This page made possible by the sponsorship of: Amateur Telescope Makers of Boston.

Edward Knight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Edward Knight Observatory clear sky chart from 2005-11-05 to 2017-08-12.

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%
14739
18%
46455
8%
14707
40%
14707
20%
30278

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%
720
19%
2506
6%
716
34%
716
16%
1619
January16th to endNight
15%
768
19%
2688
4%
768
34%
768
15%
1722
February1st to 15thNight
17%
704
16%
2198
5%
701
31%
701
16%
1482
February16th to endNight
18%
631
23%
1893
7%
631
37%
631
18%
1243
March1st to 15thNight
18%
720
17%
2160
6%
720
32%
720
16%
1489
March16th to endNight
21%
768
22%
2025
7%
768
34%
768
21%
1374
April1st to 15thNight
21%
718
18%
1796
4%
718
30%
718
22%
1265
April16th to endNight
24%
594
18%
1674
4%
594
34%
594
20%
1141
May1st to 15thNight
26%
540
15%
1620
6%
540
34%
540
18%
1122
May16th to endNight
25%
402
11%
1524
7%
401
34%
401
15%
1010
June1st to 15thNight
33%
360
12%
1440
9%
360
44%
360
15%
958
June16th to endNight
39%
360
11%
1440
12%
360
47%
360
21%
887
July1st to 15thNight
46%
340
12%
1360
15%
340
52%
340
22%
821
July16th to endNight
48%
376
15%
1418
15%
354
54%
354
22%
848
August1st to 15thNight
48%
485
17%
1534
13%
483
52%
483
27%
960
August16th to endNight
51%
664
19%
1714
10%
664
54%
664
34%
1114
September1st to 15thNight
40%
660
21%
1702
9%
660
47%
660
34%
1088
September16th to endNight
46%
657
26%
1815
16%
657
55%
657
36%
1085
October1st to 15thNight
34%
660
23%
1844
10%
660
45%
660
26%
1191
October16th to endNight
25%
704
16%
2288
6%
704
38%
704
20%
1494
November1st to 15thNight
32%
700
21%
2277
11%
700
45%
700
22%
1436
November16th to endNight
25%
720
20%
2340
8%
720
39%
720
18%
1512
December1st to 15thNight
21%
720
17%
2511
5%
720
36%
720
16%
1626
December16th to endNight
16%
768
18%
2688
5%
768
32%
768
15%
1791

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
17%5%25%43445%6%39%
47526
Night
17%6%2442445%6%41%
46625
Cloud CoverDay
17%6%25%43445%6%39%
47536
Night
17%6%24424446%41%
46630
TransparencyDay
43%5%11%21%14%2
47214
Night
43%5%11%21%15%2
46455
SeeingDay
22%13%30%20%9%3
5727
Night
21%13%32%24%7%
14739

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%6%25%5%3445%440%
1432
16th to endDay
15%5%335%2445%6%44%
1536
February1st to 15thDay
16%7%26%5%35%6%5%7%34%
1659
16th to endDay
15%9%36%5%33345%40%
1580
March1st to 15thDay
21%7%35%424346%36%
1790
16th to endDay
18%6%25%423445%41%
2198
April1st to 15thDay
18%6%36%5%34347%38%
2152
16th to endDay
18%4245%2445%6%40%
2286
May1st to 15thDay
24%7%35%434445%33%
2332
16th to endDay
17%425%6%3435%8%38%
2599
June1st to 15thDay
25%5%25%42445%7%32%
2520
16th to endDay
16%425%5%35%5%7%8%34%
2520
July1st to 15thDay
12%435%5%35%5%6%7%38%
2520
16th to endDay
12%6%245%35%5%6%9%38%
2654
August1st to 15thDay
11%5%16%5%35%6%5%8%38%
2318
16th to endDay
11%415%5%35%5%6%7%43%
2135
September1st to 15thDay
14%5%2342445%6%45%
1928
16th to endDay
14%424332435%51%
1815
October1st to 15thDay
20%424424445%41%
1785
16th to endDay
17%5%25%4245%45%42%
1584
November1st to 15thDay
19%7%23332345%44%
1581
16th to endDay
17%6%25%423325%44%
1620
December1st to 15thDay
18%7%25%424436%39%
1446
16th to endDay
23%8%25%324346%35%
1536
January1st to 15thNight
16%8%345%24346%40%
2506
16th to endNight
16%9%35%423345%41%
2688
February1st to 15thNight
14%9%35%5%35%446%38%
2197
16th to endNight
16%8%25%424445%41%
1893
March1st to 15thNight
20%6%24424346%37%
2147
16th to endNight
16%6%35%424345%44%
2025
April1st to 15thNight
17%6%25%5%3435%6%38%
1796
16th to endNight
21%6%2442335%5%40%
1674
May1st to 15thNight
21%6%25%435%35%6%36%
1620
16th to endNight
19%5%16%434448%36%
1522
June1st to 15thNight
23%5%25%423346%35%
1440
16th to endNight
15%5%245%245%5%7%40%
1440
July1st to 15thNight
12%5%25%435%5%6%6%42%
1440
16th to endNight
11%5%246%35%45%7%42%
1514
August1st to 15thNight
9%425%5%45%45%8%44%
1544
16th to endNight
12%324336%46%8%44%
1714
September1st to 15thNight
15%41332335%7%47%
1702
16th to endNight
14%413213346%53%
1815
October1st to 15thNight
20%41342345%5%45%
1844
16th to endNight
20%6%2442345%5%40%
2288
November1st to 15thNight
17%6%24333345%45%
2277
16th to endNight
18%6%25%323445%41%
2340
December1st to 15thNight
19%7%34333446%39%
2511
16th to endNight
19%8%25%42345%6%37%
2688

Percentage Cloud Cover SE (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%7%25%424445%40%
1432
16th to endDay
15%6%145%3445%5%43%
1536
February1st to 15thDay
16%7%36%425%5%5%7%34%
1659
16th to endDay
15%8%26%5%3434440%
1580
March1st to 15thDay
22%7%35%424345%36%
1800
16th to endDay
18%7%25%423345%41%
2198
April1st to 15thDay
17%7%35%5%34346%38%
2152
16th to endDay
18%4245%3445%6%40%
2286
May1st to 15thDay
24%8%25%5%2435%5%32%
2332
16th to endDay
17%435%5%3445%7%39%
2599
June1st to 15thDay
25%6%244245%5%8%32%
2520
16th to endDay
16%425%5%35%6%7%7%34%
2520
July1st to 15thDay
12%435%5%35%5%6%9%37%
2520
16th to endDay
12%5%25%5%36%5%6%9%38%
2654
August1st to 15thDay
11%425%5%35%6%5%8%39%
2318
16th to endDay
11%425%436%46%8%42%
2135
September1st to 15thDay
14%5%1442445%6%45%
1928
16th to endDay
14%5%24323245%51%
1815
October1st to 15thDay
19%5%145%24445%41%
1785
16th to endDay
16%5%25%4335%5%5%42%
1584
November1st to 15thDay
20%6%23323335%44%
1581
16th to endDay
17%6%35%324435%44%
1620
December1st to 15thDay
18%6%26%5%24436%39%
1446
16th to endDay
23%8%24425%335%35%
1536
January1st to 15thNight
16%8%25%433445%40%
2506
16th to endNight
16%9%35%423346%40%
2688
February1st to 15thNight
14%8%35%5%345%5%5%38%
2190
16th to endNight
16%9%35%423445%40%
1893
March1st to 15thNight
19%7%35%435%346%37%
2160
16th to endNight
16%7%35%423335%44%
2024
April1st to 15thNight
17%7%25%5%34346%38%
1796
16th to endNight
21%6%25%323337%40%
1674
May1st to 15thNight
21%6%25%425%445%36%
1620
16th to endNight
20%426%43445%8%35%
1522
June1st to 15thNight
23%6%2442335%7%35%
1440
16th to endNight
14%5%34425%5%5%9%39%
1440
July1st to 15thNight
12%5%245%25%46%8%41%
1440
16th to endNight
11%5%25%5%45%5%5%7%41%
1514
August1st to 15thNight
9%5%245%35%5%6%7%44%
1544
16th to endNight
12%324435%46%8%44%
1714
September1st to 15thNight
14%414413447%47%
1702
16th to endNight
14%423223346%52%
1815
October1st to 15thNight
19%5%14423345%44%
1844
16th to endNight
20%5%245%2445%6%39%
2288
November1st to 15thNight
17%5%2432334445%
2277
16th to endNight
19%6%35%423346%41%
2340
December1st to 15thNight
19%7%25%423346%39%
2511
16th to endNight
18%8%3442435%6%36%
2688

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
44%5%10%23%11%3
1433
16th to endDay
40%413%25%13%1
1536
February1st to 15thDay
47%6%12%19%12%
1662
16th to endDay
46%5%9%20%14%3
1580
March1st to 15thDay
49%311%19%12%3
1800
16th to endDay
44%49%20%14%6%
2198
April1st to 15thDay
46%49%19%15%3
2152
16th to endDay
42%6%7%20%19%3
2286
May1st to 15thDay
53%48%17%13%3
2332
16th to endDay
43%7%12%20%13%2
2603
June1st to 15thDay
49%6%13%19%10%1
2520
16th to endDay
44%8%13%19%14%
2520
July1st to 15thDay
41%7%11%22%15%1
2371
16th to endDay
40%7%15%21%12%2
2487
August1st to 15thDay
40%8%14%23%12%1
2304
16th to endDay
37%7%12%26%15%2
2135
September1st to 15thDay
37%6%10%23%20%1
1928
16th to endDay
35%38%24%24%2
1815
October1st to 15thDay
44%410%23%14%3
1785
16th to endDay
41%5%11%23%15%1
1584
November1st to 15thDay
43%312%24%14%1
1581
16th to endDay
42%413%21%16%1
1620
December1st to 15thDay
45%5%15%16%16%
1446
16th to endDay
50%6%12%19%11%
1536
January1st to 15thNight
45%412%21%12%2
2506
16th to endNight
45%411%20%15%2
2688
February1st to 15thNight
46%5%12%20%11%2
2198
16th to endNight
44%310%22%15%3
1893
March1st to 15thNight
48%5%10%19%13%3
2160
16th to endNight
42%48%18%19%5%
2025
April1st to 15thNight
46%410%18%17%2
1796
16th to endNight
45%49%18%16%4
1674
May1st to 15thNight
47%5%9%18%14%3
1620
16th to endNight
46%7%14%18%11%1
1524
June1st to 15thNight
49%6%12%20%10%1
1440
16th to endNight
41%7%12%21%15%1
1440
July1st to 15thNight
38%7%13%25%14%
1360
16th to endNight
39%6%13%27%10%2
1418
August1st to 15thNight
36%8%13%23%15%1
1534
16th to endNight
36%5%11%28%16%
1714
September1st to 15thNight
36%414%24%19%
1702
16th to endNight
32%48%24%28%1
1815
October1st to 15thNight
40%413%22%15%3
1844
16th to endNight
44%5%13%23%13%
2288
November1st to 15thNight
41%5%11%20%18%2
2277
16th to endNight
44%411%22%14%2
2340
December1st to 15thNight
46%414%21%12%
2511
16th to endNight
48%412%21%12%
2688

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%26%26%20%3
60
16th to endDay
28%18%40%12%
64
February1st to 15thDay
28%18%34%15%3
213
16th to endDay
19%22%36%17%4
239
March1st to 15thDay
30%20%30%14%2
270
16th to endDay
22%22%34%10%7%2
288
April1st to 15thDay
24%18%33%14%8%1
270
16th to endDay
24%13%33%18%6%3
270
May1st to 15thDay
34%15%25%14%6%2
288
16th to endDay
21%9%37%23%6%1
331
June1st to 15thDay
23%12%32%20%10%
315
16th to endDay
17%10%40%22%5%2
315
July1st to 15thDay
19%8%28%28%13%1
305
16th to endDay
15%6%29%31%13%3
326
August1st to 15thDay
13%8%28%27%18%3
297
16th to endDay
13%425%26%21%8%
268
September1st to 15thDay
20%6%23%25%15%8%
255
16th to endDay
18%7%25%20%14%13%
254
October1st to 15thDay
24%11%21%21%16%5%
254
16th to endDay
21%17%26%25%8%
240
November1st to 15thDay
26%9%30%20%10%2
235
16th to endDay
24%16%27%18%9%2
240
December1st to 15thDay
24%31%25%13%4
66
16th to endDay
26%26%31%14%1
64
January1st to 15thNight
22%17%37%19%2
720
16th to endNight
23%17%36%18%2
768
February1st to 15thNight
20%18%38%21%1
704
16th to endNight
22%19%34%19%3
631
March1st to 15thNight
24%19%34%18%2
720
16th to endNight
21%19%33%19%6%
768
April1st to 15thNight
21%17%36%20%4
718
16th to endNight
23%17%32%19%6%
594
May1st to 15thNight
25%13%31%23%5%
540
16th to endNight
21%9%36%27%5%
402
June1st to 15thNight
24%6%31%28%8%
360
16th to endNight
17%335%34%7%
360
July1st to 15thNight
12%427%42%11%1
340
16th to endNight
12%228%41%14%
376
August1st to 15thNight
11%329%42%12%1
485
16th to endNight
12%327%36%16%2
664
September1st to 15thNight
17%432%29%15%
660
16th to endNight
16%5%26%27%20%3
657
October1st to 15thNight
23%10%26%26%11%1
660
16th to endNight
25%12%32%23%6%
704
November1st to 15thNight
19%12%31%25%10%1
700
16th to endNight
23%16%29%22%7%1
720
December1st to 15thNight
25%16%32%20%4
720
16th to endNight
24%19%36%17%2
768
Page generated at 2017-08-13 14:16:57. Version ac04

Copyright 2017 A. Danko


Edward Knight Observatory Clear Sky Chart | Charts Home