Treadwell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Treadwell Observatory clear sky chart from 2005-11-05 to 2017-05-01.

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
37%
14293
14%
45685
10%
14262
42%
14262
18%
32389

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%
720
7%
2506
5%
716
26%
716
9%
2008
January16th to endNight
28%
768
11%
2673
8%
768
33%
768
13%
1998
February1st to 15thNight
27%
703
10%
2287
6%
701
32%
701
12%
1753
February16th to endNight
31%
631
15%
1928
9%
631
39%
631
16%
1436
March1st to 15thNight
27%
720
14%
2130
9%
720
34%
720
16%
1592
March16th to endNight
36%
768
21%
2013
11%
768
43%
768
26%
1390
April1st to 15thNight
32%
718
16%
1796
7%
718
35%
718
23%
1323
April16th to endNight
34%
645
18%
1725
8%
645
40%
645
24%
1215
May1st to 15thNight
37%
332
18%
1328
18%
332
45%
332
18%
854
May16th to endNight
44%
343
14%
1369
14%
342
54%
342
20%
864
June1st to 15thNight
40%
330
10%
1320
10%
330
47%
330
15%
917
June16th to endNight
51%
330
9%
1320
14%
330
54%
330
21%
807
July1st to 15thNight
62%
310
15%
1240
23%
310
66%
310
28%
670
July16th to endNight
60%
348
11%
1306
14%
326
63%
326
25%
739
August1st to 15thNight
61%
370
15%
1351
16%
368
64%
368
25%
827
August16th to endNight
60%
668
18%
1718
10%
668
62%
668
38%
1076
September1st to 15thNight
55%
660
16%
1650
9%
660
56%
660
30%
1144
September16th to endNight
54%
657
21%
1783
15%
657
57%
657
32%
1138
October1st to 15thNight
38%
660
19%
1980
14%
660
45%
660
22%
1359
October16th to endNight
34%
704
11%
2225
8%
704
40%
704
16%
1651
November1st to 15thNight
37%
700
17%
2309
15%
700
43%
700
18%
1616
November16th to endNight
27%
720
11%
2520
9%
720
32%
720
12%
1893
December1st to 15thNight
22%
720
8%
2520
4%
720
26%
720
10%
1974
December16th to endNight
18%
768
10%
2688
5%
768
24%
768
9%
2145

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%8%36%5%34446%32%
46090
Night
18%9%36%5%24445%32%
45854
Cloud CoverDay
18%8%36%5%3445%6%32%
46099
Night
18%9%36%5%34445%32%
45860
TransparencyDay
51%412%20%10%1
45777
Night
53%411%20%10%1
45685
SeeingDay
22%6%22%26%16%5%
4962
Night
23%5%24%32%12%
14294

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
23%15%5%8%7%25%42420%
1432
16th to endDay
16%14%5%8%6%35%44425%
1550
February1st to 15thDay
14%13%6%11%7%35%445%23%
1575
16th to endDay
18%13%5%9%6%4434426%
1545
March1st to 15thDay
17%11%5%7%7%35%44427%
1819
16th to endDay
17%8%35%433445%39%
2211
April1st to 15thDay
22%10%35%424345%34%
2152
16th to endDay
20%8%35%434445%36%
2247
May1st to 15thDay
20%6%36%5%3435%6%34%
2304
16th to endDay
20%6%345%335%47%34%
2394
June1st to 15thDay
22%7%25%5%45%5%5%7%28%
2310
16th to endDay
18%5%25%435%5%7%8%31%
2310
July1st to 15thDay
13%5%25%5%36%5%6%8%36%
2310
16th to endDay
12%6%25%6%35%7%6%9%33%
2458
August1st to 15thDay
13%5%36%6%36%5%47%36%
2268
16th to endDay
10%425%5%35%5%48%43%
2119
September1st to 15thDay
17%42443435%7%40%
1980
16th to endDay
17%5%23323337%46%
1845
October1st to 15thDay
22%7%25%5%24336%35%
1650
16th to endDay
26%10%47%5%34345%24%
1647
November1st to 15thDay
17%7%46%6%3435%6%34%
1548
16th to endDay
26%11%45%5%3334426%
1440
December1st to 15thDay
24%15%5%7%5%35%435%19%
1440
16th to endDay
24%17%48%6%2433320%
1536
January1st to 15thNight
20%16%6%8%6%3443320%
2506
16th to endNight
14%15%5%8%6%35%445%25%
2673
February1st to 15thNight
13%13%6%11%8%35%445%23%
2285
16th to endNight
17%10%5%8%7%34445%28%
1928
March1st to 15thNight
21%10%48%7%34345%28%
2117
16th to endNight
15%9%36%5%1433342%
2013
April1st to 15thNight
25%8%245%23345%34%
1796
16th to endNight
21%7%26%414435%37%
1725
May1st to 15thNight
22%6%35%413336%37%
1328
16th to endNight
19%5%245%245%46%39%
1367
June1st to 15thNight
21%7%36%5%345%6%7%30%
1320
16th to endNight
17%5%14425%6%5%7%39%
1320
July1st to 15thNight
10%4243245%5%8%47%
1320
16th to endNight
9%5%3443446%9%43%
1402
August1st to 15thNight
10%6%25%44448%8%40%
1361
16th to endNight
10%42442445%7%49%
1718
September1st to 15thNight
15%5%25%5%34446%41%
1650
16th to endNight
18%424423236%48%
1783
October1st to 15thNight
19%6%36%334346%36%
1980
16th to endNight
26%8%35%5%35%44428%
2225
November1st to 15thNight
21%7%36%6%34345%33%
2309
16th to endNight
21%11%48%6%25%44426%
2520
December1st to 15thNight
22%15%5%8%5%3443422%
2520
16th to endNight
25%16%5%7%5%2343420%
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
23%15%5%8%6%3443421%
1432
16th to endDay
16%15%5%7%6%4445%5%25%
1550
February1st to 15thDay
14%13%5%10%7%35%445%24%
1575
16th to endDay
18%13%5%9%6%4434426%
1545
March1st to 15thDay
19%10%5%8%7%36%44427%
1829
16th to endDay
17%7%35%42335%5%39%
2210
April1st to 15thDay
23%9%35%414335%34%
2152
16th to endDay
20%7%45%5%23345%36%
2247
May1st to 15thDay
20%6%35%5%3445%5%35%
2304
16th to endDay
20%6%345%3445%7%35%
2394
June1st to 15thDay
23%6%345%345%5%7%29%
2310
16th to endDay
18%5%25%5%36%6%6%8%31%
2310
July1st to 15thDay
13%425%5%36%5%5%9%37%
2310
16th to endDay
12%5%25%6%36%6%7%9%33%
2458
August1st to 15thDay
13%6%36%6%35%5%5%7%36%
2268
16th to endDay
10%435%435%45%8%44%
2119
September1st to 15thDay
17%5%2442445%7%41%
1980
16th to endDay
18%5%24323346%46%
1845
October1st to 15thDay
22%8%34424346%35%
1650
16th to endDay
25%10%47%5%3444424%
1647
November1st to 15thDay
17%7%46%5%34447%34%
1548
16th to endDay
25%13%35%5%33345%26%
1440
December1st to 15thDay
25%13%47%5%36%346%18%
1440
16th to endDay
24%18%5%8%5%3423320%
1536
January1st to 15thNight
21%16%6%8%7%3433420%
2506
16th to endNight
14%15%5%8%6%35%445%25%
2673
February1st to 15thNight
13%12%6%11%8%35%5%45%22%
2278
16th to endNight
17%10%5%8%6%35%345%28%
1928
March1st to 15thNight
21%11%47%7%2434428%
2130
16th to endNight
16%8%35%5%1334342%
2013
April1st to 15thNight
25%8%245%33335%34%
1796
16th to endNight
20%7%26%424335%38%
1725
May1st to 15thNight
21%6%35%414346%37%
1328
16th to endNight
18%6%25%4245%5%6%39%
1367
June1st to 15thNight
21%7%45%5%3445%8%29%
1320
16th to endNight
16%5%23435%5%6%7%37%
1320
July1st to 15thNight
10%5%233345%5%9%46%
1320
16th to endNight
10%434425%5%6%8%43%
1402
August1st to 15thNight
10%6%245%35%47%10%38%
1361
16th to endNight
10%4145%3345%6%49%
1718
September1st to 15thNight
15%5%15%5%345%36%41%
1650
16th to endNight
18%423423236%48%
1783
October1st to 15thNight
19%7%36%424346%37%
1980
16th to endNight
26%8%26%6%3444428%
2225
November1st to 15thNight
21%8%26%5%34346%33%
2309
16th to endNight
21%11%38%6%25%33426%
2520
December1st to 15thNight
22%15%5%7%6%23435%21%
2520
16th to endNight
25%16%5%7%5%2433421%
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
67%38%14%5%
1433
16th to endDay
60%411%14%8%
1550
February1st to 15thDay
61%213%17%4
1577
16th to endDay
60%210%15%9%1
1545
March1st to 15thDay
59%211%15%8%1
1829
16th to endDay
46%49%18%16%5%
2211
April1st to 15thDay
53%37%20%12%2
2152
16th to endDay
51%28%18%14%4
2247
May1st to 15thDay
50%47%21%15%1
2304
16th to endDay
48%5%10%21%12%1
2398
June1st to 15thDay
53%411%20%9%
2310
16th to endDay
46%6%13%20%11%
2310
July1st to 15thDay
40%5%16%26%12%
2161
16th to endDay
43%6%17%22%9%
2291
August1st to 15thDay
44%5%17%23%9%
2254
16th to endDay
36%419%27%11%
2119
September1st to 15thDay
40%414%27%12%
1980
16th to endDay
38%413%32%10%
1845
October1st to 15thDay
50%311%22%11%
1650
16th to endDay
61%312%15%5%
1647
November1st to 15thDay
50%415%19%9%
1548
16th to endDay
60%313%16%6%
1440
December1st to 15thDay
66%413%12%2
1440
16th to endDay
69%49%10%6%
1536
January1st to 15thNight
68%39%12%4
2506
16th to endNight
60%310%16%7%
2673
February1st to 15thNight
62%410%17%5%
2287
16th to endNight
57%48%19%10%
1928
March1st to 15thNight
59%29%18%8%1
2130
16th to endNight
46%38%20%16%4
2013
April1st to 15thNight
53%36%19%15%1
1796
16th to endNight
48%37%19%17%3
1725
May1st to 15thNight
48%37%19%15%5%
1328
16th to endNight
45%410%21%15%2
1369
June1st to 15thNight
52%6%10%20%9%
1320
16th to endNight
41%6%13%21%16%
1320
July1st to 15thNight
32%5%14%31%14%
1240
16th to endNight
35%5%17%24%15%
1306
August1st to 15thNight
39%6%19%24%10%
1351
16th to endNight
34%418%31%11%
1718
September1st to 15thNight
44%412%27%11%
1650
16th to endNight
38%411%33%11%
1783
October1st to 15thNight
48%410%24%11%
1980
16th to endNight
57%413%18%6%
2225
November1st to 15thNight
53%313%18%9%1
2309
16th to endNight
61%411%17%5%
2520
December1st to 15thNight
65%411%15%3
2520
16th to endNight
68%39%12%6%
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
30%11%20%26%8%3
60
16th to endDay
35%423%28%42
67
February1st to 15thDay
28%15%35%16%3
90
16th to endDay
30%6%28%29%4
204
March1st to 15thDay
25%10%29%26%8%
270
16th to endDay
23%427%26%11%5%
288
April1st to 15thDay
28%11%28%16%14%1
270
16th to endDay
20%10%24%24%16%3
270
May1st to 15thDay
24%324%28%10%6%
229
16th to endDay
20%7%26%24%16%4
267
June1st to 15thDay
27%324%27%13%4
255
16th to endDay
16%318%34%23%3
255
July1st to 15thDay
14%317%31%23%9%
245
16th to endDay
13%313%35%26%7%
270
August1st to 15thDay
16%112%30%29%8%
253
16th to endDay
12%311%33%24%14%
268
September1st to 15thDay
19%216%17%25%18%
255
16th to endDay
22%6%16%20%20%14%
254
October1st to 15thDay
25%422%25%18%3
255
16th to endDay
31%7%23%24%12%
251
November1st to 15thDay
23%5%20%28%18%2
202
16th to endDay
28%13%23%25%10%
60
December1st to 15thDay
26%8%35%23%6%
60
16th to endDay
39%12%35%12%
64
January1st to 15thNight
28%7%25%31%6%
720
16th to endNight
22%7%26%32%9%
768
February1st to 15thNight
19%7%31%31%8%
704
16th to endNight
24%5%29%32%7%
631
March1st to 15thNight
26%8%27%31%5%
720
16th to endNight
19%6%28%34%10%
768
April1st to 15thNight
28%7%26%31%6%
718
16th to endNight
25%6%26%30%10%
645
May1st to 15thNight
25%422%35%9%2
332
16th to endNight
20%424%34%15%
343
June1st to 15thNight
22%421%40%10%
330
16th to endNight
18%121%39%16%2
330
July1st to 15thNight
10%118%47%20%
310
16th to endNight
12%116%46%21%1
348
August1st to 15thNight
15%110%42%26%3
370
16th to endNight
13%116%40%25%1
668
September1st to 15thNight
19%315%39%20%1
660
16th to endNight
21%317%31%23%2
657
October1st to 15thNight
23%424%33%13%
660
16th to endNight
28%6%24%30%10%
704
November1st to 15thNight
25%5%24%30%13%
700
16th to endNight
28%7%26%24%12%
720
December1st to 15thNight
31%8%25%28%5%
720
16th to endNight
34%7%28%22%5%1
768
Page generated at 2017-05-02 08:43:33. Version ac04

Copyright 2017 A. Danko


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