What Should I Wear?
Books on weather often save the best for last. Before you finally get to weather forecasting, you've had to master volumes of basics. But why not start with dessert and get to the rest of the menu later? Let's begin with forecasting, at least a little bit about it. We'll give you all the background you could want, and then revisit it in later sections. For now our first task is to take a look outside.
Your Fifty-Year Forecast
If you just read the preceding heading, "Your Fifty-Year Forecast," I bet I know what you're thinking—he's the biggest idiot of all. How can anyone forecast 50 years in advance when most forecasts seem to depart from reality in just 50 hours?
Well, as I was putting this together, my boss at the ABC affiliate in Connecticut, WTNH-TV, asked how I might contribute to a special called Taken by Storm: 50 Years of Weather. The station was marking its fiftieth anniversary, and there were specials in the works commemorating 50 years of everything, including the weather. So in addition to a historical wrap-up of major weather events, I suggested a segment on the weather expected for the next 50 years. He had that "Are you nuts?" look on his face, but really, that ultra-long-range forecast isn't as far out as it might seem. The following figure shows a long-range prediction for some of the larger cities around the country. All you have to do is find the city closest to your location and check the numbers. The forecast of sunny and cloudy days is included among the predictions for all the other meteorological possibilities over the next 50 years. I bet the forecast comes close to at least 90 percent accuracy.
Sunny days, cloudy days: the next 50 years.
How Did He Do That?
"There is really no such thing as bad weather, only different kinds of good weather."
People often think the words "weather" and "climate" can be used interchangeably, but that's not so. Weather refers to current conditions of temperature, precipitation, humidity, and wind. Climate is an average of those conditions as they've occurred over decades.
Persistence is the assumption that weather patterns do not change rapidly. Even in mid-latitudes, where change is always expected, persistence can be surprisingly accurate. If you predict that today's weather will be repeated tomorrow, you'll be correct 67 percent of the time in places like Boston, New York, or Chicago.
This long-range forecasting is often an application of climate, which is an average of weather conditions. Weather is a snapshot of current conditions—the temperature, humidity, wind, and precipitation. But climate represents those weather conditions averaged over decades. The weather is always changing. Some days are hot. Some are wet. But as they say, "One swallow doesn't make a summer." One balmy day in winter, or even a whole balmy season, doesn't make a tropical climate.
In today's hot debate about greenhouse warming, current weather conditions are often confused with climate. The decades of the 1980s and 1990s have set some record warm temperatures. The 1990s are thought to be the warmest decade and 1998 the warmest year of the past 1,000 years. But is that a sign of climate change or just normal variability of weather?
We'll revisit that steamy topic later, but there's no doubt that having knowledge of climate is important for weather forecasting. And that 50 year forecast is simply a compilation of average annual conditions multiplied by 50. Over the long haul, climate changes slowly, even glacially. In fact, persistence is one of the most accurate short-range forecast techniques. So for example, if Atlanta receives an average of 219 sunny days each year, then over the next 50 years, this southern city should experience about 10,950 sunny days. I know that will be close—give or take a few days for leap years. The 50-year forecast is that simple. Of course if there's a sudden climate change, you'll need an update, but I have 50 years to work on that.
Certainly our first forecast doesn't exactly tell us what to wear for the next 18,250 days, but we're just getting started.