TMJC 2025 JC1H2 C2 Lecture 4 Tropical Cyclones

Higher 2 (9173) Tropical Environments TMJC Lecture 4 Tropical Cyclones 2025 Page 1 of 12 LECTURE OBJECTIVES • Explain the spatial and temporal patterns of tropical cyclones occurrence • Explain the surface and atmospheric conditions that lead to the formation of tropical cyclones (A) WHAT ARE TROPICAL CYCLONES Tropical cyclones are intense storms that originate over warm tropical seas, characterised by a low-pressure center surrounded by swirling spiral bands of thunderstorms that produce torrential rain and sustained winds of at least 119 km/hour. They are another mechanism by which surplus energy is transferred away from the tropics Fig. 1 - Aerial view of Hurricane Sandy at the east coast of the USA on October 27, 2012 Tropical cyclones are known as hurricanes and typhoons in different parts of the world. They are known as hurricanes over the North Atlantic Ocean, cyclones over the Indian Ocean and typhoons over the Pacific Ocean.

2 (B) The Structure of a Tropical Cyclone Structure Tropical cyclones have distinctive sections: the eye, eyewall and rainbands (Figs. 3-6) The centre of a tropical cyclone is called the eye. The eye is 30 -50km in diameter, has very low pressure, light winds, no clouds and no rain because of subsiding air in the eye. Due to the calm, sunny and clear conditions at the eye of the storm, it gives a deceiving impression that the storm has passed. The eye wall is a band around the eye , where there is spiralling rising air, very strong winds around 160km/hr, storm clouds and torrential rain. The greatest wind speeds and rainfall intensity within the tropical cyclone are found in the eye wall. Rainbands are found away from the eye wall. Spiralling rising air is also found here. However, wind speed and rainfall intensity is lower than in the eye wall, and temperatures are higher. Note: 1. Air is only subsiding at the eye, while rising at different parts of the tropical cyclone. 2. Temperature is highest and pressure is lowest at the eye. 3. Heaviest precipitation and greatest wind speed is at the eye walls. 4. Due to the tropical cyclone spinning anti-clockwise in the Northern Hemisphere, upon contact with the tropical cyclone, wind direction is from the north-west, followed by calm conditions at the eye, followed by wind direction from the south -east on the other side of the tropical cyclone (i.e. wind directions are reversed on opposite sides of the tropical cyclone). Size The diameter of the whole tropical cyclone may be as much as 800km across in diameter, and in terms of height it normally reaches all the way to the tropopause. Associated As the storm is passing over land, the weather phenomenon experienced is strong wind blowing from a prevailing direction bringing with it rain and storm surges. The high

3 Weather conditions intensity rainfall may exceed 500mm in 24 hours (Note: For reference, Singapore's average total annual rainfa