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 2005-11-06 to 2026-03-18.

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 Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(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

By Month

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 Forecast	Data Points
Cloud Cover	Day	6% 7% 2 6% 4 4 5% 4 6% 9% 48%	139904
	Night	8% 10% 3 7% 4 4 4 3 5% 6% 45%	139094
Transparency	Day	29% 6% 8% 17% 23% 17%	134828
	Night	36% 6% 9% 16% 19% 15%	135158
Seeing	Day	11% 11% 27% 30% 17% 4	17185
	Night	12% 7% 14% 31% 29% 7%	26634

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	15% 6% 2 6% 3 4 4 3 6% 9% 42%	5160
	11th to 20th	Day	8% 6% 2 6% 4 4 3 3 5% 6% 54%	5340
	21st to end	Day	13% 10% 3 8% 4 4 5% 4 5% 7% 37%	5843
February	1st to 10th	Day	16% 7% 2 6% 4 3 5% 4 4 5% 44%	1890
	11th to 20th	Day	13% 8% 3 7% 5% 5% 5% 3 6% 7% 38%	1924
	21st to end	Day	10% 9% 3 5% 4 4 4 4 6% 6% 45%	1730
March	1st to 10th	Day	12% 8% 4 10% 6% 6% 5% 5% 6% 8% 31%	5034
	11th to 20th	Day	9% 7% 3 6% 4 4 4 4 5% 9% 45%	5078
	21st to end	Day	10% 8% 2 8% 7% 6% 6% 5% 7% 9% 32%	5526
April	1st to 10th	Day	12% 9% 2 6% 4 4 5% 4 5% 7% 43%	2400
	11th to 20th	Day	8% 6% 2 5% 4 4 4 4 5% 9% 49%	2369
	21st to end	Day	6% 6% 2 6% 4 4 4 3 7% 9% 50%	2400
May	1st to 10th	Day	4 3 1 3 2 3 3 3 5% 7% 65%	5655
	11th to 20th	Day	3 4 2 3 3 3 4 3 5% 9% 61%	5875
	21st to end	Day	3 9% 2 5% 4 5% 6% 4 5% 6% 51%	6902
June	1st to 10th	Day	6% 7% 2 5% 4 4 6% 4 6% 9% 47%	2800
	11th to 20th	Day	1 4 1 4 3 4 4 4 6% 10% 59%	2800
	21st to end	Day	3 6% 2 5% 4 4 5% 4 4 9% 55%	2800
July	1st to 10th	Day	1 3 1 4 3 3 4 4 5% 10% 61%	6233
	11th to 20th	Day	1 3 2 6% 4 4 6% 6% 7% 15% 47%	5880
	21st to end	Day	1 16% 4 8% 4 5% 8% 7% 7% 11% 31%	6402
August	1st to 10th	Day	1 4 2 6% 4 5% 9% 6% 8% 14% 40%	5777
	11th to 20th	Day	2 7% 2 5% 3 4 5% 4 6% 10% 52%	5543
	21st to end	Day	1 4 2 6% 4 4 5% 4 8% 13% 48%	5984
September	1st to 10th	Day	2 5% 2 4 3 3 4 3 5% 9% 59%	2400
	11th to 20th	Day	4 8% 2 6% 4 4 4 4 6% 10% 47%	2400
	21st to end	Day	3 4 2 3 2 3 3 3 5% 9% 63%	2108
October	1st to 10th	Day	2 4 1 4 3 3 3 3 5% 7% 66%	3280
	11th to 20th	Day	5% 8% 2 6% 4 3 4 3 4 6% 54%	3261
	21st to end	Day	6% 5% 2 5% 4 3 3 4 6% 8% 54%	3558
November	1st to 10th	Day	6% 5% 2 6% 4 4 4 4 5% 9% 51%	2056
	11th to 20th	Day	9% 8% 2 8% 4 5% 4 4 5% 6% 45%	1890
	21st to end	Day	11% 6% 2 5% 3 3 4 4 5% 7% 51%	1706
December	1st to 10th	Day	7% 4 2 5% 3 4 3 4 7% 10% 52%	2700
	11th to 20th	Day	19% 9% 3 7% 5% 4 5% 3 7% 9% 30%	3108
	21st to end	Day	21% 11% 3 8% 5% 4 4 3 5% 7% 28%	4092
January	1st to 10th	Night	14% 8% 3 7% 4 4 4 3 5% 7% 41%	7740
	11th to 20th	Night	9% 6% 3 6% 4 3 4 3 4 6% 51%	7650
	21st to end	Night	13% 12% 4 8% 4 4 4 3 5% 7% 37%	8371
February	1st to 10th	Night	14% 8% 2 6% 4 3 4 3 5% 5% 46%	2730
	11th to 20th	Night	12% 8% 3 8% 5% 5% 4 3 5% 7% 39%	2583
	21st to end	Night	10% 9% 2 6% 5% 3 4 3 5% 7% 46%	2076
March	1st to 10th	Night	14% 13% 4 9% 5% 4 4 4 5% 6% 34%	5063
	11th to 20th	Night	13% 9% 3 6% 4 3 3 2 4 6% 47%	5077
	21st to end	Night	14% 14% 4 8% 4 3 4 3 5% 7% 35%	5447
April	1st to 10th	Night	8% 10% 3 6% 4 5% 4 4 6% 6% 44%	2000
	11th to 20th	Night	9% 9% 3 6% 3 3 4 4 4 7% 49%	1979
	21st to end	Night	5% 6% 2 5% 4 4 5% 4 4 9% 52%	2000
May	1st to 10th	Night	6% 5% 2 4 2 3 3 3 4 5% 65%	4331
	11th to 20th	Night	2 9% 2 6% 4 5% 5% 4 4 5% 54%	4032
	21st to end	Night	4 12% 3 4 2 3 2 3 4 4 58%	3972
June	1st to 10th	Night	4 9% 3 7% 5% 5% 5% 5% 5% 9% 44%	1600
	11th to 20th	Night	1 4 2 5% 3 4 5% 3 5% 7% 60%	1600
	21st to end	Night	2 10% 3 5% 4 4 4 3 5% 6% 55%	1600
July	1st to 10th	Night	7% 4 6% 4 3 4 6% 5% 6% 55%	3724
	11th to 20th	Night	1 10% 8% 10% 7% 5% 6% 5% 8% 6% 35%	4100
	21st to end	Night	2 37% 4 8% 3 4 3 4 5% 5% 26%	4472
August	1st to 10th	Night	2 10% 9% 12% 5% 4 6% 4 5% 6% 36%	4035
	11th to 20th	Night	16% 3 7% 5% 4 5% 4 5% 6% 45%	4318
	21st to end	Night	1 10% 7% 10% 5% 5% 5% 5% 5% 6% 41%	5015
September	1st to 10th	Night	2 7% 2 5% 5% 5% 4 3 5% 8% 54%	2000
	11th to 20th	Night	4 10% 4 8% 5% 6% 4 3 5% 6% 46%	2000
	21st to end	Night	2 6% 2 5% 4 3 3 2 4 7% 62%	2288
October	1st to 10th	Night	3 4 1 5% 3 2 3 3 4 5% 65%	3920
	11th to 20th	Night	4 13% 3 7% 5% 3 4 3 4 5% 50%	3897
	21st to end	Night	6% 6% 2 5% 3 4 4 4 5% 6% 56%	4253
November	1st to 10th	Night	7% 6% 2 6% 4 4 3 3 6% 7% 52%	2460
	11th to 20th	Night	10% 8% 3 6% 4 4 4 4 6% 7% 45%	2730
	21st to end	Night	9% 4 2 5% 4 4 4 3 5% 7% 53%	2909
December	1st to 10th	Night	7% 5% 2 5% 4 4 5% 3 4 7% 55%	4700
	11th to 20th	Night	20% 10% 3 7% 4 4 5% 4 6% 8% 29%	5316
	21st to end	Night	22% 9% 3 8% 6% 5% 5% 3 4 5% 30%	7106

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	39% 5% 17% 21% 14% 4	5161
	11th to 20th	Day	32% 3 6% 10% 26% 23%	5340
	21st to end	Day	45% 4 7% 14% 16% 14%	5843
February	1st to 10th	Day	41% 5% 7% 19% 17% 11%	1890
	11th to 20th	Day	43% 8% 8% 17% 19% 6%	1933
	21st to end	Day	39% 5% 9% 24% 16% 6%	1730
March	1st to 10th	Day	49% 6% 9% 17% 16% 2	5034
	11th to 20th	Day	35% 5% 15% 26% 10% 8%	5078
	21st to end	Day	43% 9% 16% 20% 10% 1	5526
April	1st to 10th	Day	39% 7% 11% 22% 18% 3	2400
	11th to 20th	Day	32% 6% 15% 23% 19% 5%	2372
	21st to end	Day	29% 7% 12% 27% 22% 4	2400
May	1st to 10th	Day	16% 5% 8% 25% 32% 14%	5655
	11th to 20th	Day	17% 6% 8% 28% 34% 7%	5878
	21st to end	Day	23% 10% 7% 24% 33% 3	6904
June	1st to 10th	Day	29% 9% 10% 23% 22% 7%	2800
	11th to 20th	Day	16% 8% 8% 20% 35% 14%	2800
	21st to end	Day	18% 11% 5% 11% 28% 28%	2800
July	1st to 10th	Day	11% 6% 4 6% 18% 56%	6166
	11th to 20th	Day	14% 6% 6% 3 12% 59%	5811
	21st to end	Day	28% 7% 7% 11% 21% 26%	6217
August	1st to 10th	Day	23% 8% 6% 6% 26% 30%	4206
	11th to 20th	Day	16% 6% 4 7% 41% 26%	4014
	21st to end	Day	19% 8% 5% 9% 31% 28%	4311
September	1st to 10th	Day	17% 7% 4 9% 27% 37%	2400
	11th to 20th	Day	26% 8% 6% 20% 26% 14%	2400
	21st to end	Day	16% 5% 5% 21% 31% 23%	2108
October	1st to 10th	Day	18% 3 5% 12% 26% 37%	3280
	11th to 20th	Day	26% 5% 6% 19% 29% 15%	3261
	21st to end	Day	26% 5% 8% 23% 25% 14%	3558
November	1st to 10th	Day	29% 6% 9% 24% 21% 10%	2056
	11th to 20th	Day	38% 6% 8% 22% 19% 7%	1890
	21st to end	Day	32% 4 10% 18% 27% 9%	1706
December	1st to 10th	Day	27% 4 8% 23% 31% 7%	2700
	11th to 20th	Day	50% 5% 9% 16% 14% 6%	3108
	21st to end	Day	55% 5% 11% 17% 10% 2	4092
January	1st to 10th	Night	42% 6% 16% 19% 14% 3	7740
	11th to 20th	Night	34% 3 9% 9% 25% 20%	7650
	21st to end	Night	47% 4 6% 13% 18% 13%	8371
February	1st to 10th	Night	40% 4 9% 17% 18% 12%	2730
	11th to 20th	Night	43% 7% 8% 18% 18% 6%	2585
	21st to end	Night	36% 6% 11% 27% 15% 5%	2076
March	1st to 10th	Night	51% 6% 11% 15% 15% 2	5063
	11th to 20th	Night	39% 5% 10% 23% 15% 8%	5077
	21st to end	Night	48% 7% 12% 19% 10% 2	5447
April	1st to 10th	Night	38% 6% 13% 22% 16% 4	2000
	11th to 20th	Night	34% 6% 11% 23% 18% 8%	1979
	21st to end	Night	27% 8% 11% 29% 20% 6%	2000
May	1st to 10th	Night	22% 4 8% 23% 29% 15%	4328
	11th to 20th	Night	27% 9% 9% 22% 24% 10%	4033
	21st to end	Night	27% 6% 9% 21% 34% 2	3972
June	1st to 10th	Night	31% 10% 9% 20% 24% 7%	1600
	11th to 20th	Night	17% 11% 7% 16% 33% 16%	1600
	21st to end	Night	23% 9% 4 10% 26% 29%	1600
July	1st to 10th	Night	18% 5% 6% 5% 16% 51%	3683
	11th to 20th	Night	32% 7% 10% 3 7% 42%	4046
	21st to end	Night	53% 6% 4 3 14% 20%	4345
August	1st to 10th	Night	41% 7% 7% 6% 19% 20%	2929
	11th to 20th	Night	26% 8% 4 4 34% 24%	3117
	21st to end	Night	27% 9% 8% 6% 25% 25%	3608
September	1st to 10th	Night	22% 8% 4 8% 22% 36%	2000
	11th to 20th	Night	32% 9% 5% 17% 22% 15%	2000
	21st to end	Night	19% 6% 6% 18% 27% 24%	2288
October	1st to 10th	Night	20% 3 6% 10% 20% 41%	3920
	11th to 20th	Night	33% 6% 5% 17% 21% 19%	3897
	21st to end	Night	27% 5% 6% 19% 24% 20%	4253
November	1st to 10th	Night	30% 6% 10% 22% 19% 12%	2460
	11th to 20th	Night	35% 6% 10% 24% 17% 8%	2730
	21st to end	Night	31% 4 9% 21% 22% 13%	2909
December	1st to 10th	Night	29% 4 6% 21% 29% 11%	4700
	11th to 20th	Night	52% 5% 8% 14% 16% 5%	5316
	21st to end	Night	55% 6% 12% 16% 8% 3	7106

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	28% 14% 25% 20% 11% 2	372
	11th to 20th	Day	17% 12% 22% 32% 15% 2	430
	21st to end	Day	17% 11% 27% 25% 17% 2	473
February	1st to 10th	Day	20% 13% 27% 24% 13% 3	430
	11th to 20th	Day	23% 18% 26% 22% 10% 1	466
	21st to end	Day	15% 18% 30% 25% 11% 1	446
March	1st to 10th	Day	22% 18% 28% 24% 8% 1	540
	11th to 20th	Day	20% 17% 25% 20% 15% 3	529
	21st to end	Day	18% 18% 31% 24% 7% 2	550
April	1st to 10th	Day	18% 16% 32% 21% 10% 2	500
	11th to 20th	Day	14% 18% 32% 22% 9% 4	492
	21st to end	Day	9% 20% 32% 21% 12% 5%	500
May	1st to 10th	Day	7% 17% 33% 29% 11% 3	503
	11th to 20th	Day	13% 14% 33% 27% 11% 2	528
	21st to end	Day	12% 16% 27% 28% 14% 4	573
June	1st to 10th	Day	8% 11% 34% 30% 14% 4	530
	11th to 20th	Day	3 13% 35% 34% 11% 3	530
	21st to end	Day	3 7% 29% 40% 18% 3	530
July	1st to 10th	Day	5% 4 25% 45% 18% 3	526
	11th to 20th	Day	1 3 23% 48% 22% 3	524
	21st to end	Day	2 4 24% 43% 21% 5%	575
August	1st to 10th	Day	4 6% 28% 39% 19% 5%	527
	11th to 20th	Day	4 3 27% 40% 21% 5%	517
	21st to end	Day	2 3 23% 44% 22% 7%	583
September	1st to 10th	Day	4 4 23% 38% 24% 7%	530
	11th to 20th	Day	6% 12% 25% 34% 17% 6%	530
	21st to end	Day	6% 8% 21% 31% 28% 7%	530
October	1st to 10th	Day	4 8% 26% 29% 24% 9%	530
	11th to 20th	Day	8% 8% 22% 28% 25% 8%	530
	21st to end	Day	9% 8% 25% 29% 22% 7%	583
November	1st to 10th	Day	9% 11% 21% 26% 25% 8%	532
	11th to 20th	Day	10% 13% 18% 31% 22% 7%	330
	21st to end	Day	10% 9% 26% 26% 27% 2	234
December	1st to 10th	Day	12% 10% 23% 30% 21% 4	220
	11th to 20th	Day	17% 13% 20% 27% 19% 5%	220
	21st to end	Day	26% 13% 29% 19% 11% 2	242
January	1st to 10th	Night	21% 8% 13% 32% 22% 5%	840
	11th to 20th	Night	15% 8% 13% 32% 26% 6%	840
	21st to end	Night	19% 6% 11% 35% 25% 5%	924
February	1st to 10th	Night	17% 8% 15% 34% 23% 4	840
	11th to 20th	Night	16% 12% 16% 29% 21% 5%	824
	21st to end	Night	14% 12% 17% 35% 19% 3	692
March	1st to 10th	Night	20% 11% 16% 29% 20% 3	840
	11th to 20th	Night	18% 10% 15% 30% 23% 5%	828
	21st to end	Night	17% 11% 20% 31% 18% 3	880
April	1st to 10th	Night	15% 10% 21% 30% 21% 3	800
	11th to 20th	Night	14% 14% 18% 27% 21% 6%	791
	21st to end	Night	9% 11% 25% 25% 23% 6%	800
May	1st to 10th	Night	7% 7% 22% 32% 26% 6%	701
	11th to 20th	Night	10% 5% 20% 31% 28% 6%	576
	21st to end	Night	11% 11% 19% 30% 22% 6%	433
June	1st to 10th	Night	7% 7% 23% 34% 26% 5%	400
	11th to 20th	Night	2 6% 19% 39% 31% 4	400
	21st to end	Night	5% 5% 11% 34% 39% 6%	400
July	1st to 10th	Night	5% 3 13% 28% 43% 9%	421
	11th to 20th	Night	3 2 4 33% 43% 15%	582
	21st to end	Night	4 5% 5% 28% 47% 11%	657
August	1st to 10th	Night	8% 5% 8% 25% 42% 11%	597
	11th to 20th	Night	4 3 11% 30% 43% 9%	704
	21st to end	Night	3 3 9% 30% 42% 13%	880
September	1st to 10th	Night	4 3 9% 29% 42% 13%	800
	11th to 20th	Night	8% 4 13% 28% 38% 9%	800
	21st to end	Night	3 4 11% 29% 41% 12%	800
October	1st to 10th	Night	8% 4 11% 30% 37% 11%	800
	11th to 20th	Night	8% 5% 10% 26% 38% 12%	800
	21st to end	Night	9% 6% 9% 32% 31% 14%	880
November	1st to 10th	Night	9% 6% 13% 25% 35% 11%	820
	11th to 20th	Night	12% 5% 13% 32% 30% 8%	840
	21st to end	Night	11% 5% 12% 40% 27% 6%	840
December	1st to 10th	Night	13% 5% 15% 33% 26% 7%	840
	11th to 20th	Night	22% 8% 13% 31% 21% 4	840
	21st to end	Night	23% 10% 19% 27% 18% 2	924

Page generated at 2026-03-14 23:23:52. Version ac07

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