Physics Lesson Please

[Tom]

Water freezes at 32 degrees right? Why then do I consistently find frost and frozen water outside my house when the overnight lows are only dropping to 36-37. I have several thermometers outside in various places and the weather reports are actually reasonably close as far as the lows are concerned.

I've been covering my planter boxes to keep the frost off. Every morning the furniture pads that I cover them with are frozen and stiff, but the temperature has not dropped below 35.

And nevermind the frozen windshields...

Can anyone help me out here?

 

[ascott]

LOL...Tom, just remember that there is usually a period of 90 minutes or so when the temps drop even lower than the average reported low for the night....in the neighborhood of 2am to 5am the temps usually drop even lower....the coldest time of the night, is actually the earliest part of the am.....like here tonight in the high desert it shows the average low will be 26 degrees, which means that there will be about 90 minutes or so of temps that will actually be about 20 degrees.....

 

[dmmj]

And here I was gonna suggest magic.

 

[ascott]

And here I was gonna suggest magic.

Ooooo, I love magic .....

 

[alita]

heheh interesting. i was going to go for magic too, but that does make sense

 

[jtrux]

Well heres my input...as the temp of the air lowers its capacity to contain moisture decreases. Thats what the dew point is, the point when the air becomes saturated and begins shedding excess water vapor in the form of condensation. When this happens it is called an exothermic reaction since heat has to be either released or absorbed in order for a chemical to change from one state of matter to another. In this example heat is being released since a gas is turning into a liquid then a solid or ice. Since the air temp is already nearly freezing im assumimg that the thermal reaction in which your encountering is on a large enough scale to cause the condensation to freeze. Hopefully that makes sense and more importantly hopefully im right lol.

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Oh and if the reaction were the other way aroumd it would be an endothermic reaction such as sweat evaporating off your skin.

 

[shellysmom]

Well heres my input...as the temp of the air lowers its capacity to contain moisture decreases. Thats what the dew point is, the point when the air becomes saturated and begins shedding excess water vapor in the form of condensation. When this happens it is called an exothermic reaction since heat has to be either released or absorbed in order for a chemical to change from one state of matter to another. In this example heat is being released since a gas is turning into a liquid then a solid or ice. Since the air temp is already nearly freezing im assumimg that the thermal reaction in which your encountering is on a large enough scale to cause the condensation to freeze. Hopefully that makes sense and more importantly hopefully im right lol.

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Oh and if the reaction were the other way aroumd it would be an endothermic reaction such as sweat evaporating off your skin.

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That's exactly what I was gonna say.

 

[jtrux]

I've been working all night and i'm still thinking about this one. I'm probably wrong. 1 degree wouldn't be a big deal but you're talking about several. Ascott is more than likely correct. I'll call my friend who is a meteorologist for the local news and ask him after I wake up.

 

[yagyujubei]

Temperature readings are taken several feet above ground, maybe five to six feet. The temperature at "ground level" is colder.

 

[Edna]

I have the opposite effect here, Tom, but for me the difference is elevation. The overnight temps can be into the mid twenties here, frost on the vehicles, but the plants on my porch don't freeze because they are 10-12 feet above street level. Talking Sept-Oct of course. Right now my entire world is a frozen wasteland.

 

[tickle]

Wind chill and most thermometers arexonly accurate within 4 degrees.ground retains heat so different levals of bight are different temps

 

[Neal]

Have you tried taking your infrared gun and pointing it at the ground at night.

In areas where it is wet, like where I watered the grass during the day, the ground is already in the mid 30's by 9pm usually, but the air temperature is still in the 40's or 50's. In the dryer areas, the ground is still colder by about 10 degrees usually.

 

[Tom]

So its colder at the ground level vs. higher up?

But the ground is warmer due to retained heat?

At 10-12 feet above street level things don't freeze as readily?

Temps get colder from 2-5am, but my thermometers don't record that?

... I'm so confused now.

Is this how the tortoise noobs feel when we tell them to house their tortoises on coir instead of sand?

The top of my car is 7' off of the ground, and it is covered in frost. My grass is on the ground and its frosty. My frozen blankets are about 12" off the ground, but the plants under them are fine, and multiple remote probes at multiple heights are recording the temps at anywhere from 35-38 over the course of the last week with the frost filled mornings. Tonight I will put a probe on top of my blankets that cover my planter boxes. And get temp readings from all over with my temp gun.

 

[Yvonne G]

Yes, I'm confused too. The roof of my house gets frost on it before the grass in my front yard.

 

[yagyujubei]

1/ Yes
2/ Yes and no. People can get frostbite eventhough their internal temp is 98.6
3/ Generally yes, though depends on air currents
Seven feet is pretty tall for a car.
But, I assume that your car is steel, and conduction is why it may be colder.
Your blankets insulate the ground. The underside would be warmer than the top. If the sun comes out, the opposite could be true.

 

[Spn785]

Okay, there are MANY factors that affect the freezing of water. Saying that water freezes at 32 degrees Fahrenheit is pure water (no minerals or other contaminents) at sea level. When at different elevations the freezing temp of water does actually change. When there is impurities in the water the freezing temp will change. The temperature of the surface on which the frost forms can be different from air temp as well. There are too many factors for me to name, but these are some of the biggest.

 

[Tom]

Seven feet is pretty tall for a car.

Yes. Yes it is.

Thanks Dennis. So are we saying that due to air currents, height and possible retained ground heat that my temperature readings are not representative of the actual temperature where the frost is occurring?

Jtrux, I'm still trying to wrap my brain around your theory. I think it my still be too early for that much science...

 

[kbaker]

... I'm so confused now.

Is this how the tortoise noobs feel when we tell them to house their tortoises on coir instead of sand?

Ha! Ha!! Yes!

Goes to show things are more complicated then we would like them. You mentioned the ground retains heat...well, yes and no...is it cement, dirt, wood, compost,...? Was it that warm during the day to retain a certain amount of heat for a certain period of time?

There is difference when the weather gets cold here (MI) and when things are chilled all the way through.

I lived in AZ for some years. It always seemed like things were in a state of drying out as to things seemed to stay wet here in MI.

Here is a weird one for you...during the spring time there is a period where we do not have any rain, but the garage floor is basically a puddle from wall to wall. Some how between the air and ground temps, the moisture is sucked to the surface.

Magic!

 

[Spn785]

Seven feet is pretty tall for a car.

Yes. Yes it is.

Thanks Dennis. So are we saying that due to air currents, height and possible retained ground heat that my temperature readings are not representative of the actual temperature where the frost is occurring?

Jtrux, I'm still trying to wrap my brain around your theory. I think it my still be too early for that much science...

One other point is that your thermometer only measures the temp when the thermometer actually is, ten feet away the temp can actually be different, usually no more than a couple degrees, but it can be different. My grandfather has about 6 thermometers around his house, and no two ever read the same.

 

[Tom]

Here is a weird one for you...during the spring time there is a period where we do not have any rain, but the garage floor is basically a puddle from wall to wall. Some how between the air and ground temps, the moisture is sucked to the surface.

We get that here too. The wind and the sun dry the ground out all day, then the coolness and stillness of night allow the lower level moisture to be "wicked" back up to the surface, resulting in wet ground in the morning. At first I thought is was dew, but the soil is wet all the way down, not just the surface.