Making your own forecast

More than ever before, it's feasible for a layperson to craft their own weather forecast. The raw material used by the professionals – output from computer models – is available for many parts of the world on the Internet. Anyone can put together a home weather station at reasonable cost and see how their out- looks compare with reality. And, as always, the sights and sounds of weather on the move are here for all to interpret and enjoy.

What's typical, what's not

The best starting point in understanding your local weather is to pay close attention to it Get to know the average conditions and how often and radically the weather departs from those averages. Mini-climatologies can be found in Weather around the World for more than 200 cities. You can also find more data for these and many other locations by checking with your local weather service, or by following Web links for climate data listed in Resources, p.385.

You can build your own climatology by making regular observations at home. For more detail on the weather instruments below, see “Taking the weather's pulse”. Some of the basic components of a home weather station include:

  • A good thermometer, ideally a “max-min'* model that records daily highs and lows.
  • A barometer, for measuring atmospheric pressure. You can keep this inside, since pressure isn't significantly different inside and outside a home (unless you live in a high-rise building).
  • A humidity-measuring device, such as a sling psychrometer.
  • An anemometer, for measuring wind speed.
  • A wind vane, for sensing its direction.
  • A rain gauge, which funnels rainfall into a narrow column so that fine-scale readings become visible. It should be placed in an open area as far from buildings as possible so that windblown rain won't be blocked.
  • Where climate dictates, a snowboard – a small, flat board painted white to reflect sunlight and stay cool. Snowboards are placed In an open area and cleared off after every observation during a snowstorm.

These instruments can be purchased a la carte and placed in a well-ventilated shelter, which you can either build or buy ready-made. Several kinds of instruments are often bundled together as part of a digital weather station; these are more expensive, but far more convenient, than manual instruments. Some digital stations can upload their data directly into a personal computer, making it easy to compile statistics on the climate at your home.

In general, the quality of weather instruments goes up with price, but you can often save money by shopping around A basic set of reliable, durable instruments could be put together by spending in the region of $100/?55. Basic all-in-one digital weather stations now sell for as little as $30/?16, while higher-quality packages go for $150/?80 and up.

Consider pooling your observations with other weather fans. Many local TV stations and newspapers across the US have informal networks of weather watchers who send in daily reports by phone or the Internet. Your nation may also have co-operative weather observers whose reports become part of the permanent climate record. If you're a student, you and your school can take part in the GLOBE Program, which collects and posts environmental observations from all over the world.

Reading the sky

What the atmosphere intends for us is very often revealed in the colour and texture of the sky. Weather lore worldwide is packed with visual references to clouds and their appearance. The images in the colour section will give you a feeling for the kinds of clouds that might pass over your area. Below are some tips to bear in mind as you watch the show unfolding above

  • Strong mid-latitude storms typically send out high clouds well ahead of any rain or snow. A gradual lowering of the clouds, from cirrus or cyrostratus to altostratus, often heralds a wet period to come in the next day or so.
  • Cumulus building on a spring or summer morning – especially if they appear as tall as they are wide – could spell heavy showers or thunderstorms later on. Watch for sharp-edged, cauliflower-shaped turrets: the crisper their appearance, the more well-defined the updraught that's feeding the storm.
  • Fast-moving clouds reveal a strong upper-level wind, which hints at the potential for strong storm development in the near future. This is especially true if the low-level clouds have a northward component to their motion (southward in the Southern Hemisphere) and if high-level clouds are moving In a different direction from low clouds.
  • Poor visibility can be a sign of moisture, pollution or both. If the air feels and smells unusually moist, an incoming storm may be able to produce heavier rain.
  • All else being equal, clear days allow for a bigger temperature rise than cloudy ones, and clear, calm conditions lead to the coldest nights.

When out in the wilderness, it's especially crucial to notice your surroundings. Countless adventurers have lost their lives through unexpectedly bad weather. Keep in mind that some of the worst weather can be surrounded by some of the best. Hurricanes, for instance, produce a zone of subsiding air around their periphery: the result can be skies so azure and conditions so balmy that long-timers call it hurricane weather. Mountain climbers may be lured onto the slopes by unseasonably warm air pulled up in advance of an autumn or spring storm that brings bitter cold and snow hours later. If your weather seems unusually nice for the time of year, consider it added incentive to check the forecast before heading into the backcountry.

How to decipher the models

They aren't for everyone, but the complex maps cranked out each day by computer models provide a wealth of information. Many of the longer-range models cover the entire globe. Alas, the pressure for government weather services to make money from their modelling has prompted some to avoid putting their model output on the Web for all to see. Still, an amazing amount of guidance is on offer if you know where to look. Resources lists some of the major websites that include model output It can take years of practice to learn how to read a model's maps quickly and skilfully. Below are some tips for beginners:

  • Each model moves the weather ahead in tiny increments, and the results are depicted at set Intervals – typically every 6,12 or 24 hours.The starting time of most model runs is either 00.00 or 12.00 Universal Time (midnight or noon in Greenwich, England, and 7.00pm or 7.00am Eastern Standard Time in the US). It normally takes anywhere from one to six hours for a model run to be completed and posted on the Web. Some governments withhold the output from the Internet for a few extra hours in order to give a first look to paying customers only.
  • Weather models analyse the atmosphere at several standard levels measured in millibars or kilopascals (units of pressure; 10mb=1 kPA).The contours on each upper-level map show how high that pressure level sits above different points on the ground.”High heights” as they're called, correspond to a high-pressure centre aloft.
  • The mid-point of the atmosphere is 500mb or 50kPa (roughly equivalent to 5.5km/3.4 miles). This is the level at which major weather systems are steered, although the strongest jet-stream winds are a bit higher up, around 250-300mb. A look at the 500mb map reveals where big storms might go. Generally speaking, the wind at 500mb flows along the height contours, with lower heights to the left of the wind. Some sites include arrows or barbs to show the wind speed and direction. On other sites, you'll see blobs of vorticlty, a combined measure of wind speed and curvature that signifies the “spin” within a storm and, indirectly, its intensity.
  • The mean sea level pressure (MSL) map is essentially the same as the standard surface map you see on TV, with highs, lows and fronts marching across the land.
  • As you follow the models, notice how they agree or disagree. If a particular model suddenly changes its tune on an upcoming system, watch its next run closely to see if the change is consistent Comparing different models for the same region and period is a great way to get a fuller picture of what might happen.