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 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	17% 8% 3 6% 4 4 4 3 5% 7% 40%	101132
	Night	18% 9% 3 6% 4 4 4 3 5% 6% 37%	97925
Transparency	Day	43% 5% 13% 22% 14% 2	104710
	Night	46% 5% 12% 21% 14% 2	102436
Seeing	Day	19% 7% 23% 30% 18% 4	15176
	Night	21% 9% 26% 27% 14% 3	24116

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% 2 5% 4 4 4 3 5% 5% 40%	2517
	11th to 20th	Day	19% 9% 2 5% 4 4 4 3 5% 6% 38%	2520
	21st to end	Day	24% 11% 5% 7% 4 4 3 2 4 5% 31%	2772
February	1st to 10th	Day	19% 7% 2 7% 5% 4 4 4 6% 6% 34%	1857
	11th to 20th	Day	15% 7% 3 6% 4 4 3 4 6% 6% 40%	1848
	21st to end	Day	21% 9% 3 5% 3 2 4 4 6% 7% 36%	1570
March	1st to 10th	Day	25% 10% 2 5% 3 3 3 3 4 6% 36%	3116
	11th to 20th	Day	11% 5% 2 5% 3 3 3 3 6% 7% 51%	3317
	21st to end	Day	22% 9% 2 6% 3 4 4 3 4 5% 37%	4054
April	1st to 10th	Day	18% 8% 2 6% 3 3 4 3 5% 6% 42%	2280
	11th to 20th	Day	18% 7% 1 5% 4 3 3 3 5% 8% 44%	2254
	21st to end	Day	19% 6% 2 5% 4 4 4 4 6% 6% 40%	2446
May	1st to 10th	Day	26% 13% 5% 8% 3 3 3 2 4 6% 28%	4106
	11th to 20th	Day	22% 10% 3 7% 5% 3 3 3 5% 6% 34%	4081
	21st to end	Day	14% 11% 3 6% 3 3 4 3 5% 8% 40%	4827
June	1st to 10th	Day	18% 7% 2 6% 5% 4 4 4 6% 8% 37%	2660
	11th to 20th	Day	13% 6% 3 7% 5% 4 4 4 5% 9% 40%	2660
	21st to end	Day	12% 7% 2 5% 4 4 4 4 6% 9% 43%	2660
July	1st to 10th	Day	15% 11% 3 7% 4 4 4 3 5% 8% 35%	4440
	11th to 20th	Day	11% 7% 2 6% 4 5% 5% 4 7% 10% 40%	4440
	21st to end	Day	18% 7% 3 6% 4 4 4 4 6% 8% 35%	4797
August	1st to 10th	Day	16% 7% 3 8% 5% 4 5% 4 5% 7% 36%	4105
	11th to 20th	Day	12% 6% 2 6% 5% 4 4 4 6% 7% 45%	3892
	21st to end	Day	7% 4 2 5% 4 4 5% 4 6% 8% 51%	4180
September	1st to 10th	Day	14% 7% 2 5% 4 4 3 4 5% 6% 47%	2279
	11th to 20th	Day	13% 6% 2 5% 3 2 3 2 5% 7% 52%	2090
	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% 46%	1708
	11th to 20th	Day	16% 8% 2 4 4 3 3 4 6% 7% 43%	1710
	21st to end	Day	16% 8% 3 6% 5% 3 3 4 5% 6% 40%	1710
December	1st to 10th	Day	20% 8% 3 8% 5% 4 4 3 5% 6% 35%	1606
	11th to 20th	Day	21% 8% 2 5% 3 3 3 2 5% 6% 41%	1906
	21st to end	Day	20% 12% 3 8% 5% 3 4 3 5% 6% 31%	2772
January	1st to 10th	Night	19% 7% 2 5% 4 3 3 3 4 6% 44%	4429
	11th to 20th	Night	19% 11% 3 8% 4 3 3 3 5% 5% 35%	4440
	21st to end	Night	24% 13% 5% 8% 4 3 3 3 5% 5% 26%	4884
February	1st to 10th	Night	16% 11% 3 8% 5% 4 4 3 5% 6% 35%	2318
	11th to 20th	Night	19% 8% 3 6% 4 4 3 4 6% 7% 37%	2218
	21st to end	Night	19% 9% 2 6% 5% 4 4 4 5% 6% 37%	1884
March	1st to 10th	Night	28% 12% 2 5% 3 3 3 3 4 5% 33%	3724
	11th to 20th	Night	11% 7% 3 7% 5% 4 5% 4 6% 7% 43%	3640
	21st to end	Night	24% 8% 3 6% 4 4 3 4 5% 5% 34%	3576
April	1st to 10th	Night	19% 9% 2 6% 4 4 4 3 4 6% 39%	1900
	11th to 20th	Night	17% 8% 2 6% 4 3 4 3 5% 5% 41%	1875
	21st to end	Night	20% 9% 3 5% 4 4 3 3 4 6% 39%	1734
May	1st to 10th	Night	24% 13% 5% 8% 5% 3 3 3 5% 6% 25%	2834
	11th to 20th	Night	24% 12% 3 8% 4 4 4 4 6% 6% 24%	2813
	21st to end	Night	17% 11% 5% 7% 4 4 5% 4 5% 6% 33%	2776
June	1st to 10th	Night	18% 10% 3 6% 4 4 5% 5% 5% 7% 35%	1520
	11th to 20th	Night	18% 7% 3 4 3 3 4 3 6% 9% 40%	1520
	21st to end	Night	14% 8% 3 7% 4 5% 4 4 6% 10% 37%	1520
July	1st to 10th	Night	19% 13% 3 7% 4 3 4 4 6% 7% 30%	2520
	11th to 20th	Night	11% 9% 3 7% 5% 5% 5% 4 7% 10% 35%	2520
	21st to end	Night	19% 9% 3 7% 5% 5% 5% 4 5% 7% 31%	2739
August	1st to 10th	Night	19% 9% 3 7% 5% 5% 5% 4 7% 8% 29%	2808
	11th to 20th	Night	7% 6% 3 6% 5% 5% 6% 5% 7% 9% 41%	2932
	21st to end	Night	8% 4 2 5% 3 4 4 4 6% 8% 53%	3476
September	1st to 10th	Night	14% 6% 3 6% 4 5% 5% 3 6% 7% 41%	1900
	11th to 20th	Night	14% 7% 2 5% 3 3 3 2 4 6% 50%	2090
	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	16% 8% 2 6% 4 4 4 2 5% 6% 44%	2470
	11th to 20th	Night	18% 9% 3 6% 4 4 3 4 5% 6% 40%	2470
	21st to end	Night	16% 7% 2 6% 4 4 4 4 6% 7% 41%	2470
December	1st to 10th	Night	21% 8% 3 6% 4 5% 4 4 5% 5% 33%	2569
	11th to 20th	Night	21% 9% 2 5% 3 4 4 4 4 7% 37%	3288
	21st to end	Night	20% 12% 4 8% 4 3 3 3 5% 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	46% 4 11% 28% 12%	2518
	11th to 20th	Day	48% 6% 12% 22% 10% 2	2520
	21st to end	Day	57% 4 10% 20% 9% 1	2772
February	1st to 10th	Day	49% 5% 16% 18% 11% 1	1857
	11th to 20th	Day	43% 5% 11% 24% 14% 2	1857
	21st to end	Day	47% 5% 14% 18% 14% 3	1570
March	1st to 10th	Day	49% 4 13% 20% 12% 3	3119
	11th to 20th	Day	30% 5% 13% 35% 15% 1	3317
	21st to end	Day	50% 3 7% 19% 16% 5%	4054
April	1st to 10th	Day	44% 4 11% 21% 18% 3	2280
	11th to 20th	Day	40% 5% 10% 23% 19% 2	2254
	21st to end	Day	43% 5% 9% 21% 18% 4	2446
May	1st to 10th	Day	59% 5% 13% 13% 9% 2	4120
	11th to 20th	Day	51% 5% 16% 17% 10% 1	4085
	21st to end	Day	42% 6% 11% 22% 17% 1	4827
June	1st to 10th	Day	45% 6% 11% 22% 13% 1	2660
	11th to 20th	Day	41% 6% 14% 24% 14% 1	2660
	21st to end	Day	38% 7% 14% 25% 15% 1	2660
July	1st to 10th	Day	49% 7% 12% 17% 14% 1	4370
	11th to 20th	Day	40% 7% 17% 23% 12% 1	4365
	21st to end	Day	46% 7% 14% 18% 14% 2	4644
August	1st to 10th	Day	46% 7% 17% 21% 9%	4092
	11th to 20th	Day	37% 7% 15% 32% 8% 1	3931
	21st to end	Day	30% 6% 14% 29% 19% 1	4180
September	1st to 10th	Day	37% 6% 11% 24% 20% 1	2279
	11th to 20th	Day	33% 5% 13% 24% 22% 3	2090
	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	42% 4 15% 20% 17% 2	1708
	11th to 20th	Day	40% 5% 16% 22% 16% 2	1710
	21st to end	Day	46% 3 14% 26% 9% 2	1710
December	1st to 10th	Day	52% 3 13% 17% 13% 1	1606
	11th to 20th	Day	47% 3 16% 15% 14% 4	1906
	21st to end	Day	53% 5% 11% 21% 9%	2772
January	1st to 10th	Night	43% 3 12% 29% 9% 3	4429
	11th to 20th	Night	51% 6% 11% 21% 10% 1	4440
	21st to end	Night	61% 4 9% 16% 9% 1	4884
February	1st to 10th	Night	50% 5% 13% 20% 10% 2	2318
	11th to 20th	Night	47% 4 10% 22% 13% 3	2220
	21st to end	Night	48% 3 10% 23% 13% 4	1884
March	1st to 10th	Night	55% 3 9% 20% 11% 2	3734
	11th to 20th	Night	40% 6% 13% 24% 15% 3	3641
	21st to end	Night	53% 3 7% 13% 19% 5%	3576
April	1st to 10th	Night	47% 5% 8% 20% 17% 3	1900
	11th to 20th	Night	45% 5% 9% 20% 19% 3	1875
	21st to end	Night	48% 4 8% 20% 16% 4	1734
May	1st to 10th	Night	59% 6% 12% 14% 8% 1	2840
	11th to 20th	Night	58% 7% 12% 14% 8% 2	2814
	21st to end	Night	49% 7% 11% 19% 12% 1	2777
June	1st to 10th	Night	47% 7% 11% 19% 15% 1	1520
	11th to 20th	Night	41% 6% 12% 26% 14% 1	1520
	21st to end	Night	42% 8% 12% 25% 13% 1	1520
July	1st to 10th	Night	52% 6% 10% 19% 12% 1	2480
	11th to 20th	Night	42% 8% 17% 24% 8% 1	2472
	21st to end	Night	50% 6% 15% 16% 12% 1	2651
August	1st to 10th	Night	51% 8% 16% 17% 8% 1	2797
	11th to 20th	Night	37% 8% 17% 29% 9%	2959
	21st to end	Night	28% 5% 16% 30% 20% 1	3476
September	1st to 10th	Night	41% 6% 9% 25% 18% 2	1900
	11th to 20th	Night	37% 4 10% 24% 23% 2	2090
	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	42% 5% 12% 22% 15% 3	2470
	11th to 20th	Night	46% 5% 11% 21% 14% 3	2470
	21st to end	Night	42% 5% 15% 25% 11% 2	2470
December	1st to 10th	Night	52% 5% 14% 17% 11% 1	2569
	11th to 20th	Night	49% 5% 13% 18% 15% 2	3288
	21st to end	Night	52% 5% 11% 22% 10%	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	21% 13% 36% 22% 8% 1	180
	11th to 20th	Day	19% 18% 37% 20% 6%	180
	21st to end	Day	19% 10% 37% 24% 8% 2	198
February	1st to 10th	Day	25% 11% 35% 23% 6%	427
	11th to 20th	Day	21% 14% 39% 24% 2	456
	21st to end	Day	24% 13% 32% 25% 6% 1	382
March	1st to 10th	Day	20% 9% 36% 28% 7%	460
	11th to 20th	Day	17% 11% 36% 30% 6% 1	460
	21st to end	Day	23% 9% 34% 24% 8% 2	506
April	1st to 10th	Day	22% 9% 29% 28% 11% 1	460
	11th to 20th	Day	19% 8% 29% 35% 9% 1	452
	21st to end	Day	23% 6% 22% 36% 12% 2	460
May	1st to 10th	Day	25% 5% 27% 30% 12%	463
	11th to 20th	Day	18% 4 27% 34% 16% 1	487
	21st to end	Day	20% 2 15% 31% 27% 5%	529
June	1st to 10th	Day	18% 3 17% 34% 25% 3	490
	11th to 20th	Day	14% 2 15% 36% 28% 5%	490
	21st to end	Day	16% 1 14% 38% 26% 5%	490
July	1st to 10th	Day	16% 3 8% 32% 33% 8%	485
	11th to 20th	Day	9% 2 6% 35% 39% 8%	484
	21st to end	Day	15% 4 9% 38% 31% 3	532
August	1st to 10th	Day	13% 2 9% 37% 32% 7%	486
	11th to 20th	Day	16% 3 7% 32% 33% 9%	479
	21st to end	Day	12% 1 11% 35% 29% 11%	539
September	1st to 10th	Day	18% 3 13% 29% 26% 10%	490
	11th to 20th	Day	16% 3 16% 27% 26% 12%	490
	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	15% 9% 25% 32% 16% 4	378
	11th to 20th	Day	23% 13% 29% 26% 9%	370
	21st to end	Day	20% 12% 30% 26% 10% 1	370
December	1st to 10th	Day	21% 11% 33% 25% 9%	266
	11th to 20th	Day	25% 14% 36% 21% 4 1	180
	21st to end	Day	25% 12% 31% 23% 8% 1	198
January	1st to 10th	Night	23% 16% 35% 22% 4	760
	11th to 20th	Night	29% 16% 33% 19% 4	760
	21st to end	Night	23% 16% 37% 20% 4 1	836
February	1st to 10th	Night	22% 14% 38% 24% 3	760
	11th to 20th	Night	24% 15% 43% 15% 3	743
	21st to end	Night	24% 14% 34% 21% 6%	628
March	1st to 10th	Night	23% 13% 34% 24% 6% 1	760
	11th to 20th	Night	20% 10% 36% 27% 6% 1	760
	21st to end	Night	23% 9% 33% 24% 9% 2	836
April	1st to 10th	Night	24% 7% 30% 31% 8% 1	760
	11th to 20th	Night	20% 7% 27% 33% 12% 1	751
	21st to end	Night	23% 5% 25% 31% 13% 2	594
May	1st to 10th	Night	24% 5% 20% 34% 14% 3	570
	11th to 20th	Night	22% 5% 23% 37% 12% 1	562
	21st to end	Night	22% 3 13% 33% 25% 4	436
June	1st to 10th	Night	20% 2 16% 35% 24% 2	380
	11th to 20th	Night	21% 3 13% 34% 28% 2	380
	21st to end	Night	17% 3 12% 38% 29% 2	380
July	1st to 10th	Night	16% 4 10% 36% 30% 4	370
	11th to 20th	Night	17% 1 8% 36% 33% 5%	368
	21st to end	Night	15% 3 10% 36% 32% 4	416
August	1st to 10th	Night	11% 3 10% 38% 33% 6%	551
	11th to 20th	Night	12% 2 10% 35% 33% 8%	647
	21st to end	Night	14% 3 8% 32% 34% 9%	836
September	1st to 10th	Night	16% 2 15% 32% 28% 7%	760
	11th to 20th	Night	16% 3 16% 29% 27% 9%	760
	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	20% 8% 28% 26% 14% 3	760
	11th to 20th	Night	23% 13% 32% 23% 8% 1	760
	21st to end	Night	18% 17% 30% 26% 8% 1	760
December	1st to 10th	Night	25% 16% 31% 22% 6% 1	760
	11th to 20th	Night	25% 19% 32% 18% 5% 1	760
	21st to end	Night	24% 15% 37% 20% 4	836

Page generated at 2024-11-03 02:11:10. Version ac07

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