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What is Humidity, Dew Point, Freezing Point, Condensation and conditions affecting condensation?

Let's Look at each of these concepts one by one.

1. Humidity :

It is the amount of Water Vapour in the air around you. The actual amount of Water vapour present in the atmosphere is known as Absolute Humidity. When water evaporates it goes to the air as Water Vapour. If there is a lot of water vapour in the air, the humidity will be high. The higher the humidity, the wetter it feels outside.

2. Why we sweat more in Humid Conditions ?

Human body (human being a warm blooded animal) via its hypothalamus regulates the body temperature and in high temperature conditions in the environment body tries to maintain its internal temperature by evaporating moisture from skin to the atmosphere and the evaporated moisture or sweat takes away with it specific heat and hence cools the body. In humid conditions where surrounding air cannot hold further water vapour, sweat evaporates slowly, if at all. As a result the body heats up. it is difficult for the sweat to evaporate and in high humidity conditions we tend to sweat more.

3. Relative Humidity We know that there is always a maximum amount of water vapour that air can hold at a particular temperature. It's not necessary that air is actually holding maximum amount of water vapour at any particular temperature. Then relative humidity is defined as the amount of water vapour actually in the air, expressed as a percentage of the maximum amount of water vapour the air can hold at the same temperature. In other words ( The actual amount of water vapour divided by the maximum amount of water vapour that air can hold at that particular temperature ) * 100. Example : Think of the air at a chilly -10 degrees Celsius (14 degrees Fahrenheit). At that temperature, let say the air can hold, at most , 2 grams of water per cubic meter. So if there are 2 grams of water per cubic meter when its -10 degrees Celsius outside, the relative humidity = (2/2)*100 = 100 %. If there was 1 grams of water in the air at -10 degrees Celsius Relative Humidity = (1/2)*100 = 50%.

4. What affects Humidity ? Temperature is a main factor that affects humidity . Higher the temperature higher is the capacity of air to hold moisture and hence higher the Humidity. It's greater over oceans and least in continents as availability of water vapour is greatest over oceans. 5. Saturated Air: When air is holding moisture at 100% capacity it's called saturated air. In other words it cannot hold more water vapour , it has reached its maximum holding capacity to hold water vapour.

6. Dew Point :

The temperature at which saturation occurs in a given sample of air at a particular pressure is known as dew point. We saw earlier how with higher temperature of air its capacity to hold water vapour increases. So it's logical that when air is cooled its capacity to hold water vapour decrease and the temperature at which its capacity to hold moisture at that particular temperature is maximum, that temperature is called Dew point. Example : Let's say air can hold 5 gm of water vapour at 10 degree celsius. Now let's assume temperature of air is 15 degree celsius and it's holding the same 5 gm of water vapour. Now when its cooled to 10 degrees celsius. It will still hold 5 gm of water vapour but now the air is saturated as at 10 degree celsius it can maximum hold 5 gm of water Vapour.

7. What happens when we further cool the air below 10 degrees celsius in above example?

Now what happens is air starts to lose its water vapour when its further cooled down below 10 degrees celsius as its capacity to hold water vapour decreases and excess water vapour gets condensed in the form of water droplets and this process is called condensation. So in this case water vapour in its gaseous state is converted to Liquid state as water droplets. Condensation results from cooling around very small particles termed as hygroscopic condensation nuclei (Particles of dust, smoke and salt from the ocean). Condensation also takes place when the moist air comes in contact with some colder object due to which its temperature decreases and it may also take place when the temperature is close to the dew point. Example : You might have seen water droplets on grass in cold winter mornings that is a result of condensation.

8. Freezing point The temperature at which a liquid becomes a solid is called Freezing point. In case of water it would be the temperature at which water freezes in to ice.

9. Factors affecting Condensation ? Condensation is influenced by the volume of air, temperature, pressure and humidity. Condensation takes place: (i) When the temperature of the air is reduced to dew point with its volume remaining constant. (ii) When both the volume and the temperature are reduced (iii) When moisture is added to the air through evaporation.

Condensation happen is in both cases: 1. When Dew point > Freezing point 2. When Dew point < Freezing point When Dew point > Freezing point, condensation forms Dew (ie. Water Droplets ) When Dew point < Freezing point, Condensation forms Frost (ie. Ice crystals or Snow).

For more explanations visit the below links : Water Vapour : Evaporation & Condensation : Subscribe on YouTube :


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