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 2024-11-02.

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%	101315
	Night	15% 12% 4 10% 6% 5% 4 3 5% 6% 30%	97744
Transparency	Day	26% 50% 6% 12% 5%	101071
	Night	29% 50% 6% 9% 5%	97607
Seeing	Day	18% 13% 30% 25% 11% 3	15613
	Night	20% 12% 29% 27% 11% 1	25100

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	13% 17% 11% 22% 9% 6% 6% 3 3 3 8%	2527
	11th to 20th	Day	13% 17% 6% 17% 15% 8% 5% 4 5% 4 8%	2530
	21st to end	Day	12% 21% 10% 15% 8% 6% 5% 4 4 4 11%	2783
February	1st to 10th	Day	9% 14% 5% 12% 7% 6% 5% 5% 6% 7% 24%	1631
	11th to 20th	Day	12% 13% 5% 11% 7% 6% 5% 4 6% 7% 24%	1787
	21st to end	Day	11% 13% 5% 11% 7% 6% 5% 4 6% 7% 25%	1570
March	1st to 10th	Day	11% 12% 3 8% 5% 5% 3 3 6% 7% 37%	3118
	11th to 20th	Day	8% 12% 5% 12% 7% 6% 5% 4 5% 6% 31%	3433
	21st to end	Day	13% 11% 7% 11% 6% 6% 5% 4 7% 7% 25%	3901
April	1st to 10th	Day	18% 11% 4 6% 5% 4 5% 4 6% 7% 31%	2280
	11th to 20th	Day	18% 9% 3 7% 5% 3 4 3 5% 6% 35%	2394
	21st to end	Day	15% 8% 3 9% 6% 5% 4 4 6% 6% 34%	2470
May	1st to 10th	Day	15% 7% 3 6% 4 4 4 3 5% 8% 41%	4108
	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%	1681
	11th to 20th	Day	14% 18% 5% 14% 7% 5% 5% 3 6% 5% 16%	1687
	21st to end	Day	20% 21% 7% 13% 6% 4 5% 3 4 5% 12%	1468
December	1st to 10th	Day	15% 23% 9% 17% 6% 5% 4 2 3 4 11%	1330
	11th to 20th	Day	16% 20% 7% 16% 9% 6% 6% 3 4 4 10%	1671
	21st to end	Day	22% 20% 6% 17% 7% 5% 4 2 3 3 9%	2445
January	1st to 10th	Night	14% 18% 8% 18% 7% 5% 5% 2 4 4 13%	4428
	11th to 20th	Night	11% 15% 6% 19% 13% 8% 5% 3 4 4 11%	4440
	21st to end	Night	15% 21% 8% 14% 8% 6% 5% 3 4 4 12%	4884
February	1st to 10th	Night	9% 16% 6% 12% 9% 6% 6% 4 5% 5% 22%	2547
	11th to 20th	Night	10% 13% 5% 12% 8% 5% 5% 4 5% 6% 25%	2274
	21st to end	Night	13% 15% 5% 9% 7% 5% 5% 4 5% 6% 25%	1884
March	1st to 10th	Night	16% 11% 3 6% 4 3 3 3 4 7% 41%	3722
	11th to 20th	Night	8% 12% 4 9% 6% 4 4 4 6% 7% 37%	3525
	21st to end	Night	14% 10% 6% 10% 4 4 6% 4 6% 6% 31%	3759
April	1st to 10th	Night	19% 10% 3 8% 5% 5% 5% 5% 6% 6% 29%	1900
	11th to 20th	Night	19% 10% 3 6% 5% 4 4 3 4 6% 36%	1735
	21st to end	Night	17% 9% 3 8% 4 4 4 4 5% 6% 36%	1710
May	1st to 10th	Night	18% 7% 3 8% 5% 4 4 4 4 6% 37%	2833
	11th to 20th	Night	12% 8% 3 8% 6% 4 5% 3 5% 7% 39%	2665
	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% 22%	2717
November	1st to 10th	Night	18% 15% 4 9% 5% 4 4 4 5% 6% 26%	2420
	11th to 20th	Night	17% 18% 4 13% 7% 6% 4 3 5% 5% 18%	2493
	21st to end	Night	20% 21% 6% 12% 7% 5% 4 3 4 4 13%	2707
December	1st to 10th	Night	14% 24% 8% 15% 9% 6% 4 3 4 4 12%	2850
	11th to 20th	Night	16% 18% 6% 17% 9% 6% 4 3 4 4 14%	3526
	21st to end	Night	23% 21% 7% 15% 7% 5% 4 3 3 3 9%	5204

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	53% 42% 3 2	2528
	11th to 20th	Day	53% 40% 4 2 1	2530
	21st to end	Day	48% 45% 4 2 1	2783
February	1st to 10th	Day	20% 67% 5% 6% 3	1631
	11th to 20th	Day	21% 65% 5% 5% 4	1797
	21st to end	Day	19% 66% 5% 6% 4	1570
March	1st to 10th	Day	27% 42% 10% 14% 7% 1	3120
	11th to 20th	Day	33% 45% 8% 11% 3 1	3433
	21st to end	Day	36% 46% 6% 7% 4 1	3901
April	1st to 10th	Day	19% 67% 4 7% 3	2280
	11th to 20th	Day	17% 66% 3 7% 5% 1	2394
	21st to end	Day	17% 66% 4 7% 6% 1	2470
May	1st to 10th	Day	22% 43% 8% 15% 10% 2	4120
	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	22% 61% 3 10% 5%	2066
	11th to 20th	Day	26% 60% 4 8% 2	1805
	21st to end	Day	29% 60% 4 4 2	1881
November	1st to 10th	Day	28% 59% 4 7% 1	1681
	11th to 20th	Day	29% 62% 5% 3 1	1687
	21st to end	Day	33% 62% 3 2 1	1468
December	1st to 10th	Day	32% 61% 4 3 1	1330
	11th to 20th	Day	40% 51% 4 4 1	1671
	21st to end	Day	52% 40% 3 3 2	2445
January	1st to 10th	Night	44% 42% 8% 6% 1	4428
	11th to 20th	Night	49% 42% 5% 3 1	4440
	21st to end	Night	47% 44% 5% 3 1	4884
February	1st to 10th	Night	21% 66% 4 5% 3	2547
	11th to 20th	Night	20% 65% 5% 8% 2	2275
	21st to end	Night	19% 66% 5% 6% 5%	1884
March	1st to 10th	Night	25% 42% 11% 16% 6%	3733
	11th to 20th	Night	26% 44% 10% 16% 5% 1	3526
	21st to end	Night	31% 45% 9% 10% 5% 1	3759
April	1st to 10th	Night	21% 66% 3 6% 4	1900
	11th to 20th	Night	19% 66% 3 8% 3 2	1735
	21st to end	Night	18% 65% 4 6% 6% 1	1710
May	1st to 10th	Night	26% 44% 6% 16% 7% 1	2840
	11th to 20th	Night	23% 44% 6% 19% 8%	2666
	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	23% 60% 4 9% 4	2113
	11th to 20th	Night	24% 60% 6% 7% 2	2370
	21st to end	Night	27% 61% 5% 5% 2	2717
November	1st to 10th	Night	28% 59% 4 5% 3	2420
	11th to 20th	Night	27% 60% 6% 5% 2	2493
	21st to end	Night	32% 61% 4 2 1	2707
December	1st to 10th	Night	30% 61% 5% 4	2850
	11th to 20th	Night	39% 52% 5% 3 1	3525
	21st to end	Night	52% 40% 3 3 1	5204

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	21% 20% 33% 21% 6%	270
	11th to 20th	Day	21% 17% 39% 17% 6% 1	270
	21st to end	Day	20% 18% 32% 24% 6%	297
February	1st to 10th	Day	17% 26% 35% 17% 4 1	270
	11th to 20th	Day	16% 28% 34% 17% 4	270
	21st to end	Day	22% 27% 34% 16%	225
March	1st to 10th	Day	19% 19% 39% 17% 6%	270
	11th to 20th	Day	18% 13% 41% 20% 6% 2	426
	21st to end	Day	24% 16% 28% 24% 7% 1	506
April	1st to 10th	Day	22% 16% 34% 20% 6% 1	460
	11th to 20th	Day	25% 18% 27% 19% 8% 2	452
	21st to end	Day	20% 17% 34% 19% 8% 2	460
May	1st to 10th	Day	22% 16% 29% 22% 9% 3	463
	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	30% 16% 33% 17% 3 1	330
November	1st to 10th	Day	24% 21% 34% 17% 4 1	273
	11th to 20th	Day	21% 29% 31% 14% 5%	259
	21st to end	Day	25% 28% 32% 14% 1	180
December	1st to 10th	Day	21% 23% 34% 16% 4 2	180
	11th to 20th	Day	25% 27% 31% 13% 3 1	180
	21st to end	Day	30% 25% 28% 12% 3 1	204
January	1st to 10th	Night	21% 18% 32% 23% 5%	950
	11th to 20th	Night	22% 17% 31% 22% 7%	950
	21st to end	Night	20% 12% 34% 27% 6% 1	1045
February	1st to 10th	Night	16% 16% 36% 26% 5%	837
	11th to 20th	Night	18% 15% 33% 26% 7%	743
	21st to end	Night	20% 14% 36% 25% 5%	628
March	1st to 10th	Night	18% 12% 34% 28% 9% 1	760
	11th to 20th	Night	15% 14% 36% 28% 7%	760
	21st to end	Night	19% 10% 30% 31% 10%	803
April	1st to 10th	Night	23% 10% 30% 29% 6% 1	570
	11th to 20th	Night	23% 12% 28% 27% 10% 1	563
	21st to end	Night	19% 10% 30% 29% 11% 1	570
May	1st to 10th	Night	18% 6% 27% 34% 14%	570
	11th to 20th	Night	19% 5% 27% 34% 14% 1	561
	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	27% 14% 33% 20% 7%	745
	11th to 20th	Night	25% 20% 30% 20% 6%	760
	21st to end	Night	31% 19% 33% 13% 3	807
December	1st to 10th	Night	26% 19% 33% 16% 6%	950
	11th to 20th	Night	26% 20% 33% 15% 5% 1	950
	21st to end	Night	31% 16% 33% 17% 3	1045

Page generated at 2024-10-30 01:22:56. Version ac07

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