Lowest pressures -- when they occur

M.T. Cranium · 15-02-2019 12:48am #1

As part of my research I obtained a file of sea-level pressures at a grid point northeast of Dublin (54N 6W) with readings every six hours taken from the maps in the NOAA 20th century archives (actually they extend back to 1851).

I plan to discuss that in my research thread but thought it might be of interest to list the summary of dates of these lowest pressures, by winter half-year. This will just be a frequency count per month with a discussion of actual dates.

Over the 164 winter seasons (broadly defined, winter half year means 1st July to following 30th June), including the first one only being a partial segment (1st Jan to 30th June 1851), and leaving out the partial data for 1 July to 31 Dec 2014 as it looks like that winter's lowest pressure would have fallen in the 2015 portion, the frequency count overall is as follows, with dates that registered a minimum pressure following the count ... some dates have more than one event and that is shown by asterisks (one for each extra event):

MONTH _ freq __ dates

__ SEP __ 1 ___ 22 (1863)

__ OCT _ 11 ___ 7,8,10,13*18,23*24,27,28

__ NOV _ 30 ___ 1,2*3,5*6,7,9,10*11*12*14***16,20*21,22,23*26,27*30

__ DEC _ 38 ___ 1,2,4,6,7,8**10*,11**12,13,14,15,16*17*18,19,20**21,22*24**25,27,28,29,30*

__ JAN _ 37 ___ 1,3*6,7**8*9,10,11*13,14,15,17*,19,20*21*22,23**24***25,26,28,31**

__ FEB _ 26 ___ 1,2,4*6,8,10*11*13**14*16,17,20*22*24,26***

__ MAR _ 18 ___ 2,3***6,9,10*12,15*17,19,22,24,27,31

__ APR __ 2 ___ 4 (1991) and 15 (1922)

__ MAY __ 1 ___ 4 (2004)

_________________________________________________

The frequency of annual lowest pressure would be similar, as about three-quarters of the annual minima count for lowest in winter half-year, and those that do not qualify have a similar distribution. This is the count (without dates this time) for the months in terms of annual lowest pressures:

SEP 1, OCT 11, NOV 21, DEC 39, JAN 47, FEB 23, MAR 19, APR 1, MAY 2

_________________________________________________

The average lowest pressure over 164 years has been 969.1 mbs and for the 164 winters, 968.9 mbs.

The lowest pressure in the series is 943.1 mb on 4 Feb 1951 followed by 946.3 mb on 26 Jan 1884. A low on 8 Dec 1886 had a central pressure in the 920s just north of this location but the gradient was so tight that it entered the list at 952.9 mb.

The highest value of an annual minimum pressure was 983.5 mb in 1892. The highest value for a winter season minimum pressure was also 983.5 mb but it was for the winter of 1890-91 (on 6 Nov 1890).

A study of five-year running averages of these minimum values shows that the decades around the 1870s and 1880s had lower values than decades on either side of that interval, but in the 20th century and so far into the 21st, the means have tended to remain fairly static when averaged over five-year intervals. There was something of a dip in pressures around 1950 and another one around 1998.

The excel file in which this study is embedded will soon be available either in the research thread, or in a link that I create (depending on whether I can upload this rather large file here or not). So if you have more questions about this subject, you might be able to answer them directly by looking at that. I am going to be posting this file within a day or two at the most.

M.T. Cranium · 15-02-2019 12:58am

Some facts to note, major storms don't always qualify in this regard (Darwin for example followed a somewhat lower pressure a few days before it, and the windstorm of 27 Feb 1903 was somewhat outdone by a less violent system in March).

Taking pressures every six hours may not capture the exact lowest pressure although at that location, most lows responsible will be moving past far enough north that it's likely the pressure minimum will flatten out for about six hours at least. If our location for assessing minimum pressure happened to be northwest of Donegal then that might not be the case.

Neither should this be interpreted as a frequency count for strongest winds, although you would think the two things should be highly correlated.

M.T. Cranium · 16-02-2019 11:33pm

So it turns out that the lowest pressure of the winter-centered year (1 July to 30 June) occurs in winter (D-J-F) over 60% of the time (101 out of 164 cases) and in autumn about one-quarter of cases (42) while spring, mostly March, gets 13% (21). This statistic has never occurred in summer although some of the higher values were almost beaten in the previous August or following June.

The 25% intervals are 30 Nov, 3 Jan, and 6 February. Basically, there is almost a 50% chance that the lowest pressure of the winter will be in December or January, as well as a 50-50 chance that any given year will have its lowest pressure in those months.

The relation between lowest winter and annual pressure appears interesting in one way. It would appear that if a winter where January records the lowest pressure is then followed by a winter with a higher minimum pressure, that one will be most likely to fall in November. This is the only rationale for the otherwise similar numbers and offset numbers for January and November.

Oneiric 3 · 17-02-2019 2:46pm

Not sure if this is of any use to you in your epic research M.T, but just a chart showing the average daily difference (81-10) in hPa between SLP at Malin Hd & Johnstown Castle. (from the ECMWF)

The data is smoothed by a running 7 day mean just for better readibilty and to better pick out trends.

M.T. Cranium · 18-02-2019 9:32pm

Just getting ready to wade into that research thread.

M.T. Cranium · 22-02-2019 8:40pm

So, I was looking on a map to see exactly where 54N 6W is located, had thought it might be off the coast near Drogheda but actually it's just about at the entrance to Carlingford Lough. This makes me a little concerned about the Dec 1886 reading in the data set because that storm has isobars lower than 940 mb, it did end up in fifth place overall. Otherwise, I have looked at maps for just about all these winter and annual min pressures and they all seem to be reliable, problem being whenever strong low pressure shows up you have a lot of isobars over a small area, but seldom was there a lower central pressure than 950 mbs anyway. I would have to think if the data set went back to 1839 that would enter as about 940 mb at that data point, think there was a reconstructed map of the Big Wind posted on the forum a while back.

Here are the actual data from my excel file. These would not fit into the 10mb limit of the excel file that I posted for use in the research thread, over on Net-weather, but I will be posting other portions of that larger file (my master file is 17 mb so basically I just need to post the other half with the data).

The data that I analyze here for lowest pressure fall into both annual and winter half-year categories. Any calendar year whose lowest pressure is not lowest for either winter half-year is marked, and you'll find at least one, in some cases two, lower pressures from that calendar year or an adjacent one as lowest of winter but not of any particular year.

The ranks apply to both sets. The first rank applies to calendar years, and the second to winter half years. In many cases, they are close to identical. A few cases of very low pressures that were not winter half year extremes jogs the order of finish a little. Ranks are read as 1 being lowest pressure to 164 being highest.

Lowest Pressures -- Calendar Years and Winter-centered 12 Months, 1851 to 2014

Note: xx after Min pres indicates this value is not lowest of its winter half-year.

Min pres ___ date ___ year ___ ranks ____ Min pres__date ____ winter half year (1 July previous to 30 June current year)

978.70xx___OCT 2___1851 __ 155 158____979.35___MAR 22___(data only Jan to June 1851)
976.04_____JAN 22__1852 __ 138 144____976.04___JAN 22____winter 1851-52
975.24_____JAN 7___1853 __ 127 133____975.24___JAN 7_____winter 1852-53
969.38_____JAN 8___1854 __ 073 077____969.38___JAN 8_____winter 1853-54
964.26_____MAR 3__ 1855 __ 035 033____964.26___MAR 3____ winter 1854-55
971.14_____JAN 24__1856 __ 090 093____971.14___JAN 24____winter 1855-56
975.69_____OCT 7 __1857 __ 134 135____975.31___DEC 25____winter 1856-57
976.69_____NOV 27_ 1858 __ 142 138____975.69___OCT 7 ____winter 1857-58
975.47xx___OCT 31 _ 1859 __ 131 148____976.69___NOV 27___winter 1858-59
966.74_____JAN 3 __ 1860 __ 054 050____966.74___JAN 3 ____winter 1859-60
982.61xx___FEB 6___1861 __ 162 127____974.20___DEC 8 ____winter 1860-61
980.19_____JAN 24__1862 __ 158 160____980.19___JAN 24 ___ winter 1861-62
975.29_____JAN 6___1863 __ 128 134____975.29___JAN 6 ____winter 1862-63
967.47_____NOV 14_ 1864 __ 060 163____981.78___SEP 22 ___winter 1863-64
970.58_____NOV 22_ 1865 __ 086 060____967.47___NOV 14 __ winter 1864-65
977.19xx___MAR 24_ 1866 __ 149 088____970.58___NOV 22 __ winter 1865-66
971.52_____FEB 6___1867 __ 096 098____971.52___FEB 6 ___ winter 1866-67
974.54_____JAN 19__1868 __ 122 130____974.54___JAN 19 __ winter 1867-68
968.76_____JAN 31__1869 __ 070 071____968.76___JAN 31 __ winter 1868-69
968.32xx___OCT 24__1870 __ 068 157____978.80___JAN 8 ___ winter 1869-70
967.04_____JAN 17__1871 __ 057 054____967.04___JAN 17 __ winter 1870-71
966.05_____JAN 24__1872 __ 048 044____966.05___JAN 24 __ winter 1871-72
958.07_____JAN 20__1873 __ 021 021____958.07___JAN 20 __ winter 1872-73
966.91_____DEC 11__1874 __ 055 126____974.19___FEB 26 __ winter 1873-74
976.80xx___JAN 24__ 1875 __ 143 052____966.91___DEC 11 __winter 1874-75
956.22_____MAR 9___1876 __ 012 012____956.22___MAR 9 __ winter 1875-76
953.36_____NOV 11__1877 __ 006 057____967.25___DEC 4 __ winter 1876-77
975.98_____OCT 10__1878 __ 137 006____953.36___NOV 11 __winter 1877-78
976.90xx___FEB 11__ 1879 __ 144 143____975.98___OCT 10 __winter 1878-79
967.94_____FEB 17__ 1880 __ 064 065____967.94___FEB 17 __ winter 1879-80
957.32_____NOV 27__1881 __ 017 125____974.12___NOV 16 __winter 1880-81
975.67xx___MAR 1 __ 1882 __ 133 017____957.32___NOV 27 __winter 1881-82
962.52xx___NOV 25 _ 1883 __ 028 153____978.00___FEB 2 ___ winter 1882-83
946.32_____JAN 26__ 1884 __ 002 002____946.32___JAN 26 __ winter 1883-84
970.04_____JAN 31__ 1885 __ 080 082____970.04___JAN 31 __ winter 1884-85
952.87_____DEC 8 __ 1886 __ 005 149____977.36___NOV 26 __ winter 1885-86
960.20_____NOV 3 __ 1887 __ 024 005____952.87___DEC 8 ___ winter 1886-87
972.08_____DEC 22 _ 1888 __ 100 023____960.20___NOV 3 ___ winter 1887-88
979.88xx___MAR 20 _ 1889 __ 156 102____972.08___DEC 22 __ winter 1888-89
976.66_____JAN 23 __1890 __ 139 147____976.66___JAN 23 __ winter 1889-90
963.55_____OCT 13 __1891 __ 032 164____983.49___NOV 6 __ winter 1890-91
983.53xx___OCT 29 __1892 __ 164 030____963.55___OCT 13 __ winter 1891-92
974.43_____DEC 10 __1893 __ 119 154____978.16___FEB 14 __ winter 1892-93
975.75xx___OCT 25 __1894 __ 135 129____974.43___DEC 10 __ winter 1893-94
966.53xx___NOV 11 __1895 __ 051 118____973.50___JAN 14 __ winter 1894-95
964.17_____MAR 3 ___1896 __ 034 032____964.17___MAR 3 ___ winter 1895-96
973.38_____MAR 3 ___1897 __ 113 117____973.38___MAR 3 ___ winter 1896-97
975.23xx___NOV 25 __1898 __ 126 139____975.75___DEC 30 __ winter 1897-98
962.34_____DEC 29 __1899 __ 027 115____973.33___FEB 13 ___winter 1898-99
965.83xx___FEB 19 __ 1900 __ 047 026____962.34___DEC 29 __ winter 1899-1900
972.63xx___MAR 1 ___1901 __ 104 067____968.18___DEC 28 __ winter 1900-01
977.23xx___DEC 29 __ 1902 __ 150 116____973.37___DEC 24 __ winter 1901-02
970.01_____MAR 2 ___1903 __ 079 081____970.01___MAR 2 ___ winter 1902-03
970.53_____FEB 13 __ 1904 __ 085 086____970.53___FEB 13 __ winter 1903-04
951.71_____MAR 15 __1905 __ 004 004____951.75___MAR 15 __ winter 1904-05
970.32_____FEB 10 __ 1906 __ 084 085____970.32___FEB 10 __ winter 1905-06
975.41_____DEC 8 ___ 1907 __ 130 155____978.46___FEB 20 __ winter 1906-07
973.75_____DEC 11 __ 1908 __ 116 136____975.41___DEC 8 ___ winter 1907-08
964.98xx___DEC 3 ___ 1909 __ 037 121____973.75___DEC 11 __ winter 1908-09
957.63_____FEB 20 __ 1910 __ 019 019____957.63___ FEB 20 __ winter 1909-10
969.55_____DEC 10 __ 1911 __ 074 063____967.77___DEC 16 __ winter 1910-11
973.15xx___ FEB 8 ___ 1912 __ 110 078____969.55___DEC 10 __ winter 1911-12
971.47_____MAR 19 __ 1913 __ 094 096____971.47___MAR 19 __ winter 1912-13
957.83_____FEB 22 ___1914 __ 020 020____957.83___FEB 22 __ winter 1913-14
954.28_____JAN 1 ___ 1915 __ 010 010____954.28___JAN 1 ____winter 1914-15
963.96_____NOV 5 ___1916 __ 033 075____969.20___DEC 24 __ winter 1915-16
975.61_____OCT 13 __1917 __ 132 031____963.96___NOV 5 ___ winter 1916-17
977.60xx___JAN 21 __ 1918 __ 152 137____975.61___OCT 13 __ winter 1917-18
972.24_____JAN 7 ___ 1919 __ 102 104____972.24___JAN 7 ___ winter 1918-19
969.11_____JAN 11 __ 1920 __ 072 074____969.11___JAN 11 __ winter 1919-20
983.38xx___JAN 31 __ 1921 __ 163 151____977.61___DEC 21 __ winter 1920-21
968.30xx___DEC 30 __ 1922 __ 067 159____979.60___APR 15 __ winter 1921-22
956.42_____FEB 26 __ 1923 __ 014 014____956.42___FEB 26 __ winter 1922-23
973.50xx___DEC 27 __ 1924 __ 114 103____972.23___OCT 23 __ winter 1923-24
961.57_____FEB 26 __ 1925 __ 026 025____961.57___ FEB 26 __ winter 1924-25
965.23_____NOV 20 __ 1926 __ 040 055____967.12___OCT 23 __ winter 1925-26
963.09_____DEC 22 __ 1927 __ 030 037____965.23___NOV 20 __ winter 1926-27
968.49_____DEC 16 __ 1928 __ 069 028____963.09___DEC 22 __ winter 1927-28
953.74_____DEC 7 ___ 1929 __ 008 069____968.49___DEC 16 __ winter 1928-29
970.01xx___FEB 1 ____1930 __ 079 008____953.74___DEC 7 ___ winter 1929-30
965.10_____NOV 10 __ 1931 __ 038 092____971.04___NOV 2 ___ winter 1930-31
976.95xx___JAN 10 ___1932 __ 146 035____965.10___NOV 10 __ winter 1931-32
978.67_____MAR 17 __ 1933 __ 154 156____978.67___MAR 17 __ winter 1932-33
964.89_____DEC 15 __ 1934 __ 036 047____966.43___MAR 15 __ winter 1933-34
968.08xx___FEB 25 ___1935 __ 066 034____964.89___DEC 15 __ winter 1934-35
965.46_____JAN 9 ____1936 __ 043 040____965.46___JAN 9 ____winter 1935-36
976.68xx___JAN 24 __ 1937 __ 141 070____968.65___NOV 7 ___ winter 1936-37
965.74_____JAN 15 __ 1938 __ 046 043____965.74___JAN 15 __ winter 1937-38
970.14xx___JAN 16 __ 1939 __ 082 059____967.28___NOV 23 __ winter 1938-39
967.25_____NOV 12 __1940 __ 058 142____975.83___NOV 5 ___ winter 1939-40
974.49xx___JAN 22 __ 1941 __ 121 057____967.25___NOV 12 __ winter 1940-41
980.40xx___MAY 28 __1942 __ 159 146____976.55___NOV 10 __ winter 1941-42
966.27_____JAN 31 __ 1943 __ 149 045____966.27___JAN 31 __ winter 1942-43
969.56_____DEC 17 __1944 __ 075 087____970.57___JAN 24 ___winter 1943-44
956.32_____DEC 18 __ 1945 __ 013 079____969.56___DEC 17 __ winter 1944-45
972.89xx___JAN 9 ____1946 __ 105 013____956.32___DEC 18 __ winter 1945-46
977.84xx___MAR 21 __ 1947 __ 153 122____973.91___NOV 20 __ winter 1946-47
967.92_____MAR 31 __ 1948 __ 063 064____967.92___MAR 31 __ winter 1947-48
953.75_____JAN 1 ____1949 __ 009 009____953.75___JAN 1 ____winter 1948-49
976.98xx___FEB 11 ___1950 __ 147 145____976.11___NOV 21 __ winter 1949-50
943.11_____FEB 4____ 1951 __ 001 001____943.11___FEB 4 ____winter 1950-51
980.15xx___DEC 17 ___1952 __ 157 051____966.78___DEC 27 __ winter 1951-52
971.18xx___SEP 21 ___1953 __ 091 105____972.45___FEB 10 ___winter 1952-53
957.55_____NOV 30 __ 1954 __ 018 068____968.29___MAR 3 ___ winter 1953-54
971.24xx___DEC 14 __ 1955 __ 092 018____957.55___NOV 30 __ winter 1954-55
967.28_____JAN 10 __ 1956 __ 059 059____967.28___JAN 10 ___winter 1955-56
968.78_____DEC 11 __ 1957 __ 071 123____974.04___FEB 13 __ winter 1956-57
967.02_____DEC 19 __ 1958 __ 056 072____968.78___DEC 11 __ winter 1957-58
966.68_____OCT 27 __ 1959 __ 053 053____967.02___DEC 19 __ winter 1958-59
967.49_____NOV 1____1960 __ 061 049____966.68___OCT 27 __ winter 1959-60
976.91xx___DEC 5____1961 __ 145 061____967.49___NOV 1 ___ winter 1960-61
957.26_____JAN 11 __ 1962 __ 016 016____957.26___JAN 11 ___winter 1961-62
972.62_____MAR 10__ 1963 __ 103 106____972.62___MAR 10 __ winter 1962-63
981.33xx___OCT 9____1964 __ 160 113____973.12___NOV 11 __ winter 1963-64
974.28_____JAN 20 __ 1965 __ 117 128____974.28___JAN 20 __ winter 1964-65
953.56_____DEC 1 ___ 1966 __ 007 095____971.41___FEB 22 __ winter 1965-66
975.35xx___FEB 19 __ 1967 __ 129 007____953.56___DEC 1 ___ winter 1966-67
975.85xx___DEC 17 __ 1968 __ 136 140____975.77___NOV 2 ___ winter 1967-68
966.59_____JAN 13 __ 1969 __ 052 048____966.59___JAN 13 ___winter 1968-69
977.08xx___NOV 18 __ 1970 __ 148 100____971.66___NOV 9 ___winter 1969-70
969.73_____JAN 21 __ 1971 __ 076 080____969.73___JAN 21 __ winter 1970-71
973.07_____MAR 6___ 1972 __ 109 112____973.07___MAR 6 ___ winter 1971-72
972.99_____FEB 14 __ 1973 __ 107 109____972.99___FEB 14 __ winter 1972-73
965.20_____FEB 11 __ 1974 __ 039 036____965.20___FEB 11 __ winter 1973-74
981.55xx___JAN 14 __ 1975 __ 161 073____968.79___NOV 14 __winter 1974-75
967.95_____DEC 6____1976 __ 065 161____980.76___MAR 12 __winter 1975-76
974.86xx___JAN 25 __ 1977 __ 123 066____967.95___DEC 6 ___ winter 1976-77
962.96_____DEC 12 __ 1978 __ 029 076____969.29___JAN 28 __ winter 1977-78
974.44xx___DEC 15 __ 1979 __ 120 027____962.96___DEC 12 __ winter 1978-79
972.90_____JAN 21 __ 1980 __ 106 108____972.90___JAN 21 ___winter 1979-80
970.89_____DEC 30 __ 1981 __ 088 131____974.86___DEC 20 __ winter 1980-81
963.42_____DEC 20 __ 1982 __ 031 090____970.89___DEC 30 __ winter 1981-82
971.25_____DEC 20 __ 1983 __ 093 029____963.42___DEC 20 __ winter 1982-83
972.11xx___JAN 23 ___1984 __ 101 094____971.25___DEC 20 __ winter 1983-84
976.67xx___NOV 5 ___ 1985 __ 140 141____975.82___OCT 18 __ winter 1984-85
973.56_____MAR 24 __ 1986 __ 115 120____973.56___MAR 24 __winter 1985-86
966.42_____MAR 27 __ 1987 __ 050 046____966.42___MAR 27 __winter 1986-87
960.57_____FEB 1____ 1988 __ 025 024____960.57___FEB 1 ___ winter 1987-88
950.08_____DEC 17 __ 1989 __ 003 022____959.00___FEB 26 __ winter 1988-89
959.62xx___JAN 25 __ 1990 __ 023 003____950.08___DEC 17 __ winter 1989-90
970.96_____APR 4____1991 __ 089 091____970.96___APR 4 ___ winter 1990-91
973.18_____DEC 2____1992 __ 112 152____977.65___NOV 12 __ winter 1991-92
971.68xx___DEC 15 __ 1993 __ 098 114____973.18___DEC 2 ___ winter 1992-93
970.05_____FEB 4____ 1994 __ 081 083____970.05___FEB 4 ____winter 1993-94
965.67_____JAN 17 ___1995 __ 045 042____965.67___JAN 17 __ winter 1994-95
970.19_____JAN 7____ 1996 __ 083 084____970.19___JAN 7 ___ winter 1995-96
971.57_____FEB 24 ___1997 __ 097 099____971.57___FEB 24 __ winter 1996-97
970.69_____JAN 3____ 1998 __ 087 089____970.69___JAN 3 ___ winter 1997-98
965.65_____DEC 24 __ 1999 __ 044 107____972.81___OCT 24 __ winter 1998-99
969.88xx___NOV 25 __ 2000 __ 077 041____965.65___DEC 24 __ winter 1999-2000
965.44_____JAN 23 ___2001 __ 041 038____965.44___JAN 23 __ winter 2000-01
971.50_____NOV 14___2002 __ 095 119____973.54___JAN 23 __ winter 2001-02
974.31xx___JAN 21 ___2003 __ 118 097____971.50___NOV 14 __ winter 2002-03
975.05_____MAY 4 ___ 2004 __ 124 132____975.05___MAY 4 ___ winter 2003-04
975.17xx___DEC 2____ 2005 __ 125 162____981.22___OCT 28 __ winter 2004-05
967.67_____NOV 23 __ 2006 __ 062 112____973.07___FEB 16 __ winter 2005-06
973.18xx___JAN 18 ___2007 __ 112 062____967.67___NOV 23 __ winter 2006-07
957.05_____MAR 10 __ 2008 __ 015 015____957.05___MAR 10 __ winter 2007-08
971.93_____JAN 25 ___2009 __ 099 101____971.93___JAN 25 ___winter 2008-09
965.46_____NOV 8 ___ 2010 __ 043 124____974.07___NOV 14 __ winter 2009-10
977.55_____DEC 13 __ 2011 __ 151 040____965.46___NOV 8 ___ winter 2010-11
973.06_____DEC 14 __ 2012 __ 108 150____977.55___DEC 13 __ winter 2011-12
959.32xx___DEC 27 __ 2013 __ 022 110____973.06___DEC 14 __ winter 2012-13
955.28_____FEB 8____ 2014 __ 011 011____955.28___FEB 8 ____winter 2013-14
--- ---- ___ ___ ___ ___ ___ ___ __ (no)____984.52___NOV 7 ___ first half winter 2014-15 (to 31 Dec 2014)

969.09 ___ ___ ___ ___ ___ ___ ___ ___ ____ 968.91
mean ___________________________________ mean

983.53 ___ ___ ____ ___ ___ ___ ___ ___ ___ 983.49
max value _______________________________ max value (2014-15 not ranked)

943.11 ___ ___ ____ _____ ___ ___ ___ ___ ___ 943.11
min value ________________________________ min value
_______________________________________________________________________

M.T. Cranium · 23-02-2019 7:11pm

Not much long-term variability in the seasonal distribution of the lowest pressure. This is easier to follow from the winter half-year timing than the annual timing.

Looking at 30-year intervals 1851 to 2000 then the final 14 years of the data ... the median of 30 years will be the average of 15th and 16th ranked dates (from start to finish of the season). For example, in 1851-1880, 15 cases were 7th Jan or earlier, the other 15 8th January or later. So the average of those is 7.5 Jan.

INTERVAL ____ median date of lowest pressure

1851-1880 ___ Jan 7.5

1881-1910 ___ Jan 6.5

1911-1940 ___ Dec 16.0

1941-1970 ___ Dec 23.0

1971-2000 ___ Jan 5.0

2001-2014 ___ Jan 3.0

______________________________________________________

The longest stretch of winters with minimum pressure before end of December is seven (winters of 1925-26 to 1931-32). The longest stretch of winters with minimum pressure in January or later is eight from 1866-67 to 1873-74.

In terms of intensity (ranking from lowest to highest values), the three winters with the most sustained intensity (all ranked higher than a given ranking) is 1987-88 to 1989-90, the rankings were 24th, 22nd and 3rd. The three least intense consecutive winters would be 1856-57 to 1858-59 which ranked 135th, 138th, 148th. More recently 2003-04 to 2005-06 ranked 132nd, 162nd and 112nd.

The most intense low pressure not to be lowest of its own winter was "Erich" of Dec 27, 2013 (959.3 mbs) which was 22nd lowest pressure but beaten twice in Feb 2014 by Darwin and a previous event on 8th Feb, and that near-miss was closely followed by the "Burns Day" storm of 25 Jan 1990 (959.6 mb) which was 23rd lowest pressure, and beaten out by a lower reading in December 1989. That may not have been true of the central pressures of those two events however. 28th lowest annual reading in November 1883 also lost out to 2nd lowest in January 1884. A fourth notable storm not to dominate its winter was the destructive gale of 27 Feb 1903 which had a similar reading to a later event on 8th of March but lost fractionally to that one. This involved readings in the low 970s but I am somewhat dubious that the 27th Feb storm had such a tame central pressure, it went quite close to the grid point for the data set. I have similar doubts about the reading for the 8th Dec 1886 event which ranks fifth but might have been as low as 930 mb at the grid point given that a reading in the 920s was taken quite close to there in the Belfast region. The maps are somewhat inconclusive on those questions.

M.T. Cranium · 12-03-2019 1:59am

I can now add to this an analysis of highest annual pressures.

The average value is 1040.9 mbs.

The highest in the period of record is 1050.75 mbs, on 9 Jan 1896.

The second highest is 1048.16 mbs on 31 Jan 1902.

The highest since 1980 is 1046.58 mbs on 26 Jan 1992.

The distribution by months was similar to low pressure, Dec and Jan are about equal (33 vs 35), Feb and Mar have 25 each, Nov 20. The rest are sprinkled among all months except July and August (which have sometimes come close). 12 were in April, 5 each in September and October, 3 in May and one in June (14th, 1959).

There was a notable decrease in the running average around the period 1914-19, when nine consecutive years failed to reach 1040 mbs and one (1916) topped out at 1028.7 mbs. Otherwise, the running average for five year periods hovers close to 1040 mbs most of the time.

Since the running mean recovered from the low in the 1910-20 interval, there has been a slight tendency for this statistic to gain value, through the rather small range of 1040 to 1042 mbs. It had topped out at 1043.8 mbs (1904-08) and has twice surpassed that former maximum (in the intervals 1960-64 to 1962-66, all averaged 1044 mbs, 1961-65 at 1044.27 mbs). An even higher running average was produced by 1997 to 2001 (1044.89 mbs).

I have not broken down these high pressure readings by winter seasons, but a quick inspection of the dates shows that quite a few winters have two of the annual maxima while an almost equal number have neither of them. These winters have two annual maxima (winter being defined as July through succeeding June, although in most cases these values are in the actual winter or at least close to that season).

1851-52, 1853-54, 1855-56, 1857-58, 1866-67, 1868-69, 1873-74, 1877-78, 1879-80, 1884-85, 1886-87, 1889-90, 1894-95 1899-1900,
1903-04, 1905-06, 1913-14, 1915-16, 1924-25, 1927-28, 1931-32, 1933-34, 1937-38, 1941-42, 1947-48, 1952-53, 1956-57, 1958-59,
1962-63, 1967-68, 1970-71, 1972-73, 1974-75, 1976-77, 1988-89, 1994-95, 1999-2000, 2006-07, 2010-11.

Winters which produced neither annual maximum always include those between couples above that are two years apart, so these qualify:

1852-53, 1854-55, 1856-57, 1867-68, 1878-79, 1885-86, 1904-05, 1914-15, 1932-33, 1957-58, 1971-72, 1973-74, 1975-76.

In addition, these winters did not produce either annual max: 1862-63, 1872-73, 1876-77, 1883-84, 1888-89, 1892-93, 1896-97, 1902-03,
1911-12, 1921-22, 1925-26, 1930-31, 1935-36, 1939-40, 1945-46, 1950-51, 1955-56, 1960-61, 1966-67, 1969-70, 1985-86, 1993-94,
1998-99, 2003-04, 2008-09, 2012-13.

The first seven years (1851 to 1857) had no maximum pressure values in meteorological winter months. The longest runs of years with maximum pressure in a winter month was six (1932-37).

Lowest pressures -- when they occur

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