The Headlands
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Headlands 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
29%
15517
11%
46476
6%
15471
33%
15471
14%
34760

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
16%
900
4%
2506
1%
895
18%
895
5%
2200
January16th to endNight
15%
960
7%
2688
2%
960
20%
960
8%
2262
February1st to 15thNight
17%
753
7%
2315
3%
750
22%
750
8%
1836
February16th to endNight
25%
631
10%
1893
5%
631
31%
631
10%
1517
March1st to 15thNight
27%
720
15%
2121
7%
720
33%
720
18%
1506
March16th to endNight
33%
747
17%
2091
10%
747
41%
747
19%
1450
April1st to 15thNight
29%
539
14%
1767
9%
539
35%
539
16%
1238
April16th to endNight
30%
540
17%
1620
6%
540
40%
540
18%
1094
May1st to 15thNight
31%
540
12%
1620
7%
540
36%
540
17%
1144
May16th to endNight
39%
506
15%
1443
8%
504
47%
504
26%
923
June1st to 15thNight
38%
360
11%
1260
7%
360
45%
360
18%
801
June16th to endNight
46%
360
11%
1260
9%
360
50%
360
21%
773
July1st to 15thNight
51%
450
13%
1300
9%
450
53%
450
29%
796
July16th to endNight
48%
564
14%
1417
9%
531
55%
531
30%
894
August1st to 15thNight
55%
527
18%
1523
14%
524
59%
524
33%
880
August16th to endNight
53%
525
16%
1600
10%
525
59%
525
29%
954
September1st to 15thNight
45%
564
15%
1719
11%
564
48%
564
23%
1107
September16th to endNight
45%
657
15%
1815
10%
657
48%
657
25%
1234
October1st to 15thNight
36%
660
15%
1884
9%
660
41%
660
18%
1359
October16th to endNight
22%
704
7%
2285
4%
704
26%
704
8%
1862
November1st to 15thNight
23%
700
13%
2276
10%
700
28%
700
11%
1742
November16th to endNight
13%
750
5%
2505
3%
750
16%
750
5%
2168
December1st to 15thNight
9%
900
2%
2700
1%
900
10%
900
3%
2461
December16th to endNight
11%
960
4%
2868
1%
960
13%
960
4%
2559

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%9%37%5%3445%6%34%
47493
Night
16%11%5%7%6%34445%29%
46644
Cloud CoverDay
13%9%36%6%3445%7%33%
47502
Night
15%10%48%7%35%446%29%
46649
TransparencyDay
50%7%12%19%9%
47179
Night
55%6%12%16%7%
46476
SeeingDay
17%16%26%24%11%3
5855
Night
18%22%23%24%10%1
15517

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 15thDay
15%16%6%15%11%46%5%3310%
1432
16th to endDay
13%14%7%13%10%5%5%44415%
1536
February1st to 15thDay
12%13%6%9%6%45%5%5%6%24%
1542
16th to endDay
12%11%410%8%45%45%7%24%
1580
March1st to 15thDay
12%10%47%7%33446%35%
1829
16th to endDay
13%8%36%5%2436%6%38%
2143
April1st to 15thDay
19%8%45%42445%7%32%
2181
16th to endDay
15%8%36%5%3445%6%35%
2340
May1st to 15thDay
15%7%45%5%34446%39%
2331
16th to endDay
12%6%244245%5%7%43%
2666
June1st to 15thDay
13%5%25%5%25%5%7%8%38%
2700
16th to endDay
13%5%25%5%35%5%5%8%40%
2700
July1st to 15thDay
9%5%2432445%7%49%
2577
16th to endDay
9%42441346%7%50%
2655
August1st to 15thDay
10%41432445%8%49%
2342
16th to endDay
12%42442346%7%47%
2248
September1st to 15thDay
16%5%25%3345%5%8%40%
1899
16th to endDay
15%5%245%245%5%9%40%
1815
October1st to 15thDay
22%9%36%5%33446%30%
1746
16th to endDay
24%14%48%7%345%5%5%17%
1584
November1st to 15thDay
19%13%5%7%6%34447%22%
1582
16th to endDay
20%20%8%11%7%4333312%
1452
December1st to 15thDay
19%18%11%14%10%333239%
1260
16th to endDay
20%20%7%12%8%3433411%
1353
January1st to 15thNight
17%15%8%12%9%46%33413%
2506
16th to endNight
15%12%7%11%10%5%6%345%16%
2688
February1st to 15thNight
10%14%7%9%8%46%5%5%6%21%
2313
16th to endNight
13%13%5%8%10%45%5%5%5%23%
1893
March1st to 15thNight
13%8%48%6%33445%37%
2108
16th to endNight
13%9%36%425%45%5%38%
2091
April1st to 15thNight
19%8%35%5%3445%6%32%
1767
16th to endNight
15%9%35%435%445%39%
1620
May1st to 15thNight
15%8%37%6%35%45%6%34%
1620
16th to endNight
11%6%25%42445%7%45%
1441
June1st to 15thNight
13%5%25%435%5%49%39%
1260
16th to endNight
12%5%245%345%6%9%40%
1260
July1st to 15thNight
9%5%25%5%245%5%6%46%
1380
16th to endNight
9%435%42445%7%48%
1513
August1st to 15thNight
6%4145%33448%53%
1532
16th to endNight
10%5%1442435%6%50%
1600
September1st to 15thNight
15%6%26%423346%43%
1719
16th to endNight
16%7%25%423446%41%
1815
October1st to 15thNight
20%8%36%5%23436%34%
1884
16th to endNight
26%12%47%6%35%435%20%
2285
November1st to 15thNight
20%14%47%5%24446%24%
2276
16th to endNight
23%19%7%10%6%3433412%
2505
December1st to 15thNight
19%20%8%12%11%3433310%
2700
16th to endNight
20%19%9%11%9%3433311%
2868

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
15%16%5%13%12%5%7%44410%
1432
16th to endDay
12%14%8%12%9%5%6%5%36%14%
1536
February1st to 15thDay
11%13%6%9%7%45%5%5%7%23%
1542
16th to endDay
12%10%5%9%9%45%45%8%23%
1580
March1st to 15thDay
11%10%47%7%3435%5%35%
1839
16th to endDay
12%8%36%6%2445%7%37%
2142
April1st to 15thDay
18%8%45%5%2445%7%32%
2181
16th to endDay
15%8%36%5%3445%6%36%
2340
May1st to 15thDay
15%7%35%435%45%6%38%
2331
16th to endDay
11%6%244245%5%8%43%
2666
June1st to 15thDay
13%5%245%35%5%6%9%38%
2700
16th to endDay
12%6%245%35%5%5%8%40%
2700
July1st to 15thDay
9%5%15%4245%47%48%
2577
16th to endDay
8%4244135%6%9%48%
2655
August1st to 15thDay
9%4143246%5%8%48%
2342
16th to endDay
10%5%1442346%8%47%
2248
September1st to 15thDay
14%5%245%35%5%5%8%40%
1899
16th to endDay
13%6%245%25%45%8%40%
1815
October1st to 15thDay
20%9%26%6%33447%29%
1746
16th to endDay
21%14%48%8%45%45%5%17%
1584
November1st to 15thDay
17%10%5%9%6%45%45%6%22%
1582
16th to endDay
19%18%8%11%8%35%44412%
1452
December1st to 15thDay
18%17%10%14%9%35%3339%
1260
16th to endDay
20%19%6%11%8%35%44311%
1353
January1st to 15thNight
18%14%7%12%9%5%5%445%13%
2506
16th to endNight
14%12%6%11%10%5%6%446%15%
2688
February1st to 15thNight
10%13%7%9%8%46%5%5%6%21%
2306
16th to endNight
13%13%49%9%45%5%46%22%
1893
March1st to 15thNight
13%8%48%6%34445%36%
2121
16th to endNight
13%8%37%5%34446%38%
2091
April1st to 15thNight
19%8%36%5%345%46%33%
1767
16th to endNight
15%9%245%25%5%45%38%
1620
May1st to 15thNight
14%8%37%6%345%5%6%33%
1619
16th to endNight
11%6%2442445%8%44%
1441
June1st to 15thNight
13%5%26%3345%5%9%38%
1260
16th to endNight
11%5%25%5%345%7%9%39%
1260
July1st to 15thNight
8%5%25%5%35%35%7%47%
1380
16th to endNight
8%425%42445%7%48%
1513
August1st to 15thNight
6%424435%348%52%
1532
16th to endNight
8%5%2442435%7%50%
1600
September1st to 15thNight
14%5%35%424346%43%
1719
16th to endNight
15%6%35%5%23347%40%
1815
October1st to 15thNight
19%7%35%5%3445%6%34%
1884
16th to endNight
23%12%48%6%35%45%420%
2285
November1st to 15thNight
19%11%38%6%35%446%24%
2276
16th to endNight
21%16%6%11%8%35%44413%
2505
December1st to 15thNight
19%18%6%13%10%45%33310%
2700
16th to endNight
19%18%8%11%10%45%33311%
2868

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
77%8%6%5%1
1433
16th to endDay
71%7%9%8%2
1536
February1st to 15thDay
62%7%13%11%5%
1544
16th to endDay
60%6%12%15%5%
1580
March1st to 15thDay
51%6%13%19%8%1
1839
16th to endDay
46%6%13%22%8%1
2143
April1st to 15thDay
50%6%10%20%10%
2181
16th to endDay
47%5%11%17%15%2
2340
May1st to 15thDay
45%6%11%21%11%3
2331
16th to endDay
40%7%13%25%12%1
2670
June1st to 15thDay
40%8%13%25%11%1
2700
16th to endDay
41%7%16%22%11%1
2700
July1st to 15thDay
33%6%14%30%13%1
2428
16th to endDay
34%7%13%27%15%
2488
August1st to 15thDay
33%8%15%29%12%
2327
16th to endDay
35%7%18%27%9%1
2248
September1st to 15thDay
43%7%14%26%8%
1899
16th to endDay
40%7%18%24%8%
1815
October1st to 15thDay
54%7%10%19%7%
1746
16th to endDay
68%8%12%7%3
1584
November1st to 15thDay
60%7%12%14%5%
1582
16th to endDay
77%7%8%5%1
1452
December1st to 15thDay
81%5%6%5%1
1260
16th to endDay
79%5%8%5%1
1353
January1st to 15thNight
73%7%9%6%1
2506
16th to endNight
68%6%11%9%3
2688
February1st to 15thNight
62%7%12%12%3
2315
16th to endNight
63%7%11%13%4
1893
March1st to 15thNight
49%5%12%21%9%
2121
16th to endNight
46%5%13%20%12%1
2091
April1st to 15thNight
50%5%9%22%11%1
1767
16th to endNight
45%6%11%18%15%2
1620
May1st to 15thNight
50%6%10%17%11%3
1620
16th to endNight
37%6%12%26%14%2
1443
June1st to 15thNight
40%8%12%21%16%
1260
16th to endNight
38%6%15%26%10%1
1260
July1st to 15thNight
34%6%12%27%17%2
1300
16th to endNight
34%7%15%24%17%
1417
August1st to 15thNight
29%6%17%32%12%1
1523
16th to endNight
33%7%19%26%12%1
1600
September1st to 15thNight
41%6%14%25%11%
1719
16th to endNight
42%6%16%25%7%
1815
October1st to 15thNight
50%6%12%21%8%1
1884
16th to endNight
65%8%11%11%3
2285
November1st to 15thNight
58%7%12%15%5%
2276
16th to endNight
73%7%10%6%2
2505
December1st to 15thNight
80%6%7%4
2700
16th to endNight
77%5%7%6%1
2868

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%15%42%17%4
90
16th to endDay
25%12%29%28%41
96
February1st to 15thDay
22%21%37%14%4
90
16th to endDay
22%17%33%23%3
81
March1st to 15thDay
16%19%29%21%13%
129
16th to endDay
22%16%22%25%9%3
288
April1st to 15thDay
22%19%22%20%13%1
270
16th to endDay
20%21%28%16%10%3
270
May1st to 15thDay
17%22%26%20%10%3
288
16th to endDay
14%19%28%25%9%3
512
June1st to 15thDay
15%18%30%26%7%1
495
16th to endDay
17%16%24%27%11%2
495
July1st to 15thDay
12%13%28%26%15%4
475
16th to endDay
10%17%23%29%12%6%
515
August1st to 15thDay
11%8%21%32%15%8%
351
16th to endDay
18%10%19%25%21%5%
268
September1st to 15thDay
15%11%18%26%17%10%
255
16th to endDay
14%12%23%27%14%5%
254
October1st to 15thDay
24%12%28%22%11%
255
16th to endDay
32%10%31%22%3
96
November1st to 15thDay
18%18%27%24%7%2
90
16th to endDay
20%20%35%20%1
67
December1st to 15thDay
25%26%36%8%3
60
16th to endDay
33%30%30%4
65
January1st to 15thNight
19%29%23%20%6%
900
16th to endNight
16%26%28%22%4
960
February1st to 15thNight
16%26%29%22%4
753
16th to endNight
17%21%25%27%7%
631
March1st to 15thNight
15%24%26%24%8%
720
16th to endNight
16%22%22%27%9%
747
April1st to 15thNight
19%22%24%25%7%
539
16th to endNight
17%23%25%20%11%
540
May1st to 15thNight
16%20%24%26%11%
540
16th to endNight
12%20%23%27%14%1
506
June1st to 15thNight
15%19%22%32%9%
360
16th to endNight
13%19%15%32%17%1
360
July1st to 15thNight
10%13%17%34%18%3
450
16th to endNight
10%17%17%32%18%3
564
August1st to 15thNight
8%11%17%33%25%3
527
16th to endNight
10%13%18%33%21%2
525
September1st to 15thNight
16%12%22%27%17%4
564
16th to endNight
18%12%17%30%15%5%
657
October1st to 15thNight
22%16%21%26%12%1
660
16th to endNight
31%19%21%22%5%
704
November1st to 15thNight
22%24%25%19%7%
700
16th to endNight
26%29%24%15%3
750
December1st to 15thNight
25%32%26%11%3
900
16th to endNight
24%27%26%15%31
960
Page generated at 2017-08-13 14:16:14. Version ac04

Copyright 2017 A. Danko


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