Gliding Weather in Denmark
- a short introduction, especially about the conditions around Arnborg
Gliding in Denmark means thermal flying. A few places offered suitable conditions for slope soaring and was used in the early days of gliding, nowadays only a cliff in North Jutland facing the North Sea is used and then by the Danish Historical Gliding Club to commemorate the old days.
Thermal flights take different forms, you have to have your senses wide open! - Once a headline in the magazine Sailplane and Gliding caught my attention, it said “Lift is where you find it”, this open approach to gliding should be printed on the dashboard in every glider!
It is necessary to have some knowledge of, how the exchange of energy between the surface and the atmosphere takes place in order to use the air currents that result for gliding, soaring and long distance cross country flights. You have to be able to apply the knowledge, in the words of a Belgian meteorologist at an OSTIV course, you must put on your "thermal spectacles", unfortunately such spectacles will not be for sale at the rally office!
Denmark has be formed by the ice ages and is surrounded by water, both have an influence on the gliding weather.
Wind and Sea
On this map the fjords, cutting deep into the coastline of eastern Jutland are seen. The fjords are actually subglacial stream trenches. Today they funnel cold air into wide areas deep inland in the spring and early summer, when the water in Kattegat is still cold. This has two opposite effects on the production of thermals on sunny days.
The cold air undercuts the air inland, and kicks off incipient thermals, but an area of air with a stable stratification is left behind in the area covered by the cold air and extinguish further formation of thermals.
At the time of the VGC Rally, this should not be a big problem, the surrounding waters have warmed up, but sea air flowing inland always tend to reduce either the formation of thermals or the hight they reach - often both!
Arnborg is located a long distance from both the west and the east coast of Jutland, and the effects just described are less pronounced and occur more seldom here.
Soil and Water
Just like the landscapes having been formed by the ice ages, these have left the diffenrent types of soil, that not only influences, what the farmers can grow, but also have a big influence on the gliding weather and the formation of thermals.
On the map - below right - the black line shows the westernmost extent of the ice shield during the last ice age. Arnborg lies just outside the ice zone on a plain of sandy soild created by the melting water running off towards what later became The North Sea and carrying sand with it, sand that was then sorted and deposited according to the weight og the particles.
To the east of the black line the landscape is undulated and the soil contains more clay, and towards the east cost the clay becomes the dominant ingreedient
The Sandy Soil
The sandy soil is porous and the water (rain) penetrats easily into deeper layers, the soil also contains much air, and hence is a good insulator. The suns rays warm up the very top layer, but the the warmth remains in a thin surface layer, which is quickly heated, but also rapidly looses its warmth towards the end of the day. - Thermals form early in the day over this surface, but the production dies out soon, when the shadows become long.
The revers is true of the clay soil. The soil is compact and water from rain forms pools on the surface, the water very slowly sinks down. After a heavy rainfall the soil remains wet, soaked, and the energy from the suns rays is used for evaporation instead of producing thermals.
In periods of dry weather on the other hand heat penetrates to greater depth by conduction and the clay becomes a reservoir of heat, thermals form at a later time in the morning, than over sand, but due to the heat coming up again late in the day, thermals keep forming until late.
Everything else being equal thermals form early in the day over sand and dies out early. Thermals take a longer time to form over soil containing clay, but production last longer in the day.
Of cause there are many things, that influence and modify, what has just been described as the typical train of evnets: Wind, cloud cover, formation of cloud streets, seabreezes and advection are essential modifiers.
In a very wet period things become questionable. Arnborg isn't too bad located in these situations! - The surface becomes relatively quickly dry enough for thermals to be formed, but over soil with clay the formation of thermals may not start at all.
A characteristic phenomena as the result of the warming and convection over Jutland, is the formation of a convergence zone north-south situated a little east of Arnborg and running almost from the top of Jutland to well down into Germany. This occurs in weather situations with a relatively slack pressure gradient, favourable for the formation of sea breezez. Sea air from both the Kattegat in the east and the North Sea to the west penetrates inland and converge, adding bouyancy to the air in between. This enhances the convection and eventually form the line to be seen on this satellite image (From the web site of DMI).
Dependent on the direction and force of the general wind, the convergence line may be pushed towards east or west.
A study of favourable conditions for cross country flights
As a measure of good gliding weather, the conditions on days where triangle and out and return flights of 300 kilometres or more in Jutland between 1970 and 1980 were carried out, this was a time where the good old wood and fabric gliders where still widely used.
Thermals are formed, when an airmass becomes unstable, this happens more easily in cold airmasses, airmass coming from colder regions and being warmed near the ground as it travels over a warmer surface, or stagnant air gradually becoming unstable by heating from the underlying surface heated by the radiation from the sun.
The map shows from which directions the airmass came prior to the flights mentioned.
Not surprisingly in 65 percent of the cases, this direction lay in the west to north north easterly sector.
In 850 hPa pressure level (1,500 metres or 5,000') the correspondig wind directions can be seen on this map:
… and the surface wind:
In addition to looking where the airmasses came from, it is good to look at the advection, when forecasting good gliding weather.
On 80 percent of the days with long distance flights, the advection in the period from midnight to noon was either cold air advection or zero advection. In only 20 percent of the days, warm air advection was recorded, and then with a maximum of 3 degrees centigrade rise in temperature.
The difference in temperature between the days surface maximum and in 850 hPa gives an indication of the stability in the layer and of the strength of the thermals, the distribution of thermals is governed by other conditions, not least the total cloud cover:
|12||13||14||15||16||17||18||19||20||Deg. C. difference surface -850 hPa
|1||3||13||14||32||21||10||2||3||Percent long flights
At 67 percent of all the long flights the temperature gradient was 1.0 to 1.13 degrees C, corresponding to a temperature difference in the layer of 15 to 17 degrees Celcius. - On many web sites with aviation weather, the surface max and the temperature in 850 hPa can be found.
75 to 80 procent of the long flights were done on days where the total cloud amount was equal to or less than 3/8.
The pressure is an important indicator of good gliding weather, look for rising pressure and - as can be seen in this graph, the majority of long flights were done when the pressure was about 1020 hPa:
The importance of rain has been mentioned earlier in this account, but to illustrate the dramatic effect of water on and in the ground, the following two graphs may serve:
87 percent of the flights were done in weather where there had been no rain or a maximum of 1 millimetre of rain the last 24 hours prior to the flights.
73 percent of the flights were done in weather where there had been no rain or a maximum of 1 millimetre of rain the last 48 hours prior to the flights.
The relative humidity at the surface (2 metres above ground) varies on days with thermals, partly due to the mixing of air from different hights within the convective layer. On the days with long flights the following variation was observed:
At 1000 LT in 70 percent of the flights the relative humidity was 45-65 %
At 1300 LT in 75 percent of the flights the relative humidity was 35-55 %
At 1600 LT in 80 percent of the flights the relative humidity was 30-55 %