THUNDERSTORMS AND LIGHTNING
Singapore has one of the highest occurrences of lightning activity in the world. Situated close to the Equator, the warm and humid tropical conditions are highly favourable for the development of thunderstorms. Thunderstorms produce lightning, and in severe cases, are accompanied by heavy rain and gusty winds. At any given time, more than 2,000 thunderstorms are estimated to be active around the globe.
On average, Singapore experiences 167 thunderstorm days per year. This refers to the number of days in which thunder is heard over Singapore, and gives an indication of lightning activity (as thunderstorms are associated with lightning). The monthly distribution of mean thunderstorm days as recorded at the climate station at Changi is shown below. More thunderstorm days are observed during the Inter-Monsoon months of April and May and October and November. The months of January and February in the later phase of the Northeast monsoon experience the least number of thunderstorm days.
Thunderstorms tend to occur between 2 pm and 6 pm in the afternoon as diurnal heating and convection play an important role in thunderstorm development.
The ingredients for thunderstorm formation are moisture, atmospheric instability and a lifting mechanism. When a parcel of warm, moist air rises vertically in an unstable atmosphere, it cools and expands. At a certain point, condensation takes place and this leads to the formation of a cumulus cloud. With sufficient instability and further uplift, the cumulus cloud grows and develops into a cumulonimbus, which is an anvil shaped cloud associated with thunderstorms. A typical cumulonimbus cloud in Singapore can reach a height of 8 to 12 km.
The three stages in the life cycle of a thunderstorm are depicted in the diagram below. A mature thunderstorm cell is characterised by vigorous updrafts and downdrafts. Updrafts are associated with inflow of humid air from the base of the cloud. When a thunderstorm matures, the falling of raindrops drags and pushes air downwards causing downdrafts. These downdrafts eventually spread throughout the entire cloud, cutting off the feed of moisture by updrafts. The thunderstorm cell then enters the dissipating stage. Each individual thunderstorm cell typically has a lifespan of less than one hour and a horizontal extent of several kilometres.
The collisions between water and ice particles within a thunderstorm cloud are believed to induce charge separation, such that the upper part of the cloud becomes positively charged and the lower part becomes negatively charged. The negative charge at the base of the cloud subsequently induces a positive charge on the ground. Once the charge difference between the cloud and ground is large enough to overcome air resistance, lightning occurs.
Initially, a small amount of charge called a “stepped leader” travels towards the ground in a forked pattern. Positive charges on the ground become attracted to the stepped leader and a channel moves up to meet it. Once they connect, a strong current propagates upwards as a brightly visible return stroke. As a result, the surrounding air is superheated to 20,000°C and rapid expansion takes place, producing a shockwave which is heard as thunder. This form of lightning is known as cloud-to-ground lightning. Lightning can also occur within a thunderstorm cloud (intra-cloud lightning) or between two different clouds (inter-cloud lightning).
Cloud-to-ground lightning poses a serious threat to live and property. Lightning strikes can be fatal and have left many victims critically injured. Generally, there is no safe place outdoors when a thunderstorm is nearby. A common misconception is that there is lightning only when there is rain. However, lightning can strike a distance away from the thunderstorm cloud where there is no rain or even where the skies appear to be clear. These so-called “bolts from the blue”, have been documented to strike even as far as 16 km from the thunderstorm cloud. Thus, if you are outdoors and hear thunder, immediately seek shelter in a safe structure such as an enclosed large building or in an enclosed metal vehicle. You should remain inside for at least 30 minutes after the last thunder is heard.
During a thunderstorm, if you are outdoors:
- Take shelter in a house, large building or car and remain inside for at least 30 minutes after the last thunder is heard
- If no structure is available in open areas, head for the lowest elevation and crouch down as low as possible, supporting yourself on the balls of your feet with heels touching (Lightning Crouch position)
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- In open water, head for the shore immediately
- Stay off bicycles, motorcycles or golf carts
- Avoid large, open areas and high ground
- Spread out if in a group
- On a golf course (without any nearby shelter), put down your clubs, take off spike shoes and adopt the lightning crouch position
- Stay away from isolated tall objects such as trees, towers or poles
- Stay away from metal conductors such as fences, pipes and rails
- Do not hold metal objects such as golf clubs, umbrellas or bicycles
- Do not handle explosive or inflammable materials
If you are indoors, keep away from any conducting path to the outside such as wires/cables and metal pipes that extend outside the structure/building. Lightning can travel through electrical, plumbing and communication reception systems. It can also travel through any metal wires or bars in concrete walls/flooring
Provision of Lightning Activity Information
Before heading outdoors, it is advisable to check the weather forecast for thunderstorms and be aware of possible lightning activity. This information can be found at:
(i) NEA website at www.nea.gov.sg
(ii) NEA’s iPhone App (myEnv)
(iii) NEA’s mobile weather service at Weather@SG (weather.nea.gov.sg)
(iv) Twitter via @NEAsg
(v) the weather forecast hotline at 6542 7788
Lightning activity can be expected when there is a forecast for thunderstorms over a specified area. There are times when lightning can be observed but no thunder is heard. This usually occurs when the thunderstorms are more than 30 km away from where one is, and due to the large distance, one would not be able to hear the thunder but only see the lightning. Such lightning occurrence is more visible at night.