The daily weather log
Dry weather with warm days and cool nights lasted most of the month. On the 13th the early morning Dew Point of -6.3° set a record for low humidity in May. The seven-day average temperature was quite low in the second week. Nine mild frosts (usually 6) all came about this time. By the 25th there had been only 57 mm of rain in 14 weeks: an average of barely 4 mm per week. A serious rainfall shortage was developing, despite months of cloudy weather.
Late in the month nights became very humid and warm. Mornings had fog on the 18th and 22nd and mist (visibility one kilometre) on the 27th and 31st. Three days had more than 5 mm of rain. In all, the month had 8 rain days, totalling 28.0 mm.
Comparing May months
This was the cloudiest May in the 11-year record. The mean maximum temperature and the mean average were normal. The mean minimum was up, reducing the daily temperature range, and the mean Dew Point was down.
The rainfall of 28.0 mm is near the median for May (51st percentile). It is below the long-term average of 40 mm, but no May total has beaten the average since 1998! Rainfall totals for groups of months show just one serious shortage. The 15-month total (538 mm) is now down to the 7th percentile.
Both graphs now include subsoil temperature measured at 750 mm depth. There the temperature barely changes during a day. It changes slowly with the seasons. Usually it ranges from 26° in February to 14° in July.
As observed in Manilla, the subsoil temperature does not relate to the normal average temperature of the air in a simple way. In May, it stays about five degrees warmer; in other autumn and winter months it is less than five degrees warmer. In spring and summer it is almost the same as the normal average air temperature.
In autumn 2007 the subsoil temperature was nearly two degrees warmer than usual; in autumn 2008 it was one degree cooler than usual. This is a puzzle, because the average air temperature did not change so much.
Data. Rainfall data is from Manilla Post Office, courtesy of Phil Pinch. Temperatures, including subsoil at 750 mm, and other data are from 3 Monash Street, Manilla.