♦ PhD. Nguyen Quang Anh

(Continued from Part 2)

River and Lake Terrain of Hanoi

thuongThe boundaries of the expanded Hanoi region are bordered by the Da River to the west and the Red River to the north. Within this area lie the Tich, Day, Nhue, and Bui rivers, along with numerous lakes. For this reason, many researchers, both domestic and international, consider Hanoi to be a “city of rivers and lakes.” However, most natural ponds and lakes have been filled in or modified, leaving only traces or lakes that have been transformed. The most prominent criterion that makes Hanoi’s rivers and lakes a valuable resource is their scientific value (particularly for natural rivers and lakes). In contrast, artificial lakes embody a combination of the various values mentioned above.

The Red River (Chuong Duong and Long Bien Bridge Area)

thuongArtificial Lakes: Most artificial lakes are located in the western part of Hanoi, where the terrain consists of hills and limestone mountains. These lakes generally have relatively large surface areas. Suoi Hai Lake covers about 700 - 800 hectares under normal conditions (and can expand to 1,200 hectares when reaching the maximum designed water level at an elevation of 24.85 m). The Quan Son - Tuy Lai lake system covers approximately 700 hectares. These lakes were constructed in the early 1960s with the primary purposes of supporting agricultural production and flood control. After more than half a century of existence, they have provided multiple benefits to the lives of local residents. Their roles in regulating hydrological regimes, moderating the microclimate, reducing floods, and serving agricultural production possess both scientific significance and practical value. The lakes themselves also function as cultural works created by humans. The Quan Son - Tuy Lai Lake has been dubbed the “Inland Ha Long Bay of Hanoi”, as it contains nearly 100 “islands” and several peninsulas, enhancing its aesthetic value. This beauty is further enriched by its wetland ecosystem functions, supporting many aquatic species such as fish, algae, lotus, and water lilies, etc. All of these values are of great importance for the development of tourism and aquaculture, generating high economic returns.

thuongNatural Lakes: Hanoi has a large number of natural lakes with very different sizes. These natural lakes are geographical entities and also important resources serving the city’s socio-economic development. In addition to providing water and aquatic biological resources, natural lakes also possess scientific, cultural - historical values and play a key role in maintaining ecosystems. From a scientific perspective, natural lakes in Hanoi are all of fluvial origin and were formed in two main ways: They are either river bends (also known as oxbow lakes), such as West Lake (Ho Tay), Linh Dam Lake, Dam Vien (Binh Minh commune), and lakes in Hat Mon commune; Or they are abandoned river channels (also called paleochannels or ancient riverbeds), such as Hoan Kiem Lake, Van Tri Lagoon (Phuc Thinh commune), Yen My Lake (Thanh Tri commune), and a series of lakes in Phuc Loc commune. Based on the distribution of these lakes, it is possible to reconstruct former river courses, especially the Day River.

thuongCultural - historical values: These values are most clearly recognized in the cases of West Lake (Ho Tay) and Hoan Kiem Lake.

thuongThe scenic beauty of West Lake and Hoan Kiem Lake has long been an inspiration for poetry, music, and painting, both in the past and today. West Lake became one of the favorite recreational and sightseeing destinations of kings and mandarins. On many such excursions, the lake was often shrouded in mist, making the scenery even more mystical and enchanting; as a result, the lake was once renamed Dam Dam. 

Climate resources

thuongHanoi’s climate clearly reflects the characteristics of the Red River Delta in the Northern climatic region - a tropical monsoon climate with a cold winter. This also gives Hà Nội a climate with four distinct seasons.

thuongLocated close to the Tropic of Cancer, the total annual solar radiation in the Hanoi area reaches 122.8 kcal/cm²/year, which is considered a moderate level in Northern Vietnam. Throughout the year, this radiation is unevenly distributed, being high during the summer - autumn months (10 - 14 kcal/cm² per month) and low during the winter - spring months (6 - 8 kcal/cm² per month). Closely related to solar radiation is sunshine duration. The total number of sunshine hours in Hanoi is approximately 1,450 - 1,530 hours per year. The summer months receive abundant sunshine, with about 5 - 6 hours of sunshine per day, while the winter months have less sunshine, with only about 2 - 3 hours per day. The period with the least sunshine coincides with the drizzle season in Northern Vietnam, which is also the time of the highest cloud cover of the year.

thuongAs a result of the Southeast Asian monsoon circulation and the characteristics of local atmospheric circulation in the Northern Vietnam region, the wind regime in Hanoi has the following features. In winter, northeasterly, northerly, and northwesterly winds prevail, among which the northeasterly wind plays a dominant role (with a frequency of approximately 23.6% - 27.3% during the months from December to February) and has a relatively high average speed of about 3.1 - 3.2 m/s. In summer, southeasterly and easterly winds are predominant, with the dominant wind being the southeasterly wind (with a frequency of 40.9% - 47.6% during the months from March to May) and an average speed of approximately 2.8 - 3.0 m/s.

thuongRegarding wind speed: While the average wind speed in the Hanoi area is relatively low, at only about 1.1 - 2.0 m/s, the maximum wind speed ever observed in Hanoi can reach 24 - 34 m/s.

thuongA notable feature of the thermal regime in the Hanoi area is the decrease in temperature during the winter months and with increasing elevation in the hilly and mountainous areas to the northwest and north. The average annual temperature in the lowland areas is generally about 23.3 - 23.7°C, ensuring a cumulative temperature total of approximately 8,500 - 8,600°C. In the low mountainous areas to the north and northwest of Hanoi (such as Soc Son and Ba Vi), temperature decreases markedly with elevation. At elevations of 500 - 700 m, the average annual temperature is about 19 - 20°C, corresponding to a cumulative temperature total of approximately 7,000°C. At elevations of 900 - 1,000 m, the average annual temperature drops to around 18°C, and the cumulative temperature total decreases to about 6,500°C.

thuongIn summer, from May to the end of September, the weather is hot, with an average air temperature of approximately 27.9 - 28.3°C. In winter, from November to March of the following year, the weather is cool to cold, with an average temperature of about 18.8°C. During winter, most of the lowland areas of Hanoi experience 2 - 3 cold months, during which the monthly mean temperature falls below 18°C. In the hilly and mountainous areas to the north and northwest, winter temperatures are even lower due to the influence of elevation, and the number of cold months in these areas can reach 4 - 5 months.

thuongObservation of the long-term variation (1930 - 2012) in the annual mean air temperature at Lang Station shows that the highest recorded annual mean air temperature was 25.1°C (in 1998), while the lowest was 22.8°C (in 1934). Although the annual mean temperature fluctuated over time, the overall trend over nearly a century has been an increase, and the trend line is expressed as: Y = 0.0125 × x + 23.152.

thuongThe absolute maximum and minimum temperature records observed across the entire Hanoi area show that the highest air temperature recorded was 42.8°C at Lang Station in May 1926, while the lowest recorded temperature was 2.7°C, also at Lang Station, on January 12, 1995.

thuongThe annual temperature range in the Hanoi area is approximately 12.5 - 12.8°C, while the average diurnal temperature range is relatively small, at only about 6.1 - 6.6°C.

thuongLike many other places, the Hanoi area has a tropical summer monsoon rainfall regime. The annual total rainfall in the lowland areas ranges from 1,500 to 1,900 mm, with approximately 140 - 157 rainy days per year. In the northern low hill areas (Soc Son) and the northwestern mountainous areas (Ba Vi), the annual rainfall increases rapidly, reaching 2,000 - 2,400 mm per year, depending on location. The rainfall regime shows a relatively clear seasonal differentiation: the rainy season lasts six months, from May to October, while the less rainy season occupies the remaining six months, from November of the previous year to April of the following year. The degree of climatic dryness in Hanoi is not pronounced; during the less rainy season, most areas experience only about two dry months (with monthly rainfall r < 25 mm).

thuongOver more than a century (1890 - 2012), the annual total rainfall in the Hanoi area has fluctuated. Taking Lang Station as an example, the year with the highest annual total rainfall recorded 2,536.0 mm (in 1994), while the year with the lowest annual total rainfall recorded 1,033.1 mm (in 1988).

thuongOverall, over more than a century, the annual total rainfall shows a slight decreasing trend, and the trend equation is expressed as Y =  - 0.5908 × x + 1718.

thuongLong - term data indicate that record daily rainfall amounts recorded were 586.8 mm/day at Lang Station in July 1902, 554.6 mm at Ba Vi on July 24, 1980, and 508.0 mm at Son Tay on July 14, 1971.

thuongThe annual mean relative humidity in the Hanoi area is approximately 83 - 85%, which is considered moderate for northern Vietnam. Although humidity does not vary greatly throughout the year, it is noticeably higher during the drizzly season in northern Vietnam (March and April), reaching about 86 - 89%. The period with the lowest humidity occurs in late autumn and early winter (November and December), when relative humidity drops to around 79–82%.

thuongAlthough dry days may still occur in the middle of the rainy season (July and August), the absolute minimum relative humidity can drop to 35 - 40%. During the dry season, absolute minimum humidity values are even lower, ranging from 15 - 25%. Notably, the lowest value recorded across the entire region was a relative humidity of 16%, observed at Lang Station on January 16, 1932.

thuongAs a consequence of the radiation regime, sunshine, rainfall, and wind patterns, annual evaporation in the Hanoi area ranges from 770 to 900 mm per year.

thuongIn the Hanoi area, the following special weather phenomena may occur:

thuongFog is a special weather phenomenon that may occur during the winter months in Hanoi, with approximately 8 - 15 foggy days per year.

thuongFrost occurs in the Hanoi area mainly in mid - winter (December - January) with a very low frequency, possibly only a few occurrences over a hundred years.

thuongDrizzle commonly occurs in Hanoi in late winter and early spring, with an average of about 7 - 30 drizzle days per year, depending on location.

thuongThunderstorms occur on average 75 - 90 days per year in Hanoi. The early summer and rainy season (May - August) usually experience the highest number of thunderstorm days.

thuongStatistical data from more than the past three decades indicate that the areas in Hanoi most frequently affected by tornadoes are Dong Da and Son Tay (including parts of the city center and the Ba Vi - Son Tay area), each experiencing six events, with an average of about one event per year, and in some years up to two events. Other areas such as Phuc Tho (part of the Phuc Tho-Ha Dong area) have experienced two events over two years, while communes and wards in Soc Son, Tu Liem, Ba Vi, Dang Phuong, My Duc, and surrounding areas (including parts of the city center; Me Linh - Dong Anh - Soc Son; Ba Vi - Son Tay; and Phuc Tho - Ha Dong) have experienced only one event over more than 30 years.

thuongIn the Hanoi area, the annual average number of hail days ranges from 0.06 to 1.1 days per year. Hail most commonly occurs during March to May, which is the transitional period between the end of the dry season and the beginning of the rainy season.

thuongStatistical data over the past more than 50 years indicate that the frequency of typhoons/tropical depressions affecting the Hanoi area is approximately 1.67 events per year. The typhoon season may begin as early as June in early - onset years and may extend as late as November in late-ending years. Peak typhoon activity occurs during July to September, with a markedly higher frequency than in the other months of the typhoon season./.

 ( To be continued...)