Olá Belém:
É só para um esclarecimento sobre os termos que utilizei. Quando digo vento, é ao movimento geral da atmosfera, neste caso, à superfície. Claro que há "ventos locais" devidos ao relevo, nas regiões fronteira com massas de água ou com solos com certas características térmicas. Serão brisas. Os ventos secos, quentes nos dias de Verão e frios nas noites de inverno a que se refere o adágio estão associados à circulação de um anticiclone sub-tropical semi-permanente no Atlântico Norte, o dos Açores. Os valores de temperatura veiculados oficialmente referem-se (procuram desesperadamente referir-se) ao ar global que passa sobre determinada região extensa. Claro que, aqui e ali, há nichos com valores diferentes. Resta saber que desafio é que se pretende "ganhar": Eu descobri um lugar onde o ambiente tem um calor de "morrer"? A minha rua é mais quente do que a tua? Ou a localidade Amareleja é mais quente do que Leiria, porque fizemos várias medições em lugares diversos de cada uma (com os mesmos sensores, à mesma hora, nas mesmas condições, com a mesma técnica de observação...) e os resultados médios assim o indicam? Esta é a minha apreciação da fragilidade da comparação de lugares quentes ou frios...
Zerrui
And exactly that last part is missing, not only in Portugal. If you look at the pictures of the IM station, they are not measuring according to WMO guidelines in so many places.
Avis Benavila: grass is 1 m high
Bragança: close to buidlings, that should at least 200 m away but oke. These are guidelines. 50 m away is to close! Radiation shield is now green from algae instead of white..
Alvega: near a large road, not just a small one. And on two sides. Stevensonscreen smashed in
Valdonas. Tomar: trees blocking free ventilation all round. At least 50 m.
In The Netherlands, the stations themselves are MUCH better maintained. The grass is always 5-7 cm. They are controlled byt two guys and if something is not okay, he notafies you and the orders you to cut the grass, clean the stations etc.
Still, there are a few stations that are surrounded by trees aswell. Or a maisfield at 4 m. And I go to those, without knowing this but just because I notice some peculiar behaviour (very high extrema on some days). And it always turns out to be correct: soemthing is not right.
Now we have to be very knowledgable and carefull when comparing station and thiking if everything is right, it must be okay. SO even all WMO standard are in order, can we compare and be sure we measure the airtemperature in our radiationscreen. No.. I am afraid we are not. Once the wind drops and it is very sunny, of course the temperature rises. A problem is that in the screen the heat builds up more and more and the screen, without any natural ventilation to speak of because the wind dropped, becoes detached form the air you want it to measure. You start to measure the radiationscreen temperature. This already occurs starting from 3 m/s wind at 10 m heigth and fro that point, the temperture rises quickly. At 1 m/s on bright days at 40 degrees lattitude, it is over 2 degrees warmer (I thought, can look it up).
So what is my point. My point is that inland there is less wind than on the coast. So coastal stations are cooler of course in summer, but not only because of the sea nearby. Because there tends to be more wind, the coastal stations actually measure the real air temperature more often thaninland station. So you should introduce ventilators that start once there is sun and no wind. Or...when there is sun only of course!! So a ventilator that works on solar energy would suffice.
Otoh: this takes even more maitainance as spiders etc could build webs when there is no wind and no sun, stalling the ventilator. Week ventilators do no warm up and have no effect on the sensor. More powerfull ones have two problems:
- Blowing not only heat but also moist over the sensor. In sunny weather, this is not a problem. During rain they would still shut down.
The Campbell sensorscreens with ventialtion have shown to be 0,5 K cooler because of the wetbulb effect caused by the strong ventilator.
- They generate more heatthemselves, warming up sensors.
I feel very few people know this, but while I constructed my own radiationshield I wnet trhough a lot of theory on that myself and came across these studies. The KNMI has a good one, but there are more.
So nothing is ideal. I have two identical radiation screens and intend to install a fan in one, just to measure the effect constantly with two, identical, calibrated dataloggers in each one. My station is 3 km away, so I can check them regularly and see if the fan stalls. If you can't do that, it is not ideal...
