FreeFly Paragliding - Paragliding Weather

Paragliders are powered by nature and therefore, weather is paramount. Typical safe paragliding conditions require winds between 0 - 25km/h with no rain, hail or snow. When there is wind, then wind-direction is important because all aircraft (like birds too), especially those launched by foot, are best to take-off into wind (this reduces the effective ground-speed required to reach flying-speed - thus reducing the need to run!). For cross-country flying greater distances can be flown when flying with the wind.

Solar Heating and Circulation

Solar heating and the resulting high- and low-pressure systems cause all weather phenomena on Earth. The Sun heats the ground, which in turn heats the air above it. Because different surfaces heat up at different rates (e.g. water, snow, green grass, and forests are slow to heat up while asphalt, dry fields, dark soil and dark rocks are far quicker), areas of relatively warmer and colder air develop. When air warms up it expands and gets lighter. The relatively warmer, lighter air begins to rise and a low-pressure vacuum is created, which effectively sucks cooler, denser air in from the surrounding areas to replace it, creating a low-high pressure loop.

This freshly-forming cumulus cloud marked the top of a perfect thermal that took me up to coudbase at 2,200m :)

Cross-country (XC) Weather

As free-flyers, we aim to maximise these natural air-flows, seeking out rising, low-pressure air pockets while evading high-pressure, sinking air that would speed our decent and could cut our flight short. For cross-country flying, un-stable air following the passing of a cold-front is particularly good for producing reliable thermals that will take us up to a high Cloudbase.

However, the same un-stable conditions that create rewarding thermals also run the risk of over-development, which causes the formation of cumulo-nimbus storm clouds with violent gust-fronts, freezing hail, vicious lightning and over-powering cloud-suck. Therefore, it is important to be vigilant at all times and aware of our limitations and that of our simple aircraft.

It would be better to be on the ground wishing you were in the sky than to be in the sky wishing you were on the ground!

Meteorological Winds

It's all about the sunshine! The heat of the sun is the primary source of energy in our world and it is the distribution and redistribution of this energy across the globe that causes all our weather. Temperatures vary constantly across the globe; while it's day-time on one side of the world, it's night-time on the other. Areas near the equator heat up more readily than areas near the poles. As mentioned above, temperature also varies from place to place due to the unequal cooling and heating of various types of terrain. Now imagine this on a scale of oceans and continents.

This process of convection plays itself out worldwide as the pressure-gradient force tries to equalise the pressure imbalance causing winds to flow out of cooler, higher air-density areas into warmer, lower air-density areas. Because of the Coriolis effect (the earth spinning round on its north-south axis), in the Northern Hemisphere the wind flows clockwise around higher air density areas, called a "high", and counter clockwise around lower air density areas or a "low" or "depression".

Fronts

A weather front is a boundary separating two masses of air of different temperatures, and therefore, densities. Fronts are depicted using various coloured lines and symbols, depending on the type of front (see diagram).

Fronts rotate around a Low pressure system in an anti-clockwise direction in the northern hemisphere and clockwise in the southern hemisphere. Both are associated with a change in temperature, cloud, a lot of rain and a shift in wind direction. When a depression or a low pressure system forms, it usually consists of a warm front and a faster moving cold front. As the depression intensifies, the cold front catches the warm front. The line where the two fronts meet is called an occluded front. When an Occluded Front passed overhead, you would feel changes in temperature and wind speed.

A warm front creates a progression of cloud types beginning with cirrus (can be as many as 600 miles ahead of the front) and ending with nimbostratus (dark low cloud deck covering the entire sky with light to moderate rain of long duration). This progression usually takes from 12 to 24 hours. Warm air rises over cool air because it is less dense.  Warm air holds moisture and that moisture will condense at high (cooler) altitudes.  

A cold front is fast moving and responsible for the vast majority of damaging weather.  Cold (dense) air will push the warm air up and out of its way cause rapid cloud and storm formation resulting in thunderstorms, severe thunderstorms, and tornadoes.

Local Winds

The same forces that dictate meteorological weather on a global scale also effect conditions locally in similar ways but on a smaller scale. Terrain features such as mountains, valleys, forests and shorelines generate local wind patterns of which all pilots, particularly free-flying pilots, need to be aware.

For example, as mentioned previously, different surfaces heat up at different rates. This warms pockets of air at different rates accordingly, which triggers the more-heated pockets of air to rise as thermals. As free-flying pilots we aim to identify and harness these natural, invisible elevators, flying into the thermal columns or bubbles and hitch a lift up to cloudbase.

We must be aware that friction occurs between the wind and the terrain surface, which acts in opposition to the wind's direction, slowing the wind speed and creating turbulence. We must anticipate areas of extreme turbulence, such as in the lee (shadow, down-wind) of big obstacles like mountains where wind is dumping in a rotor fashion.

Anabatic and Katabatic Winds

An anabatic wind is a wind which blows up a steep slope or mountain-side during the day, driven by Sunshine heating the slope. A hill or mountain-top is warmed by the Sun, which in turn heats the air just above it. This air becomes relatively warmer and lighter than other air at a similar altitude over an adjacent valley which does not get warmed as much because of the greater distance to the ground below it and / or if it is in shade. This warmer, lighter air will rise through convection, creating a lower-pressure region into which the higher-pressure air at the bottom of the slope gets drawn, causing the wind.

It is common for the air rising from the tops of large mountains to reach a height where it cools adiabatically to below its dew point and forms cumulus clouds, which mark the top of an anabatic thermal column. This is like a neon sign to a paraglider pilot - "Free Lift to Cloudbase!"

Katabatic winds are down-slope winds, frequently produced at night by the opposite effect, the air near to the ground losing heat to it faster than air at a similar altitude over adjacent low-lying land.

Valley Wind

Valley and mountain winds are a bigger, macro version of the more localised anabatic and katabatic winds described above. They represent the effect that sunshine-fuelled warming and cooling of the mountains and valleys has on the entire valley system; during the day as the Sun heats the ground, the radiated ground heats air next to the slopes of the entire mountain range (at various stages of the day). This warmer, lighter, less-dense air rises by convection causing an area of low pressure to develop. Relatively cooler, denser air from low in the valley system gets drawn up through the valley system to replace the rising air. In the mountain valley, this process causes a rotation of air. As the warm air rises from the valley floor up the mountains and cools adiabatically, it can rotate back down to the centre of the valley, completing a low-high pressure loop.

A paraglider pilot will not want to spend too much time crossing a valley full of cool, dense, sinking air or he / she will face an early landing.

Mountain Wind - At night (or during times of cloud-induced shade), the opposite movement occurs. The air on the mountain slope is cooled, becomes relatively heavier than the surrounding air and follows the mountain slope, descending down into the valley. Mountain winds are usually stronger than valley winds.

Sea-breeze

A sea-breeze (or onshore breeze) is a wind from the sea that develops over land near coasts. It is formed by increasing temperature differences between the land and water. Sunshine warms the land and the air above it faster than it does the sea and the air above it. Warming air on the land expands and rises, drawing in cooler, high-pressure air by means of a sea breeze. Generally, air temperature gets cooler relative to nearby locations as one moves closer to a large body of water.

Land breezes

At night, the land cools off quicker than the ocean due to differences in their specific heat values, which forces the dying of the daytime sea breeze. If the land cools below that of the adjacent sea surface temperature, the pressure over the water will be lower than that of the land, setting up a land breeze as long as the environmental surface wind pattern is not strong enough to oppose it. The strength of the land breeze is weaker than the sea breeze. The land breeze will die once the land warms up again the next morning.