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 2019-07-16.

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
52%
17901
19%
54069
16%
17862
55%
17862
26%
34629

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
43%
836
11%
2919
10%
836
46%
836
15%
2125
January16th to endNight
42%
896
12%
2912
11%
896
44%
896
16%
2134
February1st to 15thNight
39%
820
10%
2678
9%
818
42%
818
15%
1980
February16th to endNight
38%
736
12%
2244
9%
736
43%
736
17%
1623
March1st to 15thNight
45%
840
16%
2379
12%
840
49%
840
21%
1698
March16th to endNight
41%
896
13%
2464
8%
896
45%
896
22%
1739
April1st to 15thNight
40%
749
14%
2129
8%
749
44%
749
22%
1461
April16th to endNight
44%
624
21%
1872
14%
624
50%
624
28%
1153
May1st to 15thNight
55%
629
21%
1889
18%
629
57%
629
32%
1127
May16th to endNight
55%
658
14%
1897
13%
657
57%
657
26%
1257
June1st to 15thNight
60%
503
19%
1553
19%
503
64%
503
35%
887
June16th to endNight
70%
420
28%
1470
32%
420
77%
420
47%
647
July1st to 15thNight
79%
400
25%
1542
34%
400
82%
400
44%
688
July16th to endNight
80%
586
27%
1717
29%
553
81%
553
56%
789
August1st to 15thNight
75%
575
29%
1719
26%
572
77%
572
49%
875
August16th to endNight
76%
621
37%
1863
30%
621
78%
621
54%
894
September1st to 15thNight
70%
732
30%
2081
24%
732
75%
732
49%
1073
September16th to endNight
69%
776
33%
2145
26%
776
72%
776
47%
1146
October1st to 15thNight
59%
780
24%
2272
18%
780
63%
780
36%
1329
October16th to endNight
61%
832
27%
2554
24%
832
66%
832
34%
1472
November1st to 15thNight
51%
820
21%
2665
20%
820
54%
820
23%
1711
November16th to endNight
45%
1011
15%
2901
11%
1011
49%
1011
23%
2009
December1st to 15thNight
39%
1050
9%
2940
6%
1050
41%
1050
17%
2199
December16th to endNight
30%
1111
9%
3264
6%
1111
33%
1111
12%
2613

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%5%2445%6%44%
55234
Night
9%9%36%5%2445%6%43%
54253
TransparencyDay
39%5%10%19%16%7%
54917
Night
39%6%11%20%14%6%
54070
SeeingDay
12%16%29%26%12%3
8104
Night
12%7%19%36%20%3
17911

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%16%5%8%6%34346%30%
1675
16th to endDay
12%18%6%7%5%3434428%
2016
February1st to 15thDay
12%18%6%10%5%24345%25%
1845
16th to endDay
9%18%7%10%7%34445%24%
1799
March1st to 15thDay
9%15%6%7%6%34446%30%
2240
16th to endDay
12%16%5%8%5%2445%6%28%
2464
April1st to 15thDay
13%12%47%7%35%446%28%
2476
16th to endDay
10%11%45%5%2445%6%39%
2693
May1st to 15thDay
8%6%245%45%46%7%44%
2721
16th to endDay
12%10%36%5%35%5%6%7%33%
2918
June1st to 15thDay
9%5%243345%5%7%47%
3063
16th to endDay
3312223346%67%
3150
July1st to 15thDay
321331345%7%63%
2994
16th to endDay
331232335%6%63%
2706
August1st to 15thDay
3213423447%62%
2485
16th to endDay
3313323445%63%
2691
September1st to 15thDay
3313323447%61%
2207
16th to endDay
7%413323245%61%
2145
October1st to 15thDay
11%8%345%24336%46%
2018
16th to endDay
7%7%25%43435%7%47%
2021
November1st to 15thDay
14%11%35%424445%39%
1854
16th to endDay
9%12%46%5%3345%5%38%
1704
December1st to 15thDay
12%15%6%9%8%2445%5%25%
1680
16th to endDay
14%14%6%9%8%3445%423%
1669
January1st to 15thNight
12%14%48%5%24448%30%
2919
16th to endNight
12%16%5%7%5%24346%30%
2912
February1st to 15thNight
10%14%6%8%6%3445%5%29%
2667
16th to endNight
7%13%7%8%5%3446%6%31%
2244
March1st to 15thNight
7%10%5%7%6%35%5%5%7%34%
2379
16th to endNight
10%11%37%5%35%5%6%7%34%
2464
April1st to 15thNight
10%10%46%5%2446%6%38%
2129
16th to endNight
11%6%2442445%5%48%
1872
May1st to 15thNight
6%5%25%43445%6%50%
1889
16th to endNight
12%6%245%36%5%5%6%41%
1896
June1st to 15thNight
8%4144235%5%9%50%
1553
16th to endNight
4212413345%67%
1470
July1st to 15thNight
12123245%7%8%59%
1622
16th to endNight
213223447%66%
1824
August1st to 15thNight
3334245%7%8%57%
1728
16th to endNight
3312312346%66%
1863
September1st to 15thNight
431332334464%
2081
16th to endNight
7%313212345%63%
2145
October1st to 15thNight
9%6%25%323245%54%
2272
16th to endNight
8%5%1342345%8%50%
2554
November1st to 15thNight
10%11%35%42435%6%41%
2665
16th to endNight
9%10%46%6%3445%6%38%
2901
December1st to 15thNight
12%13%5%8%5%3445%6%29%
2940
16th to endNight
14%17%6%10%6%3433424%
3264

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
54%7%17%14%5%
1675
16th to endDay
58%5%12%15%6%1
2016
February1st to 15thDay
59%6%12%15%5%
1845
16th to endDay
61%6%12%14%4
1799
March1st to 15thDay
53%7%12%18%6%
2240
16th to endDay
55%7%12%16%7%
2464
April1st to 15thDay
55%6%12%16%8%
2476
16th to endDay
43%6%10%24%14%
2693
May1st to 15thDay
37%5%9%29%16%1
2721
16th to endDay
48%6%11%21%10%1
2921
June1st to 15thDay
34%5%12%25%18%3
3063
16th to endDay
17%46%22%35%14%
3150
July1st to 15thDay
19%45%19%30%19%
2845
16th to endDay
19%35%15%24%30%
2550
August1st to 15thDay
21%36%15%28%24%
2470
16th to endDay
22%35%17%31%18%
2691
September1st to 15thDay
22%47%17%31%16%
2207
16th to endDay
25%47%24%28%9%
2145
October1st to 15thDay
39%5%11%26%14%3
2018
16th to endDay
35%6%15%26%16%
2021
November1st to 15thDay
45%7%14%21%9%1
1854
16th to endDay
46%7%18%18%7%1
1704
December1st to 15thDay
59%8%13%14%4
1680
16th to endDay
61%7%13%12%4
1669
January1st to 15thNight
52%8%15%16%5%
2919
16th to endNight
54%6%13%17%5%2
2912
February1st to 15thNight
54%7%15%16%6%
2678
16th to endNight
51%6%15%19%5%
2244
March1st to 15thNight
46%9%11%22%8%
2379
16th to endNight
45%8%14%21%8%
2464
April1st to 15thNight
44%7%16%20%10%1
2129
16th to endNight
36%5%13%24%19%
1872
May1st to 15thNight
32%6%9%30%18%2
1890
16th to endNight
41%6%13%24%12%1
1897
June1st to 15thNight
29%6%12%23%22%5%
1553
16th to endNight
18%45%18%31%22%
1470
July1st to 15thNight
19%5%419%28%22%
1542
16th to endNight
16%35%13%27%33%
1717
August1st to 15thNight
21%46%16%23%28%
1719
16th to endNight
17%35%17%31%23%
1863
September1st to 15thNight
21%37%13%30%23%
2081
16th to endNight
22%46%25%29%10%
2145
October1st to 15thNight
33%312%27%17%5%
2272
16th to endNight
30%6%12%29%19%1
2554
November1st to 15thNight
41%6%12%22%13%2
2665
16th to endNight
44%8%14%21%9%1
2901
December1st to 15thNight
52%8%17%15%5%
2940
16th to endNight
62%6%11%14%4
3264

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
19%428%27%14%4
121
16th to endDay
21%14%28%25%10%
192
February1st to 15thDay
21%23%24%22%7%1
180
16th to endDay
17%24%34%14%6%1
302
March1st to 15thDay
18%29%25%22%4
390
16th to endDay
21%29%29%15%3
416
April1st to 15thDay
23%29%27%15%31
389
16th to endDay
18%27%27%17%6%1
378
May1st to 15thDay
9%16%38%25%8%1
408
16th to endDay
17%13%24%23%15%5%
531
June1st to 15thDay
11%12%22%30%17%4
645
16th to endDay
411%28%30%21%2
645
July1st to 15thDay
37%25%38%22%2
479
16th to endDay
37%38%34%14%1
392
August1st to 15thDay
5%8%39%31%13%1
359
16th to endDay
39%31%34%16%4
396
September1st to 15thDay
316%28%24%18%7%
375
16th to endDay
8%14%28%30%13%4
375
October1st to 15thDay
15%16%27%24%11%3
248
16th to endDay
13%11%22%28%20%3
192
November1st to 15thDay
11%12%28%23%16%8%
180
16th to endDay
17%18%30%22%7%2
180
December1st to 15thDay
13%10%27%33%13%1
180
16th to endDay
18%18%36%20%41
151
January1st to 15thNight
17%7%18%37%16%2
840
16th to endNight
19%7%19%33%17%1
896
February1st to 15thNight
15%11%21%36%13%1
824
16th to endNight
11%12%27%33%12%2
736
March1st to 15thNight
10%9%25%39%13%2
840
16th to endNight
15%11%25%34%13%1
896
April1st to 15thNight
13%13%23%32%14%2
749
16th to endNight
13%11%24%30%17%1
624
May1st to 15thNight
10%7%20%35%22%3
630
16th to endNight
15%5%16%31%25%5%
659
June1st to 15thNight
8%5%20%35%27%2
503
16th to endNight
6%214%41%29%5%
420
July1st to 15thNight
219%45%36%5%
400
16th to endNight
318%47%36%2
586
August1st to 15thNight
412%44%36%1
575
16th to endNight
3212%42%34%5%
621
September1st to 15thNight
5%314%35%32%7%
732
16th to endNight
9%311%37%27%8%
776
October1st to 15thNight
11%6%17%37%23%3
780
16th to endNight
11%6%13%38%25%3
832
November1st to 15thNight
15%8%17%38%18%2
820
16th to endNight
12%11%22%34%14%4
1011
December1st to 15thNight
17%9%22%37%12%
1050
16th to endNight
20%13%23%30%10%1
1111
Page generated at 2019-07-17 06:53:10. Version ac04

Copyright 2019 A. Danko


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