Historic Rainfall in Cwm-dyli: A Deep Dive into Wet Extremes

Precipitation in Cwm-dyli (1921–2025)

The average annual precipitation in Cwm-dyli is 3120.8 mm, according to historical Met Office data for the period 1921–2025. This makes it one of the wettest locations in the UK, comparable to other high-rainfall sites in Snowdonia. The distribution through the year is far from uniform, with a clear wet season in the winter months and a relative dry spell in late spring.

Seasonally, winter brings the most substantial rainfall, with 981 mm falling in December, January, and February combined. Autumn follows closely behind at 959 mm, while summer totals 665 mm and spring is the driest season at 579 mm. This pattern—wettest in winter, driest in spring—is typical for the maritime climate of North Wales, where Atlantic weather systems dominate from late autumn through early spring.

Average Rainfall by Month

The table below shows the long-term monthly average for total precipitation, the record daily rainfall for each month, and the average number of rain days (≥1 mm) per month.

December is the wettest month on average, with 372.2 mm of precipitation across 20.7 rain days. May is the driest, averaging 169.8 mm. Interestingly, although December is the wettest month on average, the daily records show that both June and November have seen extreme single-day totals of 164.3 mm and 165.1 mm, respectively—those extreme days match the longest-standing daily records in the dataset.

The 5 Most Intense Rainfall Days on Record

These are the top five highest daily precipitation values recorded at the cwm-dyli station between 1921 and 2025.

It's notable that four of these five events occurred before 1984, with only the New Year's Day 2025 event entering the top five. The record from 1953 remains unmatched for over seven decades.

Annual Precipitation Statistics by Year

This table presents the total annual precipitation, the number of days with rain (≥1 mm), and the highest single-day total for each hydrological year in the dataset. Note that some years have incomplete data, indicated by records where the annual total appears disproportionately low compared to neighbouring years.

The chart shows the complete annual series. The table lists the most notable years; the full series comes from Met Office.

The variability between years is substantial. 1954 stands out as the wettest year in the record with 4486.7 mm, while 1933 saw only 2138.8 mm. Years such as 1975 (780.1 mm) and 1992 (1864.8 mm) indicate gaps in the observational record rather than true drought conditions, as do 1975 and 1992 which show lower rain-day counts (49 and 115) that signal missing data.

Frequently Asked Questions

How much rain does Cwm-dyli get per year?

Average annual precipitation is 3120.8 mm at the cwm-dyli Met Office station, based on records from 1921 to 2025. This makes it substantially wetter than most of the UK; for comparison, the annual average in London is under 700 mm.

How many days of rain are there per year in Cwm-dyli?

On average, there are 198.3 rain days per year, defined as days with at least 1 mm of precipitation. This means that, on the typical year, more than half the days see measurable rainfall.

Which month is wettest?

December is the wettest month on average, with 372.2 mm of precipitation over 20.7 rain days. November is the second wettest, averaging 351.5 mm.

What was the most intense single day of rain?

That record belongs to 1953-11-27, when 165.1 mm fell in one day. This is more than double the average for the entire month of April.

Is Cwm-dyli a particularly rainy area?

Yes, very much so. Typical annual totals of 3120.8 mm place it among the wettest inhabited locations in England and Wales. The surrounding Snowdonia mountains enhance rainfall through orographic uplift, especially from autumn through winter.