Two very warm nights
On two mornings this month, the 21st and 23rd, the minimum temperature in my thermometer screen was higher than it has ever been in July. That is, in the 17-year record that I began in March 1999.
These minimum readings were 14.3° and 14.4°. The highest July minimum had been 13.5° (31/07/2010), and only five readings had ever been above 12°. Such readings are more than ten degrees higher than normal in July.
In fact, one of the nights was much warmer than the minimum temperature indicates.
Minima not always at night
High minimum temperature readings are the usual evidence of warm nights. Unfortunately, they are not the same thing. Especially in the case of very high readings, they can be misleading.
The catch is that we expect daily maximum temperature to occur in daytime, when the sun is in the sky, and daily minimum temperature to occur at night, when it is not. In Manilla’s very sunny climate, the maximum is usually about fifteen degrees higher than the minimum. We can expect the maximum about 3 pm, and the minimum about 6 am.
When a thermometer is read at 9 am (as they are), the maximum reading recorded on it is usually that of the afternoon, and the minimum reading that of just before sunrise. Usually, but not always!
Times when nights are very warm are usually cloudy. The clouds form a blanket that keeps us warm. Because they also block the sun in daytime, the daily maximum temperature may be almost the same as the daily minimum. The times when maxima and minima occur may become vague. As a complication, warm nights tend to happen when warm air comes to us on the wind. Then much colder air often follows. If the cold air arrives before 9 am, it will lower the minimum temperature reading, destroying the evidence of a warm night.
I do not have a thermograph that makes a continuous temperature trace. The trace for Tamworth Airport can be seen here. (Choose a date.)
For this month’s warmest night, the Tamworth thermograph trace shows that the daily minimum temperature value is misleading due to these factors. The night was much warmer.
Between 5:30 pm on the 22nd and 5:30 am on the 23rd, the lowest temperature, which came at 7:40 pm, was 17.8°. Most of the night, from midnight to 4:00 am, was above 20°! Yet the (Tamworth) minimum for the 24 hour period was 12.5°.
Earlier in the month, in the 24 hours to 9 am on the 6th, the conventional maximum and minimum values were highly misleading. The daily maximum was the very last reading (10.0°) and the daily minimum the very first, 24 hours earlier (6.5°). The afternoon maximum temperature was 8.0° and the pre-dawn minimum temperature was 8.2°. On that date, the day was 0.2° cooler than the night!
More frequent hot days do not come in a three year cycle, but in a 1.5 year cycle related to ENSO.
The Hot Day data set
The graph of number of hot days per year
The graph on the left is one I posted earlier. The height of each data point represents the number of hot days in a year, plotted near January. The pattern of points led me to join them by a smooth curve. This curve swings up and down rather regularly, with five peaks and five dips in the fifteen years. That is, more frequent hot days seem to come in a three-year cycle.
Is this cycle “real”? Should we look for a cause? Will the cycle continue?
Probably not! The points of measurement are one year apart. Cycles that are only three years long may be “aliases” of different and shorter undetectable cycles. (See Note below on Nyquist frequency.)
More detailed hot day data
Other graphs already shown include further data: the number of hot days in each month, and the 13-year average number of hot days in each calendar month. From these I have calculated a relative frequency. That is, the ratio of the actual number to the average number for that month.
Only the months of November, December, January and February have enough hot days to calculate a relative frequency, but these can show changes within the hotter months of each year.
The daily maximum temperature data set
A graph that I posted in “El Niño and my climate” shows a curve of smoothed monthly means of daily maximum temperature anomalies. The yearly cycle of summer-to winter temperature has been removed. I have also applied a smoothing function, which makes the monthly points of measurement effectively two or three months apart. As a result, cycles longer than about six months can be detected.
There are about 10 peaks and 10 dips in the 15.5 year curve. They define a cycle of about 1.5 years wavelength. That cycle is so much longer than the minimum-detectable six month cycle that “aliasing” is not likely.
The reality of this temperature curve is supported by its close similarity to the recognised curve of the El Niño – Southern Oscillation (ENSO), as read from NINO3.4 Pacific Ocean sea surface temperature anomalies.
A combined graph of hot day and temperature data
The graph at the top of the page presents the monthly smoothed maximum temperature anomaly again, using the scale at the left. To this I have added data on the number and frequency of hot days.
The annual number of hot days is shown in blue, in blue boxes. The boxes are placed higher or lower according to the number, but the height is adjusted to match other data better.
A “Hot Day Index” is shown by blue diamonds. This index is based on the relative frequency of hot days in each month that has data. I have re-scaled the values to improve the match. (See Note on Re-scaling below.)
Matching hot days with temperature
This post updates a similar one by including three more years to make a total of sixteen. It is in the same format as a recent post on Manilla’s frosts. Because the summer, which has the most hot days, crosses from one calendar year to the next, I have begun each year at July. I have called days warmer than 35° “hot days”, and days warmer than 40° “very hot days”.
I have analysed the pattern of hot days in more detail in a later post “Hot days and ENSO”. By finding the relative frequency of hot days in all of the hotter months, I show that there is a cyclic variation related to ENSO. The cycle period is near 1.5 years, not 3 years as the log of annual frequency of hot days (above) suggests.
Graphical log of hot and very hot days
The first graph is a log of the number of hot and very hot days in each year. The three years with the most hot days had almost the same number: the year ’02-’03 had 41, the year ’09-’10 had 44, and the year ’13-’14 had 43. The two years with the fewest were ’07-’08 which had 5, and ’11-’12, which had only 4. The 13-year average is 26. Counting only the very hot days, ’03-’04 had the most (6), and four years had none at all. On the average, two days exceeded 40° in a year. (These are thirteen-year averages, not updated.) The number of hot days per year seems to have a cyclic pattern, with a period that increases from two years to four years during this short record. This is just a curiosity. The pattern of hot days has a lot in common with the pattern of smoothed monthly temperature anomalies for all months. These are plotted here, on a graph that relates them to ENSO. The relation of Manilla daily maximum temperature to ENSO was quite close from 1999 to 2011, but failed almost completely since mid-2011. In the earlier post on frosts, no cyclic pattern can be seen, nor any relation to ENSO.
New Record hottest days
In the sixteen years, there have now been 37 days hotter than 40 degrees: that is, 2.4 days per year. It remains true that December has fewer very hot days than November or February. A new record was set on 12/1/2013 by a daily maximum temperature of 43.2 degrees, beating the 42.6 degrees of 20/11/2009. This record was broken again on 3/1/2014, with 43.7 degrees. In the latest year, the hottest day (41.1 degrees) ranked only 12th, and it was not in summer, but in November.
Three new annual graphs
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
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.
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.”
I have used my 13-year weather record to find the number of hot days in each year and in each month. Earlier I did the same for frosty mornings. Because the summer, which has the most hot days, crosses from one calendar year to the next, I have begun each year at July.
I have called days warmer than 35° “hot days”, and days warmer than 40° “very hot days”.
Total hot days
The first graph shows the number of hot and very hot days in each year. The most hot days were in the year ’09-’10, which had 44; the fewest were in the year ’11-’12, which had only 4. The 13-year average is 26, but the number of hot days is quite different from year to year.
Counting only the very hot days, ’03-’04 had the most (6), and four years had none at all. On the average, two days exceeded 40° in a year.
Months with hot days
The second graph shows how the number of hot and very hot days peaks strongly in January, with very few earlier than November or later than March. On average, Manilla’s summer has about 22 days warmer than 35°, while spring has 3, and autumn only 1.
The other graphs show how each year had a different pattern of hot days. The highest monthly peaks, each 19 hot days, came in January 2003 (following drought) and January 2007. Annual peaks also came in January in 2008 and 2012, but these peaks were extremely low: only 4 and 3 hot days. Continue reading
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.
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.
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