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 2024-03-18.

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% 8% 4 6% 5% 3 4 4 5% 7% 37%	78638
	Night	18% 10% 4 6% 5% 3 4 4 5% 6% 35%	81144
Transparency	Day	47% 6% 13% 21% 12% 1	78350
	Night	50% 6% 11% 19% 11% 2	80966
Seeing	Day	20% 8% 23% 29% 16% 3	14436
	Night	22% 10% 25% 28% 14% 2	23410

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% 8% 3 6% 5% 4 4 4 6% 6% 37%	2517
	11th to 20th	Day	20% 9% 4 7% 5% 2 4 5% 5% 6% 35%	2520
	21st to end	Day	21% 12% 7% 8% 8% 3 4 3 3 6% 26%	2935
February	1st to 10th	Day	20% 10% 4 9% 6% 3 4 5% 5% 6% 30%	1786
	11th to 20th	Day	16% 8% 3 6% 6% 4 4 4 5% 6% 37%	1848
	21st to end	Day	20% 8% 5% 6% 4 4 5% 4 5% 6% 32%	1570
March	1st to 10th	Day	19% 8% 3 5% 5% 4 4 4 5% 7% 37%	1897
	11th to 20th	Day	16% 7% 3 6% 5% 2 4 4 4 8% 42%	2031
	21st to end	Day	18% 10% 4 5% 5% 3 4 4 4 5% 39%	2204
April	1st to 10th	Day	21% 8% 3 6% 3 2 3 2 4 6% 41%	2160
	11th to 20th	Day	16% 7% 3 4 5% 3 4 4 5% 7% 43%	2186
	21st to end	Day	19% 7% 3 6% 4 3 4 3 5% 7% 37%	2340
May	1st to 10th	Day	24% 9% 3 7% 5% 3 4 4 4 6% 31%	2340
	11th to 20th	Day	19% 9% 3 5% 5% 2 3 3 5% 9% 38%	2301
	21st to end	Day	18% 8% 3 4 5% 3 5% 5% 5% 7% 38%	2595
June	1st to 10th	Day	19% 9% 4 6% 5% 4 5% 5% 5% 8% 30%	2520
	11th to 20th	Day	16% 8% 4 5% 4 3 5% 6% 6% 9% 35%	2520
	21st to end	Day	14% 8% 3 6% 5% 4 5% 5% 7% 9% 36%	2520
July	1st to 10th	Day	12% 7% 3 6% 5% 3 5% 5% 6% 8% 41%	2520
	11th to 20th	Day	10% 7% 4 6% 6% 4 6% 6% 6% 9% 35%	2520
	21st to end	Day	12% 7% 3 5% 5% 3 5% 5% 7% 8% 40%	2636
August	1st to 10th	Day	12% 5% 2 5% 6% 3 5% 5% 6% 9% 43%	2327
	11th to 20th	Day	13% 8% 3 6% 5% 4 4 4 5% 8% 41%	2304
	21st to end	Day	12% 6% 3 5% 5% 3 4 5% 7% 8% 41%	2408
September	1st to 10th	Day	13% 6% 3 5% 5% 3 5% 4 6% 7% 45%	2093
	11th to 20th	Day	13% 6% 2 6% 4 2 4 4 6% 7% 47%	1980
	21st to end	Day	19% 8% 3 4 4 3 5% 4 6% 7% 38%	1980
October	1st to 10th	Day	21% 6% 3 6% 4 3 3 3 3 6% 41%	1980
	11th to 20th	Day	16% 6% 3 4 5% 3 3 3 5% 7% 45%	1815
	21st to end	Day	18% 7% 3 7% 5% 3 4 5% 5% 7% 36%	1782
November	1st to 10th	Day	18% 7% 4 5% 5% 3 4 4 4 6% 40%	1671
	11th to 20th	Day	15% 9% 4 7% 5% 3 5% 4 5% 7% 36%	1710
	21st to end	Day	17% 7% 3 5% 6% 3 4 4 6% 6% 39%	1710
December	1st to 10th	Day	21% 10% 4 6% 5% 3 6% 5% 6% 6% 29%	1710
	11th to 20th	Day	20% 11% 5% 8% 5% 3 4 5% 4 6% 29%	1930
	21st to end	Day	19% 13% 7% 10% 5% 3 4 5% 5% 6% 23%	2772
January	1st to 10th	Night	19% 9% 3 6% 5% 3 3 4 4 5% 40%	4429
	11th to 20th	Night	19% 12% 5% 8% 5% 3 4 4 4 5% 32%	4440
	21st to end	Night	24% 13% 7% 9% 7% 3 4 3 3 4 24%	4720
February	1st to 10th	Night	18% 10% 5% 8% 7% 3 6% 4 5% 5% 29%	2393
	11th to 20th	Night	19% 9% 4 6% 5% 3 6% 5% 4 6% 33%	2218
	21st to end	Night	19% 8% 4 7% 5% 3 4 5% 5% 5% 36%	1884
March	1st to 10th	Night	18% 8% 3 6% 5% 3 4 4 5% 7% 36%	2270
	11th to 20th	Night	17% 9% 4 6% 5% 3 4 5% 5% 6% 36%	2093
	21st to end	Night	18% 9% 3 6% 5% 3 4 4 5% 5% 37%	2150
April	1st to 10th	Night	21% 12% 3 6% 5% 3 4 4 4 5% 35%	1800
	11th to 20th	Night	17% 8% 3 5% 6% 3 4 4 5% 6% 39%	1723
	21st to end	Night	20% 8% 4 7% 5% 3 4 4 4 7% 34%	1620
May	1st to 10th	Night	21% 12% 3 6% 6% 3 3 3 4 6% 33%	1620
	11th to 20th	Night	19% 9% 3 6% 5% 3 5% 4 5% 8% 33%	1593
	21st to end	Night	18% 11% 4 6% 4 3 6% 4 5% 6% 32%	1692
June	1st to 10th	Night	20% 10% 4 7% 5% 2 4 5% 5% 7% 31%	1440
	11th to 20th	Night	18% 8% 3 5% 5% 3 4 4 5% 8% 36%	1440
	21st to end	Night	15% 10% 4 7% 5% 3 5% 5% 5% 8% 34%	1440
July	1st to 10th	Night	12% 8% 3 6% 6% 3 5% 4 5% 8% 40%	1440
	11th to 20th	Night	13% 9% 4 7% 5% 3 5% 5% 6% 10% 34%	1440
	21st to end	Night	12% 8% 4 7% 6% 4 5% 4 6% 8% 37%	1664
August	1st to 10th	Night	9% 7% 3 6% 5% 4 4 5% 7% 9% 42%	1612
	11th to 20th	Night	13% 6% 2 5% 5% 3 4 6% 6% 9% 40%	1596
	21st to end	Night	12% 6% 2 5% 4 2 5% 4 6% 10% 43%	1945
September	1st to 10th	Night	15% 7% 3 5% 4 3 4 4 5% 7% 44%	1865
	11th to 20th	Night	12% 7% 3 5% 5% 3 4 3 4 7% 47%	1980
	21st to end	Night	19% 8% 2 6% 4 2 4 4 4 5% 41%	1980
October	1st to 10th	Night	23% 8% 3 5% 4 2 4 3 5% 5% 37%	1980
	11th to 20th	Night	18% 8% 3 5% 4 2 4 4 5% 7% 41%	2142
	21st to end	Night	19% 9% 3 7% 5% 3 5% 4 5% 6% 34%	2574
November	1st to 10th	Night	17% 7% 3 6% 5% 3 4 4 4 7% 40%	2407
	11th to 20th	Night	18% 9% 3 5% 5% 3 4 4 5% 7% 36%	2470
	21st to end	Night	15% 8% 4 6% 5% 3 5% 4 6% 7% 38%	2470
December	1st to 10th	Night	21% 10% 5% 7% 6% 3 5% 5% 6% 7% 27%	2470
	11th to 20th	Night	19% 12% 4 6% 5% 3 4 4 4 6% 32%	3260
	21st to end	Night	18% 14% 5% 9% 6% 3 4 4 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	49% 7% 13% 20% 10% 2	1900
	11th to 20th	Day	44% 4 13% 25% 12% 3	2031
	21st to end	Day	49% 5% 10% 17% 16% 3	2204
April	1st to 10th	Day	44% 5% 12% 19% 18% 2	2160
	11th to 20th	Day	40% 6% 12% 25% 16% 2	2186
	21st to end	Day	46% 5% 11% 20% 14% 3	2340
May	1st to 10th	Day	54% 5% 9% 19% 10% 2	2340
	11th to 20th	Day	46% 5% 9% 21% 17% 2	2305
	21st to end	Day	45% 7% 15% 21% 11% 1	2595
June	1st to 10th	Day	52% 6% 10% 21% 10% 1	2520
	11th to 20th	Day	45% 6% 14% 22% 12% 1	2520
	21st to end	Day	44% 8% 14% 22% 12%	2520
July	1st to 10th	Day	40% 8% 12% 24% 15% 2	2450
	11th to 20th	Day	43% 8% 15% 23% 10% 2	2445
	21st to end	Day	38% 9% 13% 24% 13% 3	2489
August	1st to 10th	Day	37% 7% 17% 26% 12% 1	2314
	11th to 20th	Day	42% 8% 13% 22% 13% 1	2304
	21st to end	Day	38% 9% 14% 22% 16% 1	2408
September	1st to 10th	Day	39% 7% 11% 25% 17% 2	2093
	11th to 20th	Day	36% 5% 12% 22% 22% 3	1980
	21st to end	Day	44% 6% 9% 21% 18% 2	1980
October	1st to 10th	Day	46% 5% 11% 23% 13% 3	1980
	11th to 20th	Day	41% 5% 13% 27% 13% 1	1815
	21st to end	Day	47% 6% 14% 20% 11% 2	1782
November	1st to 10th	Day	47% 6% 13% 18% 13% 2	1671
	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	49% 7% 12% 19% 11% 2	2280
	11th to 20th	Night	49% 7% 11% 17% 14% 3	2094
	21st to end	Night	50% 5% 9% 17% 16% 4	2150
April	1st to 10th	Night	52% 4 8% 17% 16% 3	1800
	11th to 20th	Night	47% 5% 9% 20% 16% 3	1723
	21st to end	Night	51% 5% 10% 18% 13% 3	1620
May	1st to 10th	Night	53% 5% 9% 18% 12% 2	1620
	11th to 20th	Night	49% 6% 11% 18% 13% 3	1594
	21st to end	Night	53% 7% 13% 16% 10% 1	1693
June	1st to 10th	Night	53% 7% 9% 20% 10% 1	1440
	11th to 20th	Night	48% 7% 9% 20% 14% 1	1440
	21st to end	Night	50% 6% 11% 21% 11% 1	1440
July	1st to 10th	Night	44% 7% 13% 22% 13% 1	1400
	11th to 20th	Night	47% 9% 10% 21% 11% 2	1392
	21st to end	Night	45% 7% 12% 22% 12% 2	1571
August	1st to 10th	Night	38% 8% 14% 25% 14% 1	1600
	11th to 20th	Night	41% 8% 16% 24% 11% 1	1596
	21st to end	Night	38% 7% 12% 22% 18% 2	1945
September	1st to 10th	Night	41% 6% 10% 24% 17% 2	1865
	11th to 20th	Night	39% 4 11% 22% 22% 2	1980
	21st to end	Night	44% 5% 8% 19% 20% 3	1980
October	1st to 10th	Night	49% 6% 10% 19% 14% 2	1980
	11th to 20th	Night	42% 6% 12% 26% 13% 1	2142
	21st to end	Night	51% 5% 11% 19% 12% 1	2574
November	1st to 10th	Night	46% 5% 13% 20% 13% 2	2407
	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% 7%	449
	21st to end	Day	20% 11% 34% 23% 9% 2	462
April	1st to 10th	Day	27% 10% 29% 25% 9% 1	420
	11th to 20th	Day	20% 8% 29% 33% 9%	412
	21st to end	Day	24% 6% 25% 34% 10% 2	420
May	1st to 10th	Day	28% 5% 26% 31% 9% 2	423
	11th to 20th	Day	19% 4 25% 36% 13% 3	447
	21st to end	Day	23% 4 14% 33% 22% 5%	485
June	1st to 10th	Day	22% 4 14% 37% 20% 2	450
	11th to 20th	Day	18% 3 15% 40% 21% 2	450
	21st to end	Day	18% 3 12% 41% 22% 5%	450
July	1st to 10th	Day	15% 3 11% 31% 31% 8%	445
	11th to 20th	Day	11% 3 6% 39% 32% 8%	444
	21st to end	Day	13% 1 12% 39% 29% 5%	488
August	1st to 10th	Day	13% 1 11% 35% 34% 6%	446
	11th to 20th	Day	17% 3 10% 33% 29% 8%	439
	21st to end	Day	16% 2 13% 28% 29% 12%	495
September	1st to 10th	Day	15% 3 16% 28% 28% 10%	450
	11th to 20th	Day	14% 4 18% 32% 23% 8%	450
	21st to end	Day	25% 4 20% 28% 21% 3	450
October	1st to 10th	Day	27% 7% 18% 23% 19% 6%	450
	11th to 20th	Day	18% 6% 25% 32% 17% 3	378
	21st to end	Day	24% 10% 28% 23% 12% 3	396
November	1st to 10th	Day	18% 10% 27% 28% 14% 3	365
	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	752
	21st to end	Night	23% 9% 31% 25% 9% 2	792
April	1st to 10th	Night	28% 9% 27% 29% 6% 1	720
	11th to 20th	Night	22% 6% 28% 35% 8%	659
	21st to end	Night	24% 6% 24% 30% 14% 2	540
May	1st to 10th	Night	26% 5% 21% 34% 12% 2	540
	11th to 20th	Night	23% 5% 26% 35% 10% 2	532
	21st to end	Night	24% 2 13% 40% 18% 3	531
June	1st to 10th	Night	25% 3 15% 33% 22% 3	360
	11th to 20th	Night	21% 3 13% 39% 21% 3	360
	21st to end	Night	17% 3 13% 39% 27% 1	360
July	1st to 10th	Night	17% 2 10% 39% 29% 3	350
	11th to 20th	Night	15% 3 8% 36% 33% 4	348
	21st to end	Night	14% 3 12% 38% 30% 3	484
August	1st to 10th	Night	12% 1 11% 39% 33% 4	538
	11th to 20th	Night	16% 2 10% 35% 33% 5%	532
	21st to end	Night	15% 2 9% 32% 32% 10%	757
September	1st to 10th	Night	17% 1 14% 32% 29% 6%	720
	11th to 20th	Night	17% 4 13% 35% 23% 8%	720
	21st to end	Night	23% 3 17% 32% 21% 4	720
October	1st to 10th	Night	27% 6% 20% 25% 19% 4	720
	11th to 20th	Night	20% 7% 25% 27% 17% 3	720
	21st to end	Night	26% 8% 28% 27% 10% 1	792
November	1st to 10th	Night	22% 8% 25% 28% 15% 2	740
	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 2024-03-15 14:01:40. Version ac07

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