National Weather Service United States Department of Commerce

The 'Parcel' Theory

It is common knowledge that warm air rises. It is normally assumed that is because warm air is lighter than cooler air. While that is true there is a more fundamental process that takes place for the cause of rising warm air.

Warm air rises primarily due its lower density as compared to cooler air. As the temperature increases, the density of the air decreases. But even air that is of a lower density will not begin to rise by itself.

Isaac Newton's first law of physics is that the velocity of an object will remain constant unless another force is exerted on that object. The more common way of saying this is 'an object at rest tends to stay at rest and an object in motion tends to stay in motion'.

This is why decreasing the density of air alone is not sufficient enough to cause air to rise. There must be another force exerting on the less dense air for it to begin its upward motion.

That force is 'gravity'. Gravity's role is its pull of cooler, denser air toward the earth's surface. As the denser air reaches the earth's surface it spreads and undercuts the less dense air which, in turn, forces the less dense air into motion causing it to rise.

This is how hot air ballooning works. A flame is used to heat the air inside of the balloon making it less dense. Outside of the balloon, the cooler, denser air is pulled down by gravity. The cooler air undercuts the warmer, less dense air trapped inside the balloon causing it to lift.

This is why thunderstorms often form along weather fronts. A front represents the boundary where cooler, more dense air undercuts less dense, warmer air forcing it up into the atmosphere forming the storms.

In meteorology, we often treat 'pockets of air' in a similar way to ballooning. We call these pockets of air "parcels". A parcel is a bubble of air of no definite size that we generally assume it retains its shape and general characteristics as it rises or sinks in the atmosphere.

The theory behind the "parcel" has several assumptions.

  • In a stable atmosphere, the rising parcel becomes cooler than the surrounding environment slowing or ending its rise (left image). In an unstable atmosphere, the temperature of the parcel is higher than the surrounding environment and as such remains buoyant and will continue to rise (right image).
    In both cases the parcel's rate of cooling remains fixed. Therefore, stability/instability is based upon the vertical temperature profile of the atmosphere.We generally assume the ratio of moist air to dry air in the parcel remains constant as it rises (or sinks) in the atmosphere.
  • We also assume there is no outside source of heating added to the parcel.
  • Any parcel that is unsaturated (relative humidity less than 100%) will cool (or lapses) at a rate of 9.8°C per 1,000 meters (5.5°F/1,000 feet) until the relative humidity becomes 100% (the air becomes saturated).
  • Any saturated parcel (parcel with 100% relative humidity) cools at a slower rate. This is because the process of water vapor condensing into a liquid releases heat. The released heat that is added to the atmosphere slows the rate of cooling.

Because of many different influences on a parcel of rising air most, if not all, of the assumptions will not be 100% true at all times. However, the 'parcel theory', while an over-simplification of real-world processes in the atmosphere, is a good way of thinking about how the atmosphere produces the weather.

Buoyancy: Positive and Negative Energy

The reason for looking at parcels is to help determine the stability of the atmosphere. As an unsaturated parcel rises it will cool at the fixed rate of 9.8°C per 1,000 meters (5.5°F/1,000 feet).

If the temperature of the rising parcel decreases to less than the surrounding atmosphere (due to its cooling) the parcel will become denser than the surrounding environment and gravity will slow, or even reverse, the rise. This is called negative energy and means the atmosphere at that level is 'stable'.

If the temperature of the rising parcel remains higher than the surrounding atmosphere (despite its cooling), the parcel, being less dense than the surrounding environment, will continue to rise. This is called positive energy and means the atmosphere at that level is 'unstable'.