WX BRIEF

Using global met resources

Access to weather information from almost anywhere in the world is just a few clicks away.




High-altitude significant weather forecast map for Nov 26, 2010 for ICAO Region G (Asia) issued by the London World Area Forecast Center. These WAFC products provide guidance of adverse weather conditions that might be encountered on a long-range flight over the region.

Literally dozens of weather websites give access to these reports. Many provide simple interfaces where you can enter the city name or even browse by country. But to gain direct access to the Metars, you need to know the ICAO airport identification code.

For example, to get the last month of data for ISB (Islamabad, Pakistan), you would use the ICAO code OPRN. A station master list is maintained by adds. aviationweather.gov. (Choose the "metars" tab and click the stations. txt link.)

From adds you can access the most recent 36 hrs for any of these stations. Other websites give access to over 1000 past reports. (The www.uswx.com/us/stn/ metar inter­face, for example provides access to the last 1440 Metars from most stations on the list.)

ICAO also coordinates 2 world area forecast centers—one at NOAA in Washington DC, the other at the Met Office in the UK. These centers are responsible for developing significant weather maps for large regions usually covering international airspace or the airspace over several countries at once.

High-altitude significant weather maps for much of the world are available from aviation­weather. gov. Washington produces low and middle-level significant weather charts for the Washington domain (105° E eastward to 65° E).

Significant weather charts for Europe, Asia, Africa and the Indian Ocean are produced by London WAFC. All the products can be accessed in T4 fax format from weather.noaa.gov/fax/wafssig. shtml.

Beyond the global products, a search on the Internet can often reveal foreign websites that provide access to regional weather information. Many of these products are available free but, in general, most national meteorological services charge for any value added product, and the information may only be available to paid subscribers.

However, many aviation websites have taken it upon themselves to provide some of this information as a free service to their visitors. For example, eurometeo.com has compiled links to open-access radar data from across Europe, while weather.org/radar.htm links to several radar sites around the world.

Terra incognita

High-altitude significant weather forecast map for Nov 26, 2010 for the Pacific Ocean. These maps are produced by ICAO's World Area Fore­cast Centers in Washington DC and London and cover most of the world.

Even though most airports we fly into and out of have weather observations, and many of these are from automated stations whose observations are collected and disseminated over the Internet in minutes, there remain some regions of the planet with very few continually operating automated weather stations. When planning flights into or across these regions, your best bet is to turn to a combination of satellite imagery and climatology.

In the absence of current conditions, and even if a location has present weather information, understanding the climatology of a region can help you know what sort of weather to expect. The climate of a location or region is a statistical summary of the observed conditions over time.

By repeated observation, seasonally likely conditions can be determined, as can the likelihood of seeing out-of-the-ordinary weather. What's more, unless there is some reason why the weather at one location should be vastly different from the weather at the nearest observing site, the climatology should provide a reasonable guess about the likely conditions in the vicinity of where you want to go.

Planning tools

There are many resources available on the Internet, such as weatherreports.com, that provide graphs or tables of monthly average temperature and precipitation. Some sites, such as the Weather Underground (wunderground. com), even provide "trip planners" that, if you select certain dates, will give you the average and extreme conditions for those dates, as well as the probabilities of experiencing certain weather conditions such as an overly hot, windy or cloudy day.

For example, if one were planning a trip to ADD (Addis Ababa, Eth­iopia) for the first week in January, climatology (using 13 years of data since 1997) would reveal that high temperatures are likely to fall between 53 and 75°F (12–24°C), winds will probably blow between 4 and 13 kts with only a 30% chance of winds exceeding 9 kts, and just a 6% chance of a cloudy day.

Supplementing climate information are satellite images. For the past 50 years, weather satellites have been taking pictures of Earth, and in recent decades, both the information and coverage they provide have improved drastically. Today, it is possible to download a satellite image that is only an hour or two old, covering just about anywhere.

The most common satellite imagery are the visible scans. These will quickly tell a pilot about the prevailing cloudiness and large-scale weather patterns over a region. With visible satellite images, someone planning a flight can even identify individual thunderstorms half a world away.

When the area being considered is cloaked by night, infrared (IR) images are able to differentiate temperatures and use that information to display nocturnal cloud patterns. These images are also useful during the day to interpret the approximate heights of cloud tops.

In an area devoid of clouds, IR imagery is also useful in estimating the surface temperature from which some approximate air temperatures may be deduced. By comparing visible and IR images, it is also possible to tell whether the area you're flying into is blanketed with snow. (Clouds and snow on the ground can be hard to differentiate in a visible satellite image.)

Water vapor images are another useful version of the standard satellite image. Similar to IR imagery, certain IR wavelengths are effectively intercepted by water vapor. The more water vapor, the greater the interception.

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