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 2017-12-20.

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
43%
16287
21%
48368
13%
16240
50%
16240
33%
28370

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
35%
900
21%
2327
8%
895
44%
895
35%
1438
January16th to endNight
30%
960
22%
2496
9%
960
38%
960
35%
1536
February1st to 15thNight
27%
880
16%
2288
6%
876
34%
876
25%
1581
February16th to endNight
30%
654
20%
1916
7%
654
40%
654
30%
1151
March1st to 15thNight
33%
540
20%
1980
13%
540
43%
540
26%
1138
March16th to endNight
28%
576
15%
2112
10%
576
36%
576
20%
1324
April1st to 15thNight
32%
539
19%
1884
12%
539
45%
539
28%
1063
April16th to endNight
31%
540
16%
1665
9%
540
42%
540
25%
1061
May1st to 15thNight
38%
539
18%
1620
13%
539
47%
539
28%
1000
May16th to endNight
46%
563
14%
1685
11%
561
50%
561
34%
1002
June1st to 15thNight
57%
540
15%
1620
9%
540
61%
540
40%
885
June16th to endNight
61%
540
18%
1620
11%
540
65%
540
50%
811
July1st to 15thNight
68%
510
16%
1530
13%
510
71%
510
41%
839
July16th to endNight
74%
564
20%
1594
20%
531
76%
531
42%
862
August1st to 15thNight
78%
539
22%
1610
26%
536
80%
536
42%
868
August16th to endNight
70%
573
26%
1783
29%
573
74%
573
40%
965
September1st to 15thNight
60%
540
24%
1903
24%
540
65%
540
30%
1074
September16th to endNight
59%
537
27%
1980
24%
537
67%
537
36%
1005
October1st to 15thNight
44%
684
27%
2124
19%
684
49%
684
32%
1243
October16th to endNight
39%
768
29%
2304
14%
768
50%
768
41%
1209
November1st to 15thNight
42%
867
26%
2387
12%
867
52%
867
38%
1355
November16th to endNight
34%
975
25%
2609
8%
975
46%
975
37%
1534
December1st to 15thNight
31%
975
19%
2730
8%
975
40%
975
30%
1682
December16th to endNight
32%
984
22%
2601
9%
984
41%
984
27%
1744

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%24416%3346%40%
48469
Night
8%413316%3345%44%
48563
Cloud CoverDay
11%5%2442445%7%46%
48467
Night
10%5%24324446%51%
48563
TransparencyDay
37%5%13%26%15%2
48167
Night
33%412%27%18%3
48368
SeeingDay
13%6%26%31%17%3
6180
Night
13%7%32%33%12%
16288

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%
2340
16th to endDay
5%414411%5%45%6%48%
2417
September1st to 15thDay
10%5%14410%3346%44%
2055
16th to endDay
6%314410%345%5%48%
1980
October1st to 15thDay
11%5%13410%3345%47%
1824
16th to endDay
8%413310%334453%
1920
November1st to 15thDay
6%413215%3335%48%
1801
16th to endDay
10%5%13216%2135%47%
1677
December1st to 15thDay
11%6%35%317%2335%37%
1560
16th to endDay
10%7%36%418%323435%
1719
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%50%
1620
16th to endNight
5%313410%2335%56%
1783
September1st to 15thNight
9%5%23310%3347%46%
1903
16th to endNight
5%314311%3446%51%
1980
October1st to 15thNight
12%423310%333448%
2124
16th to endNight
6%41229%2235%57%
2304
November1st to 15thNight
6%323315%2335%51%
2387
16th to endNight
9%412216%223349%
2609
December1st to 15thNight
10%6%24317%2345%39%
2730
16th to endNight
10%7%35%318%323437%
2601

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
8%31342446%9%48%
2340
16th to endDay
7%425%5%3445%7%49%
2417
September1st to 15thDay
12%5%24423347%48%
2055
16th to endDay
7%3145%335%5%6%52%
1980
October1st to 15thDay
12%5%14324346%51%
1824
16th to endDay
12%413313345%56%
1920
November1st to 15thDay
8%42322345%6%55%
1801
16th to endDay
12%5%23322246%54%
1677
December1st to 15thDay
13%7%35%323346%45%
1560
16th to endDay
13%8%47%434336%41%
1719
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%
1620
16th to endNight
7%314413336%58%
1783
September1st to 15thNight
11%424323347%50%
1903
16th to endNight
6%315%334447%55%
1980
October1st to 15thNight
14%41432333452%
2124
16th to endNight
9%412213235%61%
2304
November1st to 15thNight
8%423322336%58%
2387
16th to endNight
11%413313345%56%
2609
December1st to 15thNight
11%6%25%423446%47%
2730
16th to endNight
14%8%45%323346%42%
2601

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
30%6%14%28%16%2
2326
16th to endDay
32%5%11%26%16%6%
2417
September1st to 15thDay
36%5%12%23%16%5%
2055
16th to endDay
29%6%10%29%21%2
1980
October1st to 15thDay
34%310%23%22%4
1824
16th to endDay
31%310%26%22%5%
1920
November1st to 15thDay
29%411%26%21%5%
1801
16th to endDay
32%210%30%21%1
1677
December1st to 15thDay
40%414%28%11%
1560
16th to endDay
44%412%23%12%2
1719
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
24%6%17%30%17%4
1610
16th to endNight
27%48%28%23%6%
1783
September1st to 15thNight
33%5%10%25%18%7%
1903
16th to endNight
27%410%29%24%3
1980
October1st to 15thNight
35%37%23%22%7%
2124
16th to endNight
26%39%29%26%4
2304
November1st to 15thNight
28%412%25%22%7%
2387
16th to endNight
31%29%33%19%3
2609
December1st to 15thNight
36%314%29%14%
2730
16th to endNight
42%411%23%13%3
2601

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%39%32%6%
493
16th to endDay
10%116%34%29%7%
457
September1st to 15thDay
7%20%37%26%7%
210
16th to endDay
10%122%47%14%4
135
October1st to 15thDay
9%7%39%26%9%7%
135
16th to endDay
12%14%36%25%9%1
144
November1st to 15thDay
20%428%33%10%3
135
16th to endDay
27%13%34%15%8%
135
December1st to 15thDay
12%17%45%19%22
135
16th to endDay
20%17%35%24%2
108
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%45%36%1
539
16th to endNight
9%14%39%32%3
573
September1st to 15thNight
12%219%40%22%2
540
16th to endNight
6%223%39%24%3
537
October1st to 15thNight
16%430%30%16%1
684
16th to endNight
12%9%37%29%10%
768
November1st to 15thNight
11%11%33%31%11%
867
16th to endNight
15%11%37%28%6%
975
December1st to 15thNight
17%12%35%28%5%
975
16th to endNight
18%10%34%30%5%
984
Page generated at 2017-12-21 06:04:53. Version ac04

Copyright 2017 A. Danko


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