Inside the Observatory: How Hong Kong's weathermen make their predictions

The Hong Kong Observatory's central forecasting office occupies the entire seventh floor of its offices in Tsim Sha Tsui. It's a large space dominated on one side by banks of television monitors displaying real-time images of weather events captured by satellite and radar, along with other meteorological information. Coloured charts showing changes in temperature and wind patterns cover the opposite wall.

This is where scientific officer Lee Kwok-lun and his fellow meteorologists crunch numbers and analyse satellite images to tell us what we can expect from the next few days: sunny periods with isolated showers and thunderstorms, with temperatures ranging between 28 and 33 degrees Celsius.

A keen outdoorsman who spends much of his spare time hiking or diving, Lee takes a personal as well as professional interest in weather prediction.

"I like this work very much because I can have more information to have safe and fun trips," he says.

But the weather forecasters' job is usually a thankless one; no one gives them credit when they get it right; and when predictions are off base, they face a storm of condemnation.

One example saw workers sent home following the raising of a No 8 typhoon signal, only to have relatively light rain fall on the city. Another time, meteorological data called for a No 3 typhoon signal, but much more severe conditions hit the New Territories and parts of Lantau.

Still, technology has made their job much easier.

The Observatory has begun issuing nine-day forecasts, up from seven days in the past. They found, after a year of trials, that accuracy of predictions for eight to nine day periods was about the same as those of six to seven days.

In the summer, Lee says, forecasts are updated much more frequently throughout the day because factors affecting weather in localised areas can develop very quickly.

"During summertime, rainfall is the major factor; clouds can evolve into a rainstorm in a short time and in a small area … the predictability is much lower, mainly due to the low pressure systems developing at this time of year."

Small areas of low pressure, or troughs, result from columns of rising air; the air cools as it rises, moisture condenses to form clouds and precipitation. Rain patches are localised and it becomes difficult to predict where the troughs are moving, Lee says.

In the winter, when Hong Kong is under the influence of the northeasterly monsoon, an enormous cold air mass builds from Siberia all the way south to the coast of Guangdong. It forms over land, where there are many weather stations to collect data to run simulations for the super computers.

"With such a large weather system, we can normally predict accurate weather on a longer time scale," says Lee.

Despite better weather data collection and greater computing power, accurate forecasting remains an enormous challenge because meteorologists are dealing with such a vast and complex system.

Besides tracking what is going on in billions of cubic metres of air in the atmosphere, they must take into account ground and sea temperatures, ocean currents, geological formations and the like.

Even for short-term predictions, they must consider myriad interactions of temperature, clouds, wind and atmospheric pressure.

The Observatory's scientific officers develop their forecasts each day using data from weather stations (the two largest are at King's Park and the Observatory), where sensors provide readings on factors such as temperature, atmospheric pressure, humidity and wind speed. Combined with examination of radar and satellite images, the two forecasters on duty meet their supervisor and senior colleagues to finalise analysis of incoming information before issuing twice daily predictions.

The meteorologists sometimes get flak for issuing warnings for heavy storms that many fail to experience, but these typically refer to rains in a specific area or a system that is moving.

"Thunderstorms can move quickly, along an irregular path, and increase or decrease in a short time," Lee says

They have developed a smartphone app called MyObservatory that gives more localised information - for instance, whether Tai Po residents might expect showers in the afternoon, including where lightning might strike.

"That way you can get a better idea of where the rain may fall. Swimming pools, beaches and people who do water sports use our localised rainfall [forecast] system."

Summer ushers in the typhoon season. The Observatory is expecting fewer than six this year. An average of six tropical cyclones sweep into Hong Kong annually but with El Nino forming over the central Pacific, Lee expects there will be perhaps four, as trade winds weaken over the warm ocean, resulting in less rain across Southeast Asia.

All the same, meteorologists must closely examine satellite pictures of cloud bands forming over the South China Sea and northwest Pacific to see if they may develop into a low pressure system.

Scattered clouds can coalesce into a pattern resembling a tropical cyclone, so they examine the cloud features to estimate its strength. Then, gathering data from weather stations from around the world, they feed the information into super computers that use numerical modelling to run simulations to forecast cyclones.

"Whether we raise a tropical cyclone warning or switch to higher signal depends on the force of wind and other scientific information," Lee says.

"Data about wind speed is recorded at eight reference stations across Hong Kong. There's an anemometer [for measuring wind speed] at each station, which can predict if winds will get stronger over the next few hours. We don't need to wait until the winds actually get stronger to issue the signals."

So when a No 3 signal is issued, the public is told to expect significantly stronger winds within 12 hours; that's to give people enough time to make necessary preparations.

There have been discussions about modifying the system in response to complaints, but because so many institutions from businesses to schools and hospitals use the Observatory's tropical cyclone signals, Lee says, it would be difficult to change significantly.

Although some people believe that the Observatory issues typhoon signals with employers' convenience in mind, their decision is entirely based on available data, with public safety as the key consideration, Lee says.

"If we expect the wind to hit at 7am, we will issue warnings well before then. But if we expect the winds to strengthen at, say, 10am, do we let people go to work first? Of course, we have to issue before they leave home.

"In spite of all the advanced technology, we can't be 100 per cent accurate. We base our assessments on the data on hand at the time, but it's only forecasts," he says.

"We can have insufficient data and there can be a lot of short-term changes to the cyclone path. Changes in a tropical cyclone's direction, wind speed and strength can be caused by interaction with the land and by changes in temperature in the sea [along the path of the cyclone]. So there can be divergence from our expectations."

Professor Johnny Chan Chung-leung, a typhoon expert and dean of the School of Energy and Environment at City University, says that technology to interpret weather data has improved tremendously compared to two or three decades ago.

"Previously we wouldn't know there was a typhoon until it was formed. Today some models are able to predict the formation of a typhoon a few days to a week ahead. There can be false alarms but, in many cases, the model can predict the formation of tropical cyclones before they hit us," Chan says.

Typhoons move in an anti-clockwise direction and their impact varies depending on where they make landfall.

A typhoon hitting the east of Hong Kong is relatively straightforward, but a typhoon that hits the west side can create complications.

"Because the typhoon is going anti-clockwise, the winds will be coming up from the ocean … Hong Kong Island and the eastern part of Kowloon form a channel that forces wind through - all of that makes the wind become stronger."

Examining historical data, Chan believes typhoons go through 30-year cycles, noting that the number of storms forming or entering the South China Sea that could affect Hong Kong has been decreasing in the past 15 years.

"They peaked in the mid-60s and the mid-90s, but numbers have come down since then."

If the trend holds, the next peak for typhoons will be in the 2020s, he says.

Projections by the UN's intergovernmental panel on climate change suggested that the number of tropical cyclones forming annually would decline from the global average of 80, but that's not a reason to let our guard down.

"There could be fewer cyclones, but they may become stronger. We have had Haiyan and Sandy, and those were two very strong storms."

More than a few climate geeks turn to organisations such as Weather Underground for alternative forecasts. Started 19 years ago, the Hong Kong chapter, which has 78,000 Facebook followers, is run by five self-taught meteorology enthusiasts.

Jasmine Fong Laam, spokesperson of the group, says they develop forecasts using Observatory data along with information from its weather stations in Prince Edward, Tai Po and Aberdeen.

"Weather is a chaotic thing. Because of its unpredictability, no forecast - official or otherwise - is always accurate. Even a supercomputer can produce a busted forecast."
 

Take the weather with you

The MyObservatory smartphone app provides detailed information on weather changes. Besides displaying current conditions and a nine-day forecast, it includes useful features such as:

Location-based rain - provides half-hourly rainfall predictions for your area.

Lightning location - indicates where lightning strikes are occurring.

Storm Track - gives the predicted course of tropical cyclones.

Radar images - see where rain bands are and in which direction they are moving. When the patches are yellow, rain is heavy, blue is light rain.

Weather photos - images taken at various locations in the city to give users an idea of conditions at a given destination.

Astro & tide info - timetable for high and low tides in areas such as Cheung Chau, Ma Wanand Shek Pik, useful for water-based activities.

bernice.chan@scmp.com