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 2024-04-25.

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% 6% 2 5% 5% 3 5% 5% 5% 7% 43%	83455
	Night	15% 6% 2 5% 4 3 4 4 5% 7% 45%	85656
Transparency	Day	39% 7% 11% 23% 17% 4	83172
	Night	39% 6% 10% 22% 18% 4	85458
Seeing	Day	19% 11% 27% 25% 14% 4	14324
	Night	17% 10% 29% 30% 13% 1	24124

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	18% 5% 2 4 3 2 3 3 4 5% 51%	2776
	11th to 20th	Day	15% 8% 3 6% 5% 2 4 4 5% 4 44%	2840
	21st to end	Day	17% 9% 3 6% 5% 3 4 4 4 6% 39%	3124
February	1st to 10th	Day	13% 7% 3 6% 5% 3 5% 5% 5% 7% 40%	1710
	11th to 20th	Day	21% 6% 3 4 4 2 4 3 5% 6% 42%	1709
	21st to end	Day	18% 8% 2 6% 6% 3 3 5% 5% 7% 36%	1585
March	1st to 10th	Day	21% 8% 2 6% 5% 3 4 4 4 6% 37%	3435
	11th to 20th	Day	10% 4 2 4 5% 3 4 4 5% 8% 52%	3480
	21st to end	Day	21% 6% 3 5% 5% 3 4 5% 5% 6% 38%	3832
April	1st to 10th	Day	11% 5% 3 4 4 2 4 4 5% 7% 50%	2280
	11th to 20th	Day	12% 5% 2 5% 4 3 4 4 5% 7% 49%	2254
	21st to end	Day	17% 6% 2 5% 4 3 5% 4 5% 7% 42%	2210
May	1st to 10th	Day	15% 6% 2 5% 5% 4 5% 6% 5% 8% 40%	2293
	11th to 20th	Day	18% 6% 2 5% 5% 3 5% 5% 6% 7% 37%	2305
	21st to end	Day	13% 5% 2 5% 5% 3 5% 5% 7% 9% 41%	2725
June	1st to 10th	Day	11% 4 3 5% 5% 3 5% 5% 7% 8% 44%	2520
	11th to 20th	Day	11% 4 3 5% 5% 4 5% 5% 6% 11% 41%	2520
	21st to end	Day	8% 3 2 5% 5% 4 6% 7% 7% 10% 42%	2520
July	1st to 10th	Day	9% 5% 3 6% 5% 4 7% 6% 7% 9% 40%	2486
	11th to 20th	Day	6% 3 2 5% 5% 4 7% 7% 7% 13% 42%	2340
	21st to end	Day	8% 5% 2 6% 7% 4 7% 6% 7% 11% 37%	2547
August	1st to 10th	Day	10% 6% 2 6% 7% 4 6% 6% 7% 10% 35%	2327
	11th to 20th	Day	11% 5% 3 7% 7% 4 6% 6% 7% 8% 36%	2210
	21st to end	Day	10% 5% 2 5% 7% 4 5% 6% 6% 10% 41%	2376
September	1st to 10th	Day	14% 5% 2 5% 4 3 5% 5% 5% 8% 45%	2160
	11th to 20th	Day	12% 6% 2 6% 5% 2 4 4 6% 7% 46%	2155
	21st to end	Day	21% 6% 2 5% 5% 3 4 4 5% 6% 40%	1882
October	1st to 10th	Day	17% 4 2 5% 5% 3 4 5% 4 6% 47%	1800
	11th to 20th	Day	17% 4 2 3 4 3 3 4 4 6% 51%	1800
	21st to end	Day	15% 5% 2 5% 4 2 3 3 4 6% 51%	1980
November	1st to 10th	Day	13% 5% 2 3 5% 2 4 3 4 4 54%	1846
	11th to 20th	Day	14% 6% 2 4 4 2 4 3 4 6% 51%	1706
	21st to end	Day	15% 5% 2 4 4 3 3 4 4 5% 52%	1520
December	1st to 10th	Day	17% 6% 2 4 4 3 5% 4 6% 8% 40%	1520
	11th to 20th	Day	17% 6% 2 4 3 2 3 3 3 4 52%	1910
	21st to end	Day	17% 7% 3 6% 5% 3 4 4 5% 6% 40%	2772
January	1st to 10th	Night	14% 8% 2 5% 4 3 4 3 4 6% 47%	4164
	11th to 20th	Night	17% 7% 2 5% 4 2 3 4 4 4 47%	4120
	21st to end	Night	21% 8% 3 5% 4 2 3 3 4 4 43%	4532
February	1st to 10th	Night	14% 8% 2 4 5% 3 4 4 5% 7% 43%	2470
	11th to 20th	Night	19% 7% 2 5% 5% 3 3 3 4 5% 43%	2356
	21st to end	Night	17% 7% 2 6% 4 2 4 4 4 5% 46%	1865
March	1st to 10th	Night	21% 6% 3 5% 4 3 4 4 5% 6% 39%	3418
	11th to 20th	Night	14% 5% 2 4 3 2 3 4 6% 7% 50%	3480
	21st to end	Night	16% 7% 2 4 4 3 3 3 4 6% 49%	3800
April	1st to 10th	Night	15% 7% 2 5% 4 3 4 3 4 7% 46%	1900
	11th to 20th	Night	11% 5% 3 4 5% 3 4 4 5% 8% 49%	1875
	21st to end	Night	14% 6% 2 5% 5% 2 4 4 5% 7% 45%	1850
May	1st to 10th	Night	14% 6% 3 5% 5% 2 5% 5% 6% 8% 40%	1682
	11th to 20th	Night	15% 7% 2 5% 5% 4 5% 5% 5% 9% 37%	1586
	21st to end	Night	12% 5% 2 5% 7% 4 4 4 6% 9% 41%	1550
June	1st to 10th	Night	10% 5% 3 5% 5% 3 5% 5% 5% 11% 42%	1440
	11th to 20th	Night	11% 5% 2 5% 5% 4 5% 6% 6% 10% 41%	1440
	21st to end	Night	8% 5% 3 4 5% 4 5% 6% 8% 11% 43%	1440
July	1st to 10th	Night	9% 6% 3 7% 5% 3 5% 5% 6% 10% 41%	1468
	11th to 20th	Night	5% 3 2 5% 5% 4 7% 6% 7% 12% 43%	1620
	21st to end	Night	7% 4 2 6% 7% 5% 7% 7% 8% 9% 37%	1771
August	1st to 10th	Night	8% 5% 2 7% 6% 4 6% 6% 8% 11% 37%	1611
	11th to 20th	Night	13% 6% 2 5% 6% 4 5% 6% 7% 10% 37%	1671
	21st to end	Night	9% 6% 3 6% 6% 4 5% 6% 8% 10% 37%	1980
September	1st to 10th	Night	13% 5% 2 5% 5% 3 4 5% 6% 8% 44%	1800
	11th to 20th	Night	10% 6% 2 4 6% 3 4 4 5% 7% 48%	1802
	21st to end	Night	21% 7% 3 6% 5% 2 3 4 5% 6% 39%	2078
October	1st to 10th	Night	16% 6% 2 4 4 2 4 4 4 6% 48%	2160
	11th to 20th	Night	16% 5% 2 5% 4 3 3 4 5% 6% 47%	2160
	21st to end	Night	14% 5% 2 5% 4 2 4 4 4 6% 51%	2376
November	1st to 10th	Night	13% 6% 2 4 4 2 3 3 4 5% 53%	2230
	11th to 20th	Night	14% 5% 3 4 4 2 4 3 4 6% 51%	2472
	21st to end	Night	13% 4 2 4 3 2 4 5% 6% 6% 50%	2660
December	1st to 10th	Night	17% 7% 3 5% 4 3 4 4 5% 6% 42%	2660
	11th to 20th	Night	18% 6% 2 4 4 2 4 4 5% 6% 46%	3285
	21st to end	Night	15% 7% 3 5% 4 3 4 4 6% 7% 41%	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	36% 3 7% 32% 17% 5%	2777
	11th to 20th	Day	42% 5% 8% 19% 21% 5%	2840
	21st to end	Day	47% 6% 8% 21% 15% 3	3124
February	1st to 10th	Day	42% 6% 10% 27% 13% 2	1710
	11th to 20th	Day	43% 4 6% 22% 20% 5%	1718
	21st to end	Day	47% 5% 11% 16% 17% 4	1585
March	1st to 10th	Day	47% 7% 8% 19% 14% 6%	3435
	11th to 20th	Day	29% 6% 8% 25% 24% 8%	3480
	21st to end	Day	45% 7% 10% 19% 17% 3	3832
April	1st to 10th	Day	33% 6% 11% 23% 19% 8%	2280
	11th to 20th	Day	35% 6% 11% 19% 23% 6%	2254
	21st to end	Day	41% 6% 11% 23% 14% 6%	2210
May	1st to 10th	Day	40% 8% 15% 18% 15% 3	2293
	11th to 20th	Day	44% 9% 13% 22% 11% 1	2309
	21st to end	Day	39% 8% 15% 25% 13%	2725
June	1st to 10th	Day	35% 9% 15% 26% 14% 1	2520
	11th to 20th	Day	37% 10% 15% 20% 17% 1	2520
	21st to end	Day	35% 9% 14% 25% 16% 1	2520
July	1st to 10th	Day	38% 10% 14% 20% 15% 3	2417
	11th to 20th	Day	30% 11% 20% 26% 12% 2	2271
	21st to end	Day	37% 10% 14% 23% 16% 1	2405
August	1st to 10th	Day	41% 10% 16% 22% 10% 1	2314
	11th to 20th	Day	43% 10% 15% 20% 11% 1	2210
	21st to end	Day	37% 9% 16% 22% 13% 3	2376
September	1st to 10th	Day	37% 8% 14% 17% 20% 4	2160
	11th to 20th	Day	36% 7% 9% 27% 18% 4	2155
	21st to end	Day	45% 6% 9% 20% 17% 3	1882
October	1st to 10th	Day	38% 6% 9% 24% 20% 3	1800
	11th to 20th	Day	36% 5% 7% 24% 23% 4	1800
	21st to end	Day	35% 5% 6% 25% 23% 6%	1980
November	1st to 10th	Day	33% 5% 8% 24% 25% 5%	1846
	11th to 20th	Day	35% 4 10% 22% 23% 7%	1706
	21st to end	Day	35% 4 8% 25% 24% 5%	1520
December	1st to 10th	Day	40% 6% 11% 24% 14% 5%	1520
	11th to 20th	Day	37% 4 7% 30% 16% 6%	1906
	21st to end	Day	43% 6% 10% 27% 13% 1	2772
January	1st to 10th	Night	37% 5% 7% 25% 20% 5%	4164
	11th to 20th	Night	41% 5% 7% 21% 22% 5%	4120
	21st to end	Night	45% 4 6% 24% 17% 4	4532
February	1st to 10th	Night	40% 6% 10% 24% 17% 3	2470
	11th to 20th	Night	43% 4 7% 20% 19% 6%	2358
	21st to end	Night	41% 5% 9% 21% 17% 7%	1865
March	1st to 10th	Night	45% 6% 7% 23% 17% 2	3418
	11th to 20th	Night	32% 5% 8% 22% 26% 7%	3480
	21st to end	Night	38% 5% 8% 22% 22% 5%	3800
April	1st to 10th	Night	39% 6% 9% 18% 23% 6%	1900
	11th to 20th	Night	32% 6% 12% 21% 22% 6%	1875
	21st to end	Night	38% 6% 11% 23% 16% 5%	1850
May	1st to 10th	Night	40% 7% 14% 21% 15% 3	1682
	11th to 20th	Night	42% 10% 13% 19% 14% 2	1587
	21st to end	Night	38% 9% 16% 24% 12% 1	1551
June	1st to 10th	Night	36% 9% 13% 25% 15% 1	1440
	11th to 20th	Night	37% 11% 17% 19% 15% 2	1440
	21st to end	Night	34% 8% 14% 22% 18% 3	1440
July	1st to 10th	Night	37% 11% 15% 19% 14% 4	1427
	11th to 20th	Night	31% 11% 20% 24% 11% 2	1566
	21st to end	Night	36% 12% 15% 23% 13%	1672
August	1st to 10th	Night	38% 12% 16% 22% 12% 1	1600
	11th to 20th	Night	40% 11% 16% 20% 13%	1671
	21st to end	Night	37% 11% 15% 23% 11% 2	1980
September	1st to 10th	Night	37% 9% 16% 18% 15% 6%	1800
	11th to 20th	Night	35% 7% 10% 25% 20% 3	1802
	21st to end	Night	46% 5% 10% 18% 17% 4	2078
October	1st to 10th	Night	38% 5% 10% 23% 20% 4	2160
	11th to 20th	Night	38% 4 8% 24% 21% 5%	2160
	21st to end	Night	35% 5% 8% 24% 23% 6%	2376
November	1st to 10th	Night	34% 6% 9% 23% 23% 6%	2230
	11th to 20th	Night	37% 4 7% 20% 24% 8%	2472
	21st to end	Night	33% 5% 10% 25% 22% 5%	2660
December	1st to 10th	Night	43% 6% 9% 23% 16% 4	2660
	11th to 20th	Night	39% 5% 10% 20% 20% 5%	3288
	21st to end	Night	40% 7% 11% 24% 16% 2	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	29% 22% 30% 15% 4	254
	11th to 20th	Day	30% 27% 29% 12% 2	270
	21st to end	Day	24% 23% 39% 10% 3	297
February	1st to 10th	Day	21% 26% 37% 12% 3	270
	11th to 20th	Day	35% 24% 30% 10% 1	316
	21st to end	Day	30% 19% 30% 18% 3	382
March	1st to 10th	Day	22% 19% 35% 18% 5%	460
	11th to 20th	Day	22% 24% 33% 16% 4 1	460
	21st to end	Day	25% 17% 36% 17% 4 1	506
April	1st to 10th	Day	21% 20% 34% 18% 5% 1	460
	11th to 20th	Day	18% 14% 35% 21% 9% 4	452
	21st to end	Day	24% 16% 34% 18% 8% 1	437
May	1st to 10th	Day	20% 15% 31% 24% 7% 3	423
	11th to 20th	Day	21% 8% 32% 25% 13% 1	447
	21st to end	Day	19% 4 23% 29% 19% 4	485
June	1st to 10th	Day	13% 5% 26% 31% 22% 4	450
	11th to 20th	Day	12% 6% 26% 34% 16% 6%	450
	21st to end	Day	11% 6% 25% 32% 22% 4	450
July	1st to 10th	Day	11% 3 18% 32% 27% 9%	445
	11th to 20th	Day	5% 1 16% 41% 27% 9%	444
	21st to end	Day	9% 3 22% 34% 22% 10%	488
August	1st to 10th	Day	13% 3 18% 30% 27% 9%	446
	11th to 20th	Day	13% 3 17% 31% 27% 10%	439
	21st to end	Day	11% 4 17% 30% 24% 14%	495
September	1st to 10th	Day	17% 5% 16% 27% 21% 15%	450
	11th to 20th	Day	14% 4 19% 31% 23% 9%	450
	21st to end	Day	28% 7% 21% 27% 12% 4	450
October	1st to 10th	Day	21% 6% 23% 27% 16% 8%	450
	11th to 20th	Day	18% 10% 24% 28% 18% 2	450
	21st to end	Day	17% 13% 33% 25% 8% 3	495
November	1st to 10th	Day	18% 10% 31% 26% 11% 4	449
	11th to 20th	Day	18% 13% 31% 26% 10% 1	366
	21st to end	Day	21% 16% 33% 19% 9% 2	180
December	1st to 10th	Day	22% 19% 37% 15% 6% 1	180
	11th to 20th	Day	22% 16% 34% 18% 8% 1	180
	21st to end	Day	27% 15% 31% 19% 7% 1	198
January	1st to 10th	Night	22% 17% 39% 19% 3	760
	11th to 20th	Night	21% 16% 35% 24% 3	760
	21st to end	Night	21% 16% 39% 22% 2	836
February	1st to 10th	Night	19% 14% 38% 25% 3	760
	11th to 20th	Night	24% 20% 38% 17% 1	743
	21st to end	Night	22% 18% 36% 20% 3	628
March	1st to 10th	Night	18% 18% 36% 26% 3	760
	11th to 20th	Night	23% 18% 34% 19% 6%	760
	21st to end	Night	17% 16% 38% 23% 5%	836
April	1st to 10th	Night	20% 18% 38% 21% 4	760
	11th to 20th	Night	14% 14% 37% 27% 8%	751
	21st to end	Night	18% 11% 34% 30% 6%	740
May	1st to 10th	Night	18% 11% 34% 28% 8%	602
	11th to 20th	Night	17% 5% 34% 31% 13% 1	525
	21st to end	Night	16% 2 21% 37% 23% 2	389
June	1st to 10th	Night	11% 2 23% 42% 22% 1	360
	11th to 20th	Night	12% 5% 18% 42% 22% 2	360
	21st to end	Night	8% 2 24% 46% 20% 1	360
July	1st to 10th	Night	10% 3 18% 38% 27% 4	377
	11th to 20th	Night	6% 1 12% 41% 34% 7%	522
	21st to end	Night	8% 2 17% 43% 27% 3	591
August	1st to 10th	Night	10% 2 12% 44% 30% 3	537
	11th to 20th	Night	14% 3 12% 41% 27% 3	607
	21st to end	Night	10% 3 14% 38% 30% 5%	792
September	1st to 10th	Night	15% 2 15% 35% 28% 5%	720
	11th to 20th	Night	13% 4 14% 36% 29% 3	720
	21st to end	Night	26% 3 19% 33% 18% 1	720
October	1st to 10th	Night	19% 4 21% 31% 21% 3	720
	11th to 20th	Night	19% 8% 27% 30% 15% 2	720
	21st to end	Night	16% 10% 31% 30% 13% 1	792
November	1st to 10th	Night	17% 8% 30% 30% 13% 2	740
	11th to 20th	Night	17% 13% 32% 29% 9%	760
	21st to end	Night	15% 12% 32% 31% 9% 1	760
December	1st to 10th	Night	21% 13% 36% 22% 8%	760
	11th to 20th	Night	19% 12% 35% 29% 6%	760
	21st to end	Night	19% 14% 30% 33% 4	836

Page generated at 2024-04-22 09:25:40. Version ac07

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