Naked Science Forum

General Science => General Science => Topic started by: paul.fr on 16/04/2007 10:40:08

Title: hanging the laundry out.
Post by: paul.fr on 16/04/2007 10:40:08

i have just hung my laundry out, it is a beautiful sunny morning and i "know" the clothes will dry. the sun shining on them (evapoation) and a nice little warm breeze.

when i hang the laundry out on a day when it is overcast/cloudy how do my clothes dry without the aid of the sun? i know there is some wind but how is the wind removing the water?

Title: hanging the laundry out.
Post by: Karen W. on 16/04/2007 10:44:13

doesnt evaporation take place regardless of the sun.. just overtime..?

Title: hanging the laundry out.
Post by: Seany on 16/04/2007 11:31:54

Yes- Evaporation take place regardless of the sun. Say we put a glass of water on a table, and leave it there for months/years. It will eventually evaporate, although it is a lot slower than in the sun.

The wind just helps to remove the water vapour in the clothings. This is just like a hand-dryer, except without the heat. It still works, but less fast, although the wind is actually better to evaporate the water than just sheer sunlight.

Title: hanging the laundry out.
Post by: eric l on 16/04/2007 11:58:31

Air can contain a certain quantity of water, depending on temperature.
If it contains that amount of water, it is said that "relative humidity is 100 %". 
If relative humidity is not 100 %, water can evaporate from your clothes (or from that glass) until relative humidity of the air immediately around is 100 %. 
Now the effect of wind is that this air immediately around the wet clothes is constantly replaced.  So air at 100 % RH is replaced by air at lower humidity, and evaporation can go on.

Title: hanging the laundry out.
Post by: another_someone on 16/04/2007 12:14:50

Evaporation depends upon the difference between the capacity of the air to absorb water and the amount of humidity in the water itself.

If you put some ice in a frost free freezer, that ice will evaporate (the frost free freezer works by making sure the air in the freezer is dry enough that any buildup of ice will evaporate, and that there is enough air circulating around the freezer to draw the moistened air our of the freezer).

All the heat of the sun does is increase the capacity of the air to hold moisture, and if that moisture is not already in the air, then it can absorb the moisture from the damp clothing.  If you have a hot sun in air that is already saturated with humidity, it will not dry anything.

Title: hanging the laundry out.
Post by: DoctorBeaver on 16/04/2007 13:57:00

Quote from: another_someone on 16/04/2007 12:14:50

Evaporation depends upon the difference between the capacity of the air to absorb water and the amount of humidity in the water itself.

If you put some ice in a frost free freezer, that ice will evaporate (the frost free freezer works by making sure the air in the freezer is dry enough that any buildup of ice will evaporate, and that there is enough air circulating around the freezer to draw the moistened air our of the freezer).

All the heat of the sun does is increase the capacity of the air to hold moisture, and if that moisture is not already in the air, then it can absorb the moisture from the damp clothing.  If you have a hot sun in air that is already saturated with humidity, it will not dry anything.

Not much good for making ice lollies then  [>:(]

Title: hanging the laundry out.
Post by: neilep on 16/04/2007 14:31:24

They are all wrong Paul.

It is a well known fact that in times of overcast days the Laundry Fairies come out and flap their wings fervently  against the hanging attire !......this is true.

Title: hanging the laundry out.
Post by: Seany on 16/04/2007 14:33:59

Oh yes! I forgot to mention that factor Paul! *Smacks forehead*

Title: hanging the laundry out.
Post by: paul.fr on 16/04/2007 22:09:52

Quote from: another_someone on 16/04/2007 12:14:50

All the heat of the sun does is increase the capacity of the air to hold moisture, and if that moisture is not already in the air, then it can absorb the moisture from the damp clothing.  If you have a hot sun in air that is already saturated with humidity, it will not dry anything.

so this is why your clothes do not dry , even on a windy day.

Title: hanging the laundry out.
Post by: daveshorts on 16/04/2007 22:52:08

Quote

All the heat of the sun does is increase the capacity of the air to hold moisture, and if that moisture is not already in the air, then it can absorb the moisture from the damp clothing.  If you have a hot sun in air that is already saturated with humidity, it will not dry anything.

This is not strictly true the reason things dry, is that more water is evaporating from them than condensing on them, if the humidity is 100% this means that there is so much water in the air that a bowl of water at the same temperature as the air would gain as much water as it lost.

However if you bring the sun into it, the washing can be considerably warmer than the air around it. So the rate of evaporation will be increased for the clothes, but the rate of condensation cannot increase, so your clothes will dry, and the hot water molecules leaving your clothes will warm up the air they move into.

As soon as the slightly warmed air that has been past your clothes cools down, the moisture will condense out of it forming a cloud of minute water droplets. Eg if you have a bowl of 80°C water on a cold damp day it will evaporate, and then the water vapour will condese again forming a cloud downwind - or think of the clouds coming of power station cooling towers, etc etc.