kg –1, T 0 is 273.15 K, e 0 is 0.6113 kPa, and L v is the latent heat of vaporization, 2.5×10 6 J.

Where the water-vapor gas constant ℜ v is 461 J The Clausius-Clapeyron equation gives the approximate relationship between saturation vapor pressure ( e s) and temperature in the atmosphere When the partial pressure exerted by water vapor is equal to the saturation vapor pressure, the air is saturated. If we think of this quantitatively in terms of pressure, saturation vapor pressure refers to the pressure exerted by the movement of water vapor molecules exerted over a surface of liquid water. Lower temperature air cannot contain as much water vapor as higher temperature air.

The amount of water vapor that the atmosphere can contain depends on temperature. Because we are staying consistent with Roland Stull’s Practical Meteorology textbook, we will use kiloPascals (kPa) throughout this chapter. The units for vapor pressure are the same as pressure and can be in Pascals, hectoPascals, or kiloPascals. Specifically for water vapor, the more water vapor that is added to the atmosphere, the higher the vapor pressure P H2O. In the following equation, all of the gases in Earth’s atmosphere contribute to the total atmospheric pressure P atmosphere. Vapor pressure at saturationĮvery gas in the atmosphere exerts pressure, for example, vapor pressure makes up a fraction of the total atmospheric pressure. If evaporation occurs, the air cannot contain more water vapor, so some must condense. That is why condensation equals evaporation. Saturation occurs when air contains the maximum amount of water vapor possible for its given temperature. When condensation and evaporation are equal, this is called saturation. However, after some time, the number of molecules evaporating from the surface will be equal to the number of molecules condensing back into the water surface. At the initial time, more water molecules evaporate from the water surface than the number that return. Imagine a closed jar filled halfway with water. You’ve likely heard of relative humidity and dew point temperature, but what do these quantities mean physically? Saturation In general, humidity is the amount of water vapor in the air. How do they form? Why are they there? Water droplets condensed on a glass surface ( CC BY 2.0).īefore we can understand clouds in the atmosphere, we need to explore concepts like how humidity is defined and what saturation means. ( CC BY 2.0).Ĭlouds are composed of millions and billions of tiny liquid water droplets.

Phase changes of water from gas (water vapor) to liquid (water) to solid (ice) with the names for the processes also labeled. Condensation is especially important in atmospheric science because this is the process that allows clouds to form. The opposite process is called condensation, where water vapor becomes liquid water, releasing energy. As we learned, the process of liquid water becoming water vapor is called evaporation and this process absorbs or requires energy. Water can exist as a solid, liquid, or gas at typical conditions found on Earth. Use the principles of phase change and latent heating to describe why the moist adiabatic lapse rate is less than the dry adiabatic lapse rate.

Describe the conditions for saturation to occur.Differentiate between relative humidity, specific humidity, absolute humidity, wet-bulb temperature, mixing ratio, and dew point Compute saturation vapor pressure using the Clausius-Clapeyron equation.By the end of this chapter, you should be able to:

Density of water vapor in kgm3