This lecture discusses relative humidity. Water vapor plays an essential role in the Earth atmosphere system. Water gets into the atmosphere through evaporation, condenses into clouds, moves over land, and then exits the cycle through precipitation.
Once water evaporates, it usually only stays resident in the atmosphere for a week or less. The horizontal movement is called advection. Water vapor is measured with a hygrometer, and depending on latitude and season, the global distribution of water vapor ranges from 4% by volume to almost nothing in the coldest locations. Getting water vapor into the atmosphere can be challenging.
There needs to be water at the surface to evaporate. Positive relationships means that as one increases, the other increases. As more water is available, more evaporation is possible.
The warmer the water, the easier it is for the water to escape its liquid bonds and move into the atmosphere. The drier the atmosphere, the more evaporation is forced from the surface. Faster wind speeds can actually knock water molecules from their liquid bonds, forcing the water into the air. Water is transferred from the surface to the atmosphere through evaporation, the process where water changes from a liquid to a gas.
Specific humidity and the mixing ratio are both measures of the actual amount of water in the atmosphere. Specific humidity is the mass of water vapor per the total mass of air. The mixing ratio is the mass of water vapor per the mass of the dry air.
The units for both of these are grams, usually grams per kilogram. Saturation is an essential concept in understanding the distribution of water vapor. Saturation is that point where the atmosphere is holding all of the water vapor that it can for a given temperature.
Relative humidity is a measure of saturation in the atmosphere. The closer the atmosphere is to saturation, the higher the relative humidity. There's a relationship between saturation and air temperature. This relationship is shown on the Clausius-Clapeyron curve. As air temperature increases, the amount of water that could be held in the atmosphere also increases.
The line on the diagram that shows this relationship is the point of saturation, or where 100 percent relative humidity would occur. At 100% relative humidity, condensation begins. A cloud is at 100% relative humidity.
Below the line is unsaturated and relative humidity would be less than 100%. This line is the saturation curve and shows the saturation mixing ratio. The saturation mixing ratio is how much water the atmosphere could hold for a given temperature.
Another important concept is dew point temperature. The dew point temperature is the temperature when condensation or dew begins to form. The dew point temperature demonstrates saturation.
One thing to note is that the dew point temperature is a temperature, so it's given in units of Fahrenheit, Celsius, or Kelvin. It's the temperature that the air has to cool to in order to condense. The higher the dew point temperature, the more water vapor in the atmosphere, and the easier it is to force condensation. Relative humidity is one of the most reported meteorological variables, yet one of the least understood.
Relative humidity is just a measure of saturation. It's a ratio between how much moisture is actually in the atmosphere versus how much moisture the atmosphere could hold at saturation. The mixing ratio is the actual amount of water in the atmosphere.
The saturation mixing ratio is how much water the atmosphere could hold for a given temperature. The mixing ratio is measured with a hygrometer. The saturation mixing ratio comes from the Clausius-Clapeyron curve.
Relative humidity is a fraction of mixing ratio to saturation mixing ratio. It's expressed as a percentage. The relationship between air temperature and relative humidity is shown on this chart.
The warmer the air temperature, the lower the relative humidity. The colder the air temperature, the higher the relative humidity. On a diurnal basis, Relative humidity changes frequently as a result of changes to air temperature.