Nantucket Island
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nantucket Island clear sky chart from 2005-11-06 to 2026-10-29.

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% 10% 3 7% 4 4 4 3 5% 7% 36%	148966
	Night	17% 11% 4 8% 4 4 4 3 5% 6% 34%	143104
Transparency	Day	47% 6% 13% 21% 12% 1	152536
	Night	50% 5% 11% 19% 12% 2	147598
Seeing	Day	20% 8% 23% 29% 16% 4	17913
	Night	22% 10% 24% 28% 14% 2	26669

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	12% 11% 3 8% 4 4 4 3 5% 6% 39%	4661
	11th to 20th	Day	16% 9% 4 7% 4 3 3 3 5% 6% 40%	4650
	21st to end	Day	15% 9% 4 9% 6% 4 4 3 5% 6% 35%	5440
February	1st to 10th	Day	19% 10% 4 9% 5% 4 4 4 5% 5% 30%	1974
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 3 5% 6% 37%	2048
	21st to end	Day	19% 9% 5% 6% 4 5% 4 4 5% 5% 34%	1730
March	1st to 10th	Day	20% 14% 4 6% 4 3 3 2 4 6% 35%	4576
	11th to 20th	Day	15% 10% 4 8% 3 3 3 2 4 6% 41%	5034
	21st to end	Day	24% 12% 3 7% 5% 3 3 2 3 4 34%	5668
April	1st to 10th	Day	21% 10% 2 6% 3 2 3 2 5% 6% 40%	2520
	11th to 20th	Day	16% 8% 3 5% 4 4 3 3 5% 7% 43%	2555
	21st to end	Day	18% 7% 3 6% 4 4 3 3 5% 7% 39%	2730
May	1st to 10th	Day	21% 15% 4 9% 4 4 4 3 4 6% 26%	7639
	11th to 20th	Day	19% 13% 3 7% 4 4 3 2 4 6% 35%	7625
	21st to end	Day	21% 11% 4 8% 4 5% 4 3 4 6% 29%	8636
June	1st to 10th	Day	18% 9% 3 6% 4 4 5% 4 6% 8% 33%	2940
	11th to 20th	Day	15% 9% 3 5% 4 4 5% 5% 6% 8% 36%	2940
	21st to end	Day	13% 9% 3 6% 4 5% 5% 4 7% 9% 35%	2940
July	1st to 10th	Day	17% 11% 3 8% 4 4 4 3 5% 9% 32%	8280
	11th to 20th	Day	9% 9% 4 9% 5% 4 5% 4 6% 9% 36%	8280
	21st to end	Day	15% 9% 3 6% 4 4 4 4 6% 7% 38%	8636
August	1st to 10th	Day	15% 8% 3 7% 5% 4 5% 4 6% 9% 33%	6015
	11th to 20th	Day	10% 9% 3 7% 4 4 4 3 5% 8% 43%	5953
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 6% 8% 46%	6153
September	1st to 10th	Day	13% 7% 2 5% 4 4 5% 3 6% 7% 44%	2441
	11th to 20th	Day	13% 6% 2 6% 3 3 4 3 5% 7% 48%	2310
	21st to end	Day	21% 9% 3 6% 4 4 5% 3 6% 7% 34%	2310
October	1st to 10th	Day	19% 6% 3 6% 3 4 3 2 4 7% 44%	2310
	11th to 20th	Day	17% 7% 2 6% 4 3 4 3 4 6% 44%	2116
	21st to end	Day	16% 7% 3 8% 4 4 4 4 5% 7% 39%	2052
November	1st to 10th	Day	17% 7% 3 6% 5% 4 4 3 4 6% 41%	1851
	11th to 20th	Day	14% 9% 4 8% 4 5% 4 3 5% 7% 37%	1890
	21st to end	Day	17% 7% 3 6% 5% 4 4 3 6% 6% 40%	1890
December	1st to 10th	Day	16% 10% 4 7% 3 4 4 3 4 5% 39%	3020
	11th to 20th	Day	18% 12% 3 9% 4 4 3 4 3 5% 34%	3105
	21st to end	Day	18% 15% 9% 12% 4 4 3 3 5% 5% 22%	4048
January	1st to 10th	Night	15% 9% 3 8% 4 3 3 3 3 5% 44%	8234
	11th to 20th	Night	19% 11% 5% 9% 4 4 4 3 5% 5% 32%	8235
	21st to end	Night	16% 11% 5% 10% 5% 4 4 4 5% 5% 30%	8727
February	1st to 10th	Night	17% 11% 4 9% 6% 5% 5% 4 5% 5% 30%	2644
	11th to 20th	Night	19% 10% 3 6% 5% 5% 5% 4 5% 6% 33%	2458
	21st to end	Night	18% 9% 4 8% 4 4 4 4 5% 5% 35%	2076
March	1st to 10th	Night	23% 11% 3 6% 4 3 3 2 4 6% 35%	5484
	11th to 20th	Night	16% 12% 5% 8% 5% 4 4 3 4 5% 34%	5144
	21st to end	Night	23% 13% 3 7% 5% 4 3 3 4 4 31%	5503
April	1st to 10th	Night	21% 12% 2 7% 4 4 4 3 4 6% 35%	2100
	11th to 20th	Night	17% 9% 3 6% 5% 4 4 3 5% 6% 38%	2013
	21st to end	Night	19% 9% 3 7% 5% 4 4 3 4 6% 35%	1890
May	1st to 10th	Night	24% 13% 4 9% 4 3 2 2 4 5% 30%	5259
	11th to 20th	Night	20% 14% 4 7% 4 3 4 3 4 7% 31%	5235
	21st to end	Night	21% 14% 5% 10% 6% 5% 4 4 5% 5% 21%	5592
June	1st to 10th	Night	17% 11% 3 8% 4 3 5% 4 6% 7% 33%	1680
	11th to 20th	Night	17% 9% 2 6% 4 3 4 3 5% 8% 37%	1680
	21st to end	Night	15% 10% 4 8% 4 5% 4 4 5% 8% 33%	1680
July	1st to 10th	Night	20% 13% 3 7% 4 4 3 3 5% 7% 31%	4680
	11th to 20th	Night	11% 11% 4 7% 4 4 4 4 6% 9% 36%	4680
	21st to end	Night	16% 10% 3 7% 5% 4 4 3 5% 6% 35%	5382
August	1st to 10th	Night	15% 11% 3 7% 5% 5% 5% 4 6% 9% 31%	4135
	11th to 20th	Night	10% 9% 3 7% 4 4 4 4 6% 8% 42%	4092
	21st to end	Night	11% 6% 2 6% 4 4 4 4 6% 8% 45%	4968
September	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	2177
	11th to 20th	Night	13% 7% 2 6% 4 4 3 3 5% 6% 47%	2310
	21st to end	Night	20% 9% 3 7% 4 3 4 3 4 5% 38%	2310
October	1st to 10th	Night	20% 8% 3 5% 4 3 4 3 5% 5% 39%	2310
	11th to 20th	Night	17% 7% 2 6% 3 3 4 3 5% 8% 42%	2499
	21st to end	Night	17% 9% 3 7% 4 4 5% 3 5% 6% 37%	2965
November	1st to 10th	Night	16% 7% 3 6% 5% 4 4 3 4 7% 41%	2667
	11th to 20th	Night	16% 9% 3 6% 5% 4 4 4 6% 7% 36%	2730
	21st to end	Night	15% 9% 3 7% 4 4 4 3 6% 7% 38%	2730
December	1st to 10th	Night	17% 9% 3 8% 5% 4 4 4 5% 6% 35%	4380
	11th to 20th	Night	18% 13% 4 8% 4 4 4 3 4 5% 33%	5305
	21st to end	Night	16% 15% 7% 11% 5% 4 4 3 5% 6% 24%	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	47% 5% 14% 19% 15% 1	4662
	11th to 20th	Day	47% 5% 11% 20% 12% 4	4680
	21st to end	Day	53% 7% 12% 17% 11%	5440
February	1st to 10th	Day	56% 5% 12% 17% 9% 1	1974
	11th to 20th	Day	50% 5% 12% 21% 10% 1	2057
	21st to end	Day	52% 6% 11% 17% 12% 3	1730
March	1st to 10th	Day	54% 4 13% 12% 10% 7%	4579
	11th to 20th	Day	46% 4 15% 25% 9% 1	5034
	21st to end	Day	57% 4 6% 14% 15% 4	5668
April	1st to 10th	Day	46% 5% 11% 18% 18% 2	2520
	11th to 20th	Day	40% 5% 13% 24% 16% 2	2555
	21st to end	Day	45% 5% 11% 21% 16% 3	2730
May	1st to 10th	Day	59% 6% 11% 16% 7% 1	7651
	11th to 20th	Day	51% 5% 12% 21% 11% 1	7629
	21st to end	Day	55% 6% 11% 17% 12%	8637
June	1st to 10th	Day	48% 6% 10% 22% 13% 1	2940
	11th to 20th	Day	44% 6% 14% 23% 12% 1	2940
	21st to end	Day	44% 8% 14% 21% 13% 1	2940
July	1st to 10th	Day	50% 6% 14% 19% 10% 1	8210
	11th to 20th	Day	43% 7% 15% 22% 12% 1	8205
	21st to end	Day	43% 7% 12% 24% 13% 1	8446
August	1st to 10th	Day	46% 7% 18% 20% 8%	6002
	11th to 20th	Day	40% 7% 17% 29% 7%	5992
	21st to end	Day	34% 7% 15% 26% 17% 1	6153
September	1st to 10th	Day	39% 7% 10% 26% 17% 2	2441
	11th to 20th	Day	36% 4 11% 24% 22% 3	2310
	21st to end	Day	47% 6% 10% 20% 15% 2	2310
October	1st to 10th	Day	39% 5% 11% 29% 15% 2	3700
	11th to 20th	Day	35% 5% 11% 27% 21% 1	3345
	21st to end	Day	33% 5% 14% 33% 14% 2	3252
November	1st to 10th	Day	45% 6% 13% 20% 14% 2	1851
	11th to 20th	Day	46% 6% 13% 18% 15% 2	1890
	21st to end	Day	45% 7% 15% 24% 9%	1890
December	1st to 10th	Day	46% 6% 13% 25% 9%	3020
	11th to 20th	Day	54% 4 13% 16% 11% 2	3105
	21st to end	Day	65% 6% 9% 12% 6% 1	4048
January	1st to 10th	Night	45% 4 10% 26% 13% 1	8234
	11th to 20th	Night	55% 5% 11% 17% 9% 3	8280
	21st to end	Night	57% 5% 10% 15% 11% 2	8727
February	1st to 10th	Night	56% 7% 12% 16% 8% 1	2644
	11th to 20th	Night	52% 6% 10% 20% 11% 2	2460
	21st to end	Night	50% 5% 10% 22% 11% 2	2076
March	1st to 10th	Night	53% 4 11% 16% 12% 5%	5494
	11th to 20th	Night	53% 5% 14% 16% 10% 1	5145
	21st to end	Night	58% 3 5% 11% 19% 3	5503
April	1st to 10th	Night	52% 4 9% 17% 16% 2	2100
	11th to 20th	Night	47% 5% 10% 19% 15% 3	2013
	21st to end	Night	51% 5% 9% 18% 14% 3	1890
May	1st to 10th	Night	57% 5% 10% 16% 11% 1	5265
	11th to 20th	Night	55% 4 12% 18% 9% 2	5236
	21st to end	Night	63% 6% 8% 15% 7% 1	5593
June	1st to 10th	Night	49% 7% 9% 22% 12% 1	1680
	11th to 20th	Night	47% 6% 10% 21% 14% 2	1680
	21st to end	Night	50% 6% 11% 22% 11% 1	1680
July	1st to 10th	Night	55% 5% 10% 18% 12%	4640
	11th to 20th	Night	43% 7% 13% 21% 13% 1	4632
	21st to end	Night	49% 5% 11% 20% 14% 1	5258
August	1st to 10th	Night	49% 8% 18% 15% 10% 1	4123
	11th to 20th	Night	41% 6% 15% 31% 7%	4119
	21st to end	Night	37% 7% 14% 25% 16% 1	4968
September	1st to 10th	Night	40% 6% 10% 25% 17% 2	2177
	11th to 20th	Night	39% 4 10% 23% 21% 3	2310
	21st to end	Night	48% 5% 8% 18% 18% 3	2310
October	1st to 10th	Night	43% 6% 9% 27% 14% 2	3700
	11th to 20th	Night	33% 6% 11% 23% 25% 3	3984
	21st to end	Night	35% 4 11% 28% 19% 2	4715
November	1st to 10th	Night	45% 5% 12% 22% 14% 2	2667
	11th to 20th	Night	47% 6% 12% 20% 13% 2	2730
	21st to end	Night	45% 7% 14% 24% 9% 1	2730
December	1st to 10th	Night	48% 6% 12% 23% 10% 1	4380
	11th to 20th	Night	54% 5% 13% 16% 11% 1	5305
	21st to end	Night	63% 6% 12% 12% 7% 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	20% 14% 41% 20% 4	220
	11th to 20th	Day	23% 17% 35% 22% 3	220
	21st to end	Day	19% 19% 37% 18% 7%	351
February	1st to 10th	Day	27% 17% 30% 23% 3	474
	11th to 20th	Day	25% 16% 37% 19% 3	536
	21st to end	Day	23% 15% 35% 23% 3 1	446
March	1st to 10th	Day	24% 16% 33% 22% 4	540
	11th to 20th	Day	17% 16% 33% 27% 7% 1	540
	21st to end	Day	22% 14% 34% 21% 7% 2	594
April	1st to 10th	Day	27% 11% 28% 24% 8% 1	540
	11th to 20th	Day	21% 9% 29% 32% 9%	532
	21st to end	Day	22% 6% 25% 36% 10% 1	540
May	1st to 10th	Day	26% 7% 25% 33% 9% 1	543
	11th to 20th	Day	20% 4 23% 37% 13% 3	567
	21st to end	Day	22% 4 16% 35% 19% 5%	617
June	1st to 10th	Day	20% 4 14% 36% 23% 3	570
	11th to 20th	Day	16% 4 16% 40% 22% 2	570
	21st to end	Day	16% 3 11% 39% 25% 5%	570
July	1st to 10th	Day	16% 3 10% 32% 31% 7%	565
	11th to 20th	Day	11% 4 7% 38% 32% 7%	564
	21st to end	Day	15% 2 10% 38% 29% 5%	620
August	1st to 10th	Day	13% 2 10% 34% 35% 6%	566
	11th to 20th	Day	17% 3 9% 33% 30% 9%	559
	21st to end	Day	15% 2 13% 31% 29% 11%	627
September	1st to 10th	Day	16% 3 15% 31% 26% 9%	570
	11th to 20th	Day	14% 3 16% 30% 25% 10%	570
	21st to end	Day	25% 5% 19% 28% 21% 2	570
October	1st to 10th	Day	22% 6% 19% 24% 22% 7%	570
	11th to 20th	Day	19% 7% 26% 30% 15% 3	473
	21st to end	Day	21% 11% 26% 25% 13% 4	486
November	1st to 10th	Day	17% 11% 28% 28% 13% 3	425
	11th to 20th	Day	22% 15% 29% 26% 8%	430
	21st to end	Day	20% 17% 28% 25% 9% 1	430
December	1st to 10th	Day	21% 14% 31% 23% 9%	430
	11th to 20th	Day	26% 19% 30% 21% 4	246
	21st to end	Day	24% 15% 33% 18% 8% 1	242
January	1st to 10th	Night	24% 18% 33% 19% 5%	840
	11th to 20th	Night	30% 17% 31% 18% 4	840
	21st to end	Night	23% 19% 34% 19% 4 1	924
February	1st to 10th	Night	22% 18% 34% 23% 3	840
	11th to 20th	Night	24% 18% 37% 18% 3	823
	21st to end	Night	23% 16% 32% 24% 4	692
March	1st to 10th	Night	24% 17% 30% 24% 6%	840
	11th to 20th	Night	22% 11% 34% 25% 7% 1	840
	21st to end	Night	25% 10% 30% 25% 8% 2	924
April	1st to 10th	Night	28% 9% 27% 29% 6% 1	840
	11th to 20th	Night	22% 7% 28% 35% 8%	769
	21st to end	Night	23% 6% 24% 32% 13% 1	630
May	1st to 10th	Night	26% 6% 21% 34% 12% 2	630
	11th to 20th	Night	23% 5% 25% 36% 10% 1	622
	21st to end	Night	24% 2 14% 41% 17% 3	621
June	1st to 10th	Night	22% 3 15% 36% 21% 2	420
	11th to 20th	Night	20% 3 14% 39% 21% 2	420
	21st to end	Night	18% 3 13% 38% 28%	420
July	1st to 10th	Night	19% 2 9% 38% 29% 3	410
	11th to 20th	Night	14% 3 9% 37% 32% 5%	408
	21st to end	Night	15% 2 12% 38% 29% 3	565
August	1st to 10th	Night	13% 2 10% 38% 32% 4	628
	11th to 20th	Night	15% 2 9% 36% 33% 5%	622
	21st to end	Night	15% 3 9% 34% 31% 9%	884
September	1st to 10th	Night	17% 1 13% 34% 29% 5%	840
	11th to 20th	Night	17% 3 13% 32% 26% 9%	840
	21st to end	Night	25% 4 17% 30% 21% 4	840
October	1st to 10th	Night	24% 6% 20% 25% 19% 5%	840
	11th to 20th	Night	20% 8% 26% 27% 16% 3	840
	21st to end	Night	24% 7% 28% 27% 11% 2	913
November	1st to 10th	Night	21% 9% 26% 29% 14% 2	820
	11th to 20th	Night	22% 15% 29% 24% 9%	840
	21st to end	Night	19% 17% 29% 25% 9% 1	840
December	1st to 10th	Night	26% 15% 30% 21% 7% 1	840
	11th to 20th	Night	26% 19% 30% 20% 4	840
	21st to end	Night	25% 16% 32% 22% 5% 1	924

Page generated at 2026-10-26 04:37:51. Version ac07

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