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

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. Back to content 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. Back to content 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 Back to content 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
Force 1
< 2
2 ­ 6
Light
Force 2
7 ­ 12
Force 3
13 ­ 19
Moderate
Force 4
Force 5
20 ­ 30
Fresh
Force 6
31 ­ 40
41 ­ 51
Strong
Force 7
52 ­ 62
Force 8
63 ­ 75
Gale
Force 9
76 ­ 87
Force 10
88 ­ 103
Storm
Force 11
Force 12
104 ­ 117
Hurricane
>= 118
Back to content 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. Back to content 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. Back to content