James C. Veen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the James C. Veen 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	13% 11% 4 9% 5% 5% 5% 4 5% 7% 33%	125527
	Night	14% 12% 4 10% 5% 5% 4 3 5% 6% 32%	122762
Transparency	Day	49% 4 11% 21% 13% 1	129054
	Night	52% 4 11% 20% 12% 1	127296
Seeing	Day	20% 16% 30% 20% 10% 4	16849
	Night	20% 12% 30% 26% 10% 1	26678

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	12% 19% 7% 17% 7% 6% 5% 4 4 5% 15%	3617
	11th to 20th	Day	12% 15% 8% 21% 14% 11% 4 3 3 3 6%	3620
	21st to end	Day	18% 20% 9% 16% 7% 4 6% 2 3 4 12%	3982
February	1st to 10th	Day	12% 16% 6% 15% 10% 6% 5% 4 4 4 19%	1841
	11th to 20th	Day	13% 14% 7% 16% 9% 5% 4 3 3 5% 21%	1948
	21st to end	Day	15% 16% 5% 12% 6% 4 5% 4 5% 6% 22%	1650
March	1st to 10th	Day	14% 12% 4 9% 6% 5% 5% 4 4 6% 31%	4480
	11th to 20th	Day	11% 8% 4 8% 5% 5% 5% 4 6% 8% 36%	4877
	21st to end	Day	18% 15% 5% 9% 5% 4 4 3 4 5% 28%	5635
April	1st to 10th	Day	18% 15% 4 7% 5% 4 4 3 4 5% 30%	2400
	11th to 20th	Day	16% 12% 2 7% 4 4 4 3 6% 6% 36%	2443
	21st to end	Day	16% 9% 3 7% 6% 4 5% 3 5% 7% 35%	2600
May	1st to 10th	Day	19% 9% 2 6% 4 4 4 3 5% 7% 36%	5862
	11th to 20th	Day	14% 8% 2 7% 5% 4 5% 4 6% 8% 37%	5861
	21st to end	Day	15% 7% 3 6% 4 4 4 3 5% 7% 41%	6886
June	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 5% 6% 9% 42%	2800
	11th to 20th	Day	11% 6% 3 6% 4 5% 4 4 6% 8% 44%	2800
	21st to end	Day	10% 6% 3 6% 5% 5% 5% 4 6% 9% 42%	2800
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 5% 7% 10% 37%	6360
	11th to 20th	Day	7% 6% 2 6% 4 4 5% 4 6% 9% 45%	6360
	21st to end	Day	7% 6% 2 6% 4 4 5% 4 6% 10% 46%	6924
August	1st to 10th	Day	12% 5% 2 6% 4 5% 4 4 7% 9% 42%	4166
	11th to 20th	Day	10% 6% 2 7% 4 4 5% 4 5% 7% 46%	4044
	21st to end	Day	7% 6% 3 7% 5% 5% 5% 4 6% 9% 43%	4257
September	1st to 10th	Day	9% 8% 3 6% 5% 5% 4 4 6% 8% 43%	2237
	11th to 20th	Day	10% 5% 2 5% 4 3 5% 5% 6% 8% 47%	2090
	21st to end	Day	13% 7% 2 8% 5% 4 4 3 6% 8% 40%	2090
October	1st to 10th	Day	14% 7% 3 7% 5% 3 4 3 5% 6% 42%	2090
	11th to 20th	Day	20% 9% 3 8% 6% 6% 5% 4 5% 7% 27%	1853
	21st to end	Day	22% 11% 3 9% 6% 5% 5% 3 5% 7% 24%	1881
November	1st to 10th	Day	13% 12% 4 8% 4 4 4 3 5% 7% 38%	1762
	11th to 20th	Day	15% 17% 5% 10% 6% 5% 5% 3 6% 6% 23%	1800
	21st to end	Day	19% 17% 5% 10% 5% 4 4 3 4 6% 24%	1800
December	1st to 10th	Day	11% 23% 9% 14% 7% 5% 5% 3 5% 6% 14%	2721
	11th to 20th	Day	15% 22% 11% 14% 8% 4 3 3 3 4 12%	3008
	21st to end	Day	24% 22% 7% 12% 5% 4 3 3 3 4 12%	3982
January	1st to 10th	Night	12% 17% 9% 19% 6% 5% 4 3 4 4 16%	6348
	11th to 20th	Night	12% 17% 8% 20% 11% 10% 4 3 3 4 9%	6360
	21st to end	Night	16% 18% 8% 14% 6% 7% 6% 3 4 4 14%	6996
February	1st to 10th	Night	13% 18% 5% 12% 10% 6% 4 3 4 5% 20%	2553
	11th to 20th	Night	12% 15% 6% 13% 7% 5% 5% 4 4 4 25%	2338
	21st to end	Night	16% 13% 4 9% 6% 7% 5% 4 5% 6% 26%	1980
March	1st to 10th	Night	15% 9% 4 9% 5% 4 4 3 5% 6% 36%	5373
	11th to 20th	Night	11% 8% 3 6% 4 4 4 3 6% 9% 43%	5082
	21st to end	Night	17% 12% 4 8% 4 4 4 3 4 6% 33%	5337
April	1st to 10th	Night	16% 11% 3 8% 4 4 4 4 5% 5% 36%	2000
	11th to 20th	Night	19% 8% 3 7% 4 3 4 3 5% 6% 38%	1904
	21st to end	Night	17% 9% 3 6% 4 3 5% 4 5% 7% 38%	1800
May	1st to 10th	Night	16% 9% 3 8% 5% 4 5% 4 5% 6% 34%	4033
	11th to 20th	Night	16% 8% 3 7% 4 4 4 3 5% 6% 42%	4032
	21st to end	Night	17% 7% 3 6% 5% 4 4 3 4 6% 43%	3985
June	1st to 10th	Night	10% 6% 2 4 4 4 4 4 7% 7% 46%	1600
	11th to 20th	Night	10% 5% 3 7% 4 4 4 5% 6% 9% 43%	1600
	21st to end	Night	9% 5% 1 6% 4 5% 5% 5% 6% 10% 45%	1600
July	1st to 10th	Night	8% 5% 2 6% 4 5% 4 4 7% 10% 44%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 3 6% 8% 51%	3617
	21st to end	Night	9% 6% 2 7% 4 4 4 4 5% 8% 48%	3947
August	1st to 10th	Night	9% 6% 2 5% 4 4 5% 3 6% 8% 48%	2851
	11th to 20th	Night	11% 6% 2 5% 4 4 4 4 6% 7% 47%	2785
	21st to end	Night	8% 7% 3 7% 5% 4 5% 4 6% 7% 44%	3396
September	1st to 10th	Night	9% 5% 2 6% 4 4 4 3 5% 8% 51%	1923
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 5% 7% 53%	2090
	21st to end	Night	12% 7% 3 7% 4 4 4 3 4 8% 43%	2090
October	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 4 6% 48%	2090
	11th to 20th	Night	18% 10% 3 9% 4 4 4 2 4 6% 34%	2322
	21st to end	Night	22% 10% 3 7% 5% 5% 4 3 5% 6% 31%	2717
November	1st to 10th	Night	14% 10% 3 7% 5% 4 4 4 5% 6% 38%	2536
	11th to 20th	Night	15% 15% 4 10% 6% 5% 4 4 6% 6% 25%	2600
	21st to end	Night	17% 17% 5% 9% 5% 4 4 3 5% 6% 25%	2600
December	1st to 10th	Night	12% 19% 9% 15% 7% 4 4 3 4 5% 18%	4464
	11th to 20th	Night	17% 22% 7% 15% 8% 5% 4 3 3 5% 12%	5199
	21st to end	Night	26% 21% 6% 11% 4 4 4 3 4 5% 13%	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	72% 4 11% 12% 2	3618
	11th to 20th	Day	85% 3 6% 4 2	3620
	21st to end	Day	79% 3 8% 8% 2	3982
February	1st to 10th	Day	69% 4 8% 13% 5%	1841
	11th to 20th	Day	67% 4 8% 14% 7%	1958
	21st to end	Day	62% 4 12% 14% 7% 1	1650
March	1st to 10th	Day	54% 3 13% 19% 10% 1	4480
	11th to 20th	Day	46% 5% 15% 23% 10% 1	4877
	21st to end	Day	59% 3 9% 16% 11% 1	5637
April	1st to 10th	Day	56% 4 8% 20% 11% 1	2400
	11th to 20th	Day	48% 4 9% 22% 14% 2	2443
	21st to end	Day	49% 5% 11% 19% 14% 2	2600
May	1st to 10th	Day	46% 5% 10% 20% 16% 2	5897
	11th to 20th	Day	44% 5% 12% 22% 15% 2	5865
	21st to end	Day	41% 5% 10% 24% 18% 2	6900
June	1st to 10th	Day	37% 6% 12% 28% 16% 2	2800
	11th to 20th	Day	37% 5% 13% 25% 19% 1	2800
	21st to end	Day	38% 6% 13% 23% 18% 2	2800
July	1st to 10th	Day	40% 5% 12% 25% 15% 2	6291
	11th to 20th	Day	33% 6% 12% 30% 19% 1	6285
	21st to end	Day	32% 5% 14% 27% 20% 1	6728
August	1st to 10th	Day	38% 5% 12% 28% 17% 1	4152
	11th to 20th	Day	37% 5% 10% 29% 18% 1	4083
	21st to end	Day	36% 6% 13% 29% 16% 2	4256
September	1st to 10th	Day	39% 5% 12% 27% 15% 2	2237
	11th to 20th	Day	33% 5% 12% 30% 17% 2	2090
	21st to end	Day	43% 4 12% 25% 16%	2090
October	1st to 10th	Day	31% 3 8% 36% 20% 2	3480
	11th to 20th	Day	43% 4 12% 27% 15%	3043
	21st to end	Day	51% 3 12% 24% 9% 1	3081
November	1st to 10th	Day	47% 5% 12% 23% 12% 1	1762
	11th to 20th	Day	62% 4 12% 16% 6% 1	1800
	21st to end	Day	63% 3 15% 14% 5%	1800
December	1st to 10th	Day	70% 6% 13% 9% 2	2721
	11th to 20th	Day	77% 3 8% 10% 1	3005
	21st to end	Day	77% 3 6% 11% 3	3982
January	1st to 10th	Night	72% 3 8% 11% 5% 1	6348
	11th to 20th	Night	79% 3 8% 6% 4	6360
	21st to end	Night	75% 3 11% 9% 2	6996
February	1st to 10th	Night	67% 4 9% 15% 6% 1	2553
	11th to 20th	Night	62% 5% 8% 16% 8%	2339
	21st to end	Night	59% 5% 12% 17% 7% 1	1980
March	1st to 10th	Night	48% 4 13% 25% 9% 1	5373
	11th to 20th	Night	38% 5% 15% 27% 13% 2	5082
	21st to end	Night	53% 4 11% 19% 12% 1	5337
April	1st to 10th	Night	50% 4 10% 21% 14% 1	2000
	11th to 20th	Night	47% 4 9% 20% 17% 3	1904
	21st to end	Night	45% 5% 13% 20% 15% 3	1800
May	1st to 10th	Night	49% 5% 13% 15% 16% 3	4060
	11th to 20th	Night	45% 4 9% 22% 17% 3	4033
	21st to end	Night	44% 4 8% 22% 19% 3	3993
June	1st to 10th	Night	35% 6% 10% 30% 18% 2	1600
	11th to 20th	Night	36% 6% 14% 25% 18% 1	1600
	21st to end	Night	34% 6% 15% 27% 16% 2	1600
July	1st to 10th	Night	35% 5% 13% 28% 15% 5%	3580
	11th to 20th	Night	30% 5% 14% 28% 23% 1	3572
	21st to end	Night	35% 5% 13% 23% 22% 2	3832
August	1st to 10th	Night	32% 4 14% 30% 18% 1	2841
	11th to 20th	Night	34% 5% 10% 34% 16% 1	2812
	21st to end	Night	37% 5% 10% 29% 17% 2	3396
September	1st to 10th	Night	32% 6% 12% 29% 19% 2	1923
	11th to 20th	Night	30% 4 10% 34% 20% 1	2090
	21st to end	Night	41% 4 12% 28% 14% 1	2090
October	1st to 10th	Night	28% 3 9% 37% 22% 1	3480
	11th to 20th	Night	42% 4 11% 24% 19%	3858
	21st to end	Night	43% 4 10% 29% 14%	4467
November	1st to 10th	Night	46% 4 10% 24% 14% 1	2536
	11th to 20th	Night	58% 4 12% 18% 7% 1	2600
	21st to end	Night	59% 5% 14% 18% 4	2600
December	1st to 10th	Night	68% 4 13% 12% 3	4464
	11th to 20th	Night	76% 4 8% 9% 3	5201
	21st to end	Night	75% 3 9% 9% 5%	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	21% 20% 38% 15% 4 1	300
	11th to 20th	Day	22% 27% 38% 12% 1 1	300
	21st to end	Day	29% 23% 31% 15% 2	330
February	1st to 10th	Day	27% 29% 31% 11% 2 1	300
	11th to 20th	Day	27% 34% 27% 11% 1	300
	21st to end	Day	34% 33% 27% 6% 1	249
March	1st to 10th	Day	27% 32% 33% 6% 2	300
	11th to 20th	Day	28% 18% 34% 15% 5% 1	449
	21st to end	Day	33% 18% 29% 15% 4 2	550
April	1st to 10th	Day	31% 19% 28% 17% 4 1	500
	11th to 20th	Day	31% 23% 27% 11% 5% 3	492
	21st to end	Day	25% 18% 31% 19% 7% 1	500
May	1st to 10th	Day	26% 21% 27% 18% 6% 2	503
	11th to 20th	Day	21% 16% 30% 22% 8% 4	527
	21st to end	Day	16% 13% 31% 25% 12% 3	766
June	1st to 10th	Day	13% 14% 38% 24% 8% 3	730
	11th to 20th	Day	16% 13% 34% 23% 11% 3	730
	21st to end	Day	15% 12% 38% 22% 11% 3	730
July	1st to 10th	Day	9% 8% 29% 28% 19% 7%	721
	11th to 20th	Day	12% 10% 29% 27% 14% 7%	718
	21st to end	Day	9% 7% 34% 28% 16% 6%	794
August	1st to 10th	Day	10% 6% 26% 26% 21% 12%	512
	11th to 20th	Day	13% 8% 21% 27% 23% 8%	479
	21st to end	Day	13% 6% 26% 27% 19% 9%	539
September	1st to 10th	Day	14% 9% 26% 26% 18% 7%	490
	11th to 20th	Day	11% 6% 20% 30% 24% 9%	490
	21st to end	Day	20% 13% 23% 23% 14% 6%	490
October	1st to 10th	Day	20% 14% 27% 22% 13% 3	490
	11th to 20th	Day	25% 19% 32% 19% 5% 1	410
	21st to end	Day	32% 20% 35% 10% 3	330
November	1st to 10th	Day	26% 17% 38% 13% 5% 1	300
	11th to 20th	Day	26% 26% 35% 9% 3	300
	21st to end	Day	31% 22% 34% 11% 2	300
December	1st to 10th	Day	30% 29% 25% 11% 5% 1	300
	11th to 20th	Day	26% 32% 29% 10% 3	300
	21st to end	Day	30% 27% 33% 7% 2 1	330
January	1st to 10th	Night	22% 21% 31% 19% 6% 1	1000
	11th to 20th	Night	25% 19% 31% 20% 4 1	1000
	21st to end	Night	24% 13% 33% 26% 3	1100
February	1st to 10th	Night	24% 19% 32% 22% 3	808
	11th to 20th	Night	21% 19% 36% 20% 4	783
	21st to end	Night	24% 21% 36% 16% 3	660
March	1st to 10th	Night	20% 19% 31% 23% 7%	800
	11th to 20th	Night	20% 17% 35% 22% 5% 1	800
	21st to end	Night	25% 13% 32% 23% 7%	827
April	1st to 10th	Night	22% 15% 35% 23% 5% 1	600
	11th to 20th	Night	26% 13% 33% 21% 6% 1	593
	21st to end	Night	23% 13% 33% 25% 6%	600
May	1st to 10th	Night	19% 11% 35% 28% 8% 1	600
	11th to 20th	Night	20% 10% 31% 28% 11%	591
	21st to end	Night	16% 6% 28% 32% 17% 1	650
June	1st to 10th	Night	13% 8% 32% 34% 14% 1	600
	11th to 20th	Night	13% 6% 31% 35% 14% 1	600
	21st to end	Night	11% 5% 29% 37% 17% 2	600
July	1st to 10th	Night	10% 3 22% 40% 22% 3	585
	11th to 20th	Night	8% 2 26% 38% 22% 3	582
	21st to end	Night	8% 2 25% 39% 25% 2	657
August	1st to 10th	Night	10% 3 20% 39% 26% 3	579
	11th to 20th	Night	11% 2 19% 40% 23% 5%	564
	21st to end	Night	12% 3 21% 36% 25% 3	627
September	1st to 10th	Night	10% 5% 24% 35% 21% 5%	593
	11th to 20th	Night	11% 5% 22% 36% 23% 3	760
	21st to end	Night	18% 9% 22% 30% 18% 3	760
October	1st to 10th	Night	18% 7% 28% 30% 15% 1	760
	11th to 20th	Night	25% 15% 29% 23% 8%	760
	21st to end	Night	27% 13% 32% 21% 6%	836
November	1st to 10th	Night	22% 14% 31% 24% 9%	780
	11th to 20th	Night	24% 16% 34% 19% 6% 1	800
	21st to end	Night	26% 16% 33% 22% 3	800
December	1st to 10th	Night	26% 17% 30% 21% 5%	923
	11th to 20th	Night	27% 17% 35% 17% 4 1	1000
	21st to end	Night	33% 14% 34% 17% 1	1100

Page generated at 2025-08-02 01:32:27. Version ac07

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