Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: paul.fr on 14/12/2007 07:06:23

Title: how much heat do street and porch lights emit?
Post by: paul.fr on 14/12/2007 07:06:23

The temperature was -5c last night, the cars are coated with frost and ice, except those beneath street and porch lights. Do these lights really heat the surrounding area by +5c?

Title: how much heat do street and porch lights emit?
Post by: lightarrow on 14/12/2007 07:52:28

Quote from: paul.fr on 14/12/2007 07:06:23

The temperature was -5c last night, the cars are coated with frost and ice, except those beneath street and porch lights. Do these lights really heat the surrounding area by +5c?

Did all the car stay at the same distance from the houses?

Title: how much heat do street and porch lights emit?
Post by: Soul Surfer on 14/12/2007 08:41:51

The lights are a source of heat so are the buildings particularly in porches.

As to the power.  Light bulbs put out all of their energy as heat (light is a form of heat) and so the wattage of the bulb essentially defines the heat output.  Low energy bulbs put out less heat because they put out more as light and so the same light comes out of lower wattage bulbs. Street lamps are always of the low energy type nowadays and a normal yellow sodium light is around 40 watts if I remember correctly

Title: how much heat do street and porch lights emit?
Post by: daveshorts on 14/12/2007 11:10:43

I am sure they don't heat the ground by 5°C but they don't have to. They have to heat the ground to above the dew point (or in this case frost point). This will be a temperature dependent upon the water content below which water will condense onto an object. If there are enough other objects around the place, which are colder than the piece of street under the light, then water will condense on them first and the water content of the air will reduce below the dew point for your bit of street.

So you could take an object well below zero with no ice forming at all, as long as the surroundings were slightly colder, so they dry out the air for you.

This would only happen occasionally as if the air cools too quickly it will condense everywhere. If it is really common, I would have thought there was something else going on, like the lights coincide with more sheltered bits of road, or people tend to stand under them...

Title: how much heat do street and porch lights emit?
Post by: lyner on 14/12/2007 12:03:16

I can't help feeling that there is some other factor at work; it is unlikely that a 60W portch light could  make much difference to  the temperature or humidity even one metre away in free moving air.
For a 'fair test', all the other variables must be considered / controlled. Distances and actual positions of cars are very relevant to frost / dew formation. Even the angle of the window affects  frosting - side windows often stay frost-free whilst the windscreen gets covered.
You will have to park your own car beneath your portch light over a number of nights and record whether the light being on or off makes a difference, on a statistical basis.

Title: how much heat do street and porch lights emit?
Post by: lightarrow on 14/12/2007 12:38:54

Quote from: sophiecentaur on 14/12/2007 12:03:16

I can't help feeling that there is some other factor at work; it is unlikely that a 60W portch light could  make much difference to  the temperature or humidity even one metre away in free moving air.
For a 'fair test', all the other variables must be considered / controlled. Distances and actual positions of cars are very relevant to frost / dew formation. Even the angle of the window affects  frosting - side windows often stay frost-free whilst the windscreen gets covered.
You will have to park your own car beneath your portch light over a number of nights and record whether the light being on or off makes a difference, on a statistical basis.

Agree.

Title: how much heat do street and porch lights emit?
Post by: paul.fr on 14/12/2007 15:14:47

"Did all the car stay at the same distance from the houses?"

it was just too cold for me take take that much notice, sorry.
an extra factor i forgot to mention; the cars under porch light were also covered by a porch roof (sort of roof with no walls), could this have caused some sort of micro climate?

Title: how much heat do street and porch lights emit?
Post by: lyner on 14/12/2007 16:56:39

You got it! It was the roof  wot done it!