Rainfall Deficiencies I: 3-months Duration

Posted in Australian Climate, Climate Change, Manilla NSW, Rainfall from 1883 by surlybond

This is the first of four graphs that show Manilla’s history of rainfall deficiencies (rainfall droughts), for periods of duration 3 months, 12 months, 36 months, and 120 months.

This first graph includes those short periods of severe or extreme rainfall deficiency that last only three months. They are rainfall droughts for one season rather than for a year or more. Crops and pastures may fail.
In Manilla’s climate, a time of severe 3-month rainfall deficiency has a rainfall total less than 50 mm, when it normally would be 150 mm.
The graph shows that such short-term droughts have occurred in every decade, but more often in some than in others. These brief droughts were most common (in 7% of months) around 1915, 1920 and 1970. They were least common (in only 3% of months) around 1895, 1935, 1940, 1955, and (more recently) in the twenty years since 1995.
Extreme short-term droughts (3-month total less than 26 mm) were more common near those times when severe short-term droughts were more common.

Areas shown on the graph

Rainfall deficiencies are called “severe” when they are lower than are recorded for five percent of the months. I have called deficiencies “extreme” when they are lower than are recorded for one percent of the months.
In this graph, I have coloured extreme deficiencies in blue. The maroon colour is deficiencies that are severe, but not extreme. The top edge of the maroon area marks the proportion of severe deficiencies including extreme deficiencies. As an average, this line is at five percent.

Data analysis

Continue reading →

23Dec2014

Extreme Droughts by Decade at Manilla

Posted in Australian Climate, Climate Change, Manilla NSW, Rainfall from 1883, World Climate by surlybond

The record of extreme droughts at Manilla, NSW, relates to the Southern Oscillation only now and then, and relates to global warming not at all.

This graph shows some of the same data as I presented earlier in the post “Manilla’s Record of Droughts”. The graph there showed precise dates, but it was hard to tell when extreme droughts were more or less frequent. This graph adds up the number of months of extreme drought in each decade. (See Note below: How I count drought months.)

There are separate columns (getting progressively redder) for extreme droughts of duration 3 months, 1 year, 3 years, and 10 years.
Extreme droughts of 10-year duration occurred only in the 1920’s and 1940’s.
Extreme droughts of 3-year duration occurred in the 1910’s, 1940’s, and 1960’s.
Extreme droughts of 1-year duration occurred in the 1900’s, 1940’s, 1960’s and 2000’s.
Extreme droughts of 3-month duration occurred in the 1880’s, 1900’s, 1910’s, 1920’s, 1940’s, 1970’s and 2000’s.
No extreme droughts at all occurred in five of the fourteen decades: the 1890’s, 1930’s, 1980’s, 1990’s, and 2010’s.

[Note added August 2019.

More data for the decade beginning 2010.
This post, dated December 2014, shows no extreme droughts in the decade beginning 2010. Extreme droughts did occurr in 2018 and 2019, as shown in the post “Rain Shortage Jan 2000 – May 2019”.
By August 2019, some months of extreme drought at 3-month and 1-year duration had occurred, and a month at 3-year duration was imminent.]

Relation to the Southern Oscillation Index

I posted this graph of cumulative values of the SOI earlier.

The record of the Southern Oscillation Index relates to the Manilla record of extreme rainfall deficiency only now and then. Persistent El Niños from 1911 to 1915 seem to relate to four months in the decade of the 1910’s having extreme 3-year droughts, carrying forward to two months in the 1920’s having extreme 10-year droughts. Similarly, the catastrophic droughts of short to very long duration in the 1940’s relate to El Niños that persisted from 1939 to 1942.
Other major El Niño events did not produce extreme droughts at Manilla: those of 1896, 1982, and 1997.
Long term trends in the Southern Oscillation Index do not predict Manilla’s extreme droughts at all. The 1940’s droughts Continue reading →

13Dec2014

House Thermal Mass Works in Summer Too

Posted in Indoor Climate, Manilla NSW by surlybond

My house at Manilla, NSW, is in a climate with temperatures that are extreme, but comfortable on the average. To reduce extreme temperatures indoors, the house contains more than a hundred tonnes of thermal mass within a shell of insulation.
The “thermal mass” is the materials, such as bricks, stones, concrete, earth or water, that have high thermal capacity (See Notes below): they take in and give out a lot of heat.
Many people, who can see that having thermal mass inside a house will help to keep it warm in winter, think that the thermal mass will make it hard to keep the house cool in summer. They see many brick and brick-veneer houses in which thermal mass is exposed to the intense heat of the summer sun. In that case, thermal mass material does no good.

In this graph, I have used my last twelve months of temperature data to show the benefit of well-insulated thermal mass in summer as well as in winter.
Outdoor temperature in this year went as low as minus 4.0° Celsius and as high as plus 43.7°: a range of 47.7°. Continue reading →

1Dec2014

Still hot and dry in November 2014

Posted in Manilla NSW, Monthly weather by surlybond

November Silky Oaks

Although the month began near normal, with two rain days, the weather became hot and dry again. Saturday the 22nd reached 41.4°, to be the second hottest November day on this 15-year record.
There were two heat waves, with the average weekly temperature above normal by 6.1° on the 12th and by 7.0° on Sunday the 23rd. (November of 2009 had a worse heat wave, being above normal by 9.5°, and reaching 31.3°: the hottest week in any month.)
Most nights this month were warm, with low dew points. By contrast, many days were cloudy.

Comparing November months

On average, the month was not quite as hot as November 2009, the hottest this century. The air was again very dry, with an early morning dew point of 7.1°, a little higher than the lowest value (5.7°) recorded for November last year.
Following only three years after the wettest November on the 130-year record (242.9 mm), the total rainfall of only 24 mm was in the 15th percentile. That is, such low November rainfalls happen about one year in seven. However, the time since a lower November rainfall (15 mm in November 1990) is thirteen years: the longest gap ever.
For the third month in a row, little changed in rainfall totals for periods of more than one month. There is no longer a serious shortage in the 6-month total (now in the 10th percentile) but there are now serious shortages in the totals for 5 months (9th percentile), 9 months (8th percentile), 15 months (6th percentile), 18 months (6th percentile), and 30 months (9th percentile). There is one severe shortage: the total for 12 months (393 mm) is in the 4th percentile.

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.

25Nov2014

Manilla’s Record of Droughts

Posted in Australian Climate, Climate Change, Manilla NSW, Rainfall from 1883 by surlybond

In terms of rainfall alone, Manilla, NSW, had droughts between 1900 and 1950 that were more severe, and lasted very much longer than those of recent years.

Comparing droughts

It is hard to say how bad one drought is compared to another because some droughts last longer than others. A drought that lasts two months, and has only 10 mm of rain when it would normally have 100 mm, qualifies as “extreme”. In such a very short drought, rainfall as low as 10% of normal just qualifies as extreme. For a drought lasting twelve months, when there is normally 652 mm of rainfall at Manilla, there has never been a case of a twelve-month rainfall as low as 10% of that (65 mm). (The lowest ever was 288 mm, in 1964-65.) Clearly, using 10% of normal rainfall will not do to define longer-term droughts.
I find the severity of each drought, whether it is long or short, by its percentile rank.
The Bureau of Meteorology defines “Rainfall Deficiency” as:

Lowest on record – lowest since at least 1900 when the data analysed begin.
Severe deficiency – rainfalls in the lowest 5% of historical totals.
Serious deficiency – rainfalls in the lowest 10% of historical totals, but not in the lowest 5%.

On the graph, I use this code:

Extreme rainfall shortage: rainfall in the 1st percentile only.
Severe rainfall shortage: rainfall in the 2nd to 4th percentiles.
Serious rainfall shortage: rainfall in the 5th to 9th percentiles.

Major droughts

All of Manilla’s extreme rainfall droughts that lasted for six years or more happened in the first half of the 20th century. Extreme droughts lasting for thirty years ended during 1940, 1941 and 1947.
Since 1950, the longest extreme drought lasted only five years, ending in 1961. The next longest lasted three years, ending in 1968. The last forty-four years have brought only six extreme droughts, all of less than two years duration: 1971, 1974, 1982, 1984 (2 months!), 1994 and 2002. The twelve years since 2002 may be the longest period without an extreme drought in the whole record since 1883.
Extreme droughts had also been few and short in the earliest years, from 1883 to 1902.

Similar, but much improved graphs

[This graph gives the misleading impression that the longer the duration of rainfall shortage, the later it occurs. That is an artefact.

A shortage of a given duration observed in a particular month must have commenced earlier: earlier by the number of months of its duration, less one.
Graphs that remove this defect, and plot correctly the dates of onset, persistence, and breaking of rainfall shortages are “Rainfall Shortage History: Manilla” and “Rainfall Shortage Jan 2000 – Mar 2019”.

Droughts Elsewhere

At Lake George, in the southern highlands of NSW, extreme droughts of long duration were similarly restricted to the first half of the 20th century, as shown by rainfall records and lake levels.
The “Millennial Drought of southeastern Australia” was not a drought of long duration at either Manilla or Lake George.

[Note added May 2016

Graphical log of droughts, 1884 to 1916 The post “Manilla’s Droughts, 1884 to 1916” has an enlarged graph showing more detail. Of the catastrophic droughts of 1902 (“Federation Drought”) and 1912-16, the first had a sudden termination but the second had a sudden onset.]

This blog has many more posts about drought (or rainfall deficiency) at Manilla, NSW. Please use the “Search” function at the top right.