The first and last frost


The dates of the first and last frosts are important to everyone who wants to grow anything outside. Generally, the further south and nearer the coast you are, the more frost-free days you can expect, and the earlier you can put out those delicate plants.

The ground (particularly grass) is usually considerably colder than the height of standard measurements (about 4' off the ground). A reading of -0.1 or less at this height is called an air frost. The temperature at this height can be well above freezing, yet you can still have a ground frost on the surface.


Across Britain

When is the first air frost of the year? On average it is late September in the glens of the Highlands and in some parts of Wales, early October (10th) for Aberdeen, Edinburgh, Carlisle, Lincoln, Shrewsbury and Stoke; mid-October (15th) for Glasgow, Manchester, Leeds, Birmingham, Oxford, York; late October (25th) for most of the south; early November for most of the coast; mid-November or later for central London; and December for the Scillies, Cornwall, west Wales, and the smaller Scottish islands (surprisingly).

These are the data for the first frost from my previous weather station, in sunny Leamington Spa in the Midlands (central Warwickshire). This is a typical inland English location.

1988

October 28

1989

November 5

1990

November 4

1991

November 5

1992

October 15

1993

October 14

1994

October 3

1995

November 3

The average of these was the October 25th.


The last air frost from the same site:

1989

April 27

1990

April 17

1991

April 22

1992

April 22

1993

April 1

1994

April 2

1995

May 13

1996

April 4

The average of these was the April 17th.


Local: East Scotland (Pitroddie, Perth, then Lundie, then Newtyle)

Air frost

Here in east Scotland the figures for the first air frost are:

1996

November 7

1997

December 29

1998

October 20

1999

November 20

2000

December 15

2001

November 9

2002

October 19

2003

October 30

2004

November 19

2005

November 13

2006

November 2

2007

October 25

2008

October 28

2009

November 8

2010

October 20

2011

December 2

2012

October 16

2013

November 5

2014

November 6

2015

November 21

2016

25 October

2017

30 October

2018

18 October

At first I thought that the 1997 figure was extraordinary late, but after 2000 I'm not so sure. The average date for the first air frost in this part of east Scotland airfrost is currently November 7th.


When is the last air frost in this area of east Scotland? So far the data for the area are:

1997

April 20

1998

April 16

1999

April 16

2000

April 15

2001

April 18

2002

April 7

2003

April 10

2004

April 8

2005

May 18

2006

April 10

2007

April 2

2008

April 9

2009

March 29

2010

May 11

2011

May 4

2012

May 20

2013

April 30

Giving an average so far for the last air frost of April 20th. Unlike the first frost, the dates of the last frost have so far been remarkably similar, with the notable exception of the late air frost in May 2005 (beaten in 2012).


Ground frost


In east Scotland my dates for the last ground frost have been:

1997

May 10

1998

May 3

1999

May 31

2000

June 3

2001

May 7

2002

June 10

2003

May 13

2004

April 17

2005

June 7

2006

May 31

2007

May 28

2008

May 19

2009

May 11

2010

May 31

Average last ground frost: May 21st.


And for the first ground frost:

1996

October 8

1997

October 12

1998

October 8

1999

October 4

2000

October 9

2001

September 18

2002

October 13

2003

September 23

2004

October 18

2005

September 16

2006

October 27

2007

September 15

2008

October 2

2009

October 5

2010

September 25

Average first ground frost: October 4.


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