New England Sci-Tech Astronomy
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the New England Sci-Tech Astronomy clear sky chart from 2005-11-06 to 2026-01-02.

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% 39%	126979
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 40%	124395
Transparency	Day	44% 5% 11% 22% 16% 3	130512
	Night	44% 4 11% 22% 16% 3	128949
Seeing	Day	25% 14% 33% 19% 8% 2	16488
	Night	23% 12% 33% 25% 7% 1	25440

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	11% 7% 2 8% 4 4 4 4 5% 6% 43%	3605
	11th to 20th	Day	13% 7% 3 5% 3 3 4 4 4 5% 48%	3570
	21st to end	Day	17% 8% 2 5% 3 4 4 3 6% 5% 43%	3960
February	1st to 10th	Day	19% 7% 3 6% 5% 5% 5% 4 6% 5% 37%	1870
	11th to 20th	Day	13% 9% 2 6% 6% 4 5% 4 5% 7% 39%	1948
	21st to end	Day	18% 10% 3 6% 4 5% 4 3 5% 5% 38%	1650
March	1st to 10th	Day	21% 12% 3 6% 3 4 3 3 4 6% 35%	4476
	11th to 20th	Day	16% 8% 2 6% 4 5% 5% 4 6% 7% 39%	4782
	21st to end	Day	25% 9% 2 6% 4 4 4 4 6% 6% 30%	5776
April	1st to 10th	Day	20% 9% 3 7% 4 4 4 3 4 6% 37%	2400
	11th to 20th	Day	19% 7% 2 5% 4 4 5% 3 5% 7% 38%	2386
	21st to end	Day	20% 6% 3 7% 5% 5% 4 3 5% 6% 37%	2600
May	1st to 10th	Day	30% 10% 3 7% 5% 4 4 3 5% 7% 24%	5886
	11th to 20th	Day	20% 9% 3 8% 5% 5% 5% 3 5% 6% 32%	5948
	21st to end	Day	20% 7% 3 6% 4 4 5% 4 5% 8% 35%	6949
June	1st to 10th	Day	19% 6% 2 6% 5% 4 4 4 6% 9% 34%	2800
	11th to 20th	Day	16% 6% 2 6% 4 5% 5% 4 7% 7% 37%	2800
	21st to end	Day	16% 6% 2 7% 4 5% 5% 4 6% 8% 36%	2800
July	1st to 10th	Day	17% 6% 2 6% 5% 5% 5% 4 6% 9% 33%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 6% 6% 4 7% 10% 38%	6360
	21st to end	Day	15% 6% 2 6% 4 4 4 3 5% 7% 44%	6926
August	1st to 10th	Day	15% 5% 3 7% 5% 6% 6% 5% 6% 9% 33%	4277
	11th to 20th	Day	13% 8% 3 7% 4 5% 5% 4 5% 8% 38%	4115
	21st to end	Day	7% 4 2 6% 4 5% 5% 4 7% 9% 47%	4312
September	1st to 10th	Day	14% 6% 1 5% 4 4 4 4 5% 7% 46%	2395
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 4 6% 51%	2200
	21st to end	Day	18% 6% 2 6% 4 3 4 3 6% 7% 43%	2200
October	1st to 10th	Day	23% 5% 2 5% 3 3 4 3 4 6% 43%	2200
	11th to 20th	Day	16% 5% 2 6% 4 3 3 3 4 7% 46%	1955
	21st to end	Day	20% 7% 2 5% 3 4 4 3 5% 6% 41%	1980
November	1st to 10th	Day	16% 5% 2 5% 3 4 4 2 4 4 50%	1851
	11th to 20th	Day	17% 6% 2 4 3 3 3 4 5% 4 48%	1890
	21st to end	Day	18% 7% 2 5% 4 3 4 3 5% 6% 43%	1890
December	1st to 10th	Day	15% 8% 2 4 3 3 4 3 4 5% 49%	2748
	11th to 20th	Day	20% 7% 2 4 3 3 3 3 4 5% 48%	3066
	21st to end	Day	20% 11% 3 6% 4 3 3 3 4 5% 38%	4048
January	1st to 10th	Night	14% 7% 2 5% 3 3 3 3 5% 6% 49%	6364
	11th to 20th	Night	13% 9% 3 8% 5% 4 4 3 5% 6% 40%	6315
	21st to end	Night	16% 11% 3 7% 5% 5% 4 3 4 5% 38%	6996
February	1st to 10th	Night	17% 10% 2 7% 4 4 4 4 5% 6% 38%	2523
	11th to 20th	Night	15% 9% 3 7% 4 4 4 4 4 7% 39%	2338
	21st to end	Night	18% 9% 2 6% 4 4 3 3 5% 6% 40%	1980
March	1st to 10th	Night	24% 9% 2 5% 3 4 3 3 5% 5% 37%	5364
	11th to 20th	Night	16% 10% 3 7% 4 4 4 4 5% 6% 38%	5176
	21st to end	Night	25% 10% 2 6% 4 3 4 4 5% 4 33%	5152
April	1st to 10th	Night	19% 8% 3 7% 5% 4 4 3 5% 5% 37%	2000
	11th to 20th	Night	19% 8% 2 7% 4 4 4 3 4 6% 39%	1961
	21st to end	Night	20% 8% 3 6% 4 4 4 3 4 5% 40%	1800
May	1st to 10th	Night	31% 10% 3 7% 4 3 3 3 4 6% 26%	4054
	11th to 20th	Night	18% 8% 3 8% 6% 5% 5% 5% 6% 8% 28%	3987
	21st to end	Night	20% 8% 3 8% 5% 4 4 4 5% 7% 33%	3926
June	1st to 10th	Night	16% 8% 3 7% 5% 4 5% 3 5% 7% 37%	1600
	11th to 20th	Night	18% 7% 3 6% 5% 4 4 4 6% 8% 36%	1600
	21st to end	Night	14% 6% 2 6% 4 5% 4 4 6% 9% 40%	1600
July	1st to 10th	Night	13% 7% 3 7% 5% 4 5% 4 7% 9% 37%	3600
	11th to 20th	Night	12% 7% 2 7% 5% 4 4 4 7% 9% 39%	3600
	21st to end	Night	12% 7% 3 6% 4 4 4 4 5% 7% 44%	3926
August	1st to 10th	Night	19% 7% 3 8% 5% 4 5% 5% 6% 7% 33%	2840
	11th to 20th	Night	10% 7% 3 7% 6% 6% 6% 4 6% 8% 37%	2928
	21st to end	Night	9% 4 1 4 4 3 4 3 5% 8% 56%	3586
September	1st to 10th	Night	14% 5% 2 6% 4 3 4 3 6% 6% 48%	2005
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 4 6% 53%	2200
	21st to end	Night	18% 7% 2 6% 4 3 4 4 5% 5% 43%	2200
October	1st to 10th	Night	19% 6% 2 6% 4 3 3 3 4 5% 44%	2200
	11th to 20th	Night	16% 5% 2 5% 3 4 3 3 6% 6% 48%	2440
	21st to end	Night	21% 8% 2 6% 3 4 3 3 4 6% 40%	2860
November	1st to 10th	Night	15% 5% 2 5% 3 4 4 3 5% 5% 49%	2667
	11th to 20th	Night	17% 7% 2 5% 3 3 3 3 3 5% 47%	2730
	21st to end	Night	17% 7% 2 5% 4 4 4 4 4 6% 45%	2730
December	1st to 10th	Night	18% 8% 2 5% 4 4 3 3 4 5% 43%	4649
	11th to 20th	Night	22% 8% 3 5% 4 3 3 3 4 4 42%	5348
	21st to end	Night	17% 10% 3 6% 4 4 3 3 5% 5% 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	41% 3 6% 28% 19% 3	3606
	11th to 20th	Day	39% 4 10% 22% 19% 5%	3600
	21st to end	Day	42% 4 9% 22% 20% 3	3960
February	1st to 10th	Day	48% 3 13% 20% 14% 3	1870
	11th to 20th	Day	45% 4 11% 21% 15% 3	1957
	21st to end	Day	48% 3 9% 21% 16% 3	1650
March	1st to 10th	Day	51% 3 10% 16% 11% 10%	4479
	11th to 20th	Day	43% 6% 12% 27% 11% 2	4782
	21st to end	Day	54% 4 6% 15% 15% 7%	5776
April	1st to 10th	Day	50% 4 8% 19% 17% 4	2400
	11th to 20th	Day	44% 5% 10% 20% 18% 3	2386
	21st to end	Day	48% 4 8% 19% 17% 4	2600
May	1st to 10th	Day	60% 6% 12% 13% 9% 1	5897
	11th to 20th	Day	52% 5% 13% 17% 11% 2	5952
	21st to end	Day	47% 6% 10% 22% 14% 2	6949
June	1st to 10th	Day	46% 6% 12% 24% 12% 1	2800
	11th to 20th	Day	44% 5% 12% 20% 17% 2	2800
	21st to end	Day	45% 6% 12% 21% 16% 1	2800
July	1st to 10th	Day	47% 6% 10% 21% 14% 1	6290
	11th to 20th	Day	40% 6% 16% 24% 12% 1	6285
	21st to end	Day	40% 4 11% 27% 16% 2	6728
August	1st to 10th	Day	46% 7% 15% 22% 9% 1	4263
	11th to 20th	Day	43% 6% 12% 27% 11% 1	4154
	21st to end	Day	32% 5% 11% 30% 20% 1	4312
September	1st to 10th	Day	38% 5% 10% 25% 20% 2	2395
	11th to 20th	Day	35% 3 7% 27% 24% 3	2200
	21st to end	Day	42% 3 8% 23% 22% 3	2200
October	1st to 10th	Day	40% 4 8% 24% 20% 4	3590
	11th to 20th	Day	33% 3 7% 27% 27% 3	3158
	21st to end	Day	33% 3 11% 30% 20% 3	3180
November	1st to 10th	Day	40% 3 11% 25% 19% 2	1851
	11th to 20th	Day	39% 3 8% 23% 23% 3	1890
	21st to end	Day	42% 4 13% 26% 13% 2	1890
December	1st to 10th	Day	39% 3 15% 28% 15% 1	2748
	11th to 20th	Day	40% 2 14% 18% 20% 6%	3066
	21st to end	Day	50% 4 8% 19% 15% 4	4048
January	1st to 10th	Night	36% 3 9% 33% 16% 2	6364
	11th to 20th	Night	46% 4 12% 21% 14% 4	6360
	21st to end	Night	49% 3 9% 16% 18% 5%	6996
February	1st to 10th	Night	46% 4 12% 22% 14% 2	2523
	11th to 20th	Night	46% 4 9% 21% 17% 4	2340
	21st to end	Night	46% 3 7% 23% 16% 4	1980
March	1st to 10th	Night	49% 3 8% 16% 16% 7%	5374
	11th to 20th	Night	46% 5% 16% 18% 13% 2	5177
	21st to end	Night	54% 3 4 12% 20% 6%	5152
April	1st to 10th	Night	49% 4 9% 19% 16% 3	2000
	11th to 20th	Night	47% 4 8% 21% 18% 3	1961
	21st to end	Night	47% 3 10% 18% 18% 4	1800
May	1st to 10th	Night	59% 6% 12% 13% 8% 2	4060
	11th to 20th	Night	52% 7% 14% 15% 9% 2	3988
	21st to end	Night	51% 6% 12% 19% 11% 1	3927
June	1st to 10th	Night	47% 6% 12% 22% 13% 1	1600
	11th to 20th	Night	45% 5% 13% 22% 14% 1	1600
	21st to end	Night	41% 7% 13% 24% 15% 1	1600
July	1st to 10th	Night	43% 5% 13% 22% 16%	3560
	11th to 20th	Night	41% 5% 18% 21% 13% 1	3552
	21st to end	Night	39% 5% 10% 29% 15% 1	3809
August	1st to 10th	Night	50% 6% 13% 19% 11% 1	2830
	11th to 20th	Night	44% 7% 14% 25% 10%	2955
	21st to end	Night	27% 5% 15% 29% 23% 1	3586
September	1st to 10th	Night	38% 4 11% 25% 20% 2	2005
	11th to 20th	Night	34% 4 9% 24% 27% 2	2200
	21st to end	Night	43% 4 8% 23% 20% 2	2200
October	1st to 10th	Night	39% 4 13% 24% 17% 3	3590
	11th to 20th	Night	28% 3 10% 27% 29% 3	3976
	21st to end	Night	35% 2 11% 31% 19% 2	4610
November	1st to 10th	Night	38% 4 10% 24% 20% 4	2667
	11th to 20th	Night	40% 3 9% 23% 21% 4	2730
	21st to end	Night	41% 3 13% 27% 14% 2	2730
December	1st to 10th	Night	44% 3 11% 28% 13% 1	4649
	11th to 20th	Night	46% 3 14% 15% 21% 2	5348
	21st to end	Night	46% 4 8% 23% 16% 3	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	28% 23% 30% 14% 5%	202
	11th to 20th	Day	33% 17% 37% 10% 4	200
	21st to end	Day	28% 20% 36% 11% 5%	220
February	1st to 10th	Day	32% 15% 34% 17% 2	408
	11th to 20th	Day	28% 21% 38% 12% 2	496
	21st to end	Day	29% 15% 34% 16% 5%	414
March	1st to 10th	Day	31% 18% 31% 17% 3	500
	11th to 20th	Day	29% 19% 32% 14% 4 1	500
	21st to end	Day	32% 17% 33% 8% 7% 1	550
April	1st to 10th	Day	35% 20% 24% 15% 5% 1	500
	11th to 20th	Day	32% 23% 29% 11% 4 1	492
	21st to end	Day	28% 18% 33% 16% 3 2	500
May	1st to 10th	Day	35% 18% 28% 14% 4 1	503
	11th to 20th	Day	27% 16% 38% 13% 6% 1	527
	21st to end	Day	20% 13% 36% 22% 7% 2	573
June	1st to 10th	Day	23% 11% 39% 18% 8% 2	530
	11th to 20th	Day	19% 13% 38% 22% 6% 1	530
	21st to end	Day	18% 12% 37% 22% 9% 2	530
July	1st to 10th	Day	20% 8% 33% 26% 11% 2	525
	11th to 20th	Day	15% 6% 33% 31% 12% 3	524
	21st to end	Day	15% 7% 38% 30% 9% 1	576
August	1st to 10th	Day	11% 9% 32% 32% 13% 2	526
	11th to 20th	Day	15% 6% 32% 31% 13% 3	519
	21st to end	Day	13% 9% 33% 27% 14% 5%	583
September	1st to 10th	Day	19% 10% 30% 22% 15% 3	530
	11th to 20th	Day	20% 9% 27% 22% 12% 9%	530
	21st to end	Day	23% 10% 32% 21% 9% 5%	530
October	1st to 10th	Day	27% 11% 25% 21% 11% 5%	530
	11th to 20th	Day	22% 15% 30% 20% 11% 2	469
	21st to end	Day	27% 14% 32% 18% 8% 1	462
November	1st to 10th	Day	25% 14% 34% 17% 8% 2	425
	11th to 20th	Day	28% 17% 32% 18% 4 1	430
	21st to end	Day	31% 19% 27% 17% 6%	430
December	1st to 10th	Day	25% 21% 33% 18% 3	262
	11th to 20th	Day	35% 21% 31% 10% 3	220
	21st to end	Day	31% 16% 33% 14% 4 2	242
January	1st to 10th	Night	24% 17% 39% 18% 3	805
	11th to 20th	Night	29% 16% 34% 18% 3	800
	21st to end	Night	21% 17% 36% 22% 4	880
February	1st to 10th	Night	24% 16% 36% 22% 2	800
	11th to 20th	Night	25% 16% 40% 17% 2	783
	21st to end	Night	25% 17% 34% 20% 3	660
March	1st to 10th	Night	27% 16% 34% 20% 4	800
	11th to 20th	Night	24% 16% 38% 19% 4	800
	21st to end	Night	27% 15% 35% 19% 5%	880
April	1st to 10th	Night	27% 18% 30% 21% 4	800
	11th to 20th	Night	26% 17% 33% 19% 5%	777
	21st to end	Night	25% 17% 35% 20% 3	600
May	1st to 10th	Night	30% 13% 32% 21% 5%	600
	11th to 20th	Night	22% 14% 38% 22% 4	572
	21st to end	Night	21% 7% 35% 31% 5% 1	433
June	1st to 10th	Night	22% 5% 33% 32% 7% 1	400
	11th to 20th	Night	18% 7% 37% 31% 6% 1	400
	21st to end	Night	16% 5% 36% 36% 7%	400
July	1st to 10th	Night	14% 4 32% 39% 11%	390
	11th to 20th	Night	14% 3 32% 38% 13%	388
	21st to end	Night	14% 4 32% 41% 8% 1	438
August	1st to 10th	Night	13% 3 34% 40% 9% 1	548
	11th to 20th	Night	12% 5% 29% 41% 11% 1	622
	21st to end	Night	16% 4 25% 40% 15% 2	880
September	1st to 10th	Night	17% 5% 31% 34% 13% 1	800
	11th to 20th	Night	18% 6% 27% 28% 20% 2	800
	21st to end	Night	21% 7% 26% 32% 12% 2	800
October	1st to 10th	Night	23% 9% 27% 27% 13% 2	800
	11th to 20th	Night	21% 10% 31% 25% 12% 1	800
	21st to end	Night	27% 11% 32% 23% 6%	880
November	1st to 10th	Night	20% 14% 31% 27% 8% 1	820
	11th to 20th	Night	24% 16% 34% 20% 7%	840
	21st to end	Night	23% 18% 32% 21% 5% 1	840
December	1st to 10th	Night	27% 15% 33% 21% 4	840
	11th to 20th	Night	26% 17% 31% 22% 3	840
	21st to end	Night	27% 14% 34% 22% 3	924

Page generated at 2025-12-30 01:14:11. Version ac07

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