Weather forecasting is becoming more accurate. I don’t think there is much doubt about that. With the introduction of ever more sophisticated technology, both computerised and satellite, any weather professional can be more confident than ever about his ability to forecast…but only to a point.
Thanks to the chaotic nature of our climate and atmosphere a long range forecast is still not something you would bet your house on. Most weather forecasts are accurate to within two or three days, but then the percentages drop away sharply.
As the weather is global in nature it makes sense that global cooperation is required to help the forecasters. Most countries now have some form of national weather service with a remit to produce accurate forecasts for a couple of days ahead. However the measurements and forecasts observed in every county give an important contribution to the worldwide network and allow more data to be fed into the computer models.
There are three national superpowers that are particularly involved in global forecasting. In England there is the British Meteorological Office, in the US the American National Weather Centre and for Europe the ECMWF (European Centre for Medium Range Weather Forecasts) also located in England. The ECMWF concentrates on forecasts from 1 to 10 days, and covers 17 countries. This organisation spends a large amount of time collecting and analysing data from around the world and is generally considered a leader in medium term forecasting.
There is also the WMO (World Meteorological Organisation) which is an agency operated for and behalf of the United Nations with over 170 member countries. This organisation was established in 1951 and is tasked with improving weather observations, and allowing an optimum flow of information, around the world.
Observations and data for weather forecasting come from many different sources. Around 13,000 land stations, 7,500 ships, commercial airliners and satellites are also all pouring data into the computer models around the world which in turn churn out weather predictions from a few minutes (e.g. Tornadoes) to hundreds of years (e.g. global warming).
So, with all this information available, with all this hardware and software, and an incredible network of observations from the oceans, the atmosphere and from space is a 7 day forecast worth the paper it is written on? Well, maybe…you see it all depends on where you are in the world. Some climates rarely change, such as the Sahara desert and a forecast of this area will be pretty straightforward the majority of the time with perhaps just some tweaking of the high and low temperatures.
Other areas of the world however are much more changeable and harder to predict. The British Isles for instance lying on the western edge of Europe, with the jet stream taking aim from across the Atlantic and the Gulf Stream conveying warm air to our shores, is much more difficult to measure. A slight change in the direction of a depression mid-atlantic can have a marked effect on the local weather. The same is true of course for cyclones and hurricanes, and tornadoes and dust storms.
And that for me is the great thing about the weather. It is so unpredictable and chaotic. Yes, the forecasts are becoming ever more sophisticated and accurate but they will never be 100% accurate, and I wouldn’t have it any other way. So is a 7 day weather forecast worth the paper it is written on? Probably, but with so many caveats you wouldn’t put a bet on it.
Wednesday 12 November 2008
World Weather Feel the Weight on Your Shoulders
The weight of air resting on a given area of the Earth's surface is known as air pressure. Air pressure (or atmospheric pressure) is always greatest at sea level, where the air is at its most dense. Therefore at the top of a mountain the air is less dense and therefore the pressure is lower.
The air is composed of billions of tiny particles that are constantly moving in all directions, bouncing off whatever they encounter. These collisions constitute what is known as air pressure. The more collisions occurring within a certain area then the greater the air pressure will be.
We are completely unaware of this, but the air is constantly exerting pressure on us, on average this is 14 ¾ pounds per square inch. (1 kg per sq cm ) . Air molecules are naturally drawn towards the earth by gravity, and as a consequence the density of the air is greater near the surface of the earth. Therefore the number of molecules in a given area, the air pressure, decreases with altitude. These molecules are in constant motion and this prevents them from settling at ground level.
At sea level, standard air pressure is 1013, but typically the pressure varies between 980 and 1040 millibars (mb). As with any aspect of the atmosphere there are extremes and the highest and lowest recorded pressures are as follows:
The highest recorded atmospheric pressure, 1085.7 mb, occurred at Tonsontsengel, Mongolia, 19 December 2001.
The lowest sea level air pressure ever recorded was 870 mb in the eye of Typhoon Tip over the Pacific Ocean on October 12th 1979
Air pressure is measured using a barometer. Although the changes are usually too slow to observe directly, air pressure is almost always changing.
Weather maps showing the pressure at the surface are drawn using millibars. Air pressure can tell us about what kind of weather to expect as well. Winds blow in an attempt to combat the differences in air pressure. Wind is the movement of air over the surface of the Earth, from areas of high pressure to low pressure. A large change in pressure over a relatively small distance, a large pressure gradient, can result in far stronger winds. When the isobars are tightly packed, locations within that large pressure gradient can expect windy conditions. As air rises and creates an area of low pressure, water vapour in the atmosphere will condense and form clouds. However sinking air, in an area of high pressure, means that no condensation will take place. This is why low pressure is associated with cloudy skies and unsettled conditions, and high pressure is associated with clearer skies and drier conditions.
Winds near the Earth's surface rotate anti clockwise toward the centre of areas of low pressure and clockwise outward from the centre of areas of high pressure in the Northern Hemisphere, with an opposite flow (clockwise around areas of low pressure and counter clockwise around areas of high pressure) occurring in the Southern Hemisphere. The main reason for this pattern is the Coriolis force, which results from the Earth's rotation on its axis and deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
So keep tapping your barometers and feel the weight of the air pressing in around you!
The air is composed of billions of tiny particles that are constantly moving in all directions, bouncing off whatever they encounter. These collisions constitute what is known as air pressure. The more collisions occurring within a certain area then the greater the air pressure will be.
We are completely unaware of this, but the air is constantly exerting pressure on us, on average this is 14 ¾ pounds per square inch. (1 kg per sq cm ) . Air molecules are naturally drawn towards the earth by gravity, and as a consequence the density of the air is greater near the surface of the earth. Therefore the number of molecules in a given area, the air pressure, decreases with altitude. These molecules are in constant motion and this prevents them from settling at ground level.
At sea level, standard air pressure is 1013, but typically the pressure varies between 980 and 1040 millibars (mb). As with any aspect of the atmosphere there are extremes and the highest and lowest recorded pressures are as follows:
The highest recorded atmospheric pressure, 1085.7 mb, occurred at Tonsontsengel, Mongolia, 19 December 2001.
The lowest sea level air pressure ever recorded was 870 mb in the eye of Typhoon Tip over the Pacific Ocean on October 12th 1979
Air pressure is measured using a barometer
Weather maps showing the pressure at the surface are drawn using millibars. Air pressure can tell us about what kind of weather to expect as well. Winds blow in an attempt to combat the differences in air pressure. Wind is the movement of air over the surface of the Earth, from areas of high pressure to low pressure. A large change in pressure over a relatively small distance, a large pressure gradient, can result in far stronger winds. When the isobars are tightly packed, locations within that large pressure gradient can expect windy conditions. As air rises and creates an area of low pressure, water vapour in the atmosphere will condense and form clouds. However sinking air, in an area of high pressure, means that no condensation will take place. This is why low pressure is associated with cloudy skies and unsettled conditions, and high pressure is associated with clearer skies and drier conditions.
Winds near the Earth's surface rotate anti clockwise toward the centre of areas of low pressure and clockwise outward from the centre of areas of high pressure in the Northern Hemisphere, with an opposite flow (clockwise around areas of low pressure and counter clockwise around areas of high pressure) occurring in the Southern Hemisphere. The main reason for this pattern is the Coriolis force, which results from the Earth's rotation on its axis and deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
So keep tapping your barometers and feel the weight of the air pressing in around you!
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