July 2017 fine with cold nights

July morning photo of Manilla from the lookout

Manilla Prospect in July

Through most of the month, days were fine and sunny, but some days, mainly in the middle, were cloudy and some had a little rain. The highest reading, on the 16th, was only 7.4 mm.
No days were remarkable except the 28th which, at 23.7°, equalled the record for July set 31/07/14. It was 6.1° above normal.
Frosts (below +2.2° in the screen) happened on 23 mornings, 6 more than normal. However, the coldest morning, at -2.6°, was not nearly as cold as the record of -5.1° set in 2002.

Weather log

Comparing July months

Unlike July 2016, which had been cloudy with warm nights, this July was fine with cold nights. Days, at 18.1°, were not quite as warm as in July 2013 (18.9°), the warmest in the new century.
Moisture was scarce, as in the record-making July of 2002. Readings that reflected low moisture were:

Daily minimum temperature very low: +1.2° (2002: 0.9°);
Very many frosts: 23 (2002: 27);
Very low percentage of cloudy mornings: 29% (2002: 23%);
Very low early morning dew point: -1.4° (2002: -1.4°);
Very wide daily temperature range: 16.9° (2002: 18.5°);
Very low rainfall: 13.2 mm (2002: 1.0 mm).

Relative humidity in the early mornings, normally 90% in July, was 74%. That was the lowest July value in my 13-year record.
Despite the total rainfall of 13.2 mm (16th percentile) being far below the July average (41 mm), there are still no shortages of rainfall for groups of months. The most recent serious shortage was nearly two years ago. In October 2015, the 30-month total to that date (1216 mm) was still down at the 6th percentile. That shortage was carried over from an earlier extreme event: the 85 mm summer rainfall of 2013-14 that was 142 mm below average.

Climate graph for July

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

When is the First Frost?

This year (2017) the first frost at Manilla came on the 11th of May, close to the middle date for it: the 13th of May. In just half of the years, the first frost comes between ANZAC Day (the 25th of April) and the 19th of May.

Graphical record of first frost dates
(See the notes below: “Observing Frosts in Manilla.”)

The date of first frost from year to year

The graph shows the dates of first frosts in the last nineteen years. One feature stands out: from a very early date of the 4th of April in 2008, the dates got later each year to a very late date of the 8th of June in 2014. Otherwise, the dates simply jumped around.

Graphical log of frostsThe date of first frost hardly relates at all to the number of frosts in a season. This graph, copied from an earlier post, shows how poorly they match. The earliest first frost, in 2008, was in a year with a normal number of frosts. In the least frosty year, 2013, the first frost did not come late.

The central date and the spread

To find the central value and the spread of a climate item like this calls for readings for a number of years called a “Normal Period”. (See note below on Climate Normals.) I chose the first eleven years of my readings (1999 to 2009) as my Normal Period. For this period I found these five order statistics:

Lowest (earliest) value: 4th April;
First Quartile value: 25th April (ANZAC Day);
Median (middle) value: 13th May;
Third Quartile value: 19th May;
Highest (latest) value: 24th May.

These five values divide the dates of first frost into four equal groups. For example, the first frost comes before ANZAC Day in one year out of four. This could confirm what Manilla gardeners know already!

Is the first frost getting later?

Talk of global warming leads us to expect the date of first frost to get later. By how much?
Dates on the graph after 2009 seem to be later in the season than during the Normal Period. As shown, a linear trend line fitted to the data points slopes steeply down towards later dates in later years. A curved trend line (a parabola) slopes down even more steeply. However, with so few data points, these trend lines are wild guesses, not to be relied on for forecasting future frosts.
Australian Bureau of Meteorology data for NSW from 1910 shows that daily minimum temperatures have been rising at 0.11° per decade.  (That is much faster than the rate for daily maximum temperatures, which is 0.07° per decade.) To work out how this might affect the date of first frost in Manilla, one needs to know that the daily minimum temperature in this season gets lower each day by 0.15°. One day of seasonal cooling will more than cover a decade of climate warming. The effect of global warming is to make the date of first frost only one day later in fourteen years. If the middle date of first frost was the 13th of May in the Normal Period, centred on 2004, the forecast middle date of first frost next year (2018) would be the 14th of May. This is shown by the flattest of the three trend lines on the graph.

Looking ahead, it seems unlikely that the date of first frost will get later by as much as a week within a lifetime.

[Update 12 August 2019:
This estimate of the median date of first frost, 13 May, is not changed by first frosts observed on 15 May 2018 and 12 May 2019.]


1. Observing Frost in Manilla

Continue reading

Manilla’s Frosts to 2014

Graphical log of frosts

This post updates a similar one by including three more years to make a total of sixteen.

The Number of Frosts in Each Year

The first graph is a log of the number of frosts in each year. The pattern is different when counting all frosts or only severe frosts.
The log for all frosts had two periods of stable, medium numbers of frosts: from 1999 to 2003, and from 2008 to 2011. Three years had many frosts: 2004 (68), 2006 (70), and 2012 (69). The year 2007 had the fewest frosts (43) until beaten by 2013 (34).
In the logs for severe frosts below minus 2° or minus 4° in the thermometer screen, the drought year 2002 stands out as the most frosty by far. It had the coldest mornings: -5.1° on both the 2nd and 11th of July.

The Last Three Years

Monthly frosts in 2012, 2013, and 2014.

The second graph compares the mean seasonal pattern of frosts with the patterns for the three latest years: 2012, 2013, and 2014.
The frost season of 2012, which almost matched the record 70 frosts of 2006, began early and ended late. May had 13 frosts (like the 14 of 2006) and September had 10 (like the 8 of 2003).
The curve for the season of 2013 ( the new record fewest) was like that of a normal frost season, but lower.
The year 2014 was not very frosty, because the season began late, with no frosts in May and only nine in June.

Monthly frosts each year

Graphs showing the seasonal frost patterns for earlier years are copied here.







There is 2013 reserch on frost in NSW titled “Understanding frost risk in a variable and changing climate” reported here.
It is in GRDC Update Papers (Grains Research and development Corporation). The research is done by Steven Crimp and co-workers at CSIRO Climate Adaption Flagship.
Referring to the period 1960 to 2010,
“Over many parts of NSW the frost season length has broadened by as much as 40 days and the mean number of consecutive frost days has increased to 5 days.”

Manilla’s Frosts

I have a 13 year record that shows how frosty Manilla is, and how some years are frostier than others. My thermometer is on high ground, so people living where cold air collects will have had more frosts. However, my readings show changes from one time to another. As I do not have a thermometer in the grass, I have recorded a frost when my screen reading is below +2.2°.

There is a matching post for Manilla’s hot days.

Total frosts

The first graph shows the number of frosty mornings in each year. The most frosts were in the years 2004 (68) and 2006 (70); the fewest were in the years 2007 (43) and 2010 (44). The 13-year average is 54.
The graph also shows the number of mornings colder than zero, minus two, and minus four degrees. On the average, these occurred on 26, 7, and 1 mornings per year. For those colder than zero degrees, 2006 was again the frostiest, but 2002 was also very frosty. Counting only the most severe frosts (below minus two or minus four degrees) 2002 was the frostiest year. It had the coldest mornings: -5.1° on both the 2nd and 11th of July.Frosts each year, and seasonal distribution.

Frosty months

The second graph shows how most frosts come in the winter months, especially July, with some frosts in autumn, but few in spring and none in summer. Few come before Anzac Day (25th April) or after Labour Day (first Monday in October in NSW).

The graphs below show that each year was different. The drought year 2002 had the highest number of frosts in a single month: 27 in July – half of all frosts in that year. By contrast, the 70 frosts of 2006 were spread through the months of winter and autumn: there were more frosts in June and August than in July. Continue reading

Spring 2007 climate much as usual

Weather log spring 2007

The daily weather log

As sometimes happens, temperatures did not rise in September until late in the month. They then rose to several degrees above normal and stayed there until the end of October. Early November was very cool, and mid-November warm. The daily temperature range increased in dry periods and decreased in rainy periods.
There were two late frosts below 2.2° in September.

Only 2.2 mm of rain fell in September. Much more fell later, in short wet spells around October 12th and 26th, and November 7th and 24th. The two best daily falls were 17 mm each. Total spring rainfall was 122 mm, falling on 23 rain days.

Very cloudy or overcast skies became common in November.

Comparing spring seasons

Although each month this spring had unusual temperatures, the three months taken together did not. Daily maximum, minimum and mean temperatures for the spring, as well as the daily temperature range (16.0°), were all normal. In the extreme drought of spring 2002, the values had been two degrees higher, and the daily temperature range 17.1°. The spring of 2001 had the lowest temperatures in this record.

Two frosts this spring compare with three on the average. Spring 2004 had 10 frosts, including one in mid-October, and one on 3rd November!

Spring rainfall in 2007 (122 mm) fell 44 mm short of the long-term average (166 mm), due to September being 39 mm down. October was only 6 mm down, and November 2 mm up. Spring rainfall averaged over the last nine years is above the long-term average, because the droughty spring of 2002 is more than balanced by the very wet springs of 1999, 2000, and 2005. These were in the ninth decile for spring rainfall.

Climate spring 2007

Data. Rainfall data is from Manilla Post Office, courtesy of Phil Pinch. Dew Point values before August 2005 are from Tamworth Airport 6 am data supplied by the Bureau of Meteorology. Temperatures, including subsoil at 750 mm, and other data are from 3 Monash Street, Manilla.

Cold days in 2007 winter

Weather log winter 2007

The daily weather log

The temperature trends this winter look like a roller-coaster.
June had daily maximum temperatures about four degrees below normal. Two miserable days had maxima eight degrees below. Meanwhile, minimum temperatures were normal.
Maxima returned to normal in early July but, by that time, nightly minimum temperatures were well above normal. All temperatures then plummeted, losing 8°: by mid-July both maxima and minima were about 5° below normal.
By the end of July and early August maxima and minima had shot up again to well above normal. Next, maxima went up while minima went down, making the daily temperature range very large. This temperature range then collapsed to become very small. The winter ended with temperatures near normal.
The high ground in Manilla had 38 frosts* in June, July, and August. The nine-year average is 45. Seventeen nights went below zero and two nights below -4°.
There was quite a lot of rain, but not much fell in July. The amounts were 65 mm in June, 9 in July, and 81 in August. June was above average, July in the second decile, and August in the ninth decile.
There were ten rain days in June, seven in July, and five in August. One event was remarkable: rain fell continuously for more than 36 hours from the morning of 19th August, to a total of 55 mm.
Skies were overcast (8/8) around the periods of rain. In contrast, there were long sunny spells (0/8 or 1/8) in mid-July and the first half of August.

Comparing winter seasons

The average daily maximum temperature was the coldest in the last nine winters. At 16.6°, it was 1.6° below average.
Although the nights of 2005 were warmer, this winter also had warm nights. At 3.4°, they raised the daily mean temperature up to 10.0°, close to normal.
The daily temperature range in winter in Manilla is usually 15.4°. This winter’s cool days and warm nights made its daily range the lowest in these years. It was only 13.2°, typical of places nearer to the coast, such as Singleton.
The winter that stands out is 2002. With the warmest daily maxima (19.2°) and the coldest daily minima (1.7°) it had the remarkably high daily range of 17.5°. Such a range is found mainly in desert areas – which Manilla was at the time!.

Winter is the driest season in Manilla. The average winter rainfall, 125 mm, is just over half the average summer rainfall (227 mm).
This year’s winter rainfall (155 mm) was above average, and near that of 2005 (157 mm). None of the winters on the graph is far from the average except 2002.
The very dry winter of 2002, at 44 mm, was the ninth driest on the 124-year record. It was the driest since 1982 and 1972 (both 32 mm).
The graph just misses the very wet winter of 1998, nine years ago. Split Rock Dam suddenly filled up for the first time. At 304 mm, that was the second wettest winter on record. Only the winter of 1920 was wetter, with 318 mm.
The graph shows the peracentage of mornings with more than four octas of cloud. The winter average is 31%.
This winter, and winter 2000 were the cloudiest. The insulating blanket of cloud stopped the days from getting warm, and the nights from getting cold.
The winter of 2002 had the fewest cloudy days, another sign of how like a desert it was then.

Climate winter 2007

*Frost: A frost occurs when the temperature on the grass is below zero, causing water to freeze in the leaves. Not many stations measure grass temperature. My rule-of-thumb is to declare a frost if the temperature in the thermometer screen (at 1.5 metres) is below +2.2°.

Data. Rainfall data is from Manilla Post Office, courtesy of Phil Pinch. Dew Point values before August 2005 are from Tamworth Airport 6 am data supplied by the Bureau of Meteorology. Temperatures, including subsoil at 750 mm, and other data are from 3 Monash Street, Manilla.