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 2024-12-06.

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	7% 7% 2 6% 4 4 5% 4 6% 9% 48%	105265
	Night	8% 9% 3 7% 4 4 4 3 5% 6% 45%	103217
Transparency	Day	29% 7% 8% 17% 22% 17%	105027
	Night	36% 7% 9% 16% 18% 15%	103045
Seeing	Day	10% 11% 26% 31% 17% 5%	15558
	Night	11% 7% 14% 30% 30% 8%	24919

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	16% 7% 3 7% 4 4 4 3 5% 8% 39%	2767
	11th to 20th	Day	15% 10% 4 10% 6% 5% 4 3 5% 5% 32%	2860
	21st to end	Day	21% 11% 3 8% 4 5% 5% 4 5% 7% 28%	3146
February	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 5% 5% 44%	1710
	11th to 20th	Day	12% 8% 3 7% 5% 5% 4 3 6% 7% 40%	1736
	21st to end	Day	10% 9% 3 5% 4 4 3 4 5% 5% 47%	1570
March	1st to 10th	Day	13% 9% 4 10% 6% 6% 5% 4 6% 7% 28%	3428
	11th to 20th	Day	9% 7% 2 6% 4 3 3 3 5% 9% 49%	3490
	21st to end	Day	12% 9% 2 7% 6% 5% 5% 5% 7% 9% 33%	3852
April	1st to 10th	Day	12% 9% 2 6% 4 4 5% 4 5% 7% 43%	2280
	11th to 20th	Day	9% 6% 2 5% 4 4 3 4 5% 9% 49%	2249
	21st to end	Day	6% 5% 2 6% 4 4 4 3 7% 9% 51%	2280
May	1st to 10th	Day	6% 4 1 4 3 3 3 3 4 6% 65%	3950
	11th to 20th	Day	4 4 2 3 3 3 5% 3 6% 10% 57%	4092
	21st to end	Day	4 11% 2 6% 4 4 4 3 5% 6% 51%	4813
June	1st to 10th	Day	6% 7% 2 5% 4 4 5% 4 5% 9% 48%	2660
	11th to 20th	Day	1 4 1 4 3 4 4 4 5% 9% 60%	2660
	21st to end	Day	3 6% 2 5% 4 5% 5% 4 4 10% 54%	2660
July	1st to 10th	Day	1 4 2 4 2 3 3 3 4 8% 68%	4360
	11th to 20th	Day	1 4 2 5% 4 5% 5% 4 7% 14% 48%	4110
	21st to end	Day	9% 2 7% 4 5% 7% 6% 7% 12% 41%	4455
August	1st to 10th	Day	2 5% 3 7% 5% 6% 10% 7% 10% 16% 30%	4091
	11th to 20th	Day	1 6% 2 5% 3 4 5% 4 5% 9% 55%	3851
	21st to end	Day	1 5% 3 6% 4 5% 6% 4 7% 10% 49%	4180
September	1st to 10th	Day	2 5% 2 4 3 3 4 3 5% 9% 60%	2280
	11th to 20th	Day	4 8% 2 6% 4 4 4 5% 6% 10% 47%	2280
	21st to end	Day	3 3 2 3 2 3 4 3 5% 9% 64%	2003
October	1st to 10th	Day	2 4 1 4 2 3 3 3 5% 7% 66%	3180
	11th to 20th	Day	4 8% 2 6% 4 3 4 3 4 7% 54%	3161
	21st to end	Day	6% 5% 2 5% 4 3 3 4 6% 8% 54%	3448
November	1st to 10th	Day	6% 5% 2 6% 4 4 4 4 5% 9% 51%	1956
	11th to 20th	Day	8% 8% 2 7% 4 5% 4 4 5% 6% 47%	1800
	21st to end	Day	11% 6% 2 5% 3 3 4 4 5% 7% 51%	1625
December	1st to 10th	Day	11% 7% 3 6% 4 5% 4 4 6% 7% 43%	1560
	11th to 20th	Day	22% 10% 3 7% 5% 4 4 2 5% 6% 30%	1928
	21st to end	Day	17% 10% 3 8% 5% 4 5% 3 5% 8% 32%	2794
January	1st to 10th	Night	14% 9% 3 6% 4 4 4 3 5% 7% 41%	4181
	11th to 20th	Night	17% 10% 4 9% 6% 5% 5% 3 4 5% 31%	4110
	21st to end	Night	21% 13% 3 10% 5% 4 4 3 5% 5% 28%	4521
February	1st to 10th	Night	14% 8% 2 5% 4 3 4 3 5% 5% 46%	2470
	11th to 20th	Night	10% 8% 3 7% 5% 5% 5% 3 5% 8% 41%	2331
	21st to end	Night	10% 9% 2 6% 5% 3 4 3 5% 7% 47%	1884
March	1st to 10th	Night	13% 12% 4 9% 5% 4 5% 4 5% 6% 33%	3442
	11th to 20th	Night	11% 9% 3 5% 4 3 3 2 4 5% 50%	3490
	21st to end	Night	12% 16% 4 9% 5% 4 4 3 5% 6% 33%	3799
April	1st to 10th	Night	9% 10% 3 5% 4 5% 3 4 6% 6% 44%	1900
	11th to 20th	Night	9% 8% 2 5% 3 4 4 4 4 7% 49%	1879
	21st to end	Night	5% 5% 2 5% 4 4 5% 4 4 9% 53%	1900
May	1st to 10th	Night	5% 7% 2 5% 2 3 3 2 4 4 63%	3021
	11th to 20th	Night	3 9% 3 7% 5% 5% 6% 4 3 5% 52%	2806
	21st to end	Night	4 11% 3 4 3 3 3 3 3 4 59%	2762
June	1st to 10th	Night	4 9% 3 6% 5% 5% 5% 4 5% 9% 45%	1520
	11th to 20th	Night	1 4 2 6% 3 4 5% 3 5% 7% 59%	1520
	21st to end	Night	2 9% 3 5% 4 4 4 3 5% 6% 55%	1520
July	1st to 10th	Night	7% 3 5% 2 2 4 2 4 7% 64%	2596
	11th to 20th	Night	1 8% 5% 8% 5% 5% 5% 4 6% 7% 44%	2860
	21st to end	Night	1 23% 4 10% 5% 4 5% 5% 7% 8% 29%	3109
August	1st to 10th	Night	3 10% 11% 15% 6% 6% 7% 4 5% 6% 26%	2848
	11th to 20th	Night	1 11% 3 6% 5% 4 5% 4 5% 7% 49%	2994
	21st to end	Night	2 9% 5% 9% 5% 5% 4 4 7% 7% 43%	3498
September	1st to 10th	Night	1 6% 2 5% 5% 5% 4 3 5% 8% 56%	1900
	11th to 20th	Night	4 9% 4 7% 5% 6% 4 3 5% 7% 47%	1900
	21st to end	Night	2 6% 2 5% 3 3 3 2 4 7% 63%	2174
October	1st to 10th	Night	3 4 1 5% 3 2 3 3 4 5% 66%	3800
	11th to 20th	Night	4 14% 3 7% 5% 3 4 3 4 5% 49%	3777
	21st to end	Night	6% 5% 2 5% 3 4 4 4 5% 6% 57%	4121
November	1st to 10th	Night	7% 6% 2 6% 4 4 3 3 6% 7% 52%	2340
	11th to 20th	Night	9% 7% 3 6% 4 4 4 4 6% 7% 47%	2600
	21st to end	Night	9% 4 2 5% 4 4 4 3 5% 7% 53%	2770
December	1st to 10th	Night	12% 7% 3 6% 5% 4 4 3 4 6% 44%	2736
	11th to 20th	Night	25% 10% 3 7% 4 3 4 3 5% 6% 29%	3276
	21st to end	Night	16% 9% 3 8% 6% 5% 5% 4 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	43% 5% 13% 27% 8% 3	2767
	11th to 20th	Day	54% 5% 11% 13% 10% 7%	2860
	21st to end	Day	55% 6% 8% 14% 12% 5%	3146
February	1st to 10th	Day	41% 5% 7% 20% 18% 10%	1710
	11th to 20th	Day	40% 8% 8% 17% 21% 6%	1745
	21st to end	Day	38% 6% 8% 26% 17% 6%	1570
March	1st to 10th	Day	54% 7% 10% 14% 13% 2	3428
	11th to 20th	Day	31% 6% 12% 28% 12% 11%	3490
	21st to end	Day	46% 8% 16% 18% 10% 2	3852
April	1st to 10th	Day	39% 7% 12% 21% 18% 4	2280
	11th to 20th	Day	32% 6% 15% 24% 18% 5%	2252
	21st to end	Day	28% 7% 12% 27% 22% 4	2280
May	1st to 10th	Day	19% 7% 10% 27% 26% 11%	3950
	11th to 20th	Day	18% 8% 7% 23% 34% 10%	4095
	21st to end	Day	28% 12% 8% 23% 27% 3	4814
June	1st to 10th	Day	29% 10% 10% 22% 21% 8%	2660
	11th to 20th	Day	16% 8% 8% 21% 34% 12%	2660
	21st to end	Day	18% 11% 5% 11% 27% 28%	2660
July	1st to 10th	Day	11% 8% 3 4 11% 63%	4292
	11th to 20th	Day	15% 8% 6% 5% 12% 54%	4041
	21st to end	Day	19% 6% 8% 5% 25% 36%	4315
August	1st to 10th	Day	24% 9% 6% 6% 25% 31%	4076
	11th to 20th	Day	16% 6% 4 6% 41% 26%	3890
	21st to end	Day	19% 8% 5% 9% 30% 29%	4179
September	1st to 10th	Day	15% 7% 4 8% 27% 39%	2280
	11th to 20th	Day	26% 9% 6% 20% 26% 15%	2280
	21st to end	Day	15% 6% 5% 21% 32% 23%	2003
October	1st to 10th	Day	17% 3 5% 12% 26% 37%	3180
	11th to 20th	Day	26% 5% 6% 19% 29% 15%	3161
	21st to end	Day	26% 5% 8% 22% 25% 14%	3448
November	1st to 10th	Day	29% 6% 9% 25% 20% 11%	1956
	11th to 20th	Day	36% 6% 9% 23% 20% 7%	1800
	21st to end	Day	33% 4 10% 17% 27% 9%	1625
December	1st to 10th	Day	38% 6% 11% 18% 17% 10%	1560
	11th to 20th	Day	54% 4 9% 14% 14% 4	1928
	21st to end	Day	50% 5% 12% 19% 12% 3	2794
January	1st to 10th	Night	42% 6% 17% 24% 8% 3	4181
	11th to 20th	Night	54% 5% 15% 10% 9% 7%	4110
	21st to end	Night	58% 5% 7% 12% 12% 6%	4521
February	1st to 10th	Night	39% 4 9% 18% 18% 11%	2470
	11th to 20th	Night	41% 7% 8% 18% 20% 7%	2333
	21st to end	Night	35% 6% 11% 29% 15% 4	1884
March	1st to 10th	Night	51% 8% 13% 13% 12% 3	3442
	11th to 20th	Night	36% 5% 9% 23% 18% 10%	3490
	21st to end	Night	52% 7% 13% 16% 9% 3	3799
April	1st to 10th	Night	39% 6% 13% 22% 16% 4	1900
	11th to 20th	Night	34% 6% 11% 24% 18% 7%	1879
	21st to end	Night	26% 9% 11% 28% 20% 7%	1900
May	1st to 10th	Night	25% 5% 10% 24% 24% 13%	3021
	11th to 20th	Night	27% 12% 8% 15% 24% 14%	2807
	21st to end	Night	27% 8% 11% 25% 27% 2	2762
June	1st to 10th	Night	31% 10% 9% 19% 23% 8%	1520
	11th to 20th	Night	18% 11% 7% 17% 32% 16%	1520
	21st to end	Night	23% 9% 4 10% 25% 28%	1520
July	1st to 10th	Night	15% 6% 2 5% 12% 59%	2555
	11th to 20th	Night	27% 10% 7% 5% 9% 44%	2806
	21st to end	Night	42% 8% 5% 3 13% 29%	3011
August	1st to 10th	Night	42% 8% 7% 5% 18% 21%	2839
	11th to 20th	Night	26% 8% 4 4 35% 23%	3021
	21st to end	Night	27% 9% 8% 6% 24% 25%	3498
September	1st to 10th	Night	20% 8% 4 7% 23% 38%	1900
	11th to 20th	Night	30% 9% 5% 17% 22% 15%	1900
	21st to end	Night	18% 6% 6% 19% 27% 25%	2174
October	1st to 10th	Night	19% 3 6% 9% 20% 42%	3800
	11th to 20th	Night	33% 6% 5% 16% 20% 20%	3777
	21st to end	Night	26% 5% 6% 19% 23% 20%	4121
November	1st to 10th	Night	30% 6% 11% 22% 19% 12%	2340
	11th to 20th	Night	33% 7% 11% 24% 18% 8%	2600
	21st to end	Night	32% 4 9% 20% 22% 13%	2770
December	1st to 10th	Night	39% 6% 8% 17% 18% 11%	2736
	11th to 20th	Night	55% 5% 8% 14% 15% 3	3276
	21st to end	Night	49% 7% 14% 16% 10% 4	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% 15% 2	370
	21st to end	Day	17% 11% 27% 25% 18% 3	407
February	1st to 10th	Day	19% 14% 26% 24% 14% 4	370
	11th to 20th	Day	20% 17% 27% 23% 11% 1	398
	21st to end	Day	15% 18% 30% 24% 12% 2	382
March	1st to 10th	Day	23% 17% 26% 25% 8% 1	460
	11th to 20th	Day	20% 18% 25% 20% 14% 3	460
	21st to end	Day	17% 19% 32% 25% 7% 1	506
April	1st to 10th	Day	18% 16% 32% 22% 10% 2	460
	11th to 20th	Day	15% 18% 31% 23% 9% 4	452
	21st to end	Day	8% 21% 33% 22% 12% 6%	460
May	1st to 10th	Day	7% 17% 32% 30% 12% 3	463
	11th to 20th	Day	12% 15% 32% 27% 11% 2	488
	21st to end	Day	12% 16% 25% 28% 14% 4	529
June	1st to 10th	Day	8% 12% 33% 29% 13% 4	490
	11th to 20th	Day	3 13% 34% 35% 12% 4	490
	21st to end	Day	2 7% 29% 41% 18% 3	490
July	1st to 10th	Day	5% 5% 23% 45% 19% 3	486
	11th to 20th	Day	1 3 23% 48% 23% 3	484
	21st to end	Day	2 3 23% 44% 22% 6%	531
August	1st to 10th	Day	4 5% 26% 40% 20% 4	487
	11th to 20th	Day	3 3 27% 39% 21% 6%	477
	21st to end	Day	2 4 23% 44% 21% 7%	539
September	1st to 10th	Day	3 3 22% 39% 25% 8%	490
	11th to 20th	Day	5% 13% 24% 34% 17% 7%	490
	21st to end	Day	4 8% 20% 31% 29% 7%	490
October	1st to 10th	Day	3 7% 26% 30% 24% 10%	490
	11th to 20th	Day	8% 8% 21% 28% 26% 8%	490
	21st to end	Day	9% 9% 24% 28% 22% 8%	539
November	1st to 10th	Day	9% 12% 21% 26% 24% 8%	492
	11th to 20th	Day	9% 11% 16% 33% 23% 8%	300
	21st to end	Day	11% 10% 25% 23% 29% 1	213
December	1st to 10th	Day	14% 12% 18% 27% 23% 5%	190
	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%	760
	21st to end	Night	20% 6% 11% 33% 25% 5%	836
February	1st to 10th	Night	16% 8% 14% 33% 24% 4	760
	11th to 20th	Night	15% 11% 17% 29% 23% 6%	744
	21st to end	Night	14% 12% 18% 32% 20% 4	628
March	1st to 10th	Night	20% 12% 15% 29% 21% 3	760
	11th to 20th	Night	18% 10% 14% 31% 23% 5%	760
	21st to end	Night	17% 11% 20% 31% 18% 3	836
April	1st to 10th	Night	15% 10% 20% 31% 21% 4	760
	11th to 20th	Night	14% 15% 18% 27% 21% 6%	751
	21st to end	Night	8% 12% 26% 25% 23% 7%	760
May	1st to 10th	Night	8% 7% 22% 31% 27% 6%	666
	11th to 20th	Night	10% 5% 18% 31% 30% 6%	547
	21st to end	Night	11% 12% 18% 29% 23% 7%	411
June	1st to 10th	Night	7% 7% 23% 32% 26% 5%	380
	11th to 20th	Night	2 6% 17% 38% 32% 4	380
	21st to end	Night	5% 4 11% 34% 39% 6%	380
July	1st to 10th	Night	5% 3 12% 27% 44% 9%	399
	11th to 20th	Night	3 3 4 33% 42% 16%	552
	21st to end	Night	3 5% 5% 26% 48% 12%	624
August	1st to 10th	Night	9% 5% 7% 26% 43% 11%	567
	11th to 20th	Night	4 3 10% 29% 44% 9%	668
	21st to end	Night	3 3 9% 30% 42% 14%	836
September	1st to 10th	Night	3 3 8% 29% 44% 13%	760
	11th to 20th	Night	8% 4 12% 29% 38% 9%	760
	21st to end	Night	3 4 10% 29% 42% 12%	760
October	1st to 10th	Night	7% 3 10% 30% 38% 12%	760
	11th to 20th	Night	8% 5% 9% 27% 39% 12%	760
	21st to end	Night	8% 6% 8% 32% 31% 14%	836
November	1st to 10th	Night	9% 6% 13% 25% 35% 11%	780
	11th to 20th	Night	11% 5% 13% 33% 31% 8%	800
	21st to end	Night	12% 5% 12% 39% 27% 6%	800
December	1st to 10th	Night	14% 6% 15% 31% 26% 7%	782
	11th to 20th	Night	22% 9% 14% 30% 21% 4	760
	21st to end	Night	21% 10% 19% 28% 20% 3	836

Page generated at 2024-12-03 03:45:11. Version ac07

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