This page made possible by the sponsorship of:
.

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 2019-09-17.

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%
17456
15%
54662
7%
17416
37%
17416
17%
38033

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%
866
15%
3016
3%
862
33%
862
13%
2093
January16th to endNight
14%
896
15%
3035
2%
896
29%
896
12%
2150
February1st to 15thNight
15%
824
14%
2676
3%
821
29%
821
13%
1879
February16th to endNight
19%
735
15%
2267
3%
735
32%
735
14%
1594
March1st to 15thNight
19%
840
16%
2393
5%
840
31%
840
16%
1784
March16th to endNight
24%
896
17%
2464
6%
896
32%
896
20%
1731
April1st to 15thNight
20%
731
16%
2073
5%
731
30%
731
17%
1512
April16th to endNight
24%
623
14%
1869
6%
623
31%
623
18%
1342
May1st to 15thNight
30%
630
15%
1890
8%
630
37%
630
18%
1375
May16th to endNight
30%
612
11%
1741
6%
611
36%
611
17%
1291
June1st to 15thNight
34%
420
10%
1470
7%
420
39%
420
15%
1061
June16th to endNight
36%
420
9%
1470
7%
420
41%
420
16%
1055
July1st to 15thNight
49%
400
12%
1410
13%
400
53%
400
19%
925
July16th to endNight
45%
524
10%
1752
9%
496
49%
496
17%
1230
August1st to 15thNight
51%
619
11%
1848
10%
615
52%
615
22%
1291
August16th to endNight
50%
669
19%
2007
13%
669
54%
669
28%
1278
September1st to 15thNight
45%
760
18%
2205
11%
760
50%
760
27%
1440
September16th to endNight
44%
782
25%
2159
15%
782
52%
782
34%
1318
October1st to 15thNight
36%
780
21%
2294
10%
780
44%
780
25%
1499
October16th to endNight
24%
832
17%
2588
5%
832
36%
832
20%
1750
November1st to 15thNight
28%
837
18%
2725
8%
837
40%
837
17%
1853
November16th to endNight
23%
900
16%
2940
5%
900
35%
900
14%
2020
December1st to 15thNight
16%
900
16%
3010
3%
900
30%
900
13%
2160
December16th to endNight
13%
960
14%
3360
2%
960
27%
960
11%
2402

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%
56025
Night
20%7%25%424446%35%
54831
TransparencyDay
46%6%12%21%11%1
55712
Night
49%5%11%20%12%1
54662
SeeingDay
24%9%29%25%9%1
9374
Night
24%10%29%25%8%
17456

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%9%46%42335%6%36%
1578
16th to endDay
17%9%35%5%2445%6%34%
1890
February1st to 15thDay
17%9%36%5%36%447%30%
1845
16th to endDay
14%11%36%5%35%437%34%
1778
March1st to 15thDay
21%10%35%424446%32%
2215
16th to endDay
17%9%25%424336%39%
2464
April1st to 15thDay
21%9%35%424346%34%
2534
16th to endDay
24%8%15%423335%35%
2696
May1st to 15thDay
24%7%36%5%23346%32%
2722
16th to endDay
24%6%25%4245%5%7%30%
3085
June1st to 15thDay
25%6%36%5%24346%31%
3150
16th to endDay
22%6%25%5%35%347%31%
3150
July1st to 15thDay
15%423435%449%41%
3137
16th to endDay
16%6%25%5%345%5%8%36%
3002
August1st to 15thDay
16%6%246%3436%8%37%
2683
16th to endDay
13%31442445%7%46%
2899
September1st to 15thDay
14%424423446%49%
2412
16th to endDay
17%323323346%49%
2156
October1st to 15thDay
20%6%24424336%42%
1996
16th to endDay
21%6%15%434446%37%
1986
November1st to 15thDay
24%6%24323347%38%
1792
16th to endDay
21%8%2442435%6%35%
1680
December1st to 15thDay
24%8%3442335%7%32%
1607
16th to endDay
23%9%35%324337%32%
1568
January1st to 15thNight
16%9%36%43445%6%35%
3016
16th to endNight
15%11%36%5%345%5%7%31%
3035
February1st to 15thNight
16%10%25%436%5%46%34%
2675
16th to endNight
17%10%36%5%3344435%
2267
March1st to 15thNight
20%10%46%5%34445%31%
2381
16th to endNight
17%9%35%5%24346%37%
2464
April1st to 15thNight
23%8%36%5%25%345%31%
2073
16th to endNight
27%7%2432435%5%32%
1869
May1st to 15thNight
23%7%35%5%24445%32%
1890
16th to endNight
25%7%35%6%2445%6%29%
1739
June1st to 15thNight
27%6%26%32445%8%28%
1470
16th to endNight
24%7%345%24446%30%
1470
July1st to 15thNight
14%7%345%25%5%6%10%34%
1480
16th to endNight
19%7%25%435%5%6%6%33%
1854
August1st to 15thNight
16%6%25%5%245%5%8%35%
1860
16th to endNight
14%5%2442445%8%44%
2007
September1st to 15thNight
17%5%2442345%6%42%
2205
16th to endNight
15%423323346%51%
2159
October1st to 15thNight
20%5%15%42345%6%41%
2294
16th to endNight
21%6%24423445%40%
2588
November1st to 15thNight
21%6%245%24346%38%
2725
16th to endNight
21%6%25%5%25%447%35%
2940
December1st to 15thNight
23%8%35%423346%33%
3010
16th to endNight
22%8%25%424446%33%
3360

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
48%7%14%18%9%1
1579
16th to endDay
49%7%11%18%11%1
1890
February1st to 15thDay
53%6%11%17%10%1
1848
16th to endDay
48%5%12%20%10%2
1778
March1st to 15thDay
52%411%19%10%1
2226
16th to endDay
45%410%19%17%3
2464
April1st to 15thDay
49%49%18%14%2
2534
16th to endDay
49%410%18%13%3
2696
May1st to 15thDay
52%5%10%18%11%2
2722
16th to endDay
51%6%13%20%7%1
3089
June1st to 15thDay
52%5%12%17%10%1
3150
16th to endDay
51%7%10%18%11%1
3150
July1st to 15thDay
38%7%15%24%12%1
2978
16th to endDay
44%7%16%21%9%1
2842
August1st to 15thDay
42%8%15%24%8%
2670
16th to endDay
35%6%15%27%14%
2899
September1st to 15thDay
34%6%12%26%18%2
2412
16th to endDay
36%410%30%17%1
2156
October1st to 15thDay
43%412%26%12%
1996
16th to endDay
46%5%12%23%10%
1986
November1st to 15thDay
46%414%20%13%1
1792
16th to endDay
49%6%13%19%10%1
1680
December1st to 15thDay
53%6%12%19%8%
1607
16th to endDay
53%7%10%16%10%1
1568
January1st to 15thNight
49%7%11%19%10%1
3016
16th to endNight
51%6%11%19%9%
3035
February1st to 15thNight
50%6%13%17%10%1
2676
16th to endNight
51%5%10%17%13%2
2267
March1st to 15thNight
55%48%16%12%1
2393
16th to endNight
48%49%18%16%2
2464
April1st to 15thNight
54%38%15%14%3
2073
16th to endNight
53%49%16%13%2
1869
May1st to 15thNight
53%5%9%17%12%1
1890
16th to endNight
55%5%12%17%8%1
1741
June1st to 15thNight
54%6%11%17%8%1
1470
16th to endNight
55%5%10%17%10%1
1470
July1st to 15thNight
43%8%13%20%13%
1410
16th to endNight
50%7%12%19%9%1
1752
August1st to 15thNight
45%8%14%20%10%
1848
16th to endNight
38%6%13%26%13%
2007
September1st to 15thNight
41%5%10%24%16%1
2205
16th to endNight
35%310%30%18%
2159
October1st to 15thNight
42%5%10%25%14%1
2294
16th to endNight
47%411%23%12%
2588
November1st to 15thNight
47%5%12%19%13%1
2725
16th to endNight
48%6%12%18%12%
2940
December1st to 15thNight
53%5%9%21%9%
3010
16th to endNight
53%6%10%18%11%
3360

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%20%36%14%3
120
16th to endDay
23%20%37%15%2
226
February1st to 15thDay
23%20%35%16%3
326
16th to endDay
22%16%38%19%3
325
March1st to 15thDay
24%18%34%18%3
390
16th to endDay
22%17%32%18%7%1
416
April1st to 15thDay
27%14%34%17%31
390
16th to endDay
29%8%31%22%6%1
379
May1st to 15thDay
28%8%33%21%7%
408
16th to endDay
28%7%28%25%9%1
642
June1st to 15thDay
27%5%28%28%8%
645
16th to endDay
28%426%27%11%1
645
July1st to 15thDay
21%223%34%16%2
612
16th to endDay
20%325%32%16%1
454
August1st to 15thDay
17%226%35%16%1
418
16th to endDay
17%221%34%20%2
460
September1st to 15thDay
16%225%37%14%2
435
16th to endDay
21%426%27%16%3
378
October1st to 15thDay
22%10%24%24%14%3
329
16th to endDay
24%13%33%21%6%
336
November1st to 15thDay
27%16%25%20%7%2
325
16th to endDay
26%16%30%20%6%
330
December1st to 15thDay
29%12%33%18%5%
257
16th to endDay
27%28%30%10%2
128
January1st to 15thNight
22%18%33%22%3
866
16th to endNight
21%21%34%18%3
896
February1st to 15thNight
23%17%37%18%2
824
16th to endNight
24%17%32%19%4
735
March1st to 15thNight
25%12%36%23%2
840
16th to endNight
22%14%32%22%6%1
896
April1st to 15thNight
28%10%34%21%5%
731
16th to endNight
29%5%33%23%7%
623
May1st to 15thNight
25%5%29%29%9%1
630
16th to endNight
26%6%29%28%7%1
612
June1st to 15thNight
30%422%32%8%1
420
16th to endNight
28%423%31%11%1
420
July1st to 15thNight
19%417%42%15%
400
16th to endNight
23%218%39%15%
524
August1st to 15thNight
19%315%42%17%
619
16th to endNight
18%120%38%19%2
669
September1st to 15thNight
21%224%33%16%1
760
16th to endNight
17%427%32%15%2
782
October1st to 15thNight
23%429%28%12%2
780
16th to endNight
24%12%33%20%8%
832
November1st to 15thNight
24%12%29%23%9%1
837
16th to endNight
25%16%30%19%8%
900
December1st to 15thNight
29%14%33%17%4
900
16th to endNight
29%18%33%14%2
960
Page generated at 2019-09-17 21:12:15. Version ac04

Copyright 2019 A. Danko


Downeast Observatory Clear Sky Chart |Charts Home