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 2025-01-22.

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% 7% 42%	104961
	Night	14% 7% 3 6% 4 4 4 3 5% 6% 43%	104490
Transparency	Day	39% 6% 11% 25% 17% 2	108591
	Night	40% 5% 10% 24% 18% 3	109001
Seeing	Day	15% 9% 23% 26% 19% 8%	15805
	Night	17% 8% 27% 31% 15% 2	25725

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% 12% 5% 9% 4 4 4 4 5% 6% 35%	2607
	11th to 20th	Day	16% 8% 3 7% 4 6% 5% 4 5% 6% 37%	2653
	21st to end	Day	25% 17% 3 7% 3 3 3 3 3 5% 27%	3144
February	1st to 10th	Day	17% 10% 4 7% 5% 4 4 4 5% 6% 35%	1809
	11th to 20th	Day	15% 10% 3 9% 5% 3 5% 4 4 6% 37%	1848
	21st to end	Day	15% 11% 3 7% 5% 4 4 3 4 8% 37%	1570
March	1st to 10th	Day	20% 12% 3 6% 4 4 3 3 4 5% 36%	3120
	11th to 20th	Day	13% 7% 2 6% 3 3 3 4 5% 7% 47%	3338
	21st to end	Day	14% 8% 2 7% 5% 5% 5% 4 5% 7% 37%	4151
April	1st to 10th	Day	12% 7% 2 6% 4 4 4 4 6% 9% 43%	2280
	11th to 20th	Day	13% 9% 3 6% 5% 4 4 4 4 5% 42%	2254
	21st to end	Day	17% 7% 2 6% 5% 4 5% 4 5% 7% 39%	2280
May	1st to 10th	Day	15% 7% 2 7% 5% 5% 5% 4 7% 10% 33%	4015
	11th to 20th	Day	17% 7% 2 7% 4 4 5% 4 5% 8% 38%	4103
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 6% 8% 43%	4824
June	1st to 10th	Day	8% 5% 2 6% 5% 4 4 4 6% 9% 46%	2660
	11th to 20th	Day	6% 4 1 6% 5% 4 5% 4 7% 8% 50%	2660
	21st to end	Day	6% 5% 1 5% 5% 5% 5% 4 7% 10% 47%	2660
July	1st to 10th	Day	12% 4 2 5% 4 4 5% 5% 6% 9% 44%	4440
	11th to 20th	Day	6% 3 2 5% 4 4 5% 4 7% 12% 47%	4440
	21st to end	Day	7% 7% 3 8% 6% 6% 6% 5% 8% 10% 35%	4783
August	1st to 10th	Day	6% 4 1 5% 3 4 4 4 5% 10% 54%	3890
	11th to 20th	Day	9% 5% 2 7% 4 5% 5% 4 7% 10% 43%	3729
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 9% 54%	4180
September	1st to 10th	Day	11% 6% 2 5% 4 4 4 4 5% 7% 49%	2280
	11th to 20th	Day	7% 5% 2 6% 4 4 5% 4 5% 8% 49%	2180
	21st to end	Day	10% 5% 2 5% 4 3 4 4 4 8% 52%	2090
October	1st to 10th	Day	9% 4 2 5% 4 4 3 3 4 5% 58%	2024
	11th to 20th	Day	13% 5% 2 6% 3 3 3 2 5% 6% 51%	1900
	21st to end	Day	17% 7% 2 6% 3 4 3 3 4 6% 46%	1981
November	1st to 10th	Day	9% 7% 2 5% 4 4 4 3 4 6% 53%	1762
	11th to 20th	Day	14% 9% 2 5% 4 3 4 3 5% 6% 46%	1800
	21st to end	Day	16% 10% 4 6% 5% 3 4 2 5% 5% 40%	1800
December	1st to 10th	Day	10% 10% 3 5% 4 3 4 3 4 6% 49%	2716
	11th to 20th	Day	14% 10% 4 8% 4 4 5% 3 6% 6% 37%	3008
	21st to end	Day	28% 15% 5% 8% 5% 4 3 3 3 4 23%	3982
January	1st to 10th	Night	17% 9% 4 8% 4 5% 4 4 5% 6% 34%	4568
	11th to 20th	Night	14% 11% 5% 11% 5% 5% 6% 3 4 4 32%	4533
	21st to end	Night	25% 13% 6% 8% 4 3 3 3 4 6% 27%	4546
February	1st to 10th	Night	17% 11% 4 8% 5% 4 4 4 4 6% 33%	2368
	11th to 20th	Night	15% 8% 3 8% 4 4 4 3 4 6% 41%	2218
	21st to end	Night	18% 9% 3 9% 4 4 4 4 4 5% 37%	1884
March	1st to 10th	Night	21% 10% 3 6% 4 4 3 2 4 5% 38%	3733
	11th to 20th	Night	12% 5% 3 5% 3 3 3 3 5% 7% 51%	3651
	21st to end	Night	16% 7% 3 7% 5% 5% 4 4 5% 7% 38%	3480
April	1st to 10th	Night	12% 6% 2 6% 6% 3 4 3 4 6% 48%	1900
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 7% 8% 43%	1875
	21st to end	Night	19% 7% 3 7% 5% 4 4 4 4 6% 37%	1900
May	1st to 10th	Night	16% 8% 3 6% 4 4 4 4 6% 7% 38%	2877
	11th to 20th	Night	15% 7% 2 6% 4 3 4 3 4 6% 44%	2786
	21st to end	Night	9% 6% 2 7% 5% 4 4 5% 7% 9% 42%	2743
June	1st to 10th	Night	9% 5% 3 5% 4 4 5% 4 7% 8% 47%	1520
	11th to 20th	Night	4 5% 1 5% 4 3 4 4 7% 9% 54%	1520
	21st to end	Night	5% 3 1 5% 4 4 4 5% 7% 10% 52%	1520
July	1st to 10th	Night	12% 5% 2 4 3 4 4 3 5% 8% 48%	2530
	11th to 20th	Night	5% 3 1 4 3 4 4 4 5% 7% 59%	2530
	21st to end	Night	7% 6% 3 8% 6% 6% 5% 5% 7% 9% 38%	2776
August	1st to 10th	Night	4 3 2 3 3 3 5% 3 5% 10% 59%	3042
	11th to 20th	Night	8% 5% 2 5% 4 4 4 3 5% 7% 52%	3100
	21st to end	Night	4 4 1 4 3 3 3 4 6% 9% 58%	3476
September	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 7% 53%	1900
	11th to 20th	Night	7% 4 2 6% 4 4 3 4 6% 7% 53%	1981
	21st to end	Night	10% 5% 2 5% 4 3 5% 3 5% 7% 51%	2090
October	1st to 10th	Night	6% 4 2 6% 3 3 3 3 5% 6% 59%	2152
	11th to 20th	Night	14% 5% 2 5% 3 3 3 2 3 6% 55%	2280
	21st to end	Night	17% 8% 2 6% 3 3 3 3 4 5% 47%	2614
November	1st to 10th	Night	12% 5% 2 5% 3 2 3 3 4 5% 55%	2536
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 50%	2600
	21st to end	Night	18% 8% 3 6% 4 3 3 2 4 5% 45%	2600
December	1st to 10th	Night	15% 9% 3 7% 3 3 3 3 4 5% 46%	4466
	11th to 20th	Night	17% 8% 3 5% 5% 4 3 3 5% 6% 43%	5199
	21st to end	Night	28% 14% 4 8% 4 4 3 3 4 4 24%	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	49% 5% 11% 26% 9% 1	2608
	11th to 20th	Day	47% 5% 12% 22% 14% 1	2653
	21st to end	Day	61% 3 7% 18% 9% 1	3144
February	1st to 10th	Day	49% 4 11% 23% 11% 2	1809
	11th to 20th	Day	49% 5% 8% 22% 14% 2	1858
	21st to end	Day	47% 5% 9% 21% 14% 4	1570
March	1st to 10th	Day	51% 4 6% 20% 14% 5%	3120
	11th to 20th	Day	36% 5% 12% 27% 15% 4	3338
	21st to end	Day	44% 7% 16% 22% 10% 1	4151
April	1st to 10th	Day	37% 6% 13% 27% 17% 1	2280
	11th to 20th	Day	44% 4 10% 24% 15% 3	2254
	21st to end	Day	46% 5% 10% 25% 11% 2	2280
May	1st to 10th	Day	42% 10% 15% 25% 8%	4052
	11th to 20th	Day	44% 7% 11% 27% 11% 1	4107
	21st to end	Day	36% 8% 11% 22% 22% 2	4837
June	1st to 10th	Day	33% 7% 13% 27% 17% 3	2660
	11th to 20th	Day	29% 7% 13% 27% 21% 3	2660
	21st to end	Day	30% 8% 17% 24% 17% 5%	2660
July	1st to 10th	Day	36% 6% 12% 28% 15% 3	4371
	11th to 20th	Day	28% 7% 14% 33% 16% 2	4365
	21st to end	Day	41% 9% 15% 24% 11% 1	4632
August	1st to 10th	Day	26% 6% 12% 40% 14% 1	3876
	11th to 20th	Day	35% 6% 14% 26% 18% 1	3765
	21st to end	Day	26% 6% 14% 36% 15% 3	4179
September	1st to 10th	Day	35% 5% 10% 27% 19% 4	2280
	11th to 20th	Day	32% 6% 11% 25% 23% 3	2180
	21st to end	Day	32% 6% 8% 26% 25% 4	2090
October	1st to 10th	Day	18% 3 5% 25% 43% 7%	3362
	11th to 20th	Day	24% 3 7% 22% 37% 7%	3140
	21st to end	Day	33% 3 6% 25% 30% 4	3245
November	1st to 10th	Day	33% 4 12% 27% 19% 6%	1762
	11th to 20th	Day	39% 5% 8% 23% 19% 6%	1800
	21st to end	Day	47% 3 9% 25% 15% 2	1800
December	1st to 10th	Day	38% 4 12% 29% 17%	2716
	11th to 20th	Day	47% 3 10% 24% 13% 2	3005
	21st to end	Day	67% 4 10% 13% 6% 1	3982
January	1st to 10th	Night	49% 5% 11% 24% 10% 1	4568
	11th to 20th	Night	55% 5% 7% 20% 10% 2	4533
	21st to end	Night	60% 4 9% 18% 9% 1	4546
February	1st to 10th	Night	53% 4 8% 22% 12% 1	2368
	11th to 20th	Night	45% 4 9% 23% 16% 2	2219
	21st to end	Night	48% 4 10% 20% 13% 3	1884
March	1st to 10th	Night	49% 4 8% 20% 16% 2	3733
	11th to 20th	Night	34% 6% 13% 23% 21% 4	3651
	21st to end	Night	44% 6% 12% 28% 8% 1	3480
April	1st to 10th	Night	37% 4 11% 27% 17% 3	1900
	11th to 20th	Night	39% 5% 10% 24% 20% 3	1875
	21st to end	Night	47% 5% 9% 21% 14% 3	1900
May	1st to 10th	Night	42% 9% 16% 22% 9% 1	2905
	11th to 20th	Night	42% 7% 12% 25% 14% 1	2787
	21st to end	Night	35% 9% 12% 24% 17% 2	2751
June	1st to 10th	Night	31% 7% 13% 23% 22% 3	1520
	11th to 20th	Night	25% 7% 12% 25% 25% 6%	1520
	21st to end	Night	26% 7% 16% 25% 19% 7%	1520
July	1st to 10th	Night	34% 6% 11% 28% 19% 4	2490
	11th to 20th	Night	23% 6% 10% 36% 22% 3	2482
	21st to end	Night	40% 8% 17% 22% 11% 2	2688
August	1st to 10th	Night	20% 7% 17% 36% 19% 1	3032
	11th to 20th	Night	31% 5% 13% 27% 21% 2	3130
	21st to end	Night	23% 5% 13% 35% 20% 3	3476
September	1st to 10th	Night	32% 5% 7% 28% 21% 7%	1900
	11th to 20th	Night	30% 5% 7% 30% 24% 4	1981
	21st to end	Night	32% 5% 8% 21% 26% 7%	2090
October	1st to 10th	Night	16% 3 6% 17% 49% 9%	3594
	11th to 20th	Night	23% 2 7% 23% 39% 6%	3779
	21st to end	Night	31% 3 7% 25% 26% 8%	4300
November	1st to 10th	Night	32% 4 9% 25% 24% 5%	2536
	11th to 20th	Night	36% 4 8% 24% 24% 4	2600
	21st to end	Night	43% 3 10% 25% 18% 2	2600
December	1st to 10th	Night	42% 3 10% 28% 17% 1	4466
	11th to 20th	Night	43% 4 13% 22% 17%	5201
	21st to end	Night	64% 5% 9% 15% 6% 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	23% 16% 30% 23% 7% 1	300
	11th to 20th	Day	28% 16% 33% 18% 4	300
	21st to end	Day	19% 19% 39% 18% 5%	300
February	1st to 10th	Day	26% 14% 39% 18% 3	270
	11th to 20th	Day	24% 19% 35% 16% 4 1	270
	21st to end	Day	27% 17% 35% 18% 3	225
March	1st to 10th	Day	21% 17% 39% 17% 5%	270
	11th to 20th	Day	28% 12% 40% 14% 6%	270
	21st to end	Day	27% 14% 33% 19% 7% 1	492
April	1st to 10th	Day	17% 17% 30% 25% 10% 1	460
	11th to 20th	Day	19% 16% 29% 21% 13% 2	452
	21st to end	Day	22% 11% 32% 23% 9% 2	460
May	1st to 10th	Day	19% 13% 27% 27% 11% 2	463
	11th to 20th	Day	19% 6% 23% 28% 16% 8%	496
	21st to end	Day	10% 5% 25% 32% 19% 8%	734
June	1st to 10th	Day	11% 5% 17% 27% 28% 12%	680
	11th to 20th	Day	7% 3 18% 34% 28% 10%	680
	21st to end	Day	6% 4 24% 32% 25% 9%	680
July	1st to 10th	Day	11% 3 10% 27% 27% 22%	671
	11th to 20th	Day	6% 3 12% 32% 31% 16%	668
	21st to end	Day	8% 2 11% 31% 33% 15%	739
August	1st to 10th	Day	9% 2 10% 28% 34% 16%	553
	11th to 20th	Day	11% 1 14% 29% 32% 13%	479
	21st to end	Day	9% 2 8% 23% 37% 21%	539
September	1st to 10th	Day	12% 2 11% 27% 33% 14%	490
	11th to 20th	Day	8% 3 7% 28% 37% 17%	490
	21st to end	Day	14% 6% 20% 25% 22% 13%	490
October	1st to 10th	Day	10% 4 21% 34% 23% 8%	424
	11th to 20th	Day	14% 13% 32% 25% 12% 4	300
	21st to end	Day	27% 14% 27% 24% 8%	330
November	1st to 10th	Day	13% 10% 33% 23% 15% 6%	300
	11th to 20th	Day	17% 16% 30% 23% 11% 2	300
	21st to end	Day	22% 16% 29% 23% 9% 1	300
December	1st to 10th	Day	16% 17% 38% 18% 7% 4	300
	11th to 20th	Day	24% 13% 33% 24% 6%	300
	21st to end	Day	29% 17% 33% 16% 4 1	330
January	1st to 10th	Night	20% 14% 36% 25% 5%	1000
	11th to 20th	Night	23% 15% 35% 21% 5%	1000
	21st to end	Night	21% 11% 39% 27% 2	1051
February	1st to 10th	Night	22% 13% 37% 26% 1 1	848
	11th to 20th	Night	19% 15% 39% 24% 2	743
	21st to end	Night	21% 15% 38% 24% 2	628
March	1st to 10th	Night	21% 13% 38% 24% 4	760
	11th to 20th	Night	18% 10% 38% 27% 7%	676
	21st to end	Night	22% 8% 33% 29% 8%	627
April	1st to 10th	Night	16% 11% 36% 31% 7%	570
	11th to 20th	Night	18% 9% 30% 31% 11%	563
	21st to end	Night	22% 5% 33% 30% 9% 1	570
May	1st to 10th	Night	18% 5% 29% 37% 11% 1	570
	11th to 20th	Night	16% 5% 21% 37% 19% 2	561
	21st to end	Night	8% 3 20% 40% 24% 4	617
June	1st to 10th	Night	12% 2 16% 41% 28% 2	570
	11th to 20th	Night	6% 1 16% 49% 26% 2	570
	21st to end	Night	5% 2 18% 42% 31% 2	570
July	1st to 10th	Night	9% 2 11% 31% 39% 9%	555
	11th to 20th	Night	5% 2 10% 41% 37% 6%	552
	21st to end	Night	8% 3 9% 39% 35% 6%	624
August	1st to 10th	Night	7% 3 6% 41% 37% 7%	567
	11th to 20th	Night	8% 1 8% 40% 38% 5%	564
	21st to end	Night	6% 2 7% 36% 41% 7%	627
September	1st to 10th	Night	13% 2 10% 34% 35% 6%	570
	11th to 20th	Night	9% 1 12% 34% 39% 6%	651
	21st to end	Night	12% 4 20% 32% 27% 6%	760
October	1st to 10th	Night	8% 4 20% 38% 25% 5%	760
	11th to 20th	Night	16% 6% 31% 27% 18% 2	760
	21st to end	Night	21% 9% 32% 30% 7%	836
November	1st to 10th	Night	14% 6% 33% 35% 12% 1	780
	11th to 20th	Night	16% 12% 32% 30% 10%	800
	21st to end	Night	22% 11% 30% 28% 9%	800
December	1st to 10th	Night	24% 9% 33% 28% 5%	925
	11th to 20th	Night	23% 8% 31% 31% 5% 1	1000
	21st to end	Night	29% 11% 34% 22% 4	1100

Page generated at 2025-01-19 02:06:49. Version ac07

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