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 2024-11-03.

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% 9% 3 7% 4 4 4 3 5% 7% 35%	101110
	Night	18% 11% 4 7% 4 4 4 3 5% 6% 34%	97893
Transparency	Day	48% 6% 13% 20% 12% 1	100875
	Night	51% 6% 11% 19% 11% 1	97748
Seeing	Day	20% 8% 22% 30% 16% 4	15333
	Night	22% 10% 24% 28% 14% 2	24141

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	17% 9% 3 7% 4 5% 3 4 6% 6% 37%	2517
	11th to 20th	Day	20% 9% 3 8% 4 3 4 4 5% 6% 34%	2520
	21st to end	Day	21% 12% 6% 9% 6% 4 4 2 4 5% 26%	2935
February	1st to 10th	Day	19% 10% 3 10% 5% 4 4 4 5% 5% 30%	1786
	11th to 20th	Day	16% 9% 3 7% 5% 5% 4 3 5% 6% 37%	1848
	21st to end	Day	20% 9% 5% 6% 4 5% 4 4 5% 6% 32%	1570
March	1st to 10th	Day	25% 18% 5% 6% 3 3 3 2 3 4 28%	3116
	11th to 20th	Day	12% 7% 3 7% 3 3 3 3 5% 7% 46%	3439
	21st to end	Day	25% 11% 4 6% 5% 3 3 2 3 4 35%	3868
April	1st to 10th	Day	20% 10% 3 6% 3 2 3 2 4 6% 40%	2280
	11th to 20th	Day	16% 8% 3 5% 4 4 3 4 5% 7% 42%	2309
	21st to end	Day	18% 7% 3 6% 4 5% 3 3 6% 7% 38%	2470
May	1st to 10th	Day	25% 15% 4 9% 4 4 4 4 4 6% 22%	4106
	11th to 20th	Day	23% 14% 3 7% 3 3 3 2 5% 7% 28%	4081
	21st to end	Day	15% 10% 3 8% 5% 5% 4 4 5% 6% 36%	4609
June	1st to 10th	Day	19% 10% 3 6% 4 4 5% 4 6% 8% 31%	2660
	11th to 20th	Day	15% 9% 3 5% 4 4 5% 5% 6% 8% 37%	2660
	21st to end	Day	13% 9% 3 6% 4 5% 5% 4 8% 8% 35%	2660
July	1st to 10th	Day	16% 12% 4 8% 4 4 4 3 5% 7% 33%	4440
	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	18% 7% 3 6% 5% 4 4 3 4 5% 40%	1689
	11th to 20th	Day	15% 9% 4 7% 4 5% 4 3 6% 7% 36%	1710
	21st to end	Day	16% 8% 3 6% 5% 4 4 3 6% 6% 39%	1710
December	1st to 10th	Day	21% 11% 3 7% 4 5% 5% 4 6% 6% 29%	1710
	11th to 20th	Day	20% 12% 4 9% 4 4 3 4 4 6% 29%	1930
	21st to end	Day	19% 13% 7% 10% 4 4 4 4 6% 6% 23%	2772
January	1st to 10th	Night	19% 9% 3 7% 4 3 3 3 4 5% 40%	4429
	11th to 20th	Night	19% 12% 5% 9% 4 3 4 4 4 5% 31%	4440
	21st to end	Night	24% 14% 7% 10% 5% 3 4 3 3 3 24%	4720
February	1st to 10th	Night	18% 11% 4 9% 6% 5% 5% 4 5% 5% 29%	2393
	11th to 20th	Night	19% 10% 3 6% 5% 5% 5% 4 5% 6% 33%	2218
	21st to end	Night	19% 9% 4 7% 4 4 4 4 5% 5% 35%	1884
March	1st to 10th	Night	29% 13% 3 6% 4 3 3 2 4 6% 29%	3724
	11th to 20th	Night	11% 10% 4 8% 5% 4 5% 4 5% 6% 38%	3519
	21st to end	Night	26% 11% 3 7% 5% 4 3 2 4 4 31%	3762
April	1st to 10th	Night	21% 12% 2 7% 4 4 4 3 4 5% 35%	1900
	11th to 20th	Night	17% 10% 3 6% 5% 3 4 4 5% 6% 38%	1819
	21st to end	Night	19% 9% 4 7% 5% 4 4 3 5% 6% 35%	1710
May	1st to 10th	Night	25% 15% 4 11% 4 4 3 2 4 5% 24%	2834
	11th to 20th	Night	27% 14% 3 8% 4 3 4 3 4 6% 24%	2813
	21st to end	Night	15% 16% 6% 9% 4 4 4 4 5% 6% 27%	2992
June	1st to 10th	Night	19% 11% 3 8% 4 3 5% 4 5% 6% 32%	1520
	11th to 20th	Night	17% 8% 2 6% 5% 4 4 3 5% 8% 38%	1520
	21st to end	Night	15% 11% 3 8% 4 5% 4 4 5% 8% 33%	1520
July	1st to 10th	Night	23% 12% 2 6% 4 4 4 3 5% 8% 31%	2520
	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	17% 7% 3 7% 5% 4 4 3 4 7% 40%	2434
	11th to 20th	Night	18% 10% 3 6% 5% 4 3 4 5% 7% 36%	2470
	21st to end	Night	15% 9% 3 7% 4 4 4 3 6% 7% 38%	2470
December	1st to 10th	Night	21% 11% 4 7% 5% 5% 5% 4 6% 6% 27%	2470
	11th to 20th	Night	19% 13% 3 7% 4 4 4 4 4 5% 32%	3260
	21st to end	Night	18% 15% 5% 10% 5% 4 4 3 5% 6% 26%	4884

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	46% 6% 13% 24% 9% 1	2518
	11th to 20th	Day	50% 7% 15% 19% 7% 2	2520
	21st to end	Day	64% 7% 9% 13% 6%	2935
February	1st to 10th	Day	56% 6% 12% 16% 9% 1	1786
	11th to 20th	Day	51% 6% 13% 20% 10% 1	1857
	21st to end	Day	53% 6% 11% 16% 11% 2	1570
March	1st to 10th	Day	62% 5% 11% 13% 7% 2	3119
	11th to 20th	Day	39% 5% 15% 27% 12% 2	3439
	21st to end	Day	56% 3 6% 16% 16% 3	3868
April	1st to 10th	Day	45% 5% 11% 19% 18% 2	2280
	11th to 20th	Day	40% 6% 12% 24% 16% 2	2309
	21st to end	Day	45% 6% 11% 20% 15% 3	2470
May	1st to 10th	Day	63% 7% 11% 12% 7% 1	4120
	11th to 20th	Day	55% 6% 12% 16% 10% 1	4085
	21st to end	Day	46% 8% 11% 17% 18% 1	4609
June	1st to 10th	Day	51% 6% 11% 21% 11% 1	2660
	11th to 20th	Day	43% 6% 15% 23% 12% 1	2660
	21st to end	Day	44% 8% 14% 21% 12%	2660
July	1st to 10th	Day	50% 7% 11% 18% 13% 1	4370
	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	45% 5% 11% 24% 13% 2	2090
	11th to 20th	Day	40% 5% 13% 27% 14% 1	1914
	21st to end	Day	46% 6% 14% 22% 12% 2	1881
November	1st to 10th	Day	47% 6% 13% 19% 13% 2	1689
	11th to 20th	Day	47% 6% 14% 18% 13% 1	1710
	21st to end	Day	45% 7% 16% 23% 8%	1710
December	1st to 10th	Day	54% 7% 12% 16% 9% 1	1710
	11th to 20th	Day	59% 5% 15% 13% 7% 1	1930
	21st to end	Day	59% 9% 13% 14% 5%	2772
January	1st to 10th	Night	48% 5% 11% 28% 7% 2	4429
	11th to 20th	Night	55% 6% 13% 18% 7% 1	4440
	21st to end	Night	68% 5% 8% 14% 6%	4720
February	1st to 10th	Night	57% 7% 13% 15% 7% 1	2393
	11th to 20th	Night	53% 6% 10% 19% 10% 2	2220
	21st to end	Night	50% 5% 10% 22% 10% 2	1884
March	1st to 10th	Night	59% 5% 10% 16% 8% 1	3734
	11th to 20th	Night	46% 7% 14% 19% 14% 2	3520
	21st to end	Night	58% 4 7% 12% 16% 4	3762
April	1st to 10th	Night	52% 4 8% 18% 15% 2	1900
	11th to 20th	Night	48% 5% 9% 19% 16% 3	1819
	21st to end	Night	50% 5% 9% 18% 14% 3	1710
May	1st to 10th	Night	63% 5% 10% 14% 7% 1	2840
	11th to 20th	Night	61% 6% 12% 14% 7% 2	2814
	21st to end	Night	56% 7% 10% 17% 9% 1	2993
June	1st to 10th	Night	52% 7% 9% 21% 10% 1	1520
	11th to 20th	Night	46% 6% 9% 22% 15% 1	1520
	21st to end	Night	50% 6% 11% 21% 11% 1	1520
July	1st to 10th	Night	54% 5% 9% 18% 14% 1	2480
	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	49% 5% 9% 20% 14% 2	2090
	11th to 20th	Night	41% 6% 12% 25% 15% 1	2261
	21st to end	Night	50% 5% 11% 20% 13% 1	2717
November	1st to 10th	Night	46% 5% 13% 20% 14% 2	2434
	11th to 20th	Night	48% 6% 13% 19% 12% 2	2470
	21st to end	Night	45% 7% 14% 24% 8% 1	2470
December	1st to 10th	Night	57% 7% 13% 15% 8%	2470
	11th to 20th	Night	53% 6% 14% 14% 12% 1	3260
	21st to end	Night	59% 7% 14% 13% 6%	4884

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% 13% 39% 21% 4	180
	11th to 20th	Day	25% 16% 33% 23% 3	180
	21st to end	Day	21% 15% 35% 21% 8%	296
February	1st to 10th	Day	29% 17% 30% 21% 3	406
	11th to 20th	Day	25% 16% 38% 19% 3	456
	21st to end	Day	24% 17% 33% 21% 4 1	382
March	1st to 10th	Day	25% 15% 33% 22% 4	460
	11th to 20th	Day	17% 16% 33% 27% 6%	460
	21st to end	Day	21% 14% 32% 23% 8% 2	506
April	1st to 10th	Day	27% 10% 28% 26% 8% 1	460
	11th to 20th	Day	20% 8% 30% 33% 9%	452
	21st to end	Day	22% 6% 26% 34% 10% 2	460
May	1st to 10th	Day	27% 6% 25% 32% 8% 2	463
	11th to 20th	Day	21% 4 24% 35% 13% 3	487
	21st to end	Day	21% 4 14% 35% 21% 5%	529
June	1st to 10th	Day	22% 4 14% 38% 20% 2	490
	11th to 20th	Day	17% 3 14% 41% 23% 2	490
	21st to end	Day	17% 3 12% 41% 23% 4	490
July	1st to 10th	Day	16% 3 11% 31% 31% 8%	485
	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	17% 9% 28% 28% 14% 3	371
	11th to 20th	Day	23% 14% 26% 28% 9%	370
	21st to end	Day	20% 16% 25% 26% 11% 1	370
December	1st to 10th	Day	24% 13% 32% 22% 9%	370
	11th to 20th	Day	27% 18% 30% 21% 3	204
	21st to end	Day	25% 13% 35% 19% 7% 1	198
January	1st to 10th	Night	25% 17% 33% 20% 5%	760
	11th to 20th	Night	30% 17% 31% 18% 4	760
	21st to end	Night	23% 18% 33% 21% 4 1	836
February	1st to 10th	Night	23% 17% 34% 24% 3	760
	11th to 20th	Night	25% 18% 37% 17% 2	743
	21st to end	Night	23% 17% 32% 23% 5%	628
March	1st to 10th	Night	24% 16% 31% 23% 6%	760
	11th to 20th	Night	22% 11% 35% 25% 7% 1	760
	21st to end	Night	24% 10% 30% 25% 8% 2	836
April	1st to 10th	Night	28% 9% 27% 30% 6% 1	760
	11th to 20th	Night	22% 7% 28% 35% 8%	695
	21st to end	Night	22% 6% 24% 31% 15% 2	570
May	1st to 10th	Night	26% 6% 21% 34% 12% 2	570
	11th to 20th	Night	24% 5% 25% 34% 10% 1	562
	21st to end	Night	23% 2 13% 40% 18% 3	561
June	1st to 10th	Night	24% 3 15% 34% 22% 2	380
	11th to 20th	Night	20% 3 13% 38% 23% 3	380
	21st to end	Night	17% 3 13% 39% 27% 1	380
July	1st to 10th	Night	18% 2 10% 38% 29% 3	370
	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	22% 8% 26% 28% 15% 2	749
	11th to 20th	Night	23% 15% 28% 24% 9%	760
	21st to end	Night	18% 17% 30% 25% 9% 1	760
December	1st to 10th	Night	27% 15% 30% 21% 8% 1	760
	11th to 20th	Night	27% 19% 30% 20% 4	760
	21st to end	Night	26% 14% 33% 22% 5%	836

Page generated at 2024-10-31 03:39:21. Version ac07

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