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 2025-01-22.

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 9% 6% 5% 5% 4 6% 7% 28%	105211
	Night	13% 11% 5% 10% 6% 5% 5% 4 6% 7% 28%	104159
Transparency	Day	51% 6% 12% 20% 10% 1	108812
	Night	52% 6% 12% 20% 9% 1	108646
Seeing	Day	23% 15% 30% 20% 9% 3	14747
	Night	21% 14% 30% 25% 9% 1	24216

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% 23% 9% 15% 8% 4 5% 3 4 4 13%	2597
	11th to 20th	Day	9% 20% 10% 18% 9% 7% 5% 4 4 4 11%	2770
	21st to end	Day	15% 19% 10% 13% 9% 6% 6% 3 4 4 10%	3133
February	1st to 10th	Day	14% 17% 6% 15% 8% 6% 6% 4 4 4 15%	1710
	11th to 20th	Day	9% 16% 8% 17% 10% 6% 5% 4 4 5% 17%	1663
	21st to end	Day	15% 17% 5% 13% 9% 5% 3 3 5% 6% 20%	1508
March	1st to 10th	Day	16% 12% 6% 11% 6% 5% 3 3 4 5% 28%	3388
	11th to 20th	Day	8% 16% 6% 11% 7% 4 5% 4 6% 6% 28%	3480
	21st to end	Day	12% 14% 5% 13% 7% 4 4 4 6% 7% 26%	3844
April	1st to 10th	Day	14% 16% 4 8% 5% 5% 5% 3 5% 6% 29%	2280
	11th to 20th	Day	16% 12% 4 8% 5% 4 4 4 5% 6% 33%	2254
	21st to end	Day	17% 13% 3 9% 6% 4 4 3 5% 5% 31%	2430
May	1st to 10th	Day	19% 9% 3 8% 5% 4 5% 4 5% 7% 32%	4168
	11th to 20th	Day	16% 10% 4 8% 6% 6% 5% 4 6% 7% 28%	4398
	21st to end	Day	9% 6% 3 6% 5% 5% 5% 4 6% 8% 42%	4837
June	1st to 10th	Day	15% 9% 3 8% 4 4 5% 4 6% 7% 35%	2660
	11th to 20th	Day	13% 6% 2 7% 5% 6% 6% 5% 8% 9% 34%	2660
	21st to end	Day	13% 6% 2 7% 6% 5% 5% 5% 7% 8% 38%	2660
July	1st to 10th	Day	9% 5% 3 6% 5% 5% 6% 5% 7% 11% 38%	4440
	11th to 20th	Day	7% 6% 2 7% 5% 5% 7% 5% 8% 10% 36%	4440
	21st to end	Day	8% 6% 3 7% 6% 6% 6% 6% 8% 9% 36%	4600
August	1st to 10th	Day	16% 7% 3 7% 4 5% 5% 4 7% 8% 35%	4104
	11th to 20th	Day	13% 6% 3 8% 6% 7% 7% 6% 9% 9% 27%	4044
	21st to end	Day	12% 6% 3 6% 4 5% 5% 4 7% 9% 38%	4222
September	1st to 10th	Day	14% 7% 2 6% 5% 5% 5% 4 7% 7% 39%	2280
	11th to 20th	Day	13% 6% 2 5% 5% 5% 5% 4 6% 8% 41%	2258
	21st to end	Day	17% 8% 3 8% 6% 5% 5% 4 7% 7% 30%	1995
October	1st to 10th	Day	19% 9% 4 6% 4 5% 5% 4 6% 7% 33%	1900
	11th to 20th	Day	17% 12% 3 9% 6% 5% 4 4 5% 6% 28%	1900
	21st to end	Day	21% 14% 4 10% 6% 5% 5% 3 4 5% 23%	2090
November	1st to 10th	Day	12% 13% 4 10% 6% 4 4 4 5% 6% 32%	1752
	11th to 20th	Day	10% 18% 5% 10% 7% 5% 5% 3 5% 6% 24%	1600
	21st to end	Day	14% 19% 6% 11% 6% 6% 5% 4 5% 7% 19%	1600
December	1st to 10th	Day	15% 21% 8% 15% 8% 5% 5% 4 5% 5% 11%	2600
	11th to 20th	Day	16% 19% 6% 15% 8% 5% 4 3 4 5% 16%	2986
	21st to end	Day	17% 20% 8% 14% 7% 6% 5% 4 4 5% 12%	3960
January	1st to 10th	Night	13% 17% 8% 17% 8% 6% 5% 4 4 5% 13%	4569
	11th to 20th	Night	8% 18% 9% 18% 11% 7% 5% 4 5% 5% 8%	4409
	21st to end	Night	17% 15% 7% 15% 11% 6% 5% 3 4 5% 11%	4546
February	1st to 10th	Night	12% 16% 5% 13% 10% 6% 6% 4 5% 6% 17%	2470
	11th to 20th	Night	10% 11% 6% 15% 9% 6% 8% 4 6% 7% 18%	2403
	21st to end	Night	13% 13% 4 15% 8% 6% 5% 4 6% 6% 21%	1945
March	1st to 10th	Night	17% 11% 4 9% 6% 6% 4 3 4 5% 31%	3448
	11th to 20th	Night	12% 13% 7% 10% 7% 5% 5% 4 6% 6% 25%	3479
	21st to end	Night	13% 9% 3 11% 5% 5% 4 3 4 6% 37%	3786
April	1st to 10th	Night	15% 12% 4 8% 6% 6% 5% 3 5% 7% 30%	1900
	11th to 20th	Night	16% 10% 3 8% 5% 4 5% 4 6% 6% 34%	1875
	21st to end	Night	15% 8% 3 8% 6% 4 5% 4 6% 9% 34%	1767
May	1st to 10th	Night	20% 9% 3 7% 4 5% 4 3 5% 7% 32%	2770
	11th to 20th	Night	15% 8% 3 8% 5% 5% 5% 4 6% 8% 34%	2495
	21st to end	Night	9% 6% 2 6% 4 4 4 4 6% 9% 45%	2738
June	1st to 10th	Night	13% 5% 2 7% 5% 5% 5% 4 7% 9% 36%	1520
	11th to 20th	Night	10% 6% 2 6% 5% 5% 5% 5% 6% 8% 42%	1520
	21st to end	Night	10% 6% 2 7% 4 5% 5% 4 8% 9% 41%	1520
July	1st to 10th	Night	6% 7% 3 8% 5% 4 5% 5% 6% 10% 41%	2520
	11th to 20th	Night	7% 5% 2 6% 5% 4 4 4 6% 9% 47%	2520
	21st to end	Night	9% 5% 2 5% 4 5% 5% 5% 6% 8% 46%	2950
August	1st to 10th	Night	12% 7% 2 7% 4 4 4 4 7% 8% 40%	2825
	11th to 20th	Night	12% 7% 3 7% 5% 5% 5% 4 6% 9% 37%	2785
	21st to end	Night	8% 4 2 6% 4 5% 6% 5% 7% 9% 45%	3402
September	1st to 10th	Night	14% 6% 2 5% 4 3 4 4 5% 7% 46%	1900
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 6% 7% 48%	1919
	21st to end	Night	15% 7% 3 6% 4 4 6% 4 7% 7% 37%	2185
October	1st to 10th	Night	16% 8% 3 7% 5% 5% 4 4 5% 7% 36%	2280
	11th to 20th	Night	16% 11% 4 8% 5% 5% 4 3 6% 7% 31%	2280
	21st to end	Night	22% 12% 3 8% 5% 4 5% 4 6% 6% 26%	2508
November	1st to 10th	Night	12% 11% 3 9% 6% 4 5% 4 5% 6% 37%	2541
	11th to 20th	Night	13% 12% 5% 10% 6% 5% 6% 4 6% 7% 26%	2800
	21st to end	Night	13% 15% 6% 10% 7% 6% 6% 5% 6% 7% 22%	2800
December	1st to 10th	Night	10% 17% 7% 16% 8% 7% 7% 5% 6% 6% 12%	4580
	11th to 20th	Night	16% 18% 6% 15% 7% 5% 5% 3 5% 4 15%	5208
	21st to end	Night	17% 18% 9% 14% 7% 6% 4 3 5% 5% 12%	6996

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	72% 6% 10% 8% 4	2598
	11th to 20th	Day	74% 5% 7% 9% 4	2770
	21st to end	Day	77% 5% 10% 6% 2	3133
February	1st to 10th	Day	70% 5% 12% 10% 3	1710
	11th to 20th	Day	67% 6% 7% 15% 5% 1	1672
	21st to end	Day	66% 4 9% 14% 7% 1	1508
March	1st to 10th	Day	58% 4 9% 23% 6% 1	3391
	11th to 20th	Day	54% 5% 15% 18% 8% 1	3480
	21st to end	Day	55% 5% 7% 23% 8% 1	3844
April	1st to 10th	Day	55% 4 10% 19% 10% 2	2280
	11th to 20th	Day	51% 4 10% 20% 14% 2	2254
	21st to end	Day	54% 4 8% 19% 13% 2	2430
May	1st to 10th	Day	51% 5% 10% 21% 12% 2	4180
	11th to 20th	Day	53% 7% 12% 20% 8% 1	4402
	21st to end	Day	39% 4 12% 27% 14% 3	4837
June	1st to 10th	Day	46% 5% 11% 20% 16% 1	2660
	11th to 20th	Day	41% 6% 15% 23% 12% 1	2660
	21st to end	Day	41% 6% 13% 24% 14% 1	2660
July	1st to 10th	Day	37% 7% 13% 28% 14% 2	4370
	11th to 20th	Day	36% 8% 15% 25% 16%	4365
	21st to end	Day	39% 10% 14% 23% 13% 1	4453
August	1st to 10th	Day	45% 6% 13% 22% 14% 1	4091
	11th to 20th	Day	46% 8% 15% 24% 6%	4083
	21st to end	Day	39% 5% 12% 33% 10% 1	4222
September	1st to 10th	Day	43% 5% 12% 27% 11% 1	2280
	11th to 20th	Day	39% 4 13% 28% 16% 1	2258
	21st to end	Day	50% 6% 11% 20% 11% 1	1995
October	1st to 10th	Day	39% 7% 14% 25% 14% 1	3160
	11th to 20th	Day	46% 4 16% 19% 14% 1	3140
	21st to end	Day	47% 5% 14% 25% 9% 1	3428
November	1st to 10th	Day	52% 5% 11% 20% 10% 2	1752
	11th to 20th	Day	58% 5% 14% 15% 7% 1	1600
	21st to end	Day	62% 5% 13% 15% 5%	1600
December	1st to 10th	Day	69% 8% 12% 10% 1	2600
	11th to 20th	Day	68% 5% 13% 9% 5%	2986
	21st to end	Day	73% 6% 10% 8% 3	3960
January	1st to 10th	Night	70% 6% 10% 10% 4	4569
	11th to 20th	Night	73% 6% 10% 8% 2	4409
	21st to end	Night	76% 4 8% 8% 3	4546
February	1st to 10th	Night	64% 5% 12% 14% 4	2470
	11th to 20th	Night	62% 5% 12% 15% 6% 1	2405
	21st to end	Night	62% 4 12% 14% 8% 1	1945
March	1st to 10th	Night	55% 4 11% 24% 6%	3458
	11th to 20th	Night	57% 6% 12% 18% 6% 1	3480
	21st to end	Night	47% 5% 10% 23% 14% 2	3786
April	1st to 10th	Night	52% 5% 10% 21% 11% 1	1900
	11th to 20th	Night	49% 5% 11% 19% 14% 1	1875
	21st to end	Night	45% 4 13% 22% 14% 2	1767
May	1st to 10th	Night	51% 5% 11% 20% 10% 3	2777
	11th to 20th	Night	46% 5% 17% 21% 8% 1	2496
	21st to end	Night	34% 7% 15% 24% 19% 2	2739
June	1st to 10th	Night	41% 6% 12% 25% 15% 1	1520
	11th to 20th	Night	37% 6% 13% 26% 16% 2	1520
	21st to end	Night	37% 8% 15% 24% 15% 2	1520
July	1st to 10th	Night	36% 7% 15% 23% 16% 2	2480
	11th to 20th	Night	30% 7% 16% 30% 16% 1	2472
	21st to end	Night	34% 7% 15% 27% 15% 2	2856
August	1st to 10th	Night	39% 6% 15% 28% 11% 1	2814
	11th to 20th	Night	41% 6% 16% 28% 8%	2812
	21st to end	Night	31% 6% 15% 35% 12% 1	3402
September	1st to 10th	Night	35% 5% 15% 29% 14% 2	1900
	11th to 20th	Night	35% 4 14% 30% 17% 1	1919
	21st to end	Night	43% 5% 16% 23% 13% 1	2185
October	1st to 10th	Night	40% 7% 11% 27% 15% 1	3800
	11th to 20th	Night	43% 6% 13% 21% 14% 2	3779
	21st to end	Night	42% 6% 16% 26% 9% 1	4120
November	1st to 10th	Night	47% 6% 11% 24% 11% 2	2541
	11th to 20th	Night	52% 4 15% 20% 7% 2	2800
	21st to end	Night	59% 5% 16% 16% 4	2800
December	1st to 10th	Night	66% 9% 13% 10% 1	4580
	11th to 20th	Night	66% 6% 10% 13% 5%	5208
	21st to end	Night	73% 6% 8% 10% 3	6996

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	31% 22% 30% 12% 5% 1	200
	11th to 20th	Day	29% 25% 36% 9% 1	253
	21st to end	Day	28% 21% 33% 15% 4	300
February	1st to 10th	Day	34% 26% 29% 8% 2	270
	11th to 20th	Day	33% 29% 33% 5% 1	270
	21st to end	Day	39% 23% 26% 12% 1	320
March	1st to 10th	Day	27% 24% 30% 15% 4 1	460
	11th to 20th	Day	29% 23% 33% 12% 4	460
	21st to end	Day	31% 22% 28% 14% 4 1	506
April	1st to 10th	Day	30% 18% 32% 15% 5% 1	460
	11th to 20th	Day	31% 21% 24% 14% 7% 2	452
	21st to end	Day	28% 15% 34% 17% 4 1	460
May	1st to 10th	Day	27% 12% 29% 21% 7% 3	463
	11th to 20th	Day	20% 13% 32% 21% 10% 3	487
	21st to end	Day	17% 13% 31% 22% 13% 5%	529
June	1st to 10th	Day	18% 13% 35% 24% 9% 1	490
	11th to 20th	Day	16% 11% 31% 27% 13% 3	490
	21st to end	Day	14% 13% 37% 24% 9% 2	490
July	1st to 10th	Day	12% 6% 29% 29% 18% 5%	485
	11th to 20th	Day	13% 6% 30% 27% 17% 6%	484
	21st to end	Day	13% 8% 32% 29% 14% 3	532
August	1st to 10th	Day	11% 6% 27% 31% 17% 8%	486
	11th to 20th	Day	13% 8% 26% 29% 17% 8%	479
	21st to end	Day	13% 8% 27% 29% 14% 9%	539
September	1st to 10th	Day	18% 11% 22% 26% 14% 9%	490
	11th to 20th	Day	12% 8% 24% 24% 21% 11%	490
	21st to end	Day	20% 14% 26% 21% 13% 5%	490
October	1st to 10th	Day	25% 9% 29% 18% 15% 4	490
	11th to 20th	Day	25% 16% 32% 19% 7% 1	490
	21st to end	Day	31% 16% 32% 17% 4	539
November	1st to 10th	Day	23% 19% 30% 18% 8% 1	373
	11th to 20th	Day	24% 29% 32% 13% 3 1	200
	21st to end	Day	31% 24% 34% 10% 2	200
December	1st to 10th	Day	37% 17% 34% 10% 3 1	200
	11th to 20th	Day	35% 25% 26% 12% 3	200
	21st to end	Day	34% 22% 31% 10% 2 1	220
January	1st to 10th	Night	23% 25% 34% 14% 4	800
	11th to 20th	Night	25% 22% 32% 17% 3	800
	21st to end	Night	19% 21% 36% 20% 4	841
February	1st to 10th	Night	26% 23% 32% 17% 2	760
	11th to 20th	Night	23% 24% 36% 15% 2	743
	21st to end	Night	26% 22% 32% 18% 3	628
March	1st to 10th	Night	23% 20% 33% 20% 4	760
	11th to 20th	Night	28% 19% 30% 19% 3 1	760
	21st to end	Night	21% 15% 36% 21% 6% 1	836
April	1st to 10th	Night	24% 16% 35% 22% 3	760
	11th to 20th	Night	26% 13% 32% 22% 7% 1	751
	21st to end	Night	19% 10% 30% 31% 9%	627
May	1st to 10th	Night	20% 10% 23% 34% 11% 1	533
	11th to 20th	Night	15% 9% 30% 38% 7% 1	374
	21st to end	Night	11% 8% 31% 32% 18%	411
June	1st to 10th	Night	14% 6% 34% 31% 15% 1	380
	11th to 20th	Night	13% 7% 29% 38% 13%	380
	21st to end	Night	11% 7% 34% 39% 9%	380
July	1st to 10th	Night	7% 6% 23% 44% 18% 2	370
	11th to 20th	Night	10% 3 31% 37% 18% 2	368
	21st to end	Night	10% 4 24% 42% 18% 2	539
August	1st to 10th	Night	9% 3 25% 37% 23% 3	567
	11th to 20th	Night	12% 5% 20% 34% 25% 4	564
	21st to end	Night	12% 4 23% 37% 23% 2	788
September	1st to 10th	Night	15% 7% 24% 32% 20% 3	760
	11th to 20th	Night	14% 5% 19% 35% 23% 4	760
	21st to end	Night	19% 8% 23% 30% 16% 4	760
October	1st to 10th	Night	21% 9% 28% 25% 16% 2	760
	11th to 20th	Night	23% 15% 30% 25% 6% 1	760
	21st to end	Night	30% 13% 31% 24% 3	836
November	1st to 10th	Night	22% 14% 32% 25% 7% 1	780
	11th to 20th	Night	22% 18% 34% 17% 8% 2	800
	21st to end	Night	26% 19% 30% 20% 6%	800
December	1st to 10th	Night	29% 20% 31% 16% 4	800
	11th to 20th	Night	28% 21% 31% 18% 3	800
	21st to end	Night	29% 20% 33% 15% 2	880

Page generated at 2025-01-19 03:06:47. Version ac07

Copyright © 2025 Attilla Danko.