Rules of Thumb
Turbulence inversions are very interesting and very important to aviation because they can result in quite poor flying conditions with little indication shown on the available imagery, either satellite or radar. This is why getting the forecast is so crucial, because if you don’t spot any indications from the imagery, you may well decide to fly into this rubbish.
Let’s have a look at a real life situation.
This is an example from Wellington in December. If you look at the IR satellite image for 0800L (local), there is no cloud showing over the lower North Island at all, but if you looked out the window – there was 8/8 of stratus at 1000 feet, and although you couldn’t see it, there was another layer of stratocumulus at 2000 feet topping out at around 5000 feet. In addition to that, there was drizzle with visibility reductions to 6000 m forecast until 1000L.
Firstly, note the lack of cloud showing on the IR satellite image, despite it being overcast. In fact, there is no cloud showing over the North Island at all. This is because the cloud top temperatures are the same as the surface temperatures.
Now look at the visible satellite image. You can see there is a lot of cloud west of the main North Island ranges and from the Waikato to Wellington.
Let’s look at the radar image for the same time. Clearly there is some precipitation immediately to the northeast of New Plymouth, but there doesn’t appear to be any precipitation in or near Wellington. But have a look at the detail in the Wellington radar image.
Drizzle is showing up along the Tararua and Rimutaka Ranges and a little detail west of Ohariu and near Plimmerton and on the southern end of Kapiti. However, if the inversion was at 3000 feet instead of the 5000 feet it is today, then you would see nothing.
Turbulence has caused this cloud to form. This cloud is formed beneath a very weak inversion, but because the low-level air is extremely moist, the air immediately beneath the inversion didn’t need to be cooled much to reach its dew point and form cloud.
The cloud formed in a turbulence inversion is stratus and stratocumulus. Stratus has three formation processes:
- it can form as radiation fog lifts and begins to break up,
- it can form in bad weather simply because the low-level air becomes saturated by evaporating rain, or as in this case,
- it can form by turbulent mixing.Stratocumulus has two formation processes – the first is by the spreading out of cumulus clouds as they penetrate through a weak stable layer, and the second is by turbulent mixing.
Stratocumulus is known as a turbulence cloud ie, produced by turbulence, whereas cumuliform clouds are known as turbulence producing clouds.
Now the process.
- You need about 10 kts of wind minimum to cause low-level mechanical turbulence within the friction layer (A).
- Above the friction layer the air is moving faster, but is smooth and laminar (B).
- Within the friction layer in the tumbling air, some of the air is rising and some is descending (C).
- The air that is rising is cooling at the Dry Adiabatic Lapse Rate of 3°C/1000’ and the air that is descending is warming at the same rate.
- So, assuming the original Environmental Lapse Rate was something like the ISA Lapse Rate of 2°C/1000’ (D-E), the top of the layer will now cool to D1 and the bottom will warm to E1. In other words, the ELR has been modified by turbulence to approximate the DALR.
- The air above the friction layer has not been modified in any way, so we have induced a turbulence inversion from D1 – D.
- Now, if the cooling at D1 is sufficient for the parcel of air to become saturated, stratocumulus forms.
Rules of Thumb B
Rapid decreases in QNH, either actual or forecast, normally mean strong winds and bad weather is on the way. A rapid increase can indicate an imminent improvement.
Similarly, a significant QNH difference between two near locations normally means strong winds.
Temperature – Dew Point Split
The temp — dew point difference (split) is an indication of the amount of water vapour in the air.
When they are the same or close, it normally means low cloud/fog/precipitation. The smaller the split, the lower the cloud base. Pay particular attention late in the day when temperatures can drop rapidly, especially in winter.
A general rule of thumb is that winds at higher altitudes (7000 feet or more) are westerly.
In fine weather:
- heading south – fly low (2500 feet and below)
- heading north – fly high (as high as airspace, aircraft and cloud cover permit)
The 2000-foot wind is a good indicator of the gradient flow. A significant difference between the surface wind and the 2000-foot wind can indicate local wind effects, possible turbulence and windshear.
Rules of Thumb C
True or Magnetic
Make sure you know which reports and forecasts use degrees true, and which use magnetic to report wind direction. As a general rule, anything provided directly by an air traffic controller will be in magnetic.
New Zealand meteorology is strongly dominated by local wind effects, for example, anabatic winds (uphill), katabatic winds (downhill), sea and lake breezes, and venturi effects. Try and understand any effect that enhances a katabatic or anabatic wind, for example a sea or land breeze.
Monitor the surface wind – you never know when you might need to know it!
A forecast is just that — it is NOT a guarantee. Apply some common sense and a margin to the forecast. The conditions could be better or worse than forecast.
If the forecast indicates bad weather is on the way, the issue may be one of timing rather than severity.
Pilot reports are a very useful, but underutilised, report. If you come across weather that is different from forecast, and that can be better or worse, give an AIREP over the FISCOM frequency. You could be someone who benefits from another’s AIREP. Typically they include information on hazardous condition like windshear or turbulence.