Star Island
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Star Island clear sky chart from 2005-11-06 to 2026-05-02.

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	16% 7% 2 6% 4 4 4 4 5% 7% 40%	131251
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	131241
Transparency	Day	43% 5% 12% 23% 15% 2	134784
	Night	45% 5% 11% 21% 16% 2	135795
Seeing	Day	19% 4 19% 32% 21% 5%	16885
	Night	21% 7% 26% 30% 15% 2	25824

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% 8% 3 7% 5% 5% 5% 4 6% 7% 39%	4661
	11th to 20th	Day	16% 9% 2 6% 3 3 3 3 5% 6% 44%	4650
	21st to end	Day	15% 8% 2 5% 3 3 4 3 4 6% 48%	5124
February	1st to 10th	Day	17% 9% 3 6% 4 4 4 4 6% 7% 36%	1923
	11th to 20th	Day	14% 7% 2 6% 4 3 4 4 6% 8% 41%	2048
	21st to end	Day	15% 10% 3 7% 4 4 4 4 6% 6% 38%	1730
March	1st to 10th	Day	20% 10% 2 6% 4 4 5% 4 5% 6% 35%	4576
	11th to 20th	Day	17% 9% 3 6% 3 3 4 4 5% 7% 38%	4946
	21st to end	Day	28% 7% 2 4 3 4 4 3 6% 6% 33%	5791
April	1st to 10th	Day	19% 9% 2 6% 4 4 4 4 5% 6% 38%	2520
	11th to 20th	Day	17% 7% 2 5% 4 3 5% 5% 6% 6% 40%	2557
	21st to end	Day	18% 7% 2 6% 5% 4 4 3 6% 7% 39%	2730
May	1st to 10th	Day	28% 9% 3 6% 4 3 3 4 5% 7% 28%	5899
	11th to 20th	Day	18% 10% 3 9% 5% 4 4 3 5% 6% 32%	5903
	21st to end	Day	19% 7% 2 6% 4 4 4 4 5% 7% 38%	6949
June	1st to 10th	Day	20% 7% 2 6% 4 4 4 4 6% 7% 35%	2800
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 6% 8% 41%	2800
	21st to end	Day	16% 7% 2 6% 4 4 5% 4 6% 7% 38%	2800
July	1st to 10th	Day	13% 7% 3 9% 5% 5% 5% 5% 7% 9% 33%	6360
	11th to 20th	Day	6% 7% 3 7% 5% 4 5% 4 7% 11% 42%	6360
	21st to end	Day	13% 6% 2 5% 4 3 4 4 6% 8% 46%	6926
August	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 4 6% 7% 39%	4242
	11th to 20th	Day	14% 9% 3 7% 4 4 4 3 5% 8% 39%	4175
	21st to end	Day	7% 3 1 4 3 4 5% 5% 7% 10% 50%	4378
September	1st to 10th	Day	12% 4 2 5% 4 4 4 3 5% 7% 50%	2367
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 4 5% 56%	2200
	21st to end	Day	15% 7% 2 6% 4 3 4 3 4 6% 47%	2200
October	1st to 10th	Day	18% 6% 2 5% 4 3 4 3 4 5% 45%	2200
	11th to 20th	Day	16% 5% 2 5% 3 4 4 3 4 8% 46%	1947
	21st to end	Day	20% 6% 2 5% 4 3 4 3 4 6% 42%	1980
November	1st to 10th	Day	16% 6% 2 5% 3 2 3 3 4 6% 49%	1851
	11th to 20th	Day	17% 6% 2 4 3 3 3 3 4 7% 48%	1890
	21st to end	Day	20% 7% 2 5% 3 3 3 3 5% 6% 43%	1890
December	1st to 10th	Day	17% 8% 3 4 3 3 3 3 5% 6% 47%	2764
	11th to 20th	Day	21% 6% 1 4 3 2 3 3 4 6% 47%	3066
	21st to end	Day	22% 10% 2 6% 4 3 4 3 4 5% 36%	4048
January	1st to 10th	Night	15% 7% 3 5% 4 4 4 3 5% 6% 46%	8234
	11th to 20th	Night	16% 10% 3 8% 5% 5% 4 3 5% 5% 35%	8235
	21st to end	Night	16% 8% 3 6% 4 5% 4 3 5% 6% 41%	9057
February	1st to 10th	Night	16% 9% 3 7% 4 4 5% 4 6% 6% 37%	2690
	11th to 20th	Night	15% 9% 3 6% 4 4 4 4 6% 6% 39%	2458
	21st to end	Night	17% 9% 2 6% 4 3 5% 3 5% 6% 40%	2076
March	1st to 10th	Night	26% 9% 2 5% 3 3 3 3 4 5% 36%	5484
	11th to 20th	Night	14% 9% 4 8% 5% 4 4 4 5% 6% 36%	5231
	21st to end	Night	26% 9% 2 6% 4 3 4 3 5% 5% 32%	5367
April	1st to 10th	Night	21% 9% 2 7% 5% 3 3 3 5% 6% 37%	2100
	11th to 20th	Night	19% 9% 2 6% 4 3 4 3 5% 6% 39%	2011
	21st to end	Night	19% 9% 3 7% 4 4 4 3 5% 5% 37%	1890
May	1st to 10th	Night	29% 11% 3 7% 4 3 4 3 5% 7% 24%	4064
	11th to 20th	Night	16% 11% 3 8% 6% 5% 5% 4 6% 8% 28%	4033
	21st to end	Night	20% 9% 3 9% 5% 4 5% 4 5% 5% 32%	3926
June	1st to 10th	Night	20% 8% 2 7% 5% 4 4 3 5% 7% 35%	1600
	11th to 20th	Night	17% 8% 2 6% 4 3 4 5% 6% 9% 36%	1600
	21st to end	Night	18% 8% 2 6% 4 4 5% 4 6% 7% 36%	1600
July	1st to 10th	Night	11% 7% 3 7% 5% 5% 5% 5% 6% 10% 37%	3600
	11th to 20th	Night	10% 9% 2 6% 4 5% 5% 5% 6% 9% 38%	3600
	21st to end	Night	11% 6% 3 7% 4 4 4 3 6% 7% 46%	3926
August	1st to 10th	Night	17% 7% 3 8% 5% 5% 5% 4 6% 6% 34%	2873
	11th to 20th	Night	12% 8% 3 8% 5% 6% 6% 5% 7% 8% 33%	2872
	21st to end	Night	10% 5% 1 4 3 4 4 3 6% 8% 50%	3515
September	1st to 10th	Night	12% 5% 2 6% 3 4 4 4 5% 7% 48%	2032
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 4 5% 52%	2200
	21st to end	Night	15% 6% 2 5% 4 4 4 4 5% 8% 45%	2200
October	1st to 10th	Night	18% 7% 2 6% 4 3 3 3 5% 6% 43%	2200
	11th to 20th	Night	15% 5% 2 6% 4 4 4 3 5% 7% 44%	2448
	21st to end	Night	21% 8% 3 5% 4 3 3 3 5% 6% 39%	2860
November	1st to 10th	Night	15% 6% 2 5% 3 4 3 4 5% 6% 47%	2667
	11th to 20th	Night	16% 6% 2 6% 4 3 4 3 4 6% 46%	2730
	21st to end	Night	16% 8% 2 5% 4 4 4 3 5% 7% 42%	2730
December	1st to 10th	Night	19% 8% 2 5% 4 4 4 3 5% 5% 41%	4634
	11th to 20th	Night	21% 8% 3 5% 3 3 3 3 4 6% 41%	5348
	21st to end	Night	20% 9% 3 6% 4 4 4 3 4 6% 38%	7150

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	44% 4 12% 24% 15% 1	4662
	11th to 20th	Day	42% 4 10% 23% 18% 3	4680
	21st to end	Day	40% 4 8% 25% 21% 2	5124
February	1st to 10th	Day	47% 5% 11% 21% 13% 3	1923
	11th to 20th	Day	41% 5% 12% 22% 16% 3	2057
	21st to end	Day	45% 5% 11% 21% 15% 3	1730
March	1st to 10th	Day	50% 4 9% 14% 13% 10%	4579
	11th to 20th	Day	44% 6% 13% 23% 10% 4	4946
	21st to end	Day	52% 2 7% 16% 16% 7%	5791
April	1st to 10th	Day	46% 5% 9% 18% 18% 4	2520
	11th to 20th	Day	42% 5% 11% 22% 16% 4	2557
	21st to end	Day	45% 5% 8% 20% 19% 4	2730
May	1st to 10th	Day	56% 7% 13% 16% 7% 1	5910
	11th to 20th	Day	52% 5% 13% 18% 9% 2	5907
	21st to end	Day	45% 6% 13% 21% 13% 1	6949
June	1st to 10th	Day	47% 6% 11% 24% 12% 1	2800
	11th to 20th	Day	40% 6% 14% 21% 17% 3	2800
	21st to end	Day	44% 7% 12% 23% 14% 1	2800
July	1st to 10th	Day	47% 7% 12% 20% 14% 1	6290
	11th to 20th	Day	36% 7% 16% 26% 14% 1	6285
	21st to end	Day	36% 5% 14% 29% 15% 1	6728
August	1st to 10th	Day	42% 7% 16% 25% 10% 1	4228
	11th to 20th	Day	43% 6% 14% 27% 10% 1	4214
	21st to end	Day	27% 6% 15% 32% 19% 2	4378
September	1st to 10th	Day	35% 5% 11% 26% 22% 1	2367
	11th to 20th	Day	33% 4 9% 29% 23% 2	2200
	21st to end	Day	39% 5% 9% 25% 20% 2	2200
October	1st to 10th	Day	40% 4 10% 25% 19% 2	3590
	11th to 20th	Day	31% 4 7% 32% 23% 3	3150
	21st to end	Day	32% 3 10% 31% 21% 2	3180
November	1st to 10th	Day	39% 3 13% 27% 16% 2	1851
	11th to 20th	Day	38% 3 9% 22% 24% 3	1890
	21st to end	Day	43% 5% 14% 23% 14% 1	1890
December	1st to 10th	Day	39% 4 14% 30% 12% 1	2764
	11th to 20th	Day	41% 4 13% 17% 19% 6%	3066
	21st to end	Day	49% 5% 8% 20% 13% 5%	4048
January	1st to 10th	Night	39% 3 11% 29% 15% 2	8234
	11th to 20th	Night	51% 5% 9% 18% 13% 4	8280
	21st to end	Night	45% 3 8% 20% 19% 4	9057
February	1st to 10th	Night	47% 6% 12% 19% 14% 3	2690
	11th to 20th	Night	46% 5% 9% 22% 15% 3	2460
	21st to end	Night	45% 5% 8% 25% 13% 4	2076
March	1st to 10th	Night	52% 3 8% 14% 15% 7%	5494
	11th to 20th	Night	48% 6% 13% 18% 13% 2	5232
	21st to end	Night	54% 3 5% 12% 19% 6%	5367
April	1st to 10th	Night	49% 4 8% 18% 16% 4	2100
	11th to 20th	Night	46% 4 8% 20% 19% 3	2011
	21st to end	Night	49% 5% 8% 17% 19% 3	1890
May	1st to 10th	Night	60% 6% 12% 13% 8% 2	4070
	11th to 20th	Night	53% 7% 13% 16% 9% 2	4034
	21st to end	Night	53% 7% 11% 17% 11% 1	3927
June	1st to 10th	Night	49% 5% 11% 21% 13% 1	1600
	11th to 20th	Night	43% 7% 13% 20% 15% 2	1600
	21st to end	Night	46% 6% 12% 22% 13% 1	1600
July	1st to 10th	Night	43% 6% 12% 22% 16%	3560
	11th to 20th	Night	41% 7% 16% 21% 16%	3552
	21st to end	Night	37% 5% 12% 31% 14% 1	3809
August	1st to 10th	Night	49% 6% 12% 20% 12% 1	2863
	11th to 20th	Night	46% 7% 14% 21% 11% 1	2899
	21st to end	Night	31% 6% 15% 26% 22% 1	3515
September	1st to 10th	Night	35% 4 11% 28% 21% 1	2032
	11th to 20th	Night	34% 4 9% 27% 24% 2	2200
	21st to end	Night	38% 5% 9% 26% 19% 2	2200
October	1st to 10th	Night	42% 4 14% 23% 14% 2	3590
	11th to 20th	Night	29% 4 10% 28% 27% 2	3984
	21st to end	Night	35% 3 11% 31% 18% 2	4610
November	1st to 10th	Night	38% 4 11% 26% 18% 3	2667
	11th to 20th	Night	41% 4 9% 22% 20% 3	2730
	21st to end	Night	43% 4 14% 25% 13% 2	2730
December	1st to 10th	Night	46% 4 10% 29% 11% 1	4634
	11th to 20th	Night	46% 4 12% 17% 20% 1	5348
	21st to end	Night	48% 5% 10% 21% 16% 1	7150

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	22% 7% 32% 28% 10% 1	220
	11th to 20th	Day	17% 11% 35% 27% 8% 1	220
	21st to end	Day	18% 5% 39% 26% 11%	242
February	1st to 10th	Day	23% 8% 38% 27% 4	433
	11th to 20th	Day	19% 8% 38% 29% 5%	536
	21st to end	Day	20% 9% 28% 34% 9% 1	446
March	1st to 10th	Day	22% 10% 28% 30% 10% 1	540
	11th to 20th	Day	19% 6% 33% 34% 8%	540
	21st to end	Day	23% 6% 29% 31% 9% 3	594
April	1st to 10th	Day	25% 5% 25% 31% 12% 3	540
	11th to 20th	Day	20% 3 20% 42% 15% 1	532
	21st to end	Day	22% 2 17% 38% 18% 2	540
May	1st to 10th	Day	25% 3 16% 38% 15% 3	507
	11th to 20th	Day	18% 3 15% 36% 24% 5%	527
	21st to end	Day	20% 1 9% 35% 28% 6%	573
June	1st to 10th	Day	20% 2 9% 34% 29% 7%	530
	11th to 20th	Day	15% 3 11% 38% 28% 6%	530
	21st to end	Day	18% 2 8% 37% 26% 9%	530
July	1st to 10th	Day	15% 2 4 33% 36% 10%	525
	11th to 20th	Day	11% 3 5% 31% 41% 9%	524
	21st to end	Day	11% 2 6% 34% 41% 6%	576
August	1st to 10th	Day	11% 2 5% 32% 40% 10%	526
	11th to 20th	Day	14% 2 5% 30% 34% 14%	519
	21st to end	Day	11% 1 6% 37% 34% 10%	583
September	1st to 10th	Day	15% 2 10% 31% 30% 13%	530
	11th to 20th	Day	18% 1 12% 26% 29% 13%	530
	21st to end	Day	16% 3 14% 36% 23% 8%	530
October	1st to 10th	Day	21% 3 17% 28% 24% 9%	530
	11th to 20th	Day	17% 5% 21% 31% 21% 5%	445
	21st to end	Day	25% 3 24% 32% 14% 2	462
November	1st to 10th	Day	15% 7% 28% 30% 18% 2	425
	11th to 20th	Day	21% 8% 29% 33% 9%	430
	21st to end	Day	24% 8% 31% 26% 10% 1	430
December	1st to 10th	Day	20% 5% 31% 32% 10% 1	278
	11th to 20th	Day	23% 7% 36% 29% 5%	220
	21st to end	Day	24% 8% 30% 26% 12% 1	242
January	1st to 10th	Night	21% 14% 41% 20% 4	840
	11th to 20th	Night	25% 12% 38% 21% 4	840
	21st to end	Night	20% 13% 41% 21% 5%	924
February	1st to 10th	Night	22% 13% 37% 25% 4	840
	11th to 20th	Night	21% 11% 46% 18% 4	823
	21st to end	Night	21% 9% 37% 26% 7%	692
March	1st to 10th	Night	24% 10% 36% 26% 4	840
	11th to 20th	Night	21% 8% 33% 28% 9% 1	840
	21st to end	Night	25% 7% 30% 29% 8% 1	924
April	1st to 10th	Night	25% 5% 28% 32% 9% 1	840
	11th to 20th	Night	25% 3 24% 36% 11% 1	767
	21st to end	Night	23% 4 21% 36% 14% 2	630
May	1st to 10th	Night	26% 2 19% 37% 14% 3	604
	11th to 20th	Night	21% 2 18% 38% 19% 3	592
	21st to end	Night	24% 1 10% 39% 23% 3	433
June	1st to 10th	Night	25% 2 9% 40% 21% 4	400
	11th to 20th	Night	20% 2 10% 40% 26% 2	400
	21st to end	Night	19% 3 7% 43% 25% 3	400
July	1st to 10th	Night	19% 1 6% 38% 32% 4	390
	11th to 20th	Night	12% 2 4 40% 36% 5%	388
	21st to end	Night	14% 2 6% 39% 35% 4	438
August	1st to 10th	Night	13% 1 4 39% 40% 4	568
	11th to 20th	Night	11% 2 9% 37% 34% 6%	592
	21st to end	Night	15% 1 8% 35% 34% 7%	835
September	1st to 10th	Night	14% 1 13% 35% 31% 6%	800
	11th to 20th	Night	16% 3 14% 32% 25% 9%	800
	21st to end	Night	17% 1 16% 34% 26% 6%	800
October	1st to 10th	Night	21% 4 21% 28% 21% 5%	800
	11th to 20th	Night	18% 5% 25% 34% 16% 2	800
	21st to end	Night	25% 5% 28% 28% 13% 1	880
November	1st to 10th	Night	17% 5% 31% 34% 11% 2	820
	11th to 20th	Night	19% 9% 36% 26% 8% 1	840
	21st to end	Night	20% 13% 34% 27% 6% 1	840
December	1st to 10th	Night	24% 9% 38% 23% 5% 1	840
	11th to 20th	Night	23% 13% 38% 22% 4	840
	21st to end	Night	26% 12% 33% 24% 4	924

Page generated at 2026-04-29 02:21:46. Version ac07

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