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 2024-01-27.

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% 7% 3 6% 5% 3 5% 5% 5% 8% 42%	74907
	Night	14% 7% 3 5% 5% 3 4 4 5% 7% 44%	75152
Transparency	Day	40% 6% 12% 25% 15% 3	74609
	Night	41% 5% 10% 24% 17% 3	74971
Seeing	Day	15% 8% 23% 26% 20% 8%	14446
	Night	17% 8% 27% 31% 16% 2	24410

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	15% 11% 3 9% 5% 2 4 4 4 6% 36%	1517
	11th to 20th	Day	19% 9% 4 6% 4 4 4 5% 5% 6% 34%	1548
	21st to end	Day	14% 10% 5% 8% 6% 3 4 4 4 6% 36%	1836
February	1st to 10th	Day	18% 9% 5% 6% 5% 3 4 4 5% 6% 35%	1714
	11th to 20th	Day	16% 9% 4 8% 5% 3 5% 5% 4 6% 35%	1748
	21st to end	Day	15% 10% 3 7% 5% 3 5% 4 4 8% 36%	1480
March	1st to 10th	Day	15% 8% 3 5% 4 3 5% 5% 5% 6% 42%	1800
	11th to 20th	Day	20% 9% 4 7% 5% 3 4 4 4 6% 36%	1894
	21st to end	Day	20% 9% 4 7% 6% 4 5% 5% 4 6% 30%	2362
April	1st to 10th	Day	11% 6% 2 5% 5% 3 4 5% 6% 9% 44%	2160
	11th to 20th	Day	14% 8% 3 5% 6% 3 4 4 4 5% 41%	2134
	21st to end	Day	18% 7% 3 6% 5% 3 5% 4 4 7% 39%	2160
May	1st to 10th	Day	17% 6% 2 6% 6% 3 4 5% 5% 8% 37%	2300
	11th to 20th	Day	16% 7% 2 6% 5% 3 5% 5% 5% 7% 39%	2309
	21st to end	Day	8% 6% 3 6% 6% 4 6% 7% 7% 9% 37%	2725
June	1st to 10th	Day	8% 4 2 6% 5% 3 4 5% 6% 10% 47%	2520
	11th to 20th	Day	6% 3 2 5% 5% 3 5% 5% 7% 9% 50%	2520
	21st to end	Day	6% 4 2 4 5% 4 6% 6% 7% 11% 47%	2520
July	1st to 10th	Day	10% 4 2 4 5% 4 7% 6% 6% 11% 42%	2520
	11th to 20th	Day	6% 3 2 4 5% 3 5% 6% 8% 10% 48%	2520
	21st to end	Day	7% 4 2 4 6% 4 6% 6% 7% 10% 46%	2726
August	1st to 10th	Day	7% 4 2 5% 6% 4 6% 6% 6% 11% 45%	2212
	11th to 20th	Day	9% 4 2 6% 5% 3 5% 5% 7% 10% 43%	2125
	21st to end	Day	6% 5% 2 5% 5% 4 5% 5% 6% 9% 49%	2376
September	1st to 10th	Day	11% 6% 2 5% 4 4 4 5% 5% 7% 48%	2160
	11th to 20th	Day	7% 5% 3 5% 5% 3 5% 5% 6% 8% 49%	2066
	21st to end	Day	9% 4 2 4 4 2 4 5% 4 7% 54%	1980
October	1st to 10th	Day	10% 4 2 4 4 3 3 3 4 6% 56%	1918
	11th to 20th	Day	14% 5% 3 5% 4 2 3 3 5% 7% 50%	1800
	21st to end	Day	18% 7% 2 5% 4 3 3 4 3 6% 45%	1877
November	1st to 10th	Day	9% 6% 2 4 4 3 4 4 4 6% 54%	1672
	11th to 20th	Day	15% 8% 2 5% 4 2 3 4 5% 6% 45%	1710
	21st to end	Day	16% 10% 4 6% 6% 3 3 3 4 6% 41%	1710
December	1st to 10th	Day	15% 11% 5% 5% 5% 3 5% 4 4 6% 38%	1587
	11th to 20th	Day	15% 11% 4 6% 4 3 5% 5% 5% 5% 36%	1918
	21st to end	Day	23% 14% 5% 8% 6% 3 3 3 3 5% 27%	2783
January	1st to 10th	Night	16% 8% 3 7% 5% 3 4 4 6% 7% 38%	2648
	11th to 20th	Night	17% 10% 5% 8% 5% 3 4 4 4 5% 35%	2628
	21st to end	Night	16% 9% 4 7% 6% 3 4 4 5% 7% 35%	2665
February	1st to 10th	Night	18% 11% 5% 8% 5% 3 5% 4 4 6% 32%	2243
	11th to 20th	Night	16% 7% 3 6% 6% 3 4 4 4 6% 40%	2098
	21st to end	Night	19% 9% 3 8% 5% 3 4 5% 4 6% 36%	1776
March	1st to 10th	Night	16% 8% 3 5% 5% 3 4 3 3 5% 43%	2160
	11th to 20th	Night	17% 6% 4 6% 4 3 4 4 5% 7% 39%	2081
	21st to end	Night	19% 8% 3 7% 5% 3 4 5% 6% 6% 34%	1994
April	1st to 10th	Night	12% 5% 2 5% 6% 2 4 4 4 6% 49%	1800
	11th to 20th	Night	16% 5% 3 4 4 2 4 4 6% 8% 43%	1775
	21st to end	Night	20% 6% 2 6% 6% 3 4 5% 4 6% 38%	1800
May	1st to 10th	Night	16% 5% 3 4 5% 4 5% 5% 5% 6% 42%	1657
	11th to 20th	Night	12% 8% 3 6% 6% 3 5% 4 4 6% 44%	1579
	21st to end	Night	6% 4 3 6% 7% 3 4 6% 8% 11% 40%	1551
June	1st to 10th	Night	9% 5% 3 4 4 3 5% 5% 6% 8% 48%	1440
	11th to 20th	Night	4 4 2 5% 4 2 5% 5% 6% 9% 54%	1440
	21st to end	Night	5% 3 1 4 4 3 5% 5% 7% 11% 52%	1440
July	1st to 10th	Night	7% 4 2 4 4 3 5% 4 7% 10% 50%	1440
	11th to 20th	Night	5% 3 1 5% 4 3 5% 5% 6% 9% 55%	1440
	21st to end	Night	7% 3 2 5% 4 5% 4 5% 5% 9% 50%	1588
August	1st to 10th	Night	6% 3 2 3 4 3 5% 5% 5% 11% 53%	1726
	11th to 20th	Night	6% 4 2 4 5% 3 5% 4 6% 7% 52%	1776
	21st to end	Night	5% 4 2 4 4 3 3 4 6% 9% 56%	1980
September	1st to 10th	Night	11% 5% 2 4 4 2 3 4 5% 7% 52%	1800
	11th to 20th	Night	6% 4 3 5% 4 3 4 4 6% 8% 53%	1876
	21st to end	Night	9% 5% 2 4 4 3 4 4 5% 7% 54%	1980
October	1st to 10th	Night	7% 5% 2 5% 4 2 3 4 5% 7% 57%	2038
	11th to 20th	Night	14% 5% 2 5% 3 2 3 3 3 6% 53%	2160
	21st to end	Night	18% 8% 2 5% 4 2 3 3 4 5% 46%	2476
November	1st to 10th	Night	11% 5% 2 4 3 2 3 4 4 5% 55%	2406
	11th to 20th	Night	14% 5% 2 5% 4 2 3 3 4 6% 50%	2470
	21st to end	Night	18% 7% 3 5% 5% 2 3 2 3 5% 46%	2470
December	1st to 10th	Night	18% 10% 3 5% 5% 2 4 5% 5% 6% 36%	2588
	11th to 20th	Night	17% 8% 4 5% 6% 4 3 4 5% 5% 41%	3279
	21st to end	Night	22% 15% 4 8% 6% 4 4 4 4 5% 25%	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	50% 4 10% 22% 12% 2	1518
	11th to 20th	Day	51% 5% 10% 24% 10%	1548
	21st to end	Day	50% 4 10% 24% 9% 1	1836
February	1st to 10th	Day	50% 4 11% 23% 11% 1	1714
	11th to 20th	Day	51% 5% 8% 21% 13% 2	1758
	21st to end	Day	48% 5% 9% 21% 13% 3	1480
March	1st to 10th	Day	44% 5% 10% 24% 16% 2	1800
	11th to 20th	Day	51% 4 12% 20% 11% 2	1894
	21st to end	Day	55% 5% 11% 18% 9% 1	2362
April	1st to 10th	Day	35% 6% 13% 28% 17% 1	2160
	11th to 20th	Day	45% 4 9% 24% 15% 3	2134
	21st to end	Day	46% 5% 10% 25% 11% 2	2160
May	1st to 10th	Day	44% 7% 14% 24% 9% 1	2300
	11th to 20th	Day	44% 7% 13% 20% 15% 1	2313
	21st to end	Day	39% 8% 16% 22% 11% 3	2725
June	1st to 10th	Day	32% 7% 13% 28% 17% 2	2520
	11th to 20th	Day	29% 8% 13% 27% 21% 3	2520
	21st to end	Day	30% 8% 17% 23% 17% 5%	2520
July	1st to 10th	Day	34% 8% 15% 26% 13% 5%	2451
	11th to 20th	Day	28% 7% 16% 28% 17% 3	2445
	21st to end	Day	31% 8% 13% 29% 16% 2	2574
August	1st to 10th	Day	32% 8% 13% 30% 15% 1	2198
	11th to 20th	Day	34% 7% 16% 28% 13% 1	2125
	21st to end	Day	30% 6% 14% 27% 17% 6%	2376
September	1st to 10th	Day	36% 5% 10% 28% 18% 4	2160
	11th to 20th	Day	32% 6% 11% 26% 23% 3	2066
	21st to end	Day	30% 6% 7% 27% 26% 4	1980
October	1st to 10th	Day	30% 4 9% 28% 25% 4	1918
	11th to 20th	Day	34% 5% 9% 25% 22% 5%	1800
	21st to end	Day	42% 4 7% 26% 18% 3	1877
November	1st to 10th	Day	32% 4 12% 27% 19% 6%	1672
	11th to 20th	Day	39% 5% 8% 24% 19% 5%	1710
	21st to end	Day	47% 3 9% 24% 15% 2	1710
December	1st to 10th	Day	48% 5% 10% 23% 13% 1	1587
	11th to 20th	Day	48% 4 10% 25% 12% 2	1915
	21st to end	Day	61% 5% 10% 15% 9% 1	2783
January	1st to 10th	Night	46% 5% 9% 24% 14% 2	2648
	11th to 20th	Night	52% 5% 8% 22% 11% 1	2628
	21st to end	Night	49% 4 12% 22% 12% 1	2665
February	1st to 10th	Night	55% 4 8% 21% 11% 1	2243
	11th to 20th	Night	46% 4 9% 23% 15% 2	2099
	21st to end	Night	49% 4 10% 21% 12% 3	1776
March	1st to 10th	Night	45% 4 8% 21% 19% 3	2160
	11th to 20th	Night	46% 5% 12% 20% 13% 3	2081
	21st to end	Night	50% 5% 12% 21% 9% 2	1994
April	1st to 10th	Night	37% 5% 11% 27% 18% 3	1800
	11th to 20th	Night	39% 5% 10% 23% 19% 3	1775
	21st to end	Night	47% 5% 9% 22% 14% 3	1800
May	1st to 10th	Night	42% 7% 10% 27% 13% 2	1657
	11th to 20th	Night	43% 6% 11% 22% 16% 2	1580
	21st to end	Night	35% 10% 15% 21% 16% 4	1552
June	1st to 10th	Night	31% 7% 13% 24% 23% 2	1440
	11th to 20th	Night	25% 7% 13% 26% 24% 6%	1440
	21st to end	Night	25% 7% 16% 26% 18% 7%	1440
July	1st to 10th	Night	30% 7% 16% 27% 15% 5%	1400
	11th to 20th	Night	25% 8% 13% 30% 20% 5%	1392
	21st to end	Night	29% 7% 14% 29% 19% 3	1500
August	1st to 10th	Night	26% 8% 17% 27% 20% 2	1716
	11th to 20th	Night	30% 7% 13% 28% 20% 3	1776
	21st to end	Night	26% 6% 14% 27% 21% 6%	1980
September	1st to 10th	Night	32% 5% 7% 29% 19% 7%	1800
	11th to 20th	Night	29% 5% 8% 31% 23% 4	1876
	21st to end	Night	30% 5% 8% 22% 27% 7%	1980
October	1st to 10th	Night	27% 5% 10% 23% 29% 6%	2038
	11th to 20th	Night	34% 3 7% 25% 24% 7%	2160
	21st to end	Night	42% 4 8% 24% 18% 5%	2476
November	1st to 10th	Night	31% 5% 10% 25% 24% 6%	2406
	11th to 20th	Night	36% 4 8% 25% 24% 4	2470
	21st to end	Night	43% 3 9% 24% 19% 2	2470
December	1st to 10th	Night	48% 4 12% 23% 13% 1	2588
	11th to 20th	Night	45% 4 10% 24% 16% 1	3281
	21st to end	Night	62% 6% 8% 15% 8% 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	24% 14% 30% 23% 8% 1	270
	11th to 20th	Day	31% 14% 32% 18% 5%	270
	21st to end	Day	20% 20% 39% 17% 4	282
February	1st to 10th	Day	28% 15% 40% 15% 2	240
	11th to 20th	Day	26% 20% 35% 16% 3	240
	21st to end	Day	27% 16% 35% 19% 3 1	198
March	1st to 10th	Day	21% 18% 40% 16% 5%	240
	11th to 20th	Day	30% 11% 38% 14% 6%	240
	21st to end	Day	28% 13% 31% 19% 8% 1	448
April	1st to 10th	Day	15% 16% 31% 25% 11% 1	420
	11th to 20th	Day	18% 16% 28% 22% 14% 2	412
	21st to end	Day	23% 10% 31% 24% 9% 3	420
May	1st to 10th	Day	20% 13% 28% 27% 11% 2	423
	11th to 20th	Day	19% 6% 23% 28% 16% 7%	455
	21st to end	Day	10% 4 23% 33% 20% 9%	679
June	1st to 10th	Day	10% 5% 15% 27% 29% 13%	630
	11th to 20th	Day	7% 3 18% 34% 28% 10%	630
	21st to end	Day	5% 4 24% 31% 25% 10%	630
July	1st to 10th	Day	10% 2 9% 26% 29% 24%	621
	11th to 20th	Day	6% 3 12% 31% 32% 16%	618
	21st to end	Day	8% 2 10% 31% 33% 15%	684
August	1st to 10th	Day	10% 2 9% 28% 35% 16%	510
	11th to 20th	Day	11% 1 11% 29% 33% 14%	439
	21st to end	Day	9% 2 8% 24% 36% 21%	495
September	1st to 10th	Day	13% 2 11% 26% 34% 13%	450
	11th to 20th	Day	8% 3 7% 28% 36% 17%	450
	21st to end	Day	12% 6% 20% 26% 22% 14%	450
October	1st to 10th	Day	11% 4 22% 36% 21% 6%	388
	11th to 20th	Day	15% 13% 34% 25% 11% 2	270
	21st to end	Day	28% 13% 27% 23% 9%	297
November	1st to 10th	Day	12% 9% 33% 24% 16% 7%	270
	11th to 20th	Day	17% 16% 28% 26% 11% 2	270
	21st to end	Day	20% 15% 30% 24% 9% 1	270
December	1st to 10th	Day	16% 16% 39% 18% 7% 5%	270
	11th to 20th	Day	24% 13% 33% 25% 6%	270
	21st to end	Day	26% 18% 35% 16% 4 1	297
January	1st to 10th	Night	20% 14% 36% 25% 5%	950
	11th to 20th	Night	23% 15% 35% 22% 5%	950
	21st to end	Night	21% 12% 38% 28% 2	1025
February	1st to 10th	Night	23% 13% 37% 25% 1 1	803
	11th to 20th	Night	20% 15% 39% 24% 2	703
	21st to end	Night	22% 15% 37% 24% 3	592
March	1st to 10th	Night	20% 13% 38% 24% 4	720
	11th to 20th	Night	19% 10% 37% 27% 7%	641
	21st to end	Night	23% 7% 32% 30% 8%	594
April	1st to 10th	Night	15% 10% 36% 32% 7%	540
	11th to 20th	Night	18% 9% 30% 31% 12%	533
	21st to end	Night	23% 5% 31% 31% 9% 1	540
May	1st to 10th	Night	18% 5% 28% 37% 11% 1	540
	11th to 20th	Night	16% 5% 21% 37% 18% 2	531
	21st to end	Night	9% 3 20% 40% 25% 4	584
June	1st to 10th	Night	11% 2 15% 40% 29% 2	540
	11th to 20th	Night	6% 1 16% 50% 26% 2	540
	21st to end	Night	5% 2 18% 42% 31% 3	540
July	1st to 10th	Night	8% 2 9% 31% 40% 9%	525
	11th to 20th	Night	5% 2 10% 39% 38% 6%	522
	21st to end	Night	8% 3 9% 38% 35% 6%	591
August	1st to 10th	Night	7% 3 5% 40% 37% 7%	537
	11th to 20th	Night	8% 1 7% 39% 40% 5%	534
	21st to end	Night	6% 2 7% 35% 41% 8%	594
September	1st to 10th	Night	14% 2 9% 34% 35% 6%	540
	11th to 20th	Night	9% 1 12% 34% 38% 6%	616
	21st to end	Night	12% 3 20% 31% 28% 6%	720
October	1st to 10th	Night	9% 4 21% 37% 24% 5%	720
	11th to 20th	Night	17% 6% 31% 28% 16% 2	720
	21st to end	Night	22% 10% 32% 29% 7%	792
November	1st to 10th	Night	14% 6% 32% 35% 13% 1	740
	11th to 20th	Night	16% 12% 31% 30% 10%	760
	21st to end	Night	22% 10% 30% 28% 10%	760
December	1st to 10th	Night	24% 9% 33% 29% 5%	878
	11th to 20th	Night	23% 8% 31% 32% 6% 1	950
	21st to end	Night	28% 11% 35% 23% 4	1045

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