English  中文版  NCHU

Measurement of
surface wind
Measurement of
Measurement of
air pressure
Measurement of humidity Measurement of

 Measurement of surface wind

Cup anemometer and wind vane
How are winds measure?

Wind speed is measured by an anemometer.
Wind direction is measured by a wind vane.
Winds cause the cups of an anemometer to rotate.
The higher the wind speed, the faster the rate of rotation.
Wind speed is then determined based on the rate of rotation of cup anemometer.
A wind vane follows the wind flow. Its bearing gives the wind direction

 Measurement of precipitation

Ordinary rain-gauge

tipping bucket rain-gauge
How is rain measured?

A rain-gauge is used for measuring rainfall.
A conventional rain-gauge consists of a funnel-shaped collection fitted with a container underneath.
Using a measuring cylinder, the weather observer measures the amount of rain collected in the container.
A tipping bucket rain-gauge measures rainfall automatically.
The tipping bucket rain-gauge has two buckets inside but at any one times, only one collects rainwater. When one bucket is filled, it tips under the weight of the rainwater, empties the bucket, and sends off a signal. The rainfall is then determined by counting the number of tips. One tip normally corresponds to 0.5mm of rain

 Measurement of air pressure
Mercury barometer
How high is the air pressure?

Air pressure is usually measured by means of a mercury barometer.

A mercury barometer consists of an evacuated glass tube immersed in mercury. Air pressure forces the mercury up the glass tube. The higher the pressure, the greater the height of mercury column.

 Measurement of humidity

Dry-bulb and wet-bulb thermometers
How humid is it?

Humidity is a measure of how much water vapor is in the air.
Relation humidity is obtained from the dry-bulb t and wet-bulb temperatures.
The dry-bulb thermometer measures the air temperature.
The wet-bulb thermometer has its bulb wrapped with a piece of cloth, the end of which is immersed in water.
As water evaporates, the wet-bulb cools. The drier the air, the lower the wet-bulb temperature. The relative humidity is determined from the difference between the dry-bulb and wed-bulb temperature.
The dry-bulb and wet-bulb thermometers are usually housed inside a Stevenson Screen. This allows free movement of air and protects the thermometers from direct sunshine and rain.

 Measurement of temperature

Mercury-in-glass thermometer
How hot or cold is it?

Air temperature is measured by a mercury-in-glass thermometer.

Mercury expands when heated. Thus when the temperature rises, the mercury column inside the column expands and the mercury level rises. Conversely, when the temperature drops, the mercury level falls.

TEL:886-4-2284-0381   FAX:886-4-2287-6851   
ADD:145 Xingda Rd., South Dist., Taichung City 402
Copyright © 2007-2012 Department of Soil & Water Conservation    Powered by KYECC+Yabee