Beginners meteorology

 

METEOROLOGY FOR BEGINNERS PARAGLIDING PILOTS

 

            Meteorology is a science about atmospheric processes.

            The atmosphere is the air envelope around Earth. The air is a mix of gases, which molecules move around by different processes, but are also pulled down by gravity. The net effect of air molecule’s interaction with surrounding objects is called pressure.

            In the atmosphere, the pressure represents the weight force of the molecules above certain level. The higher we go, the less molecules weight above, thus pressure decreases with altitude.

decrease of pressure with height

 

Pressure is measured in Pa (Pascal) and represents  the effect of 1 N force applied to 1 m2 .

Pressure is also measured in bar. 1 milibar = 102 Pa = 1 hPa

An older unit called atmosphere represent the pressure at sea level and this is the pressure equalizing the weight of 760 mm column of mercury:

1 atm=760 mm Hg = 101 325 Pa ≈ 1 bar

 

On weather charts, altitude is given in pressure instead of meters above sea level. One reason is that in the past, airplanes used barometric altimeters to maintain certain altitude above ground. Nowadays, use of pressure as a vertical coordinate simplifies thermodynamic computations used to prepare weather charts.

The main charts are 850 mili bar (~1500 meters above sea level), 500 mb  (~5500 m asl), 300 mb (~8000 m asl).

 

            As the air is transparent, the main source of energy in the atmosphere comes from Sun’s heating of Earth’s surface. Usually, the higher we go, the colder it gets.

            Again, the Sun is the main engine of processes in the atmosphere and there is a big variety of them because of Earth’s irregularities (land and water, mountains and flatlands, white snow and dark fields, etc.) and also because of daily and seasonal changes of sun’s radiation for a certain location. 

            For example, when the Sun shines on a dry dark field, it will heat the air above, expand it and lifts number of molecules up. At certain level, there will be more molecules above and thus higher pressure, compared to a neighbor unheated area at the same altitude. Air moves from higher pressure (H) to the lower pressure (L). The adding of air above the unheated neighboring area will increase the pressure at ground level there and again air will move along the surface to the heated area.

            This is called temperature circulation cell and there are numerous examples around: sea breeze circulation, valley-mountain circulation, monsoons, equatorial circulation, etc.

elementary circulation cell

            Air circulation depends on the temperature difference and air mass properties like instability, moisture, etc. The temperature difference circulation cells appear in all kind of sizes and often suppress or enhance each other. Most visible effect on us is the horizontal airmass movement along the ground surface, which we call wind.

 

 

WIND

 

            Wind has great effect on the very light paragliders with relatively big surface. Strong winds create big forces during takeoff and landing, high ground speeds and turbulence during flight. At the same time, pilots are looking for wind and air circulations to fly higher and further. Thus, studying the wind is essential for their safety and progress.

            First, beginners start with measuring the wind on take off, then gradually making the habit of constant observation for building mental models of the wind's character until being able to find, explain and even predict the wind causing atmospheric circulations.

 

            Wind direction is given from where wind comes from.
wind directions

The wind strength is measured by anemometers (wind meters) in [m/s], [km/h] or [kt]. 1 m/s = 3.6 km/h = 2 knots

In paragliding, we call wind:

calm: up to 1 m/s (forward launch; a lot of running=big take off)

light: 1-3 m/s; (both forward and reversed launch; still good running is needed)

moderate: 4-6 m/s; (reversed launch, but forward also possible, some running needed)

strong: 7-10 m/s; (reversed launch; only experienced pilots can fly; turbulence in the air; wind is close to paraglider air speed = chance to be lifted upward in front of the hill and blown backward after takeoff)

very strong: 11-13 m/s; (too strong to take off; chance for landing backward; strong turbulence in the air; difficult to control the canopy on landing)

squall: above 14 m/s. (too dangerous to fly)

 

Wind is constant, if its speed doesn’t change with more than 2 m/s within 2 minutes, otherwise it’s gusty.

Wind is steady, if its direction doesn’t change more than 1 rumb (22.5°) within 2 minutes, otherwise it’s variable.

Gusty and variable winds are signs of turbulence!

Wind is laminar when it doesn’t change speed and direction.

 

            It is important to distinguish average wind speed from momentary gusts. For example, the gusts can make you fly backward for a moment, but if the average wind is less than your air speed, then you still go forward.

            If gusts exceed 150% of average wind speed, then air is probably too turbulent for beginners. The same is valid if wind direction changes by more than 45° .

 

            Wind can be lifty or sinky i.e. wind is not horizontal but has upward or downward vertical component.

 

            The most critical part of a flight is getting away from the ground after take off. Taking off in lifty air increases your height and gives you more time for reaction and recovery from unpredictable situations. Common reason for accidents is when pilots ignore the message of wind cycles and just look for easy wind for inflation. Experienced pilots spend years of studying the wind cycles at take off and build intuition when it’s good to take off and when it’s better to wait.

            Many accidents happen during inflation, because beginners cannot control their wing efficiently and let it take them in undesired directions. There are even good pilots who've grown on easy take offs and when they go to different places and conditions, they have surprisingly bad take offs. Air time is not substitute for ground training. Weeks and months of ground handling are needed to achieve good take off glider control.

 

            When we measure wind on the ground, we should aware the wind gradient – change of wind strength and direction with height.

            Wind gradient occurs, because the lower layers of the moving air mass are slowed down by the friction with the ground. Thus, we should expect increase of wind with height. It’s a common reason for accidents to take off in strong wind, being lifted up and blown backward by the stronger higher winds into the turbulence behind the hill.

            Wind gradient also changes our gliding trajectory close to the ground and we should expect increase of our ground speed before landing.

wind greadient

       

            Wind gradient can be easily seen outside: if you lay in the grass, you don't feel much wind, but when you sit and then stand up and then climb something high like a car, a building or a hill, then you will feel the increase of wind with height.

            Isolated gusts can cause sudden lift or sink due to increase or decrease of airspeed flow around the wing. Layers with strong wind gradient can cause the same but for longer time and magnitude.

 

            The wind is just a wind for the beginner, but for the experienced pilot wind has direction, strength (temporary and average, min and max), character (laminar, gusty, variable), gradient, ascending or descending component. Additionally, the experienced pilot is aware of tendencies of wind's parameters. Befriending with the wind happens after understanding the big picture - the circulations it participates in.

 

 

TURBULENCE

 

            When an isolated airflow moves slowly, there is no exchange within its layers and we call it laminar flow. However, if its speed is increased, beyond certain value, the flow becomes turbulent by itself. Turbulent flow means chaotic movements of air particles in random directions. The switch from laminar to turbulent depends on the flow properties (dimensions, viscosity) and roughness of neighboring surfaces. The rising (cigarette) smoke starts laminar, accelerates upward and once reaches a critical speed for the given conditions, it becomes turbulent by itself.

            Practically, winds of more than 5-6 m/s are considered to be turbulent and the intensity of turbulence increases with the square of wind speed (W2).

 

Flying in turbulence is dangerous because wing stops working like a wing if there is no smooth airflow around it. Flying the soft paragliders in turbulence means experiencing stalls, spins, collapses, sudden lift/sink/turn, not reaching safe landing because of height loss. It’s unpleasant even for experienced pilots.Even birds lose their elegant style in turbulence.

 

            Despite its chaotic nature in the invisible air, turbulence can be indirectly observed, studied, predicted and avoided.

 

            The classic case is the mechanical turbulence caused by wind’s interaction with solid objects (trees, buildings, terrain). The smoother shaped and smaller size objects create less turbulence. Objects which block wind well create bigger turbulence behind.

            The acceleration of the flow around the turbulence source creates suction zones which promote the creation of big vortexes. Their shape, dimensions and intensity depend on the object shape (profile), wind strength and airmass properties. Circular motion conserves energy and we can say that vortexes live their own life. In steady winds and stable air, some object profiles can create "attached" to them long lasting vortexes behind caller rotors. If wind drops, vortex loses momentum. If wind increases, the gust can literally push the vortex downwind where it dissipates into promoting and suppressing each other smaller and smaller vortexes. Still isolated vortexes can travel surprisingly far, since their initial circular motion helps preserve momentum.

            Both stationary (rotors) and travelling vortexes initiate smaller rotations next to them and the newly formed free air vortexes initiate more neighboring rotations.  As a result, the turbulence zone is expanding behind the obstacle but its intensity decreases further away from the turbulent source.

             As a rule of thumb, the turbulent zone extends 7 times the height of the obstacle (it’s unsafe to land within 700 meters behind a 100 meter high hill), but again it all depends on object’s shape, airmass properties and square of wind speed.  Vwind2  means if wind speed doubles, the energy of turbulence will quadriple (instead of having 1 broken limp you can have 4).

Mechanical turbulence behind a hill

            Windward sharp edges require special attention as they initiate vortexes. Even slight change of wind direction can activate one edge and deactivate another.

            A classic case is the rotor and turbulence behind the edge of a plateau or a terrace. There are many take offs at the edge of a plateau where pilots have problems inflating their wing inside the rotor and rise it in the undisturbed flow above. Top landings there should be far behind the edge. The terrain behind can increase or decrease the “edge effect”.

 

Turbulence behind an edge

            A high and well blocking the wind vertical slope can even create rotor and turbulence in front of them. It’s not so energetic like the turbulence behind the edges and pilots sometimes call it “dead air” because they may fall in parachutal stall, but collapses and surprising back wind may also occur.

 

            Multiple terraces (edges) create multiple turbulent zones which disturb the next one downwind because even slight change of wind strength and direction decide the role of the next edge and resultant intensity of its turbulent zone. Often experienced pilots fly safe close to the terrain, but unlike beginners they "read" well the terrain irregularities, resultant turbulent zones and possible variations.

 

Multiple terrace turbulence

 

            Airflow goes well around tiny single objects and doesn't make strong turbulence behind.

 

            A dense group of objects or a single wide object which block wind well, create strong and big zone of turbulence behind.

 

            A group of similar size objects with openings between them (resembling mesh grid) slow down the wind without creating strong turbulence behind.

 

Single vs multiple source turbulence

Tree lines are broadly used to protect fields and roads from strong winds which dry the soil, pile snow or cause icing. Solid walls are not so good because their vortex patterns behind, concentrate zones of strong wind streams hitting the ground. The efficient uniform decrease of wind speed behind tree lines causes pronounced vertical wind gradient, which may cause sudden stalls or increase of ground speed behind them.

 

            The turbulence behind obstacles is reduced and even disappears when many streams of wind bring order in the chaos and restore the initial wind speed and directions - like combing bushy hair.

 

            When you "read“ the terrain for possible turbulence, keep in mind the so called traps of the terrain. These are zones which can restrict your freedom of movement due to strong sink or wind against the direction you want to go.

            Normally, we fly in light and moderate winds, but convex shapes like the top of the hill or a rib from the slope compress and accelerate the nearby flow more than the average winds around. It’s called a venturi effect.

            A classic accident is taking off in strong wind, being lifted up in venture zone and then forcefully being blown backward.

traps of terrain

 

 

Apart from flow acceleration around convex shapes (venturi effect) , the concave shapes can also be a trap of the terrain. They can concentrate the flow in a narrow sections and increase its speed beyond the paraglider max airspeed.

 

Valleys as trap of terrain

 

           Also, big concave shapes like mountain valleys, ravines and gullies tend to concentrate the nearby sinking flow, which can quickly land the pilot on unsuitable place.

 

Frequent observations of water flow around river stones, smoke and other wind indicators helps understand the various interactions with the terrain. Try to explain every significant change of flow direction and speed. Find “suction zones”; circulation zones and flow penetration streams into them; creation and dead of vortexes; stationary and travelling vortexes. Don’t remember details but principles of flow nature. This may save your life one day.

 

            When an air mass layer is moving in relative to another neighboring layer, then shear turbulence occurs around the bordering surfaces. The shear turbulence intensity depends on the relative movement and different properties of air masses (density, viscosity, temperature…).

Shear turbulence

 

            Classic example is when cool sea breeze wind enters inland under warmer air above. Frontal surfaces also have shear turbulence. Even thermals’ surfaces create shear turbulence when rising through the surrounding cooler air.

 

           Early in the morning, later in the evening, in autumn or in winter and at sea coast we may experience mainly mechanical and shear turbulence. In the middle of the day, especially in summer and in mountains, the sun’s heated surfaces create warm light volumes of air which rise throughout cooler and more dense surrounding air. These are the so called thermals, which birds and gliding pilots soar to get high and fly far.

           Thermals can raise us to heaven, but can also throw us to hell. Apart from the shear turbulence at the edges of rising thermals, thermals themselves are source of thermal turbulence because these thousands of tons of air interact with surrounding air and cause chaotic vortexes and turbulence inside and outside them.

Thermal turbulence

 

           Thermals are part of the invisible air and their effect can surprise us everywhere -  on take off, higher up or near the clouds they feed.  Taking off in strong thermic conditions is the most common reason for accidents (collapses and stalls close to the ground).

           Experienced pilots, who’re using thermals to fly high and far, are ready to cope with their turbulence at all altitudes at any time during the flight. Beginner’s shouldn't envy the climbs of others, but first should learn fly actively and safe in turbulent conditions, before attempting to fly thermals.

 

           The best way to fight your enemy is to study it. Thermic turbulence comes in many variations and requires years of experience. As a beginning, observe the wind on takeoff and compare it with the behavior of other gliders in the sky. Strong turbulence causes significant pitch, roll and yaw movements. Collapses of wingtips are common even for experience pilots.

           Study general meteorology and learn to asses the atmospheric instability for the given day.

 

           When listening to other pilots, mind that unstable might have opposite meanings - good conditions for flying because there are thermals for soaring high and far or dangerous conditions because there is a possibility for thunderstorm development.  Stable doesn't always mean lack of turbulence, because local thermal bubbles might get quite deformed and turbulent when rise, hit and squeeze through a stable layer (inversion). Some bored or crazy pilots might look for more dynamic and exciting conditions, while pilots with more fear or stress might prefer relaxing conditions. Good or bad for flying conditions is quite an individual thing. It’s better to ask experienced pilots more specific questions like “When do you expect (thunderstorm) Over Development?”, “Are thermals turbulent?”, “Are there strong winds near the cloud base?”.  Gather more independent information and then decide yourself if it’s good or bad for flying.

 

           Cumulu nimbus and thunderstorm clouds have powerful vertical development and are extreme form of thermic turbulence, where apart from uncomfortable flying there are dangers of cloudsuck, getting wet (easier stalls), icing (can tear the canopy), deadly electricity and sharp increase of wind (gust front, squall).

 

Some signs of thermic turbulence are:

  • Gusty and variable wind on the ground. Especially when min and max winds are more than 50% from the average wind speed.

It’s more difficult to inflate and control your glider on take off in turbulent conditions. Thus, if you fail to take off 2-3 times, then consider it as a warning sign of turbulence and don’t fly at all. Go practice ground handling in the flats and choose calmer conditions for flying. There are pilots who compensate their lack of glider control with crazy running or just rely on lucky lift to take them off. If you cannot control your glider well on take off, you won’t have good control in the air too. Use inflation and prolong your control stage to let the glider ”speak” what’s  the air  like. Continue with the take off only if you’re comfortable with the conditions.    

  • Signs of thermic activity like: high soaring birds and gliders; dry leaves, grass or light rubbish lifted up by thermals; presence of fed by thermals cumulus clouds and their behavior; strong sun but fresh surrounding air;
  • Energetic pitch, roll, course change, collapses and stalls of paragliders in the air. Even birds lose their elegant flying style in turbulence.
  • Dust devils and other wind indicators (as tree leaves, flags, smoke, water surface) visualizing vortexes and turbulence.

 

            For less turbulent experience, as a beginner:

- Avoid flying between 11 am and 16 pm in spring and summer when sun and thermic turbulence are strongest;

- Avoid flying in more than 6 m/s average winds, especially if gusty and variable. Fast moving clouds and their shadows indicate strong winds, but there are also stationary clouds which are product of strong winds and air wave motions. They sare constantly created (condensing) on their upwind and lift side and constantly dying (evaporating) on their downwind and sink side. These are called lenticularis clouds and orographic caps.;

- Fly when ground is cool because it’s overcasted by clouds for a long time, because it’s green or wet after rain;

- Sea coast soaring is very relaxing because the laminar wind is formed and comes from the cool flat water surface. Things might be very different 5 km inland, when thermals from sunny surfaces accelerate upward in the cool sea breeze air (strong wind with strong updrafts can be very turbulent);

- Mountain terrain is more turbulent than flatland or gentle hilly terrain;   

- Humid air is less turbulent than dry air;

- In thermic conditions, it’s usually more turbulent at higher altitudes where thermals speed up and have more interactions with stronger winds or stable layers above. Of course there can be strong turbulence at lower altitudes too where thermals are still chaotic and influenced by the terrain. The most dangerous turbulence is the one close to the ground because there might not be enough time and height for the glider to recover from a situation or for opening of rescue parachute (the higher, the safer);

 

If you enter turbulent zone:

      - Trust your harness (it will hold you even upside down) and wing (it's designed for 16 G loads and in turbulence you have max 3-4 G). The paragliders are huge stable pendulums and are designed and tested for self-recovery from various situations;

      - Help the paraglider recover quicker by keeping it above your head (hands up if a gust pushes the wing backward; pull the brakes quickly to stop any dives forward and then quickly release them to let the wing regain airspeed; if one side of the wing soften, quickly weight shift your body to the other side to let it carry more of your weight;  bend your legs to let your body easily follow glider's sharp turns ). Don't panic holding the risers but work with your arms and body;

      - Find which is the turbulent source to know where to escape;

      - Exit the turbulent zone the shortest way plus slightly downwind (this increases your glide ratio). If possible, chose a direction which will increase your height above the terrain to give more space and time for recovery from collapses, cravats, stalls, spins;

- Maintain your course without directly opposing gusts and vortexes. Maintaining your airspeed is more important than your course (by pulling too much brakes). Good airspeed means high pressure inside the canopy and more resistance against collapses and stalls.

 

           If turbulent zone seems everywhere (usually because of too strong winds or too deep entry inside a turbulent zone), then choose the biggest and cleanest from obstacles landing fields. Low dense forest can provide soft landing if things go out of control (and you deploy your rescue parachute).

.......

Note: This material is written for SkyNomad 6 days paragliding beginners course assuming that this might be the only training opportunity for some of our world wide students. Therefore, this material tries to give basic paragliding micrometeorology understanding and most essential safety advices without informational overloading. For example, isobars, coriolis effect, rotation of cylcons, temperature lapse rate, etc. are not considered as life threatening factors for modern beginners, who can easily obtain consumer friendly meteo charts about wind strenght and direction, precipitation, instability.

Re-reading the material is strongly recommended as its knowledge is quite condensed and every word has its place and meaning.

Keep on learning and fly safe! 

Any feedback is welcome.

Nikolay Yotov, 2013