Plum Island
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Plum Island clear sky chart from 2005-11-06 to 2026-04-17.

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% 7% 2 6% 4 4 4 4 5% 7% 40%	131073
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	131099
Transparency	Day	43% 5% 11% 22% 15% 3	134593
	Night	44% 5% 11% 21% 16% 3	135666
Seeing	Day	20% 10% 29% 27% 11% 2	16826
	Night	21% 10% 31% 27% 10% 1	25797

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% 7% 3 7% 5% 5% 5% 4 6% 6% 40%	4661
	11th to 20th	Day	16% 9% 3 6% 3 3 3 3 4 5% 46%	4650
	21st to end	Day	14% 8% 2 6% 3 3 4 3 4 5% 48%	5124
February	1st to 10th	Day	17% 10% 2 5% 5% 4 5% 4 5% 7% 37%	1934
	11th to 20th	Day	14% 7% 2 7% 4 4 4 4 6% 7% 40%	2048
	21st to end	Day	16% 10% 3 5% 4 4 5% 4 5% 5% 38%	1730
March	1st to 10th	Day	21% 10% 2 6% 4 4 4 4 5% 6% 34%	4576
	11th to 20th	Day	16% 9% 3 7% 4 4 4 4 5% 7% 37%	4938
	21st to end	Day	28% 8% 2 5% 4 4 4 3 6% 6% 31%	5817
April	1st to 10th	Day	20% 8% 3 6% 4 5% 3 3 4 6% 38%	2520
	11th to 20th	Day	17% 7% 2 5% 5% 4 5% 4 6% 6% 39%	2495
	21st to end	Day	19% 6% 2 6% 5% 4 5% 3 5% 6% 39%	2600
May	1st to 10th	Day	28% 10% 3 6% 4 4 4 3 5% 7% 26%	5886
	11th to 20th	Day	18% 9% 3 9% 5% 5% 4 4 5% 6% 32%	5923
	21st to end	Day	19% 7% 3 6% 4 4 4 3 5% 7% 37%	6949
June	1st to 10th	Day	20% 7% 2 5% 4 4 4 4 6% 8% 35%	2800
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 6% 8% 40%	2800
	21st to end	Day	17% 6% 2 6% 5% 5% 4 3 6% 8% 37%	2800
July	1st to 10th	Day	15% 8% 3 6% 4 5% 5% 5% 7% 9% 33%	6360
	11th to 20th	Day	7% 6% 2 7% 5% 5% 5% 5% 7% 11% 40%	6360
	21st to end	Day	13% 6% 2 5% 4 4 4 3 6% 8% 45%	6926
August	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 5% 6% 8% 37%	4250
	11th to 20th	Day	14% 8% 3 7% 4 4 5% 4 5% 8% 39%	4175
	21st to end	Day	7% 3 1 4 4 5% 5% 5% 8% 10% 47%	4346
September	1st to 10th	Day	12% 5% 2 4 4 4 4 3 5% 7% 48%	2375
	11th to 20th	Day	14% 5% 2 4 3 3 2 3 3 6% 54%	2200
	21st to end	Day	17% 6% 2 5% 3 4 4 4 5% 7% 44%	2200
October	1st to 10th	Day	21% 5% 2 5% 3 3 4 4 4 6% 43%	2200
	11th to 20th	Day	17% 4 2 5% 4 4 4 3 6% 7% 46%	1948
	21st to end	Day	20% 6% 2 5% 4 4 4 3 5% 6% 41%	1980
November	1st to 10th	Day	15% 7% 2 6% 4 3 4 3 4 4 49%	1851
	11th to 20th	Day	17% 6% 2 6% 3 3 3 3 4 6% 47%	1890
	21st to end	Day	19% 7% 2 5% 3 3 3 3 5% 6% 43%	1890
December	1st to 10th	Day	15% 8% 3 5% 3 3 4 3 4 6% 48%	2757
	11th to 20th	Day	20% 7% 2 4 3 3 3 3 4 6% 47%	3066
	21st to end	Day	21% 10% 3 6% 4 3 4 4 5% 5% 37%	4048
January	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 5% 7% 46%	8234
	11th to 20th	Night	16% 10% 3 8% 4 4 4 3 5% 6% 37%	8235
	21st to end	Night	15% 8% 3 6% 4 4 5% 3 4 6% 41%	9057
February	1st to 10th	Night	16% 8% 3 6% 5% 4 4 4 5% 6% 38%	2683
	11th to 20th	Night	14% 9% 2 6% 4 5% 4 4 5% 6% 41%	2458
	21st to end	Night	16% 9% 2 5% 3 4 3 4 5% 6% 42%	2076
March	1st to 10th	Night	25% 8% 2 6% 3 4 3 3 4 5% 36%	5484
	11th to 20th	Night	14% 10% 4 8% 5% 4 5% 3 4 5% 37%	5241
	21st to end	Night	27% 8% 3 5% 4 3 4 4 6% 4 32%	5354
April	1st to 10th	Night	20% 9% 2 6% 4 4 3 4 5% 6% 36%	2100
	11th to 20th	Night	20% 9% 2 6% 4 4 4 3 4 6% 40%	1987
	21st to end	Night	19% 8% 2 8% 4 5% 4 2 6% 5% 37%	1800
May	1st to 10th	Night	31% 11% 3 7% 3 4 4 3 4 6% 25%	4054
	11th to 20th	Night	17% 10% 3 8% 4 5% 7% 4 6% 8% 28%	3999
	21st to end	Night	21% 8% 3 9% 5% 4 4 4 5% 5% 32%	3926
June	1st to 10th	Night	19% 9% 2 6% 6% 4 3 4 4 6% 37%	1600
	11th to 20th	Night	19% 6% 2 5% 4 4 4 5% 7% 9% 36%	1600
	21st to end	Night	17% 7% 3 6% 4 3 5% 4 6% 7% 37%	1600
July	1st to 10th	Night	12% 7% 3 7% 5% 5% 5% 4 6% 10% 36%	3600
	11th to 20th	Night	9% 8% 3 7% 5% 5% 6% 4 6% 10% 39%	3600
	21st to end	Night	12% 6% 3 6% 4 4 4 3 6% 7% 45%	3926
August	1st to 10th	Night	18% 7% 3 8% 4 5% 5% 4 7% 8% 33%	2868
	11th to 20th	Night	12% 8% 3 8% 6% 5% 5% 5% 6% 8% 34%	2872
	21st to end	Night	9% 5% 2 4 4 3 5% 4 6% 9% 50%	3548
September	1st to 10th	Night	13% 5% 2 6% 3 3 4 3 7% 7% 48%	2025
	11th to 20th	Night	14% 5% 2 4 3 3 3 2 5% 7% 52%	2200
	21st to end	Night	16% 6% 2 6% 4 4 4 3 5% 7% 44%	2200
October	1st to 10th	Night	20% 5% 2 7% 4 4 3 3 5% 6% 42%	2200
	11th to 20th	Night	16% 5% 2 5% 4 4 4 4 6% 6% 46%	2446
	21st to end	Night	21% 7% 2 5% 3 4 4 3 5% 5% 39%	2860
November	1st to 10th	Night	14% 5% 2 5% 4 4 4 3 5% 6% 48%	2667
	11th to 20th	Night	16% 6% 2 5% 4 4 4 3 5% 5% 47%	2730
	21st to end	Night	17% 7% 2 4 4 4 3 3 5% 7% 44%	2730
December	1st to 10th	Night	17% 8% 2 5% 4 4 4 3 4 5% 42%	4641
	11th to 20th	Night	21% 8% 2 6% 3 3 3 3 5% 5% 41%	5348
	21st to end	Night	18% 9% 3 7% 4 3 4 3 4 6% 40%	7150

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	44% 3 10% 26% 16% 1	4662
	11th to 20th	Day	41% 3 10% 21% 20% 4	4680
	21st to end	Day	39% 3 9% 24% 22% 3	5124
February	1st to 10th	Day	46% 4 12% 20% 15% 2	1934
	11th to 20th	Day	44% 5% 11% 22% 15% 3	2057
	21st to end	Day	47% 5% 10% 20% 14% 3	1730
March	1st to 10th	Day	51% 4 9% 14% 12% 10%	4579
	11th to 20th	Day	44% 5% 14% 23% 12% 2	4938
	21st to end	Day	54% 3 7% 15% 14% 8%	5817
April	1st to 10th	Day	48% 4 9% 17% 17% 4	2520
	11th to 20th	Day	43% 6% 11% 20% 16% 4	2495
	21st to end	Day	46% 5% 8% 19% 18% 3	2600
May	1st to 10th	Day	58% 8% 11% 14% 7% 1	5897
	11th to 20th	Day	51% 7% 13% 17% 10% 2	5927
	21st to end	Day	46% 7% 12% 21% 13% 1	6949
June	1st to 10th	Day	47% 6% 11% 22% 12% 1	2800
	11th to 20th	Day	41% 7% 13% 20% 16% 3	2800
	21st to end	Day	44% 8% 12% 23% 13% 1	2800
July	1st to 10th	Day	46% 7% 12% 20% 14% 1	6290
	11th to 20th	Day	37% 7% 15% 27% 13% 1	6285
	21st to end	Day	38% 5% 12% 29% 15% 1	6728
August	1st to 10th	Day	44% 8% 15% 23% 9% 1	4236
	11th to 20th	Day	43% 7% 13% 26% 10% 1	4214
	21st to end	Day	29% 7% 13% 31% 18% 2	4346
September	1st to 10th	Day	35% 5% 12% 25% 21% 1	2375
	11th to 20th	Day	34% 3 9% 28% 23% 3	2200
	21st to end	Day	40% 5% 9% 23% 21% 2	2200
October	1st to 10th	Day	41% 5% 10% 23% 18% 3	3590
	11th to 20th	Day	32% 4 7% 30% 24% 3	3138
	21st to end	Day	33% 4 10% 30% 20% 3	3180
November	1st to 10th	Day	40% 4 11% 26% 17% 2	1851
	11th to 20th	Day	40% 4 8% 22% 23% 3	1890
	21st to end	Day	43% 4 14% 23% 15% 1	1890
December	1st to 10th	Day	39% 4 13% 29% 14% 1	2757
	11th to 20th	Day	41% 3 13% 17% 20% 6%	3066
	21st to end	Day	50% 4 8% 20% 14% 5%	4048
January	1st to 10th	Night	39% 3 11% 29% 15% 2	8234
	11th to 20th	Night	49% 3 10% 18% 15% 4	8280
	21st to end	Night	46% 3 8% 19% 20% 5%	9057
February	1st to 10th	Night	46% 5% 12% 20% 15% 2	2683
	11th to 20th	Night	45% 4 11% 21% 17% 3	2460
	21st to end	Night	44% 4 8% 26% 14% 4	2076
March	1st to 10th	Night	51% 3 8% 15% 15% 8%	5494
	11th to 20th	Night	48% 6% 13% 17% 13% 2	5242
	21st to end	Night	54% 3 6% 12% 20% 6%	5354
April	1st to 10th	Night	49% 5% 9% 17% 16% 4	2100
	11th to 20th	Night	47% 4 8% 20% 18% 3	1987
	21st to end	Night	48% 5% 9% 16% 18% 3	1800
May	1st to 10th	Night	60% 6% 11% 13% 8% 2	4060
	11th to 20th	Night	52% 8% 14% 15% 9% 2	4000
	21st to end	Night	53% 6% 12% 17% 11% 1	3927
June	1st to 10th	Night	49% 5% 11% 22% 12% 1	1600
	11th to 20th	Night	43% 7% 13% 21% 15% 2	1600
	21st to end	Night	43% 9% 12% 21% 14% 1	1600
July	1st to 10th	Night	43% 6% 12% 22% 17%	3560
	11th to 20th	Night	40% 7% 18% 20% 15% 1	3552
	21st to end	Night	39% 5% 11% 30% 14% 1	3809
August	1st to 10th	Night	49% 6% 12% 20% 12% 1	2858
	11th to 20th	Night	45% 9% 14% 22% 11% 1	2899
	21st to end	Night	30% 6% 15% 26% 23% 1	3548
September	1st to 10th	Night	36% 5% 11% 26% 20% 1	2025
	11th to 20th	Night	35% 4 9% 25% 26% 2	2200
	21st to end	Night	40% 5% 8% 25% 19% 2	2200
October	1st to 10th	Night	41% 4 14% 22% 16% 2	3590
	11th to 20th	Night	29% 3 10% 29% 27% 3	3995
	21st to end	Night	35% 4 10% 30% 19% 2	4610
November	1st to 10th	Night	38% 4 11% 24% 19% 4	2667
	11th to 20th	Night	40% 4 10% 22% 21% 3	2730
	21st to end	Night	41% 4 14% 25% 14% 2	2730
December	1st to 10th	Night	45% 4 10% 29% 12% 1	4641
	11th to 20th	Night	46% 4 13% 15% 21% 1	5348
	21st to end	Night	46% 4 8% 21% 17% 2	7150

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	25% 16% 30% 22% 6%	220
	11th to 20th	Day	24% 22% 30% 18% 6%	220
	21st to end	Day	24% 18% 32% 21% 5%	242
February	1st to 10th	Day	26% 17% 36% 20% 2	435
	11th to 20th	Day	24% 18% 40% 16% 2	536
	21st to end	Day	21% 18% 33% 23% 5%	446
March	1st to 10th	Day	24% 16% 32% 23% 5%	540
	11th to 20th	Day	20% 20% 33% 23% 3	540
	21st to end	Day	27% 17% 33% 17% 5% 2	594
April	1st to 10th	Day	29% 14% 32% 17% 7% 1	540
	11th to 20th	Day	23% 14% 37% 20% 7% 1	516
	21st to end	Day	24% 9% 33% 28% 6% 1	500
May	1st to 10th	Day	28% 8% 31% 23% 8% 1	503
	11th to 20th	Day	17% 11% 30% 30% 9% 2	527
	21st to end	Day	19% 4 26% 36% 12% 2	573
June	1st to 10th	Day	22% 3 26% 32% 15% 2	530
	11th to 20th	Day	15% 7% 30% 35% 11% 2	530
	21st to end	Day	18% 6% 28% 32% 13% 3	530
July	1st to 10th	Day	18% 4 18% 37% 19% 4	525
	11th to 20th	Day	13% 4 20% 35% 22% 4	524
	21st to end	Day	12% 3 20% 46% 17% 2	576
August	1st to 10th	Day	11% 3 22% 37% 23% 4	526
	11th to 20th	Day	13% 4 21% 35% 22% 5%	519
	21st to end	Day	13% 4 22% 37% 19% 5%	583
September	1st to 10th	Day	17% 4 25% 29% 18% 6%	530
	11th to 20th	Day	18% 6% 24% 28% 18% 8%	530
	21st to end	Day	19% 3 29% 30% 14% 5%	530
October	1st to 10th	Day	24% 8% 22% 25% 17% 4	530
	11th to 20th	Day	20% 9% 25% 31% 12% 2	451
	21st to end	Day	26% 9% 31% 22% 11% 1	462
November	1st to 10th	Day	17% 13% 32% 26% 10% 3	425
	11th to 20th	Day	24% 16% 32% 21% 6% 1	430
	21st to end	Day	26% 17% 27% 21% 8% 1	430
December	1st to 10th	Day	23% 9% 39% 21% 7%	271
	11th to 20th	Day	27% 17% 36% 17% 3	220
	21st to end	Day	24% 16% 31% 22% 5% 2	242
January	1st to 10th	Night	22% 18% 40% 17% 3	840
	11th to 20th	Night	26% 17% 39% 16% 3	840
	21st to end	Night	20% 17% 40% 19% 4	924
February	1st to 10th	Night	23% 15% 39% 21% 2	840
	11th to 20th	Night	21% 15% 46% 15% 3	823
	21st to end	Night	22% 15% 39% 21% 4	692
March	1st to 10th	Night	25% 15% 36% 22% 3	840
	11th to 20th	Night	21% 13% 38% 22% 5%	840
	21st to end	Night	25% 12% 35% 22% 6%	924
April	1st to 10th	Night	26% 12% 31% 25% 5%	840
	11th to 20th	Night	25% 8% 35% 24% 8%	767
	21st to end	Night	24% 8% 33% 30% 6% 1	600
May	1st to 10th	Night	28% 6% 32% 26% 8% 1	600
	11th to 20th	Night	21% 6% 30% 33% 10% 1	584
	21st to end	Night	22% 4 23% 35% 15% 1	433
June	1st to 10th	Night	23% 4 23% 36% 12% 2	400
	11th to 20th	Night	20% 5% 23% 40% 12% 1	400
	21st to end	Night	20% 4 21% 39% 15% 1	400
July	1st to 10th	Night	17% 4 17% 44% 16% 2	390
	11th to 20th	Night	15% 3 15% 46% 20% 2	388
	21st to end	Night	15% 2 17% 47% 18%	438
August	1st to 10th	Night	12% 2 16% 46% 22% 1	563
	11th to 20th	Night	15% 3 18% 39% 23% 2	592
	21st to end	Night	15% 2 20% 36% 24% 3	855
September	1st to 10th	Night	15% 2 25% 35% 21% 3	800
	11th to 20th	Night	16% 4 23% 33% 19% 6%	800
	21st to end	Night	19% 3 24% 31% 19% 3	800
October	1st to 10th	Night	22% 7% 24% 29% 15% 3	800
	11th to 20th	Night	19% 9% 29% 30% 12% 2	800
	21st to end	Night	24% 11% 31% 25% 9%	880
November	1st to 10th	Night	17% 9% 35% 29% 9% 1	820
	11th to 20th	Night	21% 14% 37% 21% 7% 1	840
	21st to end	Night	21% 18% 34% 20% 6% 1	840
December	1st to 10th	Night	23% 13% 40% 18% 5%	840
	11th to 20th	Night	23% 17% 36% 21% 4	840
	21st to end	Night	26% 15% 38% 19% 2	924

Page generated at 2026-04-14 02:44:23. Version ac07

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