BÀI GIẢNG KHÍ TƯỢNG LÝ THUYẾT CHƯƠNG 7

Chapter Seven

CHAPTER SEVEN
Thunderstorms

INTRODUCTION
A thunderstorm is one of the more spectacular shows put on by nature. Its
potential dangers to the mariner are its sometimes torrential rain Which may
reduce visibility to zero: sudden squalls of wind: its risk of interference with
radio communications (known as “atmospherics" or "static"): and the
possibility of damage to magnetic compasses if the ship is struck by lightning
(a rare event). The causes of a thunderstorm's formation are such that in
general it is more violent when the weather is relatively warm and humid. In
the Mediterranean. for example, a violent thunderstorm may last for an hour or
two. creating serious difficulties for any ship trying to enter harbour and quite
dangerous to a small vessel especially if she is under sail. In temperate zones
thunderstorms are not infrequent at any time of the year during the passage of
a cold front. due to cold air undercutting warmer air. but that tends to be a
squally occasion in any case.
CAUSES OFTHUNDERSTORMS
1. The conditions necessary for the formation of thunderstorms are: (a)
Cumulonimbus cloud with precipitation and the base below the 0°C isotherm.
(b) The lapse rate must exceed the saturated adiabatic to a height of at least
3,000 m (10,000 ft) above the cloud base. thus facilitating vigorous
convection.
(c)An adequate supply of moisture from below to facilitate this. The latent
heat released by condensation within the cloud boosts the upward convection.
In temperate latitudes these conditions are most likely to be found in cols and
shallow depressions. but the thunderstorms which result are not as frequent or
violent as those which occur in the tropics.
2. Given the necessary conditions. as above. other favourable conditions are:

(a) High surface temperature.
(b) little surface wind.
(c) “Trigger action", such as:
(i) Horizontal convergence of surface air.
51


Chapter Seven

(ii) Orographic uplift of moist air.
(iii) Frontal uplift. e.g. . a cold front.
(iv) Insolation over land.
(v) Advective heating or
(vi) Any combination of the above.
HEAVY RAIN AND HAIL
The formation of hail is described in Chapter 6: the very heavy nature of the
rain. which is also a frequent feature of thunderstorms is due to somewhat
similar processes within the cumulonimbus cloud. Hail and heavy rain.
although often present within the cumulonimbus cloud, do not necessarily
reach the ground on all occasions.
LIGHTNING AND THUNDER
The intense activity within a Cb cloud results in the build up of tremendous
electrical charges. Scientific investigations have shown that the upper part of
a thunder cloud is charged 'with positive electricity. lower down the charge is
negative. Near the base there is often a small localised region which is
positive.
The lightning flash is an electric spark on a gigantic scale, i.e., an enormous
electrical discharge (estimated to be millions of volts) which takes place
either within the thunder cloud, between two separate clouds, or between
cloud and earth. It renders the air white hot along its channel.

Danger from lightning. The risk of a steel ship being struck by lightning is
not very great because her masts and other prominent features, being part of
her structure, are so perfectly earthed to the water in which she is floating.
Ashore, the risk to high buildings and other erections not fitted with lightning
conductors and to trees is greater.
Thunder is the sound resulting from the instantaneous expansion and
contraction of the air. The rumbling effect which we hear is because the
sound of the explosive report has to travel from different parts of the long
lightning path to the observer and there may be echoes from the clouds. Light
travels very rapidly and can be treated as though it arrived instantaneously.
bur the sound travels at 335 metres per second 1,100 ft per second). The
distance to a thunderstorm can be approximated by measuring the time in
seconds between the arrival of the light and the arrival of the sound. The
distance in miles is found by dividing this figure by 5 and the distance in
52


Chapter Seven

kilometres is found by dividing the figure by 3.
When the lightning stroke takes place between cloud and earth, or across a
clear space between two clouds, its main channel (and "branches") is directly
visible to the eye and is called forked lightning, but when the channel is
obscured by cloud, so that the emitted light is diffused, it is termed sheet
lightning.
There are various theories as to the mechanism resulting in the separation
of charges within a Cb cloud. Most of them have good experimental support
but it is thought that several of the charging processes operate together
and. in addition. there are other electrical processes which are not yet
completely understood.

TYPES OF THUNDERSTORMS
1. Heat thunderstorms develop over land in warm, moist conditions
accompanied by strong surface heating and convection. Surface air flows in
from all sides (thus to an observer positioned in advance of the storm. it
would appear to be travelling against the wind). In temperate latitudes they
are most frequent in summer. on late afternoons or evenings of warm sultry
days with light winds. Moumainous islands in the tropics are especially
favourable.
2. Coastal thunderstorms can occur in any season. by day or night. They are
most frequent in winter and are caused by a large lapse rare in Polar maritime
air. The final “strigger action" comes from the forced ascent of air crossing
the coast from seaward, They are usually slight and dissipate rapidly on
moving inland.
3. Frontal thunderstorms are more common in winter because there are
more depressions .
An active cold front is unstable. especially when there is a large
temperature difference between the warm and cold air masses and if the
frontal convergence is extremely marked. as for example in a V shaped
trough (see Line squall in Appendix 1).
Warm front thunderstorms are uncommon and less active. because the
frontal slope is gradual. They form at upper level which leaves little room for
development of Cb cloud. (Compare Figures 13.12 and 13.13).
Occlusions sometimes produce thunderstorms, more often with cold
occlusions than with warm ones.

53


Chapter Seven

GENERAL
Thunderstorms over the ocean. Except in the doldrums only the frontal type
are experienced. They are rare in high latitudes due to low temperatures and
consequent lack of moisture.
At night it is possible, over land or sea. for severe thunderstorms to develop
given favourable conditions aloft. but some initial "trigger action" is required
to start the cloud formation.
Passage of a thunderstorm is associated with a sudden fall in temperature.
which can be of the order of l0oC or 11°C. This is due to cold air from high
level being dragged down to the surface by falling precipitation. The
precipitation is localised and may be very heavy. The arrival of the cold air is
associated with a sudden veer in wind direction with a fierce squall and there
are violent squalls with large changes in wind direction as the storm passes.
The pressure generally rises at the forward edge of the storm and then there is
a "wake low" at the rear of the storm.

QUESTIONS:
1. (a) What are the three conditions essential for the development of a
thunderstorm? (b) Name the pressure systems in which these conditions are
most likely to be found in the middle-latitudes.
2 Given the essential conditions required in Question l(a), list other conditions
which are favourable for thunderstorm development.
3. Name the three types of thunderstorms and discuss the conditions and time
of day in which each is most likely to develop, also their seasons of greater
frequency.
4. Write notes on the following: (a) Squalls in thunderstorms. (b)
Thunderstorms over the ocean. (c)Thunderstorms at night. (d) lightning and
estimating its distance from the observer.

54