Wind Part I: How wind is created.

What is Wind?
You can’t see it, but you feel it. You can’t touch it, but it touches you. It has been called the “Breath of the Gods.” A killer and life giver. Gentle. Fierce. Friendly. Enemy.

It has many names like Snow Eater, Brick Fielder, Sky Sweeper and Cape Doctor. It can shatter homes and bring relief in needy times. It can spread disease and provide renewable energy. For kiters, we call it heaven. It’s our fuel. It’s the wind.

The first record of anyone giving a scientific reason for wind was the ancient Greek philosopher Anaximander. Before him, most were happy with the “breath of the Gods” explanation. When the Gods were happy, the wind was kind, when the Gods weren’t so happy, the wind got wicked and fierce. Winds are merely air movement over Earth’s surface. To get flowing, air requires some type of force. Fact is, there are many things affecting how winds weaken, strengthen, and change (we talk more about this in Part II: Wind’s Influences). But, let’s first focus on the one culprit responsible for it all. Air is set in motion by the Pressure Gradient Force (PGF).

In nature, regions of excess move toward regions of deficit. This comes from the second law of thermodynamics: higher energy states move toward lower energy states. Nature is always trying to balance. One of these balancing acts results in air moving from high to low pressure systems in attempts to equalize everything. Basically, when pressures change, air starts moving. What’s so awesome for kiters is this PFG force acts on a horizontal plane, giving us our beloved wind.

Heat Creates Wind
Earth’s uneven heating, or the warming and cooling of air, has the greatest affect. The pressure exerted by a gas, changes as it becomes denser or thinner. When cold dense air is placed next to warm thinner air, wind is nature’s way in balancing the pressure differences at each atmosphere level in. This can occur on all scales of motion. Examples: On the planetary scale, temperature gradients create the polar jet stream; on the synoptic scale it creates jet streaks; on the mesoscale it creates sea breeze circulations and low level jets; on the microscale it creates lake breezes.

Wind is basically how nature moves excess heat around. All wind is, directly or indirectly, helping to transport heat either away from the surface of the Earth, where sunlight causes an excess of energy buildup, or from warm regions (usually the tropics) to cooler regions (usually the higher latitudes). Tropical cyclones accomplish much of this heat transport outside of the tropics, while in the tropics the trade winds, monsoons, and hurricanes transport most heat.

When one area on Earth’s surface heats up more quickly than a neighboring area, the air above it expands, becoming less dense and rises. The atmospheric pressure above the “hot” area drops and air moves in from the cooler area, where pressure is higher. Technically called a thermal, this phenomenon is usually responsible for those epic steady wind sessions. You may know them better as land, country, city, or sea breezes.

An important factor here is land heats and cools faster than water. This is critical for kiting as areas with hot days and cool water typically offer those nice windy days. But remember, during the evening or as cold fronts emerge, land temperatures drop faster and air above water can become hotter than the land, so it rises and a breeze flows from the land out to the water, reversing the sea breeze effect.

Earth's Rotation
Known as the Coriolis Effect, the Earth’s rotation drags the atmosphere round with it. This frictions means air can not flow directly from high to low pressure in straight lines. Since air close to Earth’s surface is more affected by this dragging/stirring effect than air higher up in the atmosphere, there are different air speeds at various altitudes. Air ends up mixing, forming turbulence. A natural imbalance, surface winds start flowing around the high and low pressure areas.

The strange part is: Earth ends up both creating, and deflecting winds. In the Northern Hemisphere, air around high pressure circulates clockwise (anticyclonic) towards low pressure where it then flows counter clockwise (cyclonic). In the Southern hemisphere, it’s the opposite. This "feeling the Earth turn underneath" is important in directing very large and long-lived pressure systems (like continental storms). For small, short-lived systems (such as in the cold outflow of an afternoon thunderstorm) winds will flow directly from high pressure to low pressure with little Coriolis effect. You’ll learn more in WInds Part II.

Wind Types 1
The moral of all this scientific kiter craziness: understanding pressure systems is critical. They are souly responsible for kite fuel. Different wind types exist depending on the size, altitude, and occurnce of pressure systems. The following breiefly explains a few key types important to kiting. Studying them will help prepare you for Wind Part II: Wind’s Influences, in KiteWorld’s next issue.

Keep in mind that Wind is measured in the horizontal, parallel to the Earth’s surface. It’s defined as the direction it is coming from, NOT blowing to.

General Circulation winds span over the entire earth. The most well know are the Jet Streams. These upper atmospheric global winds separate different air types and help move pressure / frontal systems across the world. Typically it is wet in and near a jet stream (cool/cold to its north and warm/hot to its south). Knowing jet stream behavior can really help you forecast an upcoming session.

Wind Types 2
Synoptic winds are smaller-scale circulations that cause day-to-day wind changes. Differences in terrain elevation and/or adjacent mass temperatures (thermals) are the most common. Known for their steady and predictable breezes, they can be a daily occurrence. However, Synoptic winds may not occur when clouds, precipitation, or opposing winds are present.

• UPSLOPING WIND Air that rises with the terrain as it moves toward higher elevations. Causes cooler and wetter air. Typical as warm fronts approach and pass over mountains.
• DOWNSLOPING WINDS Air that sinks with the terrain as it moves toward the lower elevations. Downsloping air becomes warmer and drier as it decreases altitude. These are most common after passing cold fronts.
• SEA BREEZE Typically occur during daytime when land heats more than the water. Winds flow in from high pressure above the water. This is almost a daily occurrence at beaches whenever ocean temps are lower than adjacent inland areas.
• LAND BREEZE Usually happen during nights when land cools more than adjacent water masses. Winds flow out to water from high pressure above land.
• COUNTRY BREEZE The city tends to be warmer than the surrounding countryside. During a period of clear weather with light winds, a breeze will blow from the countryside toward the city. These are more typical at night.
• TRADE WIND Usually found in or near the tropics as air rushes toward the equator, from the NE in the Northern Hemisphere and from the SE in the Southern Hemisphere. They originate from two high air pressure two belts on either side of the equator. The high air pressure in these belts forces air to move toward a belt of low air pressure along the equator’s hot rising air.

Wind Types 3

• CALM WIND Because of sensor accuracy it is usually defined as wind speeds of less than 2 or 3 kts. Often found inside high pressure systems.
• CONVECTION A vertical motion of air. This generally refers to showers, thunderstorms or those infamous updrafts. Commonly associated are whirlwinds, including Eddies. They can be found spinning off water when air mixes due to different temp / moisture properties.
• GALE WIND Wind speeds from 34 kts to 47 kts usually occurring in and around storm systems.
• GUST FRONT A boundary separating the cooler downdraft air from a thunderstorm and the surrounding environmental air. Winds behind the gust front are typically gusty, hence its name. Also known as an OUTFLOW or OUTFLOW BOUNDARY.