Download Thunderstorms - Hong Kong Observatory

 Thunderstorms Thunderstorms occur mainly in spring and summer. They are characterized by flashes of
lightning and rumbles of thunder. 1. What are thunderstorms? 2. How do they form? 3. How are lightning and thunder produced? 4. How to estimate the distance of a thunderstorm? 5. What is the relationship between atmospheric stability and thunderstorm? 6. How often do thunderstorms occur in Hong Kong? 7. How to forecast thunderstorms? 1. What are thunderstorms? When bolts of lightning suddenly flashed across the sky, followed by rumbles of
thunder, thunderstorms are said to have occurred. In Hong Kong, thunderstorms
occur mainly in spring and summer. Picture of lightning taken at Fanling (Photo: Mr. CHAN Cheuk­yin, HKO) Back to content 2. How do they form? Thunderstorms are formed by the uplifting of warm and humid air. There are
many factors that lead to the uplifting of air, for example, solar heating; in the
vicinity of low pressure troughs; when two different air streams meet; or when
air is forced uphill. When humid air is lifted, it will be cooled and the moisture in the air will
condense to form cloud. Upon further uplifting, the cloud will extend higher.
Water droplets in the cloud continue to grow in size. As the cloud extends
further upward, ice crystals may form because of low temperature there. A
cumulonimbus cloud results when it grows to a height of 10 to 20 kilometres.
Thunderstorms are produced by cumulonimbus clouds. Development into a cumulonimbus cloud Please click here to view pictures of development into a cumulonimbus cloud. Back to content 3. How are lightning and thunder produced? Because it is very turbulent inside a cumulonimbus cloud, the water droplets
and ice crystals in it will break up and become electrically charged. Generally
speaking, the upper portion of the cloud is positively charged, while the middle
and lower portions are negatively charged. When the electric voltage between
the positive and negative charges is large enough, discharges take place
between clouds or between cloud and the earth's surface, and lightning is said
to have occurred. The electric current in a lightning discharge generates a large
amount of heat. This causes a very rapid expansion of the local air, resulting in
rumbles of thunder. Distribution of positive and negative charges inside a cumulonimbus cloud Thunderstorms are sometimes preceded or accompanied by heavy rain and
squalls. These arise when the water droplets in the cumulonimbus cloud can no
longer be supported by the underlying air and fall down as rain. The downward
rushing air produces squalls. Back to content 4. How to estimate the distance of a thunderstorm? Lightning and thunder practically occur at the same time. However, because
light travels much faster than sound, people always see lightning before
hearing thunder. The distance of the thunderstorm may be roughly estimated by
the time difference between the flash and the thunder. As a rule of thumb, if
the sound of thunder reaches an observer three seconds after a lightning flash,
the thunderstorm is about 1 kilometre away. Back to content 5. What is the relationship between atmospheric stability and thunderstorms? To put it simply, when the air is warmer than the surrounding, it will rise
continuously. The situation is said to be unstable. An unstable atmosphere is
favourable for the development of cumulonimbus and hence thunderstorms. When the air is colder than the surrounding, it will sink. The situation is said to
be stable. A stable atmosphere does not support air uplift and is unfavourable
for thunderstorm development. Back to content 6. How often do thunderstorms occur in Hong Kong? In Hong Kong, thunderstorms occur mainly in spring and summer. On average,
thunderstorms are reported on about 5 days each month from April to
September. Average number of thunderstorm days in each month Back to content 7. How to forecast thunderstorms? The weather forecaster at the Hong Kong Observatory makes use of radar
pictures, satellite pictures, lightning location information and weather reports
to monitor the development and movement of the thunderstorms. To forecast
thunderstorms, he/she makes use of various forecasting aids including results
from computer models and weather conditions aloft as measured with weather
balloons. Back to content Please click here to view the "Thunderstorm Warning" service and points to note.