Saturday, June 19, 2010

How to get weather info in Brazil

Many people ask me how to get the necessary information about the weather here in Brazil. I am not professionally qualified to talk about the weather, but as pilot I need to have a good notion about where things are heading, as this is crucial to aviation safety.

Here is a short description on how I approach the subject. Even though this text is in English you will find that most relevant sources are in Portuguese. Fact of life when dealing with Brazil. If your Portuguese is not sufficient, you can use Google Translator.

Having a basic understanding of how the weather works of course makes the understanding even better, but you can still get very useful information from several sources without knowing the theory.

Brazil is huge and covers several climate belts, each with its own characteristics and predominant meteorological events. Lets concentrate on the Southeast, i.e. the states of Sao Paulo, Minas Gerais, Rio de Janeiro, and Espirito Santo.

A frequent event are the Cold-fronts sweeping up along the costline starting in Argentina and moving up bound for Rio or even further North. Sometimes they get dissolved on their way or moves out over the ocean. Those with knowledge from Europe and Southern Argentina have also heard about Warm-fronts occurring in connection with low pressure frontal systems, but these are less frequents on our attitudes here, and show a somewhat different behavior.

Depending on how strong the Low Pressures are, the Cold-fronts will bring more or less rain with them, not seldom followed by thunderstorms. These fronts will in many cases cause stronger rain showers on their front side, one or two days of light rain or drizzle on their back side. Especially at coastal areas. If you want more info on Climate Belts you can check out this site on the Climate System.

Another basic feature which is interesting to observe is the effect that the sun has on the lower atmosphere mainly in the summer period.

In this region, typically the air is more humid and the sun is stronger, and this combination will create conditions for building rain storms during the day. Humidity and temperature in the different layers of the atmosphere are determent for whether or not the storms will build, but in general storms are likely to build on days with strong sun and humid air.

In order to understand the weather development the coming days I recommend to get the overall picture first. You could start looking at the continent at the Climatempo Synoptic Chart (Carta Sinotica) down on the right side. Here you can see high (A=Alto) and low (B=Baixo) pressures together with the fronts. If you want some text or explanation you can look here.

Now that you are aware about the general movement in play, it is time to take a look at the actual situation at CPTEC for example. On the left side of the top menu bar click on the file symbol called "Produtos" when you hold the mouse over it. Chose "AMS Realcada Ch4 Alta Res". Click on the play symbol and, in the bar that opens, click on the play symbol. This will load a three-picture animation of the whole of South America. You can move it with your mouse. Here, your ability of interpretation comes into play. You can see the temperature codes on the side. The colder the temperature, the higher the clouds. Large circular areas are normally thunderstorms, or Cumulus Nimbus (CB). If the temperature drops over a very short distance, with the color going directly from grey to blue or purple, a lot of vertical movement is taking place and the CB is active and probably connected with thunder, rain and strong winds. Sometimes also grain. Larger circular areas where the temperature drops more slowly towards to center of the cloud is probably an dieing CB, where the cloud debris is spread in the higher atmosphere. This application is Flash animated, so if you are using a cell phone, you can use the picture archive and choose "Realçada" and get the last picture on with "Alto" (high) resolution. Remember that all times are in Z (Zulu = UTC), which corresponds to current time plus two hours at Brazilian daylight saving time, and plus three hours during normal "winther" hours. On the picture to the left red lines point to thunderstorm building up, and the green line shows an old thunderstorm dying out.

Thereafter, you can use the regional forecast in Climatempo. By clicking through the dates you can see the weather development. I recommend that you read the text under each picture. Be aware that the pictures are sometimes not in sync with the text.

You can also get more precise information with more weather data on specific forecast and current conditions about Brazil. The Brazilian Meteorological Institute (INMET) has a good regional forecast model, which in my experience is quite accurate. Click on the tabs with the red spots in the screen dump. Previçao Numerica > MBAR > Mapa de prognostigos 25 km. A popup window will appear (so be sure to enable that in your browser), and you can chose to look at rain, as in this case, but you can also look at Wind, Cloud coverage, Temperature, Relative humidity. Zoom in on the map and play with it. In the bottom you can see that date and time the forecast was made and what time is it valid for.
Apart from the overall picture that the satellite pictures can give, you can of course also use them to get a picture of the current situation. Talking about current situation: If you are concerned with rain for this afternoons churrasco, you can look at the doubler rain radars at Redemet. Click on "Radares Meteorologicos" on the left menu bar. There you will find radar pictures from several radars in Brazil, which seldom are more than one hour old. For Sao Paulo, pick "Sao Roque/SP" > MAXCAPPI. You can chose to plot the cities and landscape on to the map to make it easier, as shown on the picture.

Webcams are becoming more normal, and if you are going to the coast, for example, there are ways to get a good impression on how the weather is right now. Check out Helpjet's site.
The weather is very cyclic, and the more time you spend following these cycles, the better you become at predicting the development during the following days. Whenever you can, try to observe the weather and how it develops. Look how the clouds develop and refer to the weather sides afterwards to compare. Look at a satellite picture taken at the time you made the observation. This way you will be better at interpreting your observations sources in the future.

As you can see the sites contain much more services and information compared to that I have mentioned here. I tried to make this a simple and straight forward, but is you have interest, do not hesitate to contact me. Enjoy yourself with the weather, and let me know if I can improve the description.

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