Van Vleck Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Van Vleck Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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	17% 7% 2 6% 4 4 4 4 5% 7% 38%	125531
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	122522
Transparency	Day	44% 6% 12% 21% 15% 3	129062
	Night	44% 5% 12% 21% 16% 3	127077
Seeing	Day	23% 18% 31% 19% 7% 2	15971
	Night	22% 15% 32% 24% 7% 1	24942

By Month

Percentage Cloud Cover NE (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% 8% 3 8% 5% 5% 5% 3 5% 5% 42%	3597
	11th to 20th	Day	14% 9% 3 7% 4 4 4 4 5% 6% 42%	3570
	21st to end	Day	19% 9% 2 7% 4 4 3 3 4 4 41%	4032
February	1st to 10th	Day	18% 8% 2 6% 4 5% 5% 4 5% 6% 37%	1881
	11th to 20th	Day	15% 7% 2 7% 5% 5% 4 3 6% 7% 39%	1948
	21st to end	Day	20% 9% 3 5% 4 4 3 3 4 6% 38%	1650
March	1st to 10th	Day	22% 9% 3 7% 3 3 3 2 4 5% 38%	4476
	11th to 20th	Day	14% 7% 3 6% 4 4 5% 4 6% 6% 40%	4645
	21st to end	Day	20% 8% 2 7% 4 3 5% 4 6% 6% 35%	5970
April	1st to 10th	Day	19% 8% 3 5% 4 4 4 4 5% 7% 37%	2400
	11th to 20th	Day	19% 7% 2 5% 4 4 4 3 5% 6% 41%	2374
	21st to end	Day	21% 5% 2 5% 4 4 5% 3 5% 5% 40%	2525
May	1st to 10th	Day	30% 9% 3 6% 4 4 4 3 5% 8% 26%	5886
	11th to 20th	Day	20% 9% 3 7% 4 4 4 4 5% 6% 33%	6103
	21st to end	Day	21% 7% 2 6% 4 4 4 4 6% 7% 34%	6949
June	1st to 10th	Day	17% 6% 2 6% 5% 4 5% 4 7% 9% 34%	2800
	11th to 20th	Day	16% 7% 3 6% 4 4 5% 4 6% 9% 36%	2800
	21st to end	Day	13% 7% 2 7% 4 5% 4 4 6% 9% 37%	2800
July	1st to 10th	Day	14% 7% 3 7% 6% 5% 6% 4 6% 9% 32%	6360
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 11% 35%	6360
	21st to end	Day	15% 6% 3 6% 4 4 4 3 5% 7% 41%	6926
August	1st to 10th	Day	18% 7% 3 7% 5% 6% 5% 4 7% 9% 29%	4296
	11th to 20th	Day	11% 6% 3 7% 5% 4 6% 3 6% 8% 41%	3775
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 9% 42%	4180
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 5% 7% 45%	2280
	11th to 20th	Day	12% 7% 2 6% 3 3 4 4 4 7% 50%	2138
	21st to end	Day	18% 7% 2 7% 4 3 3 4 5% 6% 41%	2090
October	1st to 10th	Day	21% 7% 2 6% 4 4 3 3 5% 5% 40%	2076
	11th to 20th	Day	14% 6% 1 6% 4 3 4 4 6% 7% 46%	1872
	21st to end	Day	20% 6% 3 6% 3 4 4 3 5% 6% 40%	1881
November	1st to 10th	Day	13% 7% 2 6% 4 4 4 3 4 5% 49%	1761
	11th to 20th	Day	18% 6% 2 4 3 3 3 3 3 7% 48%	1800
	21st to end	Day	16% 8% 2 6% 4 4 4 3 4 6% 44%	1784
December	1st to 10th	Day	16% 8% 2 5% 4 4 3 3 5% 6% 45%	2600
	11th to 20th	Day	21% 7% 2 4 3 3 3 2 4 6% 45%	2986
	21st to end	Day	20% 10% 3 7% 4 4 3 2 4 6% 37%	3960
January	1st to 10th	Night	14% 6% 2 7% 5% 4 4 3 5% 6% 44%	6349
	11th to 20th	Night	14% 8% 3 8% 4 4 3 3 4 5% 44%	6315
	21st to end	Night	18% 10% 3 7% 5% 4 4 3 4 6% 37%	6922
February	1st to 10th	Night	16% 10% 2 7% 4 3 4 3 5% 6% 39%	2517
	11th to 20th	Night	16% 8% 3 6% 4 4 4 4 5% 6% 40%	2338
	21st to end	Night	19% 9% 2 6% 4 4 4 3 5% 5% 39%	1980
March	1st to 10th	Night	22% 9% 3 6% 3 3 3 2 5% 6% 39%	5364
	11th to 20th	Night	16% 10% 3 6% 4 3 4 3 5% 6% 39%	5296
	21st to end	Night	22% 9% 3 6% 3 4 4 3 5% 5% 36%	4962
April	1st to 10th	Night	19% 8% 3 6% 5% 5% 5% 4 5% 6% 36%	2000
	11th to 20th	Night	19% 7% 3 7% 4 4 4 3 4 7% 39%	1975
	21st to end	Night	21% 7% 2 7% 4 3 3 3 4 6% 40%	1874
May	1st to 10th	Night	28% 10% 3 7% 4 4 4 3 5% 6% 26%	4054
	11th to 20th	Night	20% 11% 3 8% 4 4 4 3 5% 7% 30%	3833
	21st to end	Night	22% 9% 3 6% 4 3 4 4 6% 7% 33%	3926
June	1st to 10th	Night	17% 8% 2 7% 5% 4 4 5% 5% 7% 39%	1600
	11th to 20th	Night	17% 6% 3 7% 4 4 5% 4 5% 10% 37%	1600
	21st to end	Night	13% 6% 3 6% 5% 4 4 4 6% 8% 42%	1600
July	1st to 10th	Night	15% 7% 3 7% 5% 4 5% 4 6% 9% 36%	3600
	11th to 20th	Night	15% 8% 3 7% 5% 4 5% 4 7% 9% 34%	3600
	21st to end	Night	13% 7% 3 6% 5% 4 4 4 5% 6% 44%	3926
August	1st to 10th	Night	21% 6% 3 8% 5% 5% 5% 4 7% 7% 29%	2688
	11th to 20th	Night	12% 7% 3 7% 5% 5% 4 4 5% 7% 40%	3036
	21st to end	Night	9% 4 2 5% 4 4 4 4 5% 8% 53%	3476
September	1st to 10th	Night	14% 6% 2 6% 5% 4 4 4 6% 7% 43%	1900
	11th to 20th	Night	13% 6% 2 5% 4 2 3 2 4 6% 53%	2041
	21st to end	Night	17% 7% 2 6% 4 3 5% 4 4 6% 42%	2090
October	1st to 10th	Night	19% 7% 3 6% 4 4 3 3 5% 5% 41%	2100
	11th to 20th	Night	16% 6% 2 5% 3 3 4 3 5% 6% 48%	2308
	21st to end	Night	22% 7% 2 6% 3 2 4 3 5% 5% 40%	2717
November	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 5% 6% 49%	2537
	11th to 20th	Night	19% 7% 2 4 4 3 4 3 5% 5% 45%	2600
	21st to end	Night	16% 7% 2 5% 4 4 4 3 4 7% 43%	2614
December	1st to 10th	Night	18% 8% 2 5% 4 3 4 3 3 5% 46%	4580
	11th to 20th	Night	24% 8% 3 5% 3 3 3 3 3 4 41%	5208
	21st to end	Night	18% 10% 3 7% 3 4 3 3 4 5% 39%	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	41% 4 8% 28% 16% 3	3598
	11th to 20th	Day	42% 5% 10% 22% 14% 7%	3600
	21st to end	Day	47% 4 9% 23% 15% 2	4032
February	1st to 10th	Day	46% 6% 13% 20% 14% 2	1881
	11th to 20th	Day	44% 5% 10% 23% 15% 3	1957
	21st to end	Day	47% 5% 12% 20% 13% 3	1650
March	1st to 10th	Day	49% 3 11% 16% 12% 9%	4479
	11th to 20th	Day	41% 5% 15% 27% 10% 2	4645
	21st to end	Day	48% 4 8% 15% 19% 6%	5970
April	1st to 10th	Day	46% 5% 11% 19% 15% 3	2400
	11th to 20th	Day	44% 5% 11% 20% 18% 3	2374
	21st to end	Day	45% 5% 10% 21% 15% 4	2525
May	1st to 10th	Day	56% 7% 14% 13% 8% 1	5897
	11th to 20th	Day	49% 6% 13% 18% 12% 1	6107
	21st to end	Day	46% 7% 11% 22% 13% 1	6949
June	1st to 10th	Day	45% 7% 13% 23% 12% 1	2800
	11th to 20th	Day	43% 8% 14% 21% 13% 2	2800
	21st to end	Day	42% 8% 14% 21% 14% 1	2800
July	1st to 10th	Day	48% 8% 10% 19% 14% 1	6290
	11th to 20th	Day	41% 8% 17% 22% 11% 1	6285
	21st to end	Day	41% 6% 10% 22% 19% 1	6728
August	1st to 10th	Day	49% 9% 14% 19% 8%	4282
	11th to 20th	Day	39% 6% 14% 29% 11% 1	3812
	21st to end	Day	35% 8% 13% 26% 16% 2	4180
September	1st to 10th	Day	39% 7% 12% 22% 19% 2	2280
	11th to 20th	Day	34% 4 11% 23% 24% 3	2138
	21st to end	Day	44% 6% 8% 20% 20% 2	2090
October	1st to 10th	Day	39% 5% 9% 19% 23% 5%	3453
	11th to 20th	Day	29% 3 9% 24% 31% 4	3088
	21st to end	Day	32% 5% 13% 25% 22% 3	3081
November	1st to 10th	Day	39% 4 12% 21% 21% 2	1761
	11th to 20th	Day	38% 5% 10% 23% 21% 4	1800
	21st to end	Day	42% 5% 13% 26% 12% 3	1784
December	1st to 10th	Day	41% 4 15% 28% 12% 1	2600
	11th to 20th	Day	42% 3 14% 19% 18% 3	2986
	21st to end	Day	49% 4 8% 19% 15% 5%	3960
January	1st to 10th	Night	38% 4 10% 28% 17% 3	6349
	11th to 20th	Night	43% 3 13% 21% 15% 5%	6360
	21st to end	Night	49% 3 10% 18% 16% 3	6922
February	1st to 10th	Night	46% 4 12% 24% 11% 2	2517
	11th to 20th	Night	44% 5% 10% 20% 17% 4	2340
	21st to end	Night	47% 4 9% 21% 15% 4	1980
March	1st to 10th	Night	47% 4 11% 18% 14% 6%	5374
	11th to 20th	Night	45% 5% 15% 20% 11% 3	5297
	21st to end	Night	49% 4 6% 12% 24% 5%	4962
April	1st to 10th	Night	48% 5% 11% 18% 14% 4	2000
	11th to 20th	Night	45% 5% 10% 19% 19% 2	1975
	21st to end	Night	45% 5% 9% 21% 15% 4	1874
May	1st to 10th	Night	58% 6% 14% 12% 9% 1	4060
	11th to 20th	Night	53% 7% 12% 15% 10% 3	3834
	21st to end	Night	49% 7% 13% 22% 9% 1	3927
June	1st to 10th	Night	44% 7% 12% 20% 16% 1	1600
	11th to 20th	Night	43% 7% 14% 25% 11% 1	1600
	21st to end	Night	39% 8% 13% 24% 14% 1	1600
July	1st to 10th	Night	46% 7% 14% 21% 11% 1	3560
	11th to 20th	Night	47% 7% 16% 20% 9% 1	3552
	21st to end	Night	40% 6% 12% 23% 18% 1	3809
August	1st to 10th	Night	53% 8% 13% 15% 10% 1	2678
	11th to 20th	Night	43% 7% 13% 29% 8%	3064
	21st to end	Night	28% 6% 15% 32% 17% 1	3476
September	1st to 10th	Night	41% 6% 9% 25% 18% 1	1900
	11th to 20th	Night	33% 4 12% 23% 25% 3	2041
	21st to end	Night	44% 5% 8% 20% 20% 2	2090
October	1st to 10th	Night	39% 5% 12% 23% 18% 2	3503
	11th to 20th	Night	26% 4 9% 26% 31% 3	3831
	21st to end	Night	33% 3 14% 28% 20% 2	4467
November	1st to 10th	Night	36% 5% 12% 22% 21% 4	2537
	11th to 20th	Night	41% 4 10% 22% 20% 4	2600
	21st to end	Night	41% 4 13% 29% 13% 2	2614
December	1st to 10th	Night	42% 3 12% 29% 13% 1	4580
	11th to 20th	Night	48% 4 12% 16% 18% 2	5208
	21st to end	Night	47% 3 9% 19% 17% 4	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	33% 23% 28% 16% 2	200
	11th to 20th	Day	30% 31% 28% 10% 3	200
	21st to end	Day	26% 30% 30% 8% 5%	237
February	1st to 10th	Day	33% 25% 28% 14%	381
	11th to 20th	Day	31% 29% 31% 8% 1	496
	21st to end	Day	30% 26% 29% 11% 4	414
March	1st to 10th	Day	28% 24% 34% 12% 2	500
	11th to 20th	Day	27% 28% 31% 12% 2 1	500
	21st to end	Day	32% 23% 31% 9% 4 1	550
April	1st to 10th	Day	31% 28% 25% 11% 4	500
	11th to 20th	Day	31% 23% 28% 13% 5%	492
	21st to end	Day	29% 21% 33% 12% 4 1	500
May	1st to 10th	Day	33% 20% 29% 12% 6% 1	503
	11th to 20th	Day	25% 20% 35% 14% 6% 1	527
	21st to end	Day	21% 16% 31% 23% 8% 2	573
June	1st to 10th	Day	20% 14% 33% 22% 9% 2	530
	11th to 20th	Day	19% 15% 36% 22% 7% 1	530
	21st to end	Day	16% 17% 30% 27% 9% 1	530
July	1st to 10th	Day	18% 9% 31% 30% 10% 2	525
	11th to 20th	Day	13% 9% 34% 29% 13% 2	524
	21st to end	Day	16% 10% 39% 28% 7% 2	576
August	1st to 10th	Day	14% 9% 32% 31% 9% 4	502
	11th to 20th	Day	15% 9% 32% 27% 15% 4	479
	21st to end	Day	13% 10% 32% 25% 14% 6%	539
September	1st to 10th	Day	20% 10% 25% 24% 17% 3	490
	11th to 20th	Day	14% 12% 28% 24% 13% 8%	490
	21st to end	Day	21% 13% 36% 17% 10% 3	490
October	1st to 10th	Day	26% 13% 26% 19% 12% 4	490
	11th to 20th	Day	18% 18% 32% 21% 9% 2	475
	21st to end	Day	24% 17% 31% 19% 7% 2	429
November	1st to 10th	Day	21% 17% 30% 22% 8% 2	395
	11th to 20th	Day	25% 23% 28% 19% 5%	400
	21st to end	Day	25% 22% 30% 16% 6% 1	384
December	1st to 10th	Day	23% 26% 32% 15% 5%	200
	11th to 20th	Day	35% 25% 29% 10% 2	200
	21st to end	Day	26% 24% 28% 16% 4 1	220
January	1st to 10th	Night	23% 22% 37% 15% 3	800
	11th to 20th	Night	29% 19% 32% 18% 2	800
	21st to end	Night	22% 23% 33% 19% 3	880
February	1st to 10th	Night	23% 21% 34% 20% 2	800
	11th to 20th	Night	26% 23% 36% 14% 1	783
	21st to end	Night	26% 22% 34% 16% 3	660
March	1st to 10th	Night	27% 20% 34% 16% 3	800
	11th to 20th	Night	24% 20% 34% 18% 3	800
	21st to end	Night	24% 23% 34% 15% 3	880
April	1st to 10th	Night	25% 22% 33% 18% 3	800
	11th to 20th	Night	24% 21% 33% 19% 3	791
	21st to end	Night	22% 19% 36% 17% 5%	674
May	1st to 10th	Night	26% 18% 30% 20% 5%	600
	11th to 20th	Night	20% 17% 39% 20% 4	509
	21st to end	Night	21% 7% 31% 33% 7%	433
June	1st to 10th	Night	21% 6% 33% 31% 9%	400
	11th to 20th	Night	20% 6% 33% 33% 8%	400
	21st to end	Night	14% 7% 34% 38% 7%	400
July	1st to 10th	Night	14% 4 32% 37% 13%	390
	11th to 20th	Night	14% 3 27% 41% 14% 1	388
	21st to end	Night	15% 7% 29% 39% 9%	438
August	1st to 10th	Night	12% 5% 32% 40% 11% 1	471
	11th to 20th	Night	13% 5% 25% 38% 17% 1	713
	21st to end	Night	14% 5% 26% 36% 17% 2	836
September	1st to 10th	Night	17% 6% 32% 32% 12% 2	760
	11th to 20th	Night	17% 6% 26% 31% 17% 3	760
	21st to end	Night	20% 8% 28% 27% 15% 1	760
October	1st to 10th	Night	24% 9% 27% 26% 12% 2	760
	11th to 20th	Night	19% 14% 32% 26% 9% 1	760
	21st to end	Night	28% 12% 31% 22% 6%	836
November	1st to 10th	Night	17% 15% 35% 24% 9% 1	780
	11th to 20th	Night	24% 19% 32% 19% 6% 1	800
	21st to end	Night	23% 19% 32% 21% 5% 1	800
December	1st to 10th	Night	28% 17% 30% 20% 5% 1	800
	11th to 20th	Night	25% 21% 31% 20% 3	800
	21st to end	Night	27% 18% 36% 17% 2	880

Page generated at 2025-08-02 01:32:29. Version ac07

Copyright © 2025 Attilla Danko.