Sunday, 4 February 2018

Weather Report January 2018

The basic (ie without consulting my quantitative records) look and feel of this month was hot and dry.  The Gazette Report ...
 ... (almost identical to the hard copy version - a slight drop in average temperature is the only change) shows getting to 40oC (hot) and rainfall of half the average (dry).  But how hot and how dry?  Read on!

Rainfall

We scored a total of 36.2mm in the month, which is roughly halve the average since 1985.  However there have been 10 Januaries (out of 33) with lower amounts than that.  I have got a set of data giving the cumulative rainfall by day since 1985.  This enables me to do a prorata estimate of what we will end up with for the year.  Not surprisingly as we have about half the average for January we also have have about half the average for the year - 350,6mm.  It is interesting to see how the annual estimate has changed through the month.
 The spell on 14th-15th got us looking 500mm for the year but the following dry patch dropped it back, with a small pick up late in the month.

Looking at the amount of rain and the rate of fall shows a range of  'types' of rain.
By way of example on the 9th we had 8mm with a maximum rate of 100mm per hour.. The rate was slightly higher on the 25th, but very brief so only totalled 3.5mm.  In contrast the 30th scored 7.8mm but steadily, at less than 10mm per hour.

Temperatures

I'll begin with the daily extremes:
Clearly there are quite a few maxima over 30oC and some minima nudging or exceeding 20oC.  So definitely a Summery month. The low maximum of 24.9oC on the 24th was a surprise all round, but I don't think anyone complained.

Readers might be interested in this post looking at temperature ranges.

Maximum temperatures

The next chart compares the average maximum for the month in comparison with last January and the average since 2010.
 Almost spot on the average and a good bit below last year.

I define a Heat Wave as 3 or more days over 30oC and minima above 10oC.  I then count the number of days that fall within Heat Waves.  The next chart shows the number of heat wave days for Januaries since 1994.  This year was not unusually prone to heat waves (although a couple of days with maxima over 30oC were not part of a Wave as they had minima below 10 (thank you evening Easterly)!
So my conclusion is that it was a hot month but not unusually so.  That was the conclusion I reached in compiling this post in the middle of January.

Minimum Temperature

The month continued the recent sequence of months above the average minimum low.  This commenced in December 2016.  Indeed  apart from October and November 2016 (slightly below the mean average minimum - if that makes sense) in every month since January 2016 the average minimum has been warm.

Looking at the chart of extremes at the beginning of this section shows three very high minima.  These are the 3rd, 4th and 7th highest minima recorded since 1994.  In most cases such high minimum temperatures in the morning are overwritten by a cooler evening temperature.  In January 2018 that wasn't the case as the day continued hot.

Average Temperatures

I have observed several times that while maximum temperatures are close to, or slightly below, average the minima have been above average.  Taking the monthly mean temperature is one way of assessing how these balance out.  

For the period for which I have daily records from my weather station this is simply a matter of calculating the average of the hourly readings.  For the years 2010 -2012 I have approximated a daily mean as (max+min)/2 and taken the average of those values as the monthly mean.  (I was able to compare the two measures of mean for 2013 -2017 and they were very close.)

The first graph is in standard format showing 2018 above average, but below 2017.
As this is a new topic it is interesting to look at the story for January across the years since 2010.

Humidity

The usual time for assessing Humidity is 3pm (1500 hrs for advanced persons).  I work on a value of 40% rH as being "reasonable".
 Clearly there are many days last month which were well below 40% rH with the values from 17th to 21st very low.

From looking at my Weatherstation earlier in the day the morning humidity seemed very low.  Here are values for 0900hrs compared with average values (for the past 5 Januaries).
Clearly the periods marked with yellow rings are well under average.

In conjunction with the ran data this makes January 2018 a very dry month.

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