A few questions about steam, water vapor, and humidity

[itsthemac]

I'm trying to understand the differences (if there are any) between steam, water vapor, and humidity. First of all, is there any difference between the meanings of the terms steam and water vapor? Is there a difference between the terms "gas" and "vapor" in general?

Also, I was under the impression from my physics class that steam could not exist below 100 C, (at 1 atm) as it would turn back into liquid water. So then how does any humidity (which I'm guessing isn't any different from the steam/water vapor you get when you boil water) exist under 100 C? And just how cold can it get before it must condense back into water?

The thing that got me thinking about all this was when my windshield fogged up this morning as I was driving to school. I put the air on to defog it and then I wondered two things: 1) how the water vapor (steam) could exist in such cold air to begin with and 2) how does blowing room-temperature air through the vents of my car under the windshield cause the condensation to go away? Why does this make it evaporate?

Another semi-related question: I've heard people say you should use hot air and other people say you should use cold air to defog your windshield. Can someone please explain which makes more sense and why?

Thanks a lot, and sorry for asking so many questions.

 

[russ_watters]

You're on the right track: steam is a vapor that is saturated (at 100% RH).

Condensation is mostly unrelated: it occurs on a surface that is below the dew point.

 

[Naty1]

Suggest you look your terms up in Wilipedia:

Humidity is the amount of water vapor in the air.

http://en.wikipedia.org/wiki/Humidity

Steam is either mist (as seen from a kettle), or the gas phase of water (water vapor).

http://en.wikipedia.org/wiki/Steam

These are not necessarily scientifically precise terms...

I don't think steam is necessarily saturated water vapor...see the Wiki article above 'SATURATED STEAM'

it's just visible water vapor and while it is 100 degrees C upon emission from boiling water,at the standard pressure ( I have forgotten is it 1 ATM?)... anyway, it quickly drops below that temperature in typical, say kitchen, environments...but may still appear as water vapor mist.

You might also check on fog (which is also water vapor)...and clouds...

Fog is distinguished from mist only by its density

http://en.wikipedia.org/wiki/Fog

 

[Naty1]

Here's an example which may not be precisely accurate, but conveys the idea...

Say a power plant gives off hot water vapor...maybe from a smokestack...steam (water in the form f heated mist) ...it likely rises for a while and drfits around, may hang as a low (visible) cloud then cool and sink and become ground fog...which is now cooler water vapor...or it may just increase ground level humidity and not be visble...and it comes into conatct with a cool surface and condenses and appears as water droplets...

 

[PJC01]

Dear reader,

Thank you for your questions about steam, water vapor, and humidity. These terms are often used interchangeably but they do have distinct meanings in the scientific context.

Firstly, steam and water vapor are technically the same thing - they both refer to the gaseous form of water. However, steam is commonly used to describe water vapor that is at a higher temperature and pressure, such as the steam that comes from boiling water. On the other hand, water vapor can exist at various temperatures and pressures, including below 100°C and at atmospheric pressure. So, while they both refer to the same substance, the terms are often used to describe different states of water vapor.

Secondly, the terms "gas" and "vapor" do have different meanings. Gas refers to a substance in its gaseous state at any temperature and pressure, while vapor specifically refers to a substance in its gaseous state at or near its boiling point. So, while all vapors are gases, not all gases are vapors.

Regarding humidity, this is a measure of the amount of water vapor in the air. It is affected by temperature, as warmer air can hold more water vapor than cooler air. So, even at temperatures below 100°C, there can still be water vapor present in the air and therefore humidity.

As for your question about how the water vapor can exist in such cold air, it is because there is still some water vapor present in the air, even at low temperatures. When the air inside your car is warmed by the heater, it can hold more water vapor, which prevents the condensation on your windshield. When you blow room-temperature air through the vents, it mixes with the warm air and increases its capacity to hold water vapor, causing the condensation to evaporate.

Finally, regarding the use of hot or cold air to defog your windshield, both can be effective. Using hot air will warm up the air inside your car, allowing it to hold more water vapor and prevent condensation. Using cold air can also work, as it can lower the temperature of the windshield and reduce the temperature difference between the inside and outside of the car, preventing condensation. Ultimately, it depends on the specific conditions and preferences of the individual.

I hope this helps to clarify the differences between steam, water vapor, and humidity. Please let me know if you have any further questions.

Best regards,