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 2024-03-18.

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	23% 9% 3 6% 5% 3 4 4 5% 6% 30%	78357
	Night	27% 10% 3 6% 5% 3 4 4 4 5% 28%	81370
Transparency	Day	58% 5% 13% 17% 7% 1	78071
	Night	60% 5% 12% 15% 7% 1	81236
Seeing	Day	24% 10% 20% 26% 17% 4	14243
	Night	30% 9% 14% 24% 19% 3	22791

By Month

Percentage Cloud Cover NW (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	37% 16% 4 6% 5% 4 4 4 4 4 12%	2219
	11th to 20th	Day	27% 13% 3 7% 5% 3 4 4 4 6% 24%	2368
	21st to end	Day	41% 14% 4 7% 6% 3 4 4 4 4 10%	2827
February	1st to 10th	Day	32% 12% 3 7% 6% 3 5% 4 5% 6% 17%	1710
	11th to 20th	Day	29% 12% 5% 8% 6% 3 4 4 4 5% 20%	1665
	21st to end	Day	28% 15% 3 7% 6% 3 4 5% 5% 5% 18%	1464
March	1st to 10th	Day	25% 14% 5% 7% 7% 4 5% 4 4 7% 19%	1938
	11th to 20th	Day	29% 11% 4 7% 6% 3 4 4 4 4 23%	2059
	21st to end	Day	28% 12% 4 8% 7% 3 5% 5% 5% 6% 17%	2252
April	1st to 10th	Day	26% 12% 4 8% 7% 4 4 5% 6% 7% 17%	2160
	11th to 20th	Day	23% 10% 4 7% 7% 4 6% 5% 5% 6% 22%	2171
	21st to end	Day	24% 11% 4 7% 7% 4 5% 4 5% 7% 22%	2398
May	1st to 10th	Day	19% 8% 4 6% 6% 3 6% 5% 5% 7% 31%	2520
	11th to 20th	Day	20% 7% 3 6% 6% 4 4 5% 6% 8% 31%	2477
	21st to end	Day	24% 9% 4 7% 6% 4 4 5% 4 6% 30%	2723
June	1st to 10th	Day	23% 7% 3 6% 5% 4 5% 5% 5% 7% 30%	2640
	11th to 20th	Day	24% 9% 4 7% 6% 3 5% 5% 6% 7% 24%	2700
	21st to end	Day	14% 6% 3 5% 5% 3 5% 5% 7% 7% 39%	2700
July	1st to 10th	Day	11% 6% 3 4 4 3 5% 4 5% 7% 49%	2700
	11th to 20th	Day	11% 5% 3 5% 3 2 4 4 5% 6% 53%	2656
	21st to end	Day	9% 4 2 3 3 1 2 3 4 5% 64%	2741
August	1st to 10th	Day	10% 5% 2 4 3 2 3 3 4 6% 56%	2495
	11th to 20th	Day	8% 4 1 3 4 2 3 4 5% 8% 57%	2298
	21st to end	Day	13% 6% 3 5% 4 3 4 4 5% 8% 45%	2481
September	1st to 10th	Day	12% 6% 2 5% 4 2 3 4 5% 7% 49%	2160
	11th to 20th	Day	16% 7% 3 6% 5% 3 4 4 5% 7% 41%	2160
	21st to end	Day	20% 8% 2 5% 4 3 4 3 5% 7% 38%	1904
October	1st to 10th	Day	22% 8% 3 6% 5% 4 4 5% 6% 7% 32%	1800
	11th to 20th	Day	27% 9% 3 6% 6% 3 4 5% 5% 7% 26%	1800
	21st to end	Day	31% 8% 4 6% 4 3 4 5% 5% 5% 24%	1857
November	1st to 10th	Day	36% 10% 3 7% 6% 3 4 4 5% 5% 18%	1505
	11th to 20th	Day	34% 9% 4 6% 5% 3 5% 3 4 5% 21%	1520
	21st to end	Day	36% 9% 3 7% 5% 3 4 4 4 6% 20%	1520
December	1st to 10th	Day	29% 12% 3 6% 6% 2 4 4 4 4 25%	1520
	11th to 20th	Day	32% 14% 3 9% 5% 2 3 5% 4 4 19%	1807
	21st to end	Day	29% 15% 5% 8% 7% 4 5% 5% 5% 5% 13%	2442
January	1st to 10th	Night	37% 14% 4 7% 7% 4 4 4 3 3 12%	4726
	11th to 20th	Night	30% 11% 4 7% 4 2 4 4 4 6% 24%	4527
	21st to end	Night	40% 13% 4 7% 6% 3 4 3 3 4 14%	4827
February	1st to 10th	Night	34% 12% 3 6% 5% 3 4 4 4 5% 20%	2470
	11th to 20th	Night	30% 13% 4 6% 5% 3 4 3 4 5% 23%	2402
	21st to end	Night	31% 12% 3 7% 5% 3 4 4 4 5% 21%	1989
March	1st to 10th	Night	29% 11% 3 7% 5% 2 5% 5% 5% 5% 23%	2239
	11th to 20th	Night	31% 11% 3 5% 5% 4 4 4 4 6% 22%	2066
	21st to end	Night	33% 12% 3 8% 5% 4 4 4 5% 6% 17%	2103
April	1st to 10th	Night	26% 13% 4 7% 6% 4 5% 6% 5% 6% 18%	1800
	11th to 20th	Night	25% 10% 4 7% 7% 4 5% 4 4 5% 25%	1736
	21st to end	Night	29% 10% 4 8% 5% 3 3 4 4 6% 23%	1561
May	1st to 10th	Night	20% 5% 3 6% 5% 2 4 4 5% 7% 39%	1440
	11th to 20th	Night	18% 8% 2 5% 5% 3 4 6% 7% 8% 35%	1416
	21st to end	Night	21% 8% 4 7% 5% 3 4 5% 5% 7% 31%	1552
June	1st to 10th	Night	23% 6% 3 7% 6% 3 5% 4 5% 5% 33%	1316
	11th to 20th	Night	24% 8% 3 7% 6% 2 4 6% 4 7% 29%	1260
	21st to end	Night	15% 5% 2 5% 4 3 5% 5% 6% 7% 43%	1260
July	1st to 10th	Night	12% 5% 2 4 4 3 4 4 3 6% 53%	1260
	11th to 20th	Night	11% 4 2 3 5% 2 3 2 5% 6% 58%	1304
	21st to end	Night	8% 3 1 2 3 2 3 3 4 6% 66%	1576
August	1st to 10th	Night	10% 3 2 3 2 2 3 5% 5% 4 61%	1441
	11th to 20th	Night	8% 3 1 2 2 2 3 3 4 6% 65%	1602
	21st to end	Night	10% 4 2 4 5% 3 4 4 5% 7% 51%	1874
September	1st to 10th	Night	11% 5% 2 4 3 3 3 4 4 6% 55%	1800
	11th to 20th	Night	18% 7% 3 5% 5% 3 4 4 4 6% 43%	1800
	21st to end	Night	19% 8% 2 5% 4 3 3 4 5% 6% 41%	2055
October	1st to 10th	Night	20% 7% 3 5% 6% 4 4 3 4 5% 38%	2160
	11th to 20th	Night	28% 9% 2 5% 5% 2 4 4 4 5% 32%	2160
	21st to end	Night	29% 8% 3 6% 5% 3 4 4 4 5% 29%	2498
November	1st to 10th	Night	38% 10% 3 5% 6% 3 4 4 4 5% 19%	2572
	11th to 20th	Night	36% 9% 4 6% 5% 2 4 4 4 4 22%	2660
	21st to end	Night	35% 10% 4 6% 4 3 4 5% 4 5% 20%	2660
December	1st to 10th	Night	31% 9% 3 6% 5% 3 4 4 4 5% 26%	2660
	11th to 20th	Night	34% 13% 4 7% 5% 3 4 4 4 5% 17%	3384
	21st to end	Night	32% 15% 5% 8% 7% 3 4 4 4 5% 13%	5214

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	79% 5% 6% 7% 2 1	2219
	11th to 20th	Day	64% 5% 9% 13% 7% 3	2391
	21st to end	Day	80% 3 7% 7% 3	2827
February	1st to 10th	Day	71% 5% 8% 9% 5% 2	1710
	11th to 20th	Day	71% 3 8% 10% 7% 1	1674
	21st to end	Day	70% 4 8% 13% 3 1	1464
March	1st to 10th	Day	68% 5% 9% 13% 4 1	1938
	11th to 20th	Day	69% 3 10% 13% 4 1	2059
	21st to end	Day	73% 6% 10% 11% 1	2252
April	1st to 10th	Day	70% 6% 14% 9% 1	2160
	11th to 20th	Day	67% 4 9% 13% 6% 1	2171
	21st to end	Day	66% 5% 11% 12% 5% 1	2398
May	1st to 10th	Day	57% 6% 14% 17% 6%	2520
	11th to 20th	Day	55% 5% 20% 15% 5%	2480
	21st to end	Day	62% 5% 15% 16% 2	2724
June	1st to 10th	Day	58% 5% 18% 14% 6% 1	2640
	11th to 20th	Day	64% 5% 15% 12% 3	2700
	21st to end	Day	46% 7% 18% 20% 7% 1	2700
July	1st to 10th	Day	38% 6% 16% 26% 11% 2	2626
	11th to 20th	Day	35% 4 16% 32% 11% 1	2575
	21st to end	Day	26% 3 16% 37% 14% 3	2589
August	1st to 10th	Day	33% 5% 19% 29% 12% 2	2478
	11th to 20th	Day	28% 5% 22% 29% 12% 3	2298
	21st to end	Day	40% 5% 17% 29% 8% 1	2481
September	1st to 10th	Day	37% 5% 16% 24% 13% 5%	2160
	11th to 20th	Day	45% 5% 14% 18% 16% 2	2160
	21st to end	Day	48% 5% 15% 19% 11% 2	1904
October	1st to 10th	Day	52% 4 16% 19% 8%	1800
	11th to 20th	Day	58% 5% 12% 18% 5% 1	1800
	21st to end	Day	63% 5% 9% 15% 8%	1857
November	1st to 10th	Day	71% 5% 10% 9% 4 1	1505
	11th to 20th	Day	70% 4 8% 10% 6% 2	1520
	21st to end	Day	70% 4 5% 11% 9% 1	1520
December	1st to 10th	Day	65% 5% 9% 11% 9% 1	1520
	11th to 20th	Day	70% 5% 9% 12% 4 1	1809
	21st to end	Day	76% 6% 8% 8% 2	2442
January	1st to 10th	Night	79% 5% 5% 8% 2 1	4726
	11th to 20th	Night	61% 6% 8% 15% 8% 2	4566
	21st to end	Night	76% 4 7% 9% 3	4827
February	1st to 10th	Night	70% 4 9% 10% 4 2	2470
	11th to 20th	Night	67% 4 9% 11% 7% 2	2404
	21st to end	Night	68% 3 8% 15% 5% 1	1989
March	1st to 10th	Night	65% 4 12% 13% 6%	2239
	11th to 20th	Night	66% 4 11% 14% 4 1	2067
	21st to end	Night	71% 5% 14% 10% 1	2103
April	1st to 10th	Night	67% 7% 11% 13% 2	1800
	11th to 20th	Night	63% 4 13% 12% 6% 2	1736
	21st to end	Night	65% 5% 11% 12% 6% 1	1561
May	1st to 10th	Night	48% 5% 19% 21% 7% 1	1440
	11th to 20th	Night	48% 5% 25% 15% 7%	1416
	21st to end	Night	54% 5% 20% 18% 3	1552
June	1st to 10th	Night	55% 4 18% 17% 5% 2	1316
	11th to 20th	Night	57% 6% 19% 15% 3	1260
	21st to end	Night	41% 6% 23% 23% 4 3	1260
July	1st to 10th	Night	36% 6% 14% 26% 13% 5%	1225
	11th to 20th	Night	32% 4 18% 28% 16% 2	1262
	21st to end	Night	21% 5% 14% 36% 19% 5%	1488
August	1st to 10th	Night	27% 5% 17% 31% 17% 3	1433
	11th to 20th	Night	22% 4 23% 31% 16% 4	1602
	21st to end	Night	34% 4 18% 32% 10% 1	1874
September	1st to 10th	Night	33% 4 16% 29% 12% 5%	1800
	11th to 20th	Night	46% 5% 11% 19% 16% 4	1800
	21st to end	Night	46% 6% 15% 21% 10% 2	2055
October	1st to 10th	Night	50% 5% 14% 23% 8% 1	2160
	11th to 20th	Night	57% 3 14% 17% 7% 1	2160
	21st to end	Night	61% 4 10% 17% 9%	2498
November	1st to 10th	Night	71% 3 10% 11% 4 1	2572
	11th to 20th	Night	68% 4 9% 10% 6% 2	2660
	21st to end	Night	70% 4 7% 9% 8% 1	2660
December	1st to 10th	Night	62% 6% 11% 13% 8% 1	2660
	11th to 20th	Night	72% 6% 9% 9% 4 1	3381
	21st to end	Night	75% 6% 8% 8% 3 1	5214

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	39% 16% 14% 15% 10% 6%	180
	11th to 20th	Day	33% 16% 21% 12% 13% 4	180
	21st to end	Day	40% 14% 17% 14% 14% 1	218
February	1st to 10th	Day	41% 10% 21% 18% 8% 1	370
	11th to 20th	Day	38% 13% 19% 20% 10% 1	365
	21st to end	Day	34% 14% 21% 20% 10% 1	332
March	1st to 10th	Day	31% 15% 24% 19% 9% 1	460
	11th to 20th	Day	31% 12% 19% 18% 16% 2	449
	21st to end	Day	34% 15% 19% 21% 10% 1	462
April	1st to 10th	Day	30% 14% 24% 20% 10% 3	420
	11th to 20th	Day	24% 14% 23% 24% 11% 3	412
	21st to end	Day	25% 14% 25% 22% 12% 2	420
May	1st to 10th	Day	15% 11% 26% 31% 15% 2	423
	11th to 20th	Day	25% 6% 18% 28% 18% 5%	448
	21st to end	Day	21% 10% 24% 27% 13% 5%	485
June	1st to 10th	Day	21% 9% 25% 27% 17% 2	570
	11th to 20th	Day	22% 10% 25% 31% 11% 2	630
	21st to end	Day	11% 7% 22% 32% 23% 4	630
July	1st to 10th	Day	13% 3 21% 36% 24% 3	621
	11th to 20th	Day	10% 8% 19% 37% 22% 5%	574
	21st to end	Day	7% 4 16% 34% 30% 8%	487
August	1st to 10th	Day	12% 4 12% 32% 30% 10%	447
	11th to 20th	Day	7% 2 16% 34% 32% 10%	437
	21st to end	Day	10% 7% 21% 34% 22% 6%	495
September	1st to 10th	Day	17% 5% 21% 33% 20% 4	450
	11th to 20th	Day	17% 8% 18% 29% 23% 4	450
	21st to end	Day	25% 6% 21% 25% 18% 5%	450
October	1st to 10th	Day	26% 7% 18% 25% 16% 7%	450
	11th to 20th	Day	32% 10% 17% 21% 15% 5%	450
	21st to end	Day	30% 15% 20% 19% 13% 4	372
November	1st to 10th	Day	35% 15% 21% 17% 11% 1	188
	11th to 20th	Day	38% 12% 16% 13% 13% 7%	180
	21st to end	Day	34% 14% 21% 14% 9% 9%	180
December	1st to 10th	Day	45% 14% 8% 11% 13% 9%	180
	11th to 20th	Day	42% 12% 18% 15% 12% 1	180
	21st to end	Day	40% 12% 17% 25% 5% 2	198
January	1st to 10th	Night	43% 13% 17% 15% 10% 2	850
	11th to 20th	Night	40% 11% 14% 19% 14% 2	799
	21st to end	Night	40% 8% 18% 21% 11% 1	836
February	1st to 10th	Night	38% 10% 17% 23% 10% 2	760
	11th to 20th	Night	35% 10% 17% 25% 11% 1	744
	21st to end	Night	37% 9% 18% 23% 11% 2	628
March	1st to 10th	Night	34% 13% 17% 22% 12% 1	760
	11th to 20th	Night	35% 12% 16% 23% 13% 2	752
	21st to end	Night	38% 11% 18% 23% 9% 2	792
April	1st to 10th	Night	34% 10% 15% 25% 14% 3	720
	11th to 20th	Night	28% 10% 18% 26% 16% 2	668
	21st to end	Night	32% 8% 14% 32% 12% 2	481
May	1st to 10th	Night	21% 7% 11% 36% 24% 2	360
	11th to 20th	Night	23% 6% 10% 29% 28% 5%	354
	21st to end	Night	23% 6% 12% 25% 29% 4	389
June	1st to 10th	Night	24% 6% 12% 34% 21% 3	360
	11th to 20th	Night	24% 8% 11% 34% 22% 3	360
	21st to end	Night	16% 4 10% 32% 32% 6%	360
July	1st to 10th	Night	13% 4 6% 32% 40% 5%	350
	11th to 20th	Night	11% 5% 9% 33% 37% 5%	348
	21st to end	Night	9% 1 5% 32% 45% 9%	394
August	1st to 10th	Night	10% 2 5% 24% 48% 11%	367
	11th to 20th	Night	10% 2 4 21% 49% 14%	534
	21st to end	Night	13% 4 8% 31% 37% 7%	686
September	1st to 10th	Night	13% 4 7% 38% 33% 5%	720
	11th to 20th	Night	19% 6% 9% 23% 33% 11%	720
	21st to end	Night	22% 5% 13% 27% 27% 6%	720
October	1st to 10th	Night	23% 6% 13% 26% 27% 5%	720
	11th to 20th	Night	33% 7% 14% 22% 20% 5%	720
	21st to end	Night	31% 11% 18% 25% 12% 4	792
November	1st to 10th	Night	39% 12% 18% 22% 9%	740
	11th to 20th	Night	38% 13% 18% 21% 9% 2	760
	21st to end	Night	40% 9% 18% 18% 11% 4	760
December	1st to 10th	Night	34% 8% 15% 25% 15% 2	760
	11th to 20th	Night	41% 12% 21% 19% 6% 1	792
	21st to end	Night	41% 13% 21% 18% 7%	935

Page generated at 2024-03-15 11:01:40. Version ac07

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