Automated Surface Observing Systems program is a joint venture between the National Weather Service, the FAA and the Department of Defense. As the nation’s primary surface weather system ASOS largely replaced the trained weather observers who used to reside in the control towers at our nation’s airports.
The ASOS machines give the NWS a better, more reliable means of weather reporting and forecasting by simply providing them with a plethora of data, collected hourly, or as necessary, at over 1700 different locations in the United States.
But the reality is weather data is collected at airports because it is such an critical element in aviation. And while the automation of weather collection and reporting can help the NWS in providing more accurate forecasting, the limitations of the technology sometimes can impact us as pilots
George Carlin's "Hippy Dippy Weatherman" Al Sleet used to say ” The temperature at the airport is 88 degrees which is stupid because I dont know anybody who lives at the airport”.
The primary limitation with ASOS in my mind is the lack of peripheral vision at the receiving station. When the devices look skyward their sensors only see what is directly above them. If there is a hole in the clouds directly above the machine it will report sky clear. If the visibility is reduced this can be a problem. Everyone has had an experience where one end of the runway is clouded over or fogged in and the other is clear and sunny. I remember numerous trips I have made where we were IMC and the ATIS at many of the airports along our route were reporting sky clear below twelve thousand feet.
Looking at METARS which are solely derived from ASOS we can see where along our route of flight the weather is folding helping us determine where the frontal activity is at a given point in time and geography.
A holistic approach to weather evaluation should be made prior to embarking on a trip of any length. In fact it is a required preflight obligation of anyone desiring to operate an aircraft as Pilot in command.
Sec. 91.103 — Preflight action.
Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight. This information must include—
(a) For a flight under IFR or a flight not in the vicinity of an airport, weather reports and forecasts, fuel requirements, alternatives available if the planned flight cannot be completed, and any known traffic delays of which the pilot in command has been advised by ATC;
Starting with the commercial forecasts at cities along your proposed routing will present a picture of the prognostications of the forecasters. I’m fond of www.weather.com’s 10 day forecast. I compile a list of cities across where I am heading and bookmark them in my browser. I also use a NOAA site called HPC Sea Level Pressures and Fronts through Day 7 (http://www.hpc.ncep.noaa.gov/basicwx/day0-7loop.html) to determine why the weather channel guys are saying what they are. Essentially this site gives you a picture of where the forecasters are saying the pressure systems and fronts will be positioned across the contiguous United States from now through for the next seven days. The first four panes will look familiar to most of us as it is essentially the surface prog charts from the ADDS site.
Seeing the pressure and fronts and their forecasted movements and comparing this to the commercial forecasts across the routing gives you an overview picture of their best guess of what will be happening weather wise in these locations. Checking in with these regularly in advance of your departure will allow you to formulate alternate routings based on where the foul weather will not be on the given day you decide to launch. Be mindful that forecasts are considered so unreliable, by the folks that create them, that they are updated every six hours. By examining them in advance you can gain some insight as to whether the weather is moving along as expected, in advance of the expectation or behind it.
Once our preflight weather planning is complete and we make the decision to fly we pick our route, check the most current weather across that route and launch.
Light piston aircraft make best power at around seven thousand feet. Not to say that you can’t climb higher than that but you lose power in a naturally aspirated airplane. So climbing above the weather isn’t an option as it would be for a pilot flying a transport aircraft, turboprop or light jet. Rather those of us in the piston single/multi class have to fly through the weather or under it.
Using the WXWorx /XM weather most of us have in our airplanes these you can see in near real time what the weather is doing along your route of flight allowing you to alter your plan well in advance of encountering conditions you might not want to fly in. If you don’t have weather in the airplane you fly in I would suggest getting it. A new Garmin Aera 510 with XM is about $1200 and the middle of the road XM subscription is about $60 per month. In mind this is as essential to instrument flying as a good pair of active noise cancelling headsets. Since I’ve been flying with it (2006) I have become so accustomed to having this information at hand that I am uncomfortable flying in clouds without it.
Seeing the METAR icons change from Cyan to Green then Yellow and Red in a broad swath along your route of flight or at your destination prior to arriving there will help you make informed inflight decisions long in advance of encountering adverse conditions. Gone are the days of leaving the frequency to speak with a flight watch briefer as you barrel along towards your destination which unbeknownst to you went low IFR an hour ago. You can see where the weather is and plan to stop short add fuel and continue or change your destination before you actually needing to.
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