Culdrose WMO03809 – Aviation weather sites from the viewpoint of a pilot.

50.08427 -5.25716 Met Office CIMO Assessed Class 4 Installed 1/1/1952

This Met Office site is Royal Naval Air Station Culdrose perhaps more formally known as HMS Seahawk. This is a very major military site with extensive almost continuous activity. In assessing this site I have drawn heavily on the expert advice from reader and commenter “HiFast” who is both a former USAF Pilot and member of the American Meteorological Society.

Wikipedia describes Culdrose as

“The airbase is one of the largest helicopter bases in Europe. This location serves as the primary operational center for the Merlin Helicopter Force, which constitutes the Royal Navy’s fleet dedicated to anti-submarine warfare and is composed of two Fleet Air Arm front line AgustaWestland Merlin HMA2 helicopter squadrons: 814 Naval Air Squadron and 820 Naval Air Squadron. The former is typically associated with a Royal Navy frigate specialised in anti-submarine warfare, while the latter refers to the airborne unit tasked with safeguarding the carrier strike groups of HMS Queen Elizabeth and HMS Prince of Wales. Additionally, the airbase hosts 700X Naval Air Squadron, recognised as the Royal Navy’s specialist unit for drone operations.

The airbase also serves as an important training center for the Fleet Air Arm. New Merlin HMA2 aircrew and engineers receive training at 824 Naval Air Squadron before they move on to one of the frontline squadrons of the Merlin Helicopter Force. It also serves as the base for 750 Naval Air Squadron, which is responsible for training Royal Navy Observers and Royal Air Force Weapon Systems Officers (WSOs). There is also a number of non-flying units at RNAS Culdrose including 1700 Naval Air Squadron which delivers engineering, aviation, and logistical assistance for operations and exercises globally, both at sea and on land. The airbase is equipped with a substantial Air Engineering Training School, which includes a dedicated Survival Equipment Section. Additionally, it houses the Royal Naval School of Flight Deck Operations, the institution responsible for training all personnel involved in aircraft handling.“

This is a very busy operational site. The screen sits not far from the runway and typically close to the control tower. Either side are two large parking aprons for the numerous helicopters deployed there. The issue of whether or not helicopters have a significant impact of the Met Office Stevenson Screen mounted instruments needed to be clarified. My “gut feel” from personal experience of being in the vicinity of a large helicopter made it seem obvious that they would, but “gut feel” does not cut it in scientific terms. So I asked an expert.

To demonstrate the point HiFast referred me to a fascinating use of helicopters that I had never imagined- Frost Protection!

Santa Rosa California Helicopter Charters

“Visible frost occurs when atmospheric moisture freezes and forms small crystals on plants and other surfaces. Dew forms when the air temperature drops below the dew point temperature. If temperatures continue dropping on cold nights, this dew may freeze and form frost by sunrise. If the air temperature is below the freezing point (32 degrees) ice crystals rather than dew forms and the frost is called white frost. The temperature at which this occurs is referred to as the frost point. When the dew point temperature is below the freezing temperature of the air, neither frost or dew form. Such a condition is referred to as a black frost. The development of frost is dependent upon the dew point or frostpoint of the air and many farmers have to face two kinds of frost, and freezes, during the colder months of the season.

Advection frost occurs due to an invasion of a large, very cold air mass from Arctic or Canadian regions and generally stops happening before fields are planted or orchards start to bud. This event is also commonly called an advective or wind borne freeze. Wind speeds during an advective freeze are usually in excess of 5 mph. Clouds are commonly present during much or all of the event and air is usually quite dry with low dew points. Freeze protection systems are usually of limited value during this type of severe freeze.

Radiation frost occurs when clear, low-wind conditions allow heat to escape from the ground surface and into the upper atmosphere, dropping the local temperature. Radiation frost frequently has a warmer layer of air above the surface (called an inversion layer) and this is a distinct saving grace. If a strong inversion forms, temperatures aloft (usually up to 100 to 200 feet) may become ten degrees (or more) higher than surface temperatures.

This inversion layer can be tapped by helicopter rotor downwash to help prevent freezing temperatures near the ground. Circulating air that is significantly warmer downwards into your crop can save yields when an unexpected radiation frost hits out of nowhere.”

If that alone is not enough evidence that helicopters can have a very significant effect on near ground temperatures, there are also companies that supply fans akin to “reverse wind turbines” for the same purpose such as https://www.frostboss.com/

HiFast added the following further guidance.

“Lastly, there’s one other physical phenomenon to consider in helicopter hovering or hover-taxiing in “ground effect”–adiabatic heating of the rotor down wash as it gets compressed at the surface.  Light helicopter downwash may increase surface pressure by 100Pa, about 0.1% of typical ambient, sea-level pressure.  That figure would be greater for larger helicopters, and the Sea King would fit in that category.  So, any compression induced by the downwash would result in a temperature increase…likely insignificant, but still it exists.  I would guess any temperature increase from the engine exhaust would be (much?) greater than that from the adiabatic heating of the downwash.  Sounds like a possible master’s degree thesis topic. “

I feel this rather convincingly demonstrates that the regular helicopter movements at very many sites such as Shawbury, Wattisham and here at Culdrose will render readings frequently inaccurate and unrepresentative of the natural environment. The Class 4 rating implies potential additional inaccuracy due to siting of up to 2°C (3.6°F) though I would suggest these evident compromising conditions render this site as Class 5 and 5°C (9°F) error margin.

As I have discussed many times, but most notably at Heathrow, aviation facilities use their own dedicated sensors as they have much more specific needs for aviation than climate reporting sites. A naturally aspirated Met Office Stevenson Screen set at an alleged “internationally agreed distance from the runway” is simply not adequate for pilot’s needs. The Stevenson design from 1864 and adopted almost universally after minor modifications in 1884 is, as Dr Eric Huxter described it, ” the least worst possible way to measure temperature”. I put the issues of Aitken Effect and side louvres drawing in ground convected air to HiFast from both his flying and meteorological perspective.

“I’ve always wondered about Stevenson screens and how in calm conditions they could possibly capture true ambient temperatures.

Aerodrome weather sensors–They are fit for aeronautical purposes but not for climate.  As a pilot I need temperature readings for my “takeoff and landing data” (TOLD) along with wind speed/direction and pressure altitude.  If the official Stevenson screen temperature reading is skewed upward by concrete and asphalt, that’s fine with me, because the runway is likely warmer than that thermometer placed on the cooler grass.  My engines (thrust) and wings (lift) are going to perform in accordance with whatever conditions are present on the runway (probably much warmer than the Stevenson screen).  Furthermore, I’d rather walk out to my jet with upwardly-skewed (pessimistic, worst-case) TOLD.  If my jet performs better than the conditions I used to calculate my TOLD…good!

I inspected the satellite images of HMS Seahawk / RNAS Culdrose and found four F-35s here and a few (derelict?) Harriers 100 yards southeast of them.  Those VERTOL beasts present a real threat to accurate temperature readings with the hot exhaust pointed downward and spreading radially from the jet’s position.

If the Met Office is including aerodrome temperatures in its climate assessments, that’s worthy of ridicule.  The transient spikes in temperatures associated with aircraft operations or zephyr-advected pavement-heated air render those readings unfit for climate use.  For aviators…those inflated numbers are satisfactory.“

I personally find that view quite emphatic and bears out all the evidence I have come across in my research. In addition to the effects above there is also the crude meteorological averaging system the Met Office employs on two counts. Firstly the PRT continuous reading output is only averaged over 60 seconds and not the minimum 5 minutes period (and preferably 10 minutes) that the WMO recommends. Secondly this allows for very short duration spikes to be added to the mean daily temperature system that works on absolute extremes in a 24 hour period i.e. max+min divide 2.

Should it really surprise anyone that Scotland’s, England’s and Wales’ highest temperatures were all recorded at aerodromes? Is it surprising that both Heathrow and RAF Northolt in the London Urban Heat Island regularly record so hot? And from all this, is it surprising that using temperature readings from such compromised sites will skew data?

Is that white box on those spindly legs the best the Met Office has to offer?

Source: https://tallbloke.wordpress.com/2025/07/11/culdrose-wmo03809-aviation-weather-sites-from-the-viewpoint-of-a-pilot/