Hydrologic cycle: conversion of the three phases of H₂O (liquid, solid, gas) between phases.

The total amount of water is constant (with few exceptions).

Precipitation: H₂O falling from the atmosphere, either as a solid, or a liquid.  Gas to liquid or solid.
Evaporation: conversion of liquid to gas; more water molecules enter the atmosphere as a gas than return to the liquid water.
Condensation: conversion of gas to liquid; process in which more molecules return to the liquid than the gas.
Transpiration: release of H₂O from plants through their leaves in the atmosphere as a gas.
Evapotranspiration: combination of evaporation and transpiration.

When water changes phases is will either release or absorb heat:

Heat that is absorbed during the phase change is called latent heat, since it is a hidden heat and does not change the temperature during the process.  It can later be released.

Note that working our way through the water cycle will result in zero heat gain or loss (conservation of energy).

Humidity refers to the amount of water vapor (H₂O_V) present in the atmosphere.  There are five (5) different methods to express humidity:

1. Absolute humidity
2. Mixing ratio
3. Vapor pressure
4. Relative humidity
5. Dew point

Since temperatures and pressures change constantly and they change from place to place, meteorologists usually prefer mixing ratio.

Mixing ratio: mass of water vapor in a unit of air compared to dry air.  E.g., mass of water vapor (in grams)/mass of dry air (kilograms).  Not affected by changes in temperature or pressure.  Also known as specific humidity.

Pressure
Pressure is defined as a force per unit area.  That force is caused by air molecules striking your skin because they are in motion.  That's why the air pressure is lower at great altitudes - fewer air molecules.

Air pressure is dependent on the kinetic energy of the air molecules (temperature), the mass of the molecules, and gravity.

In meteorology we use units of millibars to measure air pressure.  Sea level is ~1013 mb.

Vapor pressure: that part of the total atmospheric pressure attributable to water vapor.  In chemistry we would call this the partial pressure.  We could also measure, or calculate, the partial pressures of all the other gasses in the atmosphere (e.g., N₂, O₂, Ar, CO₂, etc.) - they should all add to the total atmospheric pressure.  This is temperature dependent.

We can actually measure the number of molecules (by way of the pressure) leaving water, or entering water.  When the number of molecules leaving water is the same as the number entering, we call that saturation, and the pressure is the saturation vapor pressure.

The amount of water vapor required to saturate air at various temperatures is variable.  That's why on a very cold day the relative humidity may be close to 100 percent but it doesn't feel like a muggy 70 percent relative humidity day when it's 95°F outside.

If the amount of water vapor is constant:

Increasing temperature decreases RH
Decreasing temperature increases RH

If temperature is held constant:

Adding water vapor increases RH
Removing water vapor decreases RH

This is why we usually see higher relative humidity at night.

Since relative humidity is easily changed by temperature or amount of water vapor, we'd like to find another variable that isn't so easily changed.

Dew Point: temperature to which the air would have to be cooled, without changing pressure or moisture content, to reach saturation.

The drier the air, the more is must be cooled to reach saturation.

High dew points indicate a large amount of moisture in the air.
Low dew points indicate small amounts of moisture in the air.

Frost (hoar frost, white frost) is caused when the temperature is below freezing and the air is saturated with moisture.

Dew points are directly related to the amount of moisture in the air, so is a much better indicator of humidity.

Dew point depression, T_dd = T - T_d
Small T_dd indicate high RH
Large T_dd indicate low RH.
 

Ways to achieve saturation

• Cool the air without adding or removing moisture.
• Add water vapor without changing the temperature.

So we now know the various measurements of the amount of water vapor in the atmosphere, but how do we actually measure it?

The most common technique is to use the sling psychrometer.  The psychrometer has two thermometers that are twirled around on a sling.  One thermometer is covered with a wet cloth wick, so that as it is twirled it is cooled through evaporation, hence the name sling psychrometer (sling, psychro- cold, and -meter, to measure).

The lowest temperature recorded is called the wet bulb temperature.  We then look up the wet bulb depression (difference in temperatures between the wet bulb thermometer and the dry thermometer) versus the dry temperature to get the RH.

There are other ways to measure relative humidity:

• Electronic (semiconductors)
• Chilled mirror hygrometers
• LiCl dew cells
• Human hair

In order to make a cloud we need:

• Moisture
• Nuclei on which to condense (cloud condensation nuclei, CCN, or ice nuclei, IN)
• A method of cooling the air to saturation

As air ascends, the pressure decreases allowing the volume of air (a parcel) to increase.  As the volume increases, its temperature decreases - we call this adiabatic cooling.  Unsaturated air will cool 1°C/100 meters of ascent, or it will heat that amount upon descent.

This is called the dry adiabatic rate: 1°C/100 meters (10°C/km)

If this parcel of air cools enough it will reach its dew point and condensation, in the form of cloud development, will occur.

As air is compressed is heats up - adiabatic warming.
As air expands it is cooled - adiabatic cooling.