Headlands International Dark Sky Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Headlands International Dark Sky Park clear sky chart from 2005-11-06 to 2024-03-08.

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
31%
24328
6%
81185
5%
24220
33%
24220
8%
68244

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
15%
946
2%
4416
1%
926
16%
926
2%
3886
January11th to 20thNight
16%
944
1%
4437
0%
930
17%
930
1%
4113
January21st to endNight
19%
1042
2%
4884
1%
1023
21%
1023
3%
4430
February1st to 10thNight
20%
837
5%
2547
2%
837
23%
837
6%
2208
February11th to 20thNight
22%
742
5%
2274
2%
739
25%
739
6%
1959
February21st to endNight
22%
628
6%
1884
1%
628
27%
628
7%
1605
March1st to 10thNight
29%
751
12%
2251
7%
751
34%
751
12%
1757
March11th to 20thNight
28%
720
11%
2012
5%
720
33%
720
10%
1650
March21st to endNight
34%
761
10%
2147
6%
761
39%
761
12%
1744
April1st to 10thNight
29%
540
6%
1800
4%
540
32%
540
9%
1448
April11th to 20thNight
34%
533
13%
1643
8%
533
39%
533
12%
1305
April21st to endNight
35%
540
10%
1620
5%
540
39%
540
11%
1290
May1st to 10thNight
40%
540
10%
1620
6%
540
44%
540
11%
1295
May11th to 20thNight
39%
531
11%
1510
9%
531
42%
531
12%
1207
May21st to endNight
50%
499
12%
1467
9%
497
54%
497
16%
1094
June1st to 10thNight
47%
360
9%
1260
8%
360
51%
360
12%
944
June11th to 20thNight
51%
360
11%
1260
10%
360
54%
360
14%
919
June21st to endNight
50%
360
10%
1260
9%
360
52%
360
13%
950
July1st to 10thNight
59%
435
10%
1310
7%
435
61%
435
19%
942
July11th to 20thNight
56%
522
11%
1392
8%
522
58%
522
17%
1063
July21st to endNight
58%
591
10%
1487
8%
558
59%
558
17%
1138
August1st to 10thNight
59%
537
10%
1520
9%
534
61%
534
18%
1121
August11th to 20thNight
61%
534
13%
1598
8%
534
65%
534
20%
1131
August21st to endNight
53%
594
10%
1823
8%
594
54%
594
16%
1355
September1st to 10thNight
52%
563
11%
1823
8%
563
55%
563
15%
1326
September11th to 20thNight
46%
720
11%
1980
7%
720
49%
720
16%
1530
September21st to endNight
45%
720
9%
1980
6%
720
47%
720
14%
1585
October1st to 10thNight
37%
720
12%
2001
8%
720
41%
720
14%
1581
October11th to 20thNight
25%
720
5%
2245
2%
720
28%
720
6%
1928
October21st to endNight
21%
792
5%
2574
4%
792
24%
792
5%
2260
November1st to 10thNight
22%
740
8%
2406
6%
740
26%
740
7%
2004
November11th to 20thNight
16%
760
6%
2493
3%
760
20%
760
4%
2231
November21st to endNight
11%
807
3%
2707
2%
807
13%
807
3%
2478
December1st to 10thNight
12%
950
1%
2850
1%
950
12%
950
2%
2678
December11th to 20thNight
11%
950
1%
3513
0%
949
12%
949
2%
3200
December21st to endNight
10%
1039
1%
5191
0%
1026
11%
1026
1%
4889

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
14%10%48%7%35%5%5%7%33%
78170
Night
15%12%5%9%8%45%45%6%27%
81398
TransparencyDay
35%38%8%13%6%1
77876
Night
40%37%8%10%41
81227
SeeingDay
18%13%30%25%11%3
14597
Night
20%12%29%27%11%1
24348

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
13%16%12%20%11%46%4338%
2527
11th to 20thDay
13%16%7%13%19%6%6%5%5%48%
2530
21st to endDay
12%20%10%13%10%5%5%5%4411%
2783
February1st to 10thDay
9%13%6%10%8%5%6%6%6%7%24%
1631
11th to 20thDay
12%12%6%10%8%5%6%5%6%7%25%
1787
21st to endDay
11%13%5%9%8%45%5%6%7%26%
1570
March1st to 10thDay
12%10%47%7%5%445%6%36%
1868
11th to 20thDay
11%9%5%9%7%345%5%6%37%
1947
21st to endDay
15%9%46%5%3446%7%36%
2224
April1st to 10thDay
19%9%5%6%5%35%5%6%7%30%
2160
11th to 20thDay
18%9%46%5%3445%6%35%
2266
21st to endDay
15%8%48%7%345%6%7%33%
2340
May1st to 10thDay
15%7%46%5%3445%7%40%
2340
11th to 20thDay
15%6%45%43445%7%42%
2380
21st to endDay
11%6%25%5%345%6%9%44%
2805
June1st to 10thDay
13%5%25%435%6%7%9%43%
2700
11th to 20thDay
11%6%26%5%35%5%6%8%42%
2700
21st to endDay
14%7%25%4245%5%8%44%
2700
July1st to 10thDay
7%4244345%6%8%53%
2606
11th to 20thDay
11%5%245%35%45%8%50%
2520
21st to endDay
9%5%35%4245%6%8%50%
2743
August1st to 10thDay
9%6%14435%5%6%8%47%
2409
11th to 20thDay
9%5%24435%6%6%8%50%
2303
21st to endDay
11%5%25%5%3345%7%49%
2530
September1st to 10thDay
15%6%25%43445%8%43%
2118
11th to 20thDay
13%5%25%5%345%6%8%43%
1980
21st to endDay
15%7%36%6%35%5%5%9%37%
1980
October1st to 10thDay
19%9%37%5%3445%7%33%
1958
11th to 20thDay
23%13%48%7%45%6%46%19%
1710
21st to endDay
24%13%5%8%7%45%45%6%20%
1782
November1st to 10thDay
18%14%5%8%7%35%45%7%23%
1672
11th to 20thDay
15%16%6%13%9%45%5%5%5%17%
1687
21st to endDay
20%20%8%12%7%44435%12%
1468
December1st to 10thDay
15%20%12%16%8%5%433411%
1330
11th to 20thDay
16%18%8%14%11%45%5%4410%
1671
21st to endDay
23%19%7%15%9%443339%
2445
January1st to 10thNight
14%17%8%16%9%46%345%13%
4428
11th to 20thNight
12%14%7%15%17%6%6%5%4411%
4440
21st to endNight
15%20%8%12%10%46%44412%
4884
February1st to 10thNight
10%15%6%10%11%47%5%5%6%22%
2547
11th to 20thNight
11%12%6%11%10%45%5%5%6%25%
2274
21st to endNight
14%14%5%8%8%45%5%5%7%25%
1884
March1st to 10thNight
12%10%46%7%3445%6%40%
2240
11th to 20thNight
12%10%47%7%445%5%6%35%
2011
21st to endNight
15%9%47%5%35%45%6%37%
2147
April1st to 10thNight
20%9%47%6%45%6%6%6%29%
1800
11th to 20thNight
18%10%35%6%35%446%36%
1643
21st to endNight
17%8%47%5%445%5%6%36%
1620
May1st to 10thNight
15%7%37%6%45%5%5%6%37%
1620
11th to 20thNight
14%9%36%7%35%447%35%
1510
21st to endNight
10%7%36%5%35%46%8%44%
1466
June1st to 10thNight
14%6%25%5%25%5%6%10%41%
1260
11th to 20thNight
10%6%26%6%45%5%5%8%42%
1260
21st to endNight
14%6%35%43445%8%43%
1260
July1st to 10thNight
9%4245%245%7%9%50%
1350
11th to 20thNight
9%446%5%45%5%5%6%47%
1440
21st to endNight
10%5%36%435%45%8%49%
1575
August1st to 10thNight
7%6%25%5%3445%8%51%
1529
11th to 20thNight
8%325%5%3445%8%52%
1598
21st to endNight
13%6%25%5%3445%6%48%
1823
September1st to 10thNight
13%6%35%43445%7%46%
1823
11th to 20thNight
17%6%26%43446%7%43%
1980
21st to endNight
17%8%36%5%335%5%7%37%
1980
October1st to 10thNight
18%9%36%5%3445%6%37%
2001
11th to 20thNight
24%10%48%6%45%5%46%24%
2245
21st to endNight
27%12%6%7%6%35%445%21%
2574
November1st to 10thNight
18%14%5%8%6%3445%7%26%
2406
11th to 20thNight
18%17%5%12%8%45%45%5%18%
2493
21st to endNight
21%19%7%11%8%44445%13%
2707
December1st to 10thNight
14%21%10%13%11%45%43412%
2850
11th to 20thNight
17%17%7%15%11%45%44414%
3526
21st to endNight
23%20%8%13%8%443339%
5204

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
63%30%33
2528
11th to 20thDay
65%29%32
2530
21st to endDay
60%29%7%41
2783
February1st to 10thDay
42%38%8%11%1
1631
11th to 20thDay
41%38%9%8%31
1797
21st to endDay
39%38%8%12%2
1570
March1st to 10thDay
34%37%9%14%6%1
1870
11th to 20thDay
31%39%9%15%5%1
1947
21st to endDay
35%38%8%13%5%1
2224
April1st to 10thDay
34%38%7%14%6%1
2160
11th to 20thDay
33%38%7%14%8%
2266
21st to endDay
33%38%8%9%9%3
2340
May1st to 10thDay
29%38%8%14%10%1
2340
11th to 20thDay
28%38%7%15%11%2
2384
21st to endDay
24%40%9%16%10%2
2805
June1st to 10thDay
23%39%10%18%9%1
2700
11th to 20thDay
23%39%9%17%11%
2700
21st to endDay
27%39%11%15%7%2
2700
July1st to 10thDay
21%39%10%18%12%
2537
11th to 20thDay
20%40%8%20%10%1
2445
21st to endDay
24%40%10%18%8%
2591
August1st to 10thDay
25%39%11%16%7%1
2394
11th to 20thDay
20%39%11%21%9%1
2303
21st to endDay
22%39%10%19%8%1
2530
September1st to 10thDay
26%37%9%19%8%1
2118
11th to 20thDay
27%38%12%18%4
1980
21st to endDay
30%39%12%14%5%
1980
October1st to 10thDay
30%40%8%16%5%1
1958
11th to 20thDay
45%39%8%8%1
1710
21st to endDay
43%40%8%8%1
1782
November1st to 10thDay
35%43%7%11%3
1672
11th to 20thDay
41%43%6%6%3
1687
21st to endDay
47%43%6%3
1468
December1st to 10thDay
51%41%5%21
1330
11th to 20thDay
53%37%7%31
1671
21st to endDay
64%28%422
2445
January1st to 10thNight
55%31%8%6%
4428
11th to 20thNight
60%31%6%31
4440
21st to endNight
59%28%8%41
4884
February1st to 10thNight
43%37%8%9%3
2547
11th to 20thNight
41%39%8%8%3
2275
21st to endNight
40%38%8%11%3
1884
March1st to 10thNight
31%36%11%15%7%
2251
11th to 20thNight
33%39%8%14%5%1
2012
21st to endNight
33%38%10%13%6%1
2147
April1st to 10thNight
34%38%8%12%8%1
1800
11th to 20thNight
32%37%7%15%9%1
1643
21st to endNight
31%39%7%11%9%2
1620
May1st to 10thNight
31%39%7%12%8%2
1620
11th to 20thNight
31%39%6%12%10%2
1511
21st to endNight
23%39%8%16%11%4
1467
June1st to 10thNight
26%39%8%16%11%
1260
11th to 20thNight
22%40%9%18%10%2
1260
21st to endNight
26%39%10%17%7%1
1260
July1st to 10thNight
19%39%10%19%11%1
1310
11th to 20thNight
23%40%9%17%10%1
1392
21st to endNight
22%41%12%15%10%
1487
August1st to 10thNight
22%38%12%19%7%2
1520
11th to 20thNight
19%37%13%20%10%1
1598
21st to endNight
23%39%11%17%9%1
1823
September1st to 10thNight
24%37%8%19%10%1
1823
11th to 20thNight
26%38%13%19%4
1980
21st to endNight
29%39%12%15%4
1980
October1st to 10thNight
28%39%8%17%7%1
2001
11th to 20thNight
41%39%9%9%3
2245
21st to endNight
43%40%8%8%2
2574
November1st to 10thNight
32%44%8%12%4
2406
11th to 20thNight
43%43%6%5%4
2493
21st to endNight
45%43%7%4
2707
December1st to 10thNight
49%43%6%2
2850
11th to 20thNight
50%38%7%41
3525
21st to endNight
64%29%431
5204

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
21%20%33%21%6%
270
11th to 20thDay
21%17%39%17%6%1
270
21st to endDay
20%18%32%24%6%
297
February1st to 10thDay
17%26%35%17%41
270
11th to 20thDay
16%28%34%17%4
270
21st to endDay
22%27%34%16%
225
March1st to 10thDay
18%20%39%17%7%
261
11th to 20thDay
17%14%41%20%6%2
387
21st to endDay
25%15%26%26%8%1
462
April1st to 10thDay
23%15%33%21%6%1
420
11th to 20thDay
25%17%27%20%9%2
412
21st to endDay
20%16%34%20%9%2
420
May1st to 10thDay
22%16%29%22%9%3
423
11th to 20thDay
16%10%38%24%8%4
526
21st to endDay
13%11%29%31%13%3
679
June1st to 10thDay
14%10%32%31%11%2
630
11th to 20thDay
13%8%32%33%11%3
630
21st to endDay
19%7%29%30%13%2
630
July1st to 10thDay
9%5%31%33%18%4
621
11th to 20thDay
12%7%29%30%17%5%
618
21st to endDay
11%7%26%35%17%5%
684
August1st to 10thDay
11%5%24%35%19%6%
528
11th to 20thDay
10%7%20%31%23%9%
439
21st to endDay
16%9%23%30%20%3
495
September1st to 10thDay
17%9%24%26%17%6%
450
11th to 20thDay
14%8%24%29%19%6%
450
21st to endDay
17%12%25%28%14%4
450
October1st to 10thDay
24%14%27%21%14%2
450
11th to 20thDay
29%20%28%19%41
360
21st to endDay
33%15%31%16%31
297
November1st to 10thDay
24%20%33%17%41
270
11th to 20thDay
21%29%31%14%5%
259
21st to endDay
25%28%32%14%1
180
December1st to 10thDay
21%23%34%16%42
180
11th to 20thDay
25%27%31%13%31
180
21st to endDay
30%25%28%12%31
204
January1st to 10thNight
21%18%32%23%5%
950
11th to 20thNight
22%17%31%22%7%
950
21st to endNight
20%12%34%27%6%1
1045
February1st to 10thNight
16%16%36%26%5%
837
11th to 20thNight
18%15%33%26%7%
743
21st to endNight
20%14%36%25%5%
628
March1st to 10thNight
17%12%34%28%9%1
751
11th to 20thNight
16%14%35%27%7%
720
21st to endNight
19%9%31%31%10%1
761
April1st to 10thNight
24%10%30%29%6%1
540
11th to 20thNight
22%11%28%27%11%1
533
21st to endNight
19%10%30%30%11%1
540
May1st to 10thNight
18%6%28%34%14%
540
11th to 20thNight
19%5%28%34%14%1
531
21st to endNight
12%5%27%34%21%1
499
June1st to 10thNight
17%425%38%15%1
360
11th to 20thNight
12%425%38%19%1
360
21st to endNight
15%5%23%38%18%2
360
July1st to 10thNight
11%320%43%20%3
435
11th to 20thNight
10%423%31%27%5%
522
21st to endNight
10%320%40%24%3
591
August1st to 10thNight
9%420%41%24%2
537
11th to 20thNight
9%320%31%30%5%
534
21st to endNight
12%8%22%37%20%1
594
September1st to 10thNight
15%323%32%22%4
563
11th to 20thNight
17%8%23%30%18%3
720
21st to endNight
21%8%21%30%16%4
720
October1st to 10thNight
21%11%25%28%14%1
720
11th to 20thNight
30%14%25%24%6%
720
21st to endNight
34%12%26%23%5%
792
November1st to 10thNight
26%14%33%20%7%
740
11th to 20thNight
25%20%30%20%6%
760
21st to endNight
31%19%33%13%3
807
December1st to 10thNight
26%19%33%16%6%
950
11th to 20thNight
26%20%33%15%5%1
950
21st to endNight
31%16%33%17%3
1045
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