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  • The unsettled weather

  • Friday, 9th September 2016

In the weather report issued last weekend, this week's weather was expected to be unsettled. Do you know what "unsettled" means here?

"Unsettled" actually means that the weather can be rather changeable or can vary a lot. It may not be rainy all day long, but under suitable conditions, rain or even thunderstorms can occur. In meteorology, there is a term known as atmospheric instability. One of the ways weather forecasters analyse instability is to study the vertical variation of temperatures and the humidity of the atmosphere, which are commonly represented by the well-known K-index. It can be calculated based on upper-air measurement data, and numerical weather prediction model output from computer can also provide forecasters with the predicted values for the next few days. The K-index consists of three parts: (1) temperature lapse rate, which is the extent of drop in temperature with height; (2) moisture content at lower atmosphere; and (3) the degree of moisture saturation at the middle atmosphere. Generally speaking, K-index is higher for larger temperature lapse rate (i.e. higher temperature near surface and lower temperature aloft), higher moisture content at lower atmosphere, and closer to saturation of moisture at middle atmosphere. A high K-index means that convective activity can easily be triggered, resulting in precipitation or thunderstorms.

The Observatory operates an Automatic Upper-air Sounding System and launches sounding balloons twice a day to obtain upper-air meteorological observation. Take the sounding data in the morning of 5 September this year as an example (Figure 1), the near surface temperature was high and the temperatures aloft dropped rapidly with height (red circles in Figure 1). The warm air near the surface rose easily due to its lower density, resulting in convective activity. Besides, the sounding data showed a small difference between air temperatures and dew point temperatures, which meant that the humidity of the atmosphere was high and there was abundant moisture content for producing precipitation. In general, thunderstorms can be triggered when K-index reaches 33, and the index calculated from this set of sounding data was 37. In fact, there were showers and thunderstorms in Hong Kong on that day, bringing 30 millimetres of rainfall generally over the territory. Rainfall even exceeded 70 millimetres over parts of the territory (Figure 2).

Figure 1

Figure 1      The upper-air sounding profile in the morning of 5 September 2016. Small difference between air temperatures and dew point temperatures indicates higher humidity and near saturation in the atmosphere.

Figure 2

Figure 2      Distribution of total rainfall on 5 September 2016.

Although the atmosphere is unstable, we still need triggering mechanism to initiate precipitation and thunderstorms. This includes, for instance, a trough of low pressure or an upper-air disturbance near Hong Kong, which can enhance the uplift of air. There are other factors like mesoscale or small-scale weather systems and orographic effect, which are also considered and analysed by weather forecasters. From now on, when you learn from the Observatory that unsettled weather is expected in the next few days, remember to stay tuned to our more updated and detailed weather assessment!

Stephen P.W. Lau     L.S. Lee

Last revision date: <23 Jan 2017>