Weather & Wind Videos

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Atmospheric pressure is the force exerted against a surface by the weight of the air above that surface. The pressure on the earth’s surface increases as the weight of the air above it increases (the weight of the air varies according to its density; the colder the air, the denser and heavier it is), and vice versa. Air moves from high-pressure to the low-pressure areas. This airflow is called wind. But it cannot go there directly because of the Coriolis force that creates the rotation around low and high pressures. 

Air masses are continually moving and merging, because of the earth’s rotation on itself in the short term (the sun warms up one-half of the land at the time, but never the same), and because of the earth’s rotation around the sun in the longer term (the seasons). 

Seasons exist thanks to the interaction between the sun, the moon and the earth. The moon regulates the angle of the earth’s axis so part of the earth is tilted towards or away from the sun during its orbit around the sun. 

The more direct the sun shines over a place the warmer it is (sunrays hit the earth closer to perpendicular angle). The equator receives more energy per unit of area from the sun compared to regions closer to the poles. 

The difference in temperature between the cold and warm air generates air mass exchanges that can result in low-pressure areas (with rain, wind…) and high-pressure areas (sunny, not windy, and colder than low pressure areas). 

Clouds are air that is saturated in humidity and that condenses around dust particles and gathers to form a cloud. In other words, clouds are vapor. 

Fair Weather Cumulus: Puffy, small, low height clouds appearing on warm days. When they appear in a high-pressure, it is a sign of a potential sea breeze or thermal activity on land.

Cirrus: High-level clouds. Cirrus are an early sign of a low-pressure system approaching. They look like hair, as if they have been stretched by winds.

Nimbostratus: They are the largest cloud type. They can make it rain for days! They are so thick that sunlight cannot pass through them. 

Stratus: They are low-level horizontal layering clouds. These clouds are essentially above ground fog, formed either through the lifting of morning fog or through cold air moving at low altitudes over a region.

Cumulus: Cauliflower-shaped low-level clouds with dark bases and bright tops. When observing cumulus, you are actually observing the condensation process of rising thermals or air bubbles at a certain level in the atmosphere known as the condensation level. 

Cumulonimbus: These start as a Cumulus and become larger and larger. A Cumulonimbus will develop until it makes an anvil shape as it reaches around 10 to 12 km in height. They can generate storms, hail, heavy rain, tornados, and squalls. They can move against the wind and in any direction. You may not always see them due to the cloud cover.

The gradient of the wind: The closer to the surface (water, land, mountain, waves, objects) the slower the wind is. This is the wind gradient caused by friction. There are also more turbulences closer to the surface. As a rule of thumb, we can say that the wind is 35% stronger at 20m of height than it is at sea/ground level.

The Venturi effect: The wind speed increases when it passes between two mountains, buildings or obstacles. This is called the Venturi effect. Be careful as the wind changes direction and can becomes turbulent according to the shape of the obstacles. The main danger is the combination of the wind acceleration and turbulences.

The wind shadow:

Influence of mountains, hills, buildings and trees. The obstacles deviate the wind and create turbulence. 

Upwind of an obstacle 

Wind decelerates. Wind bounces back and causes turbulence.

On top and around obstacles

Wind changes direction downwind. There can be rolls/whirlpools that can create sudden downdrafts or updrafts which can lift up and drop the kite and kiter. After the rolls, there is turbulence.

Up and downwind shadow rule of thumb is: The downwind shadow area is 7 times the height of the obstacle while the upwind shadow is 3 times the height of the obstacle that created it. Therefore, you or your student should not launch, land or ride upwind or downwind of trees, buildings or other large obstacles because it could be dangerous. Wind flows are also turbulent all around the obstacle. Never fly or let fly a kite above or close to any wind obstacles. 

Sea Breeze: The land surface heats up the air mass above it, while the water absorbs more of this heat and keeps the air above it cooler. The warm air above the land expands and rises, decreasing the pressure over the land near the coast (when this starts to happen you can see the apparition of little cumulus on land moving toward the sea). Cold air from the sea will then rush in to fill the space, thus creating a sea breeze. Before the sea breeze starts, there may be no wind at all so the water will look like a mirror. This process is reversed at night when the land cools down more quickly than the sea. This is called a land breeze, and is potentially dangerous for kiters. The sea breeze will begin to drop off in the late afternoon, then stop altogether. A short while later, the wind may then turn offshore as the land breeze gets established.

High-pressure system (anticyclone) 

This system is the result of a high-pressure area that forms when an air mass cools over land or the ocean’s surface, causing the air layer to shrink and become thinner. This shrinking creates a space that is then filled by the surrounding air in the upper atmosphere adding weight and causing higher pressure on the earth’s surface. The high-pressure range is from 1013 hPa up to 1040 hPa. This tends to produce more stable weather and winds. High-pressure systems are cold air masses and produce smaller amounts of cloud cover. This allows the sun to heat up the earth in summer days and in certain parts of the world they can bring areas of mist and fog. 

Low-pressure system (cyclone) 

This system is the result of a low-pressure area which is a warmer air mass, either from being located over warm land or the ocean’s surface. Low-pressure systems generate rain or snow when a warm air mass meets a cold air mass, which is called a front. The warming causes the air layer to expand upward. This expansion then causes the upper atmosphere to flow away and reduce the air pressure on the surface. Surrounding high-pressure air then converges in to take its place.The low-pressure range is from 1013 hPa down to 865 hPa. It is known as a ‘depression' and tends to produce unstable weather, often warm, cloudy and wet. The faster the pressure drops, the stronger the incoming wind will be.

Trade winds dominate the tropical and subtropical regions. They describe the persistent and predominantly easterly flow of near-surface winds over the tropical oceans. 

This persistent flow is the result of sun heat that creates a thermal convection (air movement due to air mass temperature differences). Trade winds are seasonal. Their distance to the equator changes over the year. Trade winds blow predominantly from the North-East in the Northern Hemisphere and from the South-East in the Southern Hemisphere due to the Coriolis effect. 

It also creates semi-permanent high-pressure zones that exist over the tropical and subtropical oceans (that tend to have little wind).  

The trade winds meet at the doldrums, a low-pressure area of calm and light variable winds.

Among the most well-known trade winds is the Alize, a steady and mild north-easterly wind which blows across central Africa and the Caribbean. 

Remember, warm air cells around the equator, cold air cells on the poles. Meeting and mixing of warm and cold cells create low pressures. 

Without the Coriolis effect, the pressure would move directly from high to low pressure: As the air moves from the highest to the lowest air pressure area, the created flow deviates under the influence of the earth’s rotation. This is named the Coriolis effect. On earth, air mass movement deviates to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. 

This results in a clockwise rotation of high pressure and anti-clockwise rotation of low pressure in the Northern Hemisphere and the opposite in the Southern Hemisphere. 

The Buys Ballot’s Law  

‘When facing the wind in the Northern Hemisphere, the lowest pressure is always on your right and the highest pressure on your left. The opposite will happen in the Southern Hemisphere.’ 

Synoptic charts are the standard format used by weather forecasters to show how weather systems are acting and moving in a given area. The charts show areas of high and low pressure. The thin black lines drawn on charts are isobars, which represent lines of identical pressure. The numbers associated with each isobar indicate the pressure level along it. Any point lying in between two isobars will have a pressure somewhere in between. Indications of high and low wind areas can be seen on the isobaric chart, with high wind areas being shown by isobars that are closely grouped together. The closer together the lines are, the faster the wind changes and the stronger the average wind. The further apart, the lesser the change in the wind speed and the lighter the average wind.  

When an object or someone moves through the air they experience an induced wind.

The induced wind is in the opposite direction to the motion of the object. 

The induced wind speed has the same speed as the speed of the object or person in motion.

The apparent wind is the wind that we or the kite feel when we are in motion. As soon as the pilot and/or the kite moves, the kite gets wind from another direction apart from the true wind direction. This is called the apparent wind.

 The apparent wind plays an important part in upwind performance and power management.

To go upwind, the rider must go slower than the true wind, otherwise the apparent wind will not be oriented correctly.

Apparent wind = True wind + Induced wind

 While riding, the kite is on the wind window’s edge, but moves through the air. The force is generated by the speed of the kite moving through the air (speed of displacement) and the true wind.

You can observe the effect of the apparent wind when you ride by checking to see if your kite is moving forwards or backwards from the edge of the wind window. 

The faster you go, the more your kite will move backwards. Most kiters say the kite goes in the power zone. In fact, it remains at the edge of the wind window but the wind window’s center turns to be in the line of the apparent wind. 

The faster the speed of the rider, the further the wind window moves backward.