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 2024-11-05.

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	18% 9% 3 6% 4 4 4 3 5% 7% 38%	101134
	Night	19% 10% 3 6% 4 4 4 3 5% 6% 36%	97902
Transparency	Day	44% 6% 13% 21% 13% 2	104715
	Night	47% 6% 12% 20% 13% 2	102411
Seeing	Day	20% 7% 22% 30% 18% 4	15240
	Night	22% 9% 26% 27% 14% 2	24108

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% 9% 3 5% 3 4 3 3 6% 6% 41%	2517
	11th to 20th	Day	20% 8% 2 5% 3 4 4 4 5% 6% 38%	2520
	21st to end	Day	20% 12% 4 9% 4 3 3 3 5% 5% 32%	2818
February	1st to 10th	Day	19% 10% 3 7% 4 4 4 4 6% 5% 33%	1824
	11th to 20th	Day	16% 8% 2 6% 4 4 4 3 6% 7% 40%	1848
	21st to end	Day	20% 9% 3 6% 4 4 3 3 4 6% 36%	1570
March	1st to 10th	Day	25% 14% 3 6% 4 3 2 2 4 5% 33%	3116
	11th to 20th	Day	11% 6% 2 5% 3 4 3 3 6% 9% 48%	3381
	21st to end	Day	22% 11% 3 7% 5% 4 3 3 3 4 36%	3956
April	1st to 10th	Day	19% 10% 2 6% 3 2 3 3 5% 7% 40%	2280
	11th to 20th	Day	18% 8% 2 5% 3 3 4 3 5% 7% 42%	2278
	21st to end	Day	19% 8% 3 6% 3 4 3 4 5% 6% 39%	2470
May	1st to 10th	Day	26% 14% 4 8% 4 4 3 2 4 5% 26%	4106
	11th to 20th	Day	22% 13% 3 6% 4 3 3 3 4 7% 31%	4081
	21st to end	Day	16% 10% 3 7% 4 4 3 3 5% 7% 38%	4728
June	1st to 10th	Day	19% 8% 3 6% 5% 4 3 4 7% 7% 34%	2660
	11th to 20th	Day	15% 7% 2 6% 4 3 5% 5% 6% 8% 38%	2660
	21st to end	Day	14% 7% 2 6% 4 4 5% 4 6% 9% 40%	2660
July	1st to 10th	Day	16% 11% 3 8% 4 4 4 3 5% 7% 34%	4440
	11th to 20th	Day	13% 7% 3 7% 4 4 5% 4 8% 10% 37%	4440
	21st to end	Day	18% 8% 3 6% 5% 4 4 3 7% 8% 35%	4721
August	1st to 10th	Day	17% 9% 3 7% 5% 4 4 4 5% 7% 34%	4105
	11th to 20th	Day	12% 6% 3 6% 4 4 4 4 5% 8% 43%	4013
	21st to end	Day	7% 4 2 5% 4 4 4 4 6% 9% 51%	4180
September	1st to 10th	Day	14% 6% 3 4 3 4 4 4 4 7% 47%	2242
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 5% 7% 49%	2090
	21st to end	Day	20% 6% 2 6% 3 4 5% 3 6% 5% 40%	2090
October	1st to 10th	Day	22% 6% 2 5% 3 4 3 3 4 5% 42%	2090
	11th to 20th	Day	15% 7% 2 5% 4 3 3 3 5% 6% 47%	1900
	21st to end	Day	18% 7% 2 6% 4 3 3 3 5% 6% 42%	1881
November	1st to 10th	Day	18% 6% 3 6% 4 3 3 2 4 6% 44%	1708
	11th to 20th	Day	17% 9% 2 5% 4 4 4 3 5% 7% 41%	1710
	21st to end	Day	18% 8% 2 4 5% 3 4 4 4 6% 41%	1710
December	1st to 10th	Day	21% 9% 3 7% 4 4 4 4 4 5% 34%	1663
	11th to 20th	Day	21% 8% 2 5% 4 3 3 3 4 6% 41%	1906
	21st to end	Day	20% 11% 4 7% 5% 3 4 3 4 6% 32%	2772
January	1st to 10th	Night	19% 7% 2 5% 3 3 3 3 5% 6% 44%	4429
	11th to 20th	Night	18% 12% 3 8% 4 3 4 3 4 5% 37%	4440
	21st to end	Night	24% 14% 5% 8% 5% 4 3 3 4 5% 27%	4822
February	1st to 10th	Night	17% 10% 3 7% 5% 4 5% 3 6% 6% 34%	2352
	11th to 20th	Night	20% 8% 3 6% 4 3 4 4 5% 6% 37%	2218
	21st to end	Night	18% 9% 3 6% 4 4 4 3 5% 6% 38%	1884
March	1st to 10th	Night	28% 12% 3 5% 3 3 3 3 5% 6% 31%	3724
	11th to 20th	Night	11% 8% 4 7% 5% 5% 5% 4 5% 6% 39%	3577
	21st to end	Night	25% 9% 2 8% 5% 4 5% 3 4 4 32%	3672
April	1st to 10th	Night	19% 11% 3 6% 3 3 4 2 4 7% 37%	1900
	11th to 20th	Night	18% 8% 2 7% 4 4 3 4 5% 6% 38%	1851
	21st to end	Night	21% 8% 3 6% 5% 4 3 3 4 6% 37%	1710
May	1st to 10th	Night	25% 14% 4 9% 4 4 4 3 5% 6% 23%	2834
	11th to 20th	Night	26% 13% 3 7% 4 4 4 4 5% 6% 24%	2813
	21st to end	Night	16% 12% 5% 8% 5% 4 4 3 6% 7% 30%	2874
June	1st to 10th	Night	19% 12% 2 6% 5% 3 4 4 5% 8% 32%	1520
	11th to 20th	Night	17% 7% 2 5% 4 4 4 3 5% 8% 40%	1520
	21st to end	Night	18% 9% 4 6% 4 4 5% 4 6% 7% 33%	1520
July	1st to 10th	Night	23% 12% 3 6% 4 3 4 4 6% 7% 30%	2520
	11th to 20th	Night	14% 10% 3 6% 4 4 5% 4 6% 11% 34%	2520
	21st to end	Night	18% 10% 3 7% 5% 5% 5% 4 6% 6% 31%	2798
August	1st to 10th	Night	19% 10% 3 7% 4 5% 5% 3 5% 8% 30%	2825
	11th to 20th	Night	8% 7% 3 7% 5% 5% 5% 5% 7% 9% 39%	2809
	21st to end	Night	8% 4 2 5% 3 3 4 4 7% 9% 51%	3476
September	1st to 10th	Night	13% 7% 2 7% 4 4 4 4 5% 8% 42%	1938
	11th to 20th	Night	15% 7% 2 5% 3 3 4 3 4 6% 48%	2090
	21st to end	Night	20% 7% 2 6% 4 4 4 3 5% 5% 39%	2090
October	1st to 10th	Night	22% 8% 2 6% 4 3 4 3 4 6% 37%	2090
	11th to 20th	Night	16% 6% 2 5% 4 3 4 3 4 7% 47%	2275
	21st to end	Night	20% 8% 3 5% 5% 4 4 3 4 6% 37%	2717
November	1st to 10th	Night	17% 7% 3 7% 4 4 4 3 4 6% 42%	2470
	11th to 20th	Night	17% 10% 3 6% 4 4 3 3 5% 6% 40%	2470
	21st to end	Night	16% 8% 2 6% 4 4 4 3 5% 7% 41%	2470
December	1st to 10th	Night	21% 9% 3 7% 5% 4 4 4 5% 5% 34%	2512
	11th to 20th	Night	20% 10% 2 5% 4 3 4 3 4 6% 38%	3288
	21st to end	Night	19% 12% 4 8% 3 3 3 3 4 5% 34%	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	43% 5% 12% 26% 13% 1	2518
	11th to 20th	Day	47% 8% 13% 22% 8% 2	2520
	21st to end	Day	57% 5% 10% 19% 9% 1	2818
February	1st to 10th	Day	51% 6% 14% 17% 11% 1	1824
	11th to 20th	Day	44% 7% 11% 22% 14% 2	1857
	21st to end	Day	48% 6% 12% 19% 13% 3	1570
March	1st to 10th	Day	55% 4 13% 15% 11% 2	3119
	11th to 20th	Day	32% 7% 15% 31% 14% 2	3381
	21st to end	Day	54% 3 6% 17% 15% 5%	3956
April	1st to 10th	Day	44% 5% 10% 20% 18% 3	2280
	11th to 20th	Day	41% 6% 12% 22% 17% 2	2278
	21st to end	Day	43% 6% 11% 20% 16% 4	2470
May	1st to 10th	Day	60% 7% 11% 13% 8% 1	4120
	11th to 20th	Day	53% 6% 14% 16% 9% 1	4085
	21st to end	Day	46% 6% 10% 20% 17% 1	4728
June	1st to 10th	Day	47% 7% 12% 21% 12% 1	2660
	11th to 20th	Day	41% 8% 15% 23% 12% 1	2660
	21st to end	Day	40% 9% 14% 23% 13% 1	2660
July	1st to 10th	Day	50% 7% 11% 17% 14% 1	4370
	11th to 20th	Day	41% 8% 17% 22% 11% 1	4365
	21st to end	Day	46% 8% 14% 18% 13% 1	4571
August	1st to 10th	Day	49% 6% 16% 20% 8%	4092
	11th to 20th	Day	38% 7% 16% 29% 8% 1	4052
	21st to end	Day	29% 7% 16% 27% 19% 1	4180
September	1st to 10th	Day	36% 7% 11% 24% 21% 1	2242
	11th to 20th	Day	34% 6% 13% 23% 22% 3	2090
	21st to end	Day	45% 7% 9% 20% 18% 2	2090
October	1st to 10th	Day	38% 4 11% 27% 16% 4	3480
	11th to 20th	Day	31% 4 11% 28% 24% 3	3129
	21st to end	Day	31% 5% 15% 32% 16% 2	3081
November	1st to 10th	Day	43% 5% 16% 19% 15% 2	1708
	11th to 20th	Day	42% 6% 15% 21% 14% 2	1710
	21st to end	Day	45% 6% 13% 25% 9% 2	1710
December	1st to 10th	Day	53% 6% 12% 16% 12% 1	1663
	11th to 20th	Day	47% 6% 15% 15% 13% 4	1906
	21st to end	Day	53% 6% 13% 20% 8%	2772
January	1st to 10th	Night	44% 5% 11% 29% 9% 2	4429
	11th to 20th	Night	51% 5% 14% 20% 9% 1	4440
	21st to end	Night	62% 7% 8% 15% 9% 1	4822
February	1st to 10th	Night	50% 7% 13% 19% 9% 1	2352
	11th to 20th	Night	49% 4 11% 20% 13% 3	2220
	21st to end	Night	48% 5% 10% 22% 12% 3	1884
March	1st to 10th	Night	55% 5% 10% 18% 10% 2	3734
	11th to 20th	Night	41% 8% 14% 21% 14% 2	3578
	21st to end	Night	55% 4 7% 11% 18% 5%	3672
April	1st to 10th	Night	48% 4 9% 18% 17% 3	1900
	11th to 20th	Night	46% 5% 10% 20% 16% 4	1851
	21st to end	Night	48% 5% 10% 20% 14% 3	1710
May	1st to 10th	Night	62% 7% 11% 12% 7% 1	2840
	11th to 20th	Night	58% 8% 12% 13% 7% 1	2814
	21st to end	Night	52% 8% 12% 18% 11% 1	2875
June	1st to 10th	Night	50% 7% 10% 19% 13% 1	1520
	11th to 20th	Night	42% 7% 12% 25% 14% 1	1520
	21st to end	Night	47% 9% 12% 20% 12% 1	1520
July	1st to 10th	Night	53% 6% 10% 17% 14% 1	2480
	11th to 20th	Night	44% 8% 18% 21% 9%	2472
	21st to end	Night	51% 7% 14% 16% 10% 1	2709
August	1st to 10th	Night	52% 7% 14% 18% 8% 1	2813
	11th to 20th	Night	37% 10% 17% 28% 8%	2836
	21st to end	Night	28% 7% 17% 27% 21% 1	3476
September	1st to 10th	Night	39% 7% 11% 24% 17% 2	1938
	11th to 20th	Night	36% 6% 11% 23% 22% 2	2090
	21st to end	Night	46% 7% 7% 19% 18% 3	2090
October	1st to 10th	Night	42% 5% 9% 27% 15% 2	3480
	11th to 20th	Night	27% 4 10% 23% 30% 4	3785
	21st to end	Night	34% 4 12% 26% 22% 1	4467
November	1st to 10th	Night	44% 6% 12% 20% 15% 3	2470
	11th to 20th	Night	46% 7% 12% 20% 14% 3	2470
	21st to end	Night	44% 6% 14% 25% 10% 2	2470
December	1st to 10th	Night	52% 7% 14% 16% 10%	2512
	11th to 20th	Night	49% 6% 14% 16% 13% 2	3288
	21st to end	Night	52% 6% 13% 20% 8%	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	460
	11th to 20th	Day	21% 7% 30% 33% 9% 1	452
	21st to end	Day	24% 5% 25% 32% 13% 1	460
May	1st to 10th	Day	27% 7% 24% 33% 9% 1	463
	11th to 20th	Day	21% 5% 24% 36% 13% 2	487
	21st to end	Day	20% 4 13% 33% 24% 6%	529
June	1st to 10th	Day	19% 2 15% 37% 24% 2	490
	11th to 20th	Day	17% 2 14% 38% 26% 2	490
	21st to end	Day	15% 3 14% 37% 25% 5%	490
July	1st to 10th	Day	18% 3 9% 29% 34% 7%	485
	11th to 20th	Day	13% 2 8% 33% 36% 9%	484
	21st to end	Day	14% 3 12% 40% 29% 3	532
August	1st to 10th	Day	15% 2 9% 37% 31% 5%	486
	11th to 20th	Day	16% 2 10% 32% 30% 9%	479
	21st to end	Day	13% 2 14% 32% 31% 9%	539
September	1st to 10th	Day	18% 3 14% 29% 25% 12%	490
	11th to 20th	Day	16% 4 15% 27% 27% 11%	490
	21st to end	Day	25% 3 19% 29% 20% 4	490
October	1st to 10th	Day	26% 3 19% 26% 20% 6%	490
	11th to 20th	Day	16% 7% 25% 31% 17% 3	427
	21st to end	Day	23% 9% 26% 27% 13% 3	429
November	1st to 10th	Day	17% 9% 27% 29% 15% 2	378
	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% 5% 32% 31% 10%	727
	21st to end	Night	23% 6% 25% 31% 14% 2	570
May	1st to 10th	Night	27% 4 20% 32% 14% 2	570
	11th to 20th	Night	24% 6% 24% 34% 12% 1	562
	21st to end	Night	22% 2 14% 34% 24% 3	495
June	1st to 10th	Night	23% 3 16% 31% 25% 2	380
	11th to 20th	Night	20% 1 14% 41% 22% 2	380
	21st to end	Night	19% 3 13% 39% 25% 1	380
July	1st to 10th	Night	16% 2 12% 39% 28% 3	370
	11th to 20th	Night	15% 2 11% 35% 32% 5%	368
	21st to end	Night	15% 3 12% 35% 32% 3	451
August	1st to 10th	Night	12% 2 11% 39% 32% 4	568
	11th to 20th	Night	13% 2 10% 37% 32% 6%	576
	21st to end	Night	15% 2 10% 33% 32% 8%	836
September	1st to 10th	Night	16% 1 16% 33% 28% 6%	760
	11th to 20th	Night	18% 2 16% 30% 26% 8%	760
	21st to end	Night	24% 3 18% 29% 22% 3	760
October	1st to 10th	Night	27% 5% 20% 23% 20% 4	760
	11th to 20th	Night	19% 8% 27% 28% 14% 3	760
	21st to end	Night	24% 10% 27% 26% 11% 2	836
November	1st to 10th	Night	20% 8% 28% 28% 13% 3	760
	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 2024-11-03 02:11:17. Version ac07

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