Thunderstorms.
Thunderstorms or Cumulo-Nimbus can be formed in several different ways. The first is when moist air is mechanically lifted up to high levels as with a cold front or in some rare occasions a warm front. The air in front of the faster moving cold sector is pushed up vertically as the cold air wedges underneath. This warm moist air which is forced to rise reaches it’s condensation point and turns to cloud, however as the cold air just keeps wedging in underneath the warm air is forced ever higher. Creating a drop in air pressure and a marked change in the weather, an active cold front in the Alps in mid summer will almost certainly produce thunderstorms, turbulence and strong, damp & gusty winds in the valleys.
The other way thunderstorms can be formed is from a heat source, if the pressure, lapse rate and humidity are all favorable. Firstly the sun will warm the earth and produce thermals early on in the unstable air mass. Normally on this sort of day, good cumuli forming early on in the day at around 9am is not a great sign that the day is long lasting.
Inversions in the high level can halt the progress of Cbs but once the thermals become strong enough to break through you can expect a rapid change for the worse. This is because the warm air below can surge through the ruptured inversion into the colder unstable air above.
The Cause for Source Cbs.
Main things needed for to produce a ’source’ thunderstorm are a strong lapse rate, relatively moist air & strong heating of the terrain. If the dew point of a parcel of air is quite high it will produce a lower cloud base, because the temperature drop needed to reach condensation is less. That’s why dry air masses produce higher base days or even blue days.
However something else is happening to these parcels of wet air that makes them more dangerous. When the air turns to cloud as the dew point is reached the chemical reaction of condensation is heat producing. This gives this parcel of air even more energy than it got from just the Sun.
This energy boost also happens when this water vapour reaches the freezing level, giving the thermal/cloud an extra boost that can push it up over 10-12000m. What is dangerous about these extra boosts is the fact that as the air in the cloud rises faster it starts to suck air from below the cloud, known ‘cloud suck’. Once a cloud starts comes alive and starts to pull in huge amounts of air around it it no longer needs the power of the sun to start the next phases, hail, lightening, thunder & then strong rain. It’s this effect that allows Cb’s to roll on through the night, even when the sun is long gone.
The Risks.
The main danger in the Alps from Cbs is not as most pilots would think, getting sucked in. The flying media has reported on many stories of unsuspecting pilots being dragged up to ultra hypoxic levels. Some survived and some were less fortunate.
If a pilot finds him/herself in the pull of one of natures most powerful beasts, there is not a lot you can do about it expect perhaps start praying to God, Buddha or Allah as the outcome will pretty much be as simple as a roll of the dice. Poor flight planning has lead to this point and only luck will get you out. Simple.
Inside the Beast.
Pilots have reported climbs of 30m/s+ inside big clouds and even more impressive sink but scientists have suggested that in the core it could be nearer to 100m/s. This means a person could maintain altitude in freefall, without metres of sailcloth above them. Once the cloud has start to really get going it can enter the next phase, above the freezing level. The smaller rain droplets are frozen and fall out the back of the cloud if the cloud is active then they’ll be recirculated several times freezing and growing in size each time until they become too heavy for the amount of lift available. This will fall to the ground in the form of hail or rain. If a cloud is producing hail there is a huge chance that this up/down flow will cause a huge static electric charge. This is then discharged as lightening, which at 1,000,000degC burns the air and causes thunder, the surface of the Sun is only 6000degC.
Gust fronts in the mountains…
The main danger that I worry about when Cbs start to form is what is happening on the ground or in the valleys. As when the cloud reaches a critical point it will start to rain on the backside on the cloud. This falling wall of water is a physical object and will displace large amounts of air. Not such a problem in the flatlands but a big problems in the mountains. As the valleys systems act a channels and funnels for the 3-400m thick gust front. Simple rule is that big clouds in the Alps should always be treated with respect.
The parts of a thunderstorm.
At the front of the cloud the extra energy the chemical reactions has given to this system will draw in huge amount of air. This is where pilots tend to get into trouble and may have problems escaping the strong lift. As the cloud sucks air in from the front. This air will circulate up the front of the cloud, turn to rain then ice until it starts to descend as rain/hail on the backside. The strength of the up wind will determine how many times this air will circulate as the rain turns to hail, once the hail reaches such a size that the lift can’t support it it will finally fall to Earth. This falling wall of water is what will produce the gust front, a danger on the ground. At the top of the cloud the anvil head spreads out where the atmosphere actually starts to warm with height, this halts the progress and produces the classic spread out of ice crystals you see regularly at altitude. Only clouds in the tropics may challenge this strong inversion.
After a disappointing season on the Airwave FR3 I realized that although the wing had probably the best performance out there I was not comfortable flying it. Then Axis approached me to join Mark ‘Wagga’ Watts and Kai Coleman as the third member of the Axis UK team.
Sorry for the lack of daily info for the St Andre round. Time pressure & tiredness during the days caused this.