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  • How does rainstorm develop?

  • Tuesday, 21st April 2015

Still remember the Black rainstorm on 30 March last year? The torrential rain brought flashy lightning, loud thunders and roaring winds that night. Marble-sized hails that came with the heavy downpour fell over many places of Hong Kong. Locating the purple red echoes on the radar image is one of the possible methods to track hail (Figure 1). Actually, how does rainstorm form?

Figure 1

Figure 1      Radar image for 8 p.m. on 30 March 2014. Heavy rain was affecting areas covered by yellow to red echoes
while hail might have manifested themselves as purple red echoes.

The development of rainstorm depends on a number of factors. In simple terms, they can be categorized into three areas:

1. Abundant supply of moisture
In March or April every year, the southward advancement of cold air from inland China becomes less frequent while southerly airstream tends to affect the south China coast more often. This airstream normally takes a long sea track and therefore it is relatively warm and moist (Figure 2). As a result, the moisture over the south China coast increases. This ample supply of moisture is one of the critical factors favourable for precipitation.

Figure 2

Figure 2      A schematic diagram showing a warm and moist airstream generally affecting
Hong Kong in March or April.

2. Uplifting of air
There are a number of air lifting mechanisms. Trough of low pressure is amongst a relatively common weather system that triggers uplifting of air over southern China in spring and summer. The weather associated with a trough of low pressure is generally unstable. Since pressure along the trough axis is lower than that on both sides, the air in its vicinity tends to converge near the axis and is forced to rise. Water vapour in the air will condense into water droplets due to cooling in the uplifting process. The energy released during condensation will help lift the air further, forming cumulonimbus and precipitation (Figure 3). Apart from trough of low pressure, orography, high surface temperatures, frontal surface and upper-air weather systems or features such as jet streams, vortices, shear lines and upper-level divergence can also help uplift air. For example, when a trough of low pressure lingers over the south China coast in the presence of some of the lifting mechanisms mentioned above, the uplifting of air can become rather rapid and intense, triggering heavy rain.

Figure 3

Figure 3      Warm and moist air in the vicinity of a trough of low pressure lifting up to form
cumulonimbus clouds and precipitation.

3. Atmospheric instability
Warm air is less dense than cold air, and is thereby lighter. Hence warm air is easy to be lifted. When the lower atmosphere is filled with warm and moist air, the air will ascend easily and the atmosphere will become unstable. There are a number of indices commonly used to indicate the stability of the atmosphere. They include Level of Free Convection (LFC), Convection Available Potential Energy (CAPE) and K-index [1]. Their values can be calculated from the daily ascent data. Their forecasts can also be derived from numerical weather prediction models for reference by forecasters.

When the factors in the above three areas occur at the same time, the chance of development of rainstorm will increase significantly. The threats posed by rainstorms are not only limited to flooding in low-lying areas, rivers and places with poor drainage, flash floods, or landslips in the vicinity of slopes due to heavy rain, but also severe thunderstorms and high gusts accompanied with rainstorms, or even hail, tornadoes, etc. Because of the random nature and uncertainty of rainstorm in its location of occurrence and evolution, forecasters need to utilize real-time observational data to effectively evaluate the short-term changes in its intensity, duration of influence and threats to Hong Kong so as to issue the appropriate rainstorm warning in a timely manner.

Below are some tips to get prepared for heavy rain:

1. Pay attention to the Observatory's 9-day forecast to see whether heavy rain or thunderstorm is mentioned in the forecast, and also the weather system that may bring unstable weather, like trough of low pressure or cold front, is included in the general situation;

2. Keep track of the latest local weather forecast and special weather tips;

3. When heavy rain is approaching, pay particular attention to the weather warnings and the corresponding precautionary measures.

If you wish to know more comprehensive weather information, check out the weather radar images, lightning location information, rainfall distribution map, rainfall nowcasting product, maximum gust information in regional weather etc. available on the Observatory's website or "MyObservatory" App. When inclement weather occurs, it is most important to make sure that you are staying in a safe place. Remember to stay alert to upcoming changes in weather, and take appropriate precautionary measures when necessary!

YC Kong


[1] Glossary - NOAA's National Weather Service (