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 2024-08-30.

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% 4 4 4 4 5% 8% 41%	95714
	Night	14% 8% 3 6% 4 4 4 3 5% 6% 43%	93372
Transparency	Day	40% 6% 12% 25% 15% 2	95467
	Night	41% 5% 10% 24% 16% 3	93226
Seeing	Day	15% 8% 23% 26% 19% 8%	15333
	Night	16% 8% 27% 31% 16% 2	25115

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% 11% 5% 9% 4 4 4 4 5% 6% 36%	2527
	11th to 20th	Day	16% 8% 3 7% 5% 6% 5% 4 5% 6% 36%	2571
	21st to end	Day	25% 17% 3 7% 3 3 3 3 3 5% 27%	3135
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 6% 4 5% 6% 5% 5% 10% 46%	2335
	11th to 20th	Day	9% 4 2 7% 5% 5% 5% 4 7% 10% 43%	2245
	21st to end	Day	6% 5% 2 6% 4 5% 5% 4 6% 9% 49%	2485
September	1st to 10th	Day	11% 6% 2 6% 3 4 4 4 5% 7% 48%	2160
	11th to 20th	Day	7% 5% 2 6% 4 4 5% 4 6% 8% 49%	2066
	21st to end	Day	9% 5% 2 5% 4 3 4 4 4 7% 54%	1980
October	1st to 10th	Day	10% 5% 2 5% 4 4 3 3 4 6% 56%	1918
	11th to 20th	Day	13% 5% 2 6% 3 3 3 2 5% 7% 50%	1800
	21st to end	Day	18% 7% 2 6% 3 4 3 3 3 6% 45%	1877
November	1st to 10th	Day	9% 6% 2 5% 3 4 4 3 4 6% 54%	1672
	11th to 20th	Day	14% 8% 2 5% 4 3 3 3 5% 6% 45%	1710
	21st to end	Day	16% 10% 4 6% 5% 3 4 2 4 5% 41%	1710
December	1st to 10th	Day	15% 12% 4 5% 5% 4 4 3 4 6% 38%	1587
	11th to 20th	Day	15% 12% 3 7% 4 4 6% 4 5% 5% 36%	1918
	21st to end	Day	22% 14% 5% 9% 5% 3 3 3 4 5% 27%	2783
January	1st to 10th	Night	17% 9% 4 8% 4 4 4 4 5% 6% 34%	4428
	11th to 20th	Night	14% 11% 5% 11% 5% 6% 6% 3 4 5% 31%	4395
	21st to end	Night	25% 13% 6% 8% 4 3 3 3 4 6% 27%	4532
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	6% 3 2 4 3 4 5% 4 5% 10% 53%	1823
	11th to 20th	Night	6% 4 2 5% 4 5% 4 4 6% 7% 51%	1876
	21st to end	Night	5% 5% 2 4 3 4 3 4 6% 9% 55%	2080
September	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 5% 7% 52%	1800
	11th to 20th	Night	6% 4 2 5% 4 4 3 4 6% 8% 53%	1876
	21st to end	Night	9% 5% 2 4 4 3 4 3 5% 7% 54%	1980
October	1st to 10th	Night	7% 5% 2 6% 3 3 3 3 5% 7% 57%	2038
	11th to 20th	Night	14% 5% 2 6% 3 3 3 2 3 6% 53%	2160
	21st to end	Night	18% 8% 2 6% 3 3 3 3 3 5% 46%	2476
November	1st to 10th	Night	11% 5% 2 5% 3 2 3 3 5% 5% 55%	2406
	11th to 20th	Night	14% 6% 2 6% 4 3 3 3 4 6% 50%	2470
	21st to end	Night	18% 8% 3 6% 4 3 3 2 4 5% 45%	2470
December	1st to 10th	Night	17% 11% 3 6% 4 3 3 4 5% 6% 36%	2588
	11th to 20th	Night	17% 8% 3 6% 5% 4 3 3 5% 5% 41%	3279
	21st to end	Night	22% 15% 4 9% 5% 5% 4 3 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	48% 5% 11% 27% 9% 1	2528
	11th to 20th	Day	48% 5% 12% 22% 14%	2571
	21st to end	Day	61% 3 7% 18% 9% 1	3135
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	32% 8% 13% 31% 16% 1	2321
	11th to 20th	Day	34% 7% 16% 28% 14% 1	2245
	21st to end	Day	30% 6% 14% 28% 17% 6%	2485
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	49% 5% 11% 24% 10% 1	4428
	11th to 20th	Night	56% 5% 8% 20% 9% 1	4395
	21st to end	Night	60% 4 9% 18% 9% 1	4532
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	26% 8% 17% 28% 20% 2	1813
	11th to 20th	Night	30% 6% 13% 28% 21% 3	1876
	21st to end	Night	25% 6% 14% 28% 21% 6%	2080
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% 19% 39% 18% 5%	297
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%	532
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% 11% 39% 27% 2	1045
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% 42% 8%	624
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

Page generated at 2024-08-27 08:49:37. Version ac07

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