Roger Twitchell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Roger Twitchell Observatory clear sky chart from 2005-11-05 to 2017-09-17.

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
30%
14774
17%
46968
8%
14742
40%
14742
19%
31438

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
24%
720
19%
2506
9%
716
37%
716
17%
1625
January16th to endNight
17%
768
17%
2688
5%
768
33%
768
13%
1792
February1st to 15thNight
20%
704
14%
2261
5%
701
31%
701
14%
1578
February16th to endNight
20%
631
18%
1893
6%
631
35%
631
17%
1301
March1st to 15thNight
19%
720
15%
2136
5%
720
31%
720
17%
1505
March16th to endNight
26%
768
21%
2058
7%
768
36%
768
21%
1375
April1st to 15thNight
22%
718
17%
1796
4%
718
30%
718
21%
1312
April16th to endNight
25%
546
16%
1626
6%
546
34%
546
19%
1126
May1st to 15thNight
29%
540
14%
1620
7%
540
36%
540
17%
1144
May16th to endNight
32%
395
11%
1517
9%
394
40%
394
14%
1055
June1st to 15thNight
32%
360
9%
1440
7%
360
41%
360
12%
1016
June16th to endNight
46%
360
9%
1440
11%
360
51%
360
20%
921
July1st to 15thNight
52%
340
11%
1360
17%
340
55%
340
21%
843
July16th to endNight
51%
376
12%
1418
14%
354
56%
354
21%
883
August1st to 15thNight
47%
494
13%
1565
9%
492
51%
492
21%
1021
August16th to endNight
50%
651
20%
1797
11%
651
53%
651
32%
1121
September1st to 15thNight
45%
720
18%
1878
9%
720
51%
720
34%
1198
September16th to endNight
46%
665
25%
1836
16%
665
53%
665
35%
1088
October1st to 15thNight
39%
660
21%
1861
10%
660
48%
660
24%
1233
October16th to endNight
23%
704
15%
2288
6%
704
35%
704
18%
1544
November1st to 15thNight
34%
700
21%
2277
12%
700
47%
700
21%
1477
November16th to endNight
29%
720
20%
2346
10%
720
41%
720
18%
1538
December1st to 15thNight
20%
745
15%
2670
4%
745
34%
745
15%
1843
December16th to endNight
22%
769
16%
2691
6%
769
35%
769
13%
1899

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
18%6%25%43445%6%36%
47798
Night
18%7%24424446%38%
47138
Cloud CoverDay
18%7%25%5%2445%6%37%
47808
Night
17%7%24424446%38%
47143
TransparencyDay
46%6%11%21%13%1
47486
Night
45%6%11%22%13%1
46968
SeeingDay
23%12%27%21%11%3
5843
Night
22%10%30%27%9%
14774

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%344345%6%7%36%
1432
16th to endDay
13%10%36%425%446%39%
1536
February1st to 15thDay
15%9%46%5%46%5%46%32%
1596
16th to endDay
13%10%36%5%35%5%45%35%
1580
March1st to 15thDay
18%9%35%435%45%6%32%
1814
16th to endDay
15%6%445%34345%41%
2163
April1st to 15thDay
19%8%25%5%23445%37%
2152
16th to endDay
21%6%25%5%23445%38%
2334
May1st to 15thDay
26%8%26%424436%30%
2332
16th to endDay
22%5%25%5%2445%6%33%
2607
June1st to 15thDay
28%6%25%42445%6%29%
2520
16th to endDay
21%5%36%5%345%5%6%32%
2520
July1st to 15thDay
15%6%245%36%5%6%9%34%
2520
16th to endDay
14%5%26%5%35%5%7%8%34%
2654
August1st to 15thDay
15%5%26%445%6%6%8%34%
2370
16th to endDay
12%3244245%7%7%44%
2402
September1st to 15thDay
14%5%14434446%46%
2082
16th to endDay
15%423313447%50%
1830
October1st to 15thDay
21%6%25%434336%40%
1766
16th to endDay
20%6%25%5%34446%36%
1584
November1st to 15thDay
21%5%1442345%7%39%
1581
16th to endDay
21%6%2442345%6%40%
1612
December1st to 15thDay
22%9%25%5%2445%6%32%
1284
16th to endDay
21%12%46%423445%30%
1527
January1st to 15thNight
15%9%3442445%5%39%
2506
16th to endNight
15%8%36%5%35%45%6%35%
2688
February1st to 15thNight
12%9%37%6%45%5%6%5%33%
2260
16th to endNight
14%11%25%5%24435%38%
1893
March1st to 15thNight
18%9%35%5%34445%35%
2123
16th to endNight
16%6%3442444444%
2058
April1st to 15thNight
20%7%26%5%34445%36%
1796
16th to endNight
23%6%34324336%37%
1626
May1st to 15thNight
21%8%25%4245%45%35%
1620
16th to endNight
24%7%25%42445%7%30%
1515
June1st to 15thNight
26%6%35%424446%31%
1440
16th to endNight
19%5%2442445%9%36%
1440
July1st to 15thNight
14%425%5%445%5%9%38%
1440
16th to endNight
14%5%144345%5%9%41%
1514
August1st to 15thNight
14%5%245%445%5%7%39%
1575
16th to endNight
13%42442335%9%47%
1797
September1st to 15thNight
15%413333345%50%
1878
16th to endNight
14%41232235%6%53%
1836
October1st to 15thNight
18%6%1342445%5%42%
1861
16th to endNight
22%7%145%2335%6%36%
2288
November1st to 15thNight
18%7%2442435%7%40%
2277
16th to endNight
17%6%25%424346%42%
2346
December1st to 15thNight
21%9%35%424335%34%
2670
16th to endNight
21%10%45%424436%32%
2691

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
17%6%334346%6%6%37%
1432
16th to endDay
13%10%26%425%445%39%
1536
February1st to 15thDay
16%8%37%6%35%5%5%5%32%
1596
16th to endDay
14%9%46%6%35%5%46%34%
1580
March1st to 15thDay
19%9%45%425%446%32%
1824
16th to endDay
15%8%35%43334442%
2163
April1st to 15thDay
19%8%35%423346%36%
2152
16th to endDay
20%6%25%5%23346%38%
2334
May1st to 15thDay
26%7%26%424335%30%
2332
16th to endDay
21%6%35%425%45%6%33%
2607
June1st to 15thDay
27%6%25%43435%6%29%
2520
16th to endDay
21%5%346%35%5%5%6%32%
2520
July1st to 15thDay
15%5%25%5%36%5%5%9%33%
2520
16th to endDay
14%6%26%6%35%5%6%9%34%
2654
August1st to 15thDay
15%6%25%5%35%5%5%8%35%
2370
16th to endDay
11%425%4235%6%8%45%
2402
September1st to 15thDay
14%5%24424445%46%
2082
16th to endDay
15%413323446%50%
1830
October1st to 15thDay
21%6%25%422246%40%
1766
16th to endDay
19%7%245%24346%38%
1584
November1st to 15thDay
21%5%1431445%6%40%
1581
16th to endDay
21%7%13423346%40%
1612
December1st to 15thDay
22%9%35%413446%33%
1284
16th to endDay
21%12%46%42334431%
1527
January1st to 15thNight
16%9%24424445%40%
2506
16th to endNight
15%8%36%5%35%445%36%
2688
February1st to 15thNight
12%9%37%6%36%5%5%6%32%
2253
16th to endNight
14%11%35%434345%38%
1893
March1st to 15thNight
18%10%35%435%445%34%
2136
16th to endNight
16%6%3442344444%
2057
April1st to 15thNight
19%7%36%5%24445%35%
1796
16th to endNight
23%6%24434436%36%
1626
May1st to 15thNight
21%7%25%43445%5%35%
1620
16th to endNight
22%7%35%5%345%5%7%29%
1515
June1st to 15thNight
25%8%25%425%35%6%31%
1440
16th to endNight
18%6%244345%5%8%36%
1440
July1st to 15thNight
14%425%5%345%5%9%38%
1440
16th to endNight
14%4245%245%5%8%41%
1514
August1st to 15thNight
14%426%5%35%45%7%40%
1575
16th to endNight
13%41442335%9%46%
1797
September1st to 15thNight
14%5%14323345%49%
1878
16th to endNight
13%422313346%53%
1836
October1st to 15thNight
18%5%23324446%42%
1861
16th to endNight
22%7%245%22447%36%
2288
November1st to 15thNight
18%6%14424347%41%
2277
16th to endNight
17%6%25%5%2335%5%41%
2346
December1st to 15thNight
22%9%345%2434435%
2670
16th to endNight
21%10%345%25%345%33%
2691

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%7%11%21%13%1
1433
16th to endDay
44%5%12%23%12%1
1536
February1st to 15thDay
51%6%11%20%10%
1599
16th to endDay
48%5%11%17%13%3
1580
March1st to 15thDay
51%5%10%17%12%2
1824
16th to endDay
44%49%16%19%4
2163
April1st to 15thDay
48%47%20%15%3
2152
16th to endDay
47%6%9%19%14%3
2334
May1st to 15thDay
55%5%6%17%13%2
2332
16th to endDay
49%7%10%19%12%1
2611
June1st to 15thDay
54%5%12%16%9%
2520
16th to endDay
51%6%10%19%11%
2520
July1st to 15thDay
43%9%13%23%10%
2371
16th to endDay
43%8%14%22%9%
2487
August1st to 15thDay
44%8%14%23%8%
2356
16th to endDay
34%7%13%26%16%1
2402
September1st to 15thDay
38%5%11%26%17%1
2082
16th to endDay
34%48%30%20%1
1830
October1st to 15thDay
46%5%9%23%14%1
1766
16th to endDay
45%6%11%22%12%1
1584
November1st to 15thDay
42%5%14%22%12%1
1581
16th to endDay
44%6%12%19%14%1
1612
December1st to 15thDay
50%6%12%18%11%
1284
16th to endDay
55%5%10%16%11%1
1527
January1st to 15thNight
44%6%12%24%11%1
2506
16th to endNight
47%6%9%19%14%2
2688
February1st to 15thNight
49%6%12%18%11%1
2261
16th to endNight
46%6%9%21%12%3
1893
March1st to 15thNight
50%410%20%11%2
2136
16th to endNight
42%5%9%18%19%4
2058
April1st to 15thNight
50%46%19%17%2
1796
16th to endNight
48%5%10%18%13%3
1626
May1st to 15thNight
50%6%8%18%14%2
1620
16th to endNight
52%6%12%18%8%1
1517
June1st to 15thNight
54%6%10%19%8%1
1440
16th to endNight
44%6%14%22%11%1
1440
July1st to 15thNight
39%9%15%25%10%
1360
16th to endNight
39%8%16%22%11%
1418
August1st to 15thNight
41%8%13%26%10%
1565
16th to endNight
34%7%14%27%15%
1797
September1st to 15thNight
35%5%12%30%15%1
1878
16th to endNight
32%411%31%18%1
1836
October1st to 15thNight
41%6%11%23%15%1
1861
16th to endNight
47%5%10%24%10%1
2288
November1st to 15thNight
43%5%12%23%13%1
2277
16th to endNight
43%6%11%22%13%1
2346
December1st to 15thNight
51%5%10%22%9%
2670
16th to endNight
52%5%10%18%11%
2691

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%11%30%28%5%
60
16th to endDay
20%23%34%18%3
64
February1st to 15thDay
26%13%32%22%3
204
16th to endDay
18%23%28%22%6%
239
March1st to 15thDay
30%15%28%19%5%
270
16th to endDay
22%22%27%15%9%3
288
April1st to 15thDay
30%12%27%17%8%2
270
16th to endDay
26%19%28%13%8%4
270
May1st to 15thDay
38%12%23%14%7%3
288
16th to endDay
24%13%32%21%6%1
331
June1st to 15thDay
26%12%29%22%8%
315
16th to endDay
23%11%30%20%12%1
315
July1st to 15thDay
17%5%31%27%13%4
305
16th to endDay
14%5%31%30%14%3
326
August1st to 15thDay
16%8%26%26%16%5%
313
16th to endDay
15%9%18%28%19%7%
332
September1st to 15thDay
19%6%24%21%20%7%
315
16th to endDay
19%8%22%18%15%15%
259
October1st to 15thDay
22%12%22%17%20%5%
248
16th to endDay
25%11%32%20%10%
240
November1st to 15thDay
23%11%28%23%10%2
235
16th to endDay
34%13%21%20%9%
232
December1st to 15thDay
28%15%33%15%8%
60
16th to endDay
39%15%32%12%
64
January1st to 15thNight
21%14%33%26%3
720
16th to endNight
22%15%35%23%3
768
February1st to 15thNight
18%17%34%25%3
704
16th to endNight
22%15%33%22%5%
631
March1st to 15thNight
26%13%35%20%3
720
16th to endNight
22%16%30%23%6%1
768
April1st to 15thNight
24%13%34%22%4
718
16th to endNight
25%15%30%21%7%
546
May1st to 15thNight
25%10%30%26%6%
540
16th to endNight
26%7%28%30%7%
395
June1st to 15thNight
28%5%27%30%8%
360
16th to endNight
20%124%39%13%
360
July1st to 15thNight
15%123%39%19%
340
16th to endNight
16%122%38%20%
376
August1st to 15thNight
14%323%37%19%1
494
16th to endNight
14%224%35%20%2
651
September1st to 15thNight
16%427%30%18%1
720
16th to endNight
16%6%25%27%21%3
665
October1st to 15thNight
21%8%25%28%13%1
660
16th to endNight
27%10%34%22%5%
704
November1st to 15thNight
21%9%29%29%8%1
700
16th to endNight
21%13%26%26%9%1
720
December1st to 15thNight
27%12%35%20%4
745
16th to endNight
28%14%29%24%3
769
Page generated at 2017-09-18 08:38:18. Version ac04

Copyright 2017 A. Danko


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