Delta College Planetarium
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Delta College Planetarium clear sky chart from 2005-11-06 to 2024-01-15.

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
36%
24160
14%
74928
10%
24103
42%
24103
20%
52696

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
21%
947
8%
2648
3%
946
26%
946
13%
2083
January11th to 20thNight
18%
923
7%
2583
3%
923
22%
923
11%
2112
January21st to endNight
25%
920
8%
2702
4%
920
30%
920
13%
2116
February1st to 10thNight
23%
720
9%
2340
4%
720
28%
720
12%
1796
February11th to 20thNight
22%
700
13%
2147
6%
699
29%
699
13%
1642
February21st to endNight
20%
592
13%
1776
5%
592
29%
592
15%
1338
March1st to 10thNight
31%
720
17%
2147
10%
720
37%
720
21%
1492
March11th to 20thNight
33%
720
21%
1980
11%
720
40%
720
25%
1341
March21st to endNight
31%
792
15%
2178
9%
792
36%
792
19%
1597
April1st to 10thNight
29%
581
14%
1841
8%
581
36%
581
19%
1262
April11th to 20thNight
36%
533
20%
1635
10%
533
45%
533
25%
1072
April21st to endNight
37%
540
18%
1620
8%
540
45%
540
23%
1098
May1st to 10thNight
39%
540
14%
1620
9%
540
44%
540
23%
1107
May11th to 20thNight
42%
531
18%
1593
13%
531
49%
531
24%
1056
May21st to endNight
54%
583
15%
1642
11%
582
59%
582
32%
1041
June1st to 10thNight
48%
396
17%
1296
15%
396
54%
396
28%
785
June11th to 20thNight
53%
360
19%
1260
21%
360
57%
360
27%
741
June21st to endNight
59%
417
18%
1317
16%
417
64%
417
32%
779
July1st to 10thNight
66%
525
17%
1400
15%
525
68%
525
44%
809
July11th to 20thNight
65%
522
19%
1392
16%
522
68%
522
41%
826
July21st to endNight
63%
591
18%
1542
14%
558
65%
558
36%
934
August1st to 10thNight
68%
537
17%
1600
15%
534
70%
534
36%
938
August11th to 20thNight
63%
534
22%
1598
16%
534
67%
534
36%
914
August21st to endNight
60%
594
22%
1812
17%
594
65%
594
33%
1061
September1st to 10thNight
56%
616
18%
1876
15%
616
58%
616
30%
1150
September11th to 20thNight
60%
720
25%
1980
19%
720
63%
720
37%
1188
September21st to endNight
48%
720
20%
1980
16%
720
51%
720
28%
1310
October1st to 10thNight
44%
720
20%
2017
16%
720
48%
720
27%
1344
October11th to 20thNight
32%
720
14%
2160
9%
720
39%
720
17%
1559
October21st to endNight
25%
792
12%
2569
9%
792
31%
792
14%
1910
November1st to 10thNight
33%
740
21%
2406
14%
740
43%
740
22%
1560
November11th to 20thNight
25%
760
14%
2470
8%
760
33%
760
15%
1799
November21st to endNight
22%
760
10%
2605
6%
760
28%
760
12%
1963
December1st to 10thNight
21%
802
8%
2702
4%
802
25%
802
11%
2116
December11th to 20thNight
19%
950
8%
3492
2%
948
25%
948
11%
2657
December21st to endNight
15%
1042
4%
5002
2%
1026
20%
1026
5%
4200

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
13%9%47%6%45%5%6%7%35%
74604
Night
15%10%47%6%345%5%7%34%
75156
TransparencyDay
47%7%13%21%11%1
74309
Night
49%7%13%19%11%1
74972
SeeingDay
15%9%28%26%15%6%
14348
Night
18%7%30%31%12%1
24170

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 10thDay
15%13%6%10%8%46%5%5%6%22%
1517
11th to 20thDay
15%16%8%11%8%6%7%5%5%5%15%
1472
21st to endDay
14%13%8%10%9%5%6%5%36%20%
1652
February1st to 10thDay
11%15%6%10%7%6%6%5%5%8%20%
1620
11th to 20thDay
12%13%5%11%9%46%445%26%
1696
21st to endDay
13%12%6%8%7%47%5%5%6%26%
1480
March1st to 10thDay
14%10%38%8%47%46%6%30%
1813
11th to 20thDay
11%10%48%7%46%5%5%7%34%
1980
21st to endDay
16%12%48%7%35%5%5%6%30%
2178
April1st to 10thDay
17%10%48%7%45%5%47%30%
2135
11th to 20thDay
16%8%47%5%3445%7%35%
2273
21st to endDay
16%9%36%6%45%5%5%6%35%
2340
May1st to 10thDay
16%8%35%5%36%5%47%37%
2340
11th to 20thDay
16%7%36%6%44447%39%
2301
21st to endDay
12%6%35%5%35%5%6%8%43%
2630
June1st to 10thDay
11%5%25%5%445%6%8%44%
2659
11th to 20thDay
11%6%35%5%25%5%6%9%44%
2700
21st to endDay
10%425%6%35%7%8%10%41%
2636
July1st to 10thDay
6%4245%35%5%7%9%51%
2520
11th to 20thDay
8%6%25%5%35%5%5%9%47%
2520
21st to endDay
7%5%35%5%35%5%8%9%45%
2683
August1st to 10thDay
10%425%5%45%6%7%10%42%
2327
11th to 20thDay
9%5%25%5%45%5%6%9%45%
2303
21st to endDay
10%6%25%4346%6%8%44%
2540
September1st to 10thDay
10%7%27%7%4446%8%42%
2066
11th to 20thDay
12%6%15%43446%9%47%
1980
21st to endDay
13%7%36%5%35%6%6%8%38%
1980
October1st to 10thDay
14%6%27%6%445%6%6%39%
1942
11th to 20thDay
19%10%37%6%45%5%5%8%28%
1800
21st to endDay
21%10%38%7%345%5%7%27%
1782
November1st to 10thDay
12%9%47%7%45%45%8%34%
1672
11th to 20thDay
13%12%5%7%6%46%6%6%7%28%
1710
21st to endDay
17%12%6%8%8%36%5%5%7%23%
1573
December1st to 10thDay
12%16%8%11%8%5%5%45%5%21%
1473
11th to 20thDay
14%15%6%10%7%445%6%7%24%
1675
21st to endDay
26%18%7%10%7%3445%5%11%
2636
January1st to 10thNight
16%12%6%11%8%45%5%46%24%
2648
11th to 20thNight
16%18%6%10%7%4445%5%21%
2583
21st to endNight
12%14%7%11%9%5%5%5%6%6%22%
2702
February1st to 10thNight
13%14%5%10%8%5%45%6%6%23%
2340
11th to 20thNight
14%13%7%9%8%5%5%5%5%6%25%
2147
21st to endNight
17%13%5%9%8%34446%27%
1776
March1st to 10thNight
14%11%47%6%34445%38%
2147
11th to 20thNight
13%8%46%6%35%5%5%7%40%
1980
21st to endNight
19%9%37%6%345%46%34%
2178
April1st to 10thNight
18%9%46%5%2445%7%35%
1841
11th to 20thNight
19%6%25%5%345%5%7%38%
1635
21st to endNight
17%9%36%5%345%46%38%
1620
May1st to 10thNight
14%8%36%7%45%6%5%8%35%
1620
11th to 20thNight
17%6%36%5%345%5%7%39%
1592
21st to endNight
11%425%5%35%5%5%9%46%
1641
June1st to 10thNight
11%6%35%5%235%5%7%47%
1296
11th to 20thNight
11%5%245%335%7%10%44%
1260
21st to endNight
7%425%6%35%5%7%8%47%
1317
July1st to 10thNight
7%32443447%8%55%
1440
11th to 20thNight
8%424335%5%6%9%52%
1440
21st to endNight
8%4244345%6%7%52%
1633
August1st to 10thNight
8%4245%246%8%9%48%
1611
11th to 20thNight
7%426%5%245%6%9%51%
1598
21st to endNight
10%5%2443445%8%50%
1812
September1st to 10thNight
9%5%25%6%35%5%6%6%47%
1876
11th to 20thNight
11%5%2442446%8%52%
1980
21st to endNight
13%7%26%5%3445%7%43%
1980
October1st to 10thNight
17%7%245%2445%7%42%
2017
11th to 20thNight
18%9%48%5%44446%33%
2160
21st to endNight
22%9%48%6%34447%28%
2569
November1st to 10thNight
14%9%26%6%3445%7%41%
2406
11th to 20thNight
14%11%38%6%45%7%5%8%28%
2470
21st to endNight
19%13%47%7%35%5%46%26%
2605
December1st to 10thNight
17%14%8%9%7%345%5%6%22%
2702
11th to 20thNight
14%13%6%9%8%45%5%5%7%24%
3522
21st to endNight
26%19%6%9%6%3344415%
5012

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
62%8%12%12%5%1
1518
11th to 20thDay
70%7%11%9%3
1472
21st to endDay
66%6%9%13%6%1
1652
February1st to 10thDay
62%8%11%15%4
1620
11th to 20thDay
59%6%11%15%7%1
1706
21st to endDay
56%7%12%16%7%1
1480
March1st to 10thDay
53%7%13%18%8%1
1813
11th to 20thDay
49%7%12%18%11%2
1980
21st to endDay
54%6%12%19%8%1
2178
April1st to 10thDay
53%6%13%18%9%
2135
11th to 20thDay
47%6%12%19%14%2
2273
21st to endDay
48%7%14%17%12%1
2340
May1st to 10thDay
45%6%13%23%10%2
2340
11th to 20thDay
45%7%11%21%15%2
2305
21st to endDay
37%8%15%23%14%2
2630
June1st to 10thDay
36%7%13%27%14%2
2659
11th to 20thDay
36%7%14%24%18%2
2700
21st to endDay
35%9%15%25%15%1
2636
July1st to 10thDay
27%8%13%29%19%3
2451
11th to 20thDay
33%6%14%27%18%2
2445
21st to endDay
33%7%16%24%17%3
2534
August1st to 10thDay
35%9%16%25%14%1
2314
11th to 20thDay
35%7%16%27%13%2
2303
21st to endDay
35%8%14%27%14%2
2540
September1st to 10thDay
40%7%16%23%11%2
2066
11th to 20thDay
34%7%16%30%13%
1980
21st to endDay
42%7%15%24%12%
1980
October1st to 10thDay
44%7%11%24%13%1
1942
11th to 20thDay
54%7%14%19%7%
1800
21st to endDay
55%7%16%17%5%
1782
November1st to 10thDay
47%7%13%21%11%
1672
11th to 20thDay
53%7%16%16%8%
1710
21st to endDay
60%7%14%14%41
1573
December1st to 10thDay
64%7%13%12%3
1473
11th to 20thDay
58%7%14%16%31
1671
21st to endDay
75%7%9%7%3
2636
January1st to 10thNight
61%6%13%13%7%
2648
11th to 20thNight
66%6%13%10%5%
2583
21st to endNight
61%7%13%14%5%1
2702
February1st to 10thNight
60%6%11%16%6%1
2340
11th to 20thNight
60%6%9%16%8%1
2148
21st to endNight
59%411%17%9%
1776
March1st to 10thNight
48%5%12%23%10%1
2147
11th to 20thNight
44%6%13%21%15%2
1980
21st to endNight
52%6%10%20%11%2
2178
April1st to 10thNight
48%6%11%21%13%2
1841
11th to 20thNight
43%5%12%20%18%2
1635
21st to endNight
46%5%11%18%16%3
1620
May1st to 10thNight
46%6%13%19%14%2
1620
11th to 20thNight
44%6%10%21%16%4
1593
21st to endNight
35%8%12%26%15%4
1642
June1st to 10thNight
37%6%12%23%20%2
1296
11th to 20thNight
34%8%14%21%21%2
1260
21st to endNight
32%8%14%26%17%2
1317
July1st to 10thNight
25%7%12%30%21%5%
1400
11th to 20thNight
27%7%15%28%20%2
1392
21st to endNight
30%9%15%26%19%2
1542
August1st to 10thNight
28%9%17%30%14%1
1600
11th to 20thNight
30%6%17%29%15%2
1598
21st to endNight
31%8%14%28%18%1
1812
September1st to 10thNight
36%7%15%25%17%1
1876
11th to 20thNight
30%7%14%32%16%1
1980
21st to endNight
40%7%15%27%11%1
1980
October1st to 10thNight
41%7%11%25%14%1
2017
11th to 20thNight
52%6%12%20%8%1
2160
21st to endNight
56%7%13%17%7%
2569
November1st to 10thNight
43%7%13%24%13%1
2406
11th to 20thNight
51%9%14%18%7%1
2470
21st to endNight
58%7%16%14%5%
2605
December1st to 10thNight
63%6%13%13%5%
2702
11th to 20thNight
58%8%15%12%6%1
3525
21st to endNight
72%7%11%7%4
5012

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
18%11%34%26%9%2
270
11th to 20thDay
21%11%36%23%8%1
252
21st to endDay
20%11%31%30%8%1
264
February1st to 10thDay
19%16%41%21%3
240
11th to 20thDay
18%14%38%25%5%
240
21st to endDay
21%18%41%16%4
198
March1st to 10thDay
21%12%39%22%41
253
11th to 20thDay
15%11%33%26%13%1
420
21st to endDay
24%11%29%26%9%1
462
April1st to 10thDay
19%10%33%27%10%1
420
11th to 20thDay
23%13%30%21%9%5%
412
21st to endDay
20%12%30%25%11%2
420
May1st to 10thDay
20%13%31%20%12%4
423
11th to 20thDay
16%8%29%23%16%8%
447
21st to endDay
14%6%22%28%22%8%
584
June1st to 10thDay
12%7%29%31%15%6%
630
11th to 20thDay
14%8%26%30%16%6%
630
21st to endDay
11%6%27%31%19%6%
630
July1st to 10thDay
5%5%22%30%24%13%
621
11th to 20thDay
9%5%25%31%19%10%
618
21st to endDay
7%226%34%24%7%
624
August1st to 10thDay
8%419%26%27%15%
446
11th to 20thDay
10%316%28%30%13%
439
21st to endDay
11%7%20%25%26%10%
495
September1st to 10thDay
9%7%25%28%19%12%
450
11th to 20thDay
11%419%23%28%14%
450
21st to endDay
16%9%21%25%19%10%
450
October1st to 10thDay
18%9%26%22%16%8%
450
11th to 20thDay
23%15%29%25%7%1
450
21st to endDay
23%13%36%20%7%1
297
November1st to 10thDay
16%13%35%24%9%3
270
11th to 20thDay
15%20%39%19%5%2
270
21st to endDay
24%9%40%21%5%
205
December1st to 10thDay
22%13%38%18%7%1
180
11th to 20thDay
22%16%37%20%41
180
21st to endDay
23%16%36%21%3
258
January1st to 10thNight
19%11%37%25%7%1
950
11th to 20thNight
23%11%35%24%7%1
923
21st to endNight
16%7%34%34%9%1
920
February1st to 10thNight
20%8%39%30%3
720
11th to 20thNight
19%9%40%26%6%
703
21st to endNight
24%10%38%23%4
592
March1st to 10thNight
18%8%35%31%8%
720
11th to 20thNight
16%9%36%30%9%1
720
21st to endNight
22%8%31%31%7%1
792
April1st to 10thNight
19%10%36%27%7%1
581
11th to 20thNight
22%6%32%28%9%2
533
21st to endNight
23%6%29%32%9%1
540
May1st to 10thNight
16%6%32%37%9%1
540
11th to 20thNight
19%6%26%35%13%1
531
21st to endNight
14%225%36%21%2
584
June1st to 10thNight
12%5%28%40%15%
396
11th to 20thNight
12%424%42%18%
360
21st to endNight
9%323%42%21%2
417
July1st to 10thNight
8%217%42%28%3
525
11th to 20thNight
8%217%42%27%4
522
21st to endNight
10%219%46%21%2
591
August1st to 10thNight
9%215%45%27%2
537
11th to 20thNight
8%217%36%32%4
534
21st to endNight
12%320%35%28%3
594
September1st to 10thNight
10%325%34%24%5%
616
11th to 20thNight
13%418%34%27%5%
720
21st to endNight
16%6%23%31%20%4
720
October1st to 10thNight
19%8%23%31%17%2
720
11th to 20thNight
22%12%29%27%9%1
720
21st to endNight
26%11%32%25%5%
792
November1st to 10thNight
18%9%34%28%10%1
740
11th to 20thNight
19%12%38%21%9%
760
21st to endNight
23%11%38%23%4
760
December1st to 10thNight
23%10%36%25%5%1
802
11th to 20thNight
21%9%40%25%41
950
21st to endNight
29%10%35%23%3
1045
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