Mattatuck Astronomical Society Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mattatuck Astronomical Society 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
26%
14797
18%
46471
7%
14765
38%
14765
20%
30264

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
14%
720
20%
2506
6%
716
32%
716
17%
1575
January16th to endNight
13%
768
20%
2650
4%
768
33%
768
15%
1683
February1st to 15thNight
13%
704
16%
2207
3%
701
29%
701
14%
1473
February16th to endNight
16%
631
23%
1893
6%
631
35%
631
17%
1240
March1st to 15thNight
15%
720
17%
2160
6%
720
29%
720
16%
1463
March16th to endNight
19%
768
23%
2001
7%
768
32%
768
22%
1319
April1st to 15thNight
17%
718
16%
1796
3%
718
25%
718
20%
1299
April16th to endNight
22%
660
16%
1740
2%
660
31%
660
22%
1196
May1st to 15thNight
22%
517
17%
1597
6%
517
33%
517
17%
1102
May16th to endNight
31%
375
12%
1497
9%
374
39%
374
16%
957
June1st to 15thNight
30%
360
13%
1440
8%
360
43%
360
16%
954
June16th to endNight
40%
360
14%
1440
13%
360
50%
360
23%
849
July1st to 15thNight
43%
340
12%
1360
13%
340
48%
340
21%
827
July16th to endNight
48%
376
15%
1418
16%
354
54%
354
23%
852
August1st to 15thNight
47%
491
16%
1540
9%
489
53%
489
30%
913
August16th to endNight
52%
700
17%
1750
8%
700
55%
700
31%
1176
September1st to 15thNight
40%
660
22%
1663
9%
660
46%
660
30%
1098
September16th to endNight
45%
657
25%
1815
17%
657
51%
657
31%
1125
October1st to 15thNight
33%
660
21%
1900
11%
660
43%
660
24%
1238
October16th to endNight
23%
704
16%
2236
6%
704
35%
704
20%
1465
November1st to 15thNight
26%
700
21%
2277
8%
700
43%
700
20%
1455
November16th to endNight
23%
720
19%
2377
8%
720
35%
720
17%
1548
December1st to 15thNight
19%
720
16%
2520
4%
720
33%
720
14%
1674
December16th to endNight
14%
768
18%
2688
4%
768
32%
768
14%
1783

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
16%6%25%5%3445%7%37%
47511
Night
16%6%25%42445%6%40%
46641
Cloud CoverDay
16%6%25%5%3445%7%38%
47521
Night
16%6%25%42445%6%41%
46646
TransparencyDay
44%6%12%21%14%1
47199
Night
42%5%12%22%14%1
46471
SeeingDay
21%15%30%20%9%3
5655
Night
21%15%32%22%7%
14797

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
14%7%24445%5%45%39%
1432
16th to endDay
14%6%35%4245%5%7%38%
1572
February1st to 15thDay
15%6%27%6%46%45%6%34%
1654
16th to endDay
16%7%26%435%35%6%38%
1580
March1st to 15thDay
17%7%26%6%3436%7%34%
1790
16th to endDay
18%7%34424346%39%
2220
April1st to 15thDay
19%7%25%434445%38%
2152
16th to endDay
16%5%25%423446%42%
2220
May1st to 15thDay
24%6%3442436%6%33%
2364
16th to endDay
16%6%25%5%245%5%7%37%
2618
June1st to 15thDay
23%5%245%345%6%7%32%
2520
16th to endDay
14%5%26%5%35%6%6%8%34%
2520
July1st to 15thDay
13%5%25%6%35%5%5%8%37%
2520
16th to endDay
12%5%25%5%36%5%6%8%37%
2654
August1st to 15thDay
13%5%346%45%5%6%8%35%
2311
16th to endDay
12%6%25%5%35%46%8%39%
2100
September1st to 15thDay
13%6%35%5%35%447%41%
1966
16th to endDay
14%5%2332345%6%47%
1815
October1st to 15thDay
19%7%24323445%40%
1729
16th to endDay
17%6%145%35%45%6%38%
1636
November1st to 15thDay
18%6%1432325%7%42%
1581
16th to endDay
19%6%24423247%42%
1581
December1st to 15thDay
18%6%35%5%2445%6%38%
1440
16th to endDay
20%8%25%5%2445%6%32%
1536
January1st to 15thNight
16%6%25%5%2445%6%40%
2506
16th to endNight
15%8%34324446%40%
2650
February1st to 15thNight
14%8%35%435%45%6%37%
2206
16th to endNight
16%8%2442345%5%42%
1893
March1st to 15thNight
17%7%35%425%446%38%
2147
16th to endNight
16%5%25%413446%43%
2001
April1st to 15thNight
18%7%25%5%345%46%36%
1796
16th to endNight
18%5%25%43434442%
1740
May1st to 15thNight
20%7%26%434446%35%
1597
16th to endNight
18%7%344135%48%37%
1495
June1st to 15thNight
21%6%35%324447%37%
1440
16th to endNight
14%425%42446%9%42%
1440
July1st to 15thNight
13%415%425%5%6%8%41%
1440
16th to endNight
10%5%26%5%35%46%8%39%
1514
August1st to 15thNight
11%42342445%8%46%
1550
16th to endNight
13%4244345%5%7%44%
1750
September1st to 15thNight
14%5%24324337%47%
1663
16th to endNight
14%42431434451%
1815
October1st to 15thNight
17%6%25%432436%43%
1900
16th to endNight
20%5%15%424446%40%
2236
November1st to 15thNight
16%7%35%323345%43%
2277
16th to endNight
19%6%25%424345%38%
2377
December1st to 15thNight
20%6%25%5%245%5%6%35%
2520
16th to endNight
19%8%3442445%6%36%
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
15%6%24436%5%5%6%39%
1432
16th to endDay
14%6%35%425%5%5%8%39%
1572
February1st to 15thDay
14%6%27%5%45%5%5%7%34%
1654
16th to endDay
15%7%35%5%35%35%6%39%
1580
March1st to 15thDay
16%7%36%5%3445%7%34%
1800
16th to endDay
17%7%35%424436%40%
2220
April1st to 15thDay
19%7%24435%45%5%38%
2152
16th to endDay
17%5%25%324347%43%
2220
May1st to 15thDay
23%6%35%42445%6%33%
2364
16th to endDay
17%5%245%245%5%7%37%
2618
June1st to 15thDay
22%5%24435%5%47%32%
2520
16th to endDay
14%5%26%5%35%6%5%9%35%
2520
July1st to 15thDay
12%5%26%6%35%5%5%8%36%
2520
16th to endDay
12%6%35%5%36%5%6%8%37%
2654
August1st to 15thDay
13%5%25%5%36%6%6%9%35%
2311
16th to endDay
12%5%35%5%25%5%5%9%39%
2100
September1st to 15thDay
13%6%35%435%446%42%
1966
16th to endDay
13%6%2332345%7%46%
1815
October1st to 15thDay
18%8%24423345%41%
1729
16th to endDay
17%5%25%5%25%446%39%
1636
November1st to 15thDay
18%5%24412347%42%
1581
16th to endDay
18%6%23323346%42%
1581
December1st to 15thDay
17%5%35%5%25%446%39%
1440
16th to endDay
18%9%245%25%46%6%33%
1536
January1st to 15thNight
16%6%25%43345%6%41%
2506
16th to endNight
14%8%3442445%6%40%
2650
February1st to 15thNight
14%8%36%435%5%5%6%38%
2199
16th to endNight
16%7%25%5%14446%42%
1893
March1st to 15thNight
17%7%36%324446%38%
2160
16th to endNight
16%6%25%42335%7%43%
2000
April1st to 15thNight
17%7%245%345%46%37%
1796
16th to endNight
18%5%35%433435%42%
1740
May1st to 15thNight
20%7%26%434446%36%
1597
16th to endNight
19%6%2442345%7%38%
1495
June1st to 15thNight
21%7%343235%46%38%
1440
16th to endNight
13%5%25%4245%5%8%43%
1440
July1st to 15thNight
14%424425%47%9%39%
1440
16th to endNight
9%5%36%5%35%5%7%9%39%
1514
August1st to 15thNight
11%41433445%9%46%
1550
16th to endNight
12%424435%45%7%44%
1750
September1st to 15thNight
15%424333447%46%
1663
16th to endNight
14%423323345%50%
1815
October1st to 15thNight
16%6%25%423346%44%
1900
16th to endNight
20%5%24424346%41%
2236
November1st to 15thNight
16%7%35%32325%5%43%
2277
16th to endNight
18%7%36%424345%39%
2377
December1st to 15thNight
19%6%25%5%2445%6%36%
2520
16th to endNight
20%7%25%42445%6%37%
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
43%6%13%24%10%2
1433
16th to endDay
41%5%13%25%11%1
1572
February1st to 15thDay
47%5%13%19%12%
1657
16th to endDay
45%411%23%12%2
1580
March1st to 15thDay
47%5%13%22%9%2
1800
16th to endDay
44%410%19%16%3
2220
April1st to 15thDay
45%6%10%19%14%3
2152
16th to endDay
41%5%9%22%18%3
2220
May1st to 15thDay
50%5%9%18%13%2
2364
16th to endDay
44%8%12%20%12%1
2622
June1st to 15thDay
49%7%12%19%9%1
2520
16th to endDay
42%8%14%21%11%
2520
July1st to 15thDay
45%7%10%20%14%1
2371
16th to endDay
42%7%14%20%13%1
2487
August1st to 15thDay
43%6%15%20%13%
2297
16th to endDay
40%6%10%24%16%1
2100
September1st to 15thDay
41%6%10%20%19%1
1966
16th to endDay
35%49%22%24%2
1815
October1st to 15thDay
44%5%9%23%14%2
1729
16th to endDay
44%5%14%21%12%2
1636
November1st to 15thDay
41%5%13%20%16%2
1581
16th to endDay
41%312%25%13%2
1581
December1st to 15thDay
45%5%16%20%12%
1440
16th to endDay
48%7%14%18%10%
1536
January1st to 15thNight
43%413%23%12%2
2506
16th to endNight
43%5%12%22%14%2
2650
February1st to 15thNight
45%6%14%20%11%2
2207
16th to endNight
43%311%21%16%2
1893
March1st to 15thNight
45%5%13%21%11%2
2160
16th to endNight
40%310%22%18%4
2001
April1st to 15thNight
46%6%10%21%14%1
1796
16th to endNight
44%49%21%17%2
1740
May1st to 15thNight
48%5%10%18%14%2
1597
16th to endNight
43%7%17%17%11%1
1497
June1st to 15thNight
47%5%13%19%11%1
1440
16th to endNight
38%6%14%25%13%
1440
July1st to 15thNight
38%8%13%25%13%1
1360
16th to endNight
39%6%17%22%11%2
1418
August1st to 15thNight
34%6%16%25%15%1
1540
16th to endNight
37%8%10%25%16%
1750
September1st to 15thNight
38%6%12%23%18%
1663
16th to endNight
35%5%6%24%26%1
1815
October1st to 15thNight
42%410%21%17%2
1900
16th to endNight
44%415%22%12%
2236
November1st to 15thNight
42%5%11%20%17%3
2277
16th to endNight
46%412%22%11%1
2377
December1st to 15thNight
46%6%14%20%11%
2520
16th to endNight
47%5%13%21%11%
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
25%31%28%11%3
60
16th to endDay
22%28%40%8%
70
February1st to 15thDay
24%26%34%13%1
170
16th to endDay
24%23%34%15%2
239
March1st to 15thDay
25%23%31%15%3
270
16th to endDay
21%22%36%11%5%1
288
April1st to 15thDay
25%19%32%14%5%1
270
16th to endDay
21%17%33%18%6%2
270
May1st to 15thDay
36%15%22%14%9%2
288
16th to endDay
22%13%31%21%9%1
331
June1st to 15thDay
24%15%26%20%12%1
315
16th to endDay
18%12%33%26%8%
315
July1st to 15thDay
16%11%30%27%13%1
305
16th to endDay
13%7%31%28%15%3
326
August1st to 15thDay
13%6%29%28%13%7%
297
16th to endDay
13%6%25%25%18%10%
268
September1st to 15thDay
14%10%25%23%16%8%
255
16th to endDay
18%8%26%21%12%12%
254
October1st to 15thDay
23%10%25%24%10%5%
255
16th to endDay
20%21%30%21%6%
249
November1st to 15thDay
24%16%27%21%9%
235
16th to endDay
22%23%27%15%9%
201
December1st to 15thDay
18%31%35%10%5%
60
16th to endDay
31%28%28%12%
64
January1st to 15thNight
22%20%39%15%3
720
16th to endNight
23%21%36%16%1
768
February1st to 15thNight
21%21%38%17%1
704
16th to endNight
24%23%33%16%2
631
March1st to 15thNight
24%23%32%17%2
720
16th to endNight
21%23%31%18%4
768
April1st to 15thNight
23%20%36%16%2
718
16th to endNight
21%18%33%19%5%
660
May1st to 15thNight
26%14%33%20%4
517
16th to endNight
22%9%32%28%6%
375
June1st to 15thNight
24%8%31%26%9%
360
16th to endNight
17%332%37%8%
360
July1st to 15thNight
15%5%29%33%16%
340
16th to endNight
12%327%38%17%
376
August1st to 15thNight
12%330%39%13%1
491
16th to endNight
15%224%35%20%2
700
September1st to 15thNight
18%6%30%30%12%1
660
16th to endNight
16%7%25%27%19%3
657
October1st to 15thNight
21%10%31%25%10%1
660
16th to endNight
24%16%33%20%4
704
November1st to 15thNight
22%16%31%20%8%1
700
16th to endNight
25%15%31%19%7%
720
December1st to 15thNight
24%18%33%17%5%
720
16th to endNight
26%21%34%15%1
768
Page generated at 2017-08-13 14:16:27. Version ac04

Copyright 2017 A. Danko


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