Humidity is something we hear about daily in weather reports. Humidity is
to blame for that muggy, steam-room feeling you experience on certain
summer days.

Humidity can be measured in several ways, but relative humidity is the
most common. In order to understand relative humidity, it is helpful to first
understand absolute humidity.

Absolute humidity is the mass of water vapor divided by the mass of dry
air in a volume of air at a given temperature. The hotter the air is, the
more water (http://science.howstuffworks.com/h2o.htm) it can contain.

Relative humidity is the ratio of the current absolute humidity to the
highest possible absolute humidity (which depends on the current air
temperature). A reading of 100 percent relative humidity means that the
air is totally saturated with water vapor and cannot hold any more,
creating the possibility of rain.

This doesn't mean that the relative humidity must be 100 percent in order
for it to rain -- it must be 100 percent where the clouds are forming, but
the relative humidity near the ground could be much less.

Humans are very sensitive to humidity, as the skin relies on the air to get
rid of moisture. The process of sweating (http://health.howstuffworks.com/sweat.htm) is your body's attempt to keep
cool and maintain its current temperature. If the air is at 100-percent
relative humidity, sweat will not evaporate into the air. As a result, we feel
much hotter than the actual temperature when the relative humidity is
high. If the relative humidity is low, we can feel much cooler than the
actual temperature because our sweat evaporates easily, cooling *us off.

For example, if the air temperature is 75 degrees Fahrenheit (24 degrees
Celsius) and the relative humidity is zero percent, the air temperature feels
like 69 degrees Fahrenheit (21 C) to our bodies. If the air temperature is 75
degrees Fahrenheit (24 C) and the relative humidity is 100 percent, we feel
like it's 80 degrees (27 C) out.

People tend to feel most comfortable at a relative humidity of about 45
percent. Humidifiers (http://home.howstuffworks.com/humidifier.htm) and dehumidifiers help to keep indoor humidity at a
comfortable level.







If the air is at 100-percent relative humidity, sweat will not evaporate into
the air. As a result, we feel much hotter than the actual temperature when
the relative humidity is high.

thats why in london when there is a heat-wave...its 35 degrees but it feels more hot than peshawar which is 45 degrees...u dont sweat much in london and in peshawar u sweat non stop..

Can anybody tell me why the sun is so hot in London. I mean you can not stand in sun for just five minutes but in Peshawar you can stand for hours. You sweat their but you can afford the sun rays. But in London its way different. Why the sun rays are so sharp, so intense in London. Some one has told me that its because here there is no pollution so there is no dust and no smoke in the air so the rays come directly. But in Pakistan, there very few rays reaches to you.
Is it true or there is some other reason ?

I've noticed this too emkhan. I think it has the do with the latitude of where you are in relation to the angle of the sun, but this is only personal speculation. I live at roughly the same latitude as London, several people have mentioned this too.

Yes Janana, I think its because of the clear atmosphere that the sun rays touches our body directly. Lets see the expert's opinion what they say about it .

London shares the same latitude with various cities including Calgary in Canada.
The average temperature in Calgary can range from -9 degrees in Winter to 16 degrees in Summer. Whereas the average temperature in London ranges from 5 degrees in Winter and 18 degrees in Summer.

The reason for this is because London and the UK is exposed to the Gulf Stream.

Surface water that is in the Atlantic becomes cooled from winds coming from the Arctic. This makes it more salty and more dense so it sinks to the ocean floor. This cold water will then move towards the equator where it warms up. To replace this water coming from the Atlantic, the Gulf Stream moves the water to the Atlantic from the Gulf of Mexico.

This happens because of osmosis (where matter of higher concentration moves to an area of low concentration). I think osmosis was the word, can't remember. Also this is due to the changing wind patterns in the Atlantic.

This all results in the Uk and Northern/Western Europe being warmer than it actually should be (by about 5 degrees).

This contributes to an increase in humidity where this shouldn't normally be the case. For that reason it may feel as if it is very hot and stuffy here.

That is why in America the West Coast is less humid than the East Coast. Because the temperature of the water in the Atlantic is warm whereas the temperature of the water in the Pacific is relatively cool. It is also why London is warmer than most cities in Europe, Russia and North America with the same latitude.

The Sun isn't actually any brighter in London than in Pekhawar (as far as I am aware) so I assume it would be as a result of the differences in climate and possibly some other factors I didn't mention.



But I am no scientist, I haven't even studied this. I do this next year :)



Edit: Just because the temperature in somewhere such as Pekhawar for example may be 10 degrees higher than somewhere like London, that does not neccesarily mean you will feel hotter there. There are other factors at work too. I think its also adjusting to another climate emkhan. I do not think you have experienced summer here before? I remember stepping foot out of the Dubai Airport. As soon as the doors opened you could instantly feel the humidity. But I guess when you live in such a climate you become used to it and when you move somewhere else you are bound to notice at least some changes in the climate.