Wind

Wind is movement of air. We describe wind by its direction and speed.

1. What is wind?

2. Where does wind come from?

3. Wind can not be seen or caught. How do we describe wind?

4. What is the Beaufort Scale?

5. Why is the wind over sea usually stronger than that over land?

6. Whenever the weather forecast predicts northerly winds, the winds are westerly in my place. Why?

1. What is wind?

We feel winds because the air around us moves. The smoke from a lighted candle gives a good portray of the wind around us.

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2. Where does wind come from?

When we fan ourselves, we make the air move and it is felt as wind. What makes air move during windy weather? This is because of unequal heating, temperatures at different places are not the same. As the warmer air expands, the density and pressure of air decrease. The surrounding cooler air flows in to fill the void and wind develops. In Hong Kong, windy weather is usually caused by monsoon or tropical cyclone. A monsoon is a seasonal wind flow due to the difference in surface pressure caused by the differential heating of seas and lands. The northeast monsoon generally prevails over the coast of southern China in winter while the southwest monsoon dominates in summer. Tropical cyclone is a high wind speed whirlpool, developed over the warm oceans. It has vertical scale of more than ten kilometres and horizontal dimensions of hundreds of kilometres.

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3. Wind cannot be seen or caught. How do we describe wind?

Wind is described by its direction and speed. Wind direction is the direction from which the wind blows. It is usually expressed in terms of the point of compass. Air moving from east to west is called easterly wind. Wind speed is the speed of the air flow. It is usually expressed in kilometres per hour or as a force on the Beaufort Scale.



Wind Direction and Speed at Waglan Island Station
on 13 May 2004

Click here for the current wind direction and speed in Hong Kong

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4. What is the Beaufort Scale?

The Scale was introduced by Sir Francis Beaufort of the British navy in the early 19th century. It employs the speed of a fully rigged sailing vessel to describe the wind speed. The Beaufort Scale is divided into 13 levels, from the calm wind of force 0 to hurricane wind of force 12. The table below illustrates the relationship between the descriptive terms of wind speeds, the Beaufort force and wind speeds: Beaufort Force Description Wind Speed (km/h) Illustration Force 0 Calm < 2 Force 1 Light 2 - 6 Force 2 7 - 12 Force 3 Moderate 13 - 19 Force 4 20 - 30 Force 5 Fresh 31 - 40 Force 6 Strong 41 - 51 Force 7 52 - 62 Force 8 Gale 63 - 75 Force 9 76 - 87 Force 10 Storm 88 - 103 Force 11 104 - 117 Force 12 Hurricane >= 118

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5. Why is the wind over sea usually stronger than that over land?

The air flow is slowed down by the friction with adjacent objects. The sea surface has smaller friction, hence the wind over sea is usually stronger than that over land.

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6. Whenever the weather forecast predicts northerly winds, the winds are westerly in my place. Why?

The forecast wind is the prevailing wind in well exposed places over a relatively large area. The direction of local wind may be different from the prevailing wind because of such effects as terrain and temperature differences.

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