Snow is formed when temperatures are low and there is moisture – in the form of tiny ice crystals – in the atmosphere. When these tiny ice crystals collide they stick together in clouds to become snowflakes. If enough ice crystals stick together, they’ll become heavy enough to fall to the ground. Precipitation falls as snow when the air temperature is below 2°C. It is a myth that it needs to be below zero to snow. In fact the heaviest snow falls tend to occur when the air temperature is between zero and 2°C. The falling snow does begin to melt as soon as the temperature rises above freezing, but as the melting process begins, the air around the snowflake is cooled. If the temperature is warmer than 2°C then the snowflake will melt and fall as sleet rather than snow, and if it’s warmer still, it will be rain. The size and make up of a snowflake depends on how many ice crystals group together, and this will be determined by air temperatures. Snowflakes that fall through dry, cool air will be small, powdery snowflakes that don’t stick together. This ‘dry’ snow is ideal for snow sports but is more likely to drift in windy weather. When the temperature is slightly warmer than 0°C, the snowflakes will melt around the edges and stick together to become big, heavy flakes. This creates ‘wet’ snow, which sticks together easily and is good for making snowmen.

Which of the following statements about snow match the information in the passage?

a) Snow is formed from ice.
b) Falling snow melts because of warm air around it.
c) Subzero temperatures are required for snow to fall.
d) Sleet develops at less than two degrees centigrade.
e) Dry snow falls in colder temperatures than wet snow.

Water for public supply can be obtained from underground sources by wells sunk into aquifers, or from surface sources, such as purpose-built reservoirs or lakes (collecting rainwater run-off or water from streams) and rivers. The safety of the water is of utmost concern – several million people die each year after consuming contaminated water. The primary aim in water treatment is the elimination of any pathogenic micro-organisms present. All the above-mentioned sources can be subject to pollution.

In the case of underground water, polluted surface water can enter the saturation zone of an aquifer and so lead to its contamination. Pollution can come from waste containing heavy metals and organic compounds, farm run-off containing pesticides, and industrial wastes which may have been deliberately dumped down old coal mine shafts. River water can be affected by farm drainage, sewage works and industrial effluents, and also the run-off water from roads. Thus there is a need to maintain the quality of the aquatic environment to ensure that the water is suitable for treatment for public supply, and that the cost of treatment is kept as low as possible. Raw water is usually abstracted from a river and pumped to a reservoir for storage and settlement. In the reservoir, the number of bacteria is reduced through natural processes, such as ultraviolet radiation from sunlight. Also, a large portion of the suspended solids settles out. The water is then conveyed from the reservoir to a treatment works.

Which of the following statements are true according to the information in the passage?

a) There are insufficient sources of fresh water for human consumption.
b) Industrial pollution can affect both underground and river water.
c) There are numerous means by which water can become impure.
d) Rain is a safer source of water for human consumption than aquifers.
e) Water in reservoirs is chemically treated to remove harmful bacteria.