The Brower Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Brower Observatory clear sky chart from 2005-11-05 to 2017-10-06.

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
28%
14895
17%
47096
7%
14864
38%
14864
19%
31517

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 15thNight
20%
720
19%
2506
7%
716
36%
716
16%
1596
January16th to endNight
16%
768
17%
2688
4%
768
32%
768
14%
1764
February1st to 15thNight
17%
702
14%
2253
5%
701
30%
701
14%
1568
February16th to endNight
18%
631
19%
1893
5%
631
35%
631
18%
1290
March1st to 15thNight
17%
720
15%
2112
4%
720
30%
720
15%
1521
March16th to endNight
23%
768
20%
2095
7%
768
34%
768
22%
1399
April1st to 15thNight
20%
697
17%
1775
4%
697
30%
697
19%
1302
April16th to endNight
25%
540
16%
1620
5%
540
33%
540
18%
1123
May1st to 15thNight
25%
540
13%
1620
7%
540
32%
540
17%
1157
May16th to endNight
27%
436
11%
1558
9%
435
34%
435
13%
1128
June1st to 15thNight
33%
360
9%
1302
7%
360
40%
360
12%
962
June16th to endNight
39%
360
10%
1313
9%
360
45%
360
20%
880
July1st to 15thNight
45%
340
10%
1360
13%
340
50%
340
20%
849
July16th to endNight
45%
376
12%
1418
13%
354
51%
354
19%
893
August1st to 15thNight
43%
532
12%
1602
8%
529
46%
529
23%
1072
August16th to endNight
48%
614
21%
1760
12%
614
52%
614
32%
1080
September1st to 15thNight
40%
720
18%
1902
8%
720
46%
720
32%
1215
September16th to endNight
44%
717
27%
1980
15%
717
54%
717
35%
1178
October1st to 15thNight
34%
684
20%
1943
10%
684
45%
684
25%
1290
October16th to endNight
22%
704
16%
2260
6%
704
34%
704
19%
1504
November1st to 15thNight
32%
700
20%
2277
11%
700
44%
700
20%
1474
November16th to endNight
26%
731
19%
2408
8%
731
39%
731
17%
1567
December1st to 15thNight
17%
750
16%
2664
4%
750
31%
750
15%
1817
December16th to endNight
17%
785
17%
2787
4%
785
33%
785
14%
1888

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
19%6%25%424446%37%
48096
Night
18%7%25%424446%38%
47267
TransparencyDay
45%5%11%21%13%1
47785
Night
46%5%11%22%13%1
47096
SeeingDay
22%14%30%20%8%2
6250
Night
22%12%31%24%7%
14897

By Month

Percentage Cloud Cover NE (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 15thDay
16%6%345%2445%6%39%
1432
16th to endDay
16%8%25%32445%6%39%
1536
February1st to 15thDay
16%10%26%5%3435%5%35%
1606
16th to endDay
15%9%46%5%35%5%46%34%
1580
March1st to 15thDay
20%8%26%424346%34%
1838
16th to endDay
17%8%25%423445%41%
2127
April1st to 15thDay
19%7%35%424345%38%
2173
16th to endDay
21%6%24434446%36%
2340
May1st to 15thDay
26%7%35%5%33346%30%
2332
16th to endDay
22%6%25%42345%7%34%
2566
June1st to 15thDay
27%6%25%424446%29%
2655
16th to endDay
21%6%25%5%245%47%33%
2643
July1st to 15thDay
13%6%25%6%35%5%6%8%36%
2520
16th to endDay
14%5%25%5%35%5%7%8%34%
2648
August1st to 15thDay
15%5%245%45%5%5%8%36%
2340
16th to endDay
13%42432446%8%45%
2441
September1st to 15thDay
14%424434346%46%
2058
16th to endDay
14%5%23322336%52%
1980
October1st to 15thDay
19%6%15%433245%43%
1814
16th to endDay
20%6%25%423435%39%
1609
November1st to 15thDay
20%5%2432435%6%39%
1581
16th to endDay
21%6%24313235%44%
1550
December1st to 15thDay
21%9%24424336%36%
1293
16th to endDay
23%11%35%423235%34%
1434
January1st to 15thNight
15%8%2442435%7%39%
2506
16th to endNight
15%9%35%5%245%45%37%
2688
February1st to 15thNight
14%8%36%7%45%346%33%
2252
16th to endNight
15%10%45%424445%39%
1893
March1st to 15thNight
20%10%35%5%34445%33%
2099
16th to endNight
17%6%345%2335%5%42%
2095
April1st to 15thNight
20%8%26%5%34445%34%
1775
16th to endNight
23%6%25%5%24446%35%
1620
May1st to 15thNight
21%7%35%5%35%35%7%30%
1620
16th to endNight
23%8%46%5%1445%7%27%
1556
June1st to 15thNight
26%7%25%43445%6%26%
1302
16th to endNight
21%6%2443445%8%34%
1313
July1st to 15thNight
15%6%25%5%26%5%5%9%35%
1440
16th to endNight
15%5%25%5%35%5%6%9%34%
1514
August1st to 15thNight
13%5%26%5%445%6%7%38%
1613
16th to endNight
12%5%1343445%8%47%
1760
September1st to 15thNight
14%4145%23345%49%
1902
16th to endNight
12%32322245%7%52%
1980
October1st to 15thNight
20%423324445%42%
1943
16th to endNight
21%6%245%23346%38%
2260
November1st to 15thNight
19%6%23324446%41%
2277
16th to endNight
18%7%24424446%41%
2408
December1st to 15thNight
22%8%245%23336%36%
2664
16th to endNight
23%9%24423336%35%
2787

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 15thDay
43%6%11%22%13%1
1433
16th to endDay
44%412%24%12%1
1536
February1st to 15thDay
51%411%19%12%
1609
16th to endDay
49%5%10%18%12%3
1580
March1st to 15thDay
51%5%10%18%12%2
1848
16th to endDay
43%49%19%18%4
2127
April1st to 15thDay
48%47%19%16%3
2173
16th to endDay
47%5%9%19%13%3
2340
May1st to 15thDay
54%5%8%17%13%2
2332
16th to endDay
48%6%12%19%11%1
2570
June1st to 15thDay
53%5%10%19%8%1
2655
16th to endDay
49%7%10%19%12%
2643
July1st to 15thDay
41%8%13%25%11%
2371
16th to endDay
43%7%14%21%10%1
2481
August1st to 15thDay
42%8%14%24%8%
2327
16th to endDay
34%7%13%25%17%1
2441
September1st to 15thDay
37%6%10%25%18%1
2058
16th to endDay
33%49%28%22%1
1980
October1st to 15thDay
45%48%25%14%1
1814
16th to endDay
46%5%10%24%12%
1609
November1st to 15thDay
44%5%12%23%13%1
1581
16th to endDay
43%413%19%16%1
1550
December1st to 15thDay
49%5%11%21%11%
1293
16th to endDay
54%49%17%13%1
1434
January1st to 15thNight
43%5%11%24%12%1
2506
16th to endNight
47%410%20%14%2
2688
February1st to 15thNight
50%411%19%12%1
2253
16th to endNight
46%5%10%21%13%2
1893
March1st to 15thNight
52%48%19%12%2
2112
16th to endNight
43%310%19%18%4
2095
April1st to 15thNight
51%46%18%16%2
1775
16th to endNight
49%5%9%19%12%3
1620
May1st to 15thNight
51%6%9%17%13%2
1620
16th to endNight
55%6%10%16%10%
1558
June1st to 15thNight
56%7%11%15%7%1
1302
16th to endNight
45%7%13%20%11%
1313
July1st to 15thNight
42%6%16%22%10%
1360
16th to endNight
43%6%16%23%8%1
1418
August1st to 15thNight
43%7%14%23%9%
1602
16th to endNight
33%7%14%28%15%
1760
September1st to 15thNight
36%412%28%16%1
1902
16th to endNight
32%410%32%19%1
1980
October1st to 15thNight
42%5%9%25%15%1
1943
16th to endNight
46%411%23%12%1
2260
November1st to 15thNight
43%6%12%22%14%1
2277
16th to endNight
44%5%12%21%15%1
2408
December1st to 15thNight
50%49%23%11%
2664
16th to endNight
50%411%20%12%
2787

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 15thDay
16%28%30%18%6%
60
16th to endDay
21%29%31%15%1
64
February1st to 15thDay
25%18%32%20%2
239
16th to endDay
18%21%33%19%6%
239
March1st to 15thDay
25%26%30%12%4
270
16th to endDay
23%25%28%13%6%3
288
April1st to 15thDay
28%17%31%14%6%1
270
16th to endDay
22%22%32%14%5%3
270
May1st to 15thDay
35%15%28%13%5%1
288
16th to endDay
22%14%38%18%41
331
June1st to 15thDay
29%13%27%19%10%1
450
16th to endDay
22%11%31%23%10%1
438
July1st to 15thDay
18%6%38%22%10%3
305
16th to endDay
16%7%34%25%12%3
326
August1st to 15thDay
15%9%32%25%13%3
313
16th to endDay
15%9%26%28%15%4
332
September1st to 15thDay
17%6%32%26%11%5%
315
16th to endDay
16%10%30%17%14%10%
314
October1st to 15thDay
22%12%22%25%12%5%
266
16th to endDay
26%14%31%18%7%1
240
November1st to 15thDay
24%14%25%21%9%3
235
16th to endDay
26%20%22%22%7%1
240
December1st to 15thDay
27%16%37%16%2
93
16th to endDay
35%23%31%9%
64
January1st to 15thNight
20%19%34%22%3
720
16th to endNight
22%19%33%19%3
768
February1st to 15thNight
19%19%35%22%3
704
16th to endNight
21%17%37%19%4
631
March1st to 15thNight
27%15%35%16%3
720
16th to endNight
20%17%33%21%6%
768
April1st to 15thNight
24%16%33%20%5%
697
16th to endNight
27%13%30%22%5%
540
May1st to 15thNight
25%10%33%24%6%
540
16th to endNight
28%9%26%28%6%
436
June1st to 15thNight
28%6%25%30%10%
360
16th to endNight
21%230%34%9%
360
July1st to 15thNight
20%223%38%14%1
340
16th to endNight
17%224%41%13%
376
August1st to 15thNight
15%427%36%14%
532
16th to endNight
14%328%34%17%1
614
September1st to 15thNight
16%5%31%29%14%1
720
16th to endNight
14%7%29%28%17%3
717
October1st to 15thNight
23%8%26%26%12%1
684
16th to endNight
25%12%35%19%5%
704
November1st to 15thNight
22%10%29%26%9%1
700
16th to endNight
21%17%30%23%7%
731
December1st to 15thNight
27%13%36%18%4
750
16th to endNight
28%18%32%18%2
785
Page generated at 2017-10-07 14:07:50. Version ac04

Copyright 2017 A. Danko


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