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Thermometer
 Thermometer – This maximum–minimum thermometer records the highest and lowest temperatures of the day. The tube contains two small metal markers that are pushed up by the mercury as it moves around the U-shaped tube. The magnets are held in their farthest positions by a magnet behind the scale.
Metal marker
Thermometer
An instrument for measuring temperature. A number of different kinds of thermometer are used for recording the temperature of air in weather observations. Most commonly,
a pair of thermometers is used to read the maximum and minimum temperatures that occur during
a day.
Because temperature is affected
by the speed of air and the moisture in the air, different
pairs of thermometers are used to measure the temperature in dry air and in wet air. They are called dry bulb and wet bulb thermometers (see: Wet bulb temperature). The wet bulb thermometer is wrapped with a small piece of muslin cloth, one end of which is dipped in water. As water evaporates from the muslin, it pulls heat from
the bulb of the thermometer and the apparent temperature falls. The wet and dry bulbs read the same temperature only when the air is saturated with water, for example, when it is raining.
 Thunderstorm – A thunderstorm is
usually marked by very dark skies because thunderstorms result from great thicknesses of cloud that effectively block out sunlight.
Thunder
A loud, often rumbling sound that accompanies some severe storms in which lightning occurs.
A flash of lightning heats the
air to about 30,000°C. This makes the air expand violently, sending shock waves outwards that become a sound wave. It takes about three to four seconds for the sound of thunder to travel one kilometre,
so it is possible to estimate the distance of the lightning flash by finding the time between seeing the flash and hearing the thunder.
The reverberating nature of thunder is caused by the sound waves echoing off the layers of the thundercloud (see: Thunderstorm).
Thunderstorm
A storm that is localised to a single thundercloud, which is linked with tall cumulonimbus clouds. The constant, rapid movement of air
up and down inside such clouds is responsible for the formation of lightning, thunder and large raindrops or hail.
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Tornado
An extremely violent, tightly spiralling column of air that reaches down from the bottom of a
giant thundercloud. Originally the Spanish word for thunderstorm, tornadoes are violently spinning funnels of air that follow the base of severe thunderstorm clouds. Tornado winds can reach over 400km/h or more. A tornado over water sucks water from the ocean and is called a waterspout.
 Tornado – A tornado darkens the skyline. Notice the scale given by the utility poles on the right.