Twin Lakes Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Twin Lakes Observatory clear sky chart from 2005-11-05 to 2017-09-24.

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
46%
15526
23%
47017
15%
15483
55%
15483
27%
29184

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
22%
2327
9%
716
41%
716
20%
1466
January16th to endNight
24%
768
24%
2454
10%
768
41%
768
21%
1557
February1st to 15thNight
29%
704
25%
2112
9%
701
44%
701
26%
1285
February16th to endNight
23%
631
24%
1893
8%
631
38%
631
24%
1190
March1st to 15thNight
29%
720
22%
2063
11%
720
39%
720
26%
1312
March16th to endNight
38%
768
21%
2112
12%
768
45%
768
29%
1331
April1st to 15thNight
40%
638
25%
1896
16%
638
50%
638
32%
1097
April16th to endNight
48%
540
25%
1698
16%
540
57%
540
32%
985
May1st to 15thNight
50%
540
20%
1620
12%
540
57%
540
28%
1019
May16th to endNight
62%
561
20%
1683
13%
561
66%
561
31%
1045
June1st to 15thNight
69%
540
15%
1620
13%
540
71%
540
29%
1035
June16th to endNight
75%
540
17%
1620
15%
540
76%
540
33%
990
July1st to 15thNight
69%
510
13%
1530
12%
510
69%
510
27%
999
July16th to endNight
72%
564
16%
1595
13%
531
75%
531
27%
1035
August1st to 15thNight
69%
539
18%
1630
18%
536
69%
536
27%
1063
August16th to endNight
64%
573
18%
1910
21%
573
65%
573
22%
1287
September1st to 15thNight
61%
705
18%
1965
17%
705
62%
705
27%
1323
September16th to endNight
59%
693
24%
1914
18%
693
64%
693
30%
1230
October1st to 15thNight
53%
660
33%
1824
24%
660
60%
660
38%
1072
October16th to endNight
51%
704
33%
2112
25%
704
63%
704
36%
1173
November1st to 15thNight
49%
700
30%
2263
25%
700
60%
700
35%
1215
November16th to endNight
38%
720
27%
2340
17%
720
51%
720
28%
1356
December1st to 15thNight
32%
720
22%
2340
12%
720
45%
720
24%
1445
December16th to endNight
29%
768
20%
2496
13%
768
39%
768
18%
1674

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
14%5%245%35%5%5%7%40%
47906
Night
12%5%2442445%7%45%
47215
TransparencyDay
39%7%12%18%17%4
47607
Night
36%6%11%19%19%5%
47018
SeeingDay
15%6%19%23%21%13%
5402
Night
14%8%23%28%21%3
15528

By Month

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
19%6%34313346%42%
1611
16th to endDay
19%5%23323346%43%
1767
February1st to 15thDay
17%6%24315%45%6%42%
1750
16th to endDay
19%5%25%423346%42%
1580
March1st to 15thDay
17%425%333446%42%
1896
16th to endDay
17%424424447%42%
2111
April1st to 15thDay
13%5%25%43445%6%44%
2061
16th to endDay
12%4144245%46%48%
2262
May1st to 15thDay
12%5%25%42445%8%43%
2331
16th to endDay
10%5%245%36%5%6%8%40%
2431
June1st to 15thDay
8%5%25%7%46%7%8%10%33%
2340
16th to endDay
6%314437%7%8%11%40%
2340
July1st to 15thDay
11%6%25%6%5%7%7%8%11%28%
2340
16th to endDay
8%436%6%5%9%8%7%11%28%
2465
August1st to 15thDay
10%5%247%37%6%9%10%31%
2316
16th to endDay
14%5%27%8%45%6%8%9%26%
2292
September1st to 15thDay
13%6%26%5%35%5%6%7%37%
1983
16th to endDay
16%5%245%2345%6%43%
1905
October1st to 15thDay
13%41332333456%
1804
16th to endDay
12%41421222459%
1760
November1st to 15thDay
13%423312345%55%
1593
16th to endDay
15%424324446%47%
1620
December1st to 15thDay
20%6%23413446%40%
1620
16th to endDay
28%6%24323335%36%
1728
January1st to 15thNight
19%5%24324345%43%
2327
16th to endNight
17%6%34433345%42%
2454
February1st to 15thNight
15%424323346%48%
2103
16th to endNight
17%425%32435%5%45%
1893
March1st to 15thNight
14%5%2442445%5%46%
2063
16th to endNight
12%6%2442335%7%47%
2112
April1st to 15thNight
12%5%1232445%7%50%
1896
16th to endNight
10%5%144245%5%7%49%
1698
May1st to 15thNight
9%425%5%35%5%6%7%44%
1619
16th to endNight
8%4235%245%6%9%47%
1682
June1st to 15thNight
6%425%5%35%7%8%11%39%
1620
16th to endNight
4324437%5%8%11%44%
1620
July1st to 15thNight
7%325%6%37%6%7%10%38%
1620
16th to endNight
5%415%6%36%8%7%12%36%
1703
August1st to 15thNight
8%5%25%5%246%6%12%39%
1641
16th to endNight
11%5%246%446%7%10%35%
1910
September1st to 15thNight
11%425%5%45%5%6%8%40%
1965
16th to endNight
14%4135%245%5%7%45%
1914
October1st to 15thNight
11%5%1331325%6%55%
1824
16th to endNight
11%32332323458%
2112
November1st to 15thNight
12%422312236%57%
2263
16th to endNight
15%5%13322345%51%
2340
December1st to 15thNight
16%6%35%423336%43%
2340
16th to endNight
22%7%25%423346%36%
2496

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%49%15%22%4
1612
16th to endDay
41%46%19%21%6%
1767
February1st to 15thDay
40%310%20%21%3
1752
16th to endDay
43%37%20%19%5%
1580
March1st to 15thDay
41%410%18%19%5%
1896
16th to endDay
41%6%12%20%15%3
2112
April1st to 15thDay
38%411%20%19%4
2061
16th to endDay
33%7%10%22%21%5%
2262
May1st to 15thDay
37%7%14%19%18%3
2331
16th to endDay
38%9%14%21%14%2
2435
June1st to 15thDay
39%11%20%20%6%1
2340
16th to endDay
30%11%17%25%13%2
2340
July1st to 15thDay
42%11%19%18%6%1
2201
16th to endDay
42%13%18%16%8%1
2310
August1st to 15thDay
42%12%17%18%8%1
2303
16th to endDay
50%10%15%14%9%
2292
September1st to 15thDay
42%9%13%16%14%2
1983
16th to endDay
40%5%12%18%19%4
1905
October1st to 15thDay
31%5%9%17%27%8%
1804
16th to endDay
30%26%18%30%11%
1760
November1st to 15thDay
31%27%16%32%9%
1593
16th to endDay
37%47%15%26%7%
1620
December1st to 15thDay
43%5%8%18%15%7%
1620
16th to endDay
51%39%15%14%5%
1728
January1st to 15thNight
42%38%17%22%5%
2327
16th to endNight
42%35%18%22%6%
2454
February1st to 15thNight
37%38%20%20%8%
2112
16th to endNight
39%46%21%22%5%
1893
March1st to 15thNight
37%5%10%18%18%8%
2063
16th to endNight
36%5%10%23%18%4
2112
April1st to 15thNight
32%410%23%22%6%
1896
16th to endNight
32%5%11%22%19%8%
1698
May1st to 15thNight
36%7%14%20%19%2
1620
16th to endNight
32%9%14%25%14%3
1684
June1st to 15thNight
33%11%21%22%9%1
1620
16th to endNight
30%10%17%24%14%2
1620
July1st to 15thNight
36%10%20%21%8%1
1530
16th to endNight
33%13%21%20%9%2
1595
August1st to 15thNight
35%11%17%23%10%1
1630
16th to endNight
41%11%17%16%12%1
1910
September1st to 15thNight
39%8%14%17%15%3
1965
16th to endNight
36%8%11%21%16%6%
1914
October1st to 15thNight
31%412%16%27%8%
1824
16th to endNight
29%35%17%30%12%
2112
November1st to 15thNight
29%36%16%31%12%
2263
16th to endNight
35%46%17%26%9%
2340
December1st to 15thNight
41%310%18%18%7%
2340
16th to endNight
48%38%15%17%5%
2496

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
21%20%34%18%5%
90
16th to endDay
17%30%38%10%3
96
February1st to 15thDay
16%32%33%16%1
90
16th to endDay
24%22%37%14%1
81
March1st to 15thDay
22%16%31%21%6%1
186
16th to endDay
17%10%34%24%11%2
288
April1st to 15thDay
15%13%30%21%15%3
270
16th to endDay
15%5%25%27%22%4
270
May1st to 15thDay
14%9%28%20%15%12%
288
16th to endDay
15%315%28%23%14%
331
June1st to 15thDay
10%210%23%28%25%
315
16th to endDay
10%12%24%25%26%
315
July1st to 15thDay
10%8%25%30%24%
305
16th to endDay
8%7%18%36%28%
326
August1st to 15thDay
15%10%23%27%22%
313
16th to endDay
17%10%21%30%18%
332
September1st to 15thDay
15%311%23%28%18%
315
16th to endDay
18%212%25%27%12%
287
October1st to 15thDay
17%323%25%20%9%
255
16th to endDay
13%7%17%27%27%5%
272
November1st to 15thDay
26%13%21%16%17%2
101
16th to endDay
14%13%30%25%13%3
90
December1st to 15thDay
26%15%30%22%5%
90
16th to endDay
32%17%30%16%3
96
January1st to 15thNight
22%17%29%22%6%
720
16th to endNight
21%16%33%24%3
768
February1st to 15thNight
18%16%34%27%4
704
16th to endNight
18%18%37%22%3
631
March1st to 15thNight
17%14%36%25%5%
720
16th to endNight
15%12%28%30%12%
768
April1st to 15thNight
14%11%30%27%15%
638
16th to endNight
11%6%28%34%17%1
540
May1st to 15thNight
11%5%26%31%22%3
540
16th to endNight
12%116%30%32%6%
563
June1st to 15thNight
8%11%33%33%11%
540
16th to endNight
49%36%36%11%
540
July1st to 15thNight
9%7%24%46%11%
510
16th to endNight
6%16%23%49%12%
564
August1st to 15thNight
9%19%26%43%9%
539
16th to endNight
13%10%26%39%9%
573
September1st to 15thNight
13%110%28%36%9%
705
16th to endNight
16%115%35%28%2
693
October1st to 15thNight
13%423%28%24%4
660
16th to endNight
12%6%24%30%22%3
704
November1st to 15thNight
13%7%25%29%21%2
700
16th to endNight
17%13%27%25%14%
720
December1st to 15thNight
19%12%31%26%10%
720
16th to endNight
25%13%28%26%6%
768
Page generated at 2017-09-25 09:02:25. Version ac04

Copyright 2017 A. Danko


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