Moisture (H20 vapor, liquid, and solid) and Its Measurement

H20 vapor plays a critical role in the atmosphere Controls clouds and precipitation Moves energy horizontally and vertically Influences temperature Major greenhouse gas Need to have a deep understanding of its properties.

Solid versus liquid versus gas

The Concept of Saturation: A Thought Experiment Imagine an air tight box with liquid water at the bottom and completely dry air above. Keep at a constant temperature (T) that is above absolute zero. What will happen? Dry Air Liquid Water T > 0°K

A Thought Experiment H20 molecules in liquidare moving since T>0°K. Not all moving at the same speed. They also have some attraction to each other…that is what a liquid all about Once in a while some of the fastest molecules will escape the liquid and move into the air above as a gas molecule. Dry Air Liquid Water T > 0°K

A Thought Experiment More and more molecules end up in the dry air Occasionally, one head back into the liquid The more molecules in the air above, the more start heading back into the liquid Liquid Water T > 0°K

https://www.youtube.com/watch?v=YOnSISXDILU https://phet.colorado.edu/sims/html/states-of-matter-basics/latest/states-of-matter-basics_en.html

A Thought Experiment What if there was a vacuum above the liquid water? Would it make a difference? NO. Same story. Water molecules would increase in numbers until there was saturation. Water vapor Liquid Water T > 0°K

A Thought Experiment The H20 molecules above the liquid exert a pressure, called the water vapor pressure. During our thought experiment the water vapor pressure rose until it reached the saturation vapor pressure, the pressure due to water vapor when the air is saturated (holding as much water vapor as it can) Water vapor Liquid Water T > 0°K

Vapor Pressures Note that the total pressure we experience and measure is the SUM of the vapor pressures of all the gases around us. Total atmospheric pressure = Vapor Pressure of N2 + Vapor Pressure of O2+ Vapor Pressure of Ar + Vapor Pressure of H20 vapor

Back to the Thought Experiment What if we raised the temperature of the water? The water molecules in the water would speed up. The rate of escape from the liquid would INCREASE. More would go out until a new equilibrium was established, with MORE water vapor molecules than before. Water vapor Liquid Water T > 0°K

Back to the Thought Experiment A new HIGHER saturation vapor pressure would occur. To say it another way, the saturation vapor pressure increases with temperatures Water vapor Liquid Water T > 0°K

Sloppy Language Hold One way that this is often expressed is by saying that warm air can hold more water vapor than cold air BUT NOTHING IS HOLDING ANYTHING. We would have the same saturation vapor pressure if there was no other air above the liquid. Perhaps “contain” is a better word.

Quantitatively: How Does the Amount of Water Vapor that Can Be Contained in a Volume Increase with Temperature? EXPONENTIALLY!

At 35C (95F) the saturation vapor pressure is 4x more than at 10C (50F)

What happens when the saturation vapor pressure of water equals atmospheric pressure? You get boiling! For typical atmospheric pressure (~ 1000 hPa) that occurs around 100°C

As long as atmospheric pressure is greater than saturation vapor pressure bubbles of water vapor can not grow.

If atmospheric pressure is lower than temperature of boiling is lower If atmospheric pressure is lower than temperature of boiling is lower. The saturation vapor pressure needed to equal atmospheric pressure is less At the top of Mt. Everest (about 29K feet ASL) water boils at 70C (160F) Denver: 95C, 203F, not 212! This explain high-altitude cooking instructions for some recipes…need to cook longer to make up for lower boiling temperature

Needs to be patient

Measures of H20 Vapor in the Atmosphere H20 vapor pressure: air pressure contribution of water vapor only (hPa) Generally not on TV!

Mixing Ratio mixing ratio (w) = mass of water vapor in a sample (g) mass of dry air in a sample (kg) Normally in g/kg Very humid: 15 g/kg

Saturation Mixing Ratio At any temperature and pressure there is a maximum mixing ratio, which occurs when air is saturated saturation mixing ratio (ws) = mass of water vapor in a sample of saturated air(g) mass of dry air in a sample (kg) Will learn how useful this is in a few minutes…

Saturation Mixing Ratio Increases Rapidly with Temperature

Relative Humidity (RH) = 100 Relative Humidity (RH) = 100* amount of H20 vapor in the air max possible amount of H20 vapor in the air at that temp =100* w/ws RH varies during the 24-h day: lower during the day when temps are high, higher in early morning when temps are low

Relative Humidity Another way to understand the diurnal variation: The mixing ratio, w, stays relatively constant But ws, the saturation mixing ratio, depends on temperatures Thus, when temperatures rise, ws rises, w stays the same, and thus w/ws must drop. WRF example: https://atmos.washington.edu/~ovens/wxloop.cgi? wrfd3_rhsfc+2017101312///3

Dew Point (Td) or Dew Point Temperature Definition: the temperature at which air becomes saturated when it is cooled at constant pressure. Given in F or C More moisture in air gives a higher dew point Less moisture lower dew point At saturation T = Td Td does not change rapidly At night, if T drops to Td, dew or fog can form Td DOES drop if dew or fog forms (water is taken out of air)

Dew Point Seattle in summer ~45-50F E. Washington in summer ~20=30F DC in summer, 65-75F You feel uncomfortable with high dew points Reported at airports around the world

Measuring Humidity The classic approach before solid state sensors is to measure both temperature and the wet bulb temperature (Tw) at the same time Definition: Wet bulb temperature (Tw): the temperature given by a thermometer with a wet wick. Contrasts with the dry bulb temperature (Td), the temperature provided by a normal thermometer. If the air is unsaturated Td > Tw If the air is saturated Td=Tw The bigger the difference, the drier the air