Weather Alicante - Spain

Terms: Pressure/weather relation Air masses and Fronts Occluding Depression Air Pressure Pressure/barometer relation Barometer

Pressure System Atmospheric pressure as measured by a barometer is due to the weight of air above, so must always decrease with height - hence the use of barometers as altimeters in aircraft. To have comparative value therefore, all pressure readings must be referred to a common datum, and Mean Sea Level (MSL) is used for this purpose. Observations made simultaneously at different places can then be plotted on a map to give a Synoptic Chart of the weather. Pressure often differs markedly across a country, and lines of equal MSL pressure (Isobars) are drawn on these charts to outline areas of high pressure (Anticyclones) and areas of low pressure (Depressions). If wind directions and speeds are also plotted, it is immediately apparent that there is a relationship between wind and the pressure field. Wind flow roughly follows the isobars, in the Northern Hemisphere clockwise round anticyclones and anti-clockwise round depressions. Wind speeds are strong when the isobars are close together and light when the isobars are far apart.

Thanks to the regular showing of weather charts on television, this important pressure-wind relationship is now reasonably familiar, but it is none the less a somewhat surprising result. The only obvious force acting on the air is that due to the gradient of pressure, and one would expect air to flow directly from high to low pressure as it does, for example, in escaping from a tyre when the valve is loosened. But clearly this cannot happen in the atmosphere, for if it did the pressure field would quickly become uniform and barometer readings would never change. There must be some other, latent, force at work - this arises from the rotation of the earth. In the Northern Hemisphere, the rotational force acts to the right of the direction in which the air moves, and the net result of this force and the pressure force acting at the same time is the observed air flow almost along the isobars rather than directly across them. Airflow near the ground is affected by a third force, namely friction; this reduces the wind speed and also makes the winds blow slightly across the isobars towards the lower pressure. Near the surface therefore, air circulating round an anticyclone as in the image above tends to spiral outwards slightly from the centre, and air from above descends to take the place of that being removed at lower levels. In a similar manner, the surface winds blowing round a depression spiral slightly inwards, and this leads to upward air movement. As we will see these upward and downward motions of the air have important consequences in terms of cloud formation.

Air Masses and Fronts Air which stagnates for a long time over a uniform part of the globe (such as a large ocean, desert or polar region), not surprisingly acquires properties of temperature and humidity which reflect those of the underlying surface. When air moves away from one of these source regions the imprinted characteristics are only modified very slowly. In middle latitudes, air which originated in the arctic will differ markedly from air that had a tropical source; if two such air masses are brought alongside as in the image at the left that there may be a sharp discontinuity between them. Warm air is less dense than cold air and so has a natural tendency to overlie it, exactly as petrol or light oil spreads out over the surface of a puddle. In the atmosphere however, the surface of separation is not quite horizontal, (yet another consequence of the earth's rotation) but slopes gently upwards so that a wedge of cold air underlies the warm air. The line where the surface of separation between the two air masses meets the earth's surface is called a Front. The image at the left shows, as on a weather chart, how a front may be formed.

In the image at the left, isobars and therefore winds are in the same direction on both sides of the front which is thus stationary. Sooner or later, the front develops a bulge into the cold air. The isobars will then cross the front, and the two parts of the front move in the general direction of the winds; the leading part where warm air replaces cold is call a warm front, and the rear part where cold air replaces warm is called a cold front. A bulge of this kind is usually called a wave because of its shape and the fact that it progresses as do waves on the sea surface. Pressure falls at the tip of the wave which eventually develops into the centre of a depression.

In general, a cold front moves faster than a warm front, thus eventually catching up the warm front ahead. The really warm air is then lifted clear of the ground (i.e. is occluded). By this time, the cold air ahead of and behind the depression will have developed slightly different characteristics, thus maintaining a surface discontinuity which is called an occluded front or occlusion. Most commonly, the northerly flow behind the depression is colder than the southerly flow ahead, and the image below shows the appearance of a weather chart in this case.

Occluding Depression The broad belts of thick cloud, visible on the satellite pictures, which often cause rain or snow are associated with fronts. The warm air above the frontal surface is forced or triggered to move upwards on a very large scale along the front. At a warm front, air usually ascends gradually over the cold air wedge ahead as if this were a hill slope; at a cold front, the cold air wedge undercuts the warm air, forcing it upwards, and not in frequently providing the initial impetus to start a spontaneous convection process.

Air Pressure You can think of our atmosphere as a large ocean of air surrounding the earth. The air that composes the atmosphere is made of many different gases. Nitrogen accounts for as much as 78% of the volume while Oxygen accounts for 21%. The remaining 1% is composed of such gases as Argon, Carbon Dioxide, Helium, and Hydrogen. Typically, the weather of the earth is caused by processes that occur within the lowest 20 km of the atmosphere. This includes such phenomena as fog, wind, rain, storms, snow, tornadoes, and clouds. Air and consequently, our atmosphere, do have weight. This weight decreases as you go up within the atmosphere. When gravity acts on the air, the air exerts a force upon the earth called pressure. The typical pressure at sea level is 1013.25 mill bars or 14.7 pounds per square inch. A mill bar is a unit that is used to report the atmospheric pressure, as well measured in hPa (hecto Pascal). The French scientist Blaise Pascal (1623-1662) was one of the first people in the Renaissance that considered the relation between the weather and air pressure changes. You can measure air pressure with a Barometer, it will be indicated in mill bar of hPa

Pressure changes and the Barometer
The fact that frontal clouds and rain are linked with depressions together with the relationship mentioned earlier between wind and the pressure gradient, certainly is simply an association of weather with air pressure. Yet it is common experience that the familiar words "Stormy" and "Very Dry" inscribed on hall barometers are an inexact guide to the weather, for it may well be calm when the pressure is low and equally it sometimes rains when the pressure is high. The way in which air pressure changes over a period is called the Barometric Characteristic, and this is far more significant than the precise value of the pressure, both in relation to the behaviour of the wind and as a guide to the evolution of the weather.

Barometer The simplest way to determine rates of change of pressure is to use an instrument which monitors and records pressure more or less continuously. Such an instrument was first devised in 1678 by Robert Hooke, who introduced an ingenious modification to the form of mercury barometer that he had earlier invented. However mercury barometers are cumbersome and cannot readily be moved about so that nowadays (as in pressure altimeters and most hall barometers) pressure is more usually measured by means of an aneroid capsule which avoids the need to use mercury or other liquids. Aneroid Barometers were not made until the 1840's and the recording version known as a Barograph (in which a pen marks the pressure value continuously on a chart wrapped round a drum that is rotated by clockwork) first appeared in 1867. These instruments, though much less awkward than Hooke's recording barometer are still very sensitive to movement and are thus unsuitable for use on yachts or in similar unstable situations.

The first barometer was invented in 1643 by the Italian scientist Evangelista Torricelli, who used a column of water in a tube 34 ft (10.4 m) long. This inconvenient water column was soon replaced by mercury, which is denser than water and requires a tube about 3 ft (0.9 m) long. The mercurial barometer consists of a glass tube, sealed at one end and filled with pure mercury. After being heated to expel the air, it is inverted in a small cup of mercury called the cistern. The mercury in the tube sinks slightly, creating above it a vacuum (the Torricellian vacuum). Atmospheric pressure on the surface of the mercury in the cistern supports the column in the tube, which varies in height with variations in atmospheric pressure and hence with changes in elevation, generally decreasing with increases in height above sea level. Standard sea-level pressure is 14.7 lb per sq in. (1,030 grams per sq cm), which is equivalent to a column of mercury 29.92 in. (760 mm) in height; the decrease with elevation is approximately 1 in. (2.5 cm) for every 900 ft (270 m) of ascent.

In weather forecasting, barometric readings are usually measured on electronically controlled instruments often tied to computers. The results are plotted on base maps so that analyses of weather-producing pressure systems can be made. At a given location a storm is generally anticipated when the barometer is falling rapidly; when the barometer is rising, fair weather may usually be expected. The aneroid barometer is a metallic box so made that when the air has been partially removed from the box the surface depresses or expands with variation of air pressure on it; this motion is transmitted by a train of levers to a pointer which shows the pressure on a graduated scale. A barograph is a self-recording aneroid barometer; an altimeter is often an aneroid barometer used to calculate altitude.

However, that drawback affects only the recording technique and can be overcome by using electronic rather than mechanical methods to display the pressure variation over a period. i.e. to show the barometric characteristic at a glance. The barograph record which results just from the movement and development of pressure systems is normally smooth, but when a front passes, it shows an abrupt rather than a gradual change. Fronts always lie in Troughs of Low Pressure, and as in the images above shown, where an isobar crosses a front there is an angled kink pointing away from the centre of low pressure. The passage of a front is thus marked not only by a change of weather, but by an abrupt shift of wind direction in a clockwise sense (i.e. a wind veer) and also by a sharp change in the rate of fall or rise of the barometer. If the wave shown in the images above moves, without deepening, parallel to the isobars in the warm air (this is a good first approximation for a newly formed wave depression), pressure will fall ahead of the wave, become steady when the warm front passes, and start rising immediately behind the cold front. When continuous outdoor observations cannot be made, fronts can still be detected in this way from the barograph record. Pressure changes in temperate latitudes are generally irregular; being for the most part due to the movement and development of pressure systems. For much of the time, these pressure systems move in a roughly easterly direction, but occasionally this progression is halted when a large system, usually an anticyclone, becomes stationary for several days, blocking the normal mobility like a giant thrombosis; most of the spells of extreme weather mentioned in the introduction were associated with blocking of this kind. Cloud systems and their evolution provide very useful guidance about the weather for a few hours ahead, but a great deal more can be inferred if wind and pressure changes are also noted.