Short Droughts are Worst

The shorter the drought, the less rainfall there is in it. The longer the drought, the more rainfall. News reports give the false impression that hardly any rain falls during a drought, even if the drought lasts a long time. That is not true.

To prove the point, I have made graphs and a table showing the very worst droughts that Manilla ever had: the very worst short droughts, year-long droughts and 30-year droughts.

Lowest ever rainfalls

Graphs of the driest times

The first graph shows how the driest two month drought had only one millimetre of rain, while the driest longer periods had very much more, up to over 5000 mm of rain in 120 months (10 years). That may seem obvious. So long as there is a little rain in most months, the longer the period, the bigger the rainfall total. But there is more to it than that.

The second graph shows the average rate of rainfall during each worst drought: the rainfall per month. The rate is not steady as you might expect. It too becomes higher as longer droughts are measured. Through the worst two-month drought, only half a millimetre of rain fell per month. Through the worst 12-month drought no less than 24 mm fell per month. The worst 120-month drought had 47 mm per month on average. That is not far below the normal average monthly rainfall of 54.3 mm per month.

The third graph builds on this comparison. Each drought rainfall rate is shown as a percentage of the normal rainfall rate. While those worst droughts that were shorter than than five months had less than 10% of normal rainfall, no droughts that were longer than five months ever had so little. Droughts lasting for 12 months never had rainfall lower than 44% of normal. As for the decade-long droughts mentioned in the news, the driest decades in history had rainfall rates more than 90% of normal. Such record dry times are hard to see in rainfall figures, although they surely deplete surface and underground water storages.

[These graphs show clearly why droughts are not well defined by the percentage of normal rainfall. Percentile values are more satisfactory, but they too have problems.]

Manilla’s list of driest times

Table of lowest rainfalls

The table shows all the figures mentioned for each of the driest times on record in 134 years at Manilla.
Records can be broken, but it seldom happens. These records have stood for a very long time – at least the forty-six years since 1971.

Many of these record-setting droughts had dates of onset or breaking that were members of a rather small set. In particular, the year 1911 saw the onset of nearly half of them.

June 2018 in drought

An ornamental stone bridge

Grantham’s Stone Bridge

As in the month before, temperatures remained near normal. The second week was warm. There were 13 frosts: the usual number. The early morning dew point on the 25th was a new low record for June.
There were only two rain days. The wetter registered 4.4 mm (estimated) on the 28th.

Weather log June 2018

Comparing June months

In recent June months, the mean temperature has hardly changed from 11°. However, moisture has varied a lot. June 2013 was very cloudy, and June 2016 very rainy. This month was very dry, not only with little rain, but also with the lowest June early morning dew point (-0.9°). Warm days (18.8°) and cool nights (3.1°) made for a wide daily temperature range (15.7°).

The rainfall total of 5.2 mm was at the 9th percentile. This very low value has carried rainfall totals for two months, three months, and four months down to become extreme shortages. Such shortages have not been seen here since 2002.

Climate in June months

Developing Drought

The rainfall shortages that have now become extreme are covered in other posts, such as “Rainfall Shortages up to June 2018”.


Data. A Bureau of Meteorology automatic rain gauge operates in the museum yard. From 17 March 2017, 9 am daily readings are published as Manilla Museum, Station 55312.  These reports use that rainfall data when it is available.  The gauge, which had last reported on 24 September 2017, came on line again on the 16th of March. During the month of June three daily readings were blank, including that on the wettest day. I have substituted my own gauge readings for those days.

All other data, including subsoil at 750 mm, are from 3 Monash Street, Manilla.

Cycling into drought

Graph of rainfall versus temperature at Manilla

In the last three years, the climate of Manilla has moved into drought. Rainfall has become lower than normal, and days have become warmer than normal.

The pattern of change

The pattern of change is clear on this graph only because the rainfall and temperature anomalies have been smoothed. Values for the last six months cannot yet be smoothed so well. Their pattern is ragged.
The first point on the graph, July 2015, is close to the Zero-Zero point of normal climate, marked by a circle in turquoise. Since then, the climate has cycled mainly along the blue line joining the two corners marked “Hot Dry ‘Droughts'” and “Cold Wet ‘Flooding Rains'”, as in Dorothea Mackellar’s poem “My Country”.
For the first seven months, to February 2016, while rainfall hardly changed, the temperature rose to above normal. Then, by August 2016, the climate became unusually cold and wet. This first cycle ended in January 2017 at the hot-dry limit of normal climate.
From February 2017, a second cycle began with movement towards cool and wet, but that ceased in May without getting as far as normal. Since May 2017, the movement has been persistently towards hot and dry.
The final smoothed data point, December 2017, is close to the 21st Century record for both low rainfall anomaly (minus 27.1 mm/month in July 2002) and high daily maximum temperature anomaly (plus 1.39 degrees in October 2013). New records seem likely to be set when values for 2018 can be smoothed.

Length of cycles

The cycles on this graph have a period close to one year. February had the highest smoothed daily maximum temperature anomaly in 2016 and in 2017. When smoothed, the same may be true in 2018.
Historically, the cycles cold-wet to hot-dry have a period of about two years (“quasi-biennial”) at Manilla and in Australia as a whole.
The climate cycles or climate trends associated with Global Warming have periods that are very much longer. They do not show on this graph. If they did, they would show as movement on the other diagonal, between the corners marked “Cold Dry ‘Glacial'” and “Hot Wet ‘Interglacial'”.

The 2002 drought

The most recent extreme drought was in 2002. A similar graph for that drought is in the post “Profile of an Extreme Drought”.

For context, see the post “Manilla’s Record of Droughts”.

Graphs of other variables

The graph in this post is one of a set of six, showing smoothed anomalies of variables versus smoothed daily maximum temperature. The variables are: rainfall, cloudiness, dew point, daily temperature range, daily minimum temperature, and subsoil temperature.
All six graphs, with further explanation, are in another post.

3-year trends to June 2018

Now in drought

Three year trends to June 2018

June raw anomaly data (orange)

The raw rainfall anomaly for June 2018 was very low (high on the graph), as was that of the month before. This placed June in drought, although the other moisture indicators were nearer to normal. Daily maximum and minimum temperatures were also not far above normal, but subsoil temperature was very high.

 Fully smoothed data (red)

The last fully-smoothed data is for December 2017. All variables, except subsoil temperature, continued trends set in the spring.

Daily maximum temperature anomaly approached a record high value.
Rainfall anomaly approached a record low value.
Cloudiness was static near its normal maximum.
Dew point anomaly was low but slowly rising.
Daily temperature range anomaly was high but slowly falling.
Daily minimum temperature anomaly was high and rising.
Subsoil temperature anomaly, which had been falling, began to rise.


Note:

Fully smoothed data – Gaussian smoothing with half-width 6 months – are plotted in red, partly smoothed data uncoloured, and raw data for the last data point in orange. January data points are marked by squares.
Blue diamonds and the dashed blue rectangle show the extreme values in the fully smoothed data record since September 1999.

Normal values are based on averages for the decade from March 1999.* They appear on these graphs as a turquoise (turquoise) circle at the origin (0,0). A range of anomalies called “normal” is shown by a dashed rectangle in aqua (aqua). For values in degrees, the assigned normal range is +/-0.7°; for cloudiness, +/-7%; for monthly rainfall, +/-14 mm.

 * Normal values for rainfall are based on averages for the 125 years beginning 1883.

Rainfall Shortages up to June 2018

Rainfall shortage Manilla, June 2018

Since the twelve-month drought of 2002, Manilla has been free from extreme rainfall shortage until now. Such a long gap between extreme droughts has not been seen here before. [See Note below: Dry May 2016.]

Rainfall shortages now

On this graph the black line with black squares shows Manilla rainfall shortages at the end of June 2018. Shortages are shown for short terms down to one month, and for long terms up to 360 months (30 years). [Shortages at the end of May are shown in a previous post.]

Extreme shortages

Three extreme rainfall shortages have now developed, all below the 1st percentile rank:
Total for two months (May and June): 6 mm;
Total for three months (April, May and June): 24 mm;
Total for four months (March, April, May and June): 50 mm.

Severe shortages

There were five severe shortages in rainfall totals as follows:
Total for six months: 141 mm, at the 4th percentile;
Total for twelve months: 350 mm, at the 2nd percentile;
Total for fifteen months: 492 mm, at the 3rd percentile;
Total for sixty months: 2672 mm, at the 4th percentile;
Total for seventy-two months: 3317 mm, at the 4th percentile.

Serious shortages

Some other rainfall shortages were not severe, but serious:
Total for one month: 5.2 mm, at the 7th percentile;
Total for five months: 120 mm, at the 6th percentile;
Total for nine months: 464 mm, at the 10th percentile;
Total for eighteen months: 658 mm, at the 6th percentile.

Comparing June 2018 with the month before

Continue reading

Rainfall Shortages up to May 2018

Rainfall shortage Manilla May 2018

Rainfall shortages now

On this graph the black line with black squares shows Manilla rainfall shortages at the end of May 2018. Shortages are shown for short terms down to one month, and for long terms up to 360 months (30 years).

Extreme shortages

There were no extreme rainfall shortages at this date.

Severe shortages

There were severe shortages in rainfall totals as follows:
Total for one month (May): 1.2 mm, at the 2nd percentile;
Total for two months (April and May): 19 mm, at the 3rd percentile;
Total for three months (March, April and May): 45 mm, at the 4th percentile.

Serious shortages

Some other rainfall shortages were not severe, but serious:
Total for five months: 136 mm, at the 9th percentile;
Total for twelve months: 408 mm, at the 6th percentile;
Total for sixty months: 2765 mm, at the 8th percentile;
Total for seventy-two months: 3358 mm, at the 6th percentile.

General shortage

The first comment and reply below notes the fact that no rainfall total for any period reaches the 50th percentile. This has not happened for seventy years (1947).

Comparing May 2018 with September 2017

The graph also has a grey line showing similar rainfall shortages at September 2017 (See the earlier post “A drought has begun”.). In the following month, October, there were no rainfall shortages, because the rainfall, 84 mm, was far above average. November, December and February also had rainfalls above average.
Since March 2018, shortages have appeared again. By comparing the black line (May 2018) with the grey line (September 2017), you can see that the rainfall totals are now lower for nearly all periods of time. Only four totals are now higher, including the 4-month total.

What are the classes of rainfall shortage?

We need to compare rainfall shortages. The best way is not by how far below normal the rainfall is, but by how rare it is. That is, not by the percentage of normal rainfall, but by the percentile value. As an example, when the rainfall is at the fifth percentile, that means that only five percent of all such rainfall measurements were lower than that.
Once the percentile values have been worked out for the rainfall record, each new reading can be given its percentile value. Percentile values of low rainfall are classed as extreme, severe, or serious.
For a rainfall shortage to be classed as extreme, its value must be at or below the 1st percentile.
A severe rainfall shortage is one that is below the 5th percentile.
A serious rainfall shortage is one that is below the 10th percentile.
A rainfall shortage that is above the 10th percentile is not counted as serious.

Long-lasting rainfall shortages

Rainfall shortages sometimes last a long time. The same classes of shortage are used for long periods, such as a year, as for short periods, such as a month. They depend on how rare such a shortage is on the average, and they all use the same percentile values to separate extreme, severe, and serious rainfall shortages.

Autumn 2018 dry and very warm

Gum nuts and blossoms

Gumnut and Gum-Blossum

The air became four degrees warmer than normal by day and by night in mid-March and continued warm until late in April. As normal temperatures returned the climate became dry, with no rain for thirty-nine days.

Graphical weather log for autumn 2018

Both autumn 2016 and autumn 2018 had record high average temperatures. This season had the highest mean daily maximum (27.7°), but 2016 had the highest mean daily minimum (12.1°). In the combined average, 2016, with 19.9°, was warmer than 2018, with 19.7°.

All four indicators of moisture (rainfall, dew point, cloud, and daily temperature range) agree in showing this autumn to be drier than last autumn, and even than the three autumns before that.
The total rainfall of 45 mm was the 12th driest on record, but it was not the driest in the 21st century. Similarly dry autumns occurred in 2002 (60 mm), 2005 (35 mm), 2006 (53 mm), and 2008 (37 mm). That is about twice as many as history would suggest.

Climate for autumn 2018


Data. A Bureau of Meteorology automatic rain gauge operates in the museum yard. From 17 March 2017, 9 am daily readings are published as Manilla Museum, Station 55312.  These reports use that rainfall data when it is available. During this autumn season 30 daily readings were missed, and I replaced them with my own readings.

All other data, including subsoil at 750 mm, are from 3 Monash Street, Manilla.