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 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 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	8% 6% 3 5% 5% 3 5% 5% 5% 9% 47%	74695
	Night	9% 8% 3 6% 5% 3 5% 4 5% 7% 45%	75102
Transparency	Day	24% 36% 5% 11% 15% 8%	74417
	Night	26% 36% 6% 11% 12% 9%	74902
Seeing	Day	10% 11% 26% 31% 17% 5%	14330
	Night	11% 7% 13% 30% 30% 8%	23768

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	18% 7% 4 6% 5% 2 4 4 6% 9% 35%	1656
	11th to 20th	Day	11% 6% 3 5% 4 4 3 5% 5% 7% 47%	1660
	21st to end	Day	14% 7% 3 6% 5% 3 5% 5% 5% 7% 41%	1782
February	1st to 10th	Day	16% 6% 2 5% 4 2 5% 4 4 6% 45%	1620
	11th to 20th	Day	12% 7% 3 6% 4 3 5% 4 6% 7% 42%	1643
	21st to end	Day	11% 8% 3 5% 4 3 3 4 5% 6% 48%	1480
March	1st to 10th	Day	15% 8% 4 7% 5% 3 5% 4 6% 9% 35%	1966
	11th to 20th	Day	13% 9% 4 7% 6% 3 5% 5% 6% 8% 35%	1980
	21st to end	Day	11% 7% 2 5% 5% 3 5% 5% 5% 8% 43%	2193
April	1st to 10th	Day	12% 8% 3 5% 5% 3 5% 5% 5% 7% 42%	2160
	11th to 20th	Day	9% 6% 2 4 5% 3 3 4 5% 8% 51%	2129
	21st to end	Day	6% 5% 2 5% 4 2 4 4 6% 10% 51%	2160
May	1st to 10th	Day	5% 5% 2 5% 5% 3 5% 4 6% 9% 50%	2245
	11th to 20th	Day	8% 6% 3 4 4 3 5% 4 5% 10% 47%	2309
	21st to end	Day	8% 11% 3 6% 4 4 4 5% 5% 8% 43%	2723
June	1st to 10th	Day	7% 7% 2 4 5% 3 6% 5% 5% 9% 48%	2520
	11th to 20th	Day	2 3 2 4 3 3 4 5% 5% 9% 60%	2520
	21st to end	Day	3 6% 2 5% 4 3 6% 5% 4 9% 54%	2520
July	1st to 10th	Day	1 6% 3 6% 5% 4 6% 6% 5% 11% 48%	2486
	11th to 20th	Day	2 6% 2 6% 5% 4 6% 5% 7% 13% 43%	2340
	21st to end	Day	1 5% 2 4 5% 3 5% 7% 6% 14% 47%	2546
August	1st to 10th	Day	3 8% 3 7% 6% 4 7% 6% 8% 11% 38%	2328
	11th to 20th	Day	1 5% 2 4 5% 3 5% 5% 6% 12% 51%	2201
	21st to end	Day	1 6% 3 5% 4 3 6% 6% 7% 12% 47%	2376
September	1st to 10th	Day	2 4 2 3 3 2 4 4 5% 9% 61%	2160
	11th to 20th	Day	4 7% 2 5% 5% 3 5% 5% 6% 10% 48%	2160
	21st to end	Day	3 3 2 2 2 2 4 4 4 9% 65%	1899
October	1st to 10th	Day	4 4 2 4 4 2 4 3 5% 8% 62%	1800
	11th to 20th	Day	7% 5% 2 5% 4 3 3 4 5% 7% 55%	1800
	21st to end	Day	8% 5% 2 4 5% 2 4 4 5% 7% 55%	1980
November	1st to 10th	Day	7% 5% 2 5% 5% 3 4 5% 5% 10% 50%	1857
	11th to 20th	Day	8% 7% 3 6% 5% 4 5% 5% 5% 6% 47%	1710
	21st to end	Day	10% 5% 3 4 4 2 3 4 5% 7% 52%	1544
December	1st to 10th	Day	12% 6% 4 5% 5% 3 5% 5% 6% 7% 43%	1520
	11th to 20th	Day	23% 10% 4 6% 6% 3 4 3 5% 6% 30%	1928
	21st to end	Day	18% 10% 3 7% 5% 3 4 4 5% 9% 32%	2794
January	1st to 10th	Night	16% 9% 3 5% 5% 3 4 6% 6% 8% 36%	2505
	11th to 20th	Night	13% 7% 3 6% 6% 3 4 5% 4 6% 43%	2405
	21st to end	Night	15% 7% 3 7% 5% 2 4 4 5% 7% 41%	2574
February	1st to 10th	Night	13% 8% 2 5% 4 3 4 4 4 6% 47%	2340
	11th to 20th	Night	10% 7% 3 6% 5% 3 6% 4 5% 8% 42%	2204
	21st to end	Night	11% 8% 2 5% 6% 3 4 4 5% 7% 46%	1776
March	1st to 10th	Night	16% 9% 4 6% 5% 2 4 6% 6% 6% 37%	1990
	11th to 20th	Night	14% 9% 3 6% 5% 3 4 4 6% 8% 39%	1980
	21st to end	Night	12% 10% 3 5% 4 3 3 3 4 6% 46%	2161
April	1st to 10th	Night	10% 8% 4 5% 4 3 5% 5% 6% 7% 43%	1800
	11th to 20th	Night	10% 8% 2 5% 4 2 4 4 4 7% 50%	1779
	21st to end	Night	5% 5% 2 4 4 3 6% 5% 4 9% 54%	1800
May	1st to 10th	Night	4 6% 3 6% 5% 4 5% 4 5% 7% 52%	1711
	11th to 20th	Night	5% 10% 3 5% 6% 3 5% 4 4 7% 48%	1580
	21st to end	Night	7% 11% 5% 6% 5% 4 4 4 4 7% 42%	1552
June	1st to 10th	Night	4 8% 3 6% 5% 3 6% 5% 5% 9% 45%	1440
	11th to 20th	Night	1 4 2 5% 5% 3 5% 5% 5% 7% 59%	1440
	21st to end	Night	2 10% 2 5% 5% 2 4 5% 5% 6% 55%	1440
July	1st to 10th	Night	1 11% 3 5% 5% 2 5% 5% 5% 10% 48%	1468
	11th to 20th	Night	2 8% 3 5% 6% 4 8% 7% 6% 8% 43%	1620
	21st to end	Night	1 11% 5% 10% 7% 5% 6% 6% 6% 8% 34%	1773
August	1st to 10th	Night	3 13% 6% 8% 8% 5% 5% 5% 5% 9% 34%	1611
	11th to 20th	Night	1 6% 3 8% 8% 4 7% 6% 7% 10% 39%	1700
	21st to end	Night	1 8% 4 8% 7% 5% 6% 6% 5% 9% 41%	1980
September	1st to 10th	Night	1 5% 2 5% 5% 4 5% 4 4 8% 57%	1800
	11th to 20th	Night	4 9% 3 6% 6% 5% 4 4 5% 7% 48%	1800
	21st to end	Night	2 5% 2 4 4 2 3 3 4 7% 64%	2058
October	1st to 10th	Night	5% 6% 2 6% 4 2 3 4 4 5% 60%	2160
	11th to 20th	Night	5% 5% 3 6% 6% 2 4 3 5% 7% 54%	2160
	21st to end	Night	6% 6% 2 5% 4 3 4 5% 5% 6% 55%	2376
November	1st to 10th	Night	8% 6% 2 5% 4 2 4 4 5% 7% 52%	2220
	11th to 20th	Night	9% 7% 3 5% 5% 3 4 4 6% 8% 46%	2470
	21st to end	Night	9% 4 2 4 5% 3 4 4 5% 7% 55%	2631
December	1st to 10th	Night	13% 7% 3 5% 6% 3 5% 4 4 6% 44%	2660
	11th to 20th	Night	25% 9% 3 6% 5% 2 4 4 5% 6% 29%	3276
	21st to end	Night	16% 8% 3 7% 7% 4 5% 5% 5% 6% 34%	4862

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	28% 40% 10% 12% 7% 3	1656
	11th to 20th	Day	18% 41% 6% 12% 14% 9%	1660
	21st to end	Day	29% 38% 6% 7% 15% 5%	1782
February	1st to 10th	Day	25% 36% 4 13% 14% 8%	1620
	11th to 20th	Day	27% 38% 7% 13% 12% 4	1652
	21st to end	Day	28% 36% 5% 15% 12% 4	1480
March	1st to 10th	Day	28% 37% 9% 15% 10% 2	1966
	11th to 20th	Day	33% 36% 9% 13% 7% 2	1980
	21st to end	Day	29% 37% 8% 14% 10% 2	2193
April	1st to 10th	Day	29% 36% 8% 12% 12% 3	2160
	11th to 20th	Day	24% 35% 11% 14% 12% 4	2132
	21st to end	Day	21% 35% 8% 19% 14% 3	2160
May	1st to 10th	Day	21% 36% 5% 15% 19% 4	2245
	11th to 20th	Day	20% 37% 6% 18% 15% 5%	2312
	21st to end	Day	25% 37% 9% 13% 14% 1	2724
June	1st to 10th	Day	26% 36% 8% 14% 13% 2	2520
	11th to 20th	Day	17% 35% 5% 12% 22% 7%	2520
	21st to end	Day	20% 37% 1 8% 21% 12%	2520
July	1st to 10th	Day	19% 37% 1 4 15% 22%	2418
	11th to 20th	Day	22% 38% 2 5% 17% 16%	2271
	21st to end	Day	20% 38% 1 5% 18% 18%	2405
August	1st to 10th	Day	28% 36% 2 5% 18% 11%	2312
	11th to 20th	Day	16% 35% 1 8% 23% 17%	2201
	21st to end	Day	23% 35% 2 6% 18% 16%	2376
September	1st to 10th	Day	14% 36% 2 5% 20% 25%	2160
	11th to 20th	Day	25% 36% 3 13% 16% 7%	2160
	21st to end	Day	12% 35% 3 15% 20% 15%	1899
October	1st to 10th	Day	17% 36% 6% 12% 18% 11%	1800
	11th to 20th	Day	19% 36% 5% 14% 15% 12%	1800
	21st to end	Day	20% 36% 6% 14% 12% 11%	1980
November	1st to 10th	Day	25% 41% 7% 12% 9% 5%	1857
	11th to 20th	Day	23% 41% 5% 12% 14% 5%	1710
	21st to end	Day	19% 39% 6% 11% 17% 8%	1544
December	1st to 10th	Day	22% 39% 7% 13% 14% 5%	1520
	11th to 20th	Day	42% 32% 7% 10% 8% 2	1928
	21st to end	Day	43% 26% 10% 13% 6% 1	2794
January	1st to 10th	Night	28% 40% 9% 14% 6% 3	2505
	11th to 20th	Night	23% 40% 7% 10% 11% 9%	2405
	21st to end	Night	29% 38% 7% 7% 12% 8%	2574
February	1st to 10th	Night	24% 36% 5% 13% 14% 9%	2340
	11th to 20th	Night	27% 38% 5% 12% 13% 4	2206
	21st to end	Night	27% 36% 8% 16% 11% 3	1776
March	1st to 10th	Night	30% 36% 9% 13% 9% 3	1990
	11th to 20th	Night	32% 36% 6% 15% 9% 3	1980
	21st to end	Night	29% 36% 9% 13% 10% 3	2161
April	1st to 10th	Night	28% 36% 10% 13% 10% 3	1800
	11th to 20th	Night	25% 35% 8% 14% 13% 5%	1779
	21st to end	Night	19% 36% 6% 20% 13% 5%	1800
May	1st to 10th	Night	20% 35% 5% 14% 19% 7%	1711
	11th to 20th	Night	23% 37% 4 13% 18% 5%	1581
	21st to end	Night	25% 36% 8% 15% 15% 1	1552
June	1st to 10th	Night	26% 36% 7% 13% 14% 2	1440
	11th to 20th	Night	16% 36% 5% 9% 22% 12%	1440
	21st to end	Night	24% 35% 2 8% 16% 16%	1440
July	1st to 10th	Night	19% 35% 1 4 15% 26%	1427
	11th to 20th	Night	24% 38% 2 5% 9% 21%	1566
	21st to end	Night	34% 37% 1 2 11% 16%	1674
August	1st to 10th	Night	32% 35% 1 6% 12% 14%	1602
	11th to 20th	Night	26% 37% 1 4 13% 18%	1700
	21st to end	Night	25% 35% 1 4 15% 19%	1980
September	1st to 10th	Night	16% 36% 1 5% 13% 29%	1800
	11th to 20th	Night	29% 35% 3 10% 14% 9%	1800
	21st to end	Night	15% 36% 2 12% 20% 16%	2058
October	1st to 10th	Night	21% 37% 5% 11% 13% 13%	2160
	11th to 20th	Night	21% 37% 3 12% 13% 15%	2160
	21st to end	Night	22% 36% 3 12% 15% 12%	2376
November	1st to 10th	Night	24% 41% 8% 11% 10% 7%	2220
	11th to 20th	Night	21% 40% 6% 15% 12% 5%	2470
	21st to end	Night	18% 39% 6% 12% 15% 11%	2631
December	1st to 10th	Night	23% 40% 4 11% 15% 7%	2660
	11th to 20th	Night	42% 33% 6% 9% 9% 1	3276
	21st to end	Night	43% 27% 12% 10% 5% 2	4862

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% 13% 24% 21% 12% 3	317
	11th to 20th	Day	19% 12% 22% 29% 16% 2	354
	21st to end	Day	16% 11% 26% 26% 18% 3	374
February	1st to 10th	Day	19% 14% 25% 24% 15% 4	340
	11th to 20th	Day	19% 17% 27% 23% 12% 1	365
	21st to end	Day	15% 18% 29% 23% 12% 1	346
March	1st to 10th	Day	22% 18% 27% 25% 8% 1	420
	11th to 20th	Day	20% 18% 25% 20% 14% 3	420
	21st to end	Day	16% 18% 32% 25% 7% 2	462
April	1st to 10th	Day	18% 17% 32% 22% 10% 2	420
	11th to 20th	Day	16% 18% 31% 23% 8% 4	412
	21st to end	Day	7% 22% 31% 22% 11% 6%	420
May	1st to 10th	Day	7% 17% 30% 30% 12% 3	423
	11th to 20th	Day	13% 16% 34% 25% 10% 2	448
	21st to end	Day	13% 17% 25% 28% 13% 4	485
June	1st to 10th	Day	8% 12% 32% 29% 14% 4	450
	11th to 20th	Day	3 13% 33% 35% 12% 4	450
	21st to end	Day	2 8% 26% 41% 19% 4	450
July	1st to 10th	Day	5% 5% 24% 46% 18% 2	446
	11th to 20th	Day	1 3 22% 48% 23% 3	444
	21st to end	Day	1 3 22% 45% 22% 6%	487
August	1st to 10th	Day	4 6% 27% 40% 19% 4	447
	11th to 20th	Day	3 2 24% 41% 23% 6%	437
	21st to end	Day	2 3 22% 45% 21% 7%	495
September	1st to 10th	Day	4 4 20% 39% 26% 8%	450
	11th to 20th	Day	4 10% 25% 36% 18% 7%	450
	21st to end	Day	4 8% 20% 31% 29% 7%	450
October	1st to 10th	Day	4 8% 27% 30% 23% 10%	450
	11th to 20th	Day	7% 8% 20% 28% 27% 9%	450
	21st to end	Day	9% 8% 23% 29% 22% 8%	495
November	1st to 10th	Day	9% 11% 21% 27% 24% 8%	453
	11th to 20th	Day	7% 9% 16% 34% 25% 9%	270
	21st to end	Day	9% 9% 24% 24% 32% 2	192
December	1st to 10th	Day	14% 13% 18% 28% 22% 4	180
	11th to 20th	Day	16% 14% 21% 25% 19% 5%	180
	21st to end	Day	23% 11% 28% 23% 13% 3	198
January	1st to 10th	Night	21% 7% 13% 31% 23% 5%	760
	11th to 20th	Night	16% 8% 12% 31% 26% 7%	740
	21st to end	Night	19% 6% 10% 33% 26% 5%	792
February	1st to 10th	Night	15% 8% 14% 33% 25% 4	720
	11th to 20th	Night	14% 11% 16% 30% 23% 6%	704
	21st to end	Night	15% 13% 18% 31% 20% 3	592
March	1st to 10th	Night	19% 12% 15% 29% 22% 3	720
	11th to 20th	Night	18% 9% 14% 31% 23% 5%	720
	21st to end	Night	16% 11% 20% 31% 19% 3	792
April	1st to 10th	Night	15% 9% 21% 31% 21% 3	720
	11th to 20th	Night	14% 15% 18% 26% 20% 6%	711
	21st to end	Night	8% 11% 25% 26% 23% 7%	720
May	1st to 10th	Night	7% 7% 21% 31% 28% 6%	631
	11th to 20th	Night	10% 5% 19% 31% 29% 7%	518
	21st to end	Night	12% 12% 17% 30% 23% 7%	389
June	1st to 10th	Night	7% 6% 23% 32% 27% 5%	360
	11th to 20th	Night	3 6% 16% 38% 33% 4	360
	21st to end	Night	5% 4 10% 35% 39% 6%	360
July	1st to 10th	Night	6% 3 12% 26% 44% 9%	377
	11th to 20th	Night	3 3 4 31% 43% 16%	522
	21st to end	Night	3 5% 4 26% 49% 13%	591
August	1st to 10th	Night	9% 5% 6% 25% 43% 11%	537
	11th to 20th	Night	4 3 8% 29% 47% 9%	632
	21st to end	Night	2 3 8% 30% 43% 14%	792
September	1st to 10th	Night	3 2 8% 29% 43% 14%	720
	11th to 20th	Night	7% 4 11% 29% 40% 10%	720
	21st to end	Night	3 4 10% 28% 42% 13%	720
October	1st to 10th	Night	7% 3 10% 30% 37% 12%	720
	11th to 20th	Night	8% 5% 9% 25% 40% 13%	720
	21st to end	Night	8% 5% 8% 32% 31% 15%	792
November	1st to 10th	Night	9% 6% 13% 25% 35% 12%	740
	11th to 20th	Night	10% 4 11% 33% 33% 8%	760
	21st to end	Night	11% 5% 12% 39% 28% 7%	760
December	1st to 10th	Night	15% 6% 16% 31% 26% 7%	760
	11th to 20th	Night	22% 9% 14% 30% 21% 4	760
	21st to end	Night	21% 10% 19% 28% 20% 3	836

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