Meteorologists use the term "parcel" to describe a single unit of measurement for air in the atmosphere. Think of this unit as a large "bubble" of air, about the size of neighborhood (for mesoscale, or "thunderstorm scale" phenomena).
The measure of the average energy content of air.
The measure of force which air exerts on any one surface. Average pressure at mean sea level (MSL) is 1013.25 millibars (mb) or hectopascals (hPa) / 29.92 inches of mercury (inHg).
Relative Humidity (RH):
The relative measure of water content in a mass of air, described as a percentage. 100% humidity defines the point at which air has become "saturated" and cannot hold any more water (due to its volume, pressure, and temperature).
Dew Point (Td):
The point at which air has cooled sufficiently enough to condense some of its water, or create "dew." RH is 100% when air reaches its dew point.
Temperature and Pressure in the Troposphere
In the troposhere, temperature generally decreases with an increase in height. This process is known as "adiabatic cooling," wherein air parcels cool as they expand. To conceptualize this process, think about air temperature decreasing as you drive up a mountain.
Heating Air Parcels
When we think of air parcels, we are generally referring to "bubbles" of air near or at the surface of the Earth. As the Earth's surface warms during the daytime (diurnal heating), heat is exchanged conductively (solid-to-gas) with these parcels. This air expands, becomes less dense, and therefore rises. Heat is then transferred to other air molecules convectively (gas-to-gas), which allows the process to continue upward.
Development of Clouds
As an air parcel moves upward and cools (becoming more dense), the total water capacity of the parcel decreases. This process causes the relative humidity of the parcel to increase, eventually leading to the occurrence of condensation (cloud formation).