Dew Point, Temperature and Relative Humidity

Thanks for the reply.

Do you have any picture to illustrate the example?

How can I explain it more simpler to kids then in picture.

Best Regards

Hello stargateonly

All below Hyperlinks refer to water only, other fluids have different calculations

Refer - has nice graph: http://en.wikipedia.org/wiki/Dew_point

A simple calculator is here:http://einstein.atmos.colostate.edu/~mcnoldy/Humidity.html

And simple explanation also here: http://www.newton.dep.anl.gov/askasci/wea00/wea00130.htm

You haven't advised the age level/understanding level of the children.

Kind Regards....

When you are in a classroom, for example, take several glasses of "warm" water, about 10 C above the ambient temperature. Add progressive amounts of ice to each glass, with stirring, until "dew" forms on the outside of the glass. That's the "dew point".

For kids:

Put a jug of water on the table.Everything is common.Drop a block of ice on it now.Steer it.You will see the drop of water(condensed vapor) on the outer part of jug.This is due to "dew point".If you do this in the bath when the shower is open and the humidity is high ,possibly there'se no need to ice, because the humidity is more the dwe point temperature is higher .

AND Relative humidity:

Suppose you and your family are in the car in a very cold night on winter.The windshield will become full of damp immidiately,but will not happen it on summer,because relative humidity deponds on temperature.Notice ,the source of humidity in both situation are nearly the same.

That is pretty much the way I explain Dew Point to Bush Fire Fighters.

I set up a jug of cold beer on the table and ask them to watch the condensation forming on the outside of the jug. While that is happening explain how the high temperature of the air allows it to carry a lot of water in suspension. As the temperature drops, as it does with the air in contact with the cold beer, then the water falls out of suspension to form visible droplets, as seen in fog and condensation on the jug of beer.

I also show them a steam turbine blade that was just a little bit to far past the dew point. (Not many of them are interested in the turbine blade.)

Dew Point is important to forest fire fighters as this is when nature starts to lend a hand in the fire fighting. It gives the break that allows you to get lots of fire containment lines in place as the fire is not traveling as fast or as hot.

Cold nights and high humidity (and beer) are a fire fighters dream.

Have a wet one. (Beer that is)

BAB

Relative humidity is a more complex concept than dew point.

Dew point is the temperature where the current water vapor in the air will condense into droplets. On clear nights, objects at the surface will radiate the heat they have absorbed from sunlight, and thus cool down. If an object cools to the dew point or below, water vapor will condense into droplets on the object as dew.

Relative humidity is relative to absolute humidity. At a given temperature and pressure, a measure of air, often referred to as a "parcel", will "hold" only so much water vapor. The word "hold" is a misnomer because it is more a matter of the water's ability to remain in the gaseous state, and the air parcel doesn't really hold the water vapor. Absolute humidity is the volume of water vapor in a parcel of a given volume, and is expressed as a percentage. The maximum absolute humidity is typically 4% of the volume of the parcel. This number is too low to be useful for meteorological uses, so someone came up with the concept of relative humidity.

Relative humidity is the percentage of maximum absolute humidity that actually is in the air at the time of measurement, in comparason to the most that the parcel could "hold". That is, if we are dealing with a parcel of 1 kg, at the current temperature and pressure, the parcel will "hold" no more that 'x' grams of water vapor. The relative humidity then is the percentage of 'x' that is actually in the parcel. As the temperature changes, the absolute humidity will change, but the amount of water vapor in the parcel doesn't change, unless the temperature changes to cause the water vapor to condense, or more water vapor enters the parcel. This will cause a change in relative humidity. If the amount of water vapor in the air parcel is the same as the maximum the parcel could hold at the current temperature, then the relative humidity is 100%, and the temperature and the dew point are the same.

If you want a simple way to illustrate this to kids, have them make an old-fashioned sling psychrometer, take some readings, and look up the dew points on a psychrometric table, AKA as "wet-bulb temperature depression" table.

If the kids know some about Cartesian coordinate systems and graphs, you might have them look up info on thermodiagrams, especially the Skew-T diagrams.

A good website for learning about weather is www.theweatherprediction.com.

Guest,

Codemaster's answer is simple and to the point. Doug3's answer is more complete but about as wordy or technical as I make things (at times). Let me add a little:

Water, like everything else, can exist as a solid, liquid, or gas; depending on its temperature. At 100 degC and standard atmospheric pressure, all water will be in the gas form. Below this temperature both liquid and gas will exist. This gas form of water is dissolved in the air. Because water in the air is of so much interest to us, its concentration is called "humidity".

"Absolute humidity" is the amount of water which can be dissolved in the air at a specific air and water (liquid) temperature under equilibrium (steady and unchanging) conditions. This amount roughly follows a logarithmic curve with doubling for every 10 degC temperature rise (at least in the temperature range we normally live in). The units for absolute humidity are of the form: quantity per unit of volume (such as mg/l). "Vapor pressure" is a related term for describing water's absolute humidity (it is the pressure of the gas form at the stated temperature, with units of Torr).

"Relative humidity" is a ratio of the current absolute humidity of an air sample, divided by the maximum equilibrium absolute humidity of air at the sample's given temperature. There are no units for relative humidity since it is a ratio, and is usually multiplied by 100 and then given as a %RH.

The dewpoint is the temperature at which that particular sample of air has its relative humidity reach 100%.

If we follow the approximation stated above, that the equilibrium absolute humidity doubles for each 10 degC temperature rise, then air with 40%RH at 20 degC will have about 80%RH at 10 degC or 20%RH at 30 degC. This relationship is why air conditioners are set with a coil temperature near 40 degF (~5 degC); as the air passes over the coils the relative humidity of that air is 100%RH at 5 degC with the rest of the air's water content condensing on the coils. That air, heated back to 25 degC will have about 25%RH, which is quite comfortable.

Regards--JMM

Hello "Guest",

In the usual pattern for a "Guest" you haven't bothered to return and advise the precise composition of the liquid you intend using, or thank folks for trying to answer your Question.

You have yet to also advise the ages of the children.

Has your return to this Topic has been impeded by the throng of "Guests" to CR4?

If you do ever return to this Topic Thread, reply with

Kind Regards....