Half Moon Bay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Half Moon Bay clear sky chart from 2005-11-05 to 2018-03-21.

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
58%
16166
23%
49251
20%
16129
62%
16129
29%
30133

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
56%
777
18%
2554
17%
776
60%
776
24%
1612
January16th to endNight
53%
832
23%
2704
24%
832
57%
832
24%
1713
February1st to 15thNight
53%
760
24%
2483
24%
760
59%
760
25%
1551
February16th to endNight
46%
684
21%
2066
17%
684
53%
684
23%
1351
March1st to 15thNight
50%
780
17%
2152
15%
780
53%
780
24%
1475
March16th to endNight
46%
792
19%
2166
14%
792
51%
792
27%
1439
April1st to 15thNight
44%
719
21%
1799
10%
719
48%
719
28%
1207
April16th to endNight
50%
720
24%
1800
10%
720
55%
720
38%
1092
May1st to 15thNight
57%
599
23%
1679
14%
599
61%
599
38%
971
May16th to endNight
51%
422
19%
1545
15%
422
58%
422
24%
922
June1st to 15thNight
56%
360
20%
1440
19%
360
64%
360
24%
824
June16th to endNight
75%
360
30%
1440
39%
360
78%
360
39%
654
July1st to 15thNight
71%
413
23%
1433
25%
413
74%
413
30%
799
July16th to endNight
62%
564
22%
1593
21%
531
63%
531
22%
1065
August1st to 15thNight
62%
544
20%
1617
17%
541
63%
541
23%
1072
August16th to endNight
68%
764
25%
1910
15%
764
69%
764
32%
1284
September1st to 15thNight
74%
720
32%
1800
22%
720
76%
720
42%
1088
September16th to endNight
72%
716
36%
1973
31%
716
75%
716
48%
1048
October1st to 15thNight
68%
720
26%
2160
27%
720
73%
720
41%
1173
October16th to endNight
66%
768
31%
2304
33%
768
70%
768
35%
1341
November1st to 15thNight
63%
760
24%
2345
25%
760
66%
760
33%
1373
November16th to endNight
61%
780
24%
2646
25%
780
64%
780
28%
1535
December1st to 15thNight
55%
780
22%
2730
20%
780
61%
780
24%
1674
December16th to endNight
46%
832
20%
2912
18%
832
53%
832
20%
1870

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
8%6%2442445%8%47%
49591
Night
8%6%35%5%3445%8%45%
49441
TransparencyDay
34%5%8%18%21%12%
49281
Night
36%5%7%17%19%13%
49252
SeeingDay
10%11%25%31%17%4
6662
Night
10%6%12%28%32%9%
16175

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
16%5%35%423348%42%
1710
16th to endDay
15%5%35%4345%46%42%
1872
February1st to 15thDay
15%6%24425%346%42%
1711
16th to endDay
14%8%35%5%3345%6%39%
1694
March1st to 15thDay
14%8%36%5%3346%8%34%
2135
16th to endDay
11%7%35%6%35%5%47%38%
2183
April1st to 15thDay
12%6%345%245%47%41%
2149
16th to endDay
7%5%24423349%52%
2160
May1st to 15thDay
45%2442445%9%50%
2268
16th to endDay
9%9%35%333448%44%
2565
June1st to 15thDay
6%6%245%35%5%47%48%
2520
16th to endDay
34132233410%60%
2520
July1st to 15thDay
15%24435%5%5%10%50%
2437
16th to endDay
15%144345%5%12%49%
2468
August1st to 15thDay
26%25%5%36%5%6%11%45%
2334
16th to endDay
15%2443446%13%49%
2292
September1st to 15thDay
25%23324349%57%
2160
16th to endDay
4422213348%62%
1985
October1st to 15thDay
7%413323347%59%
1800
16th to endDay
8%5%2442335%6%53%
1920
November1st to 15thDay
7%5%15%4345%6%7%47%
1843
16th to endDay
11%7%35%5%23345%46%
1641
December1st to 15thDay
14%8%45%423446%40%
1560
16th to endDay
17%7%34424347%39%
1664
January1st to 15thNight
15%7%25%43445%8%39%
2554
16th to endNight
15%6%36%423346%43%
2704
February1st to 15thNight
11%7%35%43435%7%43%
2471
16th to endNight
12%8%25%5%35%45%8%38%
2066
March1st to 15thNight
15%7%45%42445%7%37%
2152
16th to endNight
12%9%26%5%3335%7%41%
2166
April1st to 15thNight
10%6%35%435%5%6%8%41%
1799
16th to endNight
7%414425%338%54%
1800
May1st to 15thNight
25%36%33435%7%53%
1679
16th to endNight
6%7%45%5%44447%46%
1544
June1st to 15thNight
36%25%5%35%5%5%8%45%
1440
16th to endNight
142442345%7%60%
1440
July1st to 15thNight
15%245%47%6%7%12%41%
1519
16th to endNight
17%410%8%48%5%6%9%32%
1701
August1st to 15thNight
27%5%9%7%5%6%5%5%11%32%
1626
16th to endNight
5%38%8%5%6%5%7%10%38%
1910
September1st to 15thNight
2325%6%5%5%45%10%49%
1800
16th to endNight
2423422348%61%
1973
October1st to 15thNight
5%425%423336%58%
2160
16th to endNight
6%5%25%42445%6%51%
2304
November1st to 15thNight
8%5%25%42435%7%51%
2345
16th to endNight
9%5%35%423347%49%
2646
December1st to 15thNight
14%7%245%24347%42%
2730
16th to endNight
17%7%25%424345%42%
2912

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
41%411%18%15%8%
1710
16th to endDay
42%5%9%12%18%11%
1872
February1st to 15thDay
42%48%18%20%6%
1711
16th to endDay
44%5%8%24%13%3
1694
March1st to 15thDay
45%5%12%19%12%4
2135
16th to endDay
45%5%13%19%13%2
2183
April1st to 15thDay
42%413%21%15%2
2149
16th to endDay
30%415%24%19%5%
2160
May1st to 15thDay
29%5%7%25%24%6%
2268
16th to endDay
38%5%10%20%19%4
2569
June1st to 15thDay
34%6%8%18%20%11%
2520
16th to endDay
21%4316%33%19%
2520
July1st to 15thDay
26%7%28%21%32%
2294
16th to endDay
27%6%310%27%25%
2313
August1st to 15thDay
32%6%110%30%18%
2318
16th to endDay
25%6%216%30%18%
2292
September1st to 15thDay
24%4214%28%25%
2160
16th to endDay
21%3425%28%16%
1985
October1st to 15thDay
24%47%19%23%19%
1800
16th to endDay
32%310%21%19%12%
1920
November1st to 15thDay
34%5%12%26%14%6%
1843
16th to endDay
39%5%8%17%24%4
1641
December1st to 15thDay
44%310%14%17%9%
1560
16th to endDay
45%411%15%17%4
1664
January1st to 15thNight
42%411%19%14%8%
2554
16th to endNight
42%49%12%16%14%
2704
February1st to 15thNight
41%47%18%19%9%
2483
16th to endNight
42%5%10%24%13%2
2066
March1st to 15thNight
45%410%19%13%5%
2152
16th to endNight
42%412%18%16%5%
2166
April1st to 15thNight
40%5%14%21%16%2
1799
16th to endNight
29%5%11%25%19%8%
1800
May1st to 15thNight
29%5%7%21%26%10%
1680
16th to endNight
36%7%10%20%18%6%
1545
June1st to 15thNight
32%10%8%16%20%12%
1440
16th to endNight
22%4213%30%25%
1440
July1st to 15thNight
27%11%16%19%33%
1433
16th to endNight
43%8%2414%25%
1593
August1st to 15thNight
41%8%18%20%20%
1617
16th to endNight
36%9%19%20%23%
1910
September1st to 15thNight
27%7%39%22%30%
1800
16th to endNight
23%3421%25%21%
1973
October1st to 15thNight
27%37%18%21%21%
2160
16th to endNight
32%45%19%20%16%
2304
November1st to 15thNight
32%6%12%25%16%6%
2345
16th to endNight
36%38%22%20%8%
2646
December1st to 15thNight
40%5%7%15%20%10%
2730
16th to endNight
44%5%10%15%17%6%
2912

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
28%8%20%24%14%3
249
16th to endDay
17%11%22%25%19%3
304
February1st to 15thDay
19%11%24%23%17%2
281
16th to endDay
18%17%28%20%12%1
286
March1st to 15thDay
20%17%22%23%13%1
330
16th to endDay
15%19%33%23%6%1
311
April1st to 15thDay
16%21%33%19%7%1
269
16th to endDay
10%24%31%24%5%2
270
May1st to 15thDay
6%18%33%30%9%1
288
16th to endDay
16%16%21%30%12%3
330
June1st to 15thDay
7%14%35%32%8%1
315
16th to endDay
15%24%43%20%3
315
July1st to 15thDay
2321%45%22%2
305
16th to endDay
1120%48%19%7%
327
August1st to 15thDay
2422%42%24%3
315
16th to endDay
1121%45%24%6%
332
September1st to 15thDay
4421%39%21%9%
315
16th to endDay
48%21%30%25%8%
315
October1st to 15thDay
6%7%27%34%18%6%
315
16th to endDay
9%9%25%24%23%7%
336
November1st to 15thDay
11%7%21%24%25%8%
259
16th to endDay
10%12%22%33%22%
109
December1st to 15thDay
14%12%17%26%20%8%
90
16th to endDay
14%15%15%17%29%7%
96
January1st to 15thNight
18%6%8%30%29%6%
780
16th to endNight
17%6%10%27%29%8%
832
February1st to 15thNight
14%8%12%29%29%5%
764
16th to endNight
16%10%17%27%22%5%
684
March1st to 15thNight
17%9%12%26%28%5%
780
16th to endNight
16%10%18%30%20%4
792
April1st to 15thNight
13%12%20%29%19%4
719
16th to endNight
9%11%21%25%24%7%
720
May1st to 15thNight
5%421%31%29%6%
600
16th to endNight
10%8%16%29%26%8%
423
June1st to 15thNight
5%7%20%35%27%5%
360
16th to endNight
149%35%43%5%
360
July1st to 15thNight
425%25%47%14%
413
16th to endNight
33322%48%18%
564
August1st to 15thNight
5%3422%50%13%
544
16th to endNight
226%25%50%12%
764
September1st to 15thNight
316%24%47%16%
720
16th to endNight
339%26%43%13%
716
October1st to 15thNight
7%49%29%38%11%
720
16th to endNight
9%5%7%26%34%16%
768
November1st to 15thNight
9%411%29%34%10%
760
16th to endNight
11%6%10%32%29%9%
780
December1st to 15thNight
15%7%12%28%27%8%
780
16th to endNight
20%8%16%28%22%3
832
Page generated at 2018-03-22 08:34:25. Version ac04

Copyright 2018 A. Danko


Half Moon Bay Clear Sky Chart |Charts Home