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THE AIR AND THE AIR PRESSURE

The fact that air causes pressure, we can simply see, if we take a full glass of water, filled till the edge and covered with a greeting card and put it upset down, without water falling out of the glass. The atmospheric pressure is even able to bear a column of water till ten meter height on such a manner (or a column of mercury till 75 cm).
We can also say that a column atmospheric air till twelve km height, exerts the same pressure as a column water till ten meter height. This one has a weight of ten thousands kg per square meter on the surface of the earth. The meteorologists call that a pressure of 1 bar = 1000 millibar ( = 1000 cm of water column = 75 cm of mercury column ).
Due to warming or cooling of the atmosphere, the air becomes lighter or heavier. That is why the pressure is not always the same. An atmospheric pressure between 1000 and 1030 millibar is normal for our changing weather. Above that, we talk of high pressure, what mostly causes fair weather, and below that, stormy weather.
A rising air pressure points to better weather (approach of a high pressure area) and a falling to badder weather (approach of a low pressure area).: Back to TEXT MENU

THE WIND AND HIGH AND LOW PRESSURE AREAS
Wind consists of air, which is moving from an area of high pressure to an area of low pressure. Because of the rotation of the earth air is flowing in a turning movement (clockwise) out from an high pressure area. Likewise air is drawed in a turning movement (counterclockwise) to a low pressure area.
An other word for a low pressure area is cyclone (or simply a low). A cyclone is characterized as a area of bad weather. The clouds in the drawed air come together in the center of the cyclone. Also because of the air in the center is lifted, cools and forms more clouds.
A high pressure area (anticyclone or simply high) is characterized by absence of clouds. The air sucked away from the center is filled up with dry air from the higher air layers, what hinders the forming of clouds.
An area of high pressure is therefore a fair weather area with in the summer warm, dry air and a lot of sunshine and in the winter cold, dry air with also a lot of sunshine.
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RAINSTORM AND THUNDERSTORM
When a cyclone is deepening, that is when the air pressure becomes very low in the center of the cyclone, then the air spiraling inward to the center can develop into a rainstorm. In the eye of the storm it is mostly calm. When the low pressure area is very large, par example as big as West-Europe, the spiraling air near the center can develop into a heavy storm with the power of a hurricane.This happens mostly in the spring or autumn.
When the low pressure area is very small and the air-pressure differences very large, the spiraling air in the center is lifted up to a height of ten kilometer or more. The air cools and forms the cumulonimbus (thunderhead) clouds for a thunderstorm.
Extending down from these clouds an intense wind vortex sometimes appears in the form of a funnel-shaped cloud. When this cloud reaches the ground, we have above land a wind-spout (or tornado) and above sea a water-spout.
The last called, more rare phenomena, appear only by strong local heating of the air in the summer or the spring.: Back to TEXT MENU

THE LIGHTNING AND THE THUNDER
When we rub a plastic comb with a woolly stuff, this comb becomes electrically loaded and the comb can attract shreds of paper. The loaded comb can even bend a small jet of water out of the tap, when she is hold near the jet. When we comb very dry hair in the dark, we see electric sparkles.
A thunder cloud is made electrical by movements of water droplets and hail stones in the rising and sinking air streams in the cloud.The cloud is able to discharge him self by the means of the lightning. Via the thunderbolt this electrical charge is displaced to an other cloud or to the earth's surface, at which the air is heated enormous, expands suddenly and implodes there after. This is accompanied by thundering violence.
It is always wise to take cover for a thunderstorm and stay out of the way of high pointed objects. Do not hide under a tree and certainly not under an oak.Because of the long roots of the oak, who reach to the ground water, this tree is a natural lightning attractor. Donar the god of thunder still is always fond of Wodan's oaks.: Back to TEXT MENU

THE RAINBOW
When we give water to the green with a sprinkler, while the sun is shining, then we see in the screen of raindrops a rainbow. Therefore it is needed well, that we stand with our back turned to the the sun, when we look at this mini rainbow ahead of us. If we pay attention to it, we see always the colors red, orange, yellow, green, blue and violet in the same sequence. We see these colors by the combined action of light and water (refraction and reflection of sunlight in the water droplets).
In the same manner the natural rainbow originates in rain droplets, where the sun is shining upon. Then also, we see the rainbow, if we are standing with our back turned to the sun. Big rain droplets give a narrow rainbow with bright colors. Small rain droplets cause a broader rainbow with fainter colors.
Also we see sometimes a rainbow in dew, haze or fog, at which the colors not always are good to distinguish. The moon is able to produce a white rainbow by hazy weather, because our eyes see, by little light, all colors as grey or white.: Back to TEXT MENU

THE CHANCE FOR THE FORMING OF FOG
At a certain temperature air can have a maximum quantity of water vapor, which is greater when the temperature is higher. When air contains more water vapor than this maximum quantity, then this shall transit in clouds of minuscule water droplets and fog originates.
In reality air shall contain less water vapor than this maximum quantity. This ratio (x 100 percent) between the real and maximum quantity of water vapor, we call the relative humidity.
When the temperature falls, then the relative humidity grows quickly in the same quantity of air. When the relative humidity becomes equal to hundred percent, then the forming of fog can occur. The temperature at which this happens, we call the dew point.
When the temperature lowers more, the fog formation grows rapidly. In fact fog is a sort of cloud formation at the cold ground.: Back to TEXT MENU

FAMILIES OF CLOUDS
We distinguish ten types or families of clouds. According to the appearance of these clouds they are found in an uniform blanket-like layer (stratus) or accumulated (cumulus).
The clouds are subdivided in high clouds (prefix: cirro), middle clouds (prefix: alto) and low clouds (no prefix).
High clouds are milk-white and consist of ice crystals.
Middle clouds are grey-white or blue-white and contain as well water droplets as little ice crystals.
Low clouds are light- to dark-grey and contain only water droplets.

High clouds are: a. Cirrus - feathery clouds b. Cirrocumulus - ribbed clouds c. Cirrostratus - milky air-layer
Middle clouds are: a. Altocumulus - fleecy clouds b. Altostratus - frosted glass air-layer
Low clouds without rain are: a. Cumulus - heaping clouds b. Stratus - grayish air-layer c. Stratocumulus - rolled clouds
Low clouds with rain (nimbus) are: a. Cumulonimbus - shower clouds b. Nimbostratus - rainy air-layer

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SORTS OF PRECIPITATION
Fog: diminution of sight to less than a kilometer because of floating tiny water droplets
Mist: diminution of sight to more than a kilometer because of floating tiny water droplets
Haze: diminution of sight to more than a kilometer because of air pollution (smoke or fine-grained sand)
Smog: town's fog caused by air pollution from the company 'Smoke & Fog'.
Dew: water vapor is condensing on objects near the ground
Hoar-frost: water vapor is freezing on objects near the earth's surface
Rough hoar-frost: supercooled water droplets carried along with the wind are freezing on bushes and trees
Rain: falling water droplets
Snow: falling flakes in the form of branched ice crystals
Hail: falling hailstones consisting of ice
Glazed frost: falling supercooled water droplets are freezing on the ground: Back to TEXT MENU

CLASSES OF AIR MASSES
When air hovers a longer time over the same region, it takes over the properties of this source region, what concerns the temperature and humidity. Depending on this source we distinguish four types of air masses.
1.Arctic air (A) coming from 70 - 90 degrees latitude (north or south).
2.Polar air (P) coming from 45 - 70 degrees latitude (north or south).
3.Tropical air (T) coming from 15 - 45 degrees latitude (north or south).
4.Equatorial air (E) coming from 20 - 0 degrees latitude (north or south).
Air coming from sea (maritime) contains much water vapor and always carries along clouds. Air coming from land (continental) is dry and mostly without clouds. On the same manner northern wind brings cold air and southern wind warm air.
The distinction between maritime (m) and continental (c) is designated by adding to the upper-case letter of the air mass (A, P or T) a lower-case letter m or c.
When two air masses meet each other, a front originates, where wild things happen (rain and thunderstorms).: Back to TEXT MENU

BASIC TYPES OF WEATHER FRONTS
Fronts originate in areas where cold and warm air masses meet each other.We distinguish three basic types of fronts
.
a. Cold front:
Cold air pushes away warm air, this is accompanied by wind, much rain and/or thunderstorm.
b. Warm front:
Warm air pushes away cold air with less rain and wind than by a cold front.
c. Occluded or mixed front:
A warm front is overtaken by a cold front, by which also wind and much rain is expected. Before the passage of a mixed front we see the mild characteristics of a warm front and after the passage the wild ones of a cold front.: Back to TEXT MENU

CLIMATES ACCORDING TO THE GERMAN KÖPPEN
In the order of decreasing temperatures on earth:
A. Tropical rain climates.
B. Dry climates.
C. Moderate sea climates.
D. Moderate land climates.
E. Polar climates.
The letters A to E determine the principal climate. A second added letter gives a further German subdivision.
w = wintertrocken (with dry winter)
s = sommertrocken (with dry summer)
f = feucht (with precipitation in all saisons)
S = Steppe (steppe)
W = Wüste (desert)
T = Tundra (tundra)
F = Frost (frost)
We distinguish like that twelve climates on earth.
Af. Tropical rainforest climate (9% of the earth's surface)
Aw. Savanna climate (11%)
BS. Steppe climate (14%)
BW. Desert climate (12%)
Cf. Nordwest-Europe climate (6%)
Cs. Mediterranean-Sea climate (2%)
Cw. China climate (8%)
Df. Snow-forest climate without dry periods (16%)
Dw. Snow-forest climate with cold dry winter (5%)
ET. Tundra climate (7%)
EF. Snow and ice climate or sometimes separately distinguished:
EH. High mountains climate (last two together 10%): Back to TEXT MENU

COMPUTER PROGRAMME WIND AND CLOUDS

These texts are all part of the computer programme 'Wind and clouds'. The computer programme consists of several simulations and texts over our weather. The programme is made for a modern MS DOS-computer with a color monitor and works also in Windows 95, Windows 98 and 3x. The computerprogramme 'Wind and clouds' is a product from Drs. J. Nentjes, Jan Jelles Hofleane 103, 8915 HN Leeuwarden, The Netherlands. For remarks over the programme and the texts or wishes for a next version you can turn to the author by letter or E-mail. This programme is no shareware and may on no way be distributed without written permission of the author. For ordering the Dutch version 'Wind en wolken' E-mail to nentjes@worldonline.nl. The English version is still under construction.

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