Phillips Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Phillips Observatory clear sky chart from 2005-11-05 to 2017-09-24.

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
51%
15519
19%
46838
16%
15480
55%
15480
27%
29918

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
40%
716
11%
2500
9%
716
43%
716
14%
1856
January16th to endNight
43%
768
13%
2496
11%
768
45%
768
16%
1807
February1st to 15thNight
39%
700
9%
2288
8%
698
42%
698
16%
1688
February16th to endNight
38%
632
13%
1926
10%
632
44%
632
19%
1349
March1st to 15thNight
43%
720
14%
2039
10%
720
48%
720
21%
1453
March16th to endNight
40%
768
15%
2112
9%
768
45%
768
22%
1479
April1st to 15thNight
42%
641
16%
1823
9%
641
45%
641
25%
1205
April16th to endNight
43%
540
21%
1620
14%
540
48%
540
27%
1012
May1st to 15thNight
53%
539
19%
1619
16%
539
56%
539
32%
966
May16th to endNight
57%
562
16%
1619
15%
561
59%
561
28%
1048
June1st to 15thNight
56%
431
18%
1331
16%
431
61%
431
33%
781
June16th to endNight
71%
360
30%
1260
35%
360
78%
360
51%
539
July1st to 15thNight
78%
340
23%
1310
31%
340
82%
340
44%
580
July16th to endNight
80%
538
28%
1567
30%
505
80%
505
56%
723
August1st to 15thNight
75%
539
28%
1611
25%
536
76%
536
48%
823
August16th to endNight
76%
573
38%
1719
31%
573
78%
573
54%
831
September1st to 15thNight
69%
676
29%
1921
23%
676
74%
676
49%
992
September16th to endNight
67%
692
33%
1914
27%
692
71%
692
45%
1038
October1st to 15thNight
60%
660
24%
1922
19%
660
64%
660
37%
1113
October16th to endNight
58%
704
25%
2161
22%
704
63%
704
32%
1290
November1st to 15thNight
50%
700
20%
2275
18%
700
53%
700
23%
1458
November16th to endNight
45%
867
15%
2487
11%
867
49%
867
23%
1716
December1st to 15thNight
36%
900
8%
2520
5%
900
38%
900
16%
1924
December16th to endNight
29%
953
9%
2798
6%
953
32%
953
12%
2247

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
8%9%35%424446%44%
48081
Night
9%9%35%42445%6%44%
47022
TransparencyDay
39%5%10%19%16%7%
47764
Night
39%6%11%20%14%7%
46839
SeeingDay
11%16%29%25%12%3
5736
Night
12%7%19%35%20%3
15529

By Month

Percentage Cloud Cover SW (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
10%17%5%8%6%34336%29%
1435
16th to endDay
13%18%5%7%5%3334429%
1728
February1st to 15thDay
13%17%6%9%5%23336%28%
1575
16th to endDay
10%18%6%9%5%24346%27%
1545
March1st to 15thDay
9%15%6%7%6%3445%6%31%
1920
16th to endDay
12%16%6%8%42445%6%28%
2112
April1st to 15thDay
11%11%47%7%35%5%47%31%
2122
16th to endDay
10%11%45%5%2445%6%37%
2340
May1st to 15thDay
8%6%245%35%46%7%43%
2331
16th to endDay
12%9%36%5%35%45%7%34%
2492
June1st to 15thDay
10%6%25%4345%5%6%45%
2625
16th to endDay
3212213346%68%
2700
July1st to 15thDay
321321346%7%63%
2566
16th to endDay
331232335%6%63%
2492
August1st to 15thDay
4313423447%61%
2340
16th to endDay
3313233345%64%
2483
September1st to 15thDay
4413423447%60%
2037
16th to endDay
8%413323245%59%
1904
October1st to 15thDay
11%8%245%24336%46%
1708
16th to endDay
8%8%25%44435%7%44%
1711
November1st to 15thDay
15%10%45%413445%40%
1584
16th to endDay
9%12%46%5%3345%5%39%
1461
December1st to 15thDay
13%17%6%9%7%24445%24%
1440
16th to endDay
14%13%6%10%7%3445%424%
1430
January1st to 15thNight
13%15%48%5%24447%29%
2500
16th to endNight
12%16%6%7%5%23345%32%
2496
February1st to 15thNight
10%15%6%9%6%2445%5%29%
2277
16th to endNight
8%12%6%7%43446%6%34%
1926
March1st to 15thNight
7%10%5%7%6%35%5%5%7%34%
2039
16th to endNight
10%11%36%4245%6%8%35%
2112
April1st to 15thNight
8%9%46%5%2446%6%41%
1823
16th to endNight
11%7%2442445%5%46%
1620
May1st to 15thNight
5%5%25%43445%6%50%
1619
16th to endNight
12%6%245%35%45%6%43%
1618
June1st to 15thNight
8%5%144235%5%9%47%
1331
16th to endNight
321231233470%
1260
July1st to 15thNight
12123245%7%8%59%
1390
16th to endNight
2113223347%66%
1674
August1st to 15thNight
3334235%7%8%56%
1620
16th to endNight
3312312347%65%
1719
September1st to 15thNight
431331244464%
1921
16th to endNight
8%313212345%61%
1914
October1st to 15thNight
9%6%24323245%55%
1922
16th to endNight
10%6%2442445%7%47%
2161
November1st to 15thNight
10%11%35%42435%6%42%
2275
16th to endNight
7%9%46%6%3445%6%39%
2487
December1st to 15thNight
14%13%5%8%5%3445%5%28%
2520
16th to endNight
15%17%6%10%6%3433424%
2798

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
56%7%17%13%5%
1435
16th to endDay
58%5%11%16%6%1
1728
February1st to 15thDay
57%6%13%16%5%
1575
16th to endDay
57%6%13%16%41
1545
March1st to 15thDay
52%7%13%18%6%1
1920
16th to endDay
54%7%12%17%7%
2112
April1st to 15thDay
51%6%13%18%9%
2122
16th to endDay
44%6%10%24%13%
2340
May1st to 15thDay
37%5%9%28%17%1
2331
16th to endDay
47%6%10%22%11%1
2495
June1st to 15thDay
38%5%11%23%17%3
2625
16th to endDay
16%45%20%37%16%
2700
July1st to 15thDay
19%45%17%31%21%
2417
16th to endDay
19%36%14%23%32%
2336
August1st to 15thDay
21%37%15%27%25%
2325
16th to endDay
21%35%18%31%19%
2483
September1st to 15thDay
22%48%18%30%15%
2037
16th to endDay
25%48%24%27%8%
1904
October1st to 15thDay
38%5%11%27%12%3
1708
16th to endDay
37%6%15%24%15%
1711
November1st to 15thDay
44%6%13%21%10%1
1584
16th to endDay
44%7%18%18%8%1
1461
December1st to 15thDay
61%8%13%13%2
1440
16th to endDay
60%7%15%11%4
1430
January1st to 15thNight
53%9%14%16%41
2500
16th to endNight
54%5%12%19%5%2
2496
February1st to 15thNight
54%7%15%16%6%
2288
16th to endNight
48%6%17%20%5%1
1926
March1st to 15thNight
46%8%12%22%8%
2039
16th to endNight
44%8%14%23%8%
2112
April1st to 15thNight
40%7%16%22%11%2
1823
16th to endNight
38%5%12%24%18%
1620
May1st to 15thNight
32%6%10%29%18%2
1620
16th to endNight
38%6%12%26%13%2
1619
June1st to 15thNight
31%6%11%23%20%5%
1331
16th to endNight
17%35%16%33%25%
1260
July1st to 15thNight
19%45%19%27%23%
1310
16th to endNight
16%3413%26%35%
1567
August1st to 15thNight
21%46%17%22%27%
1611
16th to endNight
18%35%17%29%25%
1719
September1st to 15thNight
21%48%13%30%22%
1921
16th to endNight
24%46%26%28%9%
1914
October1st to 15thNight
31%314%27%16%5%
1922
16th to endNight
33%6%11%29%18%1
2161
November1st to 15thNight
42%6%12%22%13%3
2275
16th to endNight
43%8%14%21%9%1
2487
December1st to 15thNight
54%8%17%14%4
2520
16th to endNight
63%6%11%13%3
2798

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
24%334%21%9%6%
61
16th to endDay
26%9%21%29%13%
96
February1st to 15thDay
21%15%25%24%11%2
90
16th to endDay
19%26%31%14%6%
204
March1st to 15thDay
17%32%28%18%3
270
16th to endDay
20%34%30%12%2
288
April1st to 15thDay
18%28%27%19%41
269
16th to endDay
21%29%27%15%5%
270
May1st to 15thDay
10%19%41%21%5%1
288
16th to endDay
14%15%24%24%14%5%
393
June1st to 15thDay
12%12%23%30%16%3
495
16th to endDay
411%26%31%24%3
495
July1st to 15thDay
36%25%36%23%3
351
16th to endDay
38%39%33%13%1
327
August1st to 15thDay
5%8%40%31%12%1
315
16th to endDay
39%30%34%17%5%
332
September1st to 15thDay
417%28%23%19%7%
315
16th to endDay
10%15%27%29%11%5%
288
October1st to 15thDay
16%16%24%24%12%6%
148
16th to endDay
20%10%30%20%15%2
96
November1st to 15thDay
10%7%22%22%21%16%
90
16th to endDay
14%25%41%14%4
90
December1st to 15thDay
18%16%35%26%2
90
16th to endDay
17%14%38%18%8%2
75
January1st to 15thNight
18%7%20%35%15%2
720
16th to endNight
20%7%19%32%18%2
768
February1st to 15thNight
15%10%20%37%13%1
704
16th to endNight
11%12%28%32%12%1
632
March1st to 15thNight
9%9%26%38%13%2
720
16th to endNight
14%12%25%33%12%
768
April1st to 15thNight
11%13%24%31%16%2
641
16th to endNight
12%11%25%30%17%1
540
May1st to 15thNight
9%8%22%35%20%3
540
16th to endNight
14%5%16%33%24%5%
563
June1st to 15thNight
9%5%23%36%23%2
431
16th to endNight
5%314%40%30%5%
360
July1st to 15thNight
1110%44%36%5%
340
16th to endNight
318%47%36%3
538
August1st to 15thNight
412%43%36%1
539
16th to endNight
3212%41%34%5%
573
September1st to 15thNight
6%314%35%32%7%
676
16th to endNight
10%412%35%28%9%
692
October1st to 15thNight
11%5%16%35%25%4
660
16th to endNight
13%6%15%37%24%3
704
November1st to 15thNight
15%8%18%35%19%2
700
16th to endNight
9%11%24%35%14%4
867
December1st to 15thNight
19%10%23%35%10%
900
16th to endNight
22%13%23%28%11%1
953
Page generated at 2017-09-25 07:02:28. Version ac04

Copyright 2017 A. Danko


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