What is Wind?
Wind stirs your emotions. A gentle breeze feels good, especially on a hot day. When the wind picks up and dust or sand blows into your face, it becomes annoying. A warm dry mountain wind gets on your nerves and makes you feel sick. When the wind grows even stronger – tree branches break, roof tiles blow away – it becomes frightening.
Wind is very useful. For thousands of years, mankind used the wind to sail the oceans. Windmills ground the wheat or created electric power. Aircraft like the wind on their noses for short take-offs or landings. Wind disseminates plant seeds and pollen.
Wind is cruel, too. It causes considerable damage, injury and death: ships sink, aircraft crash, buildings collapse. A tornado needs only a few minutes to flatten your home, a cyclone threatens many kilometers of coastline, while winter storms bring traffic to a halt.
What drives the wind?
Wind is movement of the air. When you and I talk of wind we consider only the horizontal movement. Air moves three-dimensionally, though. To differentiate, the non-horizontal components have other names, such as an updraft (updraught), downdraft (downdraught), thermals, turbulence, etc.
It is the vertical movement that triggers the wind. Let’s go to the supermarket car park on a hot summer’s day. The sun heats up the bitumen and the parked cars. The hot surface and vehicles pass on some of the heat to the air just above it. The warm air near the car park’s surface is less dense. It is, therefore, lighter than the slightly cooler air above it and rises. But unless the air is replenished, you would step into a vacuum when you walk to your car. This won’t happen, of course. The car park draws fresh air from somewhere. But from where?
Coincidentally, there is a playground nearby. The local council spent a considerable amount of money to plant nice green grass and many leafy trees. The air is much cooler on the playground than it is on the car park. The air won’t rise. Instead it has to move sideways to fill the ‘hole’ in the air above the car park. The movement is wind.
‘Stop right here,’ you say. ‘Now the playground is without air.’ Almost true. The playground has to supply all the rising air of the surrounding car park. Well, this rising air has to go somewhere. The upper layers of the atmosphere would be rather crowded with air if they couldn’t shed some of it. By the laws of physics, rising air cools. It cools enough to allow some of it to descend in a downdraft and replenish the air on the playground. Nature created a loop, better known as air circulation.
In other words, warm air rises in an updraft until it cools enough to travel as upper wind to a position where it can descend in a downdraft. Once the air reaches the surface, it moves as surface wind to the place where the updrafts occurred.
On a larger scale, the same happens globally. The tropics provide the rising warm air and the poles the cool descent points. In between you have the prevailing winds.
If this is true, then the prevailing winds should be blowing directly from the North Pole towards the equator in the northern hemisphere and from the opposite direction in the southern hemisphere. Gaspard Coriolis (1792-1843), a French mathematician, had the same thought. He searched for an answer and discovered the deflecting forces of the earth’s rotation – the Coriolis force.
Let’s assume that you fly from Adelaide to Darwin, which is almost due north; you are not subject to wind influences; you do not follow navigational aids on the way, and you are totally unaware of Gaspard’s discovery. In such a scenario you could not travel in a straight line. While you are on your journey, the earth rotates underneath you and Darwin has moved some distance to the east. Your path as plotted onto the surface looks more like a hook. You will travel the last few kilometers almost west to east to catch up with Darwin. Highly trained airline pilots compensate for this fact. The wind lacks pilot training and doesn’t compensate for the Coriolis effect. Most prevailing winds
on earth are, therefore, blowing from west to east. There are always exceptions, of course.
The next time you sit in your bath tub or spa bath, you can become your own discoverer. There are a lot of things to explore in the bath, but when you pull the plug you will observe that the water drains in a rotating funnel-like manner – just like looking into a tornado or a tropical cyclone from above. It’s no coincidence – the same force that makes your draining bath water spin also acts on tornadoes, hurricanes, low-pressure systems, dust devils etc.
When you watch the weather forecast on television you see low (L) and high (H) pressure systems marked on the map. They are also the products of unequal temperatures on the earth’s surface. An area with low density and rising air has less air pressure than the neighboring area with cool and descending air. Wind spirals from ‘High’ to ‘Low.’
The winds between the two pressure systems are often so strong that they override local air movements, in which case your car park doesn’t develop its own wind pattern and the sea breeze is non-existent or delayed.
The Sea Breeze
The sea breeze is another welcome example of this process. The hot air above land rises and draws the soothing breeze from the ocean.
Sea breeze – during the day warm air rises over land and draws cooler air from the sea.
Land breeze – at night the sea is warmer than the land. Rising air over the sea draws the air from the land.
Gust – a sudden increase in wind speed, lasting only a few seconds
Measuring Wind Speed and Direction
In the absence of instruments, early seafarers relied on accurate descriptions of wind strength. In 1806, Admiral Sir Francis Beaufort of the Royal Navy devised a scale and attributed numbers to the state of the sea.
For example, very light air movement results in a few ripples on the water’s surface – he named it strength 1. Only a strong breeze manages to spray water off the top of the waves – strength 6. Wind strengths of between 12 and 17 describe the different categories of tropical cyclones. The scale was so successful that observers on land adapted it to their needs. The description of the wind is still in use today.
The Beaufort Scale wasn’t accurate enough for modern usage. Unfortunately, when the time came to look for a replacement, the world couldn’t agree on one system. Following methods of measuring wind speed are now in use worldwide:
Nautical miles per hour, known as knots (kt), also measures a ship’s or aircraft’s speed. 1kt = 1.85km/h, 1.15mph
Statute miles per hour (mph), imperial unit. 1mph = 1.61km/h
Kilometres per hour (km/h), metric unit. 1km/h = 0.62mph
Meters per second (mps, m/sec), metric unit, measures horizontal and vertical air movements. 1m/sec = 3.3ft/sec
Feet per minute (ft/min), is only used to measure vertical air movements. Commonly used in aviation to indicate an aircraft’s descent and climb.
Beaufort Scale, ranges from 0-17 and indicates the force of the wind.
The wind is generally stronger the higher up it is, because the friction of trees and buildings slows it near the surface. To compare wind speeds, weather services around the world have agreed to place their wind speed measuring device 10m above the ground.
To indicate the wind direction, only two scales are in use: the 32 compass points (north, south, southwest etc.) and the 360° of a circle. When you use a compass as a navigation device you point it towards the direction you want to walk or drive – e.g. east or north, 90° or 360°. To indicate the wind direction, however, the observer measures the direction from where the wind comes from. In other words, a wind that blows in a southern direction is a northerly wind. It comes from the north and you are looking north when the wind blows into your face.