Understanding The Science
Of Thunderstorms & Lightning
- Courtesy NWS
By definition, all thunderstorms contain lightning. Lightning is a
giant spark of electricity that occurs within the atmosphere or
between the atmosphere and the ground. As lightning passes through
the air, it heats the air rapidly to a temperature of about 50,000
degrees Fahrenheit, considerably hotter than the surface of the sun.
During a lightning discharge, the sudden heating of the air causes it
to expand rapidly. After the discharge, the air contracts quickly as
it cools back to a normal temperature. This rapid expansion and
contraction of the air causes a shock wave that we hear as thunder
(this shock wave can damage walls and break glass).
All thunderstorms go through various stages of growth, development,
electrification, and dissipation. The process of thunderstorm
development often begins early in the day when the sun heats the air
near the ground and pockets of warmer air start to rise in the
atmosphere. When these pockets reach a certain level in the
atmosphere, cumulus clouds start to form. Continued heating can cause
these clouds to grow vertically upward into the atmosphere. These
"towering cumulus" clouds may be one of the first indications of a
developing thunderstorm. The final stage of development occurs as the
top of the cloud becomes anvil-shaped.
As the cloud grows, precipitation forms within the cloud with mostly
ice crystals in the upper levels of the cloud, a mixture of ice
crystals and hail the middle levels of the cloud, and a mixture of
rain and melting hail in the lower levels of the cloud. Due to air
movements and collisions between the precipitation particles near the
middle of the cloud, the various precipitation particles become
charged. The lighter ice crystals gather a positive charge and are
carried upward into the upper part of the storm by the updraft. The
heavier hail gathers a negative charge and falls toward the lower part
of the storm. The end result is that the top of the cloud becomes
positively charged and the lower part of the storm becomes negatively
Normally, the earth's surface has a slight negative charge; however,
as the negative charges build up in the lower part of the storm, the
ground beneath the base of the cloud and in the area immediately
surrounding the cloud becomes positively charged. As the cloud moves,
these induced positive charges on the ground follow the cloud like a
shadow. Farther away from the cloud base, but under the positively
charged anvil, the negative charge may be further induced.
In the initial stages of development, air acts as an insulator between
the positive and negative charges. However, when the electrical
potential between the positive and negative charges becomes too great,
there is a discharge of electricity that we know as lightning.
Lightning can occur completely within the thunderstorm cloud or
between the cloud and the ground. In-cloud lightning generally occurs
between positive charges near the top of the cloud and negative
charges near the bottom of the cloud. Cloud-to-ground lightning
occurs between the cloud and the ground and is, of course, the most
dangerous. Lightning can also occur between clouds.
Cloud-to-ground lightning can be categorized into two different types
-- the negative flash and the positive flash. The negative flash
usually occurs between the negative charges in the lower part of the
storm and the positive charges on the ground under and near the cloud
base. Positive flashes usually occur between the positively-charged
upper levels of the storm and the negatively-charged area surrounding
In the negative cloud-to-ground flash, an almost invisible,
negatively-charged channel of air forms near the cloud base and surges
downward toward the ground. As this "step leader" approaches the
ground, streamers of positive charge shoot upward from trees,
buildings, and other objects on the ground. When these streamers meet
the step leader, the connection is complete, and a surge of electrical
current moves from the ground to the cloud causing the visible "return
stroke" that we call lightning. The entire process takes only a
matter of a seconds.
The process for a positive flash is similar except that a positive
channel usually originates in the anvil of the storm and surges
downward. In this case, streamers of negative charge shoot up to meet
positively-charged channel as it approaches the ground. When a
connection is made, a positive flash of lightning occurs.
While both negative and positive flashes of lightning can be deadly,
the positive flashes generally are more destructive and are more apt
to catch people by surprise. Because the distance between the ground
and anvil is much greater than the distance between the ground and the
cloud base, a much larger electric potential is needed to initiate a
positive flash of lightning. For the same reason, positive flashes
are infrequent and widely scattered around the storm, but generally
involve the exchange of a much greater charge and are usually more
The greatest danger associated with the positive flashes, however, is
that they strike in areas where most people think they are safe from
the storm. They generally strike well beyond the area where rain is
falling and well beyond the main area where most of the lightning
(negative flashes) and thunder is occurring. Consequently, many
victims are caught completely off guard.
The best advice in order to minimize your risk of becoming a lightning
victim is to get to a safe shelter sooner and to stay there longer.
In general, if you can hear thunder, you are within striking distance
of the storm.
LIGHTNING FACT: What is often referred to as "HEAT
LIGHTNING" is simply the lightning from a distant thunderstorm that is
too far away for the resultant thunder to be heard. In most cases,
the light you observe is being reflected off clouds near the horizon.
Keep an eye on the storm though, since it may be headed in your
How can you tell how far a flash of lightning is away from you?
While you see the visible flash of lightning almost instantaneously,
the sound of the thunder travels at a speed of about 1100 feet per
second or about 1 mile in 5 seconds. For every 5 seconds between the
time you observe the lightning and the time you hear the thunder, the
lightning flash is 1 mile away. If it takes 10 seconds between the
lightning flash and the thunder, the lightning flash was 2 miles away.
For 15 seconds, the flash would be three miles away. Unfortunately,
this method only works for the previous flash and does not tell you
how close the next lightning strike will be. Generally, if you hear
thunder, you are within striking distance for the next flash of
lightning. If you are not in a safe place at the time, move to a safe
Special thanks to Lighthouse Weather for the layout of this document.