Arch Cape Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arch Cape Observatory clear sky chart from 2005-11-05 to 2017-09-10.

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
32%
14642
9%
46568
8%
14611
34%
14611
11%
36560

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
24%
735
8%
2598
6%
731
28%
731
7%
2151
January16th to endNight
26%
768
9%
2556
9%
768
28%
768
9%
2062
February1st to 15thNight
22%
700
5%
2288
5%
700
23%
700
6%
1972
February16th to endNight
23%
632
8%
2041
4%
632
28%
632
7%
1683
March1st to 15thNight
21%
720
4%
2003
4%
720
21%
720
7%
1766
March16th to endNight
17%
768
4%
2110
2%
768
18%
768
5%
1930
April1st to 15thNight
24%
719
4%
1799
2%
719
24%
719
8%
1576
April16th to endNight
30%
552
6%
1632
4%
552
31%
552
9%
1343
May1st to 15thNight
33%
419
10%
1500
11%
419
36%
419
10%
1121
May16th to endNight
30%
375
5%
1496
6%
374
32%
374
6%
1204
June1st to 15thNight
33%
360
5%
1363
5%
360
34%
360
9%
1078
June16th to endNight
37%
360
7%
1260
7%
360
41%
360
11%
941
July1st to 15thNight
57%
340
12%
1208
16%
340
60%
340
22%
735
July16th to endNight
50%
397
12%
1436
16%
374
56%
374
22%
833
August1st to 15thNight
51%
538
12%
1611
12%
535
52%
535
23%
1020
August16th to endNight
53%
620
18%
1766
14%
620
56%
620
25%
1135
September1st to 15thNight
60%
700
21%
1750
13%
700
61%
700
36%
1086
September16th to endNight
48%
656
19%
1825
14%
656
51%
656
21%
1275
October1st to 15thNight
42%
660
15%
1980
14%
660
45%
660
18%
1418
October16th to endNight
30%
704
9%
2112
8%
704
33%
704
12%
1653
November1st to 15thNight
21%
700
6%
2437
6%
700
23%
700
6%
2092
November16th to endNight
23%
720
6%
2520
5%
720
26%
720
7%
2043
December1st to 15thNight
25%
720
8%
2520
7%
720
27%
720
8%
2031
December16th to endNight
16%
779
5%
2757
4%
779
17%
779
4%
2412

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
28%9%36%5%24447%23%
48073
Night
30%10%36%5%23345%23%
46728
Cloud CoverDay
31%10%36%423345%23%
48073
Night
31%10%35%423345%24%
46731
TransparencyDay
62%6%10%12%6%1
47738
Night
62%6%9%11%7%2
46568
SeeingDay
30%14%17%23%12%2
6056
Night
31%18%10%18%17%3
14647

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 15thDay
46%8%35%42222317%
1337
16th to endDay
35%9%36%43222424%
1667
February1st to 15thDay
41%11%46%42433414%
1576
16th to endDay
34%12%37%5%3332418%
1432
March1st to 15thDay
45%11%46%42222412%
1956
16th to endDay
38%13%5%7%5%2333410%
2113
April1st to 15thDay
33%10%38%6%24346%14%
2149
16th to endDay
29%12%46%5%35%346%18%
2328
May1st to 15thDay
24%8%36%5%24446%27%
2451
16th to endDay
27%9%46%6%25%45%8%17%
2615
June1st to 15thDay
27%10%46%6%26%45%7%18%
2594
16th to endDay
22%9%47%5%3445%7%25%
2700
July1st to 15thDay
13%7%2443447%9%38%
2679
16th to endDay
13%9%25%5%2437%12%33%
2633
August1st to 15thDay
14%8%36%6%35%6%7%10%28%
2340
16th to endDay
14%7%35%4345%6%10%34%
2435
September1st to 15thDay
10%6%245%335%6%8%43%
2097
16th to endDay
21%7%25%6%3345%7%32%
1803
October1st to 15thDay
29%8%35%323346%29%
1650
16th to endDay
33%7%35%5%22446%23%
1760
November1st to 15thDay
42%8%27%5%3443412%
1417
16th to endDay
39%9%46%5%2332320%
1440
December1st to 15thDay
36%9%35%43442321%
1440
16th to endDay
42%12%35%32333316%
1461
January1st to 15thNight
41%10%36%31334418%
2598
16th to endNight
36%10%45%42323422%
2556
February1st to 15thNight
35%11%47%5%3333416%
2276
16th to endNight
33%13%46%5%2322420%
2041
March1st to 15thNight
45%12%4431322413%
2003
16th to endNight
42%13%48%5%233339%
2106
April1st to 15thNight
35%14%5%7%42323415%
1799
16th to endNight
28%11%38%435%45%7%17%
1632
May1st to 15thNight
29%9%35%6%23446%24%
1499
16th to endNight
32%8%45%6%35%5%6%5%16%
1496
June1st to 15thNight
28%10%6%8%6%2444419%
1363
16th to endNight
23%11%47%43345%7%24%
1260
July1st to 15thNight
11%7%36%5%235%5%10%37%
1279
16th to endNight
11%8%38%6%35%448%33%
1533
August1st to 15thNight
12%9%35%6%345%5%9%32%
1620
16th to endNight
13%8%26%6%33447%40%
1766
September1st to 15thNight
10%5%25%433445%50%
1750
16th to endNight
20%7%35%5%13347%36%
1825
October1st to 15thNight
24%7%35%423345%34%
1980
16th to endNight
32%9%36%5%2343424%
2112
November1st to 15thNight
40%9%36%5%3433414%
2437
16th to endNight
33%9%36%5%2433421%
2520
December1st to 15thNight
33%8%45%433436%22%
2520
16th to endNight
43%12%35%42333314%
2757

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
47%11%35%21222317%
1337
16th to endDay
36%10%36%42223323%
1667
February1st to 15thDay
42%12%46%32432412%
1576
16th to endDay
37%13%47%6%2222316%
1432
March1st to 15thDay
47%13%36%42222310%
1956
16th to endDay
44%13%37%5%243328%
2113
April1st to 15thDay
37%13%47%5%13445%11%
2149
16th to endDay
34%11%47%5%24335%16%
2328
May1st to 15thDay
29%9%36%42335%5%25%
2451
16th to endDay
34%11%5%6%6%34446%14%
2615
June1st to 15thDay
34%12%47%5%23435%15%
2594
16th to endDay
28%9%36%424446%23%
2700
July1st to 15thDay
16%7%35%31325%7%41%
2679
16th to endDay
18%8%35%42335%8%36%
2633
August1st to 15thDay
17%8%36%5%2345%8%33%
2340
16th to endDay
16%8%36%423347%38%
2435
September1st to 15thDay
12%7%24423347%46%
2097
16th to endDay
23%8%36%5%3245%6%31%
1803
October1st to 15thDay
31%8%35%323435%27%
1650
16th to endDay
35%8%26%423445%22%
1760
November1st to 15thDay
42%11%36%5%2343311%
1417
16th to endDay
42%9%36%5%2222220%
1440
December1st to 15thDay
39%9%4442342221%
1440
16th to endDay
45%14%2422322316%
1461
January1st to 15thNight
42%10%35%32333418%
2597
16th to endNight
36%11%36%42323422%
2556
February1st to 15thNight
36%11%48%5%2333315%
2276
16th to endNight
35%14%46%42322320%
2041
March1st to 15thNight
45%13%35%31223413%
2003
16th to endNight
44%15%35%4233339%
2110
April1st to 15thNight
36%14%46%5%2333415%
1799
16th to endNight
29%12%47%42445%6%18%
1632
May1st to 15thNight
29%8%45%42435%6%25%
1499
16th to endNight
32%9%45%5%245%5%5%17%
1496
June1st to 15thNight
28%10%48%6%24345%20%
1363
16th to endNight
23%9%48%5%2446%5%25%
1260
July1st to 15thNight
11%6%344335%6%8%41%
1279
16th to endNight
12%10%35%5%25%5%47%37%
1533
August1st to 15thNight
11%9%25%6%345%6%8%35%
1620
16th to endNight
14%7%26%5%24446%42%
1766
September1st to 15thNight
10%5%24323445%53%
1750
16th to endNight
21%6%35%423446%37%
1825
October1st to 15thNight
25%6%25%423345%34%
1980
16th to endNight
35%8%25%5%2233425%
2112
November1st to 15thNight
42%8%37%5%3333314%
2437
16th to endNight
36%10%35%43323321%
2520
December1st to 15thNight
35%10%35%42433422%
2520
16th to endNight
44%13%35%42313314%
2757

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
74%35%7%7%3
1337
16th to endDay
67%36%8%7%5%
1667
February1st to 15thDay
78%46%6%4
1576
16th to endDay
74%46%8%6%
1432
March1st to 15thDay
81%46%6%2
1956
16th to endDay
83%446%
2113
April1st to 15thDay
76%5%8%8%1
2149
16th to endDay
72%6%7%7%4
2328
May1st to 15thDay
59%6%12%14%6%
2451
16th to endDay
70%7%10%9%1
2619
June1st to 15thDay
69%8%8%9%3
2594
16th to endDay
59%9%14%11%41
2700
July1st to 15thDay
39%7%13%22%14%3
2521
16th to endDay
39%6%15%21%13%2
2468
August1st to 15thDay
42%9%18%17%8%3
2324
16th to endDay
42%6%15%22%10%2
2435
September1st to 15thDay
36%5%14%19%15%8%
2097
16th to endDay
51%8%11%14%11%2
1803
October1st to 15thDay
59%49%13%9%3
1650
16th to endDay
64%5%8%12%9%
1760
November1st to 15thDay
77%6%6%6%2
1417
16th to endDay
72%5%38%7%2
1440
December1st to 15thDay
69%46%10%8%
1440
16th to endDay
75%36%7%41
1461
January1st to 15thNight
71%46%6%7%3
2598
16th to endNight
67%46%6%7%7%
2556
February1st to 15thNight
74%5%5%9%31
2288
16th to endNight
70%48%9%7%
2041
March1st to 15thNight
75%5%6%9%2
2003
16th to endNight
80%5%5%6%2
2110
April1st to 15thNight
74%5%10%7%2
1799
16th to endNight
66%7%10%10%41
1632
May1st to 15thNight
57%7%11%13%8%1
1500
16th to endNight
64%9%13%8%3
1496
June1st to 15thNight
62%8%12%12%3
1363
16th to endNight
55%8%16%14%22
1260
July1st to 15thNight
35%9%15%19%16%3
1208
16th to endNight
35%8%19%19%9%6%
1436
August1st to 15thNight
35%11%19%18%9%4
1611
16th to endNight
38%7%15%22%11%4
1766
September1st to 15thNight
29%6%14%19%19%9%
1750
16th to endNight
45%7%11%16%14%5%
1825
October1st to 15thNight
50%6%9%16%14%3
1980
16th to endNight
62%49%12%9%1
2112
November1st to 15thNight
73%5%7%8%3
2437
16th to endNight
68%46%7%7%5%
2520
December1st to 15thNight
66%6%6%10%9%
2520
16th to endNight
77%5%46%5%1
2757

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
40%115%21%20%1
60
16th to endDay
44%15%15%12%10%2
195
February1st to 15thDay
48%17%11%13%8%
236
16th to endDay
40%20%16%15%6%
213
March1st to 15thDay
57%19%10%9%3
270
16th to endDay
48%18%18%11%21
288
April1st to 15thDay
37%19%18%14%8%1
269
16th to endDay
37%17%21%15%5%2
270
May1st to 15thDay
32%21%17%19%9%1
288
16th to endDay
31%14%14%23%11%3
330
June1st to 15thDay
29%16%20%22%11%
389
16th to endDay
20%16%21%29%10%1
495
July1st to 15thDay
13%10%16%38%18%1
455
16th to endDay
11%11%18%37%18%2
327
August1st to 15thDay
13%8%14%33%23%6%
315
16th to endDay
12%8%16%33%24%5%
332
September1st to 15thDay
17%7%23%27%21%3
294
16th to endDay
23%8%23%27%14%1
255
October1st to 15thDay
37%13%14%18%15%1
255
16th to endDay
37%13%19%19%9%1
272
November1st to 15thDay
46%12%17%17%6%
64
16th to endDay
40%8%16%16%16%1
60
December1st to 15thDay
61%13%11%13%
60
16th to endDay
31%20%14%20%14%
64
January1st to 15thNight
39%20%10%15%12%1
735
16th to endNight
35%19%10%15%14%4
768
February1st to 15thNight
36%20%11%15%13%2
704
16th to endNight
32%22%13%19%11%
632
March1st to 15thNight
44%21%8%12%10%1
720
16th to endNight
43%22%9%13%8%2
768
April1st to 15thNight
37%21%10%17%12%1
719
16th to endNight
30%21%10%21%13%1
552
May1st to 15thNight
30%19%11%17%18%2
420
16th to endNight
30%19%10%13%21%4
375
June1st to 15thNight
25%19%10%19%24%1
360
16th to endNight
21%19%10%23%24%1
360
July1st to 15thNight
11%12%8%22%39%5%
340
16th to endNight
9%17%8%28%31%4
397
August1st to 15thNight
15%10%7%19%36%10%
538
16th to endNight
16%10%9%21%36%5%
620
September1st to 15thNight
11%10%8%23%34%11%
700
16th to endNight
21%11%10%26%25%4
656
October1st to 15thNight
25%12%13%23%19%5%
660
16th to endNight
35%13%11%18%15%5%
704
November1st to 15thNight
38%19%12%17%9%1
700
16th to endNight
34%20%12%20%10%2
720
December1st to 15thNight
36%21%11%18%9%2
720
16th to endNight
45%21%11%11%8%1
779
Page generated at 2017-09-11 16:35:34. Version ac04

Copyright 2017 A. Danko


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