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 2024-01-27.

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	14% 11% 5% 8% 7% 4 5% 5% 6% 7% 29%	74807
	Night	13% 10% 5% 9% 7% 4 6% 5% 6% 7% 29%	75235
Transparency	Day	53% 5% 12% 19% 10% 1	74519
	Night	53% 5% 13% 19% 9% 1	75057
Seeing	Day	23% 15% 30% 20% 9% 3	13480
	Night	21% 14% 30% 25% 9% 1	22973

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	9% 16% 9% 15% 12% 3 7% 6% 5% 6% 12%	1517
	11th to 20th	Day	12% 17% 8% 14% 11% 6% 6% 5% 4 4 13%	1619
	21st to end	Day	9% 14% 10% 15% 13% 5% 7% 6% 4 4 11%	1836
February	1st to 10th	Day	15% 16% 8% 14% 9% 5% 6% 5% 4 4 14%	1620
	11th to 20th	Day	10% 15% 9% 16% 11% 5% 6% 5% 4 5% 16%	1573
	21st to end	Day	16% 16% 6% 13% 9% 4 4 3 5% 5% 19%	1422
March	1st to 10th	Day	13% 12% 7% 13% 10% 5% 5% 4 5% 5% 23%	1945
	11th to 20th	Day	12% 14% 7% 9% 8% 3 4 5% 5% 5% 27%	1980
	21st to end	Day	17% 14% 5% 9% 8% 3 5% 5% 5% 5% 26%	2194
April	1st to 10th	Day	13% 14% 6% 8% 6% 4 5% 5% 5% 6% 29%	2160
	11th to 20th	Day	16% 11% 4 7% 6% 3 4 5% 5% 6% 34%	2134
	21st to end	Day	18% 12% 4 8% 6% 3 4 4 5% 5% 31%	2302
May	1st to 10th	Day	16% 9% 4 8% 6% 3 5% 5% 5% 7% 30%	2372
	11th to 20th	Day	15% 7% 3 6% 6% 4 5% 5% 5% 7% 35%	2478
	21st to end	Day	12% 6% 3 6% 5% 4 6% 5% 6% 9% 39%	2725
June	1st to 10th	Day	15% 8% 3 7% 4 3 5% 5% 6% 7% 37%	2520
	11th to 20th	Day	13% 5% 3 5% 6% 4 6% 6% 8% 9% 35%	2520
	21st to end	Day	13% 6% 2 6% 6% 3 5% 6% 6% 8% 39%	2520
July	1st to 10th	Day	8% 5% 3 6% 5% 4 5% 6% 6% 9% 42%	2520
	11th to 20th	Day	9% 6% 3 6% 7% 4 6% 6% 7% 10% 34%	2520
	21st to end	Day	11% 6% 3 6% 6% 4 7% 7% 8% 9% 32%	2625
August	1st to 10th	Day	8% 6% 3 6% 6% 4 7% 6% 8% 9% 36%	2327
	11th to 20th	Day	12% 6% 3 6% 7% 6% 7% 7% 8% 8% 31%	2303
	21st to end	Day	14% 5% 3 6% 6% 3 6% 5% 7% 9% 36%	2404
September	1st to 10th	Day	15% 6% 3 6% 6% 3 5% 5% 7% 7% 38%	2160
	11th to 20th	Day	14% 6% 3 4 6% 4 5% 5% 6% 8% 40%	2140
	21st to end	Day	16% 7% 3 7% 6% 4 5% 5% 7% 8% 32%	1890
October	1st to 10th	Day	20% 9% 4 5% 5% 3 5% 4 5% 7% 32%	1800
	11th to 20th	Day	17% 12% 3 8% 7% 5% 5% 4 6% 7% 27%	1800
	21st to end	Day	22% 14% 5% 9% 7% 4 5% 4 4 5% 22%	1980
November	1st to 10th	Day	12% 12% 5% 9% 7% 3 5% 5% 5% 6% 32%	1663
	11th to 20th	Day	11% 17% 7% 8% 9% 4 5% 4 4 7% 24%	1520
	21st to end	Day	14% 18% 6% 10% 7% 4 5% 5% 5% 6% 20%	1520
December	1st to 10th	Day	15% 19% 8% 13% 8% 4 5% 5% 5% 5% 14%	1520
	11th to 20th	Day	14% 17% 8% 13% 9% 4 5% 4 4 5% 14%	1906
	21st to end	Day	21% 22% 9% 12% 7% 4 4 5% 4 3 10%	2772
January	1st to 10th	Night	11% 13% 7% 15% 10% 5% 7% 6% 5% 6% 15%	2649
	11th to 20th	Night	11% 16% 8% 13% 11% 5% 7% 5% 5% 6% 12%	2560
	21st to end	Night	10% 11% 8% 14% 13% 6% 7% 6% 5% 6% 14%	2664
February	1st to 10th	Night	13% 14% 7% 12% 10% 5% 7% 5% 5% 6% 16%	2340
	11th to 20th	Night	11% 11% 6% 14% 9% 4 8% 6% 6% 7% 18%	2273
	21st to end	Night	14% 12% 5% 14% 9% 4 6% 5% 5% 6% 20%	1834
March	1st to 10th	Night	11% 11% 6% 10% 9% 5% 6% 5% 4 6% 26%	1998
	11th to 20th	Night	15% 11% 6% 8% 8% 3 5% 4 5% 6% 30%	1979
	21st to end	Night	14% 10% 4 11% 8% 4 5% 4 3 6% 31%	2160
April	1st to 10th	Night	15% 10% 5% 7% 7% 4 6% 4 5% 7% 31%	1800
	11th to 20th	Night	15% 10% 3 7% 6% 3 5% 4 6% 7% 35%	1775
	21st to end	Night	16% 7% 3 7% 7% 3 5% 5% 6% 8% 33%	1674
May	1st to 10th	Night	16% 8% 4 5% 5% 4 6% 5% 6% 8% 33%	1585
	11th to 20th	Night	11% 6% 2 6% 6% 4 5% 5% 7% 8% 40%	1415
	21st to end	Night	10% 5% 3 5% 4 4 5% 5% 5% 8% 47%	1550
June	1st to 10th	Night	13% 4 2 6% 7% 3 6% 5% 8% 9% 37%	1440
	11th to 20th	Night	11% 5% 2 6% 6% 3 5% 6% 7% 8% 42%	1440
	21st to end	Night	10% 5% 2 5% 5% 3 6% 5% 7% 9% 43%	1440
July	1st to 10th	Night	6% 4 2 5% 5% 4 5% 5% 6% 10% 48%	1440
	11th to 20th	Night	9% 4 2 5% 5% 3 5% 5% 6% 10% 46%	1440
	21st to end	Night	10% 4 1 4 5% 3 5% 6% 6% 9% 46%	1690
August	1st to 10th	Night	7% 4 2 5% 5% 4 5% 5% 7% 9% 46%	1611
	11th to 20th	Night	9% 6% 3 4 6% 4 5% 4 7% 10% 41%	1598
	21st to end	Night	9% 4 3 5% 5% 3 6% 6% 7% 8% 44%	1934
September	1st to 10th	Night	14% 5% 3 4 4 3 3 5% 6% 7% 46%	1800
	11th to 20th	Night	11% 6% 2 5% 4 3 4 4 6% 7% 48%	1818
	21st to end	Night	14% 6% 3 5% 4 3 6% 5% 7% 7% 39%	2070
October	1st to 10th	Night	17% 8% 3 6% 6% 3 4 5% 5% 7% 36%	2160
	11th to 20th	Night	16% 10% 4 7% 6% 4 4 5% 6% 8% 30%	2160
	21st to end	Night	23% 12% 4 7% 6% 3 5% 5% 5% 6% 25%	2376
November	1st to 10th	Night	12% 10% 3 8% 6% 3 5% 5% 4 6% 37%	2410
	11th to 20th	Night	13% 11% 5% 8% 7% 4 6% 5% 6% 8% 27%	2660
	21st to end	Night	13% 13% 6% 10% 7% 4 6% 6% 6% 7% 22%	2660
December	1st to 10th	Night	15% 17% 7% 12% 8% 5% 6% 5% 5% 6% 15%	2660
	11th to 20th	Night	15% 16% 7% 13% 9% 3 6% 5% 5% 5% 17%	3288
	21st to end	Night	21% 17% 10% 12% 8% 3 5% 4 5% 5% 9%	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	70% 5% 10% 10% 4	1518
	11th to 20th	Day	73% 4 8% 11% 3	1619
	21st to end	Day	74% 4 9% 9% 3	1836
February	1st to 10th	Day	71% 5% 12% 9% 3	1620
	11th to 20th	Day	68% 5% 7% 15% 5% 1	1582
	21st to end	Day	68% 3 8% 13% 7% 1	1422
March	1st to 10th	Day	62% 4 11% 17% 5% 1	1948
	11th to 20th	Day	57% 4 9% 17% 10% 2	1980
	21st to end	Day	59% 5% 7% 18% 9% 2	2194
April	1st to 10th	Day	54% 4 10% 19% 11% 2	2160
	11th to 20th	Day	50% 3 10% 20% 14% 2	2134
	21st to end	Day	55% 4 8% 19% 13% 2	2302
May	1st to 10th	Day	52% 5% 9% 21% 11% 2	2372
	11th to 20th	Day	47% 5% 11% 21% 13% 2	2482
	21st to end	Day	42% 4 16% 24% 13% 2	2725
June	1st to 10th	Day	46% 5% 11% 21% 16% 1	2520
	11th to 20th	Day	41% 6% 15% 23% 12% 1	2520
	21st to end	Day	41% 6% 13% 24% 15% 1	2520
July	1st to 10th	Day	38% 7% 11% 28% 14% 3	2450
	11th to 20th	Day	41% 6% 15% 25% 13%	2445
	21st to end	Day	43% 8% 14% 21% 13% 1	2478
August	1st to 10th	Day	39% 7% 15% 25% 13% 1	2314
	11th to 20th	Day	44% 7% 15% 24% 9% 1	2303
	21st to end	Day	43% 5% 12% 27% 11% 2	2404
September	1st to 10th	Day	44% 5% 12% 26% 11% 2	2160
	11th to 20th	Day	40% 4 13% 27% 15% 1	2140
	21st to end	Day	49% 6% 11% 21% 12% 1	1890
October	1st to 10th	Day	50% 5% 13% 19% 13% 1	1800
	11th to 20th	Day	56% 4 14% 18% 7% 1	1800
	21st to end	Day	64% 4 10% 14% 7% 1	1980
November	1st to 10th	Day	52% 5% 11% 20% 10% 2	1663
	11th to 20th	Day	59% 5% 14% 15% 7% 1	1520
	21st to end	Day	63% 4 13% 15% 5%	1520
December	1st to 10th	Day	70% 4 12% 12% 2	1520
	11th to 20th	Day	70% 5% 14% 8% 3	1906
	21st to end	Day	77% 6% 7% 9% 1	2772
January	1st to 10th	Night	67% 5% 10% 12% 6%	2649
	11th to 20th	Night	71% 6% 11% 9% 3	2560
	21st to end	Night	69% 4 11% 12% 4 1	2664
February	1st to 10th	Night	66% 4 12% 13% 5%	2340
	11th to 20th	Night	62% 5% 12% 15% 6% 1	2275
	21st to end	Night	63% 4 12% 14% 8% 1	1834
March	1st to 10th	Night	57% 4 11% 19% 7% 1	2008
	11th to 20th	Night	54% 4 11% 21% 9% 2	1980
	21st to end	Night	55% 4 9% 18% 13% 2	2160
April	1st to 10th	Night	52% 6% 10% 21% 11% 1	1800
	11th to 20th	Night	49% 5% 12% 19% 14% 1	1775
	21st to end	Night	47% 4 12% 22% 14% 2	1674
May	1st to 10th	Night	48% 4 11% 22% 13% 2	1585
	11th to 20th	Night	40% 5% 14% 25% 14% 2	1416
	21st to end	Night	35% 6% 17% 23% 16% 2	1551
June	1st to 10th	Night	41% 6% 11% 26% 14% 1	1440
	11th to 20th	Night	37% 6% 13% 25% 16% 2	1440
	21st to end	Night	36% 8% 15% 25% 15% 2	1440
July	1st to 10th	Night	32% 6% 16% 26% 18% 3	1400
	11th to 20th	Night	33% 6% 13% 32% 15% 2	1392
	21st to end	Night	34% 6% 16% 26% 16% 2	1596
August	1st to 10th	Night	33% 6% 16% 31% 13% 1	1600
	11th to 20th	Night	36% 6% 17% 31% 10%	1598
	21st to end	Night	35% 5% 15% 29% 13% 2	1934
September	1st to 10th	Night	35% 5% 16% 29% 13% 2	1800
	11th to 20th	Night	35% 4 13% 30% 16% 1	1818
	21st to end	Night	40% 5% 17% 24% 13% 1	2070
October	1st to 10th	Night	47% 7% 12% 20% 14% 1	2160
	11th to 20th	Night	50% 6% 16% 19% 9% 1	2160
	21st to end	Night	57% 5% 13% 16% 7% 1	2376
November	1st to 10th	Night	47% 6% 11% 23% 11% 2	2410
	11th to 20th	Night	52% 4 15% 19% 7% 2	2660
	21st to end	Night	59% 5% 17% 15% 4	2660
December	1st to 10th	Night	67% 6% 13% 12% 2	2660
	11th to 20th	Night	66% 5% 13% 13% 3	3288
	21st to end	Night	75% 5% 9% 9% 1	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	32% 18% 32% 12% 5% 1	180
	11th to 20th	Day	29% 25% 36% 9% 1	227
	21st to end	Day	28% 22% 33% 13% 4	282
February	1st to 10th	Day	36% 28% 30% 6% 1	240
	11th to 20th	Day	33% 31% 30% 5%	240
	21st to end	Day	39% 23% 26% 12% 1	288
March	1st to 10th	Day	27% 25% 30% 14% 3	420
	11th to 20th	Day	29% 23% 32% 12% 4	420
	21st to end	Day	32% 22% 26% 15% 5% 1	462
April	1st to 10th	Day	29% 18% 32% 15% 5% 1	420
	11th to 20th	Day	30% 20% 24% 15% 8% 2	412
	21st to end	Day	28% 15% 35% 16% 4 1	420
May	1st to 10th	Day	27% 13% 30% 19% 7% 3	423
	11th to 20th	Day	20% 14% 34% 21% 9% 2	447
	21st to end	Day	16% 13% 30% 22% 13% 5%	485
June	1st to 10th	Day	16% 13% 36% 24% 9% 1	450
	11th to 20th	Day	17% 10% 31% 25% 13% 3	450
	21st to end	Day	14% 12% 37% 25% 10% 2	450
July	1st to 10th	Day	13% 4 28% 30% 20% 5%	445
	11th to 20th	Day	12% 5% 31% 28% 17% 7%	444
	21st to end	Day	13% 8% 34% 29% 13% 3	488
August	1st to 10th	Day	9% 5% 28% 33% 18% 8%	446
	11th to 20th	Day	14% 6% 26% 29% 17% 8%	439
	21st to end	Day	13% 7% 27% 30% 14% 9%	495
September	1st to 10th	Day	18% 10% 23% 27% 14% 9%	450
	11th to 20th	Day	13% 8% 25% 25% 20% 10%	450
	21st to end	Day	18% 14% 26% 21% 15% 5%	450
October	1st to 10th	Day	26% 8% 28% 17% 16% 5%	450
	11th to 20th	Day	26% 16% 32% 19% 6% 1	450
	21st to end	Day	33% 16% 31% 16% 4	495
November	1st to 10th	Day	23% 17% 30% 19% 9% 1	344
	11th to 20th	Day	25% 28% 31% 12% 3 1	180
	21st to end	Day	29% 22% 37% 10% 2	180
December	1st to 10th	Day	36% 18% 32% 10% 3 1	180
	11th to 20th	Day	34% 26% 24% 13% 3	180
	21st to end	Day	37% 21% 31% 10% 1 1	198
January	1st to 10th	Night	23% 24% 34% 15% 4	760
	11th to 20th	Night	25% 21% 32% 18% 4	760
	21st to end	Night	18% 22% 37% 20% 3	820
February	1st to 10th	Night	27% 23% 33% 15% 2	720
	11th to 20th	Night	23% 24% 36% 15% 2	703
	21st to end	Night	25% 23% 32% 18% 2	592
March	1st to 10th	Night	23% 20% 34% 19% 3	720
	11th to 20th	Night	27% 20% 30% 20% 3 1	720
	21st to end	Night	21% 15% 36% 21% 7% 1	792
April	1st to 10th	Night	23% 16% 34% 23% 3	720
	11th to 20th	Night	25% 13% 32% 22% 7% 1	711
	21st to end	Night	20% 10% 30% 31% 9%	594
May	1st to 10th	Night	20% 10% 23% 34% 12% 1	505
	11th to 20th	Night	14% 10% 31% 38% 7% 1	354
	21st to end	Night	11% 8% 32% 31% 18% 1	389
June	1st to 10th	Night	14% 6% 34% 31% 15% 1	360
	11th to 20th	Night	14% 7% 29% 38% 13%	360
	21st to end	Night	10% 6% 34% 40% 9%	360
July	1st to 10th	Night	7% 4 23% 45% 19% 2	350
	11th to 20th	Night	10% 3 31% 37% 18% 2	348
	21st to end	Night	11% 4 24% 42% 17% 3	510
August	1st to 10th	Night	9% 3 25% 37% 23% 3	537
	11th to 20th	Night	12% 4 19% 35% 26% 4	534
	21st to end	Night	12% 4 23% 38% 22% 2	746
September	1st to 10th	Night	15% 6% 24% 32% 20% 3	720
	11th to 20th	Night	15% 5% 20% 34% 22% 4	720
	21st to end	Night	18% 8% 24% 30% 16% 4	720
October	1st to 10th	Night	21% 9% 27% 25% 15% 2	720
	11th to 20th	Night	23% 15% 30% 25% 6% 1	720
	21st to end	Night	32% 12% 30% 23% 3	792
November	1st to 10th	Night	22% 13% 31% 26% 8% 1	740
	11th to 20th	Night	22% 18% 34% 17% 8% 2	760
	21st to end	Night	25% 19% 30% 20% 6%	760
December	1st to 10th	Night	29% 19% 31% 17% 4	760
	11th to 20th	Night	27% 20% 31% 19% 3	760
	21st to end	Night	30% 20% 33% 15% 2	836

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