The following tables present IEM computed monthly data summaries based on
daily data provided by or computed for the IEM. A download interface
exists for the daily summary information. The climatology is provided by the
nearest NCEI climate station () within the current 1991-2020
dataset.
To load shown data into Microsoft Excel,
highlight the table information with your mouse and then copy/paste into Excel.
Precipitation Totals [inch]
Year
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
1985
M
M
M
M
M
M
M
M
0.00
0.00
0.00
0.00
1986
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1987
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1988
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1989
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1990
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1991
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1992
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1993
0.00
0.00
M
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1994
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1995
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1996
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1997
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1998
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1999
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
2000
0.00
0.23
0.00
0.00
0.15
0.00
0.00
0.00
0.00
0.00
0.00
0.10
2001
1.91
3.35
0.00
2.09
2.48
5.89
4.54
1.58
3.49
4.02
2.49
1.56
2002
2.24
3.18
4.50
2.43
3.03
3.68
4.06
3.65
4.89
1.72
5.15
3.25
2003
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
2004
M
M
M
M
M
M
M
M
M
M
M
M
2005
M
M
M
M
M
M
M
M
M
M
M
M
2006
M
M
M
M
M
M
M
M
M
M
M
0.00
2007
0.36
0.12
0.59
2.60
1.48
2.58
5.85
3.15
3.81
4.62
8.21
3.02
2008
5.32
5.93
5.05
2.09
3.40
3.54
4.26
4.48
5.77
2.49
5.74
5.67
2009
4.07
3.21
1.96
6.88
7.25
2.98
4.96
3.14
2.05
6.17
2.10
2.71
2010
2.66
2.00
5.13
1.35
3.26
4.08
4.74
0.63
5.36
5.28
4.72
2.55
2011
1.63
2.81
5.51
5.12
6.06
4.70
6.81
4.37
4.57
3.11
1.77
4.32
2012
4.17
2.73
0.74
3.66
2.89
3.32
3.54
2.93
3.86
5.60
1.41
4.06
2013
1.24
3.95
4.40
1.55
6.19
2.31
3.98
2.73
4.34
0.83
4.42
2.99
2014
4.36
3.89
4.22
3.51
2.39
2.67
4.02
5.21
1.54
3.71
5.82
9.27
2015
4.66
5.88
1.11
6.28
1.39
3.70
3.64
2.19
4.25
4.83
4.17
3.43
2016
1.51
5.02
4.87
1.67
0.27
7.58
1.97
1.96
2.99
5.24
2.95
3.06
2017
2.84
5.48
3.70
4.89
7.80
2.74
1.03
1.69
3.80
1.81
8.27
4.29
2018
8.53
2.90
3.02
4.38
1.87
2.66
0.88
3.28
1.44
M
M
M
2019
M
M
M
M
M
M
M
M
M
M
M
M
2020
M
M
M
M
M
M
M
M
M
M
M
M
2021
M
M
M
M
M
M
M
M
M
M
M
M
2022
M
M
M
M
M
M
M
M
M
M
M
M
2023
M
M
M
M
M
M
M
M
M
M
M
M
2024
M
M
M
M
M
M
M
M
M
M
M
M
2025
M
M
M
M
M
M
M
M
M
M
M
M
Average Wind Speed [MPH]
Year
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
1985
M
M
M
M
M
M
M
M
2.3
1.0
0.5
0.8
1986
1.0
0.0
0.0
1.0
0.0
0.0
0.1
0.0
0.0
0.4
0.0
0.0
1987
0.0
0.0
0.0
0.0
0.0
0.0
0.6
0.0
1.1
0.0
0.0
12.6
1988
11.6
11.2
10.8
14.0
12.2
10.3
7.8
7.8
11.3
12.1
12.2
11.1
1989
12.8
10.5
12.0
10.8
8.8
8.3
9.0
9.9
10.5
9.9
13.7
12.7
1990
10.7
10.7
10.2
13.0
13.6
9.8
7.2
8.8
11.4
13.0
13.1
13.2
1991
10.9
10.5
13.2
11.0
9.6
10.6
7.9
10.2
9.2
12.9
12.4
12.9
1992
13.0
14.2
10.6
11.1
10.6
9.0
9.2
9.9
9.2
9.9
10.4
10.4
1993
8.1
6.6
M
10.5
9.8
8.9
8.5
7.6
9.8
11.1
12.8
14.5
1994
12.4
10.9
9.7
10.6
12.1
10.4
7.8
9.6
11.7
9.7
13.5
11.3
1995
13.0
13.3
12.4
8.5
9.5
9.0
9.2
10.7
9.8
10.8
12.3
14.3
1996
11.9
8.5
3.5
10.8
10.1
7.8
M
M
M
M
M
M
1997
M
M
M
M
M
M
M
M
M
M
M
M
1998
M
M
M
M
M
M
M
M
M
M
M
M
1999
M
M
M
M
M
M
M
M
13.5
13.0
11.9
12.1
2000
11.8
12.3
10.9
11.8
10.7
8.3
7.0
8.1
10.3
12.7
13.5
12.9
2001
9.9
11.6
12.0
11.4
9.4
8.6
8.1
8.2
10.3
10.8
11.4
10.7
2002
9.4
10.9
13.4
10.4
9.5
9.4
7.7
8.2
9.3
11.5
12.0
11.2
2003
10.2
13.6
11.1
9.8
10.9
8.1
8.1
9.1
9.0
12.5
11.9
12.9
2004
11.6
12.3
11.4
10.9
9.8
9.3
7.5
9.3
9.9
10.9
11.7
13.4
2005
10.9
8.6
13.2
10.9
12.2
10.6
8.4
9.4
11.0
12.2
13.4
13.9
2006
11.4
9.7
9.2
12.9
10.3
8.0
7.9
8.9
9.1
12.3
9.9
10.8
2007
10.6
11.4
13.3
13.2
9.8
8.8
8.1
8.5
10.1
10.0
12.7
12.3
2008
11.9
11.3
10.8
9.0
9.5
9.1
7.6
9.8
11.2
11.3
13.2
12.3
2009
11.7
11.9
11.9
12.7
9.4
9.6
8.0
8.5
9.9
10.4
10.4
13.0
2010
11.7
13.1
12.5
11.4
9.6
10.1
8.0
7.4
10.9
11.1
10.7
16.7
2011
12.2
12.6
12.0
11.2
12.3
8.7
7.6
9.1
9.6
11.5
10.4
11.9
2012
10.5
9.6
11.2
11.7
8.9
9.8
7.1
8.2
10.3
10.8
9.9
12.5
2013
11.2
13.5
14.8
9.2
10.5
9.5
8.6
8.6
11.2
9.0
10.3
11.9
2014
11.4
11.0
13.1
12.4
10.8
8.6
9.2
9.5
10.7
10.6
13.4
11.5
2015
12.0
12.1
12.8
13.5
8.5
8.5
8.9
7.8
10.3
10.9
11.0
10.7
2016
10.9
11.4
12.0
10.3
9.5
11.3
7.3
7.8
10.3
12.5
10.4
12.7
2017
12.2
11.3
11.1
9.7
11.6
8.4
7.9
7.6
7.9
10.7
11.8
11.9
2018
10.9
10.3
16.5
10.5
9.7
9.4
8.0
8.2
10.6
11.2
14.2
8.6
2019
12.5
11.3
10.9
12.3
9.8
9.0
7.6
7.4
11.1
9.4
14.0
11.1
2020
9.4
12.5
12.2
14.0
10.7
10.0
8.9
9.0
10.6
9.3
9.9
13.7
2021
10.5
10.9
12.0
14.3
10.0
8.7
9.1
7.7
11.6
8.9
11.2
11.5
2022
12.2
12.8
10.7
11.9
8.7
9.1
8.0
7.9
9.4
9.1
12.8
16.3
2023
11.9
11.2
10.7
9.8
13.0
13.1
7.1
9.8
9.5
10.8
11.8
11.9
2024
12.3
12.3
12.3
3.0
0.0
0.0
5.6
9.1
8.7
11.5
15.2
11.1
2025
11.5
11.2
8.9
11.7
11.4
9.9
M
M
M
M
M
M
Average Daily High Temperature [F]
Year
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
1985
M
M
M
M
M
M
M
M
57.00
49.00
M
M
1986
-12.00
M
M
34.00
M
M
55.00
M
M
36.00
M
M
1987
M
M
M
M
M
M
61.00
M
51.50
M
M
26.10
1988
21.70
19.62
28.19
34.61
47.52
53.63
62.19
64.26
54.70
43.97
37.27
24.13
1989
20.87
18.07
26.37
36.47
46.55
57.13
63.29
63.55
56.50
44.23
34.10
18.97
1990
21.81
17.64
25.97
34.43
45.58
57.07
45.90
67.21
58.47
48.84
36.13
28.58
1991
15.16
22.11
31.45
37.40
50.19
60.60
63.26
66.42
57.03
48.61
39.30
24.58
1992
19.42
18.07
26.68
35.73
48.77
56.96
60.14
62.61
58.60
47.52
34.23
26.68
1993
18.13
14.13
M
37.00
46.26
56.43
60.90
65.81
58.89
45.16
36.07
28.29
1994
16.06
17.04
28.39
35.80
45.48
58.57
65.62
63.44
55.25
49.16
44.55
28.92
1995
22.39
18.86
27.65
36.93
46.13
60.77
68.03
66.37
57.33
49.74
37.43
27.10
1996
21.13
22.10
28.81
37.53
46.90
60.00
61.24
64.48
57.54
46.34
37.08
32.12
1997
23.58
19.92
24.90
34.93
43.84
54.07
62.13
61.65
57.12
45.79
36.85
29.11
1998
21.94
25.79
29.89
40.11
48.52
54.73
62.59
62.93
57.45
46.23
35.81
28.61
1999
19.59
25.08
31.80
39.12
53.23
62.54
66.11
65.14
62.58
48.19
40.10
30.00
2000
23.67
23.94
32.09
36.61
45.63
57.83
63.45
64.04
58.33
47.08
39.48
28.77
2001
23.14
21.95
30.73
36.80
50.74
60.37
64.54
67.58
58.81
49.07
38.19
31.50
2002
22.15
22.06
27.43
36.70
47.62
55.55
62.41
67.66
58.23
47.62
35.96
28.63
2003
21.65
20.53
27.32
35.37
48.15
59.87
65.55
64.70
62.19
50.20
39.46
29.96
2004
17.10
23.60
M
M
M
51.28
61.04
62.10
53.94
46.13
34.41
22.39
2005
18.89
23.57
29.54
37.16
45.26
58.76
66.70
M
46.20
49.77
40.69
28.01
2006
27.79
17.60
28.47
36.60
46.85
56.46
62.88
62.19
55.32
45.21
36.51
27.52
2007
24.17
21.96
29.12
36.54
49.17
56.75
64.94
64.47
54.05
41.83
35.52
23.55
2008
20.99
21.40
22.18
38.05
41.39
M
67.15
64.63
59.74
48.95
39.06
27.45
2009
17.66
26.62
29.26
39.57
47.96
56.69
62.46
67.00
58.51
46.46
42.81
29.76
2010
28.12
29.51
34.72
43.14
51.93
58.64
67.25
66.86
59.01
49.58
39.68
36.82
2011
25.21
24.27
31.39
38.14
47.11
55.48
64.53
64.66
61.55
51.27
41.35
30.85
2012
24.72
23.66
33.27
39.52
50.09
57.57
66.68
67.10
60.86
50.05
37.62
31.00
2013
21.72
25.43
31.79
37.45
46.63
55.70
67.02
64.57
58.11
50.39
38.38
22.48
2014
22.08
21.30
24.21
37.09
46.91
58.83
67.47
65.85
60.04
49.78
34.97
28.17
2015
19.49
15.04
26.31
33.50
49.66
54.09
58.93
67.63
61.92
48.93
39.53
32.57
2016
26.29
27.05
28.14
38.23
49.12
56.25
64.29
66.83
59.58
51.52
40.78
28.76
2017
25.73
24.79
27.63
37.15
47.82
59.82
65.32
64.60
59.54
53.37
38.63
26.75
2018
22.67
25.69
31.79
36.26
48.51
55.68
67.15
67.25
59.26
44.62
35.47
30.23
2019
28.08
30.54
M
M
M
M
M
M
M
M
M
M
2020
M
M
M
M
M
M
M
64.01
59.32
47.60
39.75
33.34
2021
28.76
27.13
31.54
41.62
49.78
61.28
63.77
68.78
60.45
52.95
43.43
33.02
2022
21.47
23.60
30.41
39.39
49.82
58.62
67.36
68.38
60.34
53.09
39.37
32.89
2023
28.38
23.08
31.79
39.77
51.43
56.63
67.66
64.57
62.56
53.95
39.41
34.92
2024
27.70
28.40
33.93
40.24
49.19
61.82
70.85
69.11
61.91
52.55
41.86
32.40
2025
25.98
25.58
31.58
38.68
50.04
60.14
M
M
M
M
M
M
Average Daily Low Temperature [F]
Year
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
1985
M
M
M
M
M
M
M
M
57.00
49.00
M
M
1986
-12.00
M
M
34.00
M
M
55.00
M
M
36.00
M
M
1987
M
M
M
M
M
M
61.00
M
51.50
M
M
19.10
1988
11.10
9.03
21.32
31.50
41.32
47.97
57.74
58.26
47.90
37.87
32.10
12.68
1989
8.26
7.57
16.67
31.57
41.35
51.53
56.68
57.10
48.87
38.42
26.03
9.35
1990
11.35
4.79
18.06
30.67
36.68
46.63
41.50
55.57
47.97
38.35
28.10
14.71
1991
-1.35
7.64
21.10
27.33
38.32
48.50
52.10
54.16
44.87
39.55
30.27
10.94
1992
6.55
6.10
13.32
26.19
37.65
48.89
51.39
53.52
47.50
36.20
24.03
15.35
1993
3.03
-1.53
M
30.17
37.97
46.80
52.84
56.19
46.48
33.65
25.10
18.10
1994
-2.42
2.54
18.45
27.33
37.10
48.77
56.54
51.83
47.67
38.90
33.91
17.15
1995
10.97
2.93
17.52
27.80
37.52
49.77
58.19
55.10
44.67
41.45
27.13
15.03
1996
6.16
9.28
17.42
30.80
36.65
47.40
55.57
57.00
49.85
39.69
30.50
24.21
1997
9.54
7.83
14.80
29.68
38.29
47.82
55.00
55.26
50.54
39.10
29.89
20.86
1998
12.53
17.75
22.61
33.50
42.93
49.77
56.28
55.85
51.07
39.77
29.12
17.25
1999
7.04
16.27
26.83
31.92
43.46
51.21
57.11
55.31
54.08
35.87
28.60
19.42
2000
12.20
11.39
24.93
30.01
38.07
47.12
54.84
55.09
45.43
37.77
33.66
16.04
2001
11.50
9.30
21.14
29.34
40.49
50.30
55.01
55.91
47.19
38.92
28.98
22.89
2002
9.58
8.98
16.31
27.69
37.15
45.56
53.95
56.85
47.29
36.68
27.67
17.69
2003
-13.10
-4.89
13.96
23.08
38.07
48.23
55.95
54.34
49.75
38.42
28.88
19.61
2004
17.10
18.85
M
M
M
48.75
58.42
59.87
52.98
43.44
31.40
18.62
2005
4.07
12.14
18.48
31.07
37.77
48.63
55.78
M
43.00
41.04
30.32
19.05
2006
16.51
12.63
25.81
35.52
44.21
55.22
61.40
58.65
54.47
44.74
34.70
24.87
2007
10.50
7.05
17.24
28.09
37.90
46.89
55.83
54.01
42.84
32.39
25.38
11.31
2008
9.94
9.44
10.82
28.14
34.51
M
59.32
55.91
49.16
38.56
30.96
14.22
2009
2.82
14.46
16.60
30.53
37.87
48.47
54.06
56.36
46.77
35.46
32.26
20.15
2010
18.21
22.11
24.67
34.73
40.72
48.04
57.26
55.74
49.54
39.19
30.20
29.19
2011
12.38
10.66
19.86
29.08
39.26
47.87
54.84
56.68
49.28
41.70
31.58
18.45
2012
10.57
11.74
20.49
30.88
40.12
50.25
55.59
58.31
50.68
40.56
29.23
21.25
2013
7.11
14.07
25.37
27.95
37.67
45.70
55.61
55.32
48.19
38.61
27.22
10.52
2014
7.53
9.49
10.19
28.52
37.81
48.83
57.54
57.86
46.83
40.49
25.02
17.57
2015
4.62
1.22
13.97
25.26
37.14
45.19
49.59
58.38
50.25
36.42
30.65
24.57
2016
15.10
12.45
16.15
26.26
39.43
47.61
54.34
55.94
47.89
40.88
33.47
16.40
2017
13.61
12.66
16.60
29.56
39.54
48.59
54.38
53.82
48.91
41.19
26.48
15.28
2018
8.71
11.10
25.68
26.77
34.57
43.68
55.62
56.88
46.81
34.96
25.01
17.31
2019
9.19
9.12
M
M
M
M
M
M
M
M
M
M
2020
M
M
M
M
M
M
M
51.83
46.72
37.37
28.34
23.74
2021
19.22
15.32
19.64
34.14
38.33
50.20
54.70
59.13
51.04
43.78
32.55
20.21
2022
5.73
10.05
19.33
32.00
38.84
48.59
56.51
58.97
50.03
42.02
28.58
23.43
2023
17.65
7.94
23.86
30.21
39.30
48.86
59.82
56.05
53.73
44.52
27.06
24.68
2024
13.65
16.49
25.95
30.32
40.61
51.51
60.56
57.51
51.62
40.93
33.82
20.82
2025
15.44
11.52
20.65
28.75
38.92
48.94
M
M
M
M
M
M
Average Daily Temperature (high+low)/2 [F]
Year
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
1985
M
M
M
M
M
M
M
M
57.00
49.00
M
M
1986
-12.00
M
M
34.00
M
M
55.00
M
M
36.00
M
M
1987
M
M
M
M
M
M
61.00
M
51.50
M
M
22.60
1988
16.40
14.33
24.76
33.05
44.42
50.80
59.97
61.26
51.30
40.92
34.68
18.40
1989
14.56
12.82
21.52
34.02
43.95
54.33
59.98
60.32
52.68
41.33
30.07
14.16
1990
16.58
11.21
22.02
32.55
41.13
51.85
43.70
61.39
53.22
43.60
32.12
21.65
1991
6.90
14.88
26.27
32.37
44.26
54.55
57.68
60.29
50.95
44.08
34.78
17.76
1992
12.98
12.09
20.00
30.96
43.21
52.93
55.77
58.06
53.05
41.86
29.13
21.02
1993
10.58
6.30
M
33.58
42.11
51.62
56.87
61.00
52.69
39.40
30.58
23.19
1994
6.82
9.79
23.42
31.57
41.29
53.67
61.08
57.64
51.46
44.03
39.23
23.04
1995
16.68
10.89
22.58
32.37
41.82
55.27
63.11
60.73
51.00
45.60
32.28
21.06
1996
13.65
15.69
23.11
34.17
41.77
53.70
58.40
60.74
53.69
43.02
33.79
28.17
1997
16.56
13.88
19.85
32.30
41.06
50.95
58.57
58.45
53.83
42.45
33.37
24.98
1998
17.24
21.77
26.25
36.80
45.72
52.25
59.43
59.39
54.26
43.00
32.46
22.93
1999
13.31
20.67
29.32
35.52
48.35
56.88
61.61
60.22
58.33
42.03
34.35
24.71
2000
17.94
17.67
28.51
33.31
41.85
52.48
59.15
59.56
51.88
42.43
36.57
22.41
2001
17.32
15.62
25.94
33.07
45.61
55.34
59.77
61.75
53.00
44.00
33.58
27.20
2002
15.87
15.52
21.87
32.19
42.38
50.56
58.18
62.25
52.76
42.15
31.81
23.16
2003
4.27
7.82
20.64
29.23
43.11
54.05
60.75
59.52
55.97
44.31
34.17
24.79
2004
17.10
21.23
M
M
M
50.02
59.73
60.99
53.46
44.78
32.91
20.50
2005
11.48
17.85
24.01
34.11
41.52
53.69
61.24
M
44.60
45.41
35.50
23.53
2006
22.15
15.11
27.14
36.06
45.53
55.84
62.14
60.42
54.89
44.97
35.61
26.20
2007
17.33
14.51
23.18
32.32
43.54
51.82
60.38
59.24
48.44
37.11
30.45
17.43
2008
15.46
15.42
16.50
33.09
37.95
M
63.23
60.27
54.45
43.75
35.01
20.84
2009
10.24
20.54
22.93
35.05
42.92
52.58
58.26
61.68
52.64
40.96
37.53
24.96
2010
23.16
25.81
29.70
38.94
46.32
53.34
62.25
61.30
54.27
44.39
34.94
33.00
2011
18.79
17.46
25.62
33.61
43.18
51.68
59.68
60.67
55.42
46.48
36.47
24.65
2012
17.65
17.70
26.88
35.20
45.11
53.91
61.14
62.71
55.77
45.30
33.43
26.12
2013
14.42
19.75
28.58
32.70
42.15
50.70
61.31
59.95
53.15
44.50
32.80
16.50
2014
14.80
15.40
17.20
32.81
42.36
53.83
62.50
61.85
53.43
45.14
29.99
22.87
2015
12.05
8.13
20.14
29.38
43.40
49.64
54.26
63.01
56.09
42.68
35.09
28.57
2016
20.69
19.75
22.14
32.25
44.27
51.93
59.31
61.39
53.73
46.20
37.12
22.58
2017
19.67
18.72
22.12
33.36
43.68
54.20
59.85
59.21
54.23
47.28
32.56
21.02
2018
15.69
18.39
28.74
31.52
41.54
49.68
61.39
62.06
53.04
39.79
30.24
23.77
2019
18.64
19.83
M
M
M
M
M
M
M
M
M
M
2020
M
M
M
M
M
M
M
57.92
53.02
42.49
34.05
28.54
2021
23.99
21.23
25.59
37.88
44.05
55.74
59.24
63.96
55.75
48.37
37.99
26.61
2022
13.60
16.82
24.87
35.70
44.33
53.61
61.94
63.67
55.19
47.55
33.98
28.16
2023
23.02
15.51
27.82
34.99
45.36
52.74
63.74
60.31
58.14
49.23
33.24
29.80
2024
20.67
22.44
29.94
35.28
44.90
56.66
65.71
63.31
56.76
46.74
37.84
26.61
2025
20.71
18.55
26.11
33.72
44.48
54.54
M
M
M
M
M
M
Average Relative Humidity [%]
This value is computed via a simple average of available observations weighted
by the duration between observations.
