Sandy Flats Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sandy Flats Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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
30%
22724
15%
74768
8%
22682
38%
22682
17%
51805

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 10thNight
17%
759
11%
2649
4%
757
26%
757
9%
2009
January11th to 20thNight
17%
725
9%
2503
4%
725
24%
725
8%
1979
January21st to endNight
20%
792
14%
2574
7%
792
29%
792
13%
1897
February1st to 10thNight
12%
720
10%
2340
2%
720
21%
720
10%
1808
February11th to 20thNight
16%
702
14%
2275
4%
700
29%
700
13%
1647
February21st to endNight
18%
592
15%
1848
4%
592
31%
592
14%
1321
March1st to 10thNight
20%
720
17%
2030
7%
720
30%
720
17%
1446
March11th to 20thNight
21%
720
17%
1979
6%
720
31%
720
18%
1406
March21st to endNight
25%
792
21%
2152
7%
792
36%
792
24%
1464
April1st to 10thNight
23%
720
17%
1800
5%
720
30%
720
21%
1298
April11th to 20thNight
27%
711
17%
1775
6%
711
32%
711
23%
1254
April21st to endNight
31%
563
17%
1643
7%
563
38%
563
21%
1147
May1st to 10thNight
36%
455
15%
1535
11%
455
43%
455
19%
1016
May11th to 20thNight
39%
354
22%
1416
16%
354
55%
354
23%
817
May21st to endNight
45%
388
17%
1551
16%
388
56%
388
25%
856
June1st to 10thNight
36%
360
13%
1440
11%
360
48%
360
20%
881
June11th to 20thNight
46%
360
16%
1440
18%
360
56%
360
23%
826
June21st to endNight
44%
360
13%
1440
14%
360
54%
360
21%
865
July1st to 10thNight
54%
350
16%
1400
21%
350
61%
350
29%
743
July11th to 20thNight
52%
348
17%
1392
23%
348
58%
348
28%
762
July21st to endNight
55%
436
18%
1526
17%
412
60%
412
31%
856
August1st to 10thNight
56%
537
18%
1600
14%
534
60%
534
33%
961
August11th to 20thNight
57%
534
19%
1598
13%
534
63%
534
32%
960
August21st to endNight
50%
727
21%
1915
10%
727
55%
727
33%
1226
September1st to 10thNight
48%
720
17%
1800
7%
720
52%
720
33%
1195
September11th to 20thNight
54%
720
27%
1808
15%
720
58%
720
38%
1122
September21st to endNight
45%
720
22%
2074
17%
720
49%
720
26%
1351
October1st to 10thNight
37%
720
16%
2160
11%
720
43%
720
21%
1469
October11th to 20thNight
25%
720
13%
2160
8%
720
33%
720
17%
1574
October21st to endNight
21%
792
15%
2391
7%
792
32%
792
14%
1778
November1st to 10thNight
33%
740
18%
2478
14%
740
42%
740
19%
1622
November11th to 20thNight
21%
760
15%
2660
7%
760
32%
760
14%
1868
November21st to endNight
20%
760
11%
2660
5%
760
27%
760
12%
1965
December1st to 10thNight
15%
760
9%
2660
3%
760
23%
760
8%
2076
December11th to 20thNight
15%
759
9%
3242
2%
759
24%
759
8%
2418
December21st to endNight
12%
828
5%
4854
1%
817
20%
817
4%
3922

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
15%9%36%6%35%5%5%7%35%
74684
Night
15%9%47%6%35%5%5%7%35%
75019
TransparencyDay
48%7%12%20%11%1
74392
Night
48%7%12%20%11%1
74845
SeeingDay
20%16%31%21%9%4
13335
Night
19%13%31%25%10%1
22736

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 10thDay
14%14%5%9%5%45%446%28%
1517
11th to 20thDay
15%17%6%8%6%46%5%5%5%23%
1478
21st to endDay
12%13%6%11%9%46%5%5%5%24%
1782
February1st to 10thDay
13%15%5%11%8%36%5%5%5%23%
1620
11th to 20thDay
11%13%6%11%10%44447%27%
1573
21st to endDay
19%13%5%9%8%34435%27%
1408
March1st to 10thDay
14%11%47%8%45%5%6%6%30%
1924
11th to 20thDay
13%13%47%6%44445%35%
1980
21st to endDay
19%11%35%5%35%445%37%
2204
April1st to 10thDay
18%11%48%6%34446%33%
2160
11th to 20thDay
20%7%46%5%33346%39%
2134
21st to endDay
20%8%36%6%445%36%34%
2330
May1st to 10thDay
17%7%37%6%36%46%7%35%
2425
11th to 20thDay
14%6%26%5%246%5%7%41%
2478
21st to endDay
12%5%35%6%36%5%6%8%42%
2725
June1st to 10thDay
17%6%35%6%35%5%7%8%36%
2520
11th to 20thDay
13%6%25%5%445%6%10%41%
2520
21st to endDay
14%6%35%5%35%6%6%9%38%
2520
July1st to 10thDay
9%4245%36%6%7%8%46%
2520
11th to 20thDay
10%5%35%5%46%6%7%9%39%
2520
21st to endDay
9%5%25%6%47%7%8%10%38%
2700
August1st to 10thDay
9%5%25%6%45%6%7%9%42%
2327
11th to 20thDay
9%5%25%6%46%6%7%9%40%
2303
21st to endDay
11%6%25%5%35%6%8%9%40%
2422
September1st to 10thDay
12%6%35%6%45%5%5%8%43%
2160
11th to 20thDay
11%4244345%5%7%49%
2150
21st to endDay
14%6%35%5%35%5%5%7%41%
1886
October1st to 10thDay
18%9%36%6%35%5%5%7%34%
1800
11th to 20thDay
18%9%37%7%446%47%29%
1800
21st to endDay
25%12%36%5%45%5%45%26%
1963
November1st to 10thDay
14%9%47%6%4445%6%37%
1597
11th to 20thDay
15%10%47%6%36%6%6%7%30%
1520
21st to endDay
19%14%47%7%35%5%47%25%
1520
December1st to 10thDay
17%15%6%9%8%45%45%5%23%
1520
11th to 20thDay
17%14%5%7%6%3445%5%29%
1906
21st to endDay
27%15%5%8%5%34345%20%
2772
January1st to 10thNight
18%13%5%8%6%46%5%5%5%26%
2649
11th to 20thNight
18%14%5%10%7%45%45%5%24%
2503
21st to endNight
12%13%5%9%7%46%5%46%30%
2574
February1st to 10thNight
13%15%5%9%8%46%5%5%6%25%
2340
11th to 20thNight
13%13%5%7%7%35%6%6%5%30%
2273
21st to endNight
14%11%37%6%445%5%7%32%
1848
March1st to 10thNight
13%9%48%7%45%5%5%6%35%
2020
11th to 20thNight
16%11%46%5%3445%5%36%
1979
21st to endNight
16%7%37%5%34446%41%
2152
April1st to 10thNight
17%9%46%5%346%5%6%36%
1800
11th to 20thNight
17%7%36%6%34446%39%
1775
21st to endNight
17%7%27%5%446%6%7%35%
1643
May1st to 10thNight
18%6%26%5%35%5%6%7%35%
1535
11th to 20thNight
12%5%345%345%7%7%45%
1415
21st to endNight
10%5%35%4345%5%8%49%
1550
June1st to 10thNight
14%5%36%5%36%47%8%39%
1440
11th to 20thNight
10%5%26%5%335%6%8%45%
1440
21st to endNight
11%6%35%6%45%46%8%42%
1440
July1st to 10thNight
6%425%5%45%45%9%53%
1440
11th to 20thNight
10%5%34435%46%9%48%
1440
21st to endNight
8%5%245%35%5%6%7%50%
1616
August1st to 10thNight
8%5%35%5%2447%8%49%
1611
11th to 20thNight
8%5%26%5%336%6%8%48%
1598
21st to endNight
9%5%25%436%5%6%8%48%
1915
September1st to 10thNight
11%5%35%5%35%5%6%8%45%
1800
11th to 20thNight
12%42342446%7%52%
1808
21st to endNight
15%6%35%434436%46%
2074
October1st to 10thNight
17%6%36%5%35%45%7%39%
2160
11th to 20thNight
18%8%37%8%35%45%6%33%
2160
21st to endNight
21%9%48%6%3445%7%29%
2391
November1st to 10thNight
17%7%35%5%3446%6%39%
2478
11th to 20thNight
17%9%36%6%46%5%6%7%32%
2660
21st to endNight
19%10%46%6%35%5%6%7%27%
2660
December1st to 10thNight
18%13%5%8%7%45%6%5%6%23%
2660
11th to 20thNight
17%14%48%6%445%5%6%27%
3288
21st to endNight
28%16%5%8%5%345%46%17%
4884

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 10thDay
57%7%13%16%7%
1518
11th to 20thDay
63%7%9%11%10%1
1478
21st to endDay
61%7%9%12%9%2
1782
February1st to 10thDay
62%7%10%15%5%1
1620
11th to 20thDay
59%5%9%15%10%2
1582
21st to endDay
61%49%15%10%1
1408
March1st to 10thDay
53%7%15%15%9%2
1927
11th to 20thDay
52%6%10%19%12%2
1980
21st to endDay
50%6%7%19%15%4
2204
April1st to 10thDay
53%410%20%11%2
2160
11th to 20thDay
46%5%10%22%16%1
2134
21st to endDay
51%6%9%18%13%2
2330
May1st to 10thDay
48%5%10%19%14%3
2425
11th to 20thDay
41%8%10%25%16%1
2482
21st to endDay
39%7%15%22%14%3
2725
June1st to 10thDay
44%7%13%20%15%1
2520
11th to 20thDay
38%8%13%26%14%2
2520
21st to endDay
42%8%14%22%13%1
2520
July1st to 10thDay
32%9%13%28%17%2
2450
11th to 20thDay
37%9%15%28%10%1
2445
21st to endDay
38%11%13%26%11%1
2549
August1st to 10thDay
36%10%16%25%11%2
2314
11th to 20thDay
37%9%16%26%12%
2303
21st to endDay
37%9%13%28%12%1
2422
September1st to 10thDay
38%8%13%28%12%1
2160
11th to 20thDay
32%8%14%30%15%1
2150
21st to endDay
42%7%14%27%9%
1886
October1st to 10thDay
48%7%13%20%11%1
1800
11th to 20thDay
53%9%13%18%6%1
1800
21st to endDay
60%6%11%15%8%1
1963
November1st to 10thDay
47%8%14%20%11%1
1597
11th to 20thDay
51%8%16%15%9%1
1520
21st to endDay
58%9%12%13%7%
1520
December1st to 10thDay
63%7%11%14%5%
1520
11th to 20thDay
58%9%11%13%8%
1906
21st to endDay
67%6%11%12%4
2772
January1st to 10thNight
59%7%10%16%8%
2649
11th to 20thNight
63%7%9%13%7%1
2503
21st to endNight
56%6%9%17%11%1
2574
February1st to 10thNight
60%8%11%16%6%1
2340
11th to 20thNight
54%6%11%19%9%
2275
21st to endNight
51%7%12%20%9%1
1848
March1st to 10thNight
50%6%12%18%12%2
2030
11th to 20thNight
49%6%10%22%12%1
1980
21st to endNight
45%5%8%22%16%4
2152
April1st to 10thNight
48%6%9%20%15%2
1800
11th to 20thNight
46%6%11%19%18%2
1775
21st to endNight
46%8%11%18%15%2
1643
May1st to 10thNight
45%7%11%19%15%3
1535
11th to 20thNight
37%7%11%24%18%4
1416
21st to endNight
33%7%15%23%18%4
1551
June1st to 10thNight
40%8%12%24%14%2
1440
11th to 20thNight
34%8%13%21%21%3
1440
21st to endNight
41%6%14%23%14%2
1440
July1st to 10thNight
29%8%10%28%24%2
1400
11th to 20thNight
33%6%13%29%18%1
1392
21st to endNight
32%7%13%34%12%2
1526
August1st to 10thNight
32%8%17%29%14%1
1600
11th to 20thNight
32%8%17%28%13%2
1598
21st to endNight
32%9%12%31%15%1
1915
September1st to 10thNight
36%8%16%25%14%1
1800
11th to 20thNight
31%7%13%32%17%1
1808
21st to endNight
41%6%11%30%13%1
2074
October1st to 10thNight
44%8%12%21%14%1
2160
11th to 20thNight
52%7%13%20%7%
2160
21st to endNight
55%7%12%15%11%1
2391
November1st to 10thNight
44%7%14%21%13%1
2478
11th to 20thNight
50%8%13%18%10%2
2660
21st to endNight
54%9%16%15%5%
2660
December1st to 10thNight
60%9%12%14%5%
2660
11th to 20thNight
58%7%12%15%7%1
3288
21st to endNight
68%7%10%11%4
4884

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 10thDay
23%24%35%12%41
180
11th to 20thDay
21%32%30%14%31
176
21st to endDay
20%26%34%16%31
264
February1st to 10thDay
28%33%28%10%1
240
11th to 20thDay
24%31%35%9%
240
21st to endDay
34%26%32%8%
274
March1st to 10thDay
22%24%35%16%4
420
11th to 20thDay
24%22%34%16%3
420
21st to endDay
32%15%33%13%5%1
462
April1st to 10thDay
25%22%33%14%7%
420
11th to 20thDay
27%20%26%15%8%4
412
21st to endDay
27%20%31%15%5%1
420
May1st to 10thDay
23%17%29%20%6%5%
423
11th to 20thDay
19%12%35%21%8%4
447
21st to endDay
15%12%31%26%12%5%
485
June1st to 10thDay
16%12%36%23%11%1
450
11th to 20thDay
14%13%31%24%14%3
450
21st to endDay
18%11%30%27%11%4
450
July1st to 10thDay
9%8%27%34%17%5%
445
11th to 20thDay
15%8%27%28%16%7%
444
21st to endDay
9%10%32%32%11%5%
488
August1st to 10thDay
8%7%30%30%18%7%
446
11th to 20thDay
11%5%26%30%19%9%
439
21st to endDay
13%10%28%28%15%7%
495
September1st to 10thDay
11%11%30%26%15%7%
450
11th to 20thDay
12%8%27%25%17%11%
450
21st to endDay
14%14%31%23%12%6%
450
October1st to 10thDay
20%16%27%17%14%6%
450
11th to 20thDay
23%20%30%20%5%1
450
21st to endDay
30%19%27%18%5%
486
November1st to 10thDay
23%20%26%21%8%1
291
11th to 20thDay
23%27%34%12%21
180
21st to endDay
26%22%34%14%3
180
December1st to 10thDay
26%24%32%13%5%
180
11th to 20thDay
28%28%32%10%21
180
21st to endDay
32%25%32%11%
198
January1st to 10thNight
26%21%29%19%5%
760
11th to 20thNight
26%17%30%21%6%
725
21st to endNight
20%16%34%24%5%1
792
February1st to 10thNight
21%20%41%15%2
720
11th to 20thNight
21%19%37%19%4
703
21st to endNight
22%21%36%18%4
592
March1st to 10thNight
20%19%35%22%4
720
11th to 20thNight
21%18%36%20%41
720
21st to endNight
20%13%36%22%8%1
792
April1st to 10thNight
22%16%35%23%5%
720
11th to 20thNight
20%17%31%21%9%1
711
21st to endNight
20%11%32%26%10%
563
May1st to 10thNight
21%7%29%29%12%2
455
11th to 20thNight
13%8%33%33%12%
354
21st to endNight
11%8%30%35%15%1
389
June1st to 10thNight
12%8%35%31%13%1
360
11th to 20thNight
11%9%26%37%15%1
360
21st to endNight
13%6%29%36%16%1
360
July1st to 10thNight
5%5%27%43%18%1
350
11th to 20thNight
10%327%35%24%1
348
21st to endNight
8%329%42%17%2
436
August1st to 10thNight
11%320%37%26%2
537
11th to 20thNight
8%5%20%37%27%4
534
21st to endNight
12%5%26%33%22%2
727
September1st to 10thNight
12%5%26%34%20%3
720
11th to 20thNight
12%6%23%30%24%5%
720
21st to endNight
16%10%23%29%18%4
720
October1st to 10thNight
20%9%29%23%17%2
720
11th to 20thNight
22%15%31%23%8%1
720
21st to endNight
27%13%32%23%5%
792
November1st to 10thNight
24%13%26%27%9%1
740
11th to 20thNight
21%19%33%20%6%2
760
21st to endNight
25%16%32%21%6%
760
December1st to 10thNight
26%22%31%17%4
760
11th to 20thNight
23%19%35%18%4
760
21st to endNight
30%17%33%17%3
836
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