Why weather forecasts aren't always accurate?

Wind blowing on a woman.

You probably listen to a weather forecast every day.

Nearly everything that you do is affected by the weather.

Have you ever wondered how it is possible to predict something as changeable as the weather?

On the other hand, why is it that the weather forecaster is so often wrong?

In order to appreciate the weather forecaster’s task, we need to understand something about the nature of weather, and what causes it.

Above the earth’s surface, the atmosphere extends hundreds of miles into space.

But we are concerned here with only the bottom layer, some six to nine miles thick, called the “troposphere.”

This layer contains some five quadrillion tons of air, and it is within it that all the weather happens.

There are three primary properties of the atmosphere involved in the weather—temperature, pressure and humidity.


Temperature

Earth temperature.

The fundamental cause of all weather is the unequal heating of the atmosphere by the sun’s rays.

This comes about because the atmosphere itself is transparent and absorbs very little energy directly from sunlight.

The sun’s rays go through to the earth’s surface, where they are reflected or absorbed, in varying degrees.

Water reflects more light, and land, especially dark-colored earth, absorbs more.

If the surface absorbs a lot, it is warmed.

If it reflects a lot, it remains cooler.

The equatorial regions receive more heat because the sun’s rays hit vertically.

In the polar regions, they strike obliquely, so the heat energy in an equal “bundle” of rays is spread over a larger area.

Also, the snow cover at the poles reflects most of the sunlight.

Consequently, it is colder at the poles.

Temperature differences caused by this process, called “insolation,” put in motion a chain of consequences that brings about the great variety in our weather.

Pressure

Air pressure.

An important factor is changes in pressure.

Since air at a higher temperature is less dense than the cooler air over another area, it rises.

This movement, in turn, generates differences in pressure between different areas.

A barometer, which measures the weight of atmosphere above it, will show a lower pressure under a rising column of warm air, which displaces the cooler, relatively more dense air above it.

On the other hand, a descending column of cool air will cause higher pressure below it.

The pressure difference at the surface causes a wind to blow from the region of higher pressure toward that of lower pressure.

It is similar to what happens when an inflated balloon is opened at one end.

The high-pressure air inside rushes out into the surrounding area of lower pressure.

The greater the pressure difference, the stronger the wind.

This principle operates on a global scale.

The global movements of air masses in different directions generate turbulent motions where the main streams brush against each other.

There is a additional effects from topography—the irregular outlines of continents and the complex patterns of mountains and plains, deserts and forests.

Humidity

Water vapor.

As air masses move across the surface of the earth, they pick up water.

Most of this is vapor from oceans, but some comes from moist earth.

Since warm air has greater capacity for moisture, air masses drawn toward low-pressure areas have relatively higher humidity.

The fact that water vapor weighs less than an equivalent volume of dry air further contributes to the lightness of columns of warm air rising in low-pressure areas.

What happens when humid air rises?

Pressure on it decreases, it expands and cools.

When the temperature falls to the level where the air becomes saturated, water begins to condense in droplets or ice crystals, forming clouds.

From these, rain or snow may fall.

The air, thus dried, descends in high-pressure areas, bringing clear weather.

The Weather Chart Meteorologists note the air’s varying properties of temperature, pressure and humidity.

At each weather station there are instruments to measure all of these.

A weather forecaster also studies wind direction and speed, the nature of clouds, visibility, whether it is or has been raining or snowing, and how much.

These data are organized for comparison with readings at other locations.

To facilitate this, all weather stations have a fixed hour by Greenwich mean time for gathering the needed information.

The observer must take into account that barometric pressure decreases with altitude under constant weather conditions.

In fact, the difference even at an altitude of one thousand feet is greater than that caused by changes in weather.

So a correction is applied to the reading at each station, to put all the readings on a common basis, as if taken at sea level.

With the help of such detailed information from weather stations over a wide area, a meteorologist plots a chart in a special “language” created by the World Meteorological Organization.

In this language information appears as numbers instead of words, making possible its transmission between ships at sea and land stations in various countries without need of translation.

Weather charts are then drawn with irregular curving lines called “isobars” that connect places reporting the same pressure.

Some of these lines form closed curves around regions that are thus identified as high- or low-pressure areas or, for short, “highs” and “lows.”

This gives the weather forecaster a good picture of the weather as it was at the time of the observations.

The Weather Forecast However, discerning what the weather is at a given moment is one thing; telling what it will be tomorrow or several days hence is quite another.

This requires examining a number of charts drawn up over a period of time.

Since each chart is like a photograph that pinpoints the weather at a given moment, by arranging several charts in order the weather forecaster views a “motion picture” of atmospheric movements.

Based on the most recent shifts of the highs and lows, one marks out their probable position for tomorrow.

Thus one can get a good idea of how the weather will change in the immediate future.

In view of today’s sophisticated equipment and scientific know-how, why are weather forecasts often wrong?

Why are meteorologists at times unable to foresee devastating storms that cause vast damage to property and loss of lives?

It is important to realize that your weather forecaster can only observe what is happening and tell us what he believes will happen next, but he cannot control weather conditions.

We cannot expect, in the nature of a  weather forecaster's task pinpoint accuracy.

For instance, he may predict showers in a given area.

But these are often local manifestations.

One locality might enjoy a heavy rain, and another, only a few miles away, might remain perfectly dry.

And there are other unpredictable factors.

Just as differing currents exist at various depths of the sea, so too the atmosphere at varying altitudes has different winds and air masses that influence one another.

Charts of higher altitudes contain fewer details, making it difficult to know in advance the effect of these more remote conditions on our weather.

When developing the forecast, the meteorologist must ask himself questions such as:

Will this low-pressure center continue at the same speed tomorrow?

Or might it slow down, or turn aside?

Is there any indication that the disturbance is weakening and might disappear?

Will the approach of the “low” to a stationary “high” interfere with it?

Which will probably prevail?

Far from being simply a matter of guesswork, however, weather forecasting is an art that involves detailed knowledge and skill.

Your weather forecaster can indeed help you to plan activities in advance, but he cannot guarantee what the weather will be.

And now you know why.

There are so many factors beyond the weather forecaster's control.

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