gentle breeze.
Upper thermometer read 6.5 deg C and ground thermometer read 13 deg C a difference of 6.5 deg C, so bigger differences can occur but my results are not inversions.
Maybe conditions will present themselves over the winter to investigate further.
Cheers,
Bill.

[nitrx]

Strange it's usual that the groundtemperature (10 cm) at night is lower as the temperature on normal meauserment level (1,5 meter above the ground) especially in clear nights. Ecample Netherlands extremes http://www.weerplaza.nl/actueel/extremen.asp select min temp and 10 cm

[BillW]

Hi Ron,
For such a simple question it gets complicated quickly doesn't it!
Dry air is an excellent insulator so there will be some heat loss through conduction very close to the ground, then some will be carried away by convection and then there is thermal radiation. However the heat capacity of water is important as that will influence the rate of heat carried. So the specific or absolute humidity (the amount of actual water in a given mass or volume of air) is important (not the relative humidity as the same RH can be arrived at through an infinite combination of temperatures). Add in air motion and mixing as Gordon points out and I've no idea whats going on....!
Gordon's question might prompt others to do some tests and report their results here
It's all good fun.
cheers,
Bill.