Martha's Vineyard
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Martha's Vineyard clear sky chart from 2005-11-06 to 2024-04-10.

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% 3 5% 4 3 4 4 5% 7% 40%	82973
	Night	19% 9% 3 6% 5% 3 4 4 5% 6% 37%	85777
Transparency	Day	32% 34% 9% 15% 9% 1	82682
	Night	35% 33% 8% 14% 9% 1	85603
Seeing	Day	20% 7% 23% 30% 17% 4	14426
	Night	22% 9% 26% 27% 14% 2	23461

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	16% 8% 3 5% 3 3 4 4 6% 6% 41%	2517
	11th to 20th	Day	21% 7% 3 5% 4 3 5% 5% 4 6% 38%	2520
	21st to end	Day	20% 11% 5% 8% 5% 3 3 3 4 5% 32%	2818
February	1st to 10th	Day	19% 10% 3 6% 4 3 5% 5% 6% 6% 34%	1824
	11th to 20th	Day	16% 7% 3 5% 5% 3 4 4 6% 7% 40%	1848
	21st to end	Day	21% 9% 4 5% 5% 3 3 4 4 6% 36%	1570
March	1st to 10th	Day	25% 14% 3 5% 4 2 3 3 4 5% 33%	3116
	11th to 20th	Day	11% 6% 2 4 4 3 3 4 5% 9% 49%	3381
	21st to end	Day	22% 10% 3 6% 5% 3 4 3 3 4 36%	3956
April	1st to 10th	Day	19% 9% 2 5% 3 1 3 3 5% 7% 40%	2270
	11th to 20th	Day	18% 6% 3 4 4 2 4 4 5% 7% 43%	2156
	21st to end	Day	20% 7% 3 5% 4 3 4 5% 6% 6% 38%	2340
May	1st to 10th	Day	24% 9% 3 6% 4 3 3 4 4 6% 34%	2340
	11th to 20th	Day	18% 8% 3 6% 5% 2 3 3 5% 8% 39%	2301
	21st to end	Day	17% 9% 2 5% 5% 3 4 4 5% 6% 40%	2658
June	1st to 10th	Day	20% 8% 3 5% 5% 3 4 5% 7% 8% 33%	2520
	11th to 20th	Day	16% 7% 3 5% 5% 3 5% 6% 6% 9% 37%	2520
	21st to end	Day	14% 6% 3 5% 5% 3 5% 5% 6% 9% 40%	2520
July	1st to 10th	Day	12% 7% 3 6% 4 3 5% 5% 6% 7% 43%	2520
	11th to 20th	Day	12% 7% 3 6% 5% 3 5% 5% 7% 10% 38%	2520
	21st to end	Day	12% 6% 3 5% 5% 3 4 5% 7% 9% 41%	2690
August	1st to 10th	Day	11% 6% 3 5% 4 4 4 5% 5% 9% 44%	2327
	11th to 20th	Day	13% 7% 2 5% 5% 3 3 5% 5% 8% 44%	2286
	21st to end	Day	12% 5% 3 5% 5% 3 5% 5% 5% 8% 43%	2376
September	1st to 10th	Day	15% 6% 3 3 4 3 4 5% 4 7% 46%	2124
	11th to 20th	Day	13% 6% 2 4 4 3 4 4 5% 7% 49%	1980
	21st to end	Day	20% 6% 2 4 3 3 4 4 5% 6% 42%	1980
October	1st to 10th	Day	23% 6% 2 5% 4 2 4 3 4 5% 42%	1980
	11th to 20th	Day	15% 7% 2 4 4 3 4 3 5% 6% 46%	1801
	21st to end	Day	19% 7% 2 5% 5% 3 4 4 5% 7% 40%	1782
November	1st to 10th	Day	18% 6% 3 6% 4 2 3 3 4 6% 44%	1671
	11th to 20th	Day	17% 8% 2 5% 4 3 4 4 5% 7% 41%	1710
	21st to end	Day	18% 7% 3 4 5% 2 4 4 4 7% 41%	1710
December	1st to 10th	Day	21% 9% 3 6% 5% 2 5% 5% 4 6% 34%	1663
	11th to 20th	Day	21% 8% 3 5% 4 2 3 3 4 6% 41%	1906
	21st to end	Day	20% 11% 4 6% 6% 3 4 4 3 6% 32%	2772
January	1st to 10th	Night	19% 7% 3 4 3 2 3 4 5% 6% 44%	4429
	11th to 20th	Night	18% 11% 4 7% 5% 2 4 4 4 5% 37%	4440
	21st to end	Night	24% 13% 5% 7% 6% 3 4 4 4 5% 27%	4822
February	1st to 10th	Night	17% 9% 4 7% 5% 3 5% 3 5% 7% 34%	2352
	11th to 20th	Night	20% 8% 3 5% 5% 2 4 5% 4 6% 37%	2218
	21st to end	Night	18% 8% 4 5% 5% 3 4 4 5% 6% 38%	1884
March	1st to 10th	Night	28% 11% 3 4 3 2 3 3 5% 6% 31%	3724
	11th to 20th	Night	11% 7% 4 7% 6% 4 5% 5% 5% 6% 39%	3577
	21st to end	Night	25% 8% 2 6% 7% 3 5% 4 4 4 32%	3672
April	1st to 10th	Night	20% 10% 3 6% 4 3 4 3 4 7% 37%	1900
	11th to 20th	Night	18% 7% 3 6% 5% 2 4 5% 5% 6% 40%	1753
	21st to end	Night	22% 8% 3 5% 5% 3 4 3 4 6% 36%	1620
May	1st to 10th	Night	23% 10% 3 6% 5% 3 4 4 5% 6% 30%	1620
	11th to 20th	Night	19% 9% 3 5% 5% 3 4 5% 6% 7% 34%	1593
	21st to end	Night	19% 11% 3 6% 6% 3 5% 4 5% 6% 32%	1630
June	1st to 10th	Night	20% 11% 3 5% 5% 2 4 4 4 8% 32%	1440
	11th to 20th	Night	18% 7% 3 5% 5% 2 4 5% 5% 7% 39%	1440
	21st to end	Night	18% 8% 4 5% 4 2 5% 5% 6% 8% 34%	1440
July	1st to 10th	Night	14% 7% 3 6% 5% 3 5% 6% 6% 7% 38%	1440
	11th to 20th	Night	13% 10% 4 5% 5% 3 5% 5% 5% 11% 35%	1440
	21st to end	Night	13% 7% 3 7% 5% 4 5% 5% 5% 7% 38%	1607
August	1st to 10th	Night	8% 7% 2 5% 5% 4 5% 5% 6% 10% 42%	1612
	11th to 20th	Night	11% 7% 3 5% 6% 3 5% 6% 7% 9% 38%	1609
	21st to end	Night	13% 5% 3 4 5% 3 3 5% 6% 9% 43%	1980
September	1st to 10th	Night	14% 7% 2 6% 4 3 4 5% 5% 8% 42%	1836
	11th to 20th	Night	14% 7% 2 4 4 2 4 4 4 6% 48%	1980
	21st to end	Night	19% 6% 2 6% 4 3 4 4 5% 5% 42%	1980
October	1st to 10th	Night	24% 8% 2 5% 4 2 4 4 4 5% 37%	1980
	11th to 20th	Night	17% 6% 2 5% 4 2 4 4 4 7% 45%	2154
	21st to end	Night	21% 8% 3 5% 5% 3 4 4 4 6% 36%	2574
November	1st to 10th	Night	17% 7% 3 6% 5% 3 4 4 4 6% 42%	2407
	11th to 20th	Night	17% 9% 3 6% 4 3 4 4 4 6% 40%	2470
	21st to end	Night	16% 7% 2 5% 5% 3 4 4 5% 7% 41%	2470
December	1st to 10th	Night	21% 9% 3 6% 5% 3 4 4 5% 5% 34%	2512
	11th to 20th	Night	20% 9% 3 5% 5% 2 4 4 4 6% 38%	3288
	21st to end	Night	19% 12% 5% 7% 4 2 3 4 4 5% 35%	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	34% 25% 9% 23% 10%	2518
	11th to 20th	Day	38% 26% 10% 17% 8% 2	2520
	21st to end	Day	47% 23% 7% 17% 6%	2818
February	1st to 10th	Day	34% 36% 8% 12% 9% 1	1824
	11th to 20th	Day	28% 37% 10% 16% 8% 1	1857
	21st to end	Day	35% 34% 7% 12% 9% 2	1570
March	1st to 10th	Day	47% 22% 10% 11% 9% 1	3119
	11th to 20th	Day	27% 24% 13% 25% 10% 1	3381
	21st to end	Day	45% 21% 5% 12% 13% 4	3956
April	1st to 10th	Day	28% 35% 7% 14% 13% 2	2270
	11th to 20th	Day	29% 36% 7% 15% 10% 2	2156
	21st to end	Day	29% 37% 7% 14% 10% 3	2340
May	1st to 10th	Day	34% 36% 6% 14% 8% 1	2340
	11th to 20th	Day	30% 37% 7% 13% 11% 2	2305
	21st to end	Day	30% 37% 8% 15% 8% 1	2658
June	1st to 10th	Day	34% 37% 9% 13% 7%	2520
	11th to 20th	Day	30% 38% 11% 14% 6%	2520
	21st to end	Day	29% 39% 11% 15% 6%	2520
July	1st to 10th	Day	28% 40% 9% 15% 7% 1	2450
	11th to 20th	Day	29% 39% 8% 16% 7% 1	2445
	21st to end	Day	26% 39% 8% 15% 10% 1	2540
August	1st to 10th	Day	28% 38% 10% 17% 7%	2314
	11th to 20th	Day	27% 38% 8% 17% 9% 1	2286
	21st to end	Day	28% 39% 9% 12% 11% 1	2376
September	1st to 10th	Day	24% 38% 8% 15% 15% 1	2124
	11th to 20th	Day	23% 36% 10% 15% 14% 1	1980
	21st to end	Day	28% 38% 6% 13% 13% 2	1980
October	1st to 10th	Day	29% 36% 7% 15% 12% 2	1980
	11th to 20th	Day	27% 37% 9% 17% 8% 1	1801
	21st to end	Day	28% 37% 9% 14% 10% 1	1782
November	1st to 10th	Day	26% 41% 10% 12% 10% 1	1671
	11th to 20th	Day	25% 40% 11% 13% 9% 1	1710
	21st to end	Day	30% 41% 10% 15% 5%	1710
December	1st to 10th	Day	32% 42% 9% 10% 6% 2	1663
	11th to 20th	Day	34% 33% 12% 8% 10% 3	1906
	21st to end	Day	41% 26% 11% 15% 6%	2772
January	1st to 10th	Night	32% 25% 9% 26% 6% 2	4429
	11th to 20th	Night	39% 25% 11% 16% 8% 1	4440
	21st to end	Night	51% 23% 7% 13% 6% 1	4822
February	1st to 10th	Night	33% 36% 9% 13% 8%	2352
	11th to 20th	Night	33% 35% 8% 14% 8% 2	2220
	21st to end	Night	33% 34% 7% 16% 8% 2	1884
March	1st to 10th	Night	46% 22% 8% 15% 8% 1	3734
	11th to 20th	Night	35% 25% 11% 17% 10% 1	3578
	21st to end	Night	47% 21% 6% 8% 14% 4	3672
April	1st to 10th	Night	33% 34% 6% 14% 12% 2	1900
	11th to 20th	Night	32% 36% 5% 13% 11% 3	1753
	21st to end	Night	32% 36% 6% 13% 10% 3	1620
May	1st to 10th	Night	36% 37% 7% 10% 9% 1	1620
	11th to 20th	Night	31% 38% 8% 11% 9% 2	1594
	21st to end	Night	32% 40% 7% 12% 9%	1631
June	1st to 10th	Night	37% 38% 8% 11% 6%	1440
	11th to 20th	Night	32% 36% 9% 14% 9%	1440
	21st to end	Night	35% 38% 10% 11% 7%	1440
July	1st to 10th	Night	30% 40% 9% 14% 6%	1400
	11th to 20th	Night	30% 40% 10% 14% 6% 1	1392
	21st to end	Night	30% 40% 7% 14% 7% 1	1518
August	1st to 10th	Night	30% 39% 10% 15% 6%	1600
	11th to 20th	Night	28% 40% 9% 14% 8%	1609
	21st to end	Night	28% 39% 8% 14% 10% 1	1980
September	1st to 10th	Night	26% 39% 7% 15% 12% 1	1836
	11th to 20th	Night	24% 38% 8% 15% 14% 1	1980
	21st to end	Night	29% 37% 4 14% 14% 2	1980
October	1st to 10th	Night	31% 37% 7% 13% 11% 1	1980
	11th to 20th	Night	30% 36% 8% 17% 7%	2154
	21st to end	Night	33% 37% 8% 12% 9% 1	2574
November	1st to 10th	Night	26% 41% 8% 13% 10% 2	2407
	11th to 20th	Night	30% 40% 8% 13% 8% 1	2470
	21st to end	Night	28% 40% 10% 14% 8% 1	2470
December	1st to 10th	Night	33% 41% 10% 10% 6%	2512
	11th to 20th	Night	34% 33% 11% 9% 12% 1	3288
	21st to end	Night	42% 25% 10% 15% 7%	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	26% 12% 30% 23% 8% 1	180
	11th to 20th	Day	21% 17% 32% 24% 6% 1	180
	21st to end	Day	19% 10% 36% 24% 10% 1	231
February	1st to 10th	Day	23% 14% 32% 26% 5%	424
	11th to 20th	Day	24% 11% 39% 23% 3	456
	21st to end	Day	24% 14% 29% 26% 7% 1	382
March	1st to 10th	Day	22% 10% 33% 27% 7%	460
	11th to 20th	Day	16% 10% 36% 30% 7% 1	460
	21st to end	Day	21% 11% 33% 24% 8% 3	506
April	1st to 10th	Day	23% 10% 27% 30% 10% 1	457
	11th to 20th	Day	20% 7% 29% 33% 10% 1	412
	21st to end	Day	26% 5% 25% 32% 11% 1	420
May	1st to 10th	Day	28% 6% 24% 32% 9% 1	423
	11th to 20th	Day	20% 5% 25% 36% 12% 2	447
	21st to end	Day	22% 2 13% 32% 24% 6%	485
June	1st to 10th	Day	20% 2 15% 36% 24% 2	450
	11th to 20th	Day	19% 2 15% 37% 25% 2	450
	21st to end	Day	15% 3 14% 37% 26% 5%	450
July	1st to 10th	Day	17% 3 9% 30% 34% 8%	445
	11th to 20th	Day	12% 2 7% 32% 37% 9%	444
	21st to end	Day	13% 2 12% 40% 30% 3	488
August	1st to 10th	Day	14% 2 9% 37% 32% 6%	446
	11th to 20th	Day	17% 2 10% 32% 30% 9%	439
	21st to end	Day	14% 2 15% 32% 28% 10%	495
September	1st to 10th	Day	19% 2 14% 27% 25% 13%	450
	11th to 20th	Day	15% 5% 17% 29% 26% 9%	450
	21st to end	Day	25% 3 19% 28% 20% 5%	450
October	1st to 10th	Day	28% 3 19% 26% 18% 6%	450
	11th to 20th	Day	16% 8% 23% 32% 18% 3	394
	21st to end	Day	24% 9% 28% 24% 12% 3	396
November	1st to 10th	Day	18% 8% 27% 30% 15% 2	365
	11th to 20th	Day	23% 14% 29% 22% 11%	370
	21st to end	Day	22% 13% 27% 25% 11% 2	370
December	1st to 10th	Day	23% 12% 33% 22% 9%	323
	11th to 20th	Day	25% 13% 36% 19% 7%	180
	21st to end	Day	25% 10% 30% 25% 7% 3	198
January	1st to 10th	Night	23% 16% 35% 22% 4	760
	11th to 20th	Night	28% 17% 34% 17% 5%	760
	21st to end	Night	25% 15% 35% 21% 3 1	836
February	1st to 10th	Night	23% 15% 33% 25% 3	760
	11th to 20th	Night	23% 15% 41% 18% 3	743
	21st to end	Night	22% 15% 36% 21% 5%	628
March	1st to 10th	Night	23% 13% 32% 25% 6%	760
	11th to 20th	Night	21% 9% 36% 27% 6% 1	760
	21st to end	Night	23% 9% 33% 25% 8% 2	836
April	1st to 10th	Night	24% 8% 29% 31% 7% 1	760
	11th to 20th	Night	21% 4 32% 32% 11%	689
	21st to end	Night	24% 5% 25% 31% 14% 2	540
May	1st to 10th	Night	27% 4 21% 32% 13% 2	540
	11th to 20th	Night	22% 5% 25% 34% 12% 1	532
	21st to end	Night	23% 2 14% 32% 24% 3	469
June	1st to 10th	Night	24% 3 15% 30% 25% 2	360
	11th to 20th	Night	21% 1 15% 40% 21% 2	360
	21st to end	Night	20% 3 12% 40% 26% 1	360
July	1st to 10th	Night	15% 2 12% 41% 27% 3	350
	11th to 20th	Night	15% 2 11% 34% 33% 5%	348
	21st to end	Night	14% 2 12% 36% 33% 3	427
August	1st to 10th	Night	12% 1 11% 39% 33% 4	538
	11th to 20th	Night	14% 2 10% 37% 32% 5%	545
	21st to end	Night	16% 2 10% 32% 31% 8%	792
September	1st to 10th	Night	17% 1 15% 33% 28% 6%	720
	11th to 20th	Night	17% 2 17% 31% 25% 7%	720
	21st to end	Night	23% 3 19% 29% 23% 3	720
October	1st to 10th	Night	28% 5% 21% 23% 19% 4	720
	11th to 20th	Night	21% 8% 26% 28% 14% 3	720
	21st to end	Night	25% 10% 28% 26% 10% 1	792
November	1st to 10th	Night	20% 8% 28% 28% 14% 3	740
	11th to 20th	Night	22% 16% 30% 23% 8% 1	760
	21st to end	Night	19% 17% 32% 22% 9% 1	760
December	1st to 10th	Night	25% 16% 30% 21% 7% 1	760
	11th to 20th	Night	26% 19% 32% 18% 5%	760
	21st to end	Night	25% 14% 36% 20% 4	836

Page generated at 2024-04-07 09:28:10. Version ac07

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