Headlands International Dark Sky Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Headlands International Dark Sky Park clear sky chart from 2005-11-06 to 2026-09-14.

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	13% 10% 4 9% 5% 5% 4 4 5% 7% 35%	148981
	Night	14% 12% 5% 11% 6% 5% 5% 3 5% 6% 29%	142370
Transparency	Day	33% 34% 8% 16% 7% 1	152461
	Night	37% 35% 8% 13% 6% 1	146977
Seeing	Day	18% 13% 29% 26% 11% 3	18137
	Night	20% 12% 28% 27% 11% 1	27555

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	11% 15% 8% 22% 8% 6% 6% 4 4 4 11%	4691
	11th to 20th	Day	10% 18% 7% 20% 12% 8% 5% 4 4 4 9%	4679
	21st to end	Day	8% 14% 7% 17% 9% 8% 7% 5% 4 4 17%	5156
February	1st to 10th	Day	9% 14% 5% 12% 7% 6% 5% 5% 5% 7% 24%	1803
	11th to 20th	Day	12% 13% 5% 11% 7% 6% 5% 3 6% 7% 24%	1981
	21st to end	Day	12% 14% 5% 11% 8% 5% 5% 4 5% 7% 25%	1730
March	1st to 10th	Day	13% 10% 3 10% 5% 5% 4 3 6% 8% 33%	4578
	11th to 20th	Day	12% 11% 4 11% 7% 6% 5% 4 5% 6% 28%	5013
	21st to end	Day	18% 11% 6% 11% 6% 5% 5% 4 6% 6% 24%	5717
April	1st to 10th	Day	18% 11% 4 7% 5% 5% 5% 4 6% 6% 31%	2520
	11th to 20th	Day	18% 10% 4 7% 4 3 4 3 5% 6% 36%	2649
	21st to end	Day	14% 8% 3 9% 5% 5% 4 4 6% 7% 34%	2730
May	1st to 10th	Day	12% 7% 3 6% 4 4 4 3 5% 8% 44%	7642
	11th to 20th	Day	13% 9% 3 7% 4 5% 4 3 5% 7% 40%	7915
	21st to end	Day	10% 7% 2 6% 4 4 4 3 5% 8% 47%	9337
June	1st to 10th	Day	13% 5% 2 6% 4 4 5% 4 7% 9% 42%	3150
	11th to 20th	Day	11% 7% 2 7% 4 5% 5% 4 6% 8% 40%	3150
	21st to end	Day	15% 8% 2 6% 4 4 4 4 5% 8% 42%	3150
July	1st to 10th	Day	13% 6% 2 6% 4 3 4 3 6% 8% 46%	8547
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 3 6% 9% 49%	8280
	21st to end	Day	10% 6% 2 6% 3 3 3 3 5% 8% 51%	8992
August	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 6% 9% 48%	6225
	11th to 20th	Day	11% 5% 2 5% 3 4 4 4 5% 8% 49%	5951
	21st to end	Day	14% 9% 3 7% 4 4 4 3 5% 6% 41%	6477
September	1st to 10th	Day	14% 6% 2 6% 3 4 4 3 5% 7% 44%	2470
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 7% 8% 43%	2233
	21st to end	Day	14% 8% 3 7% 5% 5% 5% 3 5% 9% 38%	2200
October	1st to 10th	Day	18% 9% 3 7% 4 4 4 3 5% 7% 35%	2175
	11th to 20th	Day	21% 14% 4 10% 6% 5% 5% 5% 4 6% 20%	1900
	21st to end	Day	22% 13% 4 9% 6% 6% 4 3 5% 6% 21%	1980
November	1st to 10th	Day	18% 15% 5% 10% 6% 4 5% 3 5% 7% 22%	1852
	11th to 20th	Day	15% 18% 5% 14% 7% 5% 6% 3 6% 5% 17%	1863
	21st to end	Day	19% 22% 8% 14% 6% 5% 5% 3 4 4 11%	1622
December	1st to 10th	Day	14% 24% 8% 17% 7% 5% 4 3 4 4 9%	2350
	11th to 20th	Day	14% 18% 7% 19% 10% 6% 6% 3 4 4 9%	2691
	21st to end	Day	23% 21% 8% 15% 6% 4 4 3 3 3 10%	3582
January	1st to 10th	Night	11% 15% 7% 22% 9% 6% 5% 3 4 4 11%	8233
	11th to 20th	Night	9% 18% 8% 20% 12% 8% 6% 4 4 4 8%	8236
	21st to end	Night	8% 14% 6% 15% 9% 8% 7% 5% 6% 6% 17%	9058
February	1st to 10th	Night	9% 15% 6% 12% 9% 6% 6% 4 5% 5% 22%	2815
	11th to 20th	Night	10% 13% 6% 12% 8% 6% 5% 4 5% 6% 24%	2520
	21st to end	Night	13% 15% 5% 9% 7% 5% 5% 4 5% 6% 25%	2076
March	1st to 10th	Night	17% 10% 3 7% 5% 4 3 3 5% 7% 36%	5482
	11th to 20th	Night	11% 11% 4 10% 6% 5% 4 4 6% 8% 31%	5165
	21st to end	Night	20% 11% 5% 9% 4 4 5% 3 5% 5% 30%	5500
April	1st to 10th	Night	19% 11% 3 8% 6% 4 5% 4 5% 6% 29%	2100
	11th to 20th	Night	20% 11% 3 6% 5% 4 4 3 4 5% 35%	1920
	21st to end	Night	17% 9% 4 8% 4 4 4 4 5% 6% 35%	1890
May	1st to 10th	Night	13% 7% 2 7% 4 5% 5% 4 6% 7% 41%	5257
	11th to 20th	Night	14% 9% 3 7% 4 3 4 3 4 6% 42%	4957
	21st to end	Night	13% 8% 3 7% 4 3 4 3 5% 7% 44%	4853
June	1st to 10th	Night	13% 7% 2 6% 4 4 5% 4 6% 9% 40%	1470
	11th to 20th	Night	11% 7% 2 7% 5% 5% 5% 4 6% 8% 40%	1470
	21st to end	Night	14% 7% 3 6% 3 5% 4 3 5% 8% 42%	1470
July	1st to 10th	Night	12% 6% 3 7% 5% 4 4 4 6% 8% 42%	4424
	11th to 20th	Night	9% 5% 2 5% 4 4 4 3 5% 8% 52%	4710
	21st to end	Night	9% 5% 2 5% 3 3 4 3 5% 8% 51%	5118
August	1st to 10th	Night	14% 8% 2 6% 4 4 4 3 5% 7% 42%	3933
	11th to 20th	Night	10% 5% 2 6% 3 3 4 3 5% 9% 50%	4095
	21st to end	Night	14% 7% 2 6% 4 4 4 3 4 6% 46%	4646
September	1st to 10th	Night	12% 7% 2 7% 3 4 4 3 5% 7% 46%	2128
	11th to 20th	Night	15% 5% 2 6% 3 4 3 3 6% 8% 44%	2234
	21st to end	Night	16% 9% 3 7% 4 4 3 4 5% 6% 39%	2200
October	1st to 10th	Night	17% 9% 3 6% 4 4 4 3 5% 6% 39%	2224
	11th to 20th	Night	23% 11% 3 10% 5% 5% 5% 3 4 6% 24%	2495
	21st to end	Night	25% 12% 5% 8% 5% 5% 4 3 4 6% 23%	2860
November	1st to 10th	Night	18% 15% 4 8% 5% 4 4 3 5% 6% 27%	2666
	11th to 20th	Night	16% 17% 4 12% 7% 5% 5% 3 5% 5% 19%	2756
	21st to end	Night	20% 21% 6% 13% 7% 5% 4 3 4 4 13%	2993
December	1st to 10th	Night	12% 22% 8% 15% 8% 6% 5% 4 4 4 11%	5060
	11th to 20th	Night	13% 19% 7% 19% 9% 6% 5% 3 4 4 12%	5736
	21st to end	Night	23% 20% 9% 14% 7% 5% 4 3 3 3 10%	7620

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	60% 25% 8% 4 2 1	4692
	11th to 20th	Day	65% 23% 5% 6% 2	4710
	21st to end	Day	53% 26% 7% 9% 4	5156
February	1st to 10th	Day	23% 61% 5% 8% 3	1803
	11th to 20th	Day	26% 59% 5% 6% 4	1991
	21st to end	Day	24% 61% 5% 7% 4	1730
March	1st to 10th	Day	35% 30% 12% 11% 10% 2	4580
	11th to 20th	Day	41% 32% 9% 12% 5% 1	5013
	21st to end	Day	45% 32% 7% 11% 5%	5717
April	1st to 10th	Day	23% 61% 4 9% 4	2520
	11th to 20th	Day	20% 60% 3 9% 6% 1	2649
	21st to end	Day	19% 60% 6% 9% 6% 1	2730
May	1st to 10th	Day	29% 25% 8% 22% 14% 2	7651
	11th to 20th	Day	34% 25% 7% 21% 13%	7919
	21st to end	Day	28% 24% 8% 21% 16% 3	9337
June	1st to 10th	Day	16% 60% 4 11% 8% 2	3150
	11th to 20th	Day	19% 60% 5% 10% 5% 1	3150
	21st to end	Day	18% 58% 6% 12% 5% 1	3150
July	1st to 10th	Day	30% 22% 13% 23% 10% 1	8479
	11th to 20th	Day	25% 22% 13% 31% 8%	8205
	21st to end	Day	26% 21% 11% 29% 11%	8794
August	1st to 10th	Day	24% 28% 12% 24% 12%	6210
	11th to 20th	Day	27% 29% 14% 27% 4	5990
	21st to end	Day	36% 29% 11% 21% 3	6476
September	1st to 10th	Day	19% 57% 8% 12% 5%	2470
	11th to 20th	Day	15% 58% 7% 14% 6%	2233
	21st to end	Day	18% 59% 7% 12% 5%	2200
October	1st to 10th	Day	20% 37% 7% 23% 13%	3539
	11th to 20th	Day	33% 37% 7% 19% 4	3077
	21st to end	Day	39% 36% 9% 13% 3	3180
November	1st to 10th	Day	31% 54% 5% 8% 1	1852
	11th to 20th	Day	31% 57% 5% 5% 1	1863
	21st to end	Day	38% 56% 3 2 1	1622
December	1st to 10th	Day	52% 38% 7% 3 1	2350
	11th to 20th	Day	56% 34% 6% 4 1	2691
	21st to end	Day	60% 28% 3 6% 3	3582
January	1st to 10th	Night	59% 25% 8% 6% 3	8233
	11th to 20th	Night	64% 25% 6% 4 2	8280
	21st to end	Night	50% 26% 8% 10% 6%	9058
February	1st to 10th	Night	25% 60% 5% 6% 4	2815
	11th to 20th	Night	25% 59% 6% 9% 2	2521
	21st to end	Night	22% 61% 5% 7% 5%	2076
March	1st to 10th	Night	35% 29% 13% 14% 8% 1	5493
	11th to 20th	Night	37% 31% 14% 13% 5%	5166
	21st to end	Night	42% 31% 8% 12% 8%	5500
April	1st to 10th	Night	25% 60% 3 7% 5%	2100
	11th to 20th	Night	23% 60% 3 9% 4 2	1920
	21st to end	Night	22% 59% 5% 7% 6% 1	1890
May	1st to 10th	Night	30% 26% 7% 24% 11% 2	5264
	11th to 20th	Night	32% 25% 6% 24% 13%	4958
	21st to end	Night	32% 25% 7% 18% 14% 4	4854
June	1st to 10th	Night	16% 61% 4 9% 9% 1	1470
	11th to 20th	Night	20% 60% 4 10% 5% 1	1470
	21st to end	Night	18% 58% 5% 13% 5% 1	1470
July	1st to 10th	Night	34% 23% 11% 17% 12% 3	4384
	11th to 20th	Night	25% 21% 13% 29% 11%	4662
	21st to end	Night	25% 22% 12% 29% 13%	5002
August	1st to 10th	Night	34% 27% 9% 16% 13% 1	3924
	11th to 20th	Night	26% 28% 14% 26% 6%	4122
	21st to end	Night	31% 28% 9% 25% 7%	4646
September	1st to 10th	Night	17% 57% 7% 13% 6%	2128
	11th to 20th	Night	15% 57% 8% 13% 7% 1	2234
	21st to end	Night	18% 60% 6% 11% 5%	2200
October	1st to 10th	Night	26% 36% 7% 24% 8%	3640
	11th to 20th	Night	30% 37% 9% 16% 8%	4057
	21st to end	Night	34% 38% 11% 14% 3	4610
November	1st to 10th	Night	30% 54% 5% 7% 4	2666
	11th to 20th	Night	30% 55% 7% 5% 3	2756
	21st to end	Night	36% 55% 4 3 1	2993
December	1st to 10th	Night	48% 38% 9% 4 1	5060
	11th to 20th	Night	52% 34% 7% 5% 2	5735
	21st to end	Night	60% 29% 3 6% 2	7620

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	20% 21% 31% 23% 5%	330
	11th to 20th	Day	21% 18% 38% 18% 5% 1	330
	21st to end	Day	18% 18% 31% 26% 6%	363
February	1st to 10th	Day	17% 25% 36% 17% 5% 1	330
	11th to 20th	Day	17% 25% 32% 20% 5%	330
	21st to end	Day	23% 25% 33% 17% 1	273
March	1st to 10th	Day	20% 19% 38% 18% 5%	330
	11th to 20th	Day	21% 13% 40% 20% 5% 1	502
	21st to end	Day	26% 16% 28% 23% 6% 1	594
April	1st to 10th	Day	22% 17% 34% 19% 6% 1	540
	11th to 20th	Day	25% 18% 28% 20% 8% 2	532
	21st to end	Day	19% 16% 33% 21% 9% 2	540
May	1st to 10th	Day	20% 16% 28% 23% 10% 3	543
	11th to 20th	Day	16% 10% 37% 25% 9% 3	661
	21st to end	Day	14% 11% 29% 31% 12% 2	844
June	1st to 10th	Day	14% 11% 31% 31% 11% 2	780
	11th to 20th	Day	14% 8% 33% 32% 11% 2	780
	21st to end	Day	20% 7% 29% 30% 12% 2	780
July	1st to 10th	Day	10% 4 29% 34% 18% 3	771
	11th to 20th	Day	11% 7% 27% 32% 18% 4	768
	21st to end	Day	10% 7% 24% 37% 17% 4	849
August	1st to 10th	Day	10% 5% 23% 36% 20% 6%	661
	11th to 20th	Day	11% 7% 19% 32% 23% 8%	559
	21st to end	Day	16% 8% 24% 29% 19% 4	627
September	1st to 10th	Day	16% 10% 23% 27% 17% 7%	570
	11th to 20th	Day	13% 8% 22% 29% 22% 8%	542
	21st to end	Day	17% 12% 22% 29% 16% 5%	530
October	1st to 10th	Day	22% 14% 28% 21% 14% 2	530
	11th to 20th	Day	27% 18% 28% 22% 4	430
	21st to end	Day	29% 15% 32% 18% 5% 2	363
November	1st to 10th	Day	26% 20% 32% 17% 4 1	330
	11th to 20th	Day	22% 30% 29% 15% 4	315
	21st to end	Day	27% 29% 30% 14% 1	220
December	1st to 10th	Day	21% 24% 34% 14% 5% 2	220
	11th to 20th	Day	23% 30% 30% 14% 3	220
	21st to end	Day	32% 24% 27% 13% 3 1	250
January	1st to 10th	Night	20% 19% 32% 24% 5%	1050
	11th to 20th	Night	21% 18% 32% 22% 7%	1050
	21st to end	Night	18% 13% 34% 28% 5% 1	1155
February	1st to 10th	Night	17% 15% 36% 26% 6%	925
	11th to 20th	Night	18% 16% 33% 26% 7%	823
	21st to end	Night	20% 14% 36% 25% 5%	692
March	1st to 10th	Night	18% 12% 34% 28% 8% 1	840
	11th to 20th	Night	17% 14% 35% 27% 6%	840
	21st to end	Night	19% 10% 30% 31% 9%	887
April	1st to 10th	Night	23% 10% 30% 29% 6% 1	630
	11th to 20th	Night	24% 12% 26% 26% 10% 1	623
	21st to end	Night	20% 11% 28% 30% 11% 1	630
May	1st to 10th	Night	17% 6% 27% 35% 15%	630
	11th to 20th	Night	19% 5% 27% 35% 14% 1	621
	21st to end	Night	12% 6% 27% 35% 19% 1	583
June	1st to 10th	Night	15% 5% 25% 40% 15% 1	420
	11th to 20th	Night	14% 5% 26% 38% 18%	420
	21st to end	Night	16% 5% 23% 38% 17% 2	420
July	1st to 10th	Night	11% 3 19% 42% 22% 3	511
	11th to 20th	Night	10% 4 22% 34% 26% 4	612
	21st to end	Night	10% 3 20% 41% 24% 2	690
August	1st to 10th	Night	10% 4 19% 40% 24% 2	627
	11th to 20th	Night	10% 4 19% 33% 29% 5%	624
	21st to end	Night	13% 8% 20% 37% 21% 1	693
September	1st to 10th	Night	14% 5% 22% 31% 22% 5%	658
	11th to 20th	Night	15% 7% 21% 29% 23% 4	813
	21st to end	Night	21% 7% 20% 30% 18% 4	800
October	1st to 10th	Night	20% 11% 26% 28% 14% 1	800
	11th to 20th	Night	30% 14% 25% 25% 6%	800
	21st to end	Night	32% 12% 26% 24% 5% 1	880
November	1st to 10th	Night	26% 15% 32% 20% 7%	820
	11th to 20th	Night	24% 20% 31% 20% 5%	840
	21st to end	Night	32% 19% 32% 13% 3	893
December	1st to 10th	Night	25% 19% 33% 16% 6%	1050
	11th to 20th	Night	25% 20% 33% 16% 5% 1	1050
	21st to end	Night	31% 16% 32% 17% 3	1155

Page generated at 2026-09-11 04:11:53. Version ac07

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