Middlebury Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Middlebury Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	13% 11% 4 10% 6% 6% 5% 4 6% 7% 28%	132217
	Night	12% 11% 5% 11% 7% 6% 5% 4 5% 7% 27%	130573
Transparency	Day	52% 6% 11% 19% 10% 1	135806
	Night	52% 6% 12% 19% 9% 1	135070
Seeing	Day	23% 16% 30% 19% 9% 3	16452
	Night	21% 15% 30% 25% 9% 1	25906

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% 18% 7% 20% 11% 5% 4 3 4 4 11%	4661
	11th to 20th	Day	7% 19% 11% 19% 10% 10% 5% 4 4 3 9%	4965
	21st to end	Day	11% 13% 8% 15% 10% 11% 9% 4 5% 4 10%	5780
February	1st to 10th	Day	13% 16% 6% 16% 8% 6% 6% 4 4 4 16%	1890
	11th to 20th	Day	10% 16% 8% 17% 10% 6% 6% 4 4 5% 16%	1843
	21st to end	Day	14% 18% 5% 13% 9% 5% 3 3 5% 6% 19%	1662
March	1st to 10th	Day	14% 12% 7% 14% 7% 5% 3 3 4 5% 27%	4968
	11th to 20th	Day	9% 13% 6% 10% 6% 5% 5% 4 5% 6% 31%	5090
	21st to end	Day	11% 14% 5% 13% 7% 5% 5% 4 5% 6% 25%	5630
April	1st to 10th	Day	14% 15% 5% 9% 5% 5% 5% 3 5% 6% 28%	2520
	11th to 20th	Day	15% 12% 3 9% 6% 4 4 4 5% 6% 32%	2494
	21st to end	Day	17% 12% 3 9% 6% 4 4 3 5% 5% 32%	2686
May	1st to 10th	Day	23% 9% 3 7% 5% 5% 5% 4 5% 6% 28%	6108
	11th to 20th	Day	14% 11% 4 9% 6% 5% 5% 4 5% 6% 30%	6402
	21st to end	Day	16% 8% 3 7% 5% 5% 5% 4 7% 7% 33%	6949
June	1st to 10th	Day	15% 9% 3 8% 4 4 6% 4 6% 7% 36%	2800
	11th to 20th	Day	13% 6% 2 7% 5% 6% 6% 5% 8% 9% 33%	2800
	21st to end	Day	13% 6% 2 7% 6% 5% 5% 4 7% 8% 38%	2800
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 5% 6% 10% 40%	6360
	11th to 20th	Day	7% 6% 2 8% 6% 6% 7% 5% 8% 10% 36%	6360
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 8% 42%	6614
August	1st to 10th	Day	15% 7% 3 7% 4 5% 5% 4 6% 8% 36%	4234
	11th to 20th	Day	13% 6% 3 8% 6% 7% 7% 6% 9% 9% 27%	4174
	21st to end	Day	12% 6% 3 6% 5% 5% 5% 4 7% 9% 38%	4355
September	1st to 10th	Day	14% 7% 2 6% 5% 5% 5% 4 7% 7% 40%	2400
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 8% 42%	2377
	21st to end	Day	16% 7% 3 8% 6% 5% 5% 4 6% 8% 31%	2100
October	1st to 10th	Day	18% 9% 4 5% 4 5% 5% 3 5% 7% 35%	2000
	11th to 20th	Day	17% 12% 3 9% 6% 6% 5% 4 5% 6% 28%	2000
	21st to end	Day	20% 14% 4 10% 6% 5% 5% 3 4 5% 24%	2200
November	1st to 10th	Day	12% 13% 4 10% 6% 4 4 4 5% 6% 31%	1841
	11th to 20th	Day	10% 17% 5% 11% 7% 5% 5% 3 5% 6% 24%	1680
	21st to end	Day	13% 19% 6% 11% 6% 6% 5% 4 5% 6% 18%	1680
December	1st to 10th	Day	15% 21% 8% 15% 8% 5% 5% 4 5% 5% 11%	2680
	11th to 20th	Day	15% 19% 6% 15% 8% 5% 4 3 4 5% 16%	3066
	21st to end	Day	17% 20% 8% 14% 7% 5% 5% 4 4 5% 12%	4048
January	1st to 10th	Night	11% 15% 8% 22% 12% 6% 4 3 4 4 11%	8234
	11th to 20th	Night	7% 16% 9% 20% 11% 9% 6% 4 6% 5% 8%	7905
	21st to end	Night	11% 12% 6% 15% 11% 10% 9% 5% 6% 5% 11%	8401
February	1st to 10th	Night	12% 16% 5% 14% 10% 7% 6% 4 5% 6% 17%	2730
	11th to 20th	Night	10% 11% 6% 16% 9% 6% 8% 4 6% 7% 18%	2663
	21st to end	Night	12% 13% 4 16% 8% 6% 5% 4 6% 6% 20%	2143
March	1st to 10th	Night	13% 10% 4 12% 8% 7% 5% 3 5% 5% 28%	5074
	11th to 20th	Night	10% 11% 6% 9% 6% 4 5% 4 6% 7% 32%	5089
	21st to end	Night	13% 10% 5% 13% 6% 5% 4 3 4 6% 30%	5542
April	1st to 10th	Night	16% 12% 4 8% 6% 6% 5% 4 5% 7% 29%	2100
	11th to 20th	Night	15% 11% 3 8% 5% 4 5% 4 6% 7% 33%	2075
	21st to end	Night	15% 8% 3 7% 6% 4 5% 4 6% 8% 34%	1953
May	1st to 10th	Night	26% 10% 3 7% 4 4 4 3 5% 6% 28%	4050
	11th to 20th	Night	13% 8% 3 9% 5% 5% 5% 4 5% 7% 35%	3623
	21st to end	Night	14% 8% 4 7% 4 5% 4 4 5% 8% 37%	3926
June	1st to 10th	Night	14% 5% 3 7% 5% 4 5% 4 7% 9% 36%	1600
	11th to 20th	Night	10% 6% 2 7% 5% 5% 4 5% 7% 8% 41%	1600
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 8% 9% 42%	1600
July	1st to 10th	Night	7% 6% 3 8% 5% 4 5% 4 6% 11% 41%	3600
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 4 6% 9% 46%	3600
	21st to end	Night	9% 5% 2 5% 4 4 4 4 6% 8% 50%	4235
August	1st to 10th	Night	12% 7% 2 7% 4 4 4 4 7% 8% 41%	2915
	11th to 20th	Night	12% 7% 3 7% 5% 5% 5% 4 6% 9% 38%	2875
	21st to end	Night	8% 4 2 5% 4 5% 6% 5% 7% 9% 45%	3510
September	1st to 10th	Night	13% 5% 2 5% 4 3 4 4 5% 8% 47%	2000
	11th to 20th	Night	10% 6% 2 5% 4 3 4 4 6% 7% 50%	2020
	21st to end	Night	15% 7% 3 6% 4 4 5% 4 7% 7% 37%	2300
October	1st to 10th	Night	16% 7% 3 7% 5% 5% 4 3 5% 7% 38%	2400
	11th to 20th	Night	16% 10% 4 8% 5% 5% 4 3 6% 8% 32%	2400
	21st to end	Night	21% 11% 3 8% 5% 4 5% 4 6% 6% 27%	2640
November	1st to 10th	Night	12% 11% 3 9% 6% 4 4 4 5% 6% 36%	2672
	11th to 20th	Night	13% 12% 5% 10% 6% 5% 6% 4 6% 7% 26%	2940
	21st to end	Night	13% 14% 6% 11% 7% 6% 6% 5% 6% 7% 21%	2940
December	1st to 10th	Night	10% 17% 7% 16% 8% 7% 7% 5% 6% 6% 12%	4720
	11th to 20th	Night	15% 18% 6% 15% 7% 5% 5% 3 5% 4 16%	5348
	21st to end	Night	17% 18% 9% 14% 7% 6% 4 3 5% 5% 12%	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% 7% 10% 8% 3	4662
	11th to 20th	Day	74% 6% 7% 8% 5% 1	4998
	21st to end	Day	73% 7% 9% 9% 2	5780
February	1st to 10th	Day	69% 5% 11% 11% 4	1890
	11th to 20th	Day	67% 6% 7% 15% 5%	1852
	21st to end	Day	66% 4 8% 15% 7% 1	1662
March	1st to 10th	Day	58% 5% 11% 19% 7%	4971
	11th to 20th	Day	51% 5% 17% 18% 8% 1	5090
	21st to end	Day	57% 6% 6% 21% 9% 1	5630
April	1st to 10th	Day	55% 5% 10% 18% 10% 2	2520
	11th to 20th	Day	52% 4 9% 20% 13% 2	2494
	21st to end	Day	53% 4 8% 20% 13% 2	2686
May	1st to 10th	Day	55% 5% 10% 18% 10% 3	6117
	11th to 20th	Day	52% 7% 9% 19% 13% 1	6406
	21st to end	Day	47% 5% 11% 23% 11% 2	6949
June	1st to 10th	Day	46% 5% 11% 20% 16% 1	2800
	11th to 20th	Day	43% 7% 15% 23% 12% 1	2800
	21st to end	Day	41% 6% 13% 24% 15% 1	2800
July	1st to 10th	Day	37% 7% 13% 27% 15% 1	6290
	11th to 20th	Day	37% 9% 15% 23% 15% 1	6285
	21st to end	Day	35% 8% 12% 28% 16%	6425
August	1st to 10th	Day	44% 6% 13% 22% 15% 1	4221
	11th to 20th	Day	46% 8% 15% 24% 6%	4213
	21st to end	Day	39% 6% 12% 33% 10% 1	4355
September	1st to 10th	Day	42% 5% 12% 28% 12% 1	2400
	11th to 20th	Day	39% 3 13% 29% 15% 1	2377
	21st to end	Day	50% 6% 12% 20% 12% 1	2100
October	1st to 10th	Day	39% 7% 13% 25% 15% 1	3260
	11th to 20th	Day	46% 4 15% 19% 14% 1	3240
	21st to end	Day	46% 6% 14% 24% 9% 1	3538
November	1st to 10th	Day	52% 5% 11% 20% 10% 2	1841
	11th to 20th	Day	57% 5% 14% 16% 7% 1	1680
	21st to end	Day	62% 5% 13% 15% 4	1680
December	1st to 10th	Day	69% 8% 11% 10% 2	2680
	11th to 20th	Day	68% 5% 13% 9% 5%	3066
	21st to end	Day	72% 7% 10% 8% 3	4048
January	1st to 10th	Night	70% 8% 9% 10% 3	8234
	11th to 20th	Night	68% 8% 9% 10% 4	7947
	21st to end	Night	66% 7% 9% 13% 4	8401
February	1st to 10th	Night	65% 5% 12% 14% 5%	2730
	11th to 20th	Night	62% 6% 12% 14% 5% 1	2665
	21st to end	Night	62% 4 12% 14% 8% 1	2143
March	1st to 10th	Night	53% 5% 14% 21% 7%	5084
	11th to 20th	Night	48% 6% 16% 20% 7% 3	5090
	21st to end	Night	51% 6% 10% 19% 12% 1	5542
April	1st to 10th	Night	54% 6% 10% 20% 10% 1	2100
	11th to 20th	Night	49% 6% 11% 19% 13% 1	2075
	21st to end	Night	45% 5% 12% 23% 14% 2	1953
May	1st to 10th	Night	57% 5% 9% 17% 10% 3	4057
	11th to 20th	Night	45% 7% 15% 20% 12% 1	3624
	21st to end	Night	42% 7% 14% 21% 14% 2	3927
June	1st to 10th	Night	41% 6% 11% 26% 14% 1	1600
	11th to 20th	Night	37% 6% 14% 25% 16% 2	1600
	21st to end	Night	36% 8% 15% 24% 15% 2	1600
July	1st to 10th	Night	35% 7% 15% 26% 15% 2	3560
	11th to 20th	Night	31% 8% 15% 28% 17% 1	3552
	21st to end	Night	32% 6% 13% 27% 20% 2	4109
August	1st to 10th	Night	38% 6% 15% 28% 12% 1	2904
	11th to 20th	Night	40% 6% 16% 29% 8%	2902
	21st to end	Night	31% 6% 15% 35% 12% 1	3510
September	1st to 10th	Night	34% 5% 15% 31% 14% 2	2000
	11th to 20th	Night	33% 4 14% 32% 16% 1	2020
	21st to end	Night	43% 5% 16% 22% 13% 1	2300
October	1st to 10th	Night	40% 7% 10% 27% 15% 1	3920
	11th to 20th	Night	43% 6% 13% 21% 14% 2	3899
	21st to end	Night	42% 6% 15% 26% 10% 1	4252
November	1st to 10th	Night	47% 6% 10% 24% 10% 2	2672
	11th to 20th	Night	52% 5% 14% 19% 7% 2	2940
	21st to end	Night	59% 5% 16% 15% 4	2940
December	1st to 10th	Night	66% 9% 13% 10% 1	4720
	11th to 20th	Night	66% 6% 10% 13% 5%	5348
	21st to end	Night	73% 6% 8% 10% 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	30% 25% 30% 10% 4 1	220
	11th to 20th	Day	30% 27% 34% 8% 1	278
	21st to end	Day	31% 22% 31% 13% 3	363
February	1st to 10th	Day	34% 26% 29% 9% 2	330
	11th to 20th	Day	35% 27% 33% 4 1	330
	21st to end	Day	38% 24% 27% 11% 1	378
March	1st to 10th	Day	28% 25% 29% 14% 3 1	540
	11th to 20th	Day	32% 24% 30% 11% 4	540
	21st to end	Day	32% 23% 28% 13% 4 1	594
April	1st to 10th	Day	32% 19% 30% 14% 4 1	540
	11th to 20th	Day	32% 22% 25% 14% 6% 2	532
	21st to end	Day	29% 16% 33% 16% 4 1	540
May	1st to 10th	Day	28% 14% 30% 18% 6% 3	543
	11th to 20th	Day	20% 16% 31% 21% 10% 3	551
	21st to end	Day	18% 14% 31% 21% 12% 4	573
June	1st to 10th	Day	18% 14% 36% 23% 8% 1	530
	11th to 20th	Day	16% 12% 31% 25% 12% 3	530
	21st to end	Day	14% 13% 38% 24% 9% 2	530
July	1st to 10th	Day	12% 6% 30% 29% 18% 5%	525
	11th to 20th	Day	12% 6% 30% 28% 17% 6%	524
	21st to end	Day	13% 8% 33% 29% 15% 3	576
August	1st to 10th	Day	10% 6% 27% 33% 17% 8%	526
	11th to 20th	Day	14% 8% 25% 30% 16% 8%	519
	21st to end	Day	13% 8% 28% 27% 15% 9%	583
September	1st to 10th	Day	17% 11% 22% 25% 16% 9%	530
	11th to 20th	Day	11% 8% 23% 25% 22% 12%	530
	21st to end	Day	19% 14% 27% 22% 13% 5%	530
October	1st to 10th	Day	24% 9% 27% 20% 16% 5%	530
	11th to 20th	Day	25% 16% 32% 19% 6% 1	530
	21st to end	Day	31% 16% 32% 17% 4	583
November	1st to 10th	Day	24% 20% 30% 17% 8% 1	402
	11th to 20th	Day	24% 29% 31% 13% 3	220
	21st to end	Day	30% 25% 34% 9% 2	220
December	1st to 10th	Day	37% 17% 34% 9% 3	220
	11th to 20th	Day	35% 27% 25% 11% 2	220
	21st to end	Day	36% 21% 31% 10% 2 1	242
January	1st to 10th	Night	23% 26% 34% 14% 3	840
	11th to 20th	Night	26% 23% 31% 17% 3	840
	21st to end	Night	19% 23% 35% 19% 3	924
February	1st to 10th	Night	26% 23% 32% 17% 2	840
	11th to 20th	Night	24% 24% 34% 16% 2	823
	21st to end	Night	26% 23% 31% 17% 2	692
March	1st to 10th	Night	24% 21% 33% 19% 4	840
	11th to 20th	Night	28% 20% 30% 19% 3 1	840
	21st to end	Night	22% 17% 35% 19% 6% 1	924
April	1st to 10th	Night	25% 16% 34% 21% 3	840
	11th to 20th	Night	25% 14% 32% 22% 6% 1	831
	21st to end	Night	19% 11% 30% 31% 8%	693
May	1st to 10th	Night	22% 10% 24% 32% 10% 1	589
	11th to 20th	Night	15% 9% 31% 37% 7% 1	406
	21st to end	Night	12% 8% 32% 31% 17%	433
June	1st to 10th	Night	14% 6% 36% 30% 14% 1	400
	11th to 20th	Night	13% 7% 30% 37% 13%	400
	21st to end	Night	10% 7% 35% 39% 10%	400
July	1st to 10th	Night	7% 6% 24% 44% 18% 2	390
	11th to 20th	Night	10% 3 30% 39% 18% 2	388
	21st to end	Night	10% 4 23% 43% 18% 2	568
August	1st to 10th	Night	9% 3 24% 38% 23% 3	597
	11th to 20th	Night	12% 5% 19% 35% 25% 4	594
	21st to end	Night	12% 4 23% 38% 22% 2	830
September	1st to 10th	Night	14% 7% 24% 32% 20% 3	800
	11th to 20th	Night	13% 5% 19% 35% 24% 4	800
	21st to end	Night	19% 8% 23% 30% 16% 4	800
October	1st to 10th	Night	20% 9% 27% 25% 16% 2	800
	11th to 20th	Night	22% 15% 31% 25% 6% 1	800
	21st to end	Night	29% 13% 31% 24% 3	880
November	1st to 10th	Night	22% 14% 32% 24% 7% 1	820
	11th to 20th	Night	22% 19% 33% 17% 7% 1	840
	21st to end	Night	26% 19% 30% 19% 5%	840
December	1st to 10th	Night	28% 20% 31% 17% 4	840
	11th to 20th	Night	28% 21% 31% 18% 3	840
	21st to end	Night	30% 21% 33% 15% 2	924

Page generated at 2026-05-14 01:58:54. Version ac07

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