Rain days vs millimeters: why the rainfall number misleads

Rain days vs millimeters can tell different stories about the same month. We compare station tables from Vietnam's construction regulation, where averages describe a typical month, not a forecast.

Millimeters and rain days are different columns

Rain in millimeters describes the amount recorded in the monthly table. Rain days describes a separate monthly value. Neither column, by itself, tells us whether rain arrives in one heavy spell, several short showers, or light wet periods spread through the month.

The distinction is clear in Sa Pa. December records 65.7 mm of rain and 13.5 rain days. That is a relatively low monthly rainfall total in the Sa Pa record, yet the rain-day value remains present across the month. The same December row records 9.7 drizzle days and 13.3 fog days.

Da Lat gives a different comparison. January records 9.1 mm and 2.2 rain days. Its table has no drizzle-days or fog-days data, so we do not fill those gaps from the rainfall number.

Four selected months

The last column shows other signals where the station has them.

The Hue row shows a high rainfall total and many rain days in the same month. October is the wettest month in the Hue record at 772.7 mm, while November has the most rain days at 20.9. Those two facts are both useful, and neither replaces the other.

The Mekong Delta row shows how a low number can carry two meanings. February records 3.8 mm and 0.8 rain days. Its available fog value is 0.5 days, while the drizzle value is 0.

How to read Sa Pa and Da Lat

Sa Pa's December row is a reminder that a dry month by millimeters is not necessarily a clear one. January records 70.2 mm and 16.3 rain days, with 14.1 drizzle days and 17.4 fog days. February records 73.5 mm, 16.3 rain days, 13.8 drizzle days, and 16.9 fog days.

Da Lat's January row is much lower at 9.1 mm and 2.2 rain days. February records 20.5 mm and 2.5 rain days. The Da Lat sheet includes sun hours, with 237.4 in January and 231.1 in February, but it does not include fog or drizzle. That is why we describe the data as a low-rain record, not as evidence about fog.

Our Sa Pa in winter guide expands the winter table. The Da Lat dry-season article shows all twelve Da Lat rows. The Sa Pa destination page, Da Lat page, and Mekong Delta page keep the station names and map pins visible.

What the columns cannot say

The regulation does not define a rain day. We cannot say what threshold creates one, how long the rain must last, or whether a short shower counts in the same way as a longer period. The table also does not measure rainfall intensity, local drainage, cloud cover, or conditions on a particular road.

That limitation is why we avoid turning a low rainfall total into a promise of dry sightseeing. In Sa Pa, the December row combines 65.7 mm with 13.5 rain days, 9.7 drizzle days, and 13.3 fog days. In Da Lat, January combines 9.1 mm with 2.2 rain days, while the source supplies no drizzle or fog columns. The two places need different descriptions because the data available for them is different.

Hue and the Mekong Delta sit at the other end. Hue's October total is 772.7 mm with 20.8 rain days, and the Mekong Delta's February total is 3.8 mm with 0.8 rain days. Neither says how long a shower lasts.

Station location matters too. Sa Pa's station is 1,570 m above sea level and 2 km from its map pin. Da Lat's is 1,513 m and 2 km from its pin. Hue's is 17 m and 4 km from its pin. The Mekong Delta record uses Cần Thơ at 3 m and 2 km from its pin. Those records describe their stations, not every street or valley.

The regulation does not state the observation period of these tables, so we don't name one. For a wider route plan, use Vietnam's when-to-go guide.

References

• QCVN 02:2022/BXD, National technical regulation on natural physical and climatic data for construction: https://datafiles.chinhphu.vn/cpp/files/vbpq/2022/10/02-bxd.pdf. Checked 2026-09.

Photo: Trang An Landscape Complex, Ninh Binh Province, Vietnam, 20240202 1433 5283.jpg, Jakub Hałun, CC BY 4.0, https://commons.wikimedia.org/wiki/File:Trang_An_Landscape_Complex,_Ninh_Binh_Province,_Vietnam,_20240202_1433_5283.jpg

Data checked: September 2026.