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 2025-10-19.

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 6% 4 4 4 3 5% 7% 39%	126196
	Night	18% 10% 3 6% 4 4 4 3 5% 6% 37%	123483
Transparency	Day	45% 6% 12% 21% 14% 2	129759
	Night	47% 6% 12% 20% 14% 2	128008
Seeing	Day	20% 7% 22% 30% 17% 4	16481
	Night	22% 9% 26% 27% 14% 2	25309

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% 8% 3 5% 3 4 3 3 5% 6% 47%	3597
	11th to 20th	Day	16% 8% 2 5% 3 3 4 3 4 5% 47%	3570
	21st to end	Day	17% 9% 4 9% 4 3 3 3 4 4 39%	4034
February	1st to 10th	Day	18% 10% 3 7% 4 4 4 4 6% 5% 33%	1920
	11th to 20th	Day	16% 8% 2 6% 4 4 4 3 6% 7% 39%	1948
	21st to end	Day	19% 9% 3 6% 4 4 3 3 4 6% 38%	1650
March	1st to 10th	Day	20% 12% 2 6% 3 3 3 2 4 5% 39%	4476
	11th to 20th	Day	14% 10% 3 5% 3 3 3 3 5% 8% 44%	4839
	21st to end	Day	24% 11% 2 6% 4 4 3 2 3 5% 35%	5662
April	1st to 10th	Day	20% 10% 2 6% 3 2 3 3 5% 7% 40%	2400
	11th to 20th	Day	18% 8% 2 5% 3 3 4 3 5% 7% 42%	2399
	21st to end	Day	19% 8% 3 5% 3 4 3 4 6% 6% 39%	2600
May	1st to 10th	Day	26% 14% 4 8% 4 4 3 3 5% 7% 24%	5886
	11th to 20th	Day	19% 12% 4 7% 4 4 4 3 4 6% 32%	5861
	21st to end	Day	19% 11% 3 7% 4 4 3 3 5% 7% 34%	6784
June	1st to 10th	Day	19% 8% 3 6% 5% 4 3 4 7% 8% 34%	2800
	11th to 20th	Day	15% 8% 2 6% 4 4 5% 5% 6% 8% 38%	2800
	21st to end	Day	14% 7% 2 5% 4 4 5% 5% 6% 9% 40%	2800
July	1st to 10th	Day	17% 12% 3 8% 4 4 4 3 5% 8% 32%	6360
	11th to 20th	Day	12% 10% 3 7% 5% 5% 5% 4 7% 10% 32%	6360
	21st to end	Day	15% 7% 2 6% 4 3 3 3 5% 7% 44%	6791
August	1st to 10th	Day	17% 9% 3 7% 5% 4 4 4 5% 7% 35%	4235
	11th to 20th	Day	12% 7% 3 6% 4 4 4 4 5% 8% 43%	4142
	21st to end	Day	7% 4 2 5% 4 4 4 4 6% 9% 52%	4312
September	1st to 10th	Day	14% 6% 3 4 3 3 4 4 4 7% 47%	2360
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 4 7% 50%	2200
	21st to end	Day	20% 7% 2 6% 3 4 5% 4 6% 6% 40%	2200
October	1st to 10th	Day	21% 6% 2 5% 3 3 3 3 4 6% 44%	2200
	11th to 20th	Day	16% 7% 2 5% 4 3 3 3 5% 6% 47%	1981
	21st to end	Day	18% 7% 2 6% 4 3 3 3 5% 6% 42%	1881
November	1st to 10th	Day	17% 6% 3 7% 4 3 3 2 4 6% 45%	1761
	11th to 20th	Day	16% 8% 2 5% 4 4 4 3 5% 7% 43%	1800
	21st to end	Day	19% 8% 2 4 5% 3 4 4 4 6% 41%	1800
December	1st to 10th	Day	15% 9% 3 7% 4 3 3 3 3 5% 44%	2841
	11th to 20th	Day	18% 8% 3 6% 4 3 3 3 3 5% 45%	2986
	21st to end	Day	19% 12% 4 7% 4 4 4 3 4 5% 34%	3960
January	1st to 10th	Night	15% 6% 2 5% 3 3 3 3 5% 6% 49%	6349
	11th to 20th	Night	16% 12% 4 8% 4 3 3 3 4 5% 40%	6315
	21st to end	Night	18% 11% 4 7% 5% 4 4 3 5% 5% 33%	6906
February	1st to 10th	Night	18% 10% 3 7% 5% 4 5% 3 5% 6% 34%	2475
	11th to 20th	Night	20% 8% 3 6% 4 3 4 4 5% 6% 37%	2338
	21st to end	Night	17% 9% 3 7% 4 4 4 3 5% 6% 38%	1980
March	1st to 10th	Night	23% 10% 3 5% 3 3 4 3 5% 6% 36%	5364
	11th to 20th	Night	16% 11% 4 8% 4 4 4 3 4 5% 36%	5119
	21st to end	Night	22% 11% 3 7% 5% 4 4 3 4 4 33%	5266
April	1st to 10th	Night	20% 10% 3 6% 3 3 4 2 5% 7% 36%	2000
	11th to 20th	Night	19% 8% 2 6% 4 3 3 4 5% 6% 38%	1950
	21st to end	Night	20% 8% 3 6% 5% 4 4 3 5% 6% 37%	1800
May	1st to 10th	Night	25% 14% 4 9% 4 3 4 3 5% 6% 24%	4054
	11th to 20th	Night	23% 14% 4 6% 4 3 4 4 5% 6% 28%	4033
	21st to end	Night	21% 11% 4 9% 5% 4 5% 3 6% 6% 25%	4118
June	1st to 10th	Night	19% 12% 2 6% 5% 3 4 4 5% 8% 33%	1600
	11th to 20th	Night	17% 8% 3 6% 4 4 4 3 5% 8% 39%	1600
	21st to end	Night	17% 9% 4 6% 4 4 5% 4 6% 7% 34%	1600
July	1st to 10th	Night	22% 13% 3 7% 4 3 4 4 5% 7% 29%	3600
	11th to 20th	Night	16% 13% 3 6% 4 4 5% 4 6% 10% 30%	3600
	21st to end	Night	14% 8% 3 6% 5% 4 5% 4 6% 6% 39%	4013
August	1st to 10th	Night	19% 10% 3 7% 4 5% 5% 3 5% 8% 30%	2915
	11th to 20th	Night	8% 7% 3 7% 5% 5% 5% 5% 7% 9% 39%	2900
	21st to end	Night	8% 4 2 5% 3 3 4 4 7% 9% 51%	3586
September	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 5% 9% 42%	2040
	11th to 20th	Night	15% 7% 2 5% 3 3 4 3 4 6% 48%	2200
	21st to end	Night	20% 7% 2 6% 4 4 4 3 5% 5% 39%	2200
October	1st to 10th	Night	21% 8% 2 6% 4 3 4 4 4 6% 39%	2200
	11th to 20th	Night	17% 6% 2 5% 4 3 4 3 4 6% 46%	2370
	21st to end	Night	20% 8% 3 5% 5% 4 4 3 4 6% 37%	2717
November	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 4 6% 43%	2537
	11th to 20th	Night	16% 10% 3 6% 4 4 4 3 5% 6% 41%	2600
	21st to end	Night	16% 8% 2 6% 4 4 4 4 5% 7% 41%	2600
December	1st to 10th	Night	17% 9% 2 6% 3 3 3 3 4 5% 44%	4334
	11th to 20th	Night	21% 10% 3 6% 4 3 3 2 4 5% 39%	5208
	21st to end	Night	18% 10% 4 8% 3 3 3 3 5% 5% 37%	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	38% 4 9% 25% 22% 1	3598
	11th to 20th	Day	41% 6% 10% 21% 16% 6%	3600
	21st to end	Day	50% 4 10% 20% 15% 1	4034
February	1st to 10th	Day	51% 6% 14% 17% 11% 1	1920
	11th to 20th	Day	44% 7% 11% 22% 14% 2	1957
	21st to end	Day	46% 6% 12% 20% 13% 3	1650
March	1st to 10th	Day	48% 4 12% 14% 13% 10%	4479
	11th to 20th	Day	39% 5% 16% 28% 11% 1	4839
	21st to end	Day	54% 3 7% 14% 16% 4	5662
April	1st to 10th	Day	44% 5% 11% 20% 18% 3	2400
	11th to 20th	Day	41% 6% 12% 22% 18% 2	2399
	21st to end	Day	42% 6% 11% 21% 16% 4	2600
May	1st to 10th	Day	60% 7% 13% 12% 6% 1	5897
	11th to 20th	Day	52% 5% 13% 17% 11% 1	5865
	21st to end	Day	50% 5% 11% 20% 13% 1	6784
June	1st to 10th	Day	47% 7% 11% 21% 13% 1	2800
	11th to 20th	Day	42% 8% 15% 22% 12% 1	2800
	21st to end	Day	40% 8% 14% 23% 14% 1	2800
July	1st to 10th	Day	51% 6% 12% 17% 12% 2	6290
	11th to 20th	Day	48% 7% 15% 20% 8% 1	6285
	21st to end	Day	39% 6% 12% 27% 14% 1	6596
August	1st to 10th	Day	49% 6% 16% 21% 9%	4222
	11th to 20th	Day	39% 7% 16% 29% 8% 1	4181
	21st to end	Day	29% 7% 15% 27% 20% 1	4312
September	1st to 10th	Day	36% 7% 11% 25% 20% 1	2360
	11th to 20th	Day	34% 6% 13% 23% 22% 2	2200
	21st to end	Day	45% 7% 9% 20% 17% 2	2200
October	1st to 10th	Day	37% 4 11% 27% 17% 4	3590
	11th to 20th	Day	31% 4 11% 28% 23% 3	3210
	21st to end	Day	31% 5% 15% 32% 16% 2	3081
November	1st to 10th	Day	42% 5% 16% 19% 16% 2	1761
	11th to 20th	Day	41% 6% 14% 21% 16% 3	1800
	21st to end	Day	45% 5% 13% 26% 10% 2	1800
December	1st to 10th	Day	46% 4 13% 25% 11% 1	2841
	11th to 20th	Day	45% 5% 14% 16% 15% 4	2986
	21st to end	Day	54% 5% 10% 17% 12% 3	3960
January	1st to 10th	Night	39% 4 9% 30% 15% 2	6349
	11th to 20th	Night	49% 4 12% 19% 13% 3	6360
	21st to end	Night	54% 6% 9% 17% 12% 3	6906
February	1st to 10th	Night	50% 7% 13% 20% 9% 1	2475
	11th to 20th	Night	49% 4 10% 20% 14% 3	2340
	21st to end	Night	48% 5% 10% 22% 13% 3	1980
March	1st to 10th	Night	49% 4 12% 16% 13% 6%	5374
	11th to 20th	Night	48% 6% 14% 18% 11% 2	5120
	21st to end	Night	55% 4 6% 11% 21% 5%	5266
April	1st to 10th	Night	49% 4 9% 18% 17% 3	2000
	11th to 20th	Night	46% 5% 11% 19% 16% 3	1950
	21st to end	Night	48% 5% 10% 19% 14% 4	1800
May	1st to 10th	Night	61% 6% 13% 12% 8% 1	4060
	11th to 20th	Night	56% 7% 12% 15% 8% 2	4034
	21st to end	Night	57% 6% 10% 17% 9%	4119
June	1st to 10th	Night	50% 7% 10% 19% 13% 1	1600
	11th to 20th	Night	43% 7% 11% 24% 14% 1	1600
	21st to end	Night	46% 9% 12% 20% 12% 1	1600
July	1st to 10th	Night	54% 5% 10% 19% 12% 1	3560
	11th to 20th	Night	49% 7% 16% 20% 7%	3552
	21st to end	Night	44% 6% 13% 24% 12% 1	3895
August	1st to 10th	Night	52% 7% 14% 18% 9% 1	2903
	11th to 20th	Night	37% 10% 17% 29% 8%	2927
	21st to end	Night	28% 7% 16% 27% 21% 1	3586
September	1st to 10th	Night	38% 7% 12% 25% 17% 2	2040
	11th to 20th	Night	36% 6% 11% 23% 22% 2	2200
	21st to end	Night	47% 7% 7% 19% 18% 3	2200
October	1st to 10th	Night	41% 5% 9% 28% 16% 2	3590
	11th to 20th	Night	28% 4 10% 23% 30% 4	3880
	21st to end	Night	34% 4 12% 26% 22% 1	4467
November	1st to 10th	Night	43% 6% 12% 21% 15% 3	2537
	11th to 20th	Night	44% 6% 11% 20% 15% 3	2600
	21st to end	Night	44% 5% 14% 25% 11% 2	2600
December	1st to 10th	Night	46% 5% 13% 24% 11% 1	4334
	11th to 20th	Night	50% 4 12% 17% 14% 1	5208
	21st to end	Night	50% 6% 12% 18% 13% 1	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	24% 14% 34% 21% 7% 1	200
	11th to 20th	Day	19% 17% 34% 25% 5% 1	200
	21st to end	Day	18% 13% 37% 22% 9% 1	255
February	1st to 10th	Day	23% 13% 32% 26% 5%	460
	11th to 20th	Day	24% 13% 38% 23% 3	496
	21st to end	Day	22% 13% 31% 26% 7% 1	414
March	1st to 10th	Day	21% 11% 34% 26% 7%	500
	11th to 20th	Day	17% 10% 35% 30% 7% 1	500
	21st to end	Day	22% 11% 33% 24% 7% 2	550
April	1st to 10th	Day	24% 9% 26% 30% 10% 1	500
	11th to 20th	Day	21% 8% 30% 32% 8% 1	492
	21st to end	Day	24% 5% 24% 33% 13% 1	500
May	1st to 10th	Day	27% 7% 23% 34% 9% 1	503
	11th to 20th	Day	21% 5% 24% 36% 13% 2	527
	21st to end	Day	21% 4 12% 35% 22% 5%	573
June	1st to 10th	Day	19% 3 15% 37% 24% 2	530
	11th to 20th	Day	18% 2 14% 38% 25% 2	530
	21st to end	Day	15% 3 14% 36% 27% 5%	530
July	1st to 10th	Day	18% 3 9% 30% 34% 7%	525
	11th to 20th	Day	13% 2 8% 32% 36% 9%	524
	21st to end	Day	14% 3 11% 40% 29% 3	576
August	1st to 10th	Day	15% 2 9% 37% 31% 6%	526
	11th to 20th	Day	16% 2 10% 32% 31% 9%	519
	21st to end	Day	12% 2 14% 33% 30% 9%	583
September	1st to 10th	Day	18% 3 14% 29% 25% 12%	530
	11th to 20th	Day	15% 4 15% 28% 28% 10%	530
	21st to end	Day	24% 3 18% 30% 21% 4	530
October	1st to 10th	Day	25% 3 19% 25% 21% 8%	530
	11th to 20th	Day	17% 7% 24% 31% 17% 3	454
	21st to end	Day	23% 9% 26% 27% 13% 3	429
November	1st to 10th	Day	17% 9% 28% 30% 14% 2	395
	11th to 20th	Day	22% 15% 31% 23% 10%	400
	21st to end	Day	23% 14% 28% 24% 10% 2	400
December	1st to 10th	Day	22% 13% 33% 23% 10%	350
	11th to 20th	Day	25% 13% 36% 19% 7% 1	200
	21st to end	Day	24% 10% 29% 26% 7% 3	220
January	1st to 10th	Night	22% 17% 36% 21% 4	800
	11th to 20th	Night	27% 17% 34% 18% 5%	800
	21st to end	Night	24% 15% 36% 21% 3 1	880
February	1st to 10th	Night	23% 16% 34% 25% 3	800
	11th to 20th	Night	23% 16% 40% 17% 3	783
	21st to end	Night	22% 14% 36% 22% 5%	660
March	1st to 10th	Night	23% 14% 33% 25% 7%	800
	11th to 20th	Night	22% 9% 35% 27% 6% 1	800
	21st to end	Night	24% 9% 33% 25% 8% 1	880
April	1st to 10th	Night	24% 8% 29% 31% 7% 1	800
	11th to 20th	Night	22% 5% 33% 31% 10%	766
	21st to end	Night	23% 5% 25% 32% 13% 2	600
May	1st to 10th	Night	28% 5% 20% 33% 14% 2	600
	11th to 20th	Night	23% 5% 23% 35% 12% 1	592
	21st to end	Night	23% 3 15% 33% 24% 3	521
June	1st to 10th	Night	23% 3 16% 33% 24% 2	400
	11th to 20th	Night	21% 2 14% 41% 21% 2	400
	21st to end	Night	19% 4 13% 38% 27% 1	400
July	1st to 10th	Night	16% 3 12% 39% 27% 3	390
	11th to 20th	Night	16% 2 10% 35% 31% 6%	388
	21st to end	Night	15% 3 12% 37% 31% 3	475
August	1st to 10th	Night	13% 2 11% 38% 32% 4	598
	11th to 20th	Night	13% 1 10% 37% 33% 6%	607
	21st to end	Night	15% 2 11% 33% 31% 8%	880
September	1st to 10th	Night	16% 1 15% 34% 28% 6%	800
	11th to 20th	Night	17% 2 16% 30% 27% 8%	800
	21st to end	Night	24% 3 18% 30% 22% 3	800
October	1st to 10th	Night	26% 5% 21% 23% 20% 5%	800
	11th to 20th	Night	21% 8% 26% 28% 14% 3	793
	21st to end	Night	24% 10% 27% 26% 11% 2	836
November	1st to 10th	Night	19% 8% 29% 28% 13% 3	780
	11th to 20th	Night	21% 16% 31% 24% 8% 1	800
	21st to end	Night	19% 18% 31% 22% 9% 1	800
December	1st to 10th	Night	25% 15% 31% 21% 7% 1	800
	11th to 20th	Night	27% 19% 31% 19% 5%	800
	21st to end	Night	24% 14% 35% 21% 5%	880

Page generated at 2025-10-16 01:50:27. Version ac07

Copyright © 2025 Attilla Danko.