Mount Vernon
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Vernon clear sky chart from 2005-11-06 to 2026-05-17.

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	12% 7% 2 6% 4 4 4 4 5% 8% 42%	131824
	Night	14% 7% 3 7% 4 4 4 3 5% 7% 43%	131227
Transparency	Day	39% 5% 11% 25% 17% 2	135411
	Night	40% 5% 11% 24% 18% 3	135717
Seeing	Day	15% 9% 24% 25% 19% 8%	17576
	Night	16% 8% 27% 31% 15% 2	27474

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	13% 10% 4 11% 4 4 5% 4 5% 5% 35%	4691
	11th to 20th	Day	9% 7% 3 7% 5% 5% 4 4 5% 6% 44%	4807
	21st to end	Day	18% 13% 3 7% 4 4 4 3 4 6% 34%	5812
February	1st to 10th	Day	16% 10% 4 7% 5% 4 4 4 5% 6% 35%	2001
	11th to 20th	Day	16% 9% 4 9% 5% 4 5% 3 4 5% 37%	2048
	21st to end	Day	14% 10% 3 7% 5% 4 4 3 4 8% 38%	1730
March	1st to 10th	Day	17% 11% 3 5% 3 3 3 2 3 5% 44%	4580
	11th to 20th	Day	11% 7% 2 5% 3 3 4 3 5% 7% 49%	4873
	21st to end	Day	14% 8% 2 7% 5% 5% 5% 4 5% 7% 38%	6070
April	1st to 10th	Day	13% 7% 2 6% 4 4 4 4 6% 9% 42%	2520
	11th to 20th	Day	13% 9% 3 6% 5% 4 4 4 4 6% 42%	2494
	21st to end	Day	16% 7% 2 6% 5% 4 4 4 5% 7% 39%	2520
May	1st to 10th	Day	17% 8% 3 8% 5% 5% 5% 4 7% 9% 29%	5895
	11th to 20th	Day	16% 7% 2 7% 4 4 5% 4 5% 8% 36%	5975
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 5% 6% 8% 39%	6935
June	1st to 10th	Day	9% 5% 2 6% 5% 4 4 4 6% 9% 46%	2800
	11th to 20th	Day	7% 4 1 6% 5% 4 5% 4 7% 8% 49%	2800
	21st to end	Day	6% 5% 1 5% 5% 5% 5% 4 7% 10% 47%	2800
July	1st to 10th	Day	10% 4 2 6% 4 4 5% 4 6% 9% 47%	6360
	11th to 20th	Day	7% 4 2 7% 5% 5% 6% 5% 8% 12% 40%	6360
	21st to end	Day	6% 6% 3 7% 5% 5% 6% 5% 8% 11% 38%	6880
August	1st to 10th	Day	6% 4 1 5% 3 4 4 4 5% 9% 54%	4013
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 7% 10% 43%	3849
	21st to end	Day	5% 4 2 5% 4 4 4 4 6% 9% 54%	4312
September	1st to 10th	Day	10% 6% 2 5% 3 4 4 4 5% 7% 49%	2400
	11th to 20th	Day	7% 5% 2 6% 4 4 5% 4 5% 8% 50%	2295
	21st to end	Day	11% 5% 2 5% 4 3 4 4 4 7% 52%	2200
October	1st to 10th	Day	9% 4 2 4 4 3 3 3 4 5% 58%	2130
	11th to 20th	Day	13% 5% 2 6% 3 3 3 2 5% 6% 52%	2000
	21st to end	Day	18% 7% 2 5% 3 4 3 3 3 6% 46%	2085
November	1st to 10th	Day	9% 7% 2 5% 4 4 3 3 4 6% 53%	1852
	11th to 20th	Day	14% 9% 2 5% 4 3 3 3 5% 5% 45%	1890
	21st to end	Day	17% 10% 4 7% 5% 3 4 1 4 5% 40%	1890
December	1st to 10th	Day	10% 10% 3 6% 4 3 4 3 4 6% 49%	2799
	11th to 20th	Day	14% 10% 4 8% 4 4 5% 3 6% 6% 37%	3088
	21st to end	Day	28% 15% 5% 8% 5% 4 3 3 3 4 23%	4070
January	1st to 10th	Night	15% 10% 4 9% 4 4 4 3 5% 6% 37%	8232
	11th to 20th	Night	11% 9% 4 9% 4 5% 5% 3 4 5% 40%	8179
	21st to end	Night	18% 9% 4 7% 4 4 4 3 4 6% 37%	8402
February	1st to 10th	Night	17% 11% 4 8% 5% 4 4 4 4 6% 33%	2616
	11th to 20th	Night	16% 8% 2 8% 4 4 3 3 4 6% 40%	2458
	21st to end	Night	17% 8% 3 8% 4 3 4 4 4 6% 39%	2076
March	1st to 10th	Night	18% 11% 3 6% 4 4 4 3 4 5% 37%	5493
	11th to 20th	Night	11% 6% 3 5% 3 3 3 3 4 6% 50%	5337
	21st to end	Night	17% 7% 3 7% 5% 4 4 4 5% 6% 37%	5087
April	1st to 10th	Night	13% 6% 2 6% 5% 3 4 3 5% 6% 47%	2100
	11th to 20th	Night	15% 6% 2 5% 4 4 3 3 7% 7% 43%	2075
	21st to end	Night	19% 6% 3 7% 5% 4 5% 4 4 6% 38%	2100
May	1st to 10th	Night	17% 8% 3 7% 5% 5% 4 4 6% 9% 33%	4217
	11th to 20th	Night	13% 7% 3 7% 4 4 5% 4 6% 8% 40%	4047
	21st to end	Night	13% 6% 2 7% 5% 4 4 4 6% 8% 40%	3942
June	1st to 10th	Night	10% 5% 3 5% 4 4 5% 4 7% 8% 47%	1600
	11th to 20th	Night	5% 5% 1 6% 4 3 4 4 6% 9% 52%	1600
	21st to end	Night	5% 3 1 5% 4 4 4 5% 6% 10% 52%	1600
July	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 5% 8% 49%	3618
	11th to 20th	Night	6% 4 2 6% 4 4 5% 5% 7% 8% 49%	3617
	21st to end	Night	6% 5% 3 7% 5% 5% 5% 5% 8% 12% 40%	3987
August	1st to 10th	Night	4 3 2 4 3 3 5% 3 5% 10% 59%	3139
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 5% 7% 52%	3200
	21st to end	Night	4 4 1 4 3 3 3 4 6% 9% 59%	3586
September	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 5% 7% 52%	2000
	11th to 20th	Night	6% 4 2 5% 4 5% 3 4 6% 7% 54%	2085
	21st to end	Night	10% 5% 2 5% 4 3 5% 3 5% 7% 51%	2200
October	1st to 10th	Night	6% 4 2 5% 3 3 3 3 5% 6% 59%	2265
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 3 6% 55%	2400
	21st to end	Night	18% 8% 2 6% 3 3 3 3 4 5% 47%	2752
November	1st to 10th	Night	11% 5% 2 5% 3 2 3 3 5% 6% 54%	2666
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 5% 6% 50%	2730
	21st to end	Night	18% 8% 3 6% 4 3 3 2 4 5% 44%	2730
December	1st to 10th	Night	15% 9% 3 7% 3 3 3 3 4 5% 45%	4602
	11th to 20th	Night	17% 8% 3 5% 5% 4 3 3 5% 6% 43%	5339
	21st to end	Night	28% 13% 4 8% 4 4 3 3 4 4 24%	7150

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	50% 4 12% 25% 8% 1	4692
	11th to 20th	Day	40% 3 10% 27% 17% 3	4807
	21st to end	Day	52% 3 10% 18% 11% 5%	5812
February	1st to 10th	Day	48% 4 12% 23% 11% 1	2001
	11th to 20th	Day	50% 4 8% 21% 15% 2	2058
	21st to end	Day	46% 5% 9% 22% 15% 3	1730
March	1st to 10th	Day	44% 3 5% 21% 23% 4	4580
	11th to 20th	Day	34% 4 10% 28% 20% 4	4873
	21st to end	Day	44% 6% 14% 25% 11% 1	6072
April	1st to 10th	Day	38% 6% 13% 26% 17% 1	2520
	11th to 20th	Day	44% 4 10% 24% 14% 3	2494
	21st to end	Day	45% 5% 10% 27% 11% 2	2520
May	1st to 10th	Day	48% 9% 12% 20% 11%	5931
	11th to 20th	Day	45% 7% 10% 25% 12% 1	5979
	21st to end	Day	40% 6% 11% 23% 19% 1	6949
June	1st to 10th	Day	33% 7% 13% 28% 17% 3	2800
	11th to 20th	Day	30% 7% 12% 26% 21% 3	2800
	21st to end	Day	30% 8% 17% 24% 17% 5%	2800
July	1st to 10th	Day	34% 6% 11% 29% 18% 3	6291
	11th to 20th	Day	34% 8% 17% 27% 12% 1	6285
	21st to end	Day	37% 8% 18% 26% 10% 1	6684
August	1st to 10th	Day	26% 6% 12% 41% 14% 1	3999
	11th to 20th	Day	34% 6% 15% 26% 18% 1	3885
	21st to end	Day	26% 6% 14% 36% 16% 3	4311
September	1st to 10th	Day	34% 5% 10% 28% 19% 4	2400
	11th to 20th	Day	32% 5% 11% 25% 24% 3	2295
	21st to end	Day	32% 5% 7% 25% 27% 4	2200
October	1st to 10th	Day	18% 3 5% 25% 43% 7%	3468
	11th to 20th	Day	24% 3 8% 22% 37% 7%	3240
	21st to end	Day	33% 3 6% 24% 30% 4	3349
November	1st to 10th	Day	33% 4 12% 27% 19% 5%	1852
	11th to 20th	Day	40% 5% 8% 24% 19% 5%	1890
	21st to end	Day	48% 3 8% 25% 15% 2	1890
December	1st to 10th	Day	38% 4 12% 29% 17%	2799
	11th to 20th	Day	47% 3 10% 24% 13% 2	3085
	21st to end	Day	66% 4 10% 13% 6% 1	4070
January	1st to 10th	Night	49% 4 13% 25% 8% 1	8233
	11th to 20th	Night	46% 4 6% 22% 19% 3	8179
	21st to end	Night	50% 3 10% 19% 17% 1	8402
February	1st to 10th	Night	53% 4 9% 22% 11% 1	2616
	11th to 20th	Night	46% 4 9% 23% 16% 2	2459
	21st to end	Night	46% 4 10% 23% 14% 3	2076
March	1st to 10th	Night	50% 3 7% 19% 19% 2	5493
	11th to 20th	Night	35% 4 10% 23% 22% 5%	5337
	21st to end	Night	46% 6% 11% 26% 10% 1	5087
April	1st to 10th	Night	37% 4 12% 27% 17% 3	2100
	11th to 20th	Night	39% 5% 11% 23% 19% 3	2075
	21st to end	Night	47% 5% 9% 22% 14% 3	2100
May	1st to 10th	Night	46% 8% 15% 21% 9% 1	4245
	11th to 20th	Night	42% 7% 13% 24% 14% 1	4048
	21st to end	Night	40% 8% 11% 24% 15% 3	3950
June	1st to 10th	Night	32% 7% 13% 24% 21% 3	1600
	11th to 20th	Night	27% 7% 12% 25% 24% 6%	1600
	21st to end	Night	26% 7% 16% 26% 18% 7%	1600
July	1st to 10th	Night	34% 6% 9% 25% 23% 3	3580
	11th to 20th	Night	31% 8% 14% 30% 15% 2	3572
	21st to end	Night	34% 8% 20% 25% 12% 1	3872
August	1st to 10th	Night	21% 7% 17% 36% 18% 1	3129
	11th to 20th	Night	31% 5% 13% 28% 21% 2	3230
	21st to end	Night	22% 5% 13% 35% 21% 3	3586
September	1st to 10th	Night	32% 5% 7% 28% 21% 7%	2000
	11th to 20th	Night	29% 5% 8% 30% 25% 4	2085
	21st to end	Night	33% 5% 8% 20% 27% 7%	2200
October	1st to 10th	Night	16% 3 6% 17% 49% 9%	3707
	11th to 20th	Night	23% 2 7% 23% 38% 6%	3899
	21st to end	Night	32% 3 7% 24% 26% 8%	4438
November	1st to 10th	Night	32% 4 9% 26% 24% 5%	2666
	11th to 20th	Night	36% 4 8% 25% 23% 4	2730
	21st to end	Night	44% 3 9% 25% 17% 1	2730
December	1st to 10th	Night	43% 3 10% 27% 16% 1	4602
	11th to 20th	Night	43% 4 13% 23% 17%	5341
	21st to end	Night	64% 5% 9% 15% 6% 1	7150

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	22% 16% 32% 23% 7% 1	330
	11th to 20th	Day	27% 18% 35% 17% 4	330
	21st to end	Day	19% 19% 41% 17% 4	363
February	1st to 10th	Day	24% 13% 41% 19% 3	330
	11th to 20th	Day	26% 19% 37% 14% 3 1	330
	21st to end	Day	26% 17% 36% 17% 3	273
March	1st to 10th	Day	21% 19% 36% 18% 5%	330
	11th to 20th	Day	28% 14% 40% 12% 5% 1	330
	21st to end	Day	25% 16% 34% 19% 6% 1	578
April	1st to 10th	Day	18% 17% 30% 23% 9% 1	540
	11th to 20th	Day	19% 17% 30% 21% 12% 2	532
	21st to end	Day	21% 11% 32% 25% 9% 2	540
May	1st to 10th	Day	19% 14% 29% 27% 10% 2	543
	11th to 20th	Day	19% 8% 23% 28% 15% 7%	561
	21st to end	Day	11% 6% 26% 32% 18% 8%	789
June	1st to 10th	Day	11% 5% 18% 27% 27% 12%	730
	11th to 20th	Day	8% 3 17% 34% 28% 9%	730
	21st to end	Day	5% 4 23% 32% 26% 9%	730
July	1st to 10th	Day	10% 2 11% 27% 28% 22%	721
	11th to 20th	Day	6% 3 12% 32% 31% 15%	718
	21st to end	Day	8% 2 10% 30% 35% 14%	794
August	1st to 10th	Day	9% 3 10% 28% 34% 16%	596
	11th to 20th	Day	10% 1 13% 28% 33% 14%	519
	21st to end	Day	8% 2 9% 24% 37% 19%	583
September	1st to 10th	Day	12% 2 12% 29% 32% 13%	530
	11th to 20th	Day	8% 3 7% 28% 37% 18%	530
	21st to end	Day	15% 6% 19% 25% 22% 13%	530
October	1st to 10th	Day	10% 3 20% 35% 25% 8%	460
	11th to 20th	Day	14% 13% 30% 27% 12% 4	330
	21st to end	Day	28% 14% 27% 22% 8%	363
November	1st to 10th	Day	13% 11% 34% 24% 13% 5%	330
	11th to 20th	Day	18% 16% 31% 23% 10% 2	330
	21st to end	Day	24% 16% 27% 23% 9% 1	330
December	1st to 10th	Day	16% 16% 39% 19% 6% 4	330
	11th to 20th	Day	23% 14% 35% 23% 5%	330
	21st to end	Day	28% 17% 34% 16% 4 1	363
January	1st to 10th	Night	20% 14% 36% 26% 4	1050
	11th to 20th	Night	23% 16% 35% 22% 5%	1050
	21st to end	Night	20% 12% 40% 27% 2	1155
February	1st to 10th	Night	22% 12% 39% 25% 1 1	936
	11th to 20th	Night	21% 16% 37% 24% 2	823
	21st to end	Night	20% 15% 38% 24% 2	692
March	1st to 10th	Night	21% 14% 37% 25% 4	840
	11th to 20th	Night	18% 11% 38% 26% 6%	747
	21st to end	Night	22% 9% 33% 29% 7%	693
April	1st to 10th	Night	16% 11% 36% 30% 6%	630
	11th to 20th	Night	18% 10% 31% 29% 10%	623
	21st to end	Night	22% 5% 33% 30% 9% 1	630
May	1st to 10th	Night	17% 6% 29% 37% 10% 1	630
	11th to 20th	Night	15% 6% 22% 37% 18% 2	609
	21st to end	Night	9% 3 21% 40% 23% 4	650
June	1st to 10th	Night	12% 2 16% 41% 27% 2	600
	11th to 20th	Night	6% 2 17% 49% 25% 2	600
	21st to end	Night	5% 2 18% 41% 31% 3	600
July	1st to 10th	Night	9% 2 10% 31% 39% 9%	585
	11th to 20th	Night	5% 3 10% 41% 37% 6%	582
	21st to end	Night	8% 3 8% 39% 36% 6%	657
August	1st to 10th	Night	7% 3 6% 41% 37% 6%	597
	11th to 20th	Night	7% 1 8% 39% 40% 5%	594
	21st to end	Night	6% 2 7% 37% 40% 7%	660
September	1st to 10th	Night	13% 2 10% 35% 34% 6%	600
	11th to 20th	Night	8% 1 11% 34% 39% 6%	685
	21st to end	Night	13% 4 19% 32% 27% 6%	800
October	1st to 10th	Night	8% 4 20% 39% 25% 5%	800
	11th to 20th	Night	16% 6% 30% 28% 19% 2	800
	21st to end	Night	21% 10% 33% 29% 7%	880
November	1st to 10th	Night	14% 6% 33% 35% 12%	820
	11th to 20th	Night	16% 12% 33% 30% 10%	840
	21st to end	Night	22% 11% 30% 28% 9%	840
December	1st to 10th	Night	24% 9% 34% 28% 5%	971
	11th to 20th	Night	23% 9% 32% 31% 5% 1	1050
	21st to end	Night	29% 11% 35% 22% 4	1155

Page generated at 2026-05-14 00:58:54. Version ac07

Copyright © 2026 Attilla Danko.