Block Island
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Block Island clear sky chart from 2005-11-06 to 2025-09-19.

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% 40%	125911
	Night	18% 9% 3 7% 4 4 4 3 5% 6% 38%	123143
Transparency	Day	44% 5% 13% 22% 15% 2	129471
	Night	46% 5% 11% 21% 14% 2	127670
Seeing	Day	19% 7% 23% 30% 18% 4	16299
	Night	21% 9% 26% 27% 14% 2	25197

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% 9% 2 5% 4 3 4 3 6% 6% 47%	3597
	11th to 20th	Day	16% 9% 2 5% 4 4 4 3 4 5% 46%	3570
	21st to end	Day	19% 9% 5% 7% 4 4 3 3 4 5% 38%	3960
February	1st to 10th	Day	19% 8% 2 7% 5% 4 5% 5% 6% 6% 34%	1955
	11th to 20th	Day	15% 7% 3 6% 4 4 3 4 7% 6% 39%	1948
	21st to end	Day	20% 8% 3 5% 3 3 3 4 6% 6% 38%	1650
March	1st to 10th	Day	20% 9% 2 4 3 3 3 2 4 6% 43%	4476
	11th to 20th	Day	13% 9% 3 5% 3 3 3 3 5% 7% 46%	4747
	21st to end	Day	21% 10% 2 6% 3 4 4 3 4 5% 37%	5802
April	1st to 10th	Day	19% 7% 2 6% 3 3 4 3 5% 6% 42%	2400
	11th to 20th	Day	18% 7% 2 5% 3 3 3 3 5% 8% 44%	2374
	21st to end	Day	19% 6% 2 5% 4 4 4 3 6% 7% 40%	2575
May	1st to 10th	Day	25% 12% 4 7% 3 3 3 3 5% 7% 27%	5886
	11th to 20th	Day	19% 11% 3 7% 4 3 3 3 4 6% 36%	5861
	21st to end	Day	18% 11% 3 6% 4 3 4 3 5% 7% 35%	6939
June	1st to 10th	Day	18% 7% 2 6% 5% 4 4 4 6% 8% 37%	2800
	11th to 20th	Day	14% 7% 3 7% 5% 4 4 4 5% 9% 39%	2800
	21st to end	Day	12% 7% 2 5% 4 4 4 4 6% 9% 42%	2800
July	1st to 10th	Day	15% 10% 3 7% 4 4 4 3 6% 9% 34%	6360
	11th to 20th	Day	12% 10% 3 7% 5% 5% 5% 4 7% 9% 34%	6360
	21st to end	Day	15% 6% 3 5% 3 4 4 3 5% 8% 43%	6909
August	1st to 10th	Day	16% 7% 3 8% 5% 4 5% 4 5% 7% 37%	4235
	11th to 20th	Day	11% 6% 2 6% 5% 4 4 4 6% 7% 44%	4017
	21st to end	Day	7% 4 2 4 4 4 5% 4 6% 8% 51%	4312
September	1st to 10th	Day	14% 7% 2 5% 4 4 3 4 5% 6% 47%	2399
	11th to 20th	Day	12% 6% 2 5% 3 2 3 2 5% 7% 52%	2178
	21st to end	Day	19% 8% 2 6% 4 4 3 4 5% 6% 40%	2090
October	1st to 10th	Day	20% 7% 2 6% 3 3 3 3 3 6% 43%	2090
	11th to 20th	Day	14% 7% 2 5% 3 3 4 3 4 8% 48%	1891
	21st to end	Day	18% 6% 2 5% 5% 3 4 3 5% 6% 42%	1881
November	1st to 10th	Day	16% 7% 3 6% 4 3 4 2 4 5% 47%	1761
	11th to 20th	Day	15% 8% 2 4 4 3 3 3 6% 7% 45%	1800
	21st to end	Day	17% 8% 3 6% 5% 3 3 3 5% 6% 40%	1800
December	1st to 10th	Day	14% 8% 4 8% 5% 3 3 3 4 5% 43%	2742
	11th to 20th	Day	19% 10% 3 5% 3 3 3 2 3 5% 43%	2986
	21st to end	Day	20% 12% 4 10% 4 3 3 2 4 5% 33%	3960
January	1st to 10th	Night	15% 6% 2 6% 4 3 3 3 5% 6% 48%	6349
	11th to 20th	Night	17% 10% 3 8% 4 4 3 3 4 5% 39%	6315
	21st to end	Night	18% 11% 4 9% 4 4 4 4 5% 6% 31%	6996
February	1st to 10th	Night	17% 11% 3 7% 5% 4 4 3 5% 6% 35%	2439
	11th to 20th	Night	19% 8% 2 6% 4 4 3 4 6% 7% 37%	2338
	21st to end	Night	19% 9% 3 6% 5% 4 4 4 5% 6% 37%	1980
March	1st to 10th	Night	22% 9% 2 5% 3 3 3 3 4 6% 39%	5364
	11th to 20th	Night	16% 10% 3 7% 4 3 4 3 4 5% 39%	5210
	21st to end	Night	22% 9% 3 6% 4 4 4 4 5% 4 34%	5114
April	1st to 10th	Night	20% 9% 2 6% 4 4 4 3 4 6% 38%	2000
	11th to 20th	Night	18% 8% 2 6% 4 3 4 3 5% 5% 41%	1975
	21st to end	Night	20% 9% 3 5% 4 4 3 3 4 6% 39%	1825
May	1st to 10th	Night	25% 12% 4 8% 4 3 3 3 5% 7% 26%	4054
	11th to 20th	Night	23% 12% 3 8% 3 3 3 3 6% 7% 29%	4033
	21st to end	Night	21% 11% 4 7% 4 4 5% 4 5% 6% 28%	3978
June	1st to 10th	Night	18% 10% 3 6% 4 4 5% 5% 5% 6% 35%	1600
	11th to 20th	Night	19% 7% 3 5% 3 4 4 3 6% 9% 38%	1600
	21st to end	Night	13% 8% 3 7% 4 5% 4 4 6% 10% 37%	1600
July	1st to 10th	Night	17% 14% 3 8% 4 3 4 3 5% 8% 31%	3600
	11th to 20th	Night	13% 13% 3 8% 5% 5% 5% 4 7% 9% 30%	3600
	21st to end	Night	15% 7% 3 6% 4 4 5% 4 5% 7% 39%	3926
August	1st to 10th	Night	19% 9% 3 7% 4 5% 5% 4 7% 8% 29%	2897
	11th to 20th	Night	8% 6% 3 6% 4 5% 6% 5% 7% 9% 41%	3027
	21st to end	Night	8% 4 2 5% 3 4 4 4 6% 8% 53%	3586
September	1st to 10th	Night	13% 6% 2 6% 5% 5% 5% 3 6% 7% 41%	2000
	11th to 20th	Night	14% 7% 2 5% 3 3 3 2 4 6% 50%	2179
	21st to end	Night	20% 7% 3 7% 4 4 4 3 4 5% 39%	2090
October	1st to 10th	Night	21% 7% 2 6% 4 4 3 2 4 6% 40%	2090
	11th to 20th	Night	16% 6% 2 5% 4 4 3 3 5% 6% 46%	2287
	21st to end	Night	19% 7% 3 6% 4 4 4 3 5% 6% 38%	2717
November	1st to 10th	Night	15% 8% 2 6% 4 4 4 2 5% 6% 45%	2537
	11th to 20th	Night	17% 8% 3 6% 4 4 3 4 5% 5% 41%	2600
	21st to end	Night	16% 7% 2 6% 4 4 4 4 6% 7% 41%	2600
December	1st to 10th	Night	17% 10% 3 7% 5% 4 3 3 5% 4 39%	4433
	11th to 20th	Night	24% 9% 3 6% 3 4 3 3 3 5% 38%	5208
	21st to end	Night	18% 10% 4 8% 4 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	39% 3 8% 28% 20% 1	3598
	11th to 20th	Day	42% 5% 10% 19% 17% 7%	3600
	21st to end	Day	51% 3 10% 20% 15% 1	3960
February	1st to 10th	Day	49% 5% 16% 18% 11% 1	1955
	11th to 20th	Day	44% 5% 11% 23% 14% 2	1957
	21st to end	Day	45% 4 14% 18% 15% 3	1650
March	1st to 10th	Day	43% 3 13% 17% 14% 10%	4479
	11th to 20th	Day	38% 4 15% 30% 12% 1	4747
	21st to end	Day	50% 3 8% 16% 18% 5%	5802
April	1st to 10th	Day	44% 4 11% 20% 18% 3	2400
	11th to 20th	Day	40% 5% 10% 23% 19% 3	2374
	21st to end	Day	42% 5% 9% 21% 18% 4	2575
May	1st to 10th	Day	58% 5% 15% 13% 8% 1	5897
	11th to 20th	Day	49% 4 14% 19% 13% 1	5865
	21st to end	Day	48% 5% 11% 22% 12% 1	6939
June	1st to 10th	Day	45% 6% 11% 22% 14% 1	2800
	11th to 20th	Day	43% 6% 14% 23% 13% 1	2800
	21st to end	Day	38% 7% 14% 25% 15% 1	2800
July	1st to 10th	Day	47% 8% 13% 18% 13% 1	6290
	11th to 20th	Day	46% 7% 16% 22% 9% 1	6285
	21st to end	Day	39% 6% 12% 26% 16% 1	6711
August	1st to 10th	Day	46% 7% 16% 21% 10%	4222
	11th to 20th	Day	38% 7% 15% 31% 9% 1	4056
	21st to end	Day	30% 6% 14% 29% 20% 1	4312
September	1st to 10th	Day	37% 6% 11% 25% 20% 1	2399
	11th to 20th	Day	33% 4 13% 25% 22% 3	2178
	21st to end	Day	46% 5% 8% 21% 19% 1	2090
October	1st to 10th	Day	37% 4 11% 25% 19% 5%	3480
	11th to 20th	Day	29% 4 11% 26% 27% 3	3120
	21st to end	Day	32% 4 16% 29% 17% 2	3081
November	1st to 10th	Day	41% 4 16% 20% 18% 2	1761
	11th to 20th	Day	39% 5% 15% 22% 18% 3	1800
	21st to end	Day	46% 3 14% 26% 9% 2	1800
December	1st to 10th	Day	44% 3 15% 26% 11% 1	2742
	11th to 20th	Day	46% 3 14% 17% 16% 5%	2986
	21st to end	Day	54% 4 9% 17% 13% 3	3960
January	1st to 10th	Night	39% 3 11% 28% 17% 2	6349
	11th to 20th	Night	48% 4 10% 20% 14% 4	6360
	21st to end	Night	54% 4 11% 17% 12% 2	6996
February	1st to 10th	Night	50% 5% 13% 20% 10% 2	2439
	11th to 20th	Night	47% 4 10% 23% 14% 3	2340
	21st to end	Night	48% 3 10% 22% 13% 3	1980
March	1st to 10th	Night	48% 3 11% 18% 13% 7%	5374
	11th to 20th	Night	47% 5% 12% 22% 12% 3	5211
	21st to end	Night	53% 3 6% 11% 22% 5%	5114
April	1st to 10th	Night	48% 5% 8% 20% 16% 3	2000
	11th to 20th	Night	45% 5% 9% 19% 19% 3	1975
	21st to end	Night	47% 4 8% 19% 17% 4	1825
May	1st to 10th	Night	58% 6% 14% 13% 8% 1	4060
	11th to 20th	Night	54% 5% 12% 17% 8% 3	4034
	21st to end	Night	54% 6% 11% 19% 9% 1	3979
June	1st to 10th	Night	47% 7% 10% 19% 16% 1	1600
	11th to 20th	Night	43% 6% 12% 25% 14% 1	1600
	21st to end	Night	42% 8% 12% 24% 14% 1	1600
July	1st to 10th	Night	52% 6% 10% 21% 11% 1	3560
	11th to 20th	Night	50% 7% 15% 21% 7%	3552
	21st to end	Night	43% 5% 14% 24% 12% 1	3809
August	1st to 10th	Night	50% 8% 15% 18% 9% 1	2886
	11th to 20th	Night	37% 8% 17% 29% 9%	3054
	21st to end	Night	28% 5% 15% 30% 20% 1	3586
September	1st to 10th	Night	40% 6% 10% 25% 17% 2	2000
	11th to 20th	Night	37% 4 10% 24% 23% 2	2179
	21st to end	Night	48% 5% 7% 19% 18% 3	2090
October	1st to 10th	Night	42% 4 9% 28% 15% 2	3480
	11th to 20th	Night	28% 4 10% 24% 30% 5%	3797
	21st to end	Night	34% 4 14% 25% 23% 1	4467
November	1st to 10th	Night	41% 5% 12% 23% 16% 3	2537
	11th to 20th	Night	45% 5% 11% 20% 16% 3	2600
	21st to end	Night	42% 5% 15% 25% 11% 2	2600
December	1st to 10th	Night	48% 6% 13% 22% 10% 1	4433
	11th to 20th	Night	51% 4 10% 20% 14% 1	5208
	21st to end	Night	49% 4 11% 18% 16% 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	20% 17% 36% 20% 7% 1	200
	11th to 20th	Day	18% 19% 38% 21% 5%	200
	21st to end	Day	18% 13% 36% 25% 7% 1	220
February	1st to 10th	Day	24% 11% 35% 23% 6%	465
	11th to 20th	Day	21% 15% 38% 24% 2	496
	21st to end	Day	22% 13% 34% 25% 6% 1	414
March	1st to 10th	Day	19% 11% 37% 26% 6%	500
	11th to 20th	Day	17% 11% 34% 30% 7% 1	500
	21st to end	Day	23% 9% 34% 24% 8% 2	550
April	1st to 10th	Day	23% 9% 28% 28% 11% 1	500
	11th to 20th	Day	19% 9% 29% 34% 9% 1	492
	21st to end	Day	22% 6% 21% 38% 12% 1	500
May	1st to 10th	Day	24% 5% 26% 32% 12%	503
	11th to 20th	Day	18% 4 26% 35% 16% 2	527
	21st to end	Day	20% 3 14% 32% 26% 5%	573
June	1st to 10th	Day	18% 2 17% 34% 25% 3	530
	11th to 20th	Day	15% 2 15% 35% 27% 5%	530
	21st to end	Day	15% 1 14% 37% 28% 5%	530
July	1st to 10th	Day	16% 3 8% 32% 33% 8%	525
	11th to 20th	Day	10% 2 6% 35% 38% 9%	524
	21st to end	Day	14% 4 9% 37% 32% 3	576
August	1st to 10th	Day	13% 2 10% 36% 32% 7%	526
	11th to 20th	Day	15% 3 7% 32% 34% 9%	519
	21st to end	Day	12% 2 11% 36% 29% 10%	583
September	1st to 10th	Day	18% 3 13% 30% 26% 10%	530
	11th to 20th	Day	15% 3 15% 27% 28% 11%	522
	21st to end	Day	23% 3 21% 29% 19% 4	490
October	1st to 10th	Day	22% 4 19% 28% 19% 7%	490
	11th to 20th	Day	16% 7% 25% 32% 19% 3	450
	21st to end	Day	22% 9% 26% 27% 12% 3	429
November	1st to 10th	Day	14% 9% 26% 32% 15% 4	395
	11th to 20th	Day	21% 14% 30% 26% 9%	400
	21st to end	Day	21% 13% 32% 24% 10% 1	400
December	1st to 10th	Day	20% 12% 33% 26% 10%	290
	11th to 20th	Day	26% 14% 36% 20% 5% 1	200
	21st to end	Day	25% 12% 30% 24% 9% 1	220
January	1st to 10th	Night	22% 17% 36% 21% 3	800
	11th to 20th	Night	29% 16% 33% 19% 4	800
	21st to end	Night	22% 16% 38% 19% 4 1	880
February	1st to 10th	Night	23% 14% 38% 23% 3	800
	11th to 20th	Night	24% 16% 42% 15% 3	783
	21st to end	Night	23% 14% 35% 22% 6%	660
March	1st to 10th	Night	22% 14% 34% 23% 6% 1	800
	11th to 20th	Night	20% 10% 35% 27% 6% 2	800
	21st to end	Night	23% 9% 34% 24% 8% 2	880
April	1st to 10th	Night	24% 8% 30% 30% 8% 1	800
	11th to 20th	Night	20% 7% 28% 32% 12% 1	791
	21st to end	Night	23% 5% 25% 32% 13% 2	625
May	1st to 10th	Night	25% 5% 20% 34% 14% 3	600
	11th to 20th	Night	22% 5% 22% 38% 11% 1	592
	21st to end	Night	23% 3 13% 34% 24% 4	459
June	1st to 10th	Night	20% 3 16% 36% 24% 2	400
	11th to 20th	Night	21% 3 13% 35% 27% 2	400
	21st to end	Night	17% 3 12% 37% 30% 2	400
July	1st to 10th	Night	16% 4 11% 35% 30% 4	390
	11th to 20th	Night	17% 1 9% 35% 32% 6%	388
	21st to end	Night	15% 3 9% 37% 32% 4	438
August	1st to 10th	Night	11% 3 10% 37% 34% 6%	580
	11th to 20th	Night	12% 2 10% 34% 35% 8%	682
	21st to end	Night	14% 3 9% 32% 33% 8%	880
September	1st to 10th	Night	16% 2 15% 33% 28% 7%	800
	11th to 20th	Night	16% 3 15% 30% 27% 9%	793
	21st to end	Night	23% 3 20% 29% 21% 3	760
October	1st to 10th	Night	25% 6% 21% 25% 19% 5%	760
	11th to 20th	Night	18% 9% 26% 30% 14% 3	760
	21st to end	Night	21% 9% 31% 26% 10% 2	836
November	1st to 10th	Night	19% 8% 29% 27% 14% 3	780
	11th to 20th	Night	22% 14% 32% 23% 8% 1	800
	21st to end	Night	18% 18% 29% 26% 8% 1	800
December	1st to 10th	Night	24% 17% 31% 22% 6% 1	800
	11th to 20th	Night	25% 18% 32% 19% 5% 1	800
	21st to end	Night	23% 14% 36% 20% 5% 1	880

Page generated at 2025-09-16 01:49:13. Version ac07

Copyright © 2025 Attilla Danko.