Ayer Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ayer Observatory clear sky chart from 2005-11-05 to 2018-02-22.

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
27%
15544
19%
48853
8%
15512
39%
15512
21%
31639

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
15%
780
19%
2716
5%
776
32%
776
17%
1716
January16th to endNight
14%
832
18%
2912
3%
832
33%
832
15%
1886
February1st to 15thNight
15%
764
17%
2365
4%
761
31%
761
17%
1609
February16th to endNight
16%
659
23%
1976
7%
659
37%
659
19%
1309
March1st to 15thNight
17%
720
17%
2148
6%
720
31%
720
18%
1471
March16th to endNight
20%
768
23%
2043
7%
768
34%
768
22%
1384
April1st to 15thNight
20%
718
18%
1796
4%
718
29%
718
22%
1272
April16th to endNight
23%
585
18%
1665
4%
585
32%
585
21%
1127
May1st to 15thNight
23%
540
15%
1620
7%
540
31%
540
19%
1113
May16th to endNight
26%
433
11%
1555
8%
432
32%
432
18%
1026
June1st to 15thNight
30%
360
12%
1440
8%
360
40%
360
16%
959
June16th to endNight
38%
360
11%
1440
12%
360
45%
360
22%
854
July1st to 15thNight
43%
340
12%
1360
13%
340
49%
340
23%
801
July16th to endNight
46%
376
15%
1418
15%
354
52%
354
21%
875
August1st to 15thNight
46%
521
16%
1592
11%
519
50%
519
27%
980
August16th to endNight
53%
711
22%
1857
12%
711
57%
711
35%
1162
September1st to 15thNight
45%
720
19%
1870
9%
720
50%
720
34%
1197
September16th to endNight
46%
717
26%
1980
17%
717
53%
717
35%
1200
October1st to 15thNight
35%
720
23%
1998
11%
720
45%
720
28%
1292
October16th to endNight
25%
768
18%
2496
7%
768
39%
768
21%
1580
November1st to 15thNight
28%
760
19%
2472
9%
760
42%
760
21%
1580
November16th to endNight
24%
780
23%
2535
9%
780
40%
780
20%
1574
December1st to 15thNight
20%
780
18%
2687
4%
780
36%
780
16%
1757
December16th to endNight
14%
832
20%
2912
4%
832
32%
832
16%
1915

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%6%2442445%7%40%
49399
Night
16%6%25%424446%40%
49023
Cloud CoverDay
17%6%2442445%7%40%
49409
Night
17%6%25%424446%40%
49028
TransparencyDay
43%411%23%15%2
49087
Night
44%411%22%15%2
48853
SeeingDay
20%13%32%21%9%2
6387
Night
20%12%34%24%7%
15544

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
15%8%34323445%43%
1552
16th to endDay
16%6%15%42436%5%42%
1664
February1st to 15thDay
18%6%35%435%447%37%
1821
16th to endDay
14%7%34434446%42%
1644
March1st to 15thDay
19%7%34324447%35%
1802
16th to endDay
17%7%25%42435%5%42%
2178
April1st to 15thDay
17%7%36%5%24345%38%
2152
16th to endDay
17%5%2442345%7%41%
2295
May1st to 15thDay
24%8%35%423345%33%
2332
16th to endDay
17%5%245%24448%39%
2568
June1st to 15thDay
25%5%244245%5%7%33%
2520
16th to endDay
14%5%25%5%36%5%6%8%35%
2520
July1st to 15thDay
11%6%346%36%5%6%9%37%
2520
16th to endDay
11%5%25%5%36%6%6%8%37%
2654
August1st to 15thDay
12%6%25%435%6%6%9%38%
2344
16th to endDay
11%4244345%5%8%43%
2342
September1st to 15thDay
13%5%14423447%47%
2089
16th to endDay
16%413324345%49%
1980
October1st to 15thDay
21%424423335%43%
1957
16th to endDay
16%6%24423445%44%
1728
November1st to 15thDay
21%6%2332333443%
1716
16th to endDay
17%424424447%43%
1755
December1st to 15thDay
17%7%26%433436%39%
1602
16th to endDay
22%9%24424435%36%
1664
January1st to 15thNight
16%8%35%423345%41%
2716
16th to endNight
16%9%35%5%23345%39%
2912
February1st to 15thNight
16%8%25%5%44446%37%
2364
16th to endNight
16%8%34424435%41%
1976
March1st to 15thNight
18%7%345%34446%36%
2135
16th to endNight
16%8%36%41333444%
2043
April1st to 15thNight
16%7%36%43445%6%37%
1796
16th to endNight
21%5%25%5%24345%40%
1665
May1st to 15thNight
21%7%25%424346%35%
1620
16th to endNight
19%6%25%5%1446%7%35%
1553
June1st to 15thNight
23%6%2442445%6%33%
1440
16th to endNight
15%4244245%5%8%41%
1440
July1st to 15thNight
10%5%25%6%25%5%6%9%40%
1440
16th to endNight
11%426%6%46%5%5%9%36%
1514
August1st to 15thNight
8%425%6%35%5%6%8%42%
1602
16th to endNight
12%4244235%5%9%45%
1857
September1st to 15thNight
13%414425%45%7%45%
1870
16th to endNight
15%5%23223345%51%
1980
October1st to 15thNight
18%5%24423335%46%
1998
16th to endNight
17%6%2443445%5%41%
2496
November1st to 15thNight
17%6%25%433345%43%
2472
16th to endNight
17%6%24424346%43%
2535
December1st to 15thNight
18%8%24424446%38%
2687
16th to endNight
18%9%35%323446%37%
2912

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%8%3432345%5%42%
1552
16th to endDay
16%6%245%2335%6%42%
1664
February1st to 15thDay
18%7%24434447%37%
1821
16th to endDay
14%7%24334446%42%
1644
March1st to 15thDay
20%7%25%3245%47%35%
1812
16th to endDay
17%8%25%413445%42%
2178
April1st to 15thDay
18%7%35%5%3435%5%38%
2152
16th to endDay
18%5%1442345%6%42%
2295
May1st to 15thDay
25%8%34423436%33%
2332
16th to endDay
17%5%2442445%7%39%
2568
June1st to 15thDay
25%5%2442445%7%33%
2520
16th to endDay
15%415%5%35%5%6%9%36%
2520
July1st to 15thDay
12%5%25%5%36%5%5%9%37%
2520
16th to endDay
12%425%5%36%6%7%8%37%
2654
August1st to 15thDay
13%5%15%5%35%5%6%8%38%
2344
16th to endDay
12%4144245%5%8%44%
2342
September1st to 15thDay
14%424423347%47%
2089
16th to endDay
16%5%23323346%49%
1980
October1st to 15thDay
21%5%23323335%44%
1957
16th to endDay
17%6%24423446%44%
1728
November1st to 15thDay
22%6%2331334443%
1716
16th to endDay
17%5%14324446%43%
1755
December1st to 15thDay
18%6%35%433436%39%
1602
16th to endDay
22%8%24434335%36%
1664
January1st to 15thNight
16%8%35%424345%41%
2716
16th to endNight
17%9%345%23345%39%
2912
February1st to 15thNight
16%8%35%435%445%38%
2357
16th to endNight
17%7%34424435%42%
1976
March1st to 15thNight
18%7%25%5%34445%37%
2148
16th to endNight
16%8%36%41333344%
2042
April1st to 15thNight
16%8%36%5%34446%37%
1796
16th to endNight
21%6%14434346%39%
1665
May1st to 15thNight
22%7%26%42335%6%35%
1620
16th to endNight
19%6%35%5%2445%8%35%
1553
June1st to 15thNight
23%5%35%4235%5%7%34%
1440
16th to endNight
15%5%244245%5%8%41%
1440
July1st to 15thNight
11%5%25%6%245%6%8%40%
1440
16th to endNight
12%426%6%45%5%5%9%36%
1514
August1st to 15thNight
8%5%25%6%35%5%5%7%43%
1602
16th to endNight
13%42442345%7%46%
1857
September1st to 15thNight
13%4144245%5%7%45%
1870
16th to endNight
15%5%2321335%5%50%
1980
October1st to 15thNight
19%424323345%46%
1998
16th to endNight
18%5%25%333446%41%
2496
November1st to 15thNight
17%6%35%323345%43%
2472
16th to endNight
17%6%24423445%43%
2535
December1st to 15thNight
19%7%35%5%24445%39%
2687
16th to endNight
18%9%25%424436%37%
2912

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
42%5%10%25%13%3
1553
16th to endDay
43%211%26%14%1
1664
February1st to 15thDay
47%312%21%13%1
1824
16th to endDay
42%412%23%15%2
1644
March1st to 15thDay
48%312%21%10%3
1812
16th to endDay
43%310%21%15%5%
2178
April1st to 15thDay
45%39%21%16%2
2152
16th to endDay
40%39%24%17%4
2295
May1st to 15thDay
53%37%18%14%2
2332
16th to endDay
41%6%13%23%13%2
2572
June1st to 15thDay
49%5%13%20%10%1
2520
16th to endDay
41%6%14%22%14%1
2520
July1st to 15thDay
40%7%10%23%15%2
2371
16th to endDay
40%6%14%23%12%3
2487
August1st to 15thDay
40%5%13%24%14%1
2330
16th to endDay
35%5%10%27%18%1
2342
September1st to 15thDay
35%5%10%23%21%2
2089
16th to endDay
36%36%25%25%2
1980
October1st to 15thDay
43%38%26%15%3
1957
16th to endDay
41%311%25%16%2
1728
November1st to 15thDay
45%311%22%15%1
1716
16th to endDay
41%410%25%15%1
1755
December1st to 15thDay
46%214%17%17%
1602
16th to endDay
51%411%19%11%1
1664
January1st to 15thNight
45%311%23%15%1
2716
16th to endNight
47%310%20%15%2
2912
February1st to 15thNight
48%411%20%13%1
2365
16th to endNight
45%39%22%16%2
1976
March1st to 15thNight
48%39%20%14%3
2148
16th to endNight
45%27%19%20%4
2043
April1st to 15thNight
47%47%20%17%2
1796
16th to endNight
47%28%20%16%4
1665
May1st to 15thNight
49%310%17%14%3
1620
16th to endNight
46%5%16%19%9%2
1555
June1st to 15thNight
49%5%13%19%10%
1440
16th to endNight
39%5%13%23%15%1
1440
July1st to 15thNight
38%6%13%26%14%
1360
16th to endNight
42%5%12%27%9%2
1418
August1st to 15thNight
37%6%14%23%15%2
1592
16th to endNight
34%5%12%27%19%
1857
September1st to 15thNight
35%5%12%25%19%1
1870
16th to endNight
34%47%24%27%2
1980
October1st to 15thNight
40%49%24%16%3
1998
16th to endNight
43%412%24%14%
2496
November1st to 15thNight
44%310%22%15%2
2472
16th to endNight
42%312%24%15%2
2535
December1st to 15thNight
47%413%21%12%
2687
16th to endNight
47%411%21%13%
2912

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
20%34%27%14%3
90
16th to endDay
26%22%40%10%
96
February1st to 15thDay
25%22%38%11%2
292
16th to endDay
16%22%41%15%3
265
March1st to 15thDay
22%23%36%12%4
270
16th to endDay
20%24%35%10%7%1
288
April1st to 15thDay
22%15%36%17%7%
270
16th to endDay
21%12%35%23%42
270
May1st to 15thDay
33%15%26%15%6%2
288
16th to endDay
20%9%37%25%6%
331
June1st to 15thDay
26%11%31%22%7%1
315
16th to endDay
14%7%43%24%8%1
315
July1st to 15thDay
18%5%31%31%12%
305
16th to endDay
13%329%36%14%2
326
August1st to 15thDay
13%7%27%36%12%2
313
16th to endDay
14%5%26%28%21%4
332
September1st to 15thDay
17%7%27%25%17%4
315
16th to endDay
23%428%18%14%11%
314
October1st to 15thDay
23%9%23%24%12%5%
311
16th to endDay
18%14%35%21%8%1
288
November1st to 15thDay
25%12%31%19%9%2
280
16th to endDay
20%21%30%17%9%
285
December1st to 15thDay
23%21%36%12%6%
132
16th to endDay
22%29%38%8%1
96
January1st to 15thNight
20%20%39%17%2
780
16th to endNight
22%18%37%17%3
832
February1st to 15thNight
21%18%40%19%1
764
16th to endNight
23%18%37%16%3
659
March1st to 15thNight
23%17%37%18%3
720
16th to endNight
21%16%34%20%5%
768
April1st to 15thNight
19%16%39%20%4
718
16th to endNight
23%13%35%22%5%
585
May1st to 15thNight
25%10%35%21%6%
540
16th to endNight
21%8%39%25%6%
433
June1st to 15thNight
23%6%32%31%6%
360
16th to endNight
14%436%36%7%
360
July1st to 15thNight
12%235%40%10%
340
16th to endNight
12%130%43%11%
376
August1st to 15thNight
9%330%42%13%
521
16th to endNight
15%220%38%19%2
711
September1st to 15thNight
14%330%36%14%
720
16th to endNight
18%425%26%20%4
717
October1st to 15thNight
21%9%27%27%12%2
720
16th to endNight
22%13%36%19%7%1
768
November1st to 15thNight
20%14%31%22%9%1
760
16th to endNight
19%19%31%21%7%
780
December1st to 15thNight
23%17%34%19%4
780
16th to endNight
24%19%37%15%2
832
Page generated at 2018-02-23 11:21:34. Version ac04

Copyright 2018 A. Danko


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