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 2025-05-11.

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 10% 5% 5% 5% 4 5% 7% 33%	113163
	Night	15% 13% 5% 11% 6% 5% 4 3 5% 6% 27%	115936
Transparency	Day	30% 44% 7% 13% 6% 1	116691
	Night	35% 42% 7% 10% 5%	120571
Seeing	Day	18% 14% 30% 25% 11% 3	16200
	Night	20% 12% 29% 27% 11% 1	25888

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	10% 14% 9% 25% 8% 6% 6% 4 4 4 10%	3617
	11th to 20th	Day	11% 16% 6% 21% 14% 8% 5% 4 4 4 7%	3589
	21st to end	Day	10% 17% 8% 18% 8% 7% 6% 4 4 4 12%	3982
February	1st to 10th	Day	9% 14% 5% 11% 7% 6% 5% 5% 6% 7% 24%	1717
	11th to 20th	Day	12% 14% 5% 11% 8% 6% 5% 4 6% 7% 24%	1884
	21st to end	Day	11% 14% 5% 11% 7% 6% 5% 4 6% 7% 25%	1650
March	1st to 10th	Day	13% 10% 3 10% 5% 5% 4 3 6% 8% 33%	4478
	11th to 20th	Day	12% 11% 4 12% 7% 6% 5% 4 5% 6% 29%	4905
	21st to end	Day	17% 11% 6% 11% 6% 5% 5% 4 6% 6% 25%	5593
April	1st to 10th	Day	17% 11% 4 6% 5% 4 5% 4 6% 7% 31%	2400
	11th to 20th	Day	18% 9% 4 7% 5% 3 4 3 5% 6% 36%	2522
	21st to end	Day	15% 8% 3 9% 6% 5% 4 4 6% 7% 34%	2600
May	1st to 10th	Day	14% 7% 3 6% 4 4 4 3 5% 8% 41%	4238
	11th to 20th	Day	11% 7% 2 7% 4 5% 4 3 6% 9% 42%	4230
	21st to end	Day	13% 6% 2 5% 4 4 4 4 5% 9% 44%	4987
June	1st to 10th	Day	13% 5% 2 6% 4 4 5% 4 7% 9% 42%	2850
	11th to 20th	Day	11% 6% 2 7% 4 5% 5% 4 6% 8% 42%	2850
	21st to end	Day	15% 7% 2 5% 4 4 4 4 5% 8% 43%	2850
July	1st to 10th	Day	15% 8% 2 6% 4 3 4 3 5% 7% 43%	4582
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 4 5% 8% 52%	4440
	21st to end	Day	9% 5% 2 5% 3 3 3 3 5% 8% 52%	4814
August	1st to 10th	Day	7% 6% 2 6% 4 5% 6% 4 6% 9% 45%	4241
	11th to 20th	Day	12% 6% 2 5% 3 4 4 4 5% 8% 48%	4044
	21st to end	Day	12% 6% 2 6% 4 3 4 3 5% 6% 49%	4446
September	1st to 10th	Day	14% 6% 2 6% 3 4 4 3 6% 8% 44%	2235
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 7% 8% 43%	2090
	21st to end	Day	15% 7% 3 6% 5% 4 5% 3 5% 9% 38%	2090
October	1st to 10th	Day	18% 9% 3 7% 5% 4 4 3 5% 7% 35%	2066
	11th to 20th	Day	21% 14% 4 10% 6% 5% 5% 5% 4 6% 20%	1805
	21st to end	Day	23% 14% 4 9% 6% 5% 5% 3 5% 6% 20%	1881
November	1st to 10th	Day	18% 15% 5% 10% 6% 4 4 3 5% 7% 23%	1762
	11th to 20th	Day	15% 18% 5% 14% 7% 5% 5% 3 6% 5% 17%	1775
	21st to end	Day	19% 21% 7% 13% 6% 5% 5% 3 4 4 12%	1545
December	1st to 10th	Day	15% 24% 8% 17% 7% 5% 4 3 4 4 9%	2280
	11th to 20th	Day	14% 18% 7% 19% 10% 6% 6% 4 4 4 9%	2621
	21st to end	Day	23% 20% 8% 15% 6% 4 4 3 3 3 10%	3504
January	1st to 10th	Night	10% 15% 7% 25% 9% 6% 5% 3 4 4 12%	6348
	11th to 20th	Night	11% 16% 7% 20% 13% 8% 5% 3 4 4 9%	6316
	21st to end	Night	11% 17% 7% 15% 9% 8% 6% 4 5% 5% 14%	6996
February	1st to 10th	Night	9% 16% 6% 12% 9% 6% 6% 4 5% 5% 22%	2681
	11th to 20th	Night	10% 14% 5% 13% 8% 6% 5% 4 5% 6% 24%	2397
	21st to end	Night	13% 15% 5% 9% 7% 5% 5% 4 5% 7% 25%	1980
March	1st to 10th	Night	17% 10% 3 7% 5% 4 3 3 5% 7% 36%	5362
	11th to 20th	Night	11% 11% 4 10% 6% 5% 4 4 6% 8% 32%	5053
	21st to end	Night	20% 11% 5% 8% 4 4 5% 3 5% 5% 30%	5381
April	1st to 10th	Night	19% 10% 3 8% 6% 4 5% 4 5% 6% 29%	2000
	11th to 20th	Night	19% 11% 3 6% 5% 4 4 3 4 5% 36%	1827
	21st to end	Night	17% 10% 4 8% 4 4 4 4 5% 6% 35%	1800
May	1st to 10th	Night	18% 7% 3 8% 5% 4 4 4 4 6% 38%	2923
	11th to 20th	Night	12% 8% 3 8% 6% 4 5% 3 5% 7% 39%	2671
	21st to end	Night	16% 8% 2 6% 4 3 4 3 6% 8% 41%	2595
June	1st to 10th	Night	13% 7% 2 6% 4 4 5% 4 6% 9% 39%	1330
	11th to 20th	Night	11% 7% 2 7% 5% 5% 5% 4 5% 8% 41%	1330
	21st to end	Night	15% 7% 3 6% 3 5% 4 3 5% 8% 42%	1330
July	1st to 10th	Night	13% 7% 2 6% 5% 4 4 3 6% 8% 41%	2384
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 5% 7% 51%	2530
	21st to end	Night	10% 5% 2 5% 3 4 4 3 5% 8% 50%	2750
August	1st to 10th	Night	10% 7% 2 7% 5% 5% 5% 4 5% 8% 43%	2692
	11th to 20th	Night	12% 5% 3 6% 3 3 3 3 5% 8% 51%	2785
	21st to end	Night	10% 5% 2 5% 4 4 3 4 5% 7% 51%	3207
September	1st to 10th	Night	12% 7% 2 6% 3 4 4 3 5% 6% 47%	1925
	11th to 20th	Night	16% 6% 2 6% 3 3 3 3 6% 8% 44%	2090
	21st to end	Night	17% 8% 3 7% 4 4 3 4 5% 6% 39%	2090
October	1st to 10th	Night	18% 9% 3 6% 4 4 4 3 5% 6% 38%	2113
	11th to 20th	Night	23% 11% 3 10% 5% 5% 5% 3 4 6% 25%	2370
	21st to end	Night	26% 13% 5% 8% 5% 5% 4 3 4 6% 21%	2717
November	1st to 10th	Night	18% 15% 4 8% 5% 4 4 3 5% 7% 26%	2536
	11th to 20th	Night	17% 17% 4 12% 8% 5% 4 3 5% 5% 18%	2625
	21st to end	Night	20% 21% 6% 13% 7% 5% 4 3 4 4 13%	2850
December	1st to 10th	Night	12% 22% 8% 15% 8% 6% 5% 4 4 4 12%	4910
	11th to 20th	Night	13% 19% 7% 19% 9% 6% 5% 3 4 4 12%	5586
	21st to end	Night	23% 20% 9% 14% 7% 5% 4 3 3 3 10%	7456

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	57% 31% 5% 5% 2 1	3618
	11th to 20th	Day	61% 29% 5% 4 1	3620
	21st to end	Day	54% 33% 6% 4 2	3982
February	1st to 10th	Day	21% 63% 5% 7% 3	1717
	11th to 20th	Day	24% 62% 5% 5% 4	1894
	21st to end	Day	21% 63% 5% 6% 4	1650
March	1st to 10th	Day	35% 30% 12% 11% 10% 2	4480
	11th to 20th	Day	41% 32% 10% 12% 4 1	4905
	21st to end	Day	44% 33% 7% 11% 5%	5593
April	1st to 10th	Day	21% 64% 4 8% 4	2400
	11th to 20th	Day	19% 63% 3 9% 6% 1	2522
	21st to end	Day	18% 63% 5% 8% 6% 1	2600
May	1st to 10th	Day	23% 42% 8% 15% 11% 2	4250
	11th to 20th	Day	20% 43% 9% 20% 7%	4234
	21st to end	Day	21% 43% 8% 13% 13% 2	4987
June	1st to 10th	Day	14% 66% 3 9% 6% 1	2850
	11th to 20th	Day	16% 66% 4 8% 5% 1	2850
	21st to end	Day	14% 63% 6% 11% 5% 1	2850
July	1st to 10th	Day	30% 37% 10% 15% 7% 1	4513
	11th to 20th	Day	19% 37% 11% 27% 6%	4365
	21st to end	Day	19% 36% 7% 24% 14%	4662
August	1st to 10th	Day	21% 40% 9% 23% 7%	4226
	11th to 20th	Day	25% 39% 8% 21% 6%	4083
	21st to end	Day	25% 39% 11% 20% 4	4445
September	1st to 10th	Day	16% 62% 6% 11% 4	2235
	11th to 20th	Day	14% 61% 6% 13% 6%	2090
	21st to end	Day	16% 62% 7% 11% 4	2090
October	1st to 10th	Day	19% 38% 7% 23% 13%	3430
	11th to 20th	Day	33% 38% 7% 18% 4	2982
	21st to end	Day	39% 38% 9% 12% 2	3081
November	1st to 10th	Day	30% 57% 5% 7% 1	1762
	11th to 20th	Day	30% 59% 5% 5% 1	1775
	21st to end	Day	35% 59% 3 2 1	1545
December	1st to 10th	Day	51% 39% 7% 3	2280
	11th to 20th	Day	55% 35% 6% 4 1	2621
	21st to end	Day	60% 29% 3 6% 3	3504
January	1st to 10th	Night	54% 30% 7% 5% 3	6348
	11th to 20th	Night	58% 31% 6% 3 2	6360
	21st to end	Night	50% 33% 7% 7% 2	6996
February	1st to 10th	Night	23% 63% 5% 5% 4	2681
	11th to 20th	Night	23% 62% 5% 8% 2	2398
	21st to end	Night	21% 63% 5% 7% 5%	1980
March	1st to 10th	Night	35% 30% 13% 13% 8% 1	5373
	11th to 20th	Night	36% 32% 14% 13% 4	5054
	21st to end	Night	41% 32% 8% 12% 8%	5381
April	1st to 10th	Night	23% 63% 3 7% 5%	2000
	11th to 20th	Night	20% 62% 3 9% 4 2	1827
	21st to end	Night	21% 62% 4 6% 6% 1	1800
May	1st to 10th	Night	26% 42% 6% 16% 8% 1	2930
	11th to 20th	Night	23% 44% 6% 19% 8%	2672
	21st to end	Night	26% 44% 8% 9% 9% 3	2596
June	1st to 10th	Night	14% 67% 4 8% 7% 1	1330
	11th to 20th	Night	16% 66% 4 8% 5% 2	1330
	21st to end	Night	15% 64% 4 11% 5% 1	1330
July	1st to 10th	Night	30% 37% 10% 13% 9% 1	2344
	11th to 20th	Night	19% 37% 13% 20% 11%	2482
	21st to end	Night	21% 37% 9% 16% 17%	2662
August	1st to 10th	Night	27% 38% 8% 18% 9%	2683
	11th to 20th	Night	24% 38% 10% 21% 7%	2812
	21st to end	Night	20% 38% 8% 23% 10%	3207
September	1st to 10th	Night	15% 62% 5% 12% 6%	1925
	11th to 20th	Night	15% 60% 6% 12% 6% 1	2090
	21st to end	Night	16% 63% 6% 10% 5%	2090
October	1st to 10th	Night	26% 37% 6% 23% 8%	3529
	11th to 20th	Night	29% 38% 9% 16% 8%	3932
	21st to end	Night	34% 39% 11% 13% 3	4467
November	1st to 10th	Night	29% 56% 5% 7% 3	2536
	11th to 20th	Night	29% 58% 6% 5% 3	2625
	21st to end	Night	35% 58% 4 2 1	2850
December	1st to 10th	Night	47% 39% 9% 4 1	4910
	11th to 20th	Night	51% 35% 7% 5% 2	5585
	21st to end	Night	59% 29% 3 6% 2	7456

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	19% 20% 32% 23% 6%	300
	11th to 20th	Day	20% 18% 39% 18% 5% 1	300
	21st to end	Day	19% 18% 32% 25% 6%	330
February	1st to 10th	Day	17% 26% 35% 18% 4 1	300
	11th to 20th	Day	16% 28% 33% 18% 5%	300
	21st to end	Day	22% 27% 33% 17% 2	249
March	1st to 10th	Day	19% 19% 39% 17% 6%	300
	11th to 20th	Day	19% 14% 40% 20% 6% 2	464
	21st to end	Day	25% 16% 28% 24% 7% 1	550
April	1st to 10th	Day	22% 17% 33% 20% 7% 1	500
	11th to 20th	Day	25% 19% 27% 19% 8% 2	492
	21st to end	Day	20% 17% 34% 19% 8% 2	500
May	1st to 10th	Day	22% 15% 28% 23% 9% 3	503
	11th to 20th	Day	15% 10% 37% 25% 9% 4	571
	21st to end	Day	14% 12% 28% 31% 13% 3	734
June	1st to 10th	Day	14% 11% 32% 31% 10% 2	680
	11th to 20th	Day	13% 8% 33% 33% 11% 3	680
	21st to end	Day	20% 7% 29% 30% 12% 2	680
July	1st to 10th	Day	10% 5% 31% 33% 17% 4	671
	11th to 20th	Day	11% 7% 28% 31% 18% 5%	668
	21st to end	Day	11% 6% 24% 35% 19% 5%	739
August	1st to 10th	Day	10% 5% 25% 35% 19% 6%	572
	11th to 20th	Day	11% 6% 20% 32% 23% 9%	479
	21st to end	Day	15% 9% 23% 29% 19% 4	539
September	1st to 10th	Day	16% 10% 24% 27% 17% 6%	490
	11th to 20th	Day	13% 8% 23% 29% 21% 7%	490
	21st to end	Day	17% 12% 23% 28% 16% 4	490
October	1st to 10th	Day	22% 15% 28% 20% 13% 2	490
	11th to 20th	Day	27% 19% 28% 21% 4 1	395
	21st to end	Day	31% 16% 33% 17% 3 1	330
November	1st to 10th	Day	26% 20% 32% 17% 3 1	300
	11th to 20th	Day	22% 28% 31% 14% 5%	287
	21st to end	Day	26% 28% 32% 14% 1	200
December	1st to 10th	Day	22% 24% 33% 15% 5% 3	200
	11th to 20th	Day	24% 28% 31% 14% 4 1	200
	21st to end	Day	32% 24% 26% 13% 4 1	227
January	1st to 10th	Night	20% 18% 33% 24% 5%	1000
	11th to 20th	Night	21% 18% 32% 22% 7%	1000
	21st to end	Night	19% 13% 34% 27% 6% 1	1100
February	1st to 10th	Night	17% 16% 36% 26% 5%	881
	11th to 20th	Night	19% 15% 33% 25% 7%	783
	21st to end	Night	20% 14% 36% 26% 5%	660
March	1st to 10th	Night	18% 12% 34% 27% 8% 1	800
	11th to 20th	Night	16% 14% 36% 27% 7%	800
	21st to end	Night	20% 10% 30% 31% 9%	845
April	1st to 10th	Night	23% 10% 30% 30% 6% 1	600
	11th to 20th	Night	23% 12% 27% 27% 10% 1	593
	21st to end	Night	20% 11% 29% 29% 10% 1	600
May	1st to 10th	Night	18% 6% 27% 34% 15%	600
	11th to 20th	Night	18% 5% 28% 34% 14% 1	563
	21st to end	Night	12% 6% 27% 34% 20% 1	527
June	1st to 10th	Night	16% 4 27% 38% 15% 1	380
	11th to 20th	Night	12% 5% 25% 39% 18% 1	380
	21st to end	Night	16% 5% 23% 37% 17% 2	380
July	1st to 10th	Night	11% 3 20% 43% 21% 3	461
	11th to 20th	Night	10% 4 22% 32% 27% 5%	552
	21st to end	Night	11% 3 19% 39% 25% 3	624
August	1st to 10th	Night	10% 4 20% 41% 24% 2	567
	11th to 20th	Night	10% 4 20% 31% 30% 6%	564
	21st to end	Night	12% 7% 21% 36% 21% 1	627
September	1st to 10th	Night	15% 4 23% 31% 22% 4	595
	11th to 20th	Night	16% 7% 22% 30% 21% 4	760
	21st to end	Night	21% 8% 20% 31% 18% 4	760
October	1st to 10th	Night	21% 11% 26% 28% 14% 1	760
	11th to 20th	Night	29% 14% 24% 25% 6%	760
	21st to end	Night	33% 13% 26% 23% 5%	836
November	1st to 10th	Night	26% 14% 32% 21% 7%	780
	11th to 20th	Night	24% 20% 31% 20% 5%	800
	21st to end	Night	32% 20% 32% 13% 3	850
December	1st to 10th	Night	25% 19% 33% 16% 6%	1000
	11th to 20th	Night	25% 20% 33% 16% 5% 1	1000
	21st to end	Night	31% 15% 33% 17% 4	1100

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