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Downeast Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Downeast Observatory clear sky chart from 2005-11-05 to 2017-10-16.

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
28%
14978
15%
47106
7%
14938
37%
14938
17%
32840

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
18%
733
15%
2583
3%
729
33%
729
13%
1767
January16th to endNight
14%
768
15%
2601
3%
768
29%
768
13%
1824
February1st to 15thNight
15%
704
11%
2286
2%
701
27%
701
13%
1631
February16th to endNight
20%
631
14%
1946
3%
631
33%
631
13%
1386
March1st to 15thNight
20%
720
15%
2051
5%
720
31%
720
16%
1521
March16th to endNight
24%
768
17%
2112
7%
768
32%
768
19%
1492
April1st to 15thNight
22%
626
15%
1776
5%
626
31%
626
17%
1303
April16th to endNight
26%
540
15%
1620
6%
540
33%
540
20%
1139
May1st to 15thNight
30%
540
13%
1620
7%
540
36%
540
17%
1192
May16th to endNight
31%
522
10%
1485
6%
521
37%
521
17%
1100
June1st to 15thNight
33%
360
8%
1260
6%
360
37%
360
14%
932
June16th to endNight
37%
360
8%
1260
8%
360
41%
360
15%
911
July1st to 15thNight
47%
340
9%
1199
11%
340
50%
340
17%
806
July16th to endNight
44%
448
9%
1484
7%
420
48%
420
16%
1054
August1st to 15thNight
50%
539
11%
1608
9%
535
52%
535
22%
1122
August16th to endNight
50%
573
19%
1719
13%
573
54%
573
27%
1098
September1st to 15thNight
44%
652
16%
1891
10%
652
49%
652
27%
1241
September16th to endNight
44%
717
26%
1980
15%
717
53%
717
35%
1199
October1st to 15thNight
37%
720
20%
2118
10%
720
44%
720
25%
1379
October16th to endNight
24%
708
16%
2200
5%
708
35%
708
19%
1512
November1st to 15thNight
31%
709
17%
2327
9%
709
41%
709
19%
1575
November16th to endNight
25%
750
16%
2520
6%
750
37%
750
15%
1708
December1st to 15thNight
14%
750
14%
2580
3%
750
28%
750
12%
1860
December16th to endNight
13%
800
13%
2880
3%
800
27%
800
10%
2088

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%7%25%424447%36%
48310
Night
20%8%35%42445%6%35%
47275
TransparencyDay
47%6%12%21%11%1
47997
Night
49%5%11%20%11%1
47106
SeeingDay
24%9%28%25%10%1
6841
Night
24%10%29%25%8%
14978

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%8%45%5%1445%6%37%
1352
16th to endDay
16%10%35%5%2445%7%34%
1620
February1st to 15thDay
16%9%46%5%36%5%47%30%
1575
16th to endDay
14%11%36%5%35%438%33%
1527
March1st to 15thDay
21%10%35%424346%33%
1897
16th to endDay
16%9%35%434336%38%
2112
April1st to 15thDay
21%8%35%424345%35%
2171
16th to endDay
22%8%15%424436%36%
2340
May1st to 15thDay
25%8%36%423446%30%
2332
16th to endDay
24%6%25%4245%5%7%30%
2635
June1st to 15thDay
26%7%36%5%24346%28%
2700
16th to endDay
22%6%35%5%45%45%7%30%
2700
July1st to 15thDay
17%5%24435%449%38%
2689
16th to endDay
16%6%25%5%345%5%7%35%
2568
August1st to 15thDay
17%6%246%3436%7%37%
2340
16th to endDay
13%314435%45%7%46%
2483
September1st to 15thDay
14%5%24313446%49%
2066
16th to endDay
17%323322247%50%
1980
October1st to 15thDay
18%6%24424336%43%
1842
16th to endDay
22%6%15%434446%36%
1690
November1st to 15thDay
23%6%24323346%38%
1530
16th to endDay
21%8%25%42435%6%35%
1440
December1st to 15thDay
25%8%3442335%7%31%
1377
16th to endDay
24%10%46%224337%30%
1344
January1st to 15thNight
16%9%35%434446%36%
2583
16th to endNight
14%11%36%4245%6%8%31%
2601
February1st to 15thNight
16%10%25%436%5%46%32%
2285
16th to endNight
16%11%36%5%24445%34%
1946
March1st to 15thNight
19%9%46%5%34445%32%
2039
16th to endNight
18%8%35%5%2445%7%35%
2112
April1st to 15thNight
23%8%36%5%25%345%30%
1776
16th to endNight
25%7%2432445%5%34%
1620
May1st to 15thNight
24%7%35%5%35%445%30%
1620
16th to endNight
24%7%35%6%2446%6%29%
1483
June1st to 15thNight
29%7%36%32445%7%26%
1260
16th to endNight
23%7%346%35%447%29%
1260
July1st to 15thNight
15%8%345%25%5%6%10%32%
1269
16th to endNight
20%7%26%435%5%6%6%31%
1586
August1st to 15thNight
17%7%35%5%245%5%8%34%
1620
16th to endNight
15%5%2442445%8%43%
1719
September1st to 15thNight
17%5%2442345%6%42%
1891
16th to endNight
15%423323346%51%
1980
October1st to 15thNight
19%5%15%423446%42%
2118
16th to endNight
21%6%25%423435%39%
2200
November1st to 15thNight
20%6%245%2435%6%37%
2327
16th to endNight
21%6%25%5%25%446%36%
2520
December1st to 15thNight
25%8%35%423346%32%
2580
16th to endNight
24%9%25%424446%31%
2880

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
47%7%14%18%8%1
1353
16th to endDay
49%7%11%19%10%1
1620
February1st to 15thDay
52%6%11%18%9%1
1578
16th to endDay
49%6%12%20%9%1
1527
March1st to 15thDay
51%5%10%19%11%2
1908
16th to endDay
46%410%19%15%3
2112
April1st to 15thDay
49%49%19%14%2
2171
16th to endDay
47%411%18%13%4
2340
May1st to 15thDay
54%5%10%17%10%2
2332
16th to endDay
51%6%13%20%6%1
2639
June1st to 15thDay
55%5%12%16%9%1
2700
16th to endDay
52%8%11%17%10%
2700
July1st to 15thDay
40%7%16%22%11%
2530
16th to endDay
45%7%16%21%9%
2408
August1st to 15thDay
44%7%14%23%9%
2327
16th to endDay
34%6%16%28%13%
2483
September1st to 15thDay
35%6%12%27%17%1
2066
16th to endDay
36%39%31%17%1
1980
October1st to 15thDay
41%412%27%13%
1842
16th to endDay
48%5%12%23%9%
1690
November1st to 15thDay
47%414%19%13%
1530
16th to endDay
49%6%12%18%11%1
1440
December1st to 15thDay
53%6%13%18%7%
1377
16th to endDay
54%6%11%17%9%
1344
January1st to 15thNight
48%7%12%20%9%1
2583
16th to endNight
50%6%11%20%9%
2601
February1st to 15thNight
51%6%13%17%9%1
2286
16th to endNight
52%5%10%16%12%1
1946
March1st to 15thNight
54%5%8%16%12%1
2051
16th to endNight
48%5%10%20%14%1
2112
April1st to 15thNight
54%49%15%13%2
1776
16th to endNight
51%49%17%14%2
1620
May1st to 15thNight
55%5%10%16%11%1
1620
16th to endNight
55%5%13%17%7%
1485
June1st to 15thNight
56%6%11%17%6%1
1260
16th to endNight
55%5%10%17%10%
1260
July1st to 15thNight
45%9%14%18%12%
1199
16th to endNight
51%7%12%19%8%1
1484
August1st to 15thNight
46%8%14%20%10%
1608
16th to endNight
39%6%14%27%12%
1719
September1st to 15thNight
42%5%10%25%15%
1891
16th to endNight
34%311%30%19%
1980
October1st to 15thNight
42%5%10%25%15%1
2118
16th to endNight
48%410%23%11%
2200
November1st to 15thNight
47%5%13%20%12%
2327
16th to endNight
48%6%13%19%11%
2520
December1st to 15thNight
55%5%10%21%7%
2580
16th to endNight
55%6%10%17%9%
2880

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
23%16%35%21%3
60
16th to endDay
22%17%36%19%3
148
February1st to 15thDay
20%19%35%19%5%
236
16th to endDay
22%15%33%23%5%
230
March1st to 15thDay
24%20%32%17%4
270
16th to endDay
23%17%31%19%6%2
288
April1st to 15thDay
28%12%32%19%5%1
270
16th to endDay
27%7%31%22%7%2
270
May1st to 15thDay
30%6%31%20%7%1
288
16th to endDay
29%7%26%24%10%1
488
June1st to 15thDay
30%5%25%28%9%1
495
16th to endDay
29%426%26%11%2
495
July1st to 15thDay
25%222%31%16%2
464
16th to endDay
20%324%34%16%1
326
August1st to 15thDay
20%324%32%17%1
313
16th to endDay
16%321%33%23%2
332
September1st to 15thDay
18%325%35%15%1
315
16th to endDay
20%423%27%19%4
314
October1st to 15thDay
19%12%24%25%15%3
280
16th to endDay
24%12%34%20%5%1
243
November1st to 15thDay
25%14%26%22%7%2
235
16th to endDay
24%17%29%20%7%
240
December1st to 15thDay
30%15%31%18%3
177
16th to endDay
31%21%37%7%1
64
January1st to 15thNight
21%16%34%23%3
733
16th to endNight
21%20%36%18%3
768
February1st to 15thNight
23%15%40%17%3
704
16th to endNight
24%17%32%20%41
631
March1st to 15thNight
25%12%34%23%3
720
16th to endNight
22%15%31%23%6%
768
April1st to 15thNight
27%8%34%22%6%
626
16th to endNight
27%6%33%24%7%
540
May1st to 15thNight
25%5%28%29%8%1
540
16th to endNight
25%5%29%29%8%1
522
June1st to 15thNight
32%421%31%9%1
360
16th to endNight
28%322%30%13%1
360
July1st to 15thNight
21%416%42%15%
340
16th to endNight
23%218%39%15%
448
August1st to 15thNight
20%315%41%17%
539
16th to endNight
18%119%38%20%2
573
September1st to 15thNight
21%224%33%16%1
652
16th to endNight
17%428%32%16%2
717
October1st to 15thNight
22%428%28%12%2
720
16th to endNight
25%12%33%21%7%
708
November1st to 15thNight
22%10%30%25%10%1
709
16th to endNight
24%15%30%20%8%
750
December1st to 15thNight
32%14%32%17%3
750
16th to endNight
30%17%34%15%2
800
Page generated at 2017-10-17 16:35:19. Version ac04

Copyright 2017 A. Danko


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