Monday, 1 May 2017

Weather Report April 2017

As the report in the Gazette was compiled before the end of the month there have been slight changes to the average temperature and annual rainfall projection with the full month now available.
In summary a fairly normal set of readings for the month.  While rain wasn't brilliant it was better than the first two months of the year.  Possibly the most interesting attribute was the relatively mild temperatured with the average maximum below average and the average minimum above average.

Rainfall

The total fall this month of 31.6mm was below average, but better than the miserable fall recorded in April 2016.
Falls of greater than 0.2mm were recorded on 6 days with the two major episodes on 9-10 and 25-26 April.

Rainfall for the year to date is getting to be reasonable although still only 56% of the average of the 11 years we have been at our house.

 The 4 month total(143.8mm) is well below the 11 year average for our house (252.8mm) and the 33 year average for the area (221mm).  Applying a pro-rata expansion to the total for the 4 months gives a projected annual total of 408mm. 

Temperatures

Looking at daily extremes shows a fairly stable range until the 26th when a cold snap arrives!
Comparing with the averages since 1993 the average maximum temperature for April was below average ...
... while the average minimum was above average (but below last year's value).
This marks the month where interest turns from heatwaves to frosts.  I use two measures of frost with the less severe measure being  a recorded temperature of +2oC which is equivalent to a temperature at ground level of 0oC (ie freezing point).  Following terminology used by my dad in England I call this a ground frost.  A more severe frost occurs when the weather station records 0oC which (for the same reason) I call an air frost.

In summary for April 2017 the number of ground frosts and air frosts were almost identical at 3 and 2 respectively.  As shown in the next pair of charts the former was well below average while the latter was very close to average.

The value for ground-frost days interested me so I explored the 24 year time series.
This shows that the number of ground-frost days in April 2017 is very consistent with recent years.  It is however greatly below years in the early noughties and late 90s.  This made me wonder whether rainfall is an explanatory factor.  This definitely comes under the "nice try, no cigar"category.
The appearance of the graph alone suggests a low relationship and this is confirmed by the value of R2.  I'm glad I can't remember April 1994 with 95mm of rain and 11 ground frosts!

Humidity

Basically a moderate month as far as measured by 1500 relative humidity. Only one day bel;ow 40% and the 2 major peaks coinciding with rain events.
Comparing with previous years (and in this case I have 5 years of data to work with) April 2017 is a little moister than average but well above the value for 2016.

Wind

My impression was that April was not a windy month.  For once, the data supports the assertion: just about bang on average for the month and well below the average for months later in the year.


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