Naked Science Forum
General Science => General Science => Topic started by: Deserttrip on 18/11/2016 22:38:20
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I apologize if this is the wrong forum to ask. I have an expensive and otherwise highly reliable Davis Personal Weather Station in my backyard that is 50 ft from any buildings, concrete or asphalt. All summer it shows our location as COOLER than those in town, and since we are at a slightly higher elevation, that always made sense. In winter, we always show slightly warmer. I figured our slightly protected location (and cold air sinks?) helped in that.
This morning our weather station reported that our overnight low temp was 38.1°F.
However, I found a thin layer of ice on the top of the water we keep in a glass bowl on a pedestal for birds. The ice was thick enough to hold up the circle without it breaking.
The nearest reporting personal weather station is at the base of our hill and his showed 33.6° as a low.
Information that may or may not be relevant:
My elevation - 3750 ft
Cloud cover - none
Wind Speed - 0 - 2 mph
Location - in a natural bowl on the side of a steep hill
Water - desert well water - high in calcium, iron, nitrates and above normal alkalinity
Water location - in a glass bowl set on a concrete pedestal
If this isn't a possible scenario, it means I might be needing a new thermometer for my weather station. Wanted to check before spending money.
PS: This is the ice I found in the bowl when the thermometer was showing 41° outside. I imagine it was thicker before the sun came up.
Thank you for your time!
edited to add wind speed
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What are heights of the pedestal and your thermometer sensor?
In our area we often get ground frosts in autumn through spring. Ground frost is defined as up to 1m from ground. Causes that might apply to you:
- On clear nights with dry air, the ground can radiate heat faster than the air can warm it. Typically desert areas. I notice you talk about desert well water, are you desert area?
- moisture e.g. on grass, can evaporate causing local cooling. Is the pedestal porous? Could it have been wet.
- as you are on a slope land above you can cool and this air descends flowing under the warmer air. Although it warms adiabatically as it descends it can still be very cold when it gets to you and could collect in your hollow.
Just a few ideas, others folks may have more.
Certainly you need another thermometer, one is not enough to sample the area around you [:)]
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Thank you, Colin2B.
YES! I live in the Arizona High Desert. Our humidity is normally pretty low here, but this morning when the temps were at their lowest, it was pretty high (for us) at 31%. In contrast, we had 12% this afternoon.
The pedestal is 24" tall, and a sealed concrete. It was a bird bath pedestal.
The weather station/thermometer is 75" above ground.
I just got out a meat thermometer, and a portable $40 thermometer and placed them by the weather station. The meat thermometer matched the weather station (at this moment) and the portable cheapo thermometer is 2 degrees cooler. I called Davis and they suggested I place the indoor weather station console outside near the thermometer to see if the numbers match after 15 minutes, and they did.
Very odd to me. Even if the cheapo $40 thermometer is correct at 2° lower, that still would have been a freeze at 36°. This goes against everything I remember from school.
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Very odd to me. Even if the cheapo $40 thermometer is correct at 2° lower, that still would have been a freeze at 36°. This goes against everything I remember from school.
Air is a very good insulator if it is still so you can get quite a high temp gradient locally.
Here we can get a ground frost measured 5cm above ground when air temp is 4-5C at 2m, if my arithmetic is correct that would equate to an air temp of 41F, so 38 is not unreasonable.
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Hi
sometimes the temperature comes wrong on the instrument. Your area's temperature must be 32 degrees Fahrenheit or below. you should also checkout your wins speed.
However you can also check it in the following article on waterfilteronly.com
Hope it helps your query.
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Yes, that's a frost. You commonly get it on car windscreens, unless they're parked under carports or close to houses. It happens at air temperatures ABOVE freezing!
Frost happens because the temperature of the night sky, is very, very low, far below freezing. So the pond water is radiating infrared radiation in all directions, but being warmed only from below. So if it's reasonably close to freezing anyway, it will freeze, even if the air temperature is above freezing, because the air is transparent to infrared and a reasonably good insulator. Meanwhile the thermometer will be inside your device, so won't be cooled by the sky, and will reflect the air temperature and read much higher. It's supposed to work like that. It's not a fault.
This frosting effect was historically used in places like Iraq to make ice even in the summer, in desert conditions, they would make a large thin puddle of water and wait for it to freeze, and then collect the thin sheets of ice each morning and put it into ice houses.
Scientists have managed to boost this effect to a ridiculous degree, and actually come up with materials that can cool something down well below ambient temperature, even in direct sunlight, by producing a material that reflects sunlight away, while being really good emitters in the relevant infrared bands.