May is often the month which brings the first warmth of early summer, daylength is still increasing, the atmosphere warms with depth with increasing irradiance and daytime maximum temperatures respond with a 4C increase in average temperature during the month. Highest temperature is generally above 23C and can top 30C although days can be cool with maximum below 10C.
Current model forecasts show a change in circulation to mid-month a general rise in pressure as an anticyclone builds to the west and then transfers to the south, becoming slow moving. Model forecasts differ in detail and timing but all show a gradual rise in temperature early month and then a distinct sudden drop going into mid-month
which just so happens to coincide with 3 consecutive days in May know as the Ice Saints. The Ice Saints is European folk-lore and relates to a brief spell of colder weather occurring between 11th to 13th May with each day being a celebration of a particular saint, St. Mamertus (11th), St. Pancras (12th) and St. Servatus (13th).
A review of climatology for May, taking temperature measurements representative of middle England since 1881, summarised in the tables below show (left) daily maximum temperature for 10(11) day periods during the month averaged for fixed 10 year periods ending 00 together with average number of days defined as COOL and WARM and compared with the ICE SAINTS days (right). The middle graph shows the 10 (11) day maximum temperature anomaly from 100year LTA (1921-2020)
Five to note :
model : day to day variability of maximum temperature
model : forecast sudden change (5-10C) mid-month
climatology : COOL mid-month less than early and WARM less than late-month
climatology : WARM frequency highest late-month
folk-lore : Ice Saints frequency of WARM higher than COOL
JK
