Measuring the weather
Rain Gauge
A rain gauge measures the amount of rain that has fallen. This is a metal or plastic container at the top of which is a funnel. Inside the container is a glass cylinder marked in millimeters. Rain falls on the funnel and collects in the cylinder where it is measured every day. Wind vane
The simplest weather- measuring device is the wind vane. A metal arrow with a ‘tail’ is mounted on a pole or roof so that it will turn around. It points in the direction from the wind is blowing.
A thermometer measures the temperature. An ordinary thermometer, such as one we may have at home, shows the temperature at the moment we are looking at it. For weather purpose, we really need a maximum-minimum thermometer. This instantly shows us the highest and lowest temperatures since the thermometers were reset- usually every day. It has a U-shaped tube and the mercury moves up and down both ‘arms’. Two small pieces of steel float on top of the column, of mercury are called arms. As the mercury goes upwards in the hot column, it pushes its piece of steel up. When the mercury in the hot side goes back down- i.e. the temperature falls – it leaves the steel pointer behind, so that we can see the highest point it reached.
The ‘cold’ column does the same, except when the temperature rises, the little steel marker is left behind to show the lowest point it reached. The steel markers are reset to sit on the column of mercury, by pulling them down with a magnet.
This measures the weight of the air, and is probably the most important instrument for forecasting the weather. There are two kinds of barometers:
· The aneroid barometer
· The mercury barometer
The aneroid barometer
This looks a bit a clock, and can be carried about easily. It shows the weight of the air on a dial. Inside the case is a collapsible airtight box of thin metal with most of the air pumped out. As the air pressure changes, it pushes the sides of the box in or out. The end of the box has a strong attached to it, which works the dial.
The mercury barometer
This is glass tube, about a meter long which is close at one end. It is filled with mercury and the open end is placed in a bowl of mercury. This acts as a kind of scale. The weight of the air pushes the mercury up and down, so that we can see how heavy the column of air above it is. The average height is about 760 mm. The mercury barometer is fragile and can not be moved easily.
No comments:
Post a Comment