Strathmore
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Strathmore clear sky chart from 2005-11-06 to 2024-02-05.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
42%
23550
14%
80392
10%
23490
47%
23490
21%
51998

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 10thNight
36%
936
15%
4668
8%
926
41%
926
11%
3131
January11th to 20thNight
35%
942
19%
4624
9%
929
42%
929
13%
3034
January21st to endNight
33%
835
12%
4801
11%
829
44%
829
12%
2765
February1st to 10thNight
39%
738
14%
2567
13%
738
45%
738
17%
1755
February11th to 20thNight
36%
702
14%
2273
10%
700
43%
700
17%
1596
February21st to endNight
41%
592
16%
1836
11%
592
51%
592
21%
1262
March1st to 10thNight
37%
720
14%
2160
11%
720
42%
720
20%
1539
March11th to 20thNight
36%
720
19%
1985
10%
720
44%
720
23%
1382
March21st to endNight
35%
792
11%
2171
4%
792
40%
792
20%
1599
April1st to 10thNight
37%
653
15%
1733
6%
653
42%
653
21%
1246
April11th to 20thNight
38%
533
14%
1601
9%
533
43%
533
19%
1138
April21st to endNight
40%
540
12%
1620
10%
540
43%
540
20%
1110
May1st to 10thNight
41%
482
10%
1500
8%
482
46%
482
20%
1022
May11th to 20thNight
52%
354
14%
1239
14%
354
55%
354
23%
766
May21st to endNight
44%
388
6%
1358
4%
388
46%
388
18%
916
June1st to 10thNight
42%
360
2%
1260
1%
360
43%
360
18%
875
June11th to 20thNight
40%
360
1%
1260
0%
360
40%
360
17%
872
June21st to endNight
58%
360
1%
1260
0%
360
58%
360
27%
740
July1st to 10thNight
66%
350
2%
1225
0%
350
66%
350
29%
699
July11th to 20thNight
57%
348
6%
1218
4%
348
59%
348
25%
721
July21st to endNight
65%
394
14%
1336
16%
372
70%
372
37%
676
August1st to 10thNight
62%
358
13%
1424
17%
356
65%
356
26%
797
August11th to 20thNight
62%
529
17%
1597
15%
529
65%
529
38%
893
August21st to endNight
59%
594
22%
1782
17%
594
63%
594
36%
1015
September1st to 10thNight
55%
604
19%
1715
14%
604
58%
604
33%
1049
September11th to 20thNight
54%
720
19%
1980
13%
720
58%
720
33%
1253
September21st to endNight
49%
720
22%
1980
14%
720
54%
720
35%
1207
October1st to 10thNight
48%
720
20%
2074
13%
720
54%
720
28%
1344
October11th to 20thNight
44%
720
18%
2268
10%
720
53%
720
26%
1430
October21st to endNight
37%
792
18%
2574
11%
792
46%
792
22%
1688
November1st to 10thNight
34%
864
17%
2529
9%
864
42%
864
21%
1775
November11th to 20thNight
32%
950
19%
2702
9%
950
40%
950
24%
1829
November21st to endNight
37%
950
14%
2850
8%
950
43%
950
24%
1904
December1st to 10thNight
38%
950
16%
2850
10%
950
43%
950
20%
1997
December11th to 20thNight
36%
949
14%
3458
8%
949
41%
949
17%
2301
December21st to endNight
36%
1031
11%
4914
7%
1026
43%
1026
15%
2672

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
9%8%47%6%45%5%6%8%38%
77690
Night
7%9%48%7%45%6%6%8%37%
81128
TransparencyDay
42%7%15%24%10%2
77395
Night
43%7%15%23%11%2
81004
SeeingDay
12%7%25%31%19%6%
14283
Night
11%6%29%36%16%2
23606

By Month

Percentage Cloud Cover NW (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
48%7%11%6%5%6%6%8%9%31%
2219
11th to 20thDay
48%5%11%7%46%9%8%8%31%
2203
21st to endDay
5%7%48%8%45%6%7%8%38%
2794
February1st to 10thDay
5%11%6%10%9%5%7%5%6%7%29%
1486
11th to 20thDay
7%12%5%10%7%45%7%7%6%28%
1575
21st to endDay
411%8%10%9%5%7%5%7%7%29%
1417
March1st to 10thDay
5%14%5%10%7%45%7%5%6%32%
1800
11th to 20thDay
7%10%48%7%45%6%6%9%35%
1973
21st to endDay
9%14%5%8%7%35%5%6%8%29%
2182
April1st to 10thDay
9%11%6%8%7%45%7%7%7%28%
2227
11th to 20thDay
11%12%48%5%45%5%6%8%31%
2310
21st to endDay
14%11%47%7%445%6%8%31%
2340
May1st to 10thDay
14%10%48%6%46%5%6%8%31%
2457
11th to 20thDay
11%7%36%7%36%5%6%9%38%
2654
21st to endDay
19%7%36%5%45%6%6%7%32%
2917
June1st to 10thDay
17%6%26%5%46%5%6%9%32%
2700
11th to 20thDay
18%8%35%6%45%6%6%8%29%
2700
21st to endDay
8%425%5%46%6%7%10%43%
2700
July1st to 10thDay
9%325%6%45%6%7%9%45%
2700
11th to 20thDay
9%4245%35%5%7%9%47%
2700
21st to endDay
5%3134345%7%10%56%
2901
August1st to 10thDay
9%5%25%5%35%5%6%9%47%
2507
11th to 20thDay
9%324435%5%6%10%50%
2304
21st to endDay
10%4244346%5%8%50%
2574
September1st to 10thDay
11%5%35%43335%7%51%
2241
11th to 20thDay
14%7%25%5%2445%7%45%
1980
21st to endDay
10%8%35%6%2445%8%45%
1980
October1st to 10thDay
14%9%36%5%36%45%6%41%
1886
11th to 20thDay
10%7%36%6%35%45%7%42%
1689
21st to endDay
9%10%37%6%45%7%6%6%37%
1782
November1st to 10thDay
11%14%6%9%6%3446%8%29%
1531
11th to 20thDay
8%11%47%7%5%5%45%6%38%
1477
21st to endDay
5%10%5%8%6%45%6%8%9%35%
1330
December1st to 10thDay
6%13%6%9%6%47%5%5%7%31%
1330
11th to 20thDay
412%5%11%8%47%6%5%8%31%
1682
21st to endDay
48%47%6%35%6%6%8%44%
2442
January1st to 10thNight
413%6%10%9%46%6%7%10%27%
4726
11th to 20thNight
38%6%12%7%47%7%5%8%33%
4699
21st to endNight
410%5%9%8%35%5%6%8%38%
4890
February1st to 10thNight
5%9%5%11%8%46%6%6%7%31%
2567
11th to 20thNight
6%13%6%10%7%35%7%6%7%29%
2272
21st to endNight
410%6%10%8%46%5%7%8%32%
1836
March1st to 10thNight
5%11%5%9%10%45%6%6%10%29%
2160
11th to 20thNight
7%10%5%7%8%46%5%6%9%34%
1985
21st to endNight
7%13%6%9%7%36%6%6%7%28%
2171
April1st to 10thNight
8%12%5%8%7%47%5%6%10%29%
1733
11th to 20thNight
9%12%5%8%6%46%5%6%7%32%
1601
21st to endNight
11%11%46%6%45%5%7%9%32%
1620
May1st to 10thNight
12%9%5%7%5%37%6%7%7%34%
1500
11th to 20thNight
11%6%36%6%446%5%8%41%
1239
21st to endNight
18%7%36%6%45%6%6%7%33%
1358
June1st to 10thNight
15%7%37%7%36%6%7%7%31%
1260
11th to 20thNight
16%7%46%6%46%7%7%8%30%
1260
21st to endNight
8%6%345%337%8%8%44%
1260
July1st to 10thNight
9%5%24436%6%7%9%45%
1260
11th to 20thNight
7%425%5%46%8%7%9%43%
1260
21st to endNight
4324425%6%7%9%55%
1414
August1st to 10thNight
7%6%25%6%45%5%5%8%47%
1432
11th to 20thNight
7%4244245%7%11%50%
1597
21st to endNight
10%4235%345%5%8%51%
1782
September1st to 10thNight
10%6%35%5%35%5%5%7%47%
1715
11th to 20thNight
13%7%2443445%7%48%
1980
21st to endNight
10%5%24435%6%6%8%47%
1980
October1st to 10thNight
11%8%35%5%345%5%9%41%
2074
11th to 20thNight
9%7%35%5%35%6%7%8%41%
2268
21st to endNight
7%10%37%6%36%6%7%7%37%
2574
November1st to 10thNight
9%13%47%7%45%5%5%8%34%
2529
11th to 20thNight
5%10%47%7%45%6%6%8%37%
2702
21st to endNight
5%11%5%7%6%45%5%6%9%36%
2850
December1st to 10thNight
5%11%6%9%8%46%6%5%7%33%
2850
11th to 20thNight
39%5%10%8%5%6%6%6%10%32%
3510
21st to endNight
48%46%6%45%5%6%7%44%
5214

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
47%7%14%22%9%1
2219
11th to 20thDay
47%7%11%16%16%4
2224
21st to endDay
41%7%15%29%8%1
2794
February1st to 10thDay
53%7%15%15%9%2
1486
11th to 20thDay
52%7%14%20%7%1
1584
21st to endDay
53%6%12%17%12%1
1417
March1st to 10thDay
51%7%16%17%9%1
1800
11th to 20thDay
45%7%13%24%11%1
1974
21st to endDay
52%7%16%18%7%
2182
April1st to 10thDay
51%8%15%17%8%1
2227
11th to 20thDay
49%6%12%22%10%1
2310
21st to endDay
50%6%14%21%9%1
2340
May1st to 10thDay
50%7%14%20%8%1
2457
11th to 20thDay
41%7%14%26%11%1
2658
21st to endDay
48%7%14%23%7%1
2918
June1st to 10thDay
47%8%15%21%9%1
2700
11th to 20thDay
50%8%14%19%6%2
2700
21st to endDay
33%8%20%28%8%2
2700
July1st to 10thDay
32%8%21%27%10%2
2626
11th to 20thDay
32%8%20%28%11%1
2619
21st to endDay
23%7%19%34%16%2
2740
August1st to 10thDay
33%7%18%30%10%2
2490
11th to 20thDay
29%7%21%30%11%2
2304
21st to endDay
30%6%19%28%15%2
2574
September1st to 10thDay
34%5%15%28%14%5%
2241
11th to 20thDay
39%5%13%27%13%2
1980
21st to endDay
37%6%16%27%12%3
1980
October1st to 10thDay
43%6%14%26%9%1
1886
11th to 20thDay
39%8%14%27%10%2
1689
21st to endDay
44%8%13%23%11%1
1782
November1st to 10thDay
53%6%11%20%8%2
1531
11th to 20thDay
47%5%14%24%9%1
1477
21st to endDay
42%8%16%23%8%2
1330
December1st to 10thDay
53%5%12%17%10%3
1330
11th to 20thDay
49%7%14%23%6%1
1684
21st to endDay
35%7%11%24%18%5%
2442
January1st to 10thNight
51%7%13%21%6%2
4726
11th to 20thNight
49%6%11%16%15%3
4740
21st to endNight
43%6%14%25%11%1
4890
February1st to 10thNight
49%6%13%19%11%2
2567
11th to 20thNight
51%6%13%20%9%1
2273
21st to endNight
47%6%12%21%12%2
1836
March1st to 10thNight
50%7%14%22%7%1
2160
11th to 20thNight
46%6%14%21%12%1
1985
21st to endNight
51%7%17%18%7%1
2171
April1st to 10thNight
49%7%13%19%12%1
1733
11th to 20thNight
49%7%13%21%8%1
1601
21st to endNight
46%8%13%22%9%2
1620
May1st to 10thNight
47%7%13%22%11%1
1500
11th to 20thNight
40%7%13%25%13%1
1239
21st to endNight
48%6%17%20%8%2
1358
June1st to 10thNight
47%9%14%20%9%
1260
11th to 20thNight
48%9%17%17%7%2
1260
21st to endNight
34%9%21%27%9%1
1260
July1st to 10thNight
32%8%22%26%10%1
1225
11th to 20thNight
34%10%18%25%10%3
1218
21st to endNight
23%9%19%32%16%2
1336
August1st to 10thNight
34%7%19%27%11%1
1424
11th to 20thNight
27%7%21%31%13%2
1597
21st to endNight
29%6%15%30%15%4
1782
September1st to 10thNight
34%6%15%26%14%5%
1715
11th to 20thNight
37%5%14%26%13%5%
1980
21st to endNight
32%7%16%26%14%4
1980
October1st to 10thNight
40%6%14%25%13%2
2074
11th to 20thNight
38%8%15%24%12%3
2268
21st to endNight
43%8%14%25%10%1
2574
November1st to 10thNight
49%6%12%21%10%2
2529
11th to 20thNight
44%6%16%21%13%1
2702
21st to endNight
43%7%18%21%9%2
2850
December1st to 10thNight
50%5%13%20%9%3
2850
11th to 20thNight
46%8%19%22%6%1
3507
21st to endNight
38%5%12%26%15%4
5214

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
6%330%34%17%10%
180
11th to 20thDay
7%6%29%32%20%6%
180
21st to endDay
8%7%28%37%17%4
198
February1st to 10thDay
13%8%26%37%11%4
174
11th to 20thDay
10%8%36%31%13%2
240
21st to endDay
7%6%29%37%17%4
283
March1st to 10thDay
10%10%31%32%12%5%
420
11th to 20thDay
9%10%31%31%14%5%
420
21st to endDay
11%11%35%21%16%6%
462
April1st to 10thDay
13%10%31%28%15%3
420
11th to 20thDay
13%9%28%26%18%5%
412
21st to endDay
16%12%26%25%16%4
420
May1st to 10thDay
17%9%23%23%21%6%
485
11th to 20thDay
15%5%20%31%24%5%
625
21st to endDay
16%6%20%28%22%7%
679
June1st to 10thDay
15%7%22%33%19%4
630
11th to 20thDay
18%8%26%31%15%2
630
21st to endDay
10%322%32%24%8%
630
July1st to 10thDay
10%418%35%25%9%
621
11th to 20thDay
11%418%36%23%8%
618
21st to endDay
5%420%34%27%11%
647
August1st to 10thDay
8%5%22%34%22%9%
447
11th to 20thDay
8%220%33%30%8%
437
21st to endDay
9%6%19%31%22%13%
495
September1st to 10thDay
13%6%25%31%20%4
450
11th to 20thDay
17%7%20%28%22%6%
450
21st to endDay
12%12%28%29%14%5%
450
October1st to 10thDay
18%8%19%28%20%7%
356
11th to 20thDay
10%7%34%25%18%6%
270
21st to endDay
14%13%32%28%10%2
297
November1st to 10thDay
16%11%30%29%12%3
270
11th to 20thDay
15%8%37%26%13%1
249
21st to endDay
5%7%36%30%12%10%
180
December1st to 10thDay
5%9%37%32%13%4
180
11th to 20thDay
6%12%34%34%13%1
180
21st to endDay
8%424%32%27%4
198
January1st to 10thNight
8%7%34%37%12%2
950
11th to 20thNight
9%6%36%35%13%1
950
21st to endNight
11%8%33%33%13%2
851
February1st to 10thNight
9%8%30%34%17%3
738
11th to 20thNight
11%6%33%37%13%1
704
21st to endNight
7%329%35%20%5%
592
March1st to 10thNight
9%432%36%17%3
720
11th to 20thNight
9%7%34%35%13%2
720
21st to endNight
13%6%29%34%15%3
792
April1st to 10thNight
12%6%29%35%16%2
653
11th to 20thNight
15%6%31%35%13%
533
21st to endNight
15%6%31%37%11%
540
May1st to 10thNight
16%7%26%34%16%1
482
11th to 20thNight
13%325%38%21%1
354
21st to endNight
18%5%23%35%17%1
389
June1st to 10thNight
17%6%23%37%14%2
360
11th to 20thNight
15%6%30%34%14%1
360
21st to endNight
11%323%39%24%1
360
July1st to 10thNight
10%5%12%44%28%1
350
11th to 20thNight
9%321%44%21%2
348
21st to endNight
4322%43%26%2
394
August1st to 10thNight
9%415%44%26%2
358
11th to 20thNight
8%418%46%22%2
529
21st to endNight
10%421%39%26%1
594
September1st to 10thNight
11%422%41%21%2
604
11th to 20thNight
16%419%38%23%1
720
21st to endNight
10%429%37%17%2
720
October1st to 10thNight
15%424%37%17%2
720
11th to 20thNight
13%5%28%36%14%3
720
21st to endNight
10%7%34%35%13%1
792
November1st to 10thNight
15%7%34%33%10%1
864
11th to 20thNight
9%8%41%30%10%2
950
21st to endNight
10%5%36%35%12%2
950
December1st to 10thNight
10%7%31%34%16%2
950
11th to 20thNight
7%6%37%37%11%2
950
21st to endNight
12%430%36%15%2
1045
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