Montpelier Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Montpelier Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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% 6% 2 6% 4 4 4 4 6% 8% 42%	126145
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	123251
Transparency	Day	38% 7% 12% 22% 17% 4	129746
	Night	38% 6% 10% 22% 19% 5%	127720
Seeing	Day	19% 11% 27% 25% 13% 4	16288
	Night	17% 10% 29% 30% 13% 1	25920

By Month

Percentage Cloud Cover SE (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	16% 5% 1 4 3 3 3 3 4 5% 52%	3982
	11th to 20th	Day	14% 8% 2 5% 3 3 3 3 4 4 51%	4060
	21st to end	Day	14% 7% 2 6% 4 4 4 3 5% 5% 45%	4466
February	1st to 10th	Day	13% 7% 3 6% 5% 5% 5% 4 5% 7% 40%	1800
	11th to 20th	Day	21% 7% 2 4 4 3 4 2 5% 5% 41%	1802
	21st to end	Day	18% 8% 2 6% 5% 3 3 3 5% 7% 38%	1666
March	1st to 10th	Day	18% 6% 2 7% 4 4 5% 4 6% 7% 38%	4935
	11th to 20th	Day	10% 5% 2 5% 4 3 4 3 5% 7% 51%	4980
	21st to end	Day	17% 6% 2 6% 5% 4 4 4 5% 6% 41%	5482
April	1st to 10th	Day	12% 6% 2 6% 4 3 4 3 5% 6% 49%	2400
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 7% 48%	2374
	21st to end	Day	15% 6% 2 5% 4 5% 4 4 5% 7% 42%	2400
May	1st to 10th	Day	15% 7% 2 7% 5% 5% 6% 5% 7% 9% 32%	5760
	11th to 20th	Day	19% 8% 2 6% 4 4 5% 4 7% 9% 31%	5879
	21st to end	Day	15% 5% 2 5% 4 4 4 4 6% 10% 41%	6934
June	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 7% 9% 44%	2800
	11th to 20th	Day	11% 4 2 5% 5% 5% 4 4 7% 11% 42%	2800
	21st to end	Day	8% 4 1 5% 5% 5% 6% 5% 7% 10% 43%	2800
July	1st to 10th	Day	7% 5% 2 5% 4 5% 6% 5% 8% 11% 43%	6270
	11th to 20th	Day	10% 6% 2 7% 5% 6% 7% 6% 9% 13% 28%	5900
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 10% 41%	6424
August	1st to 10th	Day	16% 7% 3 6% 5% 4 6% 4 8% 10% 31%	4234
	11th to 20th	Day	10% 5% 3 8% 5% 5% 6% 5% 7% 9% 37%	3992
	21st to end	Day	9% 5% 1 5% 4 4 4 4 5% 10% 49%	4312
September	1st to 10th	Day	13% 5% 2 6% 4 4 4 4 5% 8% 46%	2400
	11th to 20th	Day	13% 6% 2 7% 3 4 4 3 6% 7% 45%	2393
	21st to end	Day	22% 7% 2 7% 4 4 4 3 5% 6% 37%	2090
October	1st to 10th	Day	16% 4 2 6% 4 4 4 4 4 5% 48%	1930
	11th to 20th	Day	16% 4 2 3 4 4 3 3 4 6% 52%	1900
	21st to end	Day	14% 5% 2 6% 3 3 3 2 4 6% 53%	2090
November	1st to 10th	Day	12% 5% 2 5% 4 3 4 3 4 4 54%	1945
	11th to 20th	Day	14% 7% 2 4 3 4 3 3 4 6% 50%	1795
	21st to end	Day	14% 5% 2 5% 4 3 4 3 4 5% 52%	1600
December	1st to 10th	Day	11% 5% 2 5% 4 3 4 3 5% 7% 50%	2600
	11th to 20th	Day	19% 7% 2 5% 3 3 3 3 4 5% 47%	2990
	21st to end	Day	15% 7% 2 6% 4 4 4 3 6% 6% 42%	3960
January	1st to 10th	Night	13% 7% 1 4 3 3 3 2 4 5% 54%	5958
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 4 55%	5900
	21st to end	Night	16% 7% 2 5% 3 3 3 3 4 5% 49%	6490
February	1st to 10th	Night	15% 8% 2 4 4 5% 4 3 6% 7% 42%	2600
	11th to 20th	Night	20% 7% 2 6% 4 3 3 3 4 5% 43%	2483
	21st to end	Night	16% 7% 3 6% 4 3 4 3 4 5% 46%	1960
March	1st to 10th	Night	16% 5% 3 5% 3 3 3 3 5% 6% 47%	4918
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 6% 7% 51%	4980
	21st to end	Night	13% 7% 2 5% 4 4 4 3 4 6% 47%	5450
April	1st to 10th	Night	15% 7% 2 5% 4 4 3 3 4 7% 46%	2000
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 5% 8% 48%	1975
	21st to end	Night	13% 6% 2 6% 4 3 4 3 6% 7% 45%	2000
May	1st to 10th	Night	15% 8% 3 7% 5% 4 5% 4 6% 9% 33%	4177
	11th to 20th	Night	21% 8% 3 7% 4 4 5% 3 6% 7% 32%	3998
	21st to end	Night	15% 7% 2 6% 5% 4 4 3 6% 8% 38%	3920
June	1st to 10th	Night	10% 6% 2 7% 5% 4 5% 4 6% 10% 41%	1600
	11th to 20th	Night	11% 5% 2 5% 5% 6% 4 5% 6% 9% 42%	1600
	21st to end	Night	8% 5% 2 4 5% 5% 4 5% 8% 10% 45%	1600
July	1st to 10th	Night	9% 6% 2 7% 5% 6% 5% 5% 8% 10% 37%	3682
	11th to 20th	Night	11% 8% 3 8% 5% 6% 6% 5% 7% 11% 28%	4057
	21st to end	Night	7% 6% 3 8% 6% 7% 7% 5% 8% 8% 34%	4427
August	1st to 10th	Night	16% 7% 3 8% 4 5% 5% 4 7% 9% 32%	2915
	11th to 20th	Night	11% 7% 2 7% 5% 5% 5% 5% 6% 10% 37%	3022
	21st to end	Night	9% 6% 2 6% 5% 4 5% 4 7% 9% 45%	3586
September	1st to 10th	Night	13% 5% 2 6% 4 4 4 4 5% 8% 46%	2000
	11th to 20th	Night	11% 6% 2 5% 5% 4 4 3 5% 7% 47%	2003
	21st to end	Night	22% 7% 3 6% 4 3 3 3 5% 5% 37%	2310
October	1st to 10th	Night	16% 7% 2 5% 3 3 4 3 4 6% 48%	2317
	11th to 20th	Night	16% 5% 1 5% 4 3 3 3 5% 5% 49%	2280
	21st to end	Night	13% 5% 2 5% 3 3 4 2 4 6% 53%	2508
November	1st to 10th	Night	12% 7% 2 5% 4 3 3 3 4 5% 53%	2351
	11th to 20th	Night	14% 6% 2 5% 4 4 3 3 4 5% 50%	2603
	21st to end	Night	12% 5% 2 5% 3 3 4 4 6% 6% 51%	2800
December	1st to 10th	Night	10% 5% 2 4 3 3 3 3 5% 6% 54%	4580
	11th to 20th	Night	20% 7% 2 6% 4 4 4 3 5% 5% 40%	5205
	21st to end	Night	15% 7% 2 5% 3 3 3 3 6% 7% 44%	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	35% 2 5% 26% 22% 10%	3983
	11th to 20th	Day	37% 4 5% 19% 24% 10%	4060
	21st to end	Day	41% 5% 8% 25% 18% 3	4466
February	1st to 10th	Day	42% 6% 10% 27% 13% 2	1800
	11th to 20th	Day	45% 4 6% 21% 19% 5%	1811
	21st to end	Day	46% 5% 10% 17% 17% 5%	1666
March	1st to 10th	Day	44% 6% 7% 18% 17% 8%	4935
	11th to 20th	Day	31% 5% 8% 24% 22% 9%	4980
	21st to end	Day	43% 6% 10% 18% 18% 6%	5482
April	1st to 10th	Day	34% 5% 11% 22% 19% 8%	2400
	11th to 20th	Day	35% 6% 12% 20% 23% 6%	2374
	21st to end	Day	41% 6% 11% 24% 14% 5%	2400
May	1st to 10th	Day	45% 9% 15% 18% 11% 1	5794
	11th to 20th	Day	47% 8% 13% 21% 11% 1	5883
	21st to end	Day	37% 8% 13% 22% 18% 2	6949
June	1st to 10th	Day	35% 9% 16% 25% 14% 2	2800
	11th to 20th	Day	35% 9% 16% 22% 17% 1	2800
	21st to end	Day	34% 9% 15% 25% 17% 1	2800
July	1st to 10th	Day	33% 9% 21% 20% 13% 4	6201
	11th to 20th	Day	42% 12% 22% 18% 5% 1	5831
	21st to end	Day	35% 8% 14% 23% 19% 1	6240
August	1st to 10th	Day	45% 8% 17% 23% 6%	4221
	11th to 20th	Day	39% 8% 15% 24% 12% 1	4031
	21st to end	Day	30% 7% 16% 28% 18% 2	4312
September	1st to 10th	Day	36% 8% 14% 17% 21% 4	2400
	11th to 20th	Day	37% 6% 9% 26% 18% 4	2393
	21st to end	Day	47% 6% 10% 18% 15% 3	2090
October	1st to 10th	Day	36% 4 7% 19% 28% 5%	3190
	11th to 20th	Day	26% 4 5% 21% 32% 13%	3140
	21st to end	Day	21% 3 9% 29% 25% 13%	3428
November	1st to 10th	Day	33% 5% 8% 24% 25% 4	1945
	11th to 20th	Day	36% 4 10% 21% 23% 7%	1795
	21st to end	Day	35% 4 8% 25% 24% 5%	1600
December	1st to 10th	Day	33% 4 9% 28% 18% 8%	2600
	11th to 20th	Day	41% 4 12% 24% 16% 4	2986
	21st to end	Day	40% 6% 12% 26% 16% 1	3960
January	1st to 10th	Night	33% 3 5% 22% 27% 10%	5958
	11th to 20th	Night	34% 3 5% 23% 26% 9%	5900
	21st to end	Night	39% 3 6% 26% 21% 4	6490
February	1st to 10th	Night	41% 6% 11% 24% 16% 2	2600
	11th to 20th	Night	44% 4 7% 20% 19% 6%	2485
	21st to end	Night	41% 5% 9% 21% 17% 8%	1960
March	1st to 10th	Night	38% 4 6% 22% 22% 7%	4918
	11th to 20th	Night	32% 4 8% 21% 25% 10%	4980
	21st to end	Night	39% 5% 7% 22% 21% 6%	5450
April	1st to 10th	Night	40% 5% 9% 18% 22% 6%	2000
	11th to 20th	Night	33% 6% 12% 21% 22% 6%	1975
	21st to end	Night	38% 6% 11% 23% 17% 5%	2000
May	1st to 10th	Night	46% 8% 16% 18% 10% 2	4206
	11th to 20th	Night	49% 8% 11% 22% 10% 1	3999
	21st to end	Night	42% 8% 14% 24% 12% 1	3927
June	1st to 10th	Night	37% 9% 13% 24% 16% 1	1600
	11th to 20th	Night	36% 10% 17% 21% 15% 1	1600
	21st to end	Night	33% 8% 15% 23% 18% 3	1600
July	1st to 10th	Night	39% 11% 17% 17% 13% 3	3643
	11th to 20th	Night	47% 12% 21% 16% 5% 1	4006
	21st to end	Night	43% 10% 12% 20% 15%	4297
August	1st to 10th	Night	47% 9% 15% 22% 7%	2904
	11th to 20th	Night	40% 9% 15% 24% 12%	3049
	21st to end	Night	35% 7% 11% 28% 18% 1	3586
September	1st to 10th	Night	36% 9% 15% 19% 16% 6%	2000
	11th to 20th	Night	36% 6% 10% 25% 20% 3	2003
	21st to end	Night	48% 5% 11% 17% 15% 3	2310
October	1st to 10th	Night	40% 5% 10% 18% 22% 6%	3837
	11th to 20th	Night	25% 3 8% 21% 31% 11%	3779
	21st to end	Night	21% 3 6% 31% 26% 12%	4120
November	1st to 10th	Night	34% 6% 9% 23% 24% 5%	2351
	11th to 20th	Night	37% 4 7% 20% 23% 8%	2603
	21st to end	Night	33% 4 10% 26% 22% 5%	2800
December	1st to 10th	Night	30% 4 9% 28% 25% 5%	4580
	11th to 20th	Night	45% 4 10% 19% 15% 6%	5208
	21st to end	Night	38% 6% 11% 27% 17% 1	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	29% 25% 29% 14% 3	283
	11th to 20th	Day	29% 29% 29% 12% 2	300
	21st to end	Day	24% 25% 38% 10% 2	330
February	1st to 10th	Day	23% 27% 36% 11% 3	300
	11th to 20th	Day	37% 24% 29% 9% 1	349
	21st to end	Day	29% 19% 31% 18% 3	414
March	1st to 10th	Day	23% 20% 35% 18% 4	500
	11th to 20th	Day	22% 23% 34% 15% 5% 1	500
	21st to end	Day	26% 17% 37% 16% 4 1	550
April	1st to 10th	Day	22% 21% 34% 17% 5% 1	500
	11th to 20th	Day	19% 15% 34% 21% 8% 3	492
	21st to end	Day	22% 15% 33% 20% 8% 1	500
May	1st to 10th	Day	20% 16% 31% 23% 7% 3	503
	11th to 20th	Day	21% 9% 31% 24% 12% 2	527
	21st to end	Day	20% 6% 25% 28% 17% 4	573
June	1st to 10th	Day	11% 6% 30% 29% 21% 4	530
	11th to 20th	Day	12% 5% 25% 36% 16% 5%	530
	21st to end	Day	10% 6% 24% 34% 22% 5%	530
July	1st to 10th	Day	11% 3 17% 35% 26% 8%	525
	11th to 20th	Day	6% 2 17% 39% 27% 10%	524
	21st to end	Day	9% 3 21% 33% 23% 10%	576
August	1st to 10th	Day	14% 3 20% 30% 24% 8%	526
	11th to 20th	Day	13% 3 19% 32% 26% 8%	519
	21st to end	Day	10% 3 19% 31% 23% 13%	583
September	1st to 10th	Day	16% 5% 18% 29% 19% 13%	530
	11th to 20th	Day	16% 4 19% 29% 22% 9%	530
	21st to end	Day	30% 8% 23% 25% 11% 4	530
October	1st to 10th	Day	20% 6% 23% 27% 16% 8%	502
	11th to 20th	Day	17% 11% 24% 28% 18% 2	490
	21st to end	Day	16% 12% 33% 26% 9% 4	539
November	1st to 10th	Day	17% 10% 32% 27% 11% 4	488
	11th to 20th	Day	19% 13% 32% 26% 10% 1	395
	21st to end	Day	22% 18% 32% 18% 9% 2	200
December	1st to 10th	Day	22% 20% 38% 15% 6% 1	200
	11th to 20th	Day	22% 16% 36% 19% 8% 1	200
	21st to end	Day	26% 15% 30% 20% 8% 1	220
January	1st to 10th	Night	21% 18% 40% 18% 3	800
	11th to 20th	Night	21% 17% 36% 24% 3	800
	21st to end	Night	21% 16% 39% 22% 2	880
February	1st to 10th	Night	20% 16% 37% 24% 3	800
	11th to 20th	Night	25% 20% 37% 17% 1	783
	21st to end	Night	21% 18% 37% 21% 3	660
March	1st to 10th	Night	18% 19% 36% 25% 3	800
	11th to 20th	Night	23% 17% 34% 20% 6%	800
	21st to end	Night	17% 17% 39% 22% 5%	880
April	1st to 10th	Night	20% 18% 38% 20% 4	800
	11th to 20th	Night	15% 14% 37% 26% 7%	791
	21st to end	Night	17% 11% 36% 30% 7%	800
May	1st to 10th	Night	18% 10% 34% 29% 8%	668
	11th to 20th	Night	17% 5% 34% 30% 13% 1	583
	21st to end	Night	16% 2 23% 36% 22% 1	433
June	1st to 10th	Night	11% 2 24% 42% 20% 1	400
	11th to 20th	Night	12% 5% 20% 42% 21% 2	400
	21st to end	Night	8% 2 24% 46% 20% 1	400
July	1st to 10th	Night	10% 3 17% 39% 27% 4	421
	11th to 20th	Night	7% 1 13% 41% 32% 6%	582
	21st to end	Night	8% 2 17% 44% 26% 3	657
August	1st to 10th	Night	11% 3 12% 44% 28% 3	597
	11th to 20th	Night	15% 3 12% 41% 26% 3	676
	21st to end	Night	11% 3 14% 39% 29% 5%	880
September	1st to 10th	Night	14% 2 16% 37% 27% 4	800
	11th to 20th	Night	15% 4 14% 36% 28% 3	800
	21st to end	Night	27% 4 19% 33% 17% 1	800
October	1st to 10th	Night	19% 4 21% 31% 21% 4	773
	11th to 20th	Night	18% 9% 27% 30% 15% 2	760
	21st to end	Night	15% 10% 30% 31% 14% 1	836
November	1st to 10th	Night	17% 8% 30% 30% 13% 2	780
	11th to 20th	Night	17% 12% 33% 29% 8%	800
	21st to end	Night	15% 13% 33% 30% 8% 1	800
December	1st to 10th	Night	21% 13% 37% 22% 8%	800
	11th to 20th	Night	19% 12% 35% 29% 6%	800
	21st to end	Night	20% 13% 30% 33% 4	880

Page generated at 2025-10-01 01:39:07. Version ac07

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