Water Temp

CanisLupusArctos · 06-29-2007, 11:39 AM

In the past couple months since I installed a water temperature probe on my weather station, I discovered something I never suspected before: The water temp seems to be more affected by the dewpoint & humidity than by the air temp.

On May 25 the air temp reached 87 but the lake temp stayed around 56. The dewpoint was 44 - the kind of dryness of a fall day.

Earlier this month I noticed the water temp rising overnight. The air temp wasn't much warmer than the water but the dewpoint was in the low 60s - typical summer stickyness.

Yesterday the dewpoint rose to its max of the year so far, 69 (very humid) while the air temp reached 82. The water temp jumped to 71, also its max of the year. On Mon-Tues when the weather was hotter but not as humid, the water only managed to rise 2 or 3 degrees, to 66.

Last night at 10 p.m. the air was still 70 but the dewpoint was 57, and the water had chilled to 68, still dropping.

Anyway, I found it interesting so thought I'd post it. Anyone else ever noticed this?

Rose · 06-29-2007, 12:24 PM

The reason is due to evaporative cooling. When the air is drier, it's easier for water from the lake to evaporate into the air. In the process, it takes heat from the water to evaporate, thus cooling the water.

SIKSUKR · 06-29-2007, 12:52 PM

I was just going to say the same thing Rose posted.It's actually the same process that our own bodies use to cool and why it's hard to cool down when it's very humid out.I'm interested though at what level and how far out you have the sensor?

CanisLupusArctos · 06-29-2007, 01:00 PM

The sensor is about 20 feet from shore, at a depth of 3 feet.

I had suspected evap. cooling but just amazed that its effect on the lake was this much greater than the air temp. If this much evaporation is taking place, then I wonder how much moisture it's adding to any storms overhead?

SIKSUKR · 06-29-2007, 02:29 PM

Quote:

Originally Posted by CanisLupusArctos

The sensor is about 20 feet from shore, at a depth of 3 feet.

I had suspected evap. cooling but just amazed that its effect on the lake was this much greater than the air temp. If this much evaporation is taking place, then I wonder how much moisture it's adding to any storms overhead?

Probably not that much because the highest evaporation is happening when the air is driest which usually means high pressure.Rose?

Lakegeezer · 06-29-2007, 05:27 PM

Just a guess, but is it more windy when the humidity is low? If so, the water would churn and keep the temperature more even. When humidity is high and wind is calm, the surface temp can go up. With the probe three feet down, its hard to imagine that humidity has a big impact.

CanisLupusArctos · 06-30-2007, 01:03 AM

I've noticed the same effect whether it's windy or calm. The only difference is that the lake's cooling speeds up when it's windy. No doubt churning of colder water from below has something to do with it, but if the evap. cooling is the primary thing, then I think the wind would just speed up that process like it does to your skin when you get out of the water on a dry day. Any thoughts?

Rose · 06-30-2007, 08:16 AM

Quote:

Originally Posted by SIKSUKR

Probably not that much because the highest evaporation is happening when the air is driest which usually means high pressure.Rose?

If there's a storm overhead, then the difference between the air temperature and the wet bulb temperature is probably fairly small, which inhibits evaporational cooling. It may feed storms downstream in the long run.

Just getting back on line from home due to a lightning strike Wednesday evening. It hit a power pole 200 feet from the house, blowing the top 3-4 feet off. Fried the wireless router (which was on a surge protector), a TV (not the plasma, thank God), and part of the weather station on the roof. And, of course, I missed the fun by about 15 minutes.

06-29-2007, 11:39 AM	#1
CanisLupusArctos Senior Member Join Date: Jan 2007 Location: Center Harbor Posts: 1,049 Thanks: 15 Thanked 472 Times in 107 Posts	Water Temp In the past couple months since I installed a water temperature probe on my weather station, I discovered something I never suspected before: The water temp seems to be more affected by the dewpoint & humidity than by the air temp. On May 25 the air temp reached 87 but the lake temp stayed around 56. The dewpoint was 44 - the kind of dryness of a fall day. Earlier this month I noticed the water temp rising overnight. The air temp wasn't much warmer than the water but the dewpoint was in the low 60s - typical summer stickyness. Yesterday the dewpoint rose to its max of the year so far, 69 (very humid) while the air temp reached 82. The water temp jumped to 71, also its max of the year. On Mon-Tues when the weather was hotter but not as humid, the water only managed to rise 2 or 3 degrees, to 66. Last night at 10 p.m. the air was still 70 but the dewpoint was 57, and the water had chilled to 68, still dropping. Anyway, I found it interesting so thought I'd post it. Anyone else ever noticed this?

06-29-2007, 12:24 PM	#2
Rose Senior Member Join Date: Aug 2002 Posts: 498 Thanks: 62 Thanked 71 Times in 32 Posts	Evaporative cooling The reason is due to evaporative cooling. When the air is drier, it's easier for water from the lake to evaporate into the air. In the process, it takes heat from the water to evaporate, thus cooling the water.

06-29-2007, 12:52 PM	#3
SIKSUKR Senior Member Join Date: Apr 2004 Posts: 5,075 Thanks: 215 Thanked 903 Times in 509 Posts	I was just going to say the same thing Rose posted.It's actually the same process that our own bodies use to cool and why it's hard to cool down when it's very humid out.I'm interested though at what level and how far out you have the sensor? __________________ SIKSUKR

06-29-2007, 01:00 PM	#4
CanisLupusArctos Senior Member Join Date: Jan 2007 Location: Center Harbor Posts: 1,049 Thanks: 15 Thanked 472 Times in 107 Posts	sensor depth The sensor is about 20 feet from shore, at a depth of 3 feet. I had suspected evap. cooling but just amazed that its effect on the lake was this much greater than the air temp. If this much evaporation is taking place, then I wonder how much moisture it's adding to any storms overhead?

06-29-2007, 05:27 PM	#6
Lakegeezer Senior Member Join Date: Feb 2003 Location: Moultonboro, NH Posts: 1,678 Blog Entries: 1 Thanks: 354 Thanked 639 Times in 290 Posts	wind? Just a guess, but is it more windy when the humidity is low? If so, the water would churn and keep the temperature more even. When humidity is high and wind is calm, the surface temp can go up. With the probe three feet down, its hard to imagine that humidity has a big impact. __________________ -lg

06-30-2007, 01:03 AM	#7
CanisLupusArctos Senior Member Join Date: Jan 2007 Location: Center Harbor Posts: 1,049 Thanks: 15 Thanked 472 Times in 107 Posts	wind I've noticed the same effect whether it's windy or calm. The only difference is that the lake's cooling speeds up when it's windy. No doubt churning of colder water from below has something to do with it, but if the evap. cooling is the primary thing, then I think the wind would just speed up that process like it does to your skin when you get out of the water on a dry day. Any thoughts?