Thanks, Checked and saw that the Reynolds number was around 3x10^5 which confirms what you said.
Agreed, however the comment was about the amount of water it can carry:
Vapor Pressure of water, at temperature T = Vp
Total Pressure of air, at temperature T = P
Saturated Volume fraction of water vapor in air = F
Then Vp = F * P
So as P goes down, air will be able to pickup more water before it reaches saturation.
In other words, 100% Relative Humidity Air will be less than 100% Relative humidity, if pressure is reduced. (assuming the same temp)
Relative humidity is not the same thing as saturated volume fraction.
The way I read this statement was : that the amount of water carried by a unit volume of air (at saturation) is the same at 2 different pressures ?
At two different total air pressures, yes. The partial pressure of water (assuming that there’s an available body of liquid water and that the liquid and gas are in equilibrium) depends only on temperature, not on whatever other gases may or may not be present.
I think you mean vapor pressure and not partial pressure.
I think he had it correct.
Vapor pressure is how much pressure is exerted by the evaporation of a fluid into its surroundings. The vapor pressure of ice water is low, the vapor pressure of boiling water is high.
Partial pressure is how much of a gas is in the air. The higher the humidity, the more partial pressure of water in the air.
Glider pilots (at least, this one) don’t. Either there are clouds marking the thermals or there are not. Assuming that there are thermals present, that is. But knowing predicted dew points can give you an idea at what altitude the cloud bases will be. That, in turn, gives you an idea how far you might be able to fly cross country. The higher you can climb, the greater your glide range to the next thermal. Really high dew points may mean cloud bases below 3000’. Since you you are legally required to remain 500’ below clouds, you have little to work with. When its that oppressive thermals are very weak or non-existent anyway. Better to go boating on days like that,
Thank you for the detailed explanation. I am fascinated by gliders; their determination, courage and finesse is admirable
I’ve mentioned before that dew point and humidity are never mentioned in general-purpose weather forecasts where I live. I just checked online and I find that the humidity is 72% and the dew point is 8 degC. Really those numbers mean very little to me in terms of comfort.
Basically, the dew point is going to be the temperature that the surface of your skin is approaching.
This may be an unhelpfully nit-picky point; if so I apologise. Several posters have explained dewpoint and relative humidity in terms of how much water vapour the air can hold at a given temperature. This is misleading because it implies that the ability to “hold” water vapour is a property of the air. In fact the air is irrelevant. The relative humidity and dewpoint would be the same if there were no air at all, or if the air were replaced by some other mixture of gases. What is being measured is how much water vapour can exist at a given temperature without condensing. It’s not “how much H2O per cubic metre of air” but simply “how much H2O per cubic metre”.
I would categorize that as a very helpful and useful nitpick.
By those numbers, the temperature is shown as 12.9 deg C. Here in Taiwan, that is our winter temperature and I found that I don’t get a feeling for how much comfort or discomfort a low dew point means to me.
The dew point here now at 8:30 am is 28.1 deg C, and I can tell it’s muggy.
Converting my rough rules-of-thumb from earlier in this thread to Celsius:
- Generally, a dew point that’s below 10C is very comfortable for most people, and if it’s well below 10C, it may even feel noticeably dry (i.e., your skin feels dry, etc.).
- Once the dew point reaches around 15C, the humidity starts to feel noticeable to many people.
- A dew point around 18C is noticeably muggy.
- A dew point at 20C or higher is oppressively humid.
Edit, based on @TokyoBayer’s post: a dewpoint of 28.1C would make me melt. 
At the higher end of the temperature scale, then the dew point is meaningful for me, although it’s really a question if it feels horrible or absolutely horrible.
Down at the lower end, I also feel uncomfortable with a high humidity, but the wetness is more uncomfortable than the temperature. Here in Taiwan, our winters are more in the 15C range but as low as 8C.
To provide a bit of historical context, the dew point is simply a convoluted way to measure absolute humidity. Relative Humidity and Absolute Humidity each have different uses but in general, Absolute Humidity is a more important number. The reason why we use Relative Humidity in most places is because the earliest scientific instruments we built measured Relative Humidity by measuring the tension on a length of human hair. Thus, the majority of scientific literature was written in reference to Relative Humidity with Dew Point used as a standing in Absolute Humidity when needed.
Nowadays, I believe most electronic sensors actually measure Absolute Humidity and derive the Relative Humidity number through combining it with a temperature measurement. Pretty much every electronic device that measures humidity will also measure temperature because the two sensors need to be packaged together for them to function.
Things would have been a whole lot simpler if we started off with Absolute Humidity and just used lookup tables whenever we needed to compute a Relative Humidity number but that’s not how historical contingency worked out.
I agree with you that given a certain temperature what you say there is true. However, in most practical cases that involves humans and transpiration, we are looking at cases nearing adiabatic, and in such cases the gas composition matters because the specific heat of the gas matters. Look at the following two examples :
Case 1 : 100 ft3 of dry air in a room (70F and 14.7psia), and spray in 3.24 lb of water (70F and 14.7psia). The final equilibrium temperature of air, as well as water is 44F. Saturated Air will take most of the water, and there will be 0.55 lb of water puddled on the floor (in dynamic equilibrium )
Case 2 : 100 ft3 of dry Helium in a room (70F and 14.7psia), and spray in 3.24 lb of water (70F and 14.7psia). The final equilibrium temperature of Helium, as well as water is ~40F. Saturated Helium will take most of the water, and there will be 1 lb of water puddled on the floor (in dynamic equilibrium)
So, the notion of the ability to “hold” water vapour is not entirely wrong.
But (at least under normal circumstances) you can’t have a low temperature with a high dew point.
True. But you can have 40F / 5C and high humidity. It doesn’t subjectively feel the same (damp <> muggy), but it can still be uncomfortable.
I grew up at the seashore. In an area (SoCal) with cold water and warmish air. 50F/10C and highly humid at the shore in the rain is a different beast than 50F/10C and nil humidity in the desert.