Explain how and why some atmospheric conditions create hazards.
Atmospheric conditions may be classified simply as being either stable or unstable. Stability is when a parcel of air rises at the DALR and may sink again to ground level. Instability is when a rising parcel of air goes up at the
DALR and on reaching condensation point rises at the SALR, which is faster than the ELR, thus causing the likelihood of rainfall.
In conditions of stability there are two situations which may cause hazards.
The first is during an anticyclone in winter. Usually in the UK anticyclones come from the European continent and are cold heavy stable air rotating (as seen from above) gently in a clockwise motion. The descending air causes calm conditions but may lead to fog or mist in the mornings, accompanied by very low temperatures which will freeze the fog. On UK’s motorways this is a lethal combination. Poor visibility and icy roads lead to motorway pile ups and it is this atmospheric condition which causes this hazard to be present.
Also, temperature inversions are common in winter during stable atmospheric conditions. Clear skies in winter evenings lead to rapid heat loss from the ground, because there are no blanketing clouds to keep in the heat.
Air which has been heated in contact with the ground the previous day rises, and as the ground cools overnight, so does the air in contact with it. The warmer air now lies above the cold air, and any smoke from car engines or chimneys, is trapped by the warmer air lying above the inversion layer. This is hazardous to health and people with breathing defects such as bronchitis and asthma may suffer badly from the accumulating air pollution.
Conditions of atmospheric instability are also likely to cause hazards to people, sometimes more dramatic than those mentioned already. On the night of January 31 st , 1953 an intense depression crossed the UK and was centered above the North Sea. The higher pressure of air to the north of
Scotland caused much sea water to enter the North Sea through between
Scotland and Norway and at the centre of the depression the water was up to
2.5 meters higher than MSL. As this depression moved to the south of the
North Sea, the “mound” of seawater moved with it, and being unable to enter the English Channel because of the constriction of the Straits of Dover, much of the seawater, coupled with a very high tide, crossed sea defences in
SE England in Suffolk, Norfolk, and Essex as well as submerging parts of
Central London. More importantly, over 3500 people were drowned in the

Netherlands that night. The storm surge created by the atmospheric condition of an intense depression created a very dangerous hazard.
Even more intense low pressure systems are called regionally typhoons, cyclones and hurricanes. The atmospheric conditions are created by air rising rapidly from the surface of a very warm sea of more than 27 degrees
C, and this very humid air cools rapidly with height. Relative humidity rises to 100% and water vapor condenses to form water droplets, kept aloft by the swirling winds within the centre of the typhoon. Continued heating of the sea surface causes thousands of tons of water to be evaporated and then condensed into the atmosphere in a hugely expanding rotating weather system. These are very dangerous when funneled towards the low lying island of the Bay of Bengal in B’desh and if the rivers from the Himalayas are also swollen with Monsoon rain, then the two water masses meet and flooding on a massive scale results. Hong Kong and Florida also regularly receive typhoons and hurricanes, but the loss of life is much less than in a
LEDC because the level of preparedness is so much more.
Thus the hazard created by the atmospheric conditions is much more intense in most LEDC’s which suffer from typhoons. Nowadays the greatest danger from typhoons in HK is landslides, but the HK Govt geotechnical department has taken many precautionary measures so that a repeat of the Po
Shan slide of 1976 is unlikely. The landslide occurred at around 8am one morning and two blocks of flats were swept away, causing the deaths of over
80 people.
A further intense low pressure system gives rise to another sever hazard, that of tornadoes. These develop in the continental interiors of USA and Russia, but we hear more about them in Oklahoma and Nebraska than in Russia. A vortex of extremely low pressure is able to sweep up into the atmosphere cars, animals, trees, houses and humans. People are well warned of approaching tornadoes and most have well constructed underground shelters, but if the tornado strikes in the middle of the night then there may be a lot of deaths, as happened in the Mid West during the summer of 2002.
A sudden rise in temperature after a heavy snowfall may be the atmospheric condition necessary to create that most dangerous of hazards, the avalanche.
Rising temperatures melt the snow, causing a lubricating layer between the new snow and the old snow. This can cause great danger and is a very common occurrence in the European ski resorts such as Davos and

Chamonix. Chamonix was devastated by an avalanche in 2002, in spite of a great deal of precautionary measures such as snow fences and small avalanches being triggered to stop a major one.
So it can be seen that atmospheric conditions can create many and varied hazards for people, more in LEDC’s than MEDC’s.