the weather
Written by: Hong Kong Observatory
2020-06-10 08:00
Last update date: 2020-06-10 08:00Stepping into May, after several hot days, and then being baptized by the continuous showers brought by the trough of low pressure, the summer breath gradually became stronger. In the rainy season, the heavy rain that comes in the morning is always unpredictable. Even if the citizens who have already started to go to work and bring their umbrellas, they will be drenched by the violent wind and rain, and they will be embarrassed.
(The original text is taken from the Observatory's website, headline, introduction and text, which have been edited and revised by Hong Kong 01)
Watching the process of thunderstorms along the coast of Guangdong recently on night shifts, I vaguely remembered such an interesting thing: When I was woken up by thunder in the middle of the night when I was a kid, I would go to the living room to turn on the TV and watched inexplicably. A black and white Cantonese feature film, paying attention to the rainstorm warning sign in the corner of the picture, until it turns into a red rainstorm, before going to sleep with peace of mind (otherwise you have to doze off and go to school). I had a myth at that time. The thunder coming late at night was the prelude to the suspension of classes? Did the heavy rain appear in the morning?
▼▼▼Before and after the black rain, the number of cloud-to-ground lightning in Hong Kong exceeded 14,000 times▼▼▼
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In past statistics (1992 to 2010), the red and black rainstorm warnings were effective in the morning (00 to 12 o'clock) 1.5 and 2.5 times the afternoon (12 to 24 o'clock), respectively. From 2000 to 2019, based on the time when each rainstorm issued red and black rainstorms for the first time, the number of issuances in the morning was about 2 times and 1.5 times that in the afternoon. From the statistics, the chance of severe rainstorm in the morning seems to be greater.
▼▼▼ Related photos : How to use the “Predicted Rainfall Forecast” at the Observatory App▼▼▼
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Let's start with the monsoon. The monsoon is a low-level atmospheric airflow caused by the temperature difference between the ocean and the land. Every year in May or June, the prevailing wind in Hong Kong will change from the northeast monsoon from the land to the southwest monsoon from the tropical ocean. The warm and humid characteristics of the southwest monsoon provide favorable energy and water vapor conditions for strong convection.
Heavy rain is a strong convective weather, but the trigger mechanism that appears in the morning, there is still no consensus in the academic community. Studies have pointed out that at night, the warm and humid southwesterly winds on the ocean surface and along the coast keep the lower atmospheric layer relatively warm, but the cloud top will cool down due to radiant cooling, and the colder the night, the wider the temperature difference between the lower atmospheric layer and the cloud top. Atmospheric instability.
The radiant cooling of the cloud tops at night will increase the instability of the atmosphere, and the southwesterly winds will converge on the coastal areas, making it easier for warm and humid air to rise. (Provided by Hong Kong Observatory, Hao Mengqian assisted in drawing pictures)
In addition, the accumulation of southwesterly winds along the coastal areas also helps warm and humid air to increase convection. As this mechanism of triggering convection decreases during the day, the showers along the coast will generally weaken. At the same time, inland areas heat up faster than oceans during the day, and high temperatures trigger convection, which explains why showers in inland areas occur more often in the afternoon. Of course, the conditions that trigger or exacerbate strong convection also include high-altitude divergence and vertical wind shear, but relatively speaking, these factors are less affected by inter-day changes.
Although the rainstorm in Hong Kong occurred in the morning in the morning, the public should not be taken lightly at other times. Thunderstorms are developing rapidly and often come uninvited! Since I was a child, I was expecting the change of rainstorm warning, and now I am always on duty in the forecast center, monitoring the development of the rain area and the changes of the situation every minute, every second. It is a kind of growth, and it also adds a sense of responsibility and mission.
▼▼▼How to use the lightning position function of the Observatory App ▼▼▼
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Original: Back to school first rain? Author: Hong Kong Observatory Zhuang people promise
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