13．Acid rain is a very serious environmental threat faced by our planet today．Millions of people get affected by it on a daily basis．This problem is more serious in the northern hemisphere （半球） where there's a huge growth in the number of industries．Winds absorb more and more quantities of harmful gases produced by various kinds of industries．Acid rain occurs when high levels of sulfur dioxide and nitrogen dioxide enter in the atmosphere and start chemical transformation．These harmful gases are absorbed by water droplets in the clouds and they fall on the earth during rains．They can damage the natural soil by increasing the acidity levels and pollute rivers and lakes．Dead and weakened trees are a very common sign of acid rain，while in cities worn off or scarred buildings are also a result of acid rain．
Acid rain is actually a broad term which is used to describe different types of acid that fall from the atmosphere on earth．It's mainly divided into wet and dry．Parts of the world which experience wet weather have acid rain，acid snow and fog．Other parts which experience dry weather for most of the year experience acid gas and acid dust．
One of the main causes of acid rain is a sudden drop in the pH levels of water and that happens because of absorption of harmful gases by the atmosphere．The best way to bring back the pH balance of water is to limit the use of items which run on petrol．Instead use products which run on electricity．
Another great thing that all people can do is to learn how to drive cars efficiently．To reduce smoke emission from cars，drivers should ease up as soon as the yellow light rather than slamming on the brakes and they shouldn't rush away as soon as the lights turn green．These small practices help reduce the wastage of petrol．
Limit the use of buses or cars for small things，people can also make use of cycles or walk to cover small distances．
Another great tip which almost all of us can practice is to always keep the vehicle tyres full of air，which will provide users with a better fuel economy．
Turn off your lights，computers and other appliances when not in use．
Learn about the different sources of energy besides fossil （化石） fuels and try to use them in daily life．Many nations like France and Germany are using electrical，nuclear，wind and solar energy on large basis．
Acid Rain
| Problem | Acid rain （71）affects/influences millions of people and becomes a serious environmental threat to our planet today． |
| Occurrence | ●Harmful gases （72）from industries go into the atmosphere and chemical transformation happens．
●Water droplets in the clouds that have taken in the harmful gases come to the （73）earth/ground when it rains． |
| Results | ●It damages the soil and causes water （74）pollution as well．
●It can weaken and even （75）kill/ruin/destroy trees，and will damage buildings in cities． |
| （76）Types/Kinds/Sorts/Categories | ●Wet-in the form of acid rain，acid snow and fog．
●Dry-in the form of acid gas and acid dust． |
| Prevention | ●Use more electricity than petrol．
●Drive vehicles in an （77）efficient way．
●Go to places （78）nearby on foot or by bike．
●Keep the vehicle tyres full of air all the time．
●Turn off any electrical device the moment you stop （79）using it．
●Try to （80）replace fossil fuels with other forms of energy，such as those produced from wind and the sun，wherever possible． |
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