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 2024-11-05.

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 6% 4 4 4 4 6% 8% 42%	101075
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 43%	97926
Transparency	Day	38% 7% 12% 23% 17% 4	104720
	Night	38% 6% 11% 22% 18% 4	102421
Seeing	Day	18% 11% 27% 25% 14% 4	15100
	Night	17% 10% 29% 30% 13% 1	24746

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% 6% 1 4 3 3 3 3 4 5% 51%	2776
	11th to 20th	Day	15% 8% 3 6% 4 4 4 3 5% 4 43%	2840
	21st to end	Day	17% 10% 3 7% 4 4 4 3 5% 5% 39%	3124
February	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 4 5% 7% 40%	1710
	11th to 20th	Day	20% 7% 2 4 4 3 4 2 5% 6% 42%	1709
	21st to end	Day	18% 9% 3 7% 5% 4 3 3 5% 7% 36%	1585
March	1st to 10th	Day	21% 8% 2 7% 4 3 4 3 5% 6% 37%	3435
	11th to 20th	Day	10% 4 2 5% 4 4 4 3 5% 8% 52%	3480
	21st to end	Day	21% 6% 3 6% 4 4 4 4 5% 6% 38%	3832
April	1st to 10th	Day	11% 5% 2 5% 4 3 4 3 5% 7% 50%	2280
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 7% 48%	2254
	21st to end	Day	16% 6% 2 5% 4 5% 4 3 5% 7% 42%	2280
May	1st to 10th	Day	17% 8% 2 8% 5% 5% 5% 4 6% 7% 32%	4008
	11th to 20th	Day	20% 8% 2 6% 4 4 5% 4 6% 9% 31%	4099
	21st to end	Day	9% 4 2 5% 4 4 4 4 7% 11% 45%	4823
June	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 7% 9% 44%	2660
	11th to 20th	Day	10% 4 2 5% 5% 5% 4 4 7% 11% 42%	2660
	21st to end	Day	8% 4 2 6% 5% 5% 6% 5% 7% 10% 42%	2660
July	1st to 10th	Day	6% 4 2 5% 4 5% 6% 5% 8% 12% 42%	4378
	11th to 20th	Day	11% 5% 2 6% 4 5% 6% 5% 8% 13% 36%	4120
	21st to end	Day	11% 7% 3 7% 6% 6% 6% 5% 7% 9% 34%	4466
August	1st to 10th	Day	16% 7% 3 6% 5% 4 6% 4 8% 10% 30%	4104
	11th to 20th	Day	9% 5% 3 8% 5% 5% 6% 5% 7% 9% 38%	3867
	21st to end	Day	9% 5% 1 5% 4 4 4 4 5% 10% 49%	4180
September	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 5% 8% 45%	2280
	11th to 20th	Day	13% 6% 2 7% 3 4 4 3 6% 7% 45%	2274
	21st to end	Day	22% 6% 2 6% 4 4 4 3 5% 6% 38%	1986
October	1st to 10th	Day	16% 4 2 6% 4 4 3 3 4 5% 48%	1900
	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	13% 5% 2 4 4 3 4 2 4 4 54%	1887
	11th to 20th	Day	14% 7% 2 4 3 4 3 3 4 6% 51%	1706
	21st to end	Day	15% 5% 2 5% 3 3 3 3 4 4 52%	1520
December	1st to 10th	Day	17% 6% 2 5% 4 4 5% 4 6% 8% 40%	1520
	11th to 20th	Day	16% 7% 2 5% 3 3 3 3 3 5% 51%	1910
	21st to end	Day	17% 8% 2 6% 5% 4 4 3 6% 6% 39%	2772
January	1st to 10th	Night	14% 8% 2 5% 3 4 4 3 5% 6% 47%	4164
	11th to 20th	Night	17% 7% 2 6% 3 3 3 3 4 4 47%	4120
	21st to end	Night	20% 9% 2 6% 3 3 3 3 4 4 43%	4532
February	1st to 10th	Night	14% 8% 2 4 4 5% 4 3 6% 7% 43%	2470
	11th to 20th	Night	19% 8% 2 6% 4 3 3 3 4 5% 43%	2356
	21st to end	Night	16% 7% 3 6% 4 3 4 3 4 5% 46%	1865
March	1st to 10th	Night	21% 7% 3 6% 4 4 4 3 5% 6% 39%	3418
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 6% 7% 50%	3480
	21st to end	Night	15% 8% 2 5% 4 4 3 2 4 6% 48%	3800
April	1st to 10th	Night	14% 7% 2 5% 4 4 4 3 4 7% 46%	1900
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 5% 8% 49%	1875
	21st to end	Night	14% 6% 2 6% 4 3 4 3 6% 7% 44%	1900
May	1st to 10th	Night	18% 8% 3 6% 5% 4 4 3 5% 8% 34%	2915
	11th to 20th	Night	22% 8% 3 6% 4 4 5% 3 5% 7% 33%	2792
	21st to end	Night	10% 5% 2 6% 5% 5% 4 4 7% 10% 44%	2732
June	1st to 10th	Night	10% 6% 2 7% 5% 4 6% 4 6% 10% 41%	1520
	11th to 20th	Night	10% 5% 2 5% 5% 6% 4 5% 7% 10% 42%	1520
	21st to end	Night	8% 5% 2 4 5% 5% 4 5% 8% 10% 44%	1520
July	1st to 10th	Night	6% 5% 2 7% 6% 5% 5% 5% 8% 10% 39%	2576
	11th to 20th	Night	9% 5% 2 7% 5% 6% 6% 5% 7% 12% 36%	2840
	21st to end	Night	10% 8% 3 9% 7% 7% 7% 5% 7% 7% 30%	3087
August	1st to 10th	Night	16% 7% 2 8% 4 5% 5% 4 7% 10% 31%	2825
	11th to 20th	Night	11% 7% 2 7% 5% 5% 5% 5% 6% 9% 38%	2928
	21st to end	Night	9% 6% 2 6% 4 4 5% 4 7% 9% 45%	3476
September	1st to 10th	Night	13% 5% 2 6% 4 4 5% 4 6% 8% 44%	1900
	11th to 20th	Night	11% 6% 2 5% 5% 4 4 3 5% 7% 47%	1903
	21st to end	Night	22% 7% 3 6% 4 3 3 3 5% 5% 38%	2194
October	1st to 10th	Night	16% 7% 2 5% 3 3 4 3 4 6% 48%	2280
	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	13% 6% 2 5% 4 3 3 3 4 5% 53%	2289
	11th to 20th	Night	14% 6% 2 5% 4 4 3 3 4 5% 50%	2472
	21st to end	Night	13% 5% 2 5% 3 3 4 4 6% 6% 50%	2660
December	1st to 10th	Night	17% 8% 3 6% 4 4 4 3 5% 6% 42%	2660
	11th to 20th	Night	18% 7% 2 5% 3 4 3 3 5% 6% 45%	3285
	21st to end	Night	15% 8% 3 6% 4 4 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% 24% 14% 5%	2280
May	1st to 10th	Day	48% 9% 12% 19% 10% 2	4045
	11th to 20th	Day	46% 9% 13% 22% 9% 1	4103
	21st to end	Day	30% 9% 15% 24% 19% 2	4837
June	1st to 10th	Day	35% 9% 16% 25% 14% 2	2660
	11th to 20th	Day	35% 10% 15% 22% 17% 1	2660
	21st to end	Day	34% 9% 14% 24% 16% 2	2660
July	1st to 10th	Day	32% 10% 24% 18% 11% 5%	4309
	11th to 20th	Day	36% 9% 22% 24% 7% 1	4051
	21st to end	Day	43% 9% 12% 20% 15% 1	4324
August	1st to 10th	Day	46% 8% 17% 22% 6%	4091
	11th to 20th	Day	39% 8% 15% 24% 13% 1	3906
	21st to end	Day	30% 7% 16% 28% 18% 2	4180
September	1st to 10th	Day	37% 8% 14% 17% 21% 4	2280
	11th to 20th	Day	37% 6% 8% 26% 18% 4	2274
	21st to end	Day	47% 6% 9% 19% 16% 4	1986
October	1st to 10th	Day	36% 4 7% 19% 29% 5%	3160
	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	1887
	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%	1900
May	1st to 10th	Night	46% 8% 15% 19% 11% 2	2944
	11th to 20th	Night	49% 9% 10% 22% 9% 1	2793
	21st to end	Night	33% 9% 16% 25% 15% 1	2739
June	1st to 10th	Night	36% 10% 13% 24% 16% 1	1520
	11th to 20th	Night	35% 11% 17% 21% 15% 1	1520
	21st to end	Night	34% 8% 14% 22% 18% 3	1520
July	1st to 10th	Night	37% 12% 19% 18% 11% 3	2535
	11th to 20th	Night	39% 10% 22% 21% 7% 1	2786
	21st to end	Night	49% 10% 12% 17% 12%	2988
August	1st to 10th	Night	47% 9% 16% 22% 7%	2814
	11th to 20th	Night	40% 9% 14% 24% 13%	2955
	21st to end	Night	34% 8% 11% 28% 18% 1	3476
September	1st to 10th	Night	37% 9% 15% 18% 16% 6%	1900
	11th to 20th	Night	36% 7% 10% 24% 20% 3	1903
	21st to end	Night	47% 5% 10% 18% 16% 3	2194
October	1st to 10th	Night	40% 5% 10% 18% 22% 6%	3800
	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%	2289
	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	23% 16% 33% 20% 7% 1	460
May	1st to 10th	Day	20% 16% 30% 24% 7% 3	463
	11th to 20th	Day	22% 8% 31% 25% 13% 2	487
	21st to end	Day	18% 5% 25% 30% 18% 5%	529
June	1st to 10th	Day	12% 6% 27% 30% 21% 4	490
	11th to 20th	Day	11% 5% 25% 36% 16% 6%	490
	21st to end	Day	10% 6% 25% 33% 21% 4	490
July	1st to 10th	Day	11% 4 17% 34% 27% 8%	485
	11th to 20th	Day	7% 1 17% 40% 26% 9%	484
	21st to end	Day	10% 3 22% 33% 22% 9%	532
August	1st to 10th	Day	13% 3 20% 30% 25% 8%	486
	11th to 20th	Day	12% 3 20% 31% 25% 9%	479
	21st to end	Day	11% 4 17% 31% 24% 14%	539
September	1st to 10th	Day	16% 4 17% 28% 20% 14%	490
	11th to 20th	Day	15% 4 19% 30% 22% 9%	490
	21st to end	Day	30% 8% 22% 25% 11% 4	490
October	1st to 10th	Day	20% 6% 23% 27% 16% 8%	490
	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% 30% 27% 12% 4	466
	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% 35% 29% 6%	760
May	1st to 10th	Night	18% 11% 34% 29% 8%	635
	11th to 20th	Night	17% 5% 33% 31% 13% 1	554
	21st to end	Night	15% 2 22% 37% 23% 1	411
June	1st to 10th	Night	11% 2 23% 42% 21% 1	380
	11th to 20th	Night	11% 5% 19% 42% 21% 2	380
	21st to end	Night	8% 2 24% 46% 19% 1	380
July	1st to 10th	Night	9% 3 18% 39% 27% 4	399
	11th to 20th	Night	6% 1 13% 41% 33% 6%	552
	21st to end	Night	8% 2 18% 44% 25% 3	624
August	1st to 10th	Night	11% 2 12% 44% 29% 3	567
	11th to 20th	Night	14% 3 13% 41% 26% 3	642
	21st to end	Night	11% 3 13% 39% 30% 5%	836
September	1st to 10th	Night	14% 2 15% 37% 28% 4	760
	11th to 20th	Night	14% 4 14% 35% 29% 3	760
	21st to end	Night	27% 3 19% 32% 18% 1	760
October	1st to 10th	Night	19% 4 22% 31% 21% 4	760
	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	760
	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 2024-11-03 02:11:09. Version ac07

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