METEOROLOGIST JEFF HABY
The concept of a parcel is fundamental to the understanding of convective processes. The concept of a parcel seems unusual at first since
when viewing the atmosphere outside there are no clear and clean boundaries between a parcel and the air surrounding the parcel before clouds
and storms develop.
To understanding the concept of a parcel, it will be defined and also compared to air surrounding the parcel. The term “environment” refers
to the air surrounding the parcel. The environment typically consists of a larger volume of air as compared to the parcel(s). As an analogy,
the parcels are small bubbles rising in a big pot of boiling water. Unlike boiling water, the process that occurs in the atmosphere occurs
slower and both the parcel and environment are composed of air instead of bubbles of air within water. A parcel is a local mass of air
with temperature and/or moisture characteristics that are different from the surrounding air.
A parcel is different from the environmental air in that it will have a different temperature and/or dewpoint. Also, the parcel moves
within the environmental air while the environmental air stays relatively still in comparison. In a convective process such as in
the release of CAPE (Convective Available Potential Energy), the parcel of air moves vertically upwards within the environmental air. This
can be thought of as the pockets of air that produce cumulus clouds.
How much a parcel rises depends on if it can continue to remain less dense than the surrounding air. Parcels can originate as rising
thermals from on or near the ground surface from daytime heating or a lifting process. A parcel has an original temperature and
dewpoint that it obtains as it breaks away from the surface and rises. As long as the parcel remains less dense than the surrounding
environmental air then it will continue to rise. A storm is produced from parcel(s) rising throughout much of the troposphere. A parcel
is less dense by being warmer than the surrounding environmental air. Having a higher dewpoint also helps it to be less dense than
the surrounding environmental air. Air that is warmer expands and takes up a larger volume and thus becomes less dense. As the moisture
content of the air increases, this will also decrease the density of air since gaseous H2O has a lower molecular weight than the gases
of diatomic Oxygen and Nitrogen that it displaces. A parcel rises for the same reason a helium balloon rises and this is due to being
less dense than the surrounding environmental air.
Parcel theory is used to estimate how strong updrafts in convective storms will be. Stronger updrafts tend to produce stronger storms. Index
values such as CAPE and LI (Lifted Index) are used to estimate the convective updraft potential.