Measuring rainfall: How the system works

Highlights

• Patenga recorded 1,354mm of rainfall between 6:00am on 5 July and 3:00pm on Sunday.
• Rainfall is measured by the depth of water that collects on a flat surface without flowing away.
• Bangladesh uses manual rain gauges, automatic rain gauges and Automatic Weather Stations.
• One millimetre of rainfall equals about one litre of water over every square metre.
• Heavy rainfall ranges from 44mm to 88mm in 24 hours. Higher amounts are classified as very heavy rainfall.

Days of continuous rain have brought severe disruption to Chattogram. Between 6am on 5 July and 3pm yesterday (12 July), the Bangladesh Meteorological Department's (BMD) Patenga weather office recorded 1,354 millimetres of rainfall.

As rainfall totals continue to feature prominently in weather reports, many people ask what a rainfall measurement in millimetres actually means.

Former BMD Director and former Principal of the Bangladesh Meteorological Training Institute Shadekul Alam said the concept is straightforward.

"If rain falling on a flat surface did not flow away, the height of the accumulated water would represent the amount of rainfall," he told The Business Standard. "That height is measured in millimetres under the international standard, although the United States generally uses inches."

The BMD measures rainfall using both traditional and modern equipment. The manual rain gauge is the oldest internationally recognised method. Automatic rain gauges provide continuous records, while the Automatic Weather Station (AWS) is the department's most advanced rainfall measuring system.

Mahmudul Hasan, an official at the BMD's Chattogram office, said every meteorological station has a Stevenson screen area, or observatory. It is an open and secure site without buildings or trees so rainfall can reach the measuring instruments without obstruction. The area also contains instruments used to measure temperature, humidity and evaporation.

The standard manual rain gauge consists of a 51-centimetre glass bottle placed inside an iron cylinder. The bottle measures 21 centimetres in diameter and is fitted with a funnel whose opening is ten times larger than the bottle's opening. The funnel is installed about 30 centimetres above the ground to prevent water other than rainfall from entering the gauge.

Rainwater collected through the funnel is stored in the bottle. Observers then use a calibrated measuring scale to measure the height of the collected water, which becomes the official rainfall reading.

Hasan said the automatic rain gauge uses a rotating mechanism fitted with graph paper. As rain falls, a pen marks the paper continuously, showing both the timing and amount of rainfall. The graph paper is replaced every seven days.

According to Alam, the AWS uses sensors to provide hourly digital rainfall data while also measuring wind speed, solar radiation and temperature. A built-in data logger processes information from the sensors and sends coded data to a central server, allowing rainfall information to be generated automatically every minute.

He said manual rain gauges follow a globally accepted standard and are recognised uniformly around the world. Manual gauges record rainfall every three hours, while automatic rain gauges provide hourly measurements.

The BMD also collects rainfall data from weather observation centres and more than 225 Automatic Weather Stations across the country. The information is compiled to prepare national rainfall records and weather forecasts. The Department of Agricultural Extension has also installed rain gauges in many upazilas.

Former BMD Director Asadur Rahman said rainfall intensity is classified based on the total rainfall recorded over a 24-hour period. Rainfall between 44mm and 88mm is classified as heavy rainfall, while any amount above 88mm is considered very heavy rainfall.

He said rainfall measurements also provide a simple way to understand how much water reaches the ground. One millimetre of rainfall is equivalent to about one litre of water falling on every square metre of land.

Officials also said areas receiving less than 254mm of annual rainfall are classified as deserts. Areas receiving between 254mm and 508mm can support limited vegetation, while annual rainfall exceeding 508mm is generally required for agriculture.