Rain Days at Manilla: I.

Rain per rain day graph

The annual pattern of rain day rainfall

In Manilla, the mean pattern of rainfall on rain days through the months of the year is simple and regular. This pattern can be worked out from the 125-year rainfall record of Manilla Post Office, Station 055031, beginning in 1883.
The graph above shows that, on the average, on a day when rain falls in January, the total in the day is about thirteen millimetres. When rain falls in July, the total in a day is about half of that: that is, six and a half millimetres. The pattern through the year is close to a perfect harmonic cycle, with a maximum in the third week of January, (four weeks after the longest day) and a minimum exactly six months later, in the third week of July. Only two of the monthly readings do not match the pattern well: January has about one millimetre more than would fit the curve, and December about half a millimetre less.
Of course, most people in the district realise that heavier rain falls in summer, but few would know any details. I do not think that the Bureau of Meteorology has ever worked through these figures. [See note below about the use of “rain days” in the Bureau.]
This very simple pattern of mean rainfall per rain day is the more remarkable because it comes from two other patterns that are not so simple.

MeanRainEachMonthThe second graph is the pattern of monthly rainfall totals through the year. Manilla has two peaks of rainfall volume in the year. The major peak comes in the last days of December, a few days after the longest day, and a minor peak just six months later, at the end of June. Winter is marked, not by a minimum of rainfall, but by a secondary maximum. Much more detail is given in the post “A seasonal rainfall model for Manilla”
and in the post “Manilla 30-year Monthly Rainfall Anomalies”.

The final graph shows simply how many days of rain there are in each calendar month, on the average. This pattern is quite strange. Most months of the year have about six rain days. April has fewer: Continue reading

February 2015 a very dry month

Photo of a baby Kookaburra

Baby Laughing Kookaburra

After a very cool first week, normal temperatures returned, both by day (33°) and by night (18°) . Early on, most mornings were sunny. Later, many were almost overcast, but only three days had light rain, with a maximum of 3.4 mm.

Weather log  February 2015

Comparing February months

Most of the last seven February months have been unusual in some way. This month was very dry, with near-record low rainfall and very low humidity. By contrast, February 2012 was very wet, as well as very cool. Both 2011 and 2014 were very warm, but 2011 was also very cloudy and humid. February 2013 was the coolest, without being wet like 2012.
The total rainfall of 7.6 mm this month was very low (6th percentile): this was the 8th driest February, and the driest since 1989, which (like 1974) had 7.0 mm. Still, taking rainfall totals for groups of months, only the 24-month total (985 mm) is a serious shortage, in the 9th percentile.

Climate forFebruary 2015


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.

With the retirement of the Post-master, Phil Pinch, the future of rainfall observations at Manilla Post Office (055031), kept for 132 years since March 1883, is uncertain.

January 2015: cool rainy days

Crepe myrtle shrubs

Crepe Myrtles in Arthur Street

Only five days were warmer than average. The final five days and four nights were very cool, taking the weekly average (normally the hottest of the year) down to five degrees below normal. (This would be normal for the end of March, not January!) There were fifteen rain days, equalling the 125-year record number for January, set in 1941. Rain was spread evenly through the month, with the highest reading 29.2 mm on the 2nd.

Weather log January 2015

Comparing January months

The mean daily maximum temperature (31.4°) was well below the average of 33.8°. Since nights, at 17.6°, were near normal, the daily temperature range was the record narrow value for January of 13.8°.
The dew point (humidity) returned to a normal value of 12.9°, after last January’s arid 6.9°. Subsoil temperature (25.3°) fell to normal after two January months above normal.
The total rainfall of 117.4 mm was in the 75th percentile, well above the average of 87 mm. This clears all shortages in rainfall totals for groups of months. The lowest percentile value remaining (15th) is for the 18 month total of 746 mm.

Climate for January 2015


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.