Do you like the dew (Romania)


If yes let’s make dew !

What you need ?


A metal cup with lustrous outer surface filled until half with water at room’s temperature

A cup with cold water (~ 0°C ).

( For example a cup with saulted water cooled in the freezer ).

Good light conditions for observing the outer lateral face of the cup

What you can do

Level 1


Step 1: Measure and record the room’s temperature t1 =........0C


Step 2: Check that the outer surface of the cup be dry . Introduce the thermometer in the water from the cup and read the temperature .


Step 3: Lower slowly the water temperature from the cup , pouring small quantities of cold water, mixing and watching the temperature. Watch carefully the outer surface of the cup and take care to not splash it during the experiment .


Step 4: Repet the step 3 until the moment at which the outer surface of the cup become matt and whitish . Measure the temperature in this moment and record as t2 =........°C.


Step 5: Pour in the cup a final quantity of cold water,to down the temperature with still 1-2 °C. You will see that the phenomenon observed at the surface of the cup from the step 4 become more pronounced.

Well : You have just produced dew !
The deposit from the surface of the cup is dew. You can test it with your finger and realize that the surface is wet, or lower more the temperature and you’ll see small drops of water. But from where have the water drops appeared on the outer surface of the cup? They appeared from air because we know in air there are water vapors : many when we say that the air is wet and few when we say that the air is dry.
The water vapors from air condensed on the cold surface of the cup .

Step 6: Watch further the wet surface of the cup and wait to warm slowly .You find out that after few minutes the wet disappear and the cup become again lustrous and dry . This thing is due to the water evaporation from the outer surface of the cup during warming . Now, measure again the temperature. You find out it became equal again with t2, i.e. the temperature at which the dew appear is equal with the temperature at which it disappear in your lab .
The temperature t2 you have just determined, is called “The dew point temperature“
You can repeat this experiment in different conditions: in rainy or sunny days . You’ll find out :.
If: t2 – t1 = 2 – 3 °C
It is: Rainy day
That means: Wet air
t2 – t1 = 10 – 15°C
Sunny day
Dry air
Behold that The dew point temperature tells us if the air is dry or wet
Level 2.

But: Can this Dew point temperature really tells us more ?
You’ll see that the answer is yes .
Remember the Experiment ― Boyling temperature of water ― and the diagram from there:

In this experiment we are interested in the marked zone of the diagram which is shown in more details below .
Accept ( learn ) that: for a given pressure of water vapors, the boyling temperature is equal with the temperature at which the water condense, i.e. with the dew point temperature.

Step 1: In this diagram we marked the temperatures determined in one of our experiment:
t1 = air temperature in the lab.
t2 = the dew point temperature


Step 2: From the diagram we read the pressures P1 and P2 coresponding to the two temperatures , as illustrated .
For example we determined in the lab.: t1 = 26°C and from the diagram we read P1 =25 torr
t2 = 13°C and from the diagram we read P2 =12 torr


Step 3: We calculate the ratio :
R.H. = X 100 %
(In our experiment: R.H. = X100%=48%)
Well , in this formula R.H. is nothing else that The Relative Humidity of the air, that you can often hear at The Weather News on TV.
So , EVRIKA !, you have just discovered how can determine yourself, only with a thermometer and a cup , the humidity of the air .
Do you still like the dew?

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