Sandlot Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sandlot Observatory clear sky chart from 2005-11-05 to 2017-08-12.

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
42%
15774
23%
46825
14%
15727
50%
15727
33%
27566

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
34%
900
23%
2327
9%
895
44%
895
35%
1438
January16th to endNight
29%
960
27%
2496
10%
960
40%
960
35%
1536
February1st to 15thNight
26%
880
17%
2288
6%
876
33%
876
25%
1581
February16th to endNight
29%
654
25%
1916
9%
654
41%
654
30%
1151
March1st to 15thNight
33%
540
23%
1980
14%
540
45%
540
26%
1138
March16th to endNight
27%
576
16%
2112
10%
576
37%
576
20%
1324
April1st to 15thNight
31%
539
21%
1884
13%
539
46%
539
28%
1063
April16th to endNight
31%
540
22%
1665
13%
540
44%
540
25%
1061
May1st to 15thNight
36%
539
20%
1620
15%
539
45%
539
28%
1000
May16th to endNight
44%
563
18%
1685
14%
561
50%
561
34%
1002
June1st to 15thNight
55%
540
17%
1620
10%
540
60%
540
40%
885
June16th to endNight
60%
540
23%
1620
15%
540
65%
540
50%
811
July1st to 15thNight
67%
510
18%
1530
14%
510
69%
510
41%
839
July16th to endNight
72%
564
24%
1594
24%
531
74%
531
42%
862
August1st to 15thNight
77%
527
23%
1575
26%
524
79%
524
41%
854
August16th to endNight
68%
525
26%
1633
28%
525
72%
525
39%
892
September1st to 15thNight
56%
495
26%
1744
26%
495
62%
495
28%
1017
September16th to endNight
59%
492
29%
1815
27%
492
67%
492
36%
919
October1st to 15thNight
44%
627
29%
1947
21%
627
50%
627
32%
1135
October16th to endNight
39%
704
30%
2112
14%
704
50%
704
40%
1124
November1st to 15thNight
43%
799
30%
2199
13%
799
54%
799
40%
1224
November16th to endNight
34%
900
27%
2408
9%
900
46%
900
37%
1420
December1st to 15thNight
30%
900
20%
2520
8%
900
40%
900
28%
1589
December16th to endNight
31%
960
24%
2535
10%
960
42%
960
27%
1701

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
9%5%24417%3346%39%
47144
Night
8%413317%3345%43%
47020
Cloud CoverDay
11%6%2442445%7%46%
47142
Night
10%5%24424446%50%
47020
TransparencyDay
37%5%13%26%15%2
46842
Night
33%412%27%18%3
46825
SeeingDay
13%6%26%31%17%3
5751
Night
13%7%32%32%12%
15775

By Month

Percentage Cloud Cover SW (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
7%5%25%419%3345%40%
1611
16th to endDay
7%7%24318%2335%39%
1728
February1st to 15thDay
7%7%36%418%2235%34%
1575
16th to endDay
8%7%35%418%3235%37%
1567
March1st to 15thDay
8%7%25%5%19%3436%33%
1980
16th to endDay
14%8%25%318%223432%
2112
April1st to 15thDay
12%7%14318%3235%35%
2064
16th to endDay
14%6%35%419%3346%28%
2292
May1st to 15thDay
11%6%24418%3345%35%
2331
16th to endDay
12%6%24419%45%46%30%
2430
June1st to 15thDay
6%313419%336%6%40%
2340
16th to endDay
222319%3348%46%
2340
July1st to 15thDay
7%313318%446%8%37%
2340
16th to endDay
5%312319%346%7%42%
2466
August1st to 15thDay
7%314411%435%8%45%
2289
16th to endDay
6%414411%445%6%47%
2215
September1st to 15thDay
11%6%14411%3336%42%
1884
16th to endDay
5%314411%3445%48%
1815
October1st to 15thDay
10%5%13311%2345%47%
1672
16th to endDay
9%413310%334451%
1760
November1st to 15thDay
6%313216%3335%50%
1659
16th to endDay
11%5%13218%2125%45%
1548
December1st to 15thDay
12%6%35%318%2335%34%
1440
16th to endDay
10%7%36%419%323435%
1686
January1st to 15thNight
6%414320%3335%43%
2327
16th to endNight
6%5%24318%3335%42%
2496
February1st to 15thNight
8%8%36%418%333336%
2278
16th to endNight
9%5%14317%3345%41%
1916
March1st to 15thNight
8%6%24318%333439%
1980
16th to endNight
14%6%13218%3336%35%
2112
April1st to 15thNight
8%415%318%3335%42%
1884
16th to endNight
14%6%14319%3335%33%
1665
May1st to 15thNight
10%6%13318%3246%39%
1619
16th to endNight
9%313319%335%8%38%
1683
June1st to 15thNight
5%212418%435%7%45%
1620
16th to endNight
112318%235%8%50%
1620
July1st to 15thNight
5%312218%3447%46%
1620
16th to endNight
4213318%446%6%44%
1702
August1st to 15thNight
322411%446%9%49%
1585
16th to endNight
6%313411%2335%55%
1633
September1st to 15thNight
10%5%24311%3347%44%
1744
16th to endNight
5%314312%3446%51%
1815
October1st to 15thNight
11%423411%233448%
1947
16th to endNight
7%412210%223456%
2112
November1st to 15thNight
5%313216%2335%52%
2199
16th to endNight
10%412217%223349%
2408
December1st to 15thNight
11%6%24318%2345%36%
2520
16th to endNight
10%7%35%319%323437%
2535

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
9%6%35%424445%49%
1611
16th to endDay
9%8%35%324346%47%
1728
February1st to 15thDay
9%9%5%7%5%33337%41%
1575
16th to endDay
9%8%36%423346%45%
1567
March1st to 15thDay
11%8%36%5%34447%39%
1980
16th to endDay
19%9%36%423335%37%
2110
April1st to 15thDay
15%8%14424336%43%
2064
16th to endDay
18%6%35%6%24446%36%
2292
May1st to 15thDay
16%7%345%3445%5%39%
2331
16th to endDay
15%6%345%345%5%8%37%
2430
June1st to 15thDay
9%413435%46%8%47%
2340
16th to endDay
5%3134245%5%9%54%
2340
July1st to 15thDay
9%413425%5%7%9%45%
2340
16th to endDay
7%31343447%8%51%
2466
August1st to 15thDay
9%31442446%9%49%
2289
16th to endDay
7%425%5%3445%7%49%
2215
September1st to 15thDay
14%6%25%423347%46%
1884
16th to endDay
7%3145%335%46%53%
1815
October1st to 15thDay
11%6%14323346%51%
1672
16th to endDay
12%413313345%55%
1760
November1st to 15thDay
8%31332345%6%57%
1659
16th to endDay
13%5%33322246%53%
1548
December1st to 15thDay
14%7%35%323346%43%
1440
16th to endDay
13%8%47%434337%40%
1686
January1st to 15thNight
8%5%24435%346%51%
2327
16th to endNight
8%5%26%424345%53%
2496
February1st to 15thNight
10%10%46%5%24445%41%
2278
16th to endNight
11%5%24324445%49%
1916
March1st to 15thNight
10%6%25%4245%46%47%
1980
16th to endNight
18%7%24324347%41%
2112
April1st to 15thNight
10%424424346%50%
1884
16th to endNight
18%6%14424446%42%
1665
May1st to 15thNight
13%6%3432435%7%44%
1619
16th to endNight
10%32433346%10%47%
1683
June1st to 15thNight
6%2134345%5%8%53%
1620
16th to endNight
4113135%6%10%59%
1620
July1st to 15thNight
7%31332445%9%54%
1620
16th to endNight
5%223425%5%7%9%52%
1702
August1st to 15thNight
32125%35%47%10%53%
1585
16th to endNight
7%414413346%57%
1633
September1st to 15thNight
12%5%24323447%48%
1744
16th to endNight
6%315%324447%56%
1815
October1st to 15thNight
13%41432334452%
1947
16th to endNight
10%41231323461%
2112
November1st to 15thNight
7%423322336%59%
2199
16th to endNight
12%423213335%56%
2408
December1st to 15thNight
12%6%35%423446%44%
2520
16th to endNight
14%8%45%323346%42%
2535

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
36%314%26%16%1
1612
16th to endDay
38%315%26%13%1
1728
February1st to 15thDay
44%414%25%8%2
1576
16th to endDay
40%312%27%14%2
1567
March1st to 15thDay
43%412%23%13%2
1980
16th to endDay
50%312%22%9%1
2112
April1st to 15thDay
42%411%23%17%
2064
16th to endDay
48%412%22%11%
2292
May1st to 15thDay
45%6%11%23%12%
2331
16th to endDay
44%7%15%20%11%1
2434
June1st to 15thDay
31%7%15%28%16%2
2340
16th to endDay
25%5%15%30%20%1
2340
July1st to 15thDay
32%6%15%29%12%2
2201
16th to endDay
26%6%15%33%15%2
2311
August1st to 15thDay
31%6%13%28%16%2
2275
16th to endDay
33%5%12%25%16%6%
2215
September1st to 15thDay
38%411%21%16%6%
1884
16th to endDay
29%6%10%28%22%2
1815
October1st to 15thDay
33%410%24%22%4
1672
16th to endDay
32%311%26%21%4
1760
November1st to 15thDay
27%410%27%22%6%
1659
16th to endDay
34%210%30%20%1
1548
December1st to 15thDay
42%414%27%10%
1440
16th to endDay
44%412%23%12%2
1686
January1st to 15thNight
34%314%26%19%2
2327
16th to endNight
33%414%27%18%3
2496
February1st to 15thNight
44%411%24%12%2
2288
16th to endNight
35%312%25%19%3
1916
March1st to 15thNight
36%411%24%17%5%
1980
16th to endNight
43%5%13%22%12%2
2112
April1st to 15thNight
34%311%25%21%2
1884
16th to endNight
41%410%25%16%1
1665
May1st to 15thNight
38%414%23%18%1
1620
16th to endNight
32%6%20%24%14%1
1685
June1st to 15thNight
25%6%19%30%15%2
1620
16th to endNight
19%5%14%34%22%2
1620
July1st to 15thNight
27%413%31%18%4
1530
16th to endNight
24%6%13%31%21%2
1594
August1st to 15thNight
25%6%16%30%17%4
1575
16th to endNight
27%48%28%22%7%
1633
September1st to 15thNight
36%5%9%22%18%7%
1744
16th to endNight
27%410%28%25%3
1815
October1st to 15thNight
35%37%24%22%6%
1947
16th to endNight
27%29%31%25%4
2112
November1st to 15thNight
26%312%25%23%8%
2199
16th to endNight
31%29%33%18%3
2408
December1st to 15thNight
38%315%28%13%
2520
16th to endNight
43%412%23%13%3
2535

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%13%44%17%4
90
16th to endDay
13%16%44%18%6%
96
February1st to 15thDay
8%25%41%21%21
90
16th to endDay
9%22%41%14%11%
81
March1st to 15thDay
15%18%48%14%2
90
16th to endDay
29%13%40%11%5%
96
April1st to 15thDay
20%12%28%30%7%1
183
16th to endDay
22%11%36%20%6%1
402
May1st to 15thDay
16%10%32%28%10%1
468
16th to endDay
19%8%31%24%14%2
517
June1st to 15thDay
9%5%24%36%18%4
495
16th to endDay
7%329%38%17%3
495
July1st to 15thDay
11%18%39%24%5%
475
16th to endDay
6%19%41%34%6%
515
August1st to 15thDay
8%13%38%32%6%
474
16th to endDay
10%216%33%30%7%
383
September1st to 15thDay
9%23%40%20%5%
159
16th to endDay
7%221%47%14%6%
90
October1st to 15thDay
38%30%35%11%11%
90
16th to endDay
13%7%37%30%9%2
96
November1st to 15thDay
11%226%40%14%5%
90
16th to endDay
36%10%35%14%21
90
December1st to 15thDay
18%12%44%16%43
90
16th to endDay
19%19%32%25%3
96
January1st to 15thNight
11%12%37%30%7%1
900
16th to endNight
12%10%40%29%6%
960
February1st to 15thNight
15%11%39%29%3
880
16th to endNight
14%12%39%27%5%
654
March1st to 15thNight
13%9%39%30%6%
540
16th to endNight
22%9%38%24%4
576
April1st to 15thNight
13%12%37%30%5%
539
16th to endNight
22%8%35%25%7%
540
May1st to 15thNight
17%6%35%29%10%1
540
16th to endNight
12%434%31%15%1
563
June1st to 15thNight
7%228%39%21%
540
16th to endNight
5%229%45%17%
540
July1st to 15thNight
7%117%47%24%1
510
16th to endNight
6%12%46%30%2
564
August1st to 15thNight
5%10%44%37%1
527
16th to endNight
9%14%37%33%3
525
September1st to 15thNight
13%220%40%20%2
495
16th to endNight
6%222%39%25%4
492
October1st to 15thNight
15%331%30%17%1
627
16th to endNight
12%9%37%29%11%
704
November1st to 15thNight
9%11%32%32%12%
799
16th to endNight
15%11%36%28%6%
900
December1st to 15thNight
18%12%35%28%5%
900
16th to endNight
18%10%32%31%6%
960
Page generated at 2017-08-13 13:16:28. Version ac04

Copyright 2017 A. Danko


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