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 2025-06-10.

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	21% 9% 3 7% 4 4 4 3 5% 6% 32%	123467
	Night	25% 10% 3 7% 4 4 4 3 4 5% 30%	123461
Transparency	Day	55% 4 14% 18% 8% 1	123220
	Night	58% 4 13% 17% 7% 1	123351
Seeing	Day	24% 9% 20% 26% 16% 4	15876
	Night	30% 8% 14% 25% 19% 3	24281

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	38% 15% 3 7% 5% 5% 5% 3 4 4 10%	3179
	11th to 20th	Day	20% 11% 3 8% 4 4 4 3 5% 7% 30%	3412
	21st to end	Day	33% 11% 3 6% 4 4 4 2 3 4 26%	4065
February	1st to 10th	Day	30% 13% 3 8% 5% 4 4 3 5% 6% 18%	1800
	11th to 20th	Day	29% 12% 5% 9% 5% 4 3 4 5% 5% 20%	1755
	21st to end	Day	28% 16% 3 8% 5% 5% 5% 4 4 5% 17%	1539
March	1st to 10th	Day	23% 15% 4 9% 5% 5% 5% 3 5% 7% 18%	4606
	11th to 20th	Day	21% 14% 4 9% 5% 5% 5% 3 5% 4 25%	5000
	21st to end	Day	26% 12% 4 9% 6% 6% 5% 5% 5% 6% 15%	5676
April	1st to 10th	Day	25% 13% 4 8% 7% 5% 4 4 6% 7% 17%	2400
	11th to 20th	Day	21% 11% 3 8% 6% 5% 5% 4 5% 6% 25%	2417
	21st to end	Day	24% 11% 4 8% 6% 5% 5% 3 6% 6% 23%	2665
May	1st to 10th	Day	17% 9% 3 7% 5% 4 4 3 4 6% 39%	6320
	11th to 20th	Day	20% 10% 3 8% 5% 5% 4 4 5% 7% 28%	6277
	21st to end	Day	23% 8% 3 7% 4 4 4 4 6% 9% 28%	6903
June	1st to 10th	Day	22% 8% 3 7% 4 4 4 4 5% 7% 32%	2923
	11th to 20th	Day	24% 10% 4 8% 5% 5% 5% 4 6% 6% 24%	2850
	21st to end	Day	14% 7% 2 6% 4 5% 5% 4 7% 7% 38%	2850
July	1st to 10th	Day	7% 4 2 3 2 3 3 2 3 5% 65%	4740
	11th to 20th	Day	6% 3 1 3 2 2 3 2 4 5% 69%	4654
	21st to end	Day	12% 5% 2 5% 3 3 3 3 4 6% 55%	4790
August	1st to 10th	Day	6% 3 2 4 2 3 3 2 5% 6% 64%	4376
	11th to 20th	Day	10% 6% 2 7% 5% 4 4 4 6% 8% 44%	4026
	21st to end	Day	17% 7% 3 6% 3 4 3 3 4 7% 42%	4350
September	1st to 10th	Day	12% 6% 2 6% 4 3 4 3 5% 7% 49%	2280
	11th to 20th	Day	15% 7% 3 7% 4 4 4 3 5% 7% 40%	2280
	21st to end	Day	20% 8% 2 6% 3 4 4 3 5% 7% 37%	2009
October	1st to 10th	Day	19% 7% 2 7% 4 4 5% 5% 7% 9% 32%	3180
	11th to 20th	Day	32% 9% 3 6% 4 3 3 4 4 6% 25%	3161
	21st to end	Day	28% 10% 4 8% 5% 5% 5% 4 5% 6% 19%	3230
November	1st to 10th	Day	36% 10% 3 8% 5% 4 5% 4 5% 4 18%	1585
	11th to 20th	Day	34% 9% 3 7% 5% 5% 5% 3 4 5% 20%	1600
	21st to end	Day	34% 10% 3 8% 4 4 5% 3 4 6% 18%	1600
December	1st to 10th	Day	22% 10% 2 7% 5% 5% 5% 3 5% 7% 28%	2620
	11th to 20th	Day	34% 15% 4 10% 5% 4 4 3 4 4 13%	2851
	21st to end	Day	33% 15% 5% 9% 6% 5% 4 3 4 5% 10%	3498
January	1st to 10th	Night	36% 14% 4 9% 5% 5% 4 3 4 4 13%	6766
	11th to 20th	Night	24% 10% 3 6% 3 3 4 3 5% 7% 31%	6483
	21st to end	Night	30% 10% 3 7% 4 3 3 3 3 4 30%	6891
February	1st to 10th	Night	32% 13% 3 7% 5% 4 4 3 4 5% 20%	2600
	11th to 20th	Night	30% 14% 3 7% 5% 3 4 3 4 5% 23%	2532
	21st to end	Night	31% 13% 3 7% 4 4 4 2 5% 5% 21%	2090
March	1st to 10th	Night	19% 15% 4 9% 5% 5% 5% 4 6% 6% 22%	5273
	11th to 20th	Night	24% 15% 3 7% 4 4 4 3 4 5% 27%	4999
	21st to end	Night	34% 14% 3 9% 5% 5% 5% 3 4 4 14%	5298
April	1st to 10th	Night	27% 14% 3 8% 5% 5% 4 5% 5% 5% 18%	2000
	11th to 20th	Night	23% 11% 3 7% 6% 5% 5% 3 4 5% 28%	1930
	21st to end	Night	28% 11% 3 9% 5% 4 3 3 5% 6% 24%	1733
May	1st to 10th	Night	18% 8% 3 7% 3 3 3 2 4 4 44%	3640
	11th to 20th	Night	20% 10% 3 8% 5% 4 4 4 5% 6% 31%	3616
	21st to end	Night	19% 8% 4 8% 5% 6% 6% 4 7% 6% 26%	3972
June	1st to 10th	Night	22% 7% 2 7% 6% 5% 4 3 5% 5% 34%	1457
	11th to 20th	Night	23% 8% 3 7% 5% 4 4 5% 4 7% 29%	1330
	21st to end	Night	15% 6% 2 6% 4 4 5% 4 6% 7% 42%	1330
July	1st to 10th	Night	7% 4 2 3 2 3 3 2 3 5% 66%	2220
	11th to 20th	Night	6% 3 1 3 2 1 2 1 4 4 74%	2306
	21st to end	Night	12% 4 1 4 2 3 3 2 4 5% 59%	2762
August	1st to 10th	Night	5% 2 1 2 1 2 3 3 5% 5% 70%	2552
	11th to 20th	Night	11% 6% 2 6% 3 3 3 3 5% 6% 52%	2812
	21st to end	Night	16% 6% 2 6% 3 4 3 3 5% 7% 46%	3322
September	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 4 6% 55%	1900
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 4 6% 42%	1900
	21st to end	Night	19% 8% 2 5% 3 4 4 3 5% 6% 41%	2170
October	1st to 10th	Night	19% 6% 3 7% 5% 5% 5% 3 5% 7% 36%	3800
	11th to 20th	Night	32% 8% 2 5% 3 3 3 3 4 5% 32%	3777
	21st to end	Night	28% 11% 4 9% 5% 5% 4 3 4 6% 22%	4331
November	1st to 10th	Night	38% 10% 3 6% 5% 4 3 3 4 5% 19%	2711
	11th to 20th	Night	36% 11% 4 7% 4 3 4 3 4 4 21%	2800
	21st to end	Night	34% 11% 3 7% 4 5% 4 4 5% 5% 19%	2800
December	1st to 10th	Night	24% 9% 2 7% 4 4 4 3 4 6% 31%	4560
	11th to 20th	Night	35% 14% 4 9% 5% 4 4 3 4 5% 14%	5340
	21st to end	Night	32% 15% 5% 9% 6% 5% 5% 4 4 4 11%	7458

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% 3 7% 8% 2	3179
	11th to 20th	Day	54% 4 13% 19% 8% 2	3435
	21st to end	Day	65% 3 7% 16% 9%	4065
February	1st to 10th	Day	70% 5% 8% 10% 5% 2	1800
	11th to 20th	Day	70% 3 8% 10% 7% 1	1764
	21st to end	Day	71% 4 8% 12% 3 1	1539
March	1st to 10th	Day	66% 5% 10% 15% 4	4606
	11th to 20th	Day	65% 3 12% 17% 2	5000
	21st to end	Day	70% 6% 13% 10% 1	5676
April	1st to 10th	Day	70% 6% 14% 9% 1	2400
	11th to 20th	Day	63% 4 10% 14% 7% 2	2417
	21st to end	Day	64% 5% 11% 14% 5% 1	2665
May	1st to 10th	Day	50% 5% 12% 24% 9%	6320
	11th to 20th	Day	57% 6% 19% 15% 3	6280
	21st to end	Day	56% 6% 23% 14% 1	6904
June	1st to 10th	Day	55% 5% 18% 15% 6% 1	2923
	11th to 20th	Day	64% 5% 16% 12% 4	2850
	21st to end	Day	47% 6% 18% 20% 7% 1	2850
July	1st to 10th	Day	26% 4 11% 23% 22% 14%	4666
	11th to 20th	Day	21% 3 11% 34% 29% 3	4573
	21st to end	Day	33% 3 20% 31% 12% 2	4639
August	1st to 10th	Day	23% 4 16% 40% 17% 1	4359
	11th to 20th	Day	40% 3 25% 22% 7% 2	4065
	21st to end	Day	45% 3 14% 25% 11%	4349
September	1st to 10th	Day	37% 5% 16% 23% 13% 5%	2280
	11th to 20th	Day	46% 5% 14% 18% 15% 2	2280
	21st to end	Day	49% 5% 16% 18% 10% 2	2009
October	1st to 10th	Day	49% 3 19% 24% 5%	3180
	11th to 20th	Day	62% 4 12% 15% 8% 1	3161
	21st to end	Day	66% 3 12% 12% 7%	3230
November	1st to 10th	Day	71% 5% 11% 9% 4 1	1585
	11th to 20th	Day	71% 4 8% 9% 6% 2	1600
	21st to end	Day	70% 4 6% 10% 9% 1	1600
December	1st to 10th	Day	56% 3 10% 18% 12%	2620
	11th to 20th	Day	75% 4 10% 9% 2	2853
	21st to end	Day	79% 4 9% 6% 2	3498
January	1st to 10th	Night	76% 3 7% 9% 3 1	6766
	11th to 20th	Night	53% 6% 14% 18% 9% 2	6522
	21st to end	Night	60% 3 10% 17% 9%	6891
February	1st to 10th	Night	69% 4 9% 11% 6% 2	2600
	11th to 20th	Night	67% 4 9% 11% 7% 2	2534
	21st to end	Night	69% 3 8% 15% 4 1	2090
March	1st to 10th	Night	63% 3 13% 14% 7%	5273
	11th to 20th	Night	61% 4 14% 18% 2	5000
	21st to end	Night	73% 4 13% 8% 1	5298
April	1st to 10th	Night	68% 6% 12% 12% 2	2000
	11th to 20th	Night	61% 4 14% 12% 8% 2	1930
	21st to end	Night	64% 5% 12% 13% 5% 1	1733
May	1st to 10th	Night	47% 4 12% 30% 7%	3637
	11th to 20th	Night	55% 6% 20% 15% 3	3616
	21st to end	Night	57% 7% 22% 13% 1	3972
June	1st to 10th	Night	53% 4 19% 17% 6% 1	1457
	11th to 20th	Night	56% 6% 18% 16% 3	1330
	21st to end	Night	42% 6% 22% 23% 4 3	1330
July	1st to 10th	Night	24% 4 9% 25% 21% 18%	2185
	11th to 20th	Night	18% 3 11% 29% 37% 2	2264
	21st to end	Night	29% 4 14% 30% 20% 3	2674
August	1st to 10th	Night	18% 4 16% 42% 19% 2	2544
	11th to 20th	Night	32% 3 29% 25% 9% 2	2839
	21st to end	Night	41% 4 18% 27% 10% 1	3322
September	1st to 10th	Night	33% 4 16% 28% 13% 5%	1900
	11th to 20th	Night	46% 5% 12% 18% 15% 4	1900
	21st to end	Night	46% 6% 16% 21% 10% 2	2170
October	1st to 10th	Night	49% 4 18% 24% 5%	3800
	11th to 20th	Night	57% 3 14% 17% 9% 1	3777
	21st to end	Night	66% 3 12% 12% 7%	4331
November	1st to 10th	Night	71% 3 11% 11% 4 1	2711
	11th to 20th	Night	69% 4 9% 10% 6% 2	2800
	21st to end	Night	70% 4 8% 9% 8% 1	2800
December	1st to 10th	Night	55% 5% 10% 22% 8% 1	4560
	11th to 20th	Night	75% 4 9% 10% 3	5337
	21st to end	Night	76% 5% 10% 7% 2	7458

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	40% 14% 14% 17% 10% 6%	200
	11th to 20th	Day	32% 15% 20% 14% 14% 6%	200
	21st to end	Day	38% 13% 15% 17% 14% 3	242
February	1st to 10th	Day	40% 10% 21% 19% 9% 2	400
	11th to 20th	Day	38% 12% 19% 19% 12% 1	395
	21st to end	Day	35% 13% 21% 20% 10% 1	359
March	1st to 10th	Day	31% 14% 24% 20% 9% 1	500
	11th to 20th	Day	32% 12% 19% 18% 17% 2	500
	21st to end	Day	34% 14% 20% 22% 9% 1	550
April	1st to 10th	Day	28% 14% 23% 21% 10% 3	500
	11th to 20th	Day	22% 14% 24% 24% 12% 3	492
	21st to end	Day	26% 13% 24% 21% 13% 3	500
May	1st to 10th	Day	16% 11% 25% 31% 14% 3	503
	11th to 20th	Day	25% 7% 19% 27% 16% 5%	528
	21st to end	Day	23% 10% 23% 25% 13% 5%	573
June	1st to 10th	Day	21% 8% 23% 29% 17% 2	661
	11th to 20th	Day	22% 10% 25% 30% 11% 2	680
	21st to end	Day	13% 7% 22% 31% 23% 4	680
July	1st to 10th	Day	12% 3 21% 36% 24% 4	671
	11th to 20th	Day	9% 7% 18% 38% 22% 6%	621
	21st to end	Day	8% 4 17% 34% 30% 7%	531
August	1st to 10th	Day	11% 4 12% 33% 30% 10%	487
	11th to 20th	Day	7% 3 16% 32% 32% 10%	477
	21st to end	Day	11% 6% 21% 33% 21% 7%	539
September	1st to 10th	Day	17% 5% 20% 33% 20% 5%	490
	11th to 20th	Day	17% 7% 18% 30% 23% 4	490
	21st to end	Day	25% 6% 21% 25% 18% 6%	490
October	1st to 10th	Day	25% 7% 19% 25% 17% 7%	490
	11th to 20th	Day	33% 10% 16% 21% 15% 5%	490
	21st to end	Day	29% 13% 21% 20% 13% 3	409
November	1st to 10th	Day	35% 13% 22% 17% 11% 1	208
	11th to 20th	Day	39% 12% 17% 14% 13% 7%	200
	21st to end	Day	34% 13% 22% 15% 9% 10%	200
December	1st to 10th	Day	43% 13% 8% 11% 13% 14%	200
	11th to 20th	Day	45% 12% 16% 16% 12% 1	200
	21st to end	Day	41% 11% 18% 23% 5% 2	220
January	1st to 10th	Night	43% 12% 17% 17% 10% 1	900
	11th to 20th	Night	39% 11% 14% 22% 14% 2	843
	21st to end	Night	38% 8% 18% 22% 13% 2	880
February	1st to 10th	Night	37% 10% 17% 24% 10% 2	800
	11th to 20th	Night	36% 9% 17% 26% 12% 1	784
	21st to end	Night	37% 9% 18% 22% 10% 2	660
March	1st to 10th	Night	34% 12% 17% 23% 12% 1	800
	11th to 20th	Night	35% 11% 16% 23% 13% 1	800
	21st to end	Night	38% 11% 18% 23% 9% 1	880
April	1st to 10th	Night	34% 10% 15% 25% 13% 3	800
	11th to 20th	Night	27% 10% 19% 26% 15% 2	742
	21st to end	Night	31% 8% 14% 31% 14% 2	533
May	1st to 10th	Night	21% 7% 12% 36% 23% 2	400
	11th to 20th	Night	24% 6% 11% 29% 26% 5%	394
	21st to end	Night	22% 7% 12% 27% 28% 4	433
June	1st to 10th	Night	24% 5% 12% 35% 21% 3	399
	11th to 20th	Night	23% 7% 10% 35% 21% 3	380
	21st to end	Night	17% 4 10% 32% 31% 5%	380
July	1st to 10th	Night	12% 4 6% 34% 40% 5%	370
	11th to 20th	Night	10% 5% 9% 33% 38% 6%	368
	21st to end	Night	10% 1 5% 32% 44% 8%	416
August	1st to 10th	Night	10% 2 4 23% 50% 11%	388
	11th to 20th	Night	10% 2 4 22% 49% 14%	564
	21st to end	Night	14% 4 8% 31% 37% 7%	725
September	1st to 10th	Night	13% 4 7% 37% 34% 6%	760
	11th to 20th	Night	19% 5% 9% 24% 32% 11%	760
	21st to end	Night	22% 5% 13% 27% 27% 6%	760
October	1st to 10th	Night	22% 6% 13% 27% 26% 5%	760
	11th to 20th	Night	33% 6% 14% 22% 20% 5%	760
	21st to end	Night	32% 10% 17% 25% 12% 3	836
November	1st to 10th	Night	40% 11% 18% 22% 9%	780
	11th to 20th	Night	39% 13% 17% 20% 9% 2	800
	21st to end	Night	39% 9% 17% 19% 12% 4	800
December	1st to 10th	Night	34% 8% 15% 24% 16% 3	800
	11th to 20th	Night	41% 12% 21% 19% 7% 1	836
	21st to end	Night	41% 13% 21% 18% 7%	990

Page generated at 2025-06-07 07:28:15. Version ac07

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