Saturation mixing rate

Water is a very interesting substance. It has many interesting properties, as well. It is the basis for the Celsius temperature scale. We know that at 0 degrees C, the water freeze to ice, or melting to liquid water if the temperature is increasing from temperatures below zero. And at 100 degrees C, the water start to boil or evaporate. Most of us know that water will boil at lower temperature if we boil it at Mt.Everest and that this is because of lower air pressure the higher you get. And most of us know that we can increase the temperature for boiling water, if we use a pressure cooker. The water also has several phases in solid state, depending on the pressure, but that is not so important for us to know. What is important for us to know, is that the change from liquid water to vapor, and back again, is dependent on both the temperature and the pressure. That makes it easier for us to accept that the air can contain different amount of vapor pending on the temperature and the pressure.

Density of dry air is about 1,2 kg/m3. That means that for 1 kg of air you have a little less than 1 m3 of air. If you add vapor to it, the density actually goes down, since the H2O-molecule is lighter than both N2 and O2. Isn't that strange? But that is how it is.

The mixing ratio between vapor and dry air is given by mass of vapor pr mass of dry air. To get suitable numbers we choose to use grams of H2O pr 1 kg dry air. Typical value for this is 1 - 20 g/kg. What the mixing ratio actually is, is depended of the mass of vapor in the airmasses that is coming in with the winds. Any water on the ground is negligible to the mixing ratio. Although we need the ground to dry up before thermals start, the amount of water is so small that it does not affect the mixing ratio.

Saturation happens when the air can not solve more vapor, and the condensation starts. In our term, that is when clouds starts to form. Saturation may happen by cooling the air, by decompressing the air, or by a combination of both. The last one is what we deal with when talking about clouds formation. The mixing ratio for saturation increases with higher temperature and with higher pressure.

An air mass with a given mixing ratio from the ground, will reach the dew point, if it is allowed to rise to a level with low enough pressure and low enough temperature. The mixing ratio for saturation is given in the Skew-T diagram.

Paragliding Greece mixing ratio

The mixing ratio

The mixing ratio lines in the diagram is more vertical than the lines for the temperature. This tell us that saturation is reached at a lower temperature, when the pressure decreases. The lines stops at 700 mbar, but that is just to simplify the diagram. You may extend them if necessary.

The lower end of the green line, show us the actual mixing ratio of the air close to the ground, in this example about 16 g/kg. It also tells us that if the temperature at ground level drops to 21 degree C, there would be condensation. But that will not happen. What happens is that the air will rise with the thermal. Still it contains 16 g/kg. So if the air rise to 700 mbar, it need less than 15 degrees C to condensate. If the air rise to 600 mbar, it needs less than 12 degrees C to condensate.

Cloudbase (LCL - Lifting Condensation Level) will be determined by the level where the required pressure and temperature is met by the rising air, which contain a certain amount of vapor, the mixing ratio. Some days the air don't meet these necessary conditions for cloud formation, typical blue day. When that happen, it is because there is an inversion that stop the thermal from going high enough. By knowing the humidity and the temperature on the ground, you can tell how high the cloud base will be.