Altocumulus lenticularis and hints of rotor cloud.
While the wind on the water appears to be light the lenticularis cloud alone suggests a very strong wind aloft — at least 20 kts at ridge-top level — but probably much stronger given the apparent displacement of the waves in relation to the mountain ranges. Occasional severe turbulence is probable and the hints of rotor cloud confirm that severe turbulence is a certainty.
Rotor cloud in an otherwise clear sky
The chaotic nature of the cloud in this image confirms the strong turbulence present. In the absence of this cloud however, caution should still be exercised as the cloud will only form if there is sufficient moisture in the air. Without that moisture, but in similar wind conditions, the rotor will still exist — it just won’t be visible.
This cloud formation is most often observed on the underside of CB anvils, but it may also be found under lenticular clouds at times. It forms when cold air sinks rapidly and drags the cloud down with it. While this example is spectacular (it occurred in Hastings, Nebraska, USA), it is not uncommon in Aotearoa New Zealand in a less spectacular form.
This example is associated with a CB anvil, and so it not only indicates moderate to severe turbulence in the vicinity of the cloud, but also indicates that any hazards associated with CB clouds are lurking close by.
Turbulence in mammatus clouds is generally limited to mid or high level, but turbulence associated with the attendant CB can be found from ground level to the moon.
Kelvin Helmholtz waves or billows
These cloud formations occur at any level in the troposphere. Lab experiments have concluded that the turbulence (particularly an overturning motion) in these clouds is as severe as it can get. Indeed the internal turbulence is so great that these clouds only last about five minutes before they destroy themselves, but during those five minutes they should be avoided like the plague.
Such clouds are rare but they do occur all around the world in mid-latitudes. Greg Reeve of MetService says “In my 34 years in meteorology I have observed these cloud formations only about 10 times.”
This is a roll cloud on the leading edge of a traveling CB cloud (in this case approaching Ohakea from the northwest). Note the flag is blowing into the approaching storm.
The approaching CB itself should be enough to put the fear of God into any pilot, but the presence of the roll cloud adds extra knowledge to be wary of.
Ahead of the roll cloud is the Gust Front (or First Gust). In America, the low-level windshear between the warm inflow and the cold outflow may be as much as 100 kts. In NZ this windshear may well be in the order of 50 kts or so. The Americans have investigated the effects of low-level windshear on aircraft and they have concluded that a 35-knot windshear is sufficient to cause most pilots to lose control of the aircraft and crash at low level.
This image tells us a couple of things. We can assume that there will be moderate, bordering on isolated severe turbulence within the cloud and in the column of smoke underneath it. The smoke is thin — you can see through it, which may entice an inexperienced pilot to fly through it.
The second indicator here is that the air is conditionally unstable (neutral). The column is rising vertically and the cloud is ballooning. Any more vertical development of the cloud may lead to the air within the column becoming unstable due to the release of latent heat, and thus the cloud could conceivably develop into a full blown Cb.
While this image does not involve visible moisture, the smoke column fulfils the same role. This image demonstrates the presence of an inversion where warm air is overlying cold air. This is an extreme manifestation of stability within the atmosphere.
Inversions, especially at low-level, are areas prone to turbulence due to fast moving air overlying still or much slower moving air. Usually such turbulence is only light to moderate, but in special circumstances, it may border on severe.
A secondary hazard associated with this image is that visibility may become seriously reduced beneath the inversion.
Although this is an extremely old photograph taken somewhere in Great Britain, it clearly shows low-level windshear associated with an inversion. The inversion in this case is at about the level of 1½ times the height of the higher chimney.
Not visible moisture, but the smoke gives us the same indication.
- severe turbulence
- severe icing
- electrical phenomena
- gust front (or first gust)
- poor visibility
Although from this perspective it is hard to say how close this aircraft is to the anvil there’s a good chance it’s close enough to encounter severe hail.
It is said the US Airforce has a standing order that says in effect — except under a war scenario, thou shalt not fly within 20 NM of a CB anvil. This is because they have had destructive encounters with hail as far as 10-15 NM from the edge of large CB anvils.
Some years back, a turboprop aircraft suffered severe damage when it encountered heavy hail about 2 NM from a CB anvil. The pilot had no idea that such an occurrence was possible.
The official/unofficial eleventh cloud type. Asperatus is an enhanced form of lenticular cloud where the terrain induced turbulence below has developed to such a great depth it impacts on the wave cloud above. The turbulent nature of the lower atmosphere is clearly reflected in the bizarre shapes imparted on the waves above. This is the ultimate indication of extremely severe turbulence.
Incidentally, this image also tells us some other interesting points. It is taken in the South Island in Hanmer Springs, looking towards the west southwest. How do we know this? The Speights logo is a clue to the South Island location, but there are two other points of interest in this photo. We’re looking at the southern end of the Nor’west Arch (the clear area on the right hand horizon), and the second and more important fact is the ‘flagging’ of the tree on the left. Flagging like this indicates that the prevailing wind is very strong and very dominant — in this case from the northwest.
Yes, this is the Matterhorn, Europe, but we have any number of less famous peaks where the same cloud indicator may be present. The wind is blowing from right to left. The decrease in air pressure on the lee side of the peak is causing cooling, leading to condensation and thus cloud formation. The air within this cloud is very turbulent, but its presence is also an indicator of possible moderate to severe turbulence nearby.
Most microbursts in New Zealand are wet microbursts, clearly identified by the shaft of heavy rain falling beneath the CB cell. However, dry microbursts are much more dangerous. As the rain falls from a high main CB cloud base into dry air beneath, it evaporates, thus cooling the descending air (offsetting the warming effect due to increasing air pressure as the air descends) which in turn accelerates the downdraught. So when it hits the ground, this microburst may be descending at a far greater rate than simple gravity should account for.
And because it’s dry there may be no indications of its presence or severe intensity. At best, in such circumstances, we can hope for some disturbed dust to indicate that it exists. In the continental United States, a lack of low level moisture may mean that traveling CBs have a base as high as 25,000 feet (with tops up around 70-90,000 feet). The microbursts from these CBs are horrendous.
In New Zealand, we can’t get CBs as dramatic as those, but it’s not impossible to get summer CBs in the central Otago region with bases at perhaps 10,000 feet agl and hence dry microbursts from these cells are a possibility.
“On January 10, 1964, Boeing civilian test pilot Chuck Fisher and his three man crew lost their tail, the tail of their B-52H Stratofortress that is, at about 14,000 ft. over northern New Mexico's Sangre de Christo Mountains. Their mission was to shake, rattle and roll this big beast at high speed and low altitude to record sensor data on how such a profile affected the B-52's structure. They did their job. The vertical stabilizer blew off. Six hours later and after a lot of engineering on the ground and in the air, Fisher brought his B-52 home, with the coveted data.”