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 2025-07-06.

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	17% 10% 3 7% 4 4 4 3 5% 7% 36%	119501
	Night	18% 11% 4 8% 4 4 4 3 5% 6% 34%	119301
Transparency	Day	48% 6% 12% 20% 12% 2	123112
	Night	50% 5% 11% 19% 12% 2	123826
Seeing	Day	20% 8% 23% 29% 16% 4	16172
	Night	22% 10% 24% 28% 13% 2	25003

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% 9% 3 7% 4 4 4 4 5% 6% 41%	3597
	11th to 20th	Day	15% 9% 3 7% 4 3 3 3 5% 6% 41%	3570
	21st to end	Day	18% 10% 5% 10% 6% 4 4 2 4 5% 31%	4207
February	1st to 10th	Day	19% 10% 4 9% 5% 5% 4 4 6% 5% 30%	1880
	11th to 20th	Day	17% 8% 3 7% 5% 5% 4 3 5% 6% 37%	1948
	21st to end	Day	19% 9% 5% 6% 4 5% 4 4 5% 6% 34%	1650
March	1st to 10th	Day	19% 14% 4 6% 3 3 3 2 4 6% 35%	4476
	11th to 20th	Day	15% 10% 4 8% 3 3 3 2 4 6% 41%	4925
	21st to end	Day	24% 12% 3 7% 5% 3 3 2 3 4 34%	5546
April	1st to 10th	Day	21% 10% 3 6% 3 3 3 2 5% 6% 40%	2400
	11th to 20th	Day	16% 8% 3 5% 4 3 3 3 5% 7% 43%	2432
	21st to end	Day	18% 7% 3 6% 4 5% 3 3 5% 7% 39%	2600
May	1st to 10th	Day	22% 16% 4 9% 4 4 4 3 5% 7% 21%	5886
	11th to 20th	Day	20% 15% 4 7% 4 4 4 3 5% 6% 30%	5861
	21st to end	Day	18% 12% 4 8% 5% 5% 4 3 4 6% 32%	6623
June	1st to 10th	Day	19% 10% 3 6% 4 4 5% 4 5% 8% 32%	2800
	11th to 20th	Day	15% 9% 3 5% 4 4 5% 5% 6% 8% 36%	2800
	21st to end	Day	13% 9% 3 6% 4 5% 5% 4 7% 9% 35%	2800
July	1st to 10th	Day	16% 12% 4 8% 4 4 4 3 5% 7% 34%	4510
	11th to 20th	Day	9% 7% 3 8% 5% 5% 6% 5% 7% 10% 35%	4440
	21st to end	Day	17% 9% 3 6% 4 4 4 4 7% 8% 34%	4625
August	1st to 10th	Day	19% 9% 3 8% 5% 4 4 3 5% 7% 32%	4105
	11th to 20th	Day	11% 9% 3 8% 4 5% 4 3 6% 8% 40%	4045
	21st to end	Day	7% 4 2 5% 4 4 5% 5% 7% 9% 49%	4226
September	1st to 10th	Day	13% 6% 2 6% 4 4 5% 3 6% 7% 45%	2209
	11th to 20th	Day	13% 6% 2 6% 3 3 4 3 6% 7% 47%	2090
	21st to end	Day	20% 8% 3 5% 4 4 5% 3 6% 7% 36%	2090
October	1st to 10th	Day	20% 6% 3 6% 3 4 3 2 4 7% 41%	2090
	11th to 20th	Day	15% 6% 2 5% 4 3 4 2 5% 7% 46%	1914
	21st to end	Day	17% 7% 3 8% 4 4 4 4 5% 7% 38%	1881
November	1st to 10th	Day	17% 7% 3 6% 5% 4 4 3 4 6% 41%	1761
	11th to 20th	Day	15% 9% 4 7% 4 5% 4 4 6% 7% 37%	1800
	21st to end	Day	17% 8% 3 6% 5% 4 4 3 6% 6% 39%	1800
December	1st to 10th	Day	17% 10% 4 7% 3 4 4 3 4 5% 39%	2930
	11th to 20th	Day	18% 12% 3 9% 4 4 3 4 3 5% 34%	3024
	21st to end	Day	18% 15% 9% 12% 4 4 4 3 5% 5% 21%	3960
January	1st to 10th	Night	15% 8% 2 8% 4 4 3 3 4 5% 44%	6349
	11th to 20th	Night	17% 12% 5% 9% 4 3 4 3 4 5% 33%	6315
	21st to end	Night	18% 11% 6% 9% 5% 4 4 3 5% 5% 30%	6748
February	1st to 10th	Night	18% 11% 4 9% 6% 5% 5% 4 5% 5% 29%	2518
	11th to 20th	Night	19% 10% 3 6% 5% 5% 5% 4 5% 6% 33%	2338
	21st to end	Night	18% 9% 4 8% 4 4 4 4 5% 5% 35%	1980
March	1st to 10th	Night	23% 11% 3 6% 4 3 3 2 4 6% 35%	5364
	11th to 20th	Night	16% 12% 5% 9% 5% 4 4 3 4 5% 34%	5033
	21st to end	Night	23% 13% 3 7% 5% 4 3 3 4 4 32%	5384
April	1st to 10th	Night	21% 11% 2 7% 4 4 4 3 4 6% 34%	2000
	11th to 20th	Night	18% 9% 3 6% 5% 3 4 3 4 6% 38%	1916
	21st to end	Night	19% 9% 4 8% 5% 4 4 4 5% 6% 35%	1800
May	1st to 10th	Night	24% 14% 4 11% 4 3 3 2 4 5% 25%	4054
	11th to 20th	Night	23% 15% 4 8% 4 4 4 3 4 6% 27%	4033
	21st to end	Night	17% 15% 6% 10% 5% 4 4 4 5% 6% 24%	4292
June	1st to 10th	Night	18% 11% 3 8% 4 3 5% 4 6% 7% 32%	1600
	11th to 20th	Night	17% 9% 2 6% 4 4 4 3 5% 8% 37%	1600
	21st to end	Night	15% 11% 3 8% 4 5% 4 4 6% 8% 33%	1600
July	1st to 10th	Night	22% 12% 3 6% 4 4 4 3 5% 8% 31%	2560
	11th to 20th	Night	11% 10% 3 7% 4 5% 5% 4 7% 11% 33%	2520
	21st to end	Night	17% 10% 3 7% 5% 5% 4 4 5% 7% 32%	2897
August	1st to 10th	Night	19% 11% 3 7% 5% 5% 4 3 6% 8% 29%	2825
	11th to 20th	Night	11% 9% 3 7% 4 4 4 4 6% 7% 41%	2782
	21st to end	Night	9% 5% 2 5% 3 3 4 4 6% 8% 51%	3427
September	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	1969
	11th to 20th	Night	13% 7% 3 6% 4 4 3 3 4 6% 48%	2090
	21st to end	Night	19% 8% 3 7% 4 3 5% 3 4 5% 39%	2090
October	1st to 10th	Night	21% 9% 3 6% 4 3 4 3 5% 5% 37%	2090
	11th to 20th	Night	16% 8% 2 6% 3 3 4 3 5% 7% 43%	2261
	21st to end	Night	18% 9% 3 7% 4 4 5% 3 5% 6% 36%	2717
November	1st to 10th	Night	16% 7% 3 7% 5% 4 4 3 4 7% 41%	2537
	11th to 20th	Night	17% 10% 3 6% 5% 4 3 4 6% 7% 36%	2600
	21st to end	Night	15% 9% 3 7% 4 4 4 3 6% 7% 38%	2600
December	1st to 10th	Night	17% 9% 3 7% 5% 4 4 4 5% 6% 35%	4250
	11th to 20th	Night	18% 13% 4 8% 4 4 4 3 4 5% 33%	5166
	21st to end	Night	16% 15% 7% 11% 5% 4 4 3 5% 6% 24%	6996

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	45% 6% 10% 21% 17% 1	3598
	11th to 20th	Day	46% 6% 13% 21% 9% 5%	3600
	21st to end	Day	58% 6% 9% 15% 11% 1	4207
February	1st to 10th	Day	56% 6% 13% 16% 9% 1	1880
	11th to 20th	Day	51% 5% 13% 20% 10% 1	1957
	21st to end	Day	52% 6% 11% 17% 12% 3	1650
March	1st to 10th	Day	54% 4 13% 12% 10% 7%	4479
	11th to 20th	Day	46% 4 15% 25% 9% 1	4925
	21st to end	Day	57% 4 6% 14% 15% 4	5546
April	1st to 10th	Day	46% 4 11% 19% 18% 2	2400
	11th to 20th	Day	40% 6% 12% 24% 16% 2	2432
	21st to end	Day	44% 5% 11% 21% 16% 3	2600
May	1st to 10th	Day	63% 7% 13% 12% 5% 1	5897
	11th to 20th	Day	55% 5% 11% 17% 11% 1	5865
	21st to end	Day	52% 6% 11% 16% 14% 1	6623
June	1st to 10th	Day	50% 6% 10% 21% 12% 1	2800
	11th to 20th	Day	44% 6% 15% 23% 12% 1	2800
	21st to end	Day	44% 8% 14% 21% 12% 1	2800
July	1st to 10th	Day	50% 7% 11% 18% 13% 1	4440
	11th to 20th	Day	41% 9% 19% 21% 10% 1	4365
	21st to end	Day	45% 8% 15% 20% 11% 1	4478
August	1st to 10th	Day	50% 6% 16% 20% 7%	4092
	11th to 20th	Day	42% 7% 15% 27% 8%	4084
	21st to end	Day	29% 8% 15% 24% 22% 1	4226
September	1st to 10th	Day	38% 7% 11% 24% 18% 2	2209
	11th to 20th	Day	36% 5% 12% 23% 21% 3	2090
	21st to end	Day	46% 6% 9% 20% 17% 2	2090
October	1st to 10th	Day	40% 6% 11% 28% 13% 2	3480
	11th to 20th	Day	33% 5% 11% 27% 22% 1	3143
	21st to end	Day	33% 5% 14% 32% 14% 1	3081
November	1st to 10th	Day	46% 6% 13% 20% 14% 2	1761
	11th to 20th	Day	46% 6% 13% 18% 15% 2	1800
	21st to end	Day	45% 7% 16% 23% 9%	1800
December	1st to 10th	Day	46% 6% 13% 25% 10%	2930
	11th to 20th	Day	54% 4 13% 16% 10% 2	3024
	21st to end	Day	65% 6% 9% 12% 6% 1	3960
January	1st to 10th	Night	43% 5% 10% 29% 12% 2	6349
	11th to 20th	Night	54% 6% 12% 17% 9% 3	6360
	21st to end	Night	58% 4 10% 16% 10% 2	6748
February	1st to 10th	Night	57% 7% 13% 16% 7% 1	2518
	11th to 20th	Night	53% 6% 10% 19% 11% 2	2340
	21st to end	Night	51% 5% 10% 22% 11% 2	1980
March	1st to 10th	Night	53% 4 11% 15% 12% 5%	5374
	11th to 20th	Night	53% 5% 14% 16% 10% 1	5034
	21st to end	Night	58% 3 5% 11% 19% 4	5384
April	1st to 10th	Night	52% 4 8% 18% 15% 3	2000
	11th to 20th	Night	48% 5% 10% 19% 16% 3	1916
	21st to end	Night	50% 5% 9% 18% 14% 3	1800
May	1st to 10th	Night	62% 6% 11% 12% 8% 1	4060
	11th to 20th	Night	59% 5% 11% 15% 7% 2	4034
	21st to end	Night	61% 6% 9% 16% 8% 1	4293
June	1st to 10th	Night	51% 7% 9% 21% 11% 1	1600
	11th to 20th	Night	47% 6% 10% 21% 14% 1	1600
	21st to end	Night	49% 6% 11% 22% 11% 1	1600
July	1st to 10th	Night	54% 5% 9% 18% 14% 1	2520
	11th to 20th	Night	45% 9% 17% 18% 10% 1	2472
	21st to end	Night	51% 6% 15% 16% 10% 1	2804
August	1st to 10th	Night	53% 7% 14% 16% 9%	2813
	11th to 20th	Night	42% 6% 15% 30% 7%	2809
	21st to end	Night	30% 6% 14% 26% 21% 2	3427
September	1st to 10th	Night	40% 6% 9% 24% 19% 2	1969
	11th to 20th	Night	39% 4 10% 23% 21% 2	2090
	21st to end	Night	47% 5% 8% 18% 19% 3	2090
October	1st to 10th	Night	44% 6% 9% 26% 14% 1	3480
	11th to 20th	Night	32% 6% 11% 23% 25% 3	3746
	21st to end	Night	36% 4 11% 28% 19% 1	4467
November	1st to 10th	Night	45% 5% 13% 21% 14% 2	2537
	11th to 20th	Night	48% 6% 12% 19% 13% 2	2600
	21st to end	Night	45% 7% 14% 24% 9% 1	2600
December	1st to 10th	Night	48% 6% 12% 24% 10%	4250
	11th to 20th	Night	54% 5% 13% 16% 11% 1	5166
	21st to end	Night	63% 6% 12% 12% 6%	6996

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% 15% 42% 19% 4	200
	11th to 20th	Day	23% 17% 34% 24% 3	200
	21st to end	Day	20% 17% 35% 19% 8%	324
February	1st to 10th	Day	28% 17% 30% 22% 3	440
	11th to 20th	Day	26% 17% 36% 18% 3	496
	21st to end	Day	22% 16% 35% 23% 3 1	414
March	1st to 10th	Day	24% 16% 35% 21% 4	500
	11th to 20th	Day	18% 15% 33% 27% 7% 1	500
	21st to end	Day	22% 14% 33% 22% 7% 2	550
April	1st to 10th	Day	27% 11% 28% 25% 8% 1	500
	11th to 20th	Day	21% 9% 30% 32% 8%	492
	21st to end	Day	22% 6% 25% 36% 10% 1	500
May	1st to 10th	Day	26% 7% 24% 33% 9% 1	503
	11th to 20th	Day	20% 5% 23% 36% 13% 2	527
	21st to end	Day	21% 4 15% 35% 21% 5%	573
June	1st to 10th	Day	22% 4 14% 37% 21% 2	530
	11th to 20th	Day	17% 4 15% 40% 22% 2	530
	21st to end	Day	17% 3 12% 40% 24% 5%	530
July	1st to 10th	Day	16% 3 11% 32% 30% 8%	505
	11th to 20th	Day	11% 4 6% 40% 32% 8%	484
	21st to end	Day	14% 2 11% 39% 29% 5%	532
August	1st to 10th	Day	13% 1 12% 36% 33% 5%	486
	11th to 20th	Day	17% 4 10% 32% 29% 9%	479
	21st to end	Day	14% 2 13% 29% 31% 12%	539
September	1st to 10th	Day	14% 3 16% 30% 27% 10%	490
	11th to 20th	Day	15% 4 17% 30% 24% 11%	490
	21st to end	Day	25% 5% 19% 28% 20% 2	490
October	1st to 10th	Day	25% 6% 18% 23% 21% 7%	490
	11th to 20th	Day	17% 6% 26% 31% 16% 3	409
	21st to end	Day	23% 9% 27% 25% 13% 4	429
November	1st to 10th	Day	16% 10% 28% 29% 14% 3	395
	11th to 20th	Day	22% 14% 28% 28% 8%	400
	21st to end	Day	20% 17% 27% 25% 10% 1	400
December	1st to 10th	Day	23% 14% 31% 23% 10%	400
	11th to 20th	Day	27% 18% 30% 21% 4	225
	21st to end	Day	25% 12% 34% 20% 9% 1	220
January	1st to 10th	Night	24% 18% 34% 19% 5%	800
	11th to 20th	Night	30% 17% 31% 18% 4	800
	21st to end	Night	23% 18% 34% 20% 4 1	880
February	1st to 10th	Night	23% 18% 33% 24% 3	800
	11th to 20th	Night	25% 19% 37% 17% 2	783
	21st to end	Night	23% 16% 33% 24% 5%	660
March	1st to 10th	Night	23% 17% 31% 23% 6%	800
	11th to 20th	Night	22% 11% 35% 25% 7% 1	800
	21st to end	Night	25% 10% 30% 25% 8% 2	880
April	1st to 10th	Night	28% 9% 27% 30% 6% 1	800
	11th to 20th	Night	23% 7% 28% 34% 8%	732
	21st to end	Night	22% 6% 25% 32% 14% 2	600
May	1st to 10th	Night	26% 6% 21% 34% 12% 2	600
	11th to 20th	Night	23% 5% 24% 35% 10% 1	592
	21st to end	Night	23% 2 14% 40% 18% 3	591
June	1st to 10th	Night	23% 3 15% 36% 21% 2	400
	11th to 20th	Night	21% 3 14% 39% 22% 3	400
	21st to end	Night	17% 3 13% 38% 29% 1	400
July	1st to 10th	Night	18% 2 10% 38% 28% 3	380
	11th to 20th	Night	15% 3 9% 37% 32% 4	368
	21st to end	Night	15% 3 12% 38% 29% 3	511
August	1st to 10th	Night	13% 2 11% 39% 31% 4	568
	11th to 20th	Night	16% 2 10% 35% 32% 6%	562
	21st to end	Night	14% 2 9% 33% 32% 10%	800
September	1st to 10th	Night	17% 1 14% 33% 29% 6%	760
	11th to 20th	Night	17% 4 13% 33% 24% 9%	760
	21st to end	Night	24% 3 16% 31% 21% 4	760
October	1st to 10th	Night	26% 6% 19% 25% 20% 4	760
	11th to 20th	Night	19% 8% 26% 27% 16% 4	760
	21st to end	Night	25% 8% 28% 27% 11% 2	836
November	1st to 10th	Night	21% 8% 26% 29% 14% 2	780
	11th to 20th	Night	22% 15% 29% 25% 9%	800
	21st to end	Night	19% 17% 29% 25% 9% 1	800
December	1st to 10th	Night	26% 15% 30% 21% 7% 1	800
	11th to 20th	Night	26% 19% 30% 21% 4	800
	21st to end	Night	26% 14% 32% 22% 5% 1	880

Page generated at 2025-07-03 01:30:11. Version ac07

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