A parcel is large enough to contain a very great number of molecules, but small enough so that the properties assigned to it are approximately uniform within it and so that its motions with respect to the surrounding atmosphere do not induce marked compensatory movements.

A parcel of air expands and becomes less dense as it rises. This occurs because the air pressure lowers around the parcel as it increases in altitude. The volume of the parcel increases since it is expanding. The temperature of a rising parcel always cools even though it is becoming less dense.

If the temperature of the air in a parcel becomes warmer than the surrounding envrionmental air, the air parcel becomes buoyant, and accelerates upward. In other words, a warm parcel is less dense than the cooler air surrounding it, so the parcel gets forced upward by the more dense surrounding fluid.

Negative buoyancy occurs when an object is denser than the fluid it displaces. The object will sink because its weight is greater than the buoyant force. A submarine is designed to operate underwater by storing and releasing water through ballast tanks.

Above the LFC and the Meaning of Buoyancy. The gas law predicts that at a given pressure elevation, an air parcel that is cold relative to its suroundings will be more dense (and heavier) than its surroundings.

If the air comes back to where it started, the atmosphere is stable. If the air continues to rise the atmosphere is unstable. In the figure above the air in the parcel has ended up colder and denser than the surrounding air. In this case the parcel would sink back to the ground.

The usual way of finding the LFC is to lift a parcel from a lower level along the dry adiabatic lapse rate until it crosses the saturated mixing ratio line of the parcel: this is the lifted condensation level (LCL).

Clouds in a stable atmosphere tend to spread out horizontally, resulting in cirrostratus, altostratus, nimbostratus or stratus forming in the stable air. Clouds are more likely to grow vertically in an unstable/conditionally unstable atmosphere, as the warm air parcel from the ground tends to keep rising upwards.

Why do most thunderstorms have flat tops? The reason for this shape is due to the fact that the cloud has reached the stable part of the atmosphere, and the rising air is unable to puncture very far into this stable layer. the diameter of a typical cloud droplet is 100 times smaller than a typical raindrop.

All thunderstorms require instability (potential) and lift. The lift is the mechanism that releases the instability. Lift is produced by such things as fronts and low pressure troughs, or by air rising upslope. The upward moving air in a thunderstorm is known as the updraft, while downward moving air is the downdraft.

The tropopause is characterized by a strong temperature inversion. Beyond the tropopause, the air no longer gets colder as altitude increases. The tropopause halts further upward motion of the cloud mass. The cloud tops flatten and spread into an anvil shape, as illustrated by this astronaut photograph.