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 2026-10-29.

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	19% 9% 3 7% 4 4 4 3 5% 7% 35%	154073
	Night	23% 10% 3 7% 4 4 4 3 4 5% 32%	146721
Transparency	Day	52% 4 16% 19% 8% 1	153785
	Night	55% 4 14% 18% 8% 1	146591
Seeing	Day	24% 9% 20% 26% 17% 5%	17762
	Night	30% 8% 14% 25% 19% 3	25960

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	41% 16% 4 7% 4 5% 4 3 3 4 9%	4125
	11th to 20th	Day	19% 9% 2 8% 4 4 4 3 5% 7% 34%	4456
	21st to end	Day	30% 11% 3 7% 4 4 4 3 4 5% 25%	5261
February	1st to 10th	Day	30% 13% 3 8% 5% 5% 4 3 5% 6% 18%	1890
	11th to 20th	Day	28% 12% 5% 9% 5% 5% 3 3 5% 5% 21%	1845
	21st to end	Day	28% 15% 3 9% 5% 4 5% 4 4 5% 18%	1614
March	1st to 10th	Day	23% 15% 4 9% 5% 5% 5% 3 5% 7% 18%	4708
	11th to 20th	Day	22% 14% 4 9% 5% 5% 5% 3 5% 4 25%	5110
	21st to end	Day	26% 12% 4 9% 6% 6% 5% 5% 5% 7% 15%	5801
April	1st to 10th	Day	25% 13% 4 8% 6% 5% 4 4 6% 7% 18%	2520
	11th to 20th	Day	22% 11% 3 8% 6% 5% 5% 4 5% 7% 24%	2539
	21st to end	Day	23% 11% 4 8% 6% 5% 5% 3 6% 6% 24%	2798
May	1st to 10th	Day	14% 8% 3 7% 5% 5% 4 3 5% 6% 40%	8195
	11th to 20th	Day	20% 9% 3 8% 5% 5% 4 4 5% 8% 29%	8160
	21st to end	Day	20% 8% 3 6% 4 4 4 4 6% 9% 31%	8993
June	1st to 10th	Day	22% 8% 3 7% 5% 4 4 4 6% 7% 32%	3081
	11th to 20th	Day	22% 9% 3 8% 5% 5% 5% 4 6% 6% 27%	3150
	21st to end	Day	15% 8% 3 6% 4 5% 5% 4 7% 7% 37%	3150
July	1st to 10th	Day	9% 5% 2 4 3 3 3 3 5% 7% 57%	8820
	11th to 20th	Day	5% 4 2 4 3 2 3 2 4 5% 67%	8650
	21st to end	Day	9% 4 2 5% 3 3 4 3 5% 7% 54%	8916
August	1st to 10th	Day	6% 3 2 4 2 2 3 2 4 6% 65%	6402
	11th to 20th	Day	7% 5% 2 6% 5% 4 4 4 6% 7% 50%	5923
	21st to end	Day	16% 7% 3 7% 4 4 4 3 5% 8% 40%	6328
September	1st to 10th	Day	12% 6% 2 6% 4 3 4 3 6% 7% 47%	2520
	11th to 20th	Day	15% 7% 3 7% 4 4 4 3 5% 7% 41%	2520
	21st to end	Day	20% 8% 2 6% 3 4 4 3 5% 7% 37%	2220
October	1st to 10th	Day	18% 7% 2 7% 4 4 5% 5% 7% 9% 32%	3380
	11th to 20th	Day	31% 8% 3 6% 4 3 3 4 4 6% 27%	3361
	21st to end	Day	29% 10% 4 9% 5% 5% 4 4 5% 6% 19%	3409
November	1st to 10th	Day	35% 10% 3 8% 5% 4 5% 4 5% 5% 18%	1666
	11th to 20th	Day	35% 10% 3 7% 5% 4 4 3 4 5% 20%	1680
	21st to end	Day	35% 10% 3 8% 4 4 5% 3 4 6% 18%	1680
December	1st to 10th	Day	23% 11% 2 7% 5% 5% 5% 3 5% 7% 27%	2700
	11th to 20th	Day	34% 15% 4 10% 5% 4 4 3 4 4 13%	2927
	21st to end	Day	33% 15% 5% 9% 6% 5% 5% 3 4 5% 10%	3575
January	1st to 10th	Night	35% 15% 5% 9% 5% 4 4 3 4 4 11%	8770
	11th to 20th	Night	24% 9% 3 7% 3 3 4 3 5% 6% 34%	8439
	21st to end	Night	29% 10% 3 7% 4 3 3 3 4 4 30%	8920
February	1st to 10th	Night	32% 13% 3 7% 5% 4 4 3 4 5% 21%	2730
	11th to 20th	Night	29% 14% 3 7% 5% 4 4 3 4 5% 23%	2662
	21st to end	Night	31% 13% 3 8% 4 4 4 2 5% 5% 20%	2191
March	1st to 10th	Night	20% 15% 4 9% 5% 5% 5% 4 6% 6% 22%	5391
	11th to 20th	Night	25% 14% 3 7% 4 4 4 3 4 5% 27%	5109
	21st to end	Night	33% 14% 3 9% 5% 5% 5% 3 4 5% 14%	5415
April	1st to 10th	Night	27% 14% 3 8% 5% 5% 4 5% 5% 5% 19%	2100
	11th to 20th	Night	23% 11% 3 8% 6% 5% 5% 3 4 5% 27%	2028
	21st to end	Night	28% 10% 3 9% 5% 4 3 3 5% 6% 25%	1820
May	1st to 10th	Night	15% 7% 2 7% 4 4 4 3 4 5% 46%	4724
	11th to 20th	Night	21% 10% 3 8% 5% 5% 4 4 5% 6% 30%	4700
	21st to end	Night	16% 7% 3 8% 5% 6% 6% 5% 7% 7% 31%	5178
June	1st to 10th	Night	24% 6% 2 7% 5% 5% 4 4 5% 5% 33%	1535
	11th to 20th	Night	21% 8% 3 7% 5% 4 4 4 5% 7% 32%	1470
	21st to end	Night	17% 6% 2 6% 3 4 5% 4 6% 7% 41%	1470
July	1st to 10th	Night	8% 6% 2 5% 3 3 3 3 4 6% 58%	4140
	11th to 20th	Night	5% 4 1 3 2 2 2 2 4 4 71%	4296
	21st to end	Night	8% 4 2 5% 3 3 4 3 5% 7% 57%	5152
August	1st to 10th	Night	5% 2 1 2 1 2 3 3 4 5% 74%	3747
	11th to 20th	Night	8% 4 2 5% 3 4 3 3 5% 7% 56%	4142
	21st to end	Night	17% 6% 3 5% 3 4 3 3 4 7% 44%	4830
September	1st to 10th	Night	10% 4 2 5% 3 3 4 4 4 6% 54%	2100
	11th to 20th	Night	17% 8% 3 6% 4 4 4 3 4 6% 43%	2100
	21st to end	Night	19% 8% 2 5% 3 4 4 3 5% 6% 40%	2398
October	1st to 10th	Night	18% 6% 3 7% 5% 5% 5% 3 5% 7% 36%	4040
	11th to 20th	Night	31% 8% 2 5% 3 3 3 3 4 5% 33%	4017
	21st to end	Night	28% 11% 4 8% 5% 5% 4 3 4 6% 23%	4570
November	1st to 10th	Night	38% 10% 3 7% 5% 4 4 3 4 5% 19%	2850
	11th to 20th	Night	37% 11% 4 7% 4 3 4 3 4 4 20%	2940
	21st to end	Night	34% 11% 3 7% 4 5% 4 3 5% 5% 19%	2940
December	1st to 10th	Night	26% 9% 2 7% 4 4 4 3 4 6% 30%	4700
	11th to 20th	Night	35% 14% 4 9% 5% 4 4 3 4 5% 14%	5484
	21st to end	Night	31% 15% 5% 9% 6% 5% 5% 4 4 4 11%	7623

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	82% 3 7% 7% 2	4125
	11th to 20th	Day	49% 3 12% 21% 13% 2	4479
	21st to end	Day	64% 2 7% 18% 9%	5262
February	1st to 10th	Day	69% 5% 8% 10% 6% 2	1890
	11th to 20th	Day	69% 3 8% 11% 7% 1	1854
	21st to end	Day	71% 4 8% 12% 4 1	1614
March	1st to 10th	Day	66% 5% 10% 15% 4	4708
	11th to 20th	Day	65% 3 12% 17% 2	5110
	21st to end	Day	70% 6% 13% 10% 1	5801
April	1st to 10th	Day	68% 6% 15% 10% 1	2520
	11th to 20th	Day	63% 4 11% 13% 7% 2	2539
	21st to end	Day	63% 5% 11% 15% 5% 1	2798
May	1st to 10th	Day	47% 4 16% 25% 8%	8195
	11th to 20th	Day	56% 6% 21% 14% 2	8163
	21st to end	Day	52% 5% 26% 15% 1	8994
June	1st to 10th	Day	56% 5% 18% 15% 6% 1	3081
	11th to 20th	Day	61% 5% 15% 15% 4	3150
	21st to end	Day	48% 6% 18% 20% 7% 1	3150
July	1st to 10th	Day	30% 3 23% 21% 15% 7%	8746
	11th to 20th	Day	23% 2 16% 32% 25% 2	8569
	21st to end	Day	31% 2 28% 29% 9% 1	8723
August	1st to 10th	Day	23% 3 16% 34% 21% 3	6385
	11th to 20th	Day	34% 2 32% 25% 6% 1	5962
	21st to end	Day	45% 3 18% 25% 9%	6327
September	1st to 10th	Day	38% 5% 17% 23% 13% 5%	2520
	11th to 20th	Day	45% 5% 14% 20% 14% 2	2520
	21st to end	Day	49% 4 17% 19% 10% 2	2220
October	1st to 10th	Day	49% 3 19% 24% 5%	3380
	11th to 20th	Day	60% 4 11% 16% 9% 1	3361
	21st to end	Day	67% 3 12% 11% 7%	3409
November	1st to 10th	Day	70% 5% 11% 10% 4 1	1666
	11th to 20th	Day	71% 4 8% 9% 6% 2	1680
	21st to end	Day	70% 4 6% 10% 9% 1	1680
December	1st to 10th	Day	58% 3 10% 17% 12%	2700
	11th to 20th	Day	75% 4 10% 9% 2	2929
	21st to end	Day	79% 4 10% 6% 2	3575
January	1st to 10th	Night	80% 3 7% 7% 3	8770
	11th to 20th	Night	49% 4 11% 18% 16% 2	8478
	21st to end	Night	59% 2 10% 19% 10%	8920
February	1st to 10th	Night	69% 4 9% 11% 6% 2	2730
	11th to 20th	Night	66% 4 9% 11% 8% 2	2664
	21st to end	Night	69% 3 8% 14% 5% 1	2191
March	1st to 10th	Night	63% 3 13% 14% 7%	5391
	11th to 20th	Night	62% 4 14% 18% 2	5110
	21st to end	Night	73% 4 13% 8% 1	5415
April	1st to 10th	Night	67% 6% 12% 13% 2	2100
	11th to 20th	Night	61% 4 14% 12% 7% 2	2028
	21st to end	Night	62% 5% 13% 14% 6% 1	1820
May	1st to 10th	Night	44% 3 13% 31% 8%	4721
	11th to 20th	Night	56% 7% 19% 14% 3	4700
	21st to end	Night	51% 6% 25% 16% 2	5182
June	1st to 10th	Night	54% 4 18% 16% 6% 1	1535
	11th to 20th	Night	53% 6% 18% 20% 4	1470
	21st to end	Night	44% 5% 21% 23% 5% 3	1470
July	1st to 10th	Night	29% 2 21% 24% 14% 9%	4105
	11th to 20th	Night	20% 3 14% 31% 31% 2	4254
	21st to end	Night	28% 3 23% 31% 13% 2	5040
August	1st to 10th	Night	16% 2 17% 41% 20% 4	3739
	11th to 20th	Night	27% 2 36% 26% 7% 2	4169
	21st to end	Night	41% 4 21% 25% 7% 1	4830
September	1st to 10th	Night	32% 4 19% 28% 12% 5%	2100
	11th to 20th	Night	45% 4 12% 20% 15% 3	2100
	21st to end	Night	47% 5% 16% 21% 9% 2	2398
October	1st to 10th	Night	49% 4 19% 24% 4	4040
	11th to 20th	Night	56% 3 14% 17% 10% 1	4017
	21st to end	Night	65% 3 12% 12% 7%	4570
November	1st to 10th	Night	71% 3 10% 11% 4 1	2850
	11th to 20th	Night	70% 4 9% 10% 6% 2	2940
	21st to end	Night	70% 3 8% 9% 8% 1	2940
December	1st to 10th	Night	56% 4 10% 22% 7% 1	4700
	11th to 20th	Night	75% 4 9% 9% 2	5481
	21st to end	Night	76% 5% 10% 7% 2	7623

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	43% 13% 14% 16% 9% 5%	220
	11th to 20th	Day	30% 14% 19% 13% 16% 8%	220
	21st to end	Day	37% 12% 15% 17% 15% 4	265
February	1st to 10th	Day	40% 9% 20% 20% 9% 2	430
	11th to 20th	Day	36% 11% 19% 20% 12% 1	425
	21st to end	Day	36% 12% 22% 19% 9% 1	386
March	1st to 10th	Day	33% 14% 24% 20% 9% 1	540
	11th to 20th	Day	35% 12% 18% 17% 16% 2	540
	21st to end	Day	34% 13% 21% 22% 9% 1	594
April	1st to 10th	Day	27% 14% 23% 23% 10% 4	540
	11th to 20th	Day	24% 13% 24% 24% 12% 3	532
	21st to end	Day	24% 13% 25% 22% 12% 4	540
May	1st to 10th	Day	15% 10% 25% 31% 15% 3	543
	11th to 20th	Day	26% 7% 19% 28% 15% 5%	568
	21st to end	Day	23% 10% 24% 25% 14% 5%	617
June	1st to 10th	Day	21% 8% 23% 28% 16% 3	711
	11th to 20th	Day	19% 10% 26% 31% 11% 2	780
	21st to end	Day	14% 6% 21% 31% 23% 4	780
July	1st to 10th	Day	12% 3 22% 36% 23% 4	771
	11th to 20th	Day	9% 7% 19% 38% 21% 6%	715
	21st to end	Day	9% 4 17% 35% 27% 8%	619
August	1st to 10th	Day	13% 4 13% 32% 29% 10%	567
	11th to 20th	Day	8% 2 16% 31% 32% 11%	557
	21st to end	Day	12% 6% 19% 34% 21% 7%	627
September	1st to 10th	Day	16% 5% 18% 33% 21% 7%	570
	11th to 20th	Day	17% 6% 18% 30% 24% 5%	570
	21st to end	Day	25% 6% 22% 24% 17% 5%	570
October	1st to 10th	Day	24% 6% 18% 26% 18% 7%	570
	11th to 20th	Day	30% 9% 18% 23% 15% 4	570
	21st to end	Day	31% 13% 21% 20% 12% 3	474
November	1st to 10th	Day	35% 14% 21% 18% 10% 2	229
	11th to 20th	Day	40% 10% 16% 14% 14% 6%	220
	21st to end	Day	37% 12% 20% 14% 9% 9%	220
December	1st to 10th	Day	45% 12% 7% 11% 12% 12%	220
	11th to 20th	Day	45% 13% 15% 15% 11%	220
	21st to end	Day	41% 10% 17% 22% 6% 4	242
January	1st to 10th	Night	43% 12% 17% 17% 10% 1	950
	11th to 20th	Night	39% 10% 13% 21% 15% 2	887
	21st to end	Night	38% 8% 17% 23% 14% 2	924
February	1st to 10th	Night	38% 9% 17% 24% 10% 2	840
	11th to 20th	Night	35% 9% 16% 26% 12% 1	824
	21st to end	Night	38% 9% 18% 23% 10% 2	692
March	1st to 10th	Night	35% 12% 17% 24% 12% 1	840
	11th to 20th	Night	37% 11% 16% 23% 12% 1	840
	21st to end	Night	37% 11% 18% 24% 9% 1	924
April	1st to 10th	Night	34% 9% 15% 25% 14% 3	840
	11th to 20th	Night	27% 10% 19% 26% 17% 2	780
	21st to end	Night	31% 8% 14% 33% 13% 2	560
May	1st to 10th	Night	20% 7% 11% 35% 25% 2	420
	11th to 20th	Night	24% 6% 11% 30% 25% 4	414
	21st to end	Night	21% 7% 11% 29% 28% 4	455
June	1st to 10th	Night	25% 5% 11% 35% 21% 3	420
	11th to 20th	Night	21% 6% 10% 37% 21% 3	420
	21st to end	Night	19% 4 10% 31% 32% 5%	420
July	1st to 10th	Night	13% 3 6% 35% 39% 4	410
	11th to 20th	Night	10% 4 10% 35% 37% 5%	408
	21st to end	Night	10% 1 5% 32% 45% 8%	460
August	1st to 10th	Night	10% 2 5% 24% 48% 11%	430
	11th to 20th	Night	11% 2 3 22% 48% 13%	624
	21st to end	Night	15% 4 7% 30% 37% 7%	801
September	1st to 10th	Night	12% 4 7% 36% 35% 6%	840
	11th to 20th	Night	19% 5% 9% 25% 33% 10%	840
	21st to end	Night	22% 5% 13% 29% 26% 5%	840
October	1st to 10th	Night	22% 6% 13% 28% 27% 5%	840
	11th to 20th	Night	32% 6% 13% 25% 19% 5%	840
	21st to end	Night	32% 10% 17% 26% 13% 3	912
November	1st to 10th	Night	40% 10% 17% 23% 9%	820
	11th to 20th	Night	40% 12% 17% 20% 9% 2	840
	21st to end	Night	39% 9% 17% 21% 11% 4	840
December	1st to 10th	Night	36% 8% 15% 24% 16% 3	840
	11th to 20th	Night	43% 12% 20% 18% 7% 1	880
	21st to end	Night	40% 13% 21% 18% 8%	1045

Page generated at 2026-10-26 01:37:47. Version ac07

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