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 2018-10-08.

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
31%
16105
17%
51138
8%
16073
41%
16073
19%
34201

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
23%
780
19%
2716
8%
776
37%
776
17%
1753
January16th to endNight
18%
832
16%
2912
4%
832
33%
832
13%
1957
February1st to 15thNight
20%
764
15%
2453
5%
761
32%
761
14%
1713
February16th to endNight
20%
683
18%
2049
6%
683
34%
683
17%
1409
March1st to 15thNight
19%
780
14%
2314
5%
780
30%
780
16%
1658
March16th to endNight
25%
832
22%
2230
7%
832
37%
832
21%
1492
April1st to 15thNight
23%
778
17%
1946
4%
778
31%
778
21%
1424
April16th to endNight
25%
585
15%
1741
6%
585
33%
585
19%
1210
May1st to 15thNight
29%
585
15%
1755
8%
585
36%
585
18%
1230
May16th to endNight
33%
429
11%
1647
10%
428
41%
428
14%
1134
June1st to 15thNight
33%
390
10%
1560
8%
390
42%
390
13%
1090
June16th to endNight
47%
390
10%
1560
12%
390
52%
390
20%
998
July1st to 15thNight
54%
370
13%
1480
19%
370
58%
370
22%
890
July16th to endNight
52%
408
13%
1546
15%
386
57%
386
21%
962
August1st to 15thNight
48%
525
13%
1666
9%
523
51%
523
21%
1098
August16th to endNight
51%
706
20%
1948
11%
706
54%
706
32%
1208
September1st to 15thNight
47%
780
19%
2034
9%
780
53%
780
34%
1303
September16th to endNight
45%
777
25%
2145
15%
777
52%
777
33%
1296
October1st to 15thNight
40%
752
20%
2118
10%
752
49%
752
25%
1408
October16th to endNight
23%
768
16%
2496
6%
768
35%
768
17%
1684
November1st to 15thNight
33%
760
21%
2472
12%
760
45%
760
21%
1608
November16th to endNight
29%
780
21%
2542
11%
780
41%
780
19%
1641
December1st to 15thNight
20%
818
15%
2893
3%
818
33%
818
15%
2004
December16th to endNight
21%
833
17%
2915
5%
833
36%
833
13%
2031

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%37%
51989
Night
17%7%25%424446%38%
51308
Cloud CoverDay
18%7%25%5%2445%6%37%
51999
Night
17%7%24424446%38%
51313
TransparencyDay
46%6%11%20%13%1
51677
Night
45%6%11%22%13%1
51138
SeeingDay
23%13%28%20%10%3
7175
Night
21%10%29%27%9%
16105

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%7%24435%5%6%7%35%
1552
16th to endDay
14%9%36%425%446%39%
1664
February1st to 15thDay
15%9%36%5%35%5%46%32%
1734
16th to endDay
13%10%36%5%35%5%45%35%
1710
March1st to 15thDay
18%10%45%5%35%45%6%30%
1966
16th to endDay
14%7%45%5%34345%41%
2343
April1st to 15thDay
19%9%35%5%33445%36%
2332
16th to endDay
21%6%25%5%23445%38%
2494
May1st to 15thDay
25%8%26%424436%31%
2527
16th to endDay
22%5%25%5%2445%6%34%
2830
June1st to 15thDay
27%6%25%42445%5%30%
2730
16th to endDay
20%5%36%5%35%5%5%6%32%
2730
July1st to 15thDay
14%5%245%36%5%6%9%35%
2730
16th to endDay
15%5%26%5%35%5%7%8%34%
2878
August1st to 15thDay
16%5%26%5%45%6%6%8%33%
2521
16th to endDay
12%3244245%7%7%45%
2603
September1st to 15thDay
13%5%15%434446%46%
2256
16th to endDay
14%424413347%49%
2145
October1st to 15thDay
21%5%25%434235%41%
2009
16th to endDay
20%6%25%433446%38%
1728
November1st to 15thDay
21%5%1442345%7%39%
1716
16th to endDay
20%6%2442345%6%40%
1746
December1st to 15thDay
21%9%35%5%2445%6%33%
1391
16th to endDay
20%12%47%423445%30%
1654
January1st to 15thNight
15%9%3443445%6%40%
2716
16th to endNight
14%8%36%5%35%45%6%35%
2912
February1st to 15thNight
13%9%37%6%45%5%6%5%33%
2452
16th to endNight
14%11%25%5%24436%37%
2049
March1st to 15thNight
18%9%35%5%34445%33%
2301
16th to endNight
16%6%3442444444%
2230
April1st to 15thNight
20%8%25%5%34445%35%
1946
16th to endNight
23%6%34324335%37%
1741
May1st to 15thNight
20%7%25%5%25%5%45%35%
1755
16th to endNight
23%7%25%42445%7%31%
1645
June1st to 15thNight
25%6%35%434446%31%
1560
16th to endNight
19%5%2442445%8%37%
1560
July1st to 15thNight
13%425%5%3445%10%40%
1560
16th to endNight
14%5%244345%5%8%41%
1642
August1st to 15thNight
14%5%245%345%5%7%39%
1676
16th to endNight
13%41442335%9%47%
1948
September1st to 15thNight
14%413333345%49%
2034
16th to endNight
14%41332235%6%50%
2145
October1st to 15thNight
17%5%2342445%6%42%
2118
16th to endNight
23%7%1442435%6%36%
2496
November1st to 15thNight
18%7%2442435%6%40%
2472
16th to endNight
16%6%25%42335%6%42%
2542
December1st to 15thNight
21%9%35%424435%34%
2893
16th to endNight
20%9%45%425%445%33%
2915

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%36%
1552
16th to endDay
14%9%26%425%446%39%
1664
February1st to 15thDay
16%8%37%6%35%45%5%32%
1734
16th to endDay
14%9%36%5%35%5%46%34%
1710
March1st to 15thDay
19%9%45%5%25%446%30%
1976
16th to endDay
14%8%35%43334442%
2343
April1st to 15thDay
19%8%35%5%24346%36%
2332
16th to endDay
21%7%25%5%23345%38%
2494
May1st to 15thDay
25%7%25%424335%32%
2527
16th to endDay
20%6%35%5%25%45%6%34%
2830
June1st to 15thDay
26%6%25%43436%5%30%
2730
16th to endDay
20%5%35%6%25%5%46%32%
2730
July1st to 15thDay
14%5%245%36%5%5%10%35%
2730
16th to endDay
14%5%26%6%35%5%6%8%34%
2878
August1st to 15thDay
15%6%25%5%35%5%5%7%34%
2521
16th to endDay
11%425%4335%6%7%46%
2603
September1st to 15thDay
14%5%2442445%5%46%
2256
16th to endDay
14%414323446%49%
2145
October1st to 15thDay
20%6%25%423245%41%
2009
16th to endDay
20%6%14423346%39%
1728
November1st to 15thDay
20%5%1441435%6%40%
1716
16th to endDay
20%7%13423346%41%
1746
December1st to 15thDay
21%9%35%423446%34%
1391
16th to endDay
21%11%46%5%1334432%
1654
January1st to 15thNight
15%9%24424445%40%
2716
16th to endNight
15%9%36%5%35%446%35%
2912
February1st to 15thNight
13%8%36%6%36%5%5%6%33%
2445
16th to endNight
14%11%35%434345%37%
2049
March1st to 15thNight
18%11%35%435%445%33%
2314
16th to endNight
16%6%35%42344444%
2229
April1st to 15thNight
19%8%35%5%24445%35%
1946
16th to endNight
24%6%24434336%36%
1741
May1st to 15thNight
19%7%25%43445%6%35%
1755
16th to endNight
22%7%35%5%345%5%6%30%
1645
June1st to 15thNight
25%8%25%425%35%6%32%
1560
16th to endNight
18%6%244345%5%8%36%
1560
July1st to 15thNight
13%4245%345%5%9%40%
1560
16th to endNight
15%4244245%6%7%41%
1642
August1st to 15thNight
14%5%26%6%35%45%7%39%
1676
16th to endNight
12%41442345%9%46%
1948
September1st to 15thNight
14%5%14323345%49%
2034
16th to endNight
13%5%2331335%6%51%
2145
October1st to 15thNight
17%5%23435%446%42%
2118
16th to endNight
22%6%235%22446%37%
2496
November1st to 15thNight
18%6%14424347%41%
2472
16th to endNight
16%6%25%5%2335%6%42%
2542
December1st to 15thNight
21%9%35%5%2444435%
2893
16th to endNight
20%9%345%24445%34%
2915

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%8%11%21%13%1
1553
16th to endDay
44%5%12%22%12%1
1664
February1st to 15thDay
51%5%11%19%10%1
1737
16th to endDay
48%6%11%16%13%3
1710
March1st to 15thDay
53%5%10%16%11%2
1976
16th to endDay
44%49%16%20%5%
2343
April1st to 15thDay
48%47%19%15%3
2332
16th to endDay
47%5%9%19%14%3
2494
May1st to 15thDay
53%5%6%17%14%1
2527
16th to endDay
48%7%10%19%12%1
2834
June1st to 15thDay
53%6%11%16%10%
2730
16th to endDay
51%6%9%19%11%
2730
July1st to 15thDay
41%8%12%23%12%
2581
16th to endDay
43%8%14%21%10%
2711
August1st to 15thDay
45%8%14%23%7%
2507
16th to endDay
33%7%13%26%17%1
2603
September1st to 15thDay
37%5%12%25%16%1
2256
16th to endDay
34%49%29%20%1
2145
October1st to 15thDay
45%5%9%23%14%1
2009
16th to endDay
44%6%11%23%12%1
1728
November1st to 15thDay
43%5%14%21%12%1
1716
16th to endDay
44%5%13%19%15%1
1746
December1st to 15thDay
49%6%12%19%11%
1391
16th to endDay
54%5%9%15%12%1
1654
January1st to 15thNight
43%6%12%23%11%1
2716
16th to endNight
47%6%9%18%14%2
2912
February1st to 15thNight
49%6%12%18%12%1
2453
16th to endNight
47%6%9%21%12%3
2049
March1st to 15thNight
52%410%20%10%2
2314
16th to endNight
42%49%18%20%4
2230
April1st to 15thNight
50%46%19%17%1
1946
16th to endNight
48%5%10%18%13%3
1741
May1st to 15thNight
49%6%8%18%15%2
1755
16th to endNight
51%6%12%18%9%1
1647
June1st to 15thNight
53%6%10%18%9%1
1560
16th to endNight
43%6%13%22%11%1
1560
July1st to 15thNight
37%8%14%25%12%
1480
16th to endNight
39%8%16%22%11%
1546
August1st to 15thNight
42%8%13%25%9%
1666
16th to endNight
33%7%14%28%16%
1948
September1st to 15thNight
35%5%12%29%15%1
2034
16th to endNight
33%5%11%30%17%1
2145
October1st to 15thNight
41%6%11%23%14%1
2118
16th to endNight
47%6%10%23%11%
2496
November1st to 15thNight
43%5%11%24%13%1
2472
16th to endNight
42%6%12%23%14%1
2542
December1st to 15thNight
51%5%10%21%10%
2893
16th to endNight
51%5%9%18%12%1
2915

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
26%15%28%24%4
90
16th to endDay
20%26%32%18%2
96
February1st to 15thDay
27%15%33%20%3
252
16th to endDay
20%23%29%20%5%
291
March1st to 15thDay
30%16%31%17%4
330
16th to endDay
22%22%28%15%8%2
352
April1st to 15thDay
31%15%26%16%7%2
330
16th to endDay
27%17%29%13%8%3
319
May1st to 15thDay
35%14%24%14%6%3
348
16th to endDay
23%15%34%19%6%1
395
June1st to 15thDay
25%12%30%21%8%
375
16th to endDay
21%13%31%20%11%1
375
July1st to 15thDay
15%5%31%28%14%4
365
16th to endDay
15%6%31%28%14%4
390
August1st to 15thDay
16%7%26%27%17%4
358
16th to endDay
14%9%20%29%19%7%
396
September1st to 15thDay
17%6%24%22%20%7%
375
16th to endDay
17%10%24%19%13%13%
374
October1st to 15thDay
22%12%23%20%17%4
334
16th to endDay
27%11%30%19%10%
288
November1st to 15thDay
24%11%29%21%10%2
280
16th to endDay
31%13%23%21%8%
276
December1st to 15thDay
25%14%36%14%8%
90
16th to endDay
39%17%29%13%
96
January1st to 15thNight
21%15%33%25%3
780
16th to endNight
22%15%35%23%3
832
February1st to 15thNight
19%17%35%25%3
764
16th to endNight
23%15%33%22%5%
683
March1st to 15thNight
27%13%36%19%3
780
16th to endNight
21%16%30%23%6%1
832
April1st to 15thNight
24%13%34%23%4
778
16th to endNight
26%14%29%21%6%
585
May1st to 15thNight
24%10%30%27%6%
585
16th to endNight
25%7%28%30%7%
429
June1st to 15thNight
27%5%27%31%8%
390
16th to endNight
21%123%40%12%
390
July1st to 15thNight
14%123%40%19%
370
16th to endNight
16%121%39%19%1
408
August1st to 15thNight
14%323%37%19%
525
16th to endNight
14%223%35%20%2
706
September1st to 15thNight
16%427%31%18%1
780
16th to endNight
16%7%25%27%20%3
777
October1st to 15thNight
21%8%24%29%13%1
752
16th to endNight
27%11%33%22%6%
768
November1st to 15thNight
22%10%28%28%8%1
760
16th to endNight
21%14%27%26%8%1
780
December1st to 15thNight
27%12%34%20%4
818
16th to endNight
27%15%29%23%3
833
Page generated at 2018-10-09 10:39:13. Version ac04

Copyright 2018 A. Danko


Roger Twitchell Observatory Clear Sky Chart |Charts Home