Green Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Green Mountain Observatory clear sky chart from 2005-11-06 to 2026-06-16.

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% 11% 4 9% 5% 5% 5% 4 5% 6% 28%	137627
	Night	18% 14% 5% 11% 6% 5% 4 3 4 5% 25%	134430
Transparency	Day	54% 6% 12% 19% 8% 1	141110
	Night	60% 5% 11% 16% 8% 1	139036
Seeing	Day	27% 18% 29% 17% 7% 2	16710
	Night	26% 15% 27% 23% 8% 1	25711

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% 16% 7% 19% 9% 6% 4 3 4 4 15%	4661
	11th to 20th	Day	10% 15% 6% 20% 10% 6% 5% 3 4 4 17%	4650
	21st to end	Day	10% 15% 7% 13% 9% 8% 6% 3 4 5% 20%	5124
February	1st to 10th	Day	13% 14% 5% 15% 8% 6% 4 4 6% 5% 21%	1848
	11th to 20th	Day	11% 17% 6% 15% 8% 5% 4 3 5% 5% 22%	2017
	21st to end	Day	14% 17% 5% 12% 8% 5% 5% 2 4 4 24%	1730
March	1st to 10th	Day	21% 16% 6% 13% 5% 4 4 3 3 4 21%	4576
	11th to 20th	Day	13% 17% 5% 10% 5% 5% 4 3 5% 5% 27%	4770
	21st to end	Day	21% 13% 5% 10% 5% 3 4 3 4 4 27%	6102
April	1st to 10th	Day	21% 15% 4 8% 5% 4 3 3 4 6% 28%	2520
	11th to 20th	Day	19% 13% 4 8% 4 4 4 2 4 5% 32%	2494
	21st to end	Day	24% 12% 3 7% 4 3 4 2 4 5% 31%	2693
May	1st to 10th	Day	27% 13% 4 8% 5% 5% 4 3 5% 5% 21%	7850
	11th to 20th	Day	19% 9% 3 8% 5% 5% 5% 4 5% 7% 29%	8221
	21st to end	Day	20% 10% 3 7% 4 4 5% 4 6% 7% 30%	9060
June	1st to 10th	Day	24% 7% 3 6% 5% 5% 5% 4 6% 8% 28%	2940
	11th to 20th	Day	18% 6% 2 6% 5% 5% 5% 5% 6% 9% 34%	2870
	21st to end	Day	20% 8% 3 7% 6% 4 5% 4 7% 7% 29%	2800
July	1st to 10th	Day	13% 7% 2 7% 6% 6% 6% 5% 8% 9% 32%	6360
	11th to 20th	Day	12% 6% 2 6% 5% 5% 6% 5% 7% 10% 35%	6360
	21st to end	Day	14% 6% 2 6% 4 4 5% 4 7% 8% 39%	6926
August	1st to 10th	Day	17% 8% 3 7% 5% 4 5% 4 6% 8% 33%	4563
	11th to 20th	Day	16% 8% 3 8% 5% 4 5% 4 5% 8% 33%	4022
	21st to end	Day	14% 7% 2 5% 4 4 4 4 6% 8% 41%	4312
September	1st to 10th	Day	17% 6% 1 5% 4 3 3 3 5% 7% 46%	2400
	11th to 20th	Day	15% 7% 2 4 3 3 4 2 5% 7% 48%	2284
	21st to end	Day	16% 8% 2 6% 4 4 4 5% 6% 7% 38%	2200
October	1st to 10th	Day	23% 10% 3 6% 4 4 4 3 4 5% 34%	2120
	11th to 20th	Day	18% 13% 3 7% 5% 4 4 3 5% 6% 32%	1945
	21st to end	Day	25% 14% 4 8% 5% 4 4 3 4 4 25%	1980
November	1st to 10th	Day	18% 15% 4 8% 5% 5% 4 3 4 5% 30%	1851
	11th to 20th	Day	18% 17% 5% 9% 5% 4 4 3 4 5% 25%	1890
	21st to end	Day	19% 16% 6% 11% 5% 4 4 4 5% 5% 21%	1694
December	1st to 10th	Day	14% 18% 7% 20% 8% 7% 4 3 3 4 12%	2680
	11th to 20th	Day	18% 16% 7% 14% 5% 4 4 3 4 4 21%	3066
	21st to end	Day	18% 17% 6% 13% 6% 5% 6% 3 4 5% 18%	4048
January	1st to 10th	Night	12% 18% 10% 20% 8% 5% 4 2 3 3 15%	8234
	11th to 20th	Night	11% 20% 9% 21% 9% 6% 4 3 4 3 11%	8235
	21st to end	Night	11% 13% 5% 15% 12% 10% 6% 4 4 5% 15%	9057
February	1st to 10th	Night	14% 15% 6% 16% 8% 5% 4 3 4 4 20%	2767
	11th to 20th	Night	12% 16% 7% 14% 9% 5% 4 3 4 4 21%	2488
	21st to end	Night	14% 17% 6% 12% 6% 5% 4 3 4 5% 26%	2076
March	1st to 10th	Night	19% 16% 6% 11% 7% 6% 3 2 4 4 22%	5484
	11th to 20th	Night	15% 18% 4 9% 5% 4 4 3 4 5% 28%	5405
	21st to end	Night	20% 18% 5% 12% 5% 4 3 2 3 3 26%	5072
April	1st to 10th	Night	22% 16% 3 8% 4 4 4 3 4 5% 27%	2100
	11th to 20th	Night	25% 14% 3 8% 4 4 3 3 3 5% 29%	2075
	21st to end	Night	22% 12% 3 7% 4 3 4 3 4 4 34%	1925
May	1st to 10th	Night	27% 13% 3 8% 5% 4 4 4 5% 6% 21%	5104
	11th to 20th	Night	21% 11% 3 8% 5% 4 4 4 4 7% 28%	4639
	21st to end	Night	22% 10% 3 6% 4 4 4 3 5% 7% 31%	5114
June	1st to 10th	Night	26% 7% 3 6% 4 4 5% 3 4 6% 32%	1680
	11th to 20th	Night	20% 5% 2 7% 5% 3 4 4 6% 8% 35%	1640
	21st to end	Night	21% 7% 2 7% 5% 4 4 4 7% 6% 32%	1600
July	1st to 10th	Night	14% 6% 2 7% 5% 5% 7% 5% 7% 9% 34%	3600
	11th to 20th	Night	17% 7% 3 6% 5% 5% 5% 4 6% 7% 36%	3600
	21st to end	Night	13% 7% 3 8% 4 4 5% 3 6% 8% 39%	3926
August	1st to 10th	Night	20% 10% 2 7% 5% 4 4 3 5% 6% 35%	2586
	11th to 20th	Night	16% 8% 3 8% 4 4 4 3 5% 7% 37%	2990
	21st to end	Night	15% 6% 2 5% 3 4 4 4 6% 9% 42%	3586
September	1st to 10th	Night	16% 6% 2 6% 4 4 4 3 4 6% 46%	2000
	11th to 20th	Night	15% 6% 3 5% 4 3 4 3 4 7% 48%	2115
	21st to end	Night	18% 7% 2 6% 4 5% 4 3 5% 6% 42%	2200
October	1st to 10th	Night	23% 9% 2 5% 4 5% 4 4 5% 5% 33%	2278
	11th to 20th	Night	22% 12% 2 7% 5% 4 5% 3 5% 5% 31%	2455
	21st to end	Night	27% 14% 3 8% 4 4 4 3 4 5% 25%	2860
November	1st to 10th	Night	22% 15% 4 8% 4 3 3 3 4 5% 29%	2667
	11th to 20th	Night	19% 17% 5% 9% 5% 4 3 2 4 5% 27%	2730
	21st to end	Night	19% 18% 6% 11% 5% 4 4 3 5% 5% 20%	2924
December	1st to 10th	Night	16% 22% 11% 18% 7% 5% 4 2 3 3 10%	4720
	11th to 20th	Night	22% 18% 5% 13% 6% 5% 4 3 3 3 19%	5348
	21st to end	Night	18% 18% 6% 13% 7% 6% 5% 3 3 4 18%	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	71% 5% 10% 11% 2	4662
	11th to 20th	Day	69% 5% 10% 9% 6% 1	4680
	21st to end	Day	65% 5% 10% 11% 9%	5124
February	1st to 10th	Day	62% 8% 9% 15% 5% 2	1848
	11th to 20th	Day	64% 4 11% 16% 6%	2026
	21st to end	Day	63% 5% 10% 11% 9% 2	1730
March	1st to 10th	Day	67% 5% 9% 10% 6% 4	4579
	11th to 20th	Day	57% 6% 13% 14% 9% 2	4770
	21st to end	Day	59% 4 8% 13% 15% 2	6102
April	1st to 10th	Day	58% 4 9% 18% 9% 2	2520
	11th to 20th	Day	54% 4 8% 17% 15% 1	2494
	21st to end	Day	55% 5% 10% 18% 11% 2	2693
May	1st to 10th	Day	63% 6% 8% 15% 8%	7862
	11th to 20th	Day	52% 7% 13% 19% 8% 1	8225
	21st to end	Day	51% 8% 10% 22% 8% 1	9061
June	1st to 10th	Day	52% 8% 12% 18% 10%	2940
	11th to 20th	Day	46% 8% 12% 21% 12% 2	2870
	21st to end	Day	52% 7% 12% 19% 9% 1	2800
July	1st to 10th	Day	45% 7% 16% 23% 9%	6290
	11th to 20th	Day	41% 9% 14% 27% 9%	6285
	21st to end	Day	41% 7% 12% 33% 6%	6728
August	1st to 10th	Day	47% 8% 16% 22% 8%	4549
	11th to 20th	Day	48% 6% 16% 24% 6%	4061
	21st to end	Day	39% 6% 18% 23% 13% 1	4312
September	1st to 10th	Day	37% 6% 13% 30% 12% 1	2400
	11th to 20th	Day	37% 5% 12% 31% 15% 1	2284
	21st to end	Day	42% 8% 16% 21% 11% 2	2200
October	1st to 10th	Day	46% 5% 10% 27% 11%	3458
	11th to 20th	Day	48% 5% 15% 21% 11%	3148
	21st to end	Day	49% 5% 13% 25% 7% 1	3180
November	1st to 10th	Day	56% 7% 12% 17% 8%	1851
	11th to 20th	Day	60% 6% 12% 12% 8% 2	1890
	21st to end	Day	63% 6% 15% 13% 3	1694
December	1st to 10th	Day	75% 7% 8% 8% 2	2680
	11th to 20th	Day	66% 4 9% 14% 5%	3066
	21st to end	Day	67% 7% 9% 10% 7% 1	4048
January	1st to 10th	Night	75% 4 11% 7% 3	8234
	11th to 20th	Night	76% 4 9% 6% 4 1	8280
	21st to end	Night	70% 4 9% 8% 8% 1	9057
February	1st to 10th	Night	66% 6% 9% 12% 6% 1	2767
	11th to 20th	Night	65% 4 10% 14% 7%	2490
	21st to end	Night	61% 5% 10% 16% 7% 1	2076
March	1st to 10th	Night	67% 3 8% 12% 9% 1	5494
	11th to 20th	Night	58% 6% 13% 14% 8% 1	5406
	21st to end	Night	63% 3 6% 14% 13% 2	5072
April	1st to 10th	Night	60% 4 8% 16% 11% 1	2100
	11th to 20th	Night	58% 6% 7% 16% 12%	2075
	21st to end	Night	53% 5% 10% 17% 13% 3	1925
May	1st to 10th	Night	62% 7% 10% 14% 7% 1	5110
	11th to 20th	Night	54% 7% 10% 20% 8% 1	4640
	21st to end	Night	52% 6% 9% 22% 11% 1	5115
June	1st to 10th	Night	52% 7% 10% 19% 11% 1	1680
	11th to 20th	Night	45% 7% 14% 21% 10% 2	1640
	21st to end	Night	48% 9% 12% 20% 9% 1	1600
July	1st to 10th	Night	43% 7% 18% 22% 9%	3560
	11th to 20th	Night	45% 7% 13% 24% 10%	3552
	21st to end	Night	42% 8% 13% 29% 8%	3809
August	1st to 10th	Night	48% 7% 14% 22% 8%	2576
	11th to 20th	Night	47% 6% 15% 21% 9% 1	3017
	21st to end	Night	39% 7% 16% 25% 12% 1	3586
September	1st to 10th	Night	39% 6% 14% 28% 12% 1	2000
	11th to 20th	Night	37% 5% 14% 31% 13%	2115
	21st to end	Night	42% 7% 12% 23% 14% 2	2200
October	1st to 10th	Night	54% 5% 10% 21% 9%	3720
	11th to 20th	Night	52% 5% 10% 21% 12%	3991
	21st to end	Night	54% 4 12% 22% 7%	4610
November	1st to 10th	Night	57% 6% 11% 17% 8% 1	2667
	11th to 20th	Night	59% 5% 11% 15% 8% 2	2730
	21st to end	Night	65% 6% 11% 13% 4	2924
December	1st to 10th	Night	80% 5% 7% 7% 1	4720
	11th to 20th	Night	70% 5% 8% 11% 7%	5348
	21st to end	Night	70% 5% 8% 10% 5% 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	26% 27% 30% 14% 3	220
	11th to 20th	Day	35% 25% 28% 11% 1 1	220
	21st to end	Day	29% 26% 32% 9% 4	242
February	1st to 10th	Day	31% 32% 27% 8% 2	308
	11th to 20th	Day	32% 26% 31% 9% 2	505
	21st to end	Day	31% 24% 35% 9% 2	446
March	1st to 10th	Day	34% 23% 26% 15% 2	540
	11th to 20th	Day	35% 24% 26% 14% 2	540
	21st to end	Day	37% 22% 25% 10% 4 1	594
April	1st to 10th	Day	39% 20% 24% 12% 4 1	540
	11th to 20th	Day	38% 20% 23% 12% 5% 2	532
	21st to end	Day	33% 22% 26% 12% 4 2	540
May	1st to 10th	Day	36% 19% 25% 13% 6% 2	543
	11th to 20th	Day	24% 17% 33% 19% 5% 1	567
	21st to end	Day	23% 18% 33% 19% 6% 1	617
June	1st to 10th	Day	29% 15% 32% 18% 6% 1	570
	11th to 20th	Day	22% 16% 35% 19% 6% 2	550
	21st to end	Day	24% 17% 34% 18% 6% 1	530
July	1st to 10th	Day	17% 11% 36% 24% 10% 2	525
	11th to 20th	Day	14% 13% 36% 26% 10% 1	524
	21st to end	Day	17% 9% 38% 28% 7% 1	576
August	1st to 10th	Day	15% 11% 29% 28% 15% 2	526
	11th to 20th	Day	19% 8% 31% 25% 14% 5%	519
	21st to end	Day	16% 15% 29% 25% 12% 4	583
September	1st to 10th	Day	17% 15% 31% 21% 12% 5%	530
	11th to 20th	Day	15% 13% 23% 23% 19% 7%	530
	21st to end	Day	20% 13% 30% 21% 12% 4	530
October	1st to 10th	Day	26% 15% 26% 16% 13% 3	530
	11th to 20th	Day	28% 19% 27% 19% 7% 1	500
	21st to end	Day	38% 16% 26% 17% 3	462
November	1st to 10th	Day	33% 20% 25% 16% 4	425
	11th to 20th	Day	29% 25% 25% 15% 4 1	430
	21st to end	Day	39% 19% 29% 12% 1	234
December	1st to 10th	Day	32% 21% 30% 14% 3	220
	11th to 20th	Day	34% 28% 29% 8%	220
	21st to end	Day	32% 24% 30% 11% 3	242
January	1st to 10th	Night	26% 25% 33% 15% 2	840
	11th to 20th	Night	29% 22% 29% 16% 3	840
	21st to end	Night	21% 20% 34% 20% 5%	924
February	1st to 10th	Night	24% 17% 36% 20% 2	840
	11th to 20th	Night	23% 22% 36% 17% 2	823
	21st to end	Night	28% 19% 30% 19% 4	692
March	1st to 10th	Night	29% 18% 29% 22% 2	840
	11th to 20th	Night	28% 23% 25% 19% 4	840
	21st to end	Night	32% 17% 27% 18% 6% 1	924
April	1st to 10th	Night	31% 18% 27% 21% 4	840
	11th to 20th	Night	34% 18% 23% 17% 7% 1	831
	21st to end	Night	30% 16% 26% 22% 6%	665
May	1st to 10th	Night	31% 14% 24% 23% 7%	578
	11th to 20th	Night	21% 11% 35% 25% 8% 1	414
	21st to end	Night	22% 9% 30% 30% 8% 1	455
June	1st to 10th	Night	26% 9% 28% 28% 9%	420
	11th to 20th	Night	22% 7% 27% 33% 10%	410
	21st to end	Night	23% 7% 24% 37% 9% 1	400
July	1st to 10th	Night	15% 5% 27% 41% 12% 1	390
	11th to 20th	Night	15% 4 25% 38% 17% 1	388
	21st to end	Night	15% 6% 23% 45% 11%	438
August	1st to 10th	Night	17% 4 18% 42% 18% 1	398
	11th to 20th	Night	16% 5% 22% 36% 19% 2	657
	21st to end	Night	16% 7% 25% 32% 19% 1	880
September	1st to 10th	Night	18% 9% 25% 33% 15% 2	800
	11th to 20th	Night	18% 7% 20% 31% 21% 3	800
	21st to end	Night	20% 10% 26% 27% 16% 2	800
October	1st to 10th	Night	28% 10% 24% 24% 15% 1	800
	11th to 20th	Night	29% 12% 24% 25% 9% 1	800
	21st to end	Night	34% 14% 28% 19% 5%	880
November	1st to 10th	Night	30% 17% 29% 16% 7%	820
	11th to 20th	Night	28% 19% 28% 17% 8% 1	840
	21st to end	Night	30% 22% 27% 15% 5% 1	840
December	1st to 10th	Night	31% 24% 26% 16% 3	840
	11th to 20th	Night	29% 23% 28% 18% 3	840
	21st to end	Night	33% 21% 28% 15% 3	924

Page generated at 2026-06-13 02:31:42. Version ac07

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