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 2024-11-04.

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%	101232
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	97787
Transparency	Day	43% 6% 12% 22% 15% 2	104781
	Night	44% 5% 12% 22% 15% 2	102326
Seeing	Day	22% 18% 31% 19% 8% 2	15030
	Night	22% 15% 32% 24% 7% 1	24032

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	15% 7% 2 6% 3 4 4 3 5% 4 45%	2517
	11th to 20th	Day	17% 9% 3 8% 5% 4 5% 4 5% 5% 36%	2520
	21st to end	Day	22% 13% 3 7% 4 3 3 3 5% 4 33%	2816
February	1st to 10th	Day	19% 8% 2 6% 4 5% 5% 4 5% 6% 37%	1786
	11th to 20th	Day	14% 7% 2 7% 5% 5% 4 3 6% 7% 41%	1848
	21st to end	Day	21% 9% 3 5% 4 5% 3 3 4 5% 37%	1570
March	1st to 10th	Day	26% 10% 3 8% 4 3 3 2 3 5% 33%	3116
	11th to 20th	Day	11% 5% 2 7% 5% 5% 6% 5% 7% 7% 41%	3257
	21st to end	Day	21% 7% 2 7% 4 3 4 3 5% 6% 37%	4166
April	1st to 10th	Day	19% 9% 3 5% 4 4 4 4 5% 7% 37%	2280
	11th to 20th	Day	19% 6% 2 5% 4 4 4 3 5% 6% 41%	2254
	21st to end	Day	21% 5% 2 5% 4 4 5% 3 5% 5% 40%	2399
May	1st to 10th	Day	27% 9% 3 6% 3 4 4 3 5% 7% 29%	4106
	11th to 20th	Day	22% 9% 3 7% 4 4 4 3 5% 6% 33%	4253
	21st to end	Day	15% 6% 2 5% 4 4 4 4 5% 8% 45%	4837
June	1st to 10th	Day	17% 6% 2 6% 5% 4 5% 4 7% 9% 35%	2660
	11th to 20th	Day	15% 7% 3 6% 4 4 5% 4 7% 9% 36%	2660
	21st to end	Day	13% 7% 2 7% 4 5% 4 4 6% 9% 37%	2660
July	1st to 10th	Day	12% 6% 3 8% 6% 5% 6% 5% 6% 9% 34%	4440
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 5% 7% 10% 38%	4440
	21st to end	Day	17% 7% 3 7% 5% 4 5% 4 6% 7% 34%	4814
August	1st to 10th	Day	19% 7% 3 7% 5% 5% 5% 4 7% 9% 28%	4240
	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	14% 7% 2 5% 4 4 4 3 4 5% 48%	1698
	11th to 20th	Day	19% 7% 2 4 3 3 3 3 3 7% 46%	1710
	21st to end	Day	15% 8% 2 6% 4 4 4 3 4 6% 44%	1695
December	1st to 10th	Day	19% 8% 2 6% 5% 5% 3 3 5% 6% 37%	1520
	11th to 20th	Day	22% 7% 2 4 3 2 3 2 3 6% 44%	1906
	21st to end	Day	20% 11% 4 8% 5% 4 3 2 4 6% 32%	2772
January	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 6% 6% 44%	4429
	11th to 20th	Night	16% 10% 3 7% 3 4 3 3 3 4 43%	4440
	21st to end	Night	24% 12% 3 7% 4 3 3 3 3 5% 32%	4838
February	1st to 10th	Night	16% 10% 2 7% 4 3 4 3 5% 6% 40%	2392
	11th to 20th	Night	16% 8% 3 6% 4 4 4 4 5% 6% 40%	2218
	21st to end	Night	20% 9% 2 6% 4 3 4 3 5% 5% 40%	1884
March	1st to 10th	Night	27% 10% 2 6% 3 3 3 2 5% 5% 33%	3724
	11th to 20th	Night	11% 8% 3 7% 5% 4 4 4 6% 7% 41%	3698
	21st to end	Night	22% 8% 3 5% 3 4 3 3 5% 5% 38%	3466
April	1st to 10th	Night	18% 8% 3 6% 5% 5% 5% 3 5% 6% 36%	1900
	11th to 20th	Night	19% 7% 3 7% 4 4 4 3 4 7% 40%	1875
	21st to end	Night	21% 7% 2 7% 4 3 3 3 4 6% 40%	1781
May	1st to 10th	Night	24% 10% 3 7% 5% 5% 4 3 5% 6% 28%	2834
	11th to 20th	Night	21% 10% 3 8% 5% 4 4 4 5% 7% 29%	2669
	21st to end	Night	17% 7% 3 4 3 3 4 4 6% 7% 41%	2738
June	1st to 10th	Night	17% 8% 2 7% 4 3 4 5% 5% 7% 39%	1520
	11th to 20th	Night	16% 6% 3 7% 4 4 4 4 5% 10% 38%	1520
	21st to end	Night	13% 6% 3 6% 5% 4 4 4 6% 8% 41%	1520
July	1st to 10th	Night	13% 7% 2 7% 6% 5% 5% 4 7% 10% 34%	2520
	11th to 20th	Night	16% 6% 2 7% 5% 4 4 4 6% 9% 36%	2520
	21st to end	Night	18% 9% 3 8% 5% 4 4 4 6% 7% 33%	2739
August	1st to 10th	Night	21% 6% 3 8% 5% 5% 5% 4 7% 7% 29%	2656
	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%	2453
	11th to 20th	Night	19% 7% 2 4 4 3 4 3 5% 5% 44%	2470
	21st to end	Night	16% 7% 2 5% 4 4 4 3 5% 7% 44%	2483
December	1st to 10th	Night	21% 8% 2 6% 4 3 4 4 4 5% 39%	2660
	11th to 20th	Night	20% 7% 2 5% 4 4 4 3 4 5% 41%	3288
	21st to end	Night	22% 11% 3 7% 4 4 3 3 4 4 35%	4884

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 10% 31% 11% 3	2518
	11th to 20th	Day	48% 6% 10% 24% 11% 1	2520
	21st to end	Day	54% 5% 8% 21% 10% 2	2816
February	1st to 10th	Day	46% 6% 13% 20% 14% 2	1786
	11th to 20th	Day	43% 5% 11% 23% 16% 3	1857
	21st to end	Day	48% 5% 11% 20% 12% 3	1570
March	1st to 10th	Day	56% 3 12% 17% 10% 2	3119
	11th to 20th	Day	37% 6% 16% 28% 12% 2	3257
	21st to end	Day	47% 4 8% 18% 19% 4	4166
April	1st to 10th	Day	46% 6% 11% 19% 15% 3	2280
	11th to 20th	Day	43% 5% 11% 20% 18% 3	2254
	21st to end	Day	45% 6% 10% 21% 15% 4	2399
May	1st to 10th	Day	53% 7% 14% 14% 11% 1	4120
	11th to 20th	Day	50% 6% 14% 18% 9% 2	4257
	21st to end	Day	36% 7% 13% 25% 18% 2	4837
June	1st to 10th	Day	44% 7% 13% 23% 11% 1	2660
	11th to 20th	Day	42% 8% 14% 21% 12% 2	2660
	21st to end	Day	42% 8% 14% 21% 14% 1	2660
July	1st to 10th	Day	46% 8% 11% 19% 15% 1	4370
	11th to 20th	Day	40% 7% 16% 22% 13% 1	4365
	21st to end	Day	48% 7% 11% 17% 15% 2	4661
August	1st to 10th	Day	50% 9% 14% 20% 8%	4226
	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% 5% 12% 22% 20% 2	1698
	11th to 20th	Day	40% 5% 11% 23% 19% 3	1710
	21st to end	Day	41% 5% 13% 26% 13% 3	1695
December	1st to 10th	Day	48% 5% 14% 17% 15% 1	1520
	11th to 20th	Day	44% 3 15% 19% 17% 2	1906
	21st to end	Day	53% 4 11% 21% 10% 1	2772
January	1st to 10th	Night	39% 5% 11% 31% 11% 3	4429
	11th to 20th	Night	46% 4 12% 23% 12% 3	4440
	21st to end	Night	56% 3 9% 19% 11% 2	4838
February	1st to 10th	Night	46% 5% 12% 24% 12% 2	2392
	11th to 20th	Night	44% 5% 11% 20% 17% 4	2220
	21st to end	Night	46% 4 9% 22% 14% 4	1884
March	1st to 10th	Night	53% 4 10% 19% 11% 2	3734
	11th to 20th	Night	39% 7% 15% 22% 14% 2	3699
	21st to end	Night	47% 3 8% 13% 24% 5%	3466
April	1st to 10th	Night	47% 6% 11% 18% 15% 3	1900
	11th to 20th	Night	45% 5% 10% 20% 19% 2	1875
	21st to end	Night	46% 5% 9% 21% 14% 5%	1781
May	1st to 10th	Night	55% 7% 13% 14% 9% 1	2840
	11th to 20th	Night	54% 8% 13% 14% 9% 2	2670
	21st to end	Night	40% 7% 15% 24% 12% 1	2739
June	1st to 10th	Night	44% 7% 12% 20% 16% 1	1520
	11th to 20th	Night	42% 7% 14% 26% 11% 1	1520
	21st to end	Night	40% 8% 13% 25% 14%	1520
July	1st to 10th	Night	45% 9% 15% 18% 12% 1	2480
	11th to 20th	Night	43% 7% 16% 23% 9% 1	2472
	21st to end	Night	49% 6% 13% 15% 16% 2	2651
August	1st to 10th	Night	53% 8% 13% 15% 9% 1	2646
	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% 20% 4	2453
	11th to 20th	Night	42% 4 10% 22% 18% 3	2470
	21st to end	Night	41% 4 13% 29% 12% 2	2483
December	1st to 10th	Night	47% 5% 13% 21% 14% 1	2660
	11th to 20th	Night	45% 5% 14% 18% 17% 2	3288
	21st to end	Night	52% 4 10% 22% 11%	4884

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	29% 23% 29% 17% 2	180
	11th to 20th	Day	29% 29% 28% 11% 3	180
	21st to end	Day	24% 30% 32% 8% 5%	213
February	1st to 10th	Day	32% 25% 28% 15%	346
	11th to 20th	Day	28% 29% 33% 9% 1	456
	21st to end	Day	31% 25% 29% 11% 4	382
March	1st to 10th	Day	26% 24% 35% 13% 2	460
	11th to 20th	Day	27% 28% 31% 12% 2 1	460
	21st to end	Day	31% 23% 31% 9% 4 1	506
April	1st to 10th	Day	29% 29% 26% 12% 4	460
	11th to 20th	Day	29% 23% 29% 14% 5%	452
	21st to end	Day	28% 21% 32% 13% 4 2	460
May	1st to 10th	Day	32% 21% 28% 12% 6% 1	463
	11th to 20th	Day	24% 18% 37% 15% 6% 1	487
	21st to end	Day	20% 16% 31% 24% 8% 2	529
June	1st to 10th	Day	19% 14% 33% 23% 10% 2	490
	11th to 20th	Day	17% 16% 37% 21% 8% 1	490
	21st to end	Day	16% 18% 31% 25% 9% 1	490
July	1st to 10th	Day	18% 8% 30% 30% 11% 2	485
	11th to 20th	Day	14% 8% 34% 29% 13% 2	484
	21st to end	Day	16% 10% 39% 27% 7% 2	532
August	1st to 10th	Day	14% 9% 32% 32% 9% 4	486
	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% 29% 23% 8% 2	374
	11th to 20th	Day	26% 22% 27% 19% 5%	370
	21st to end	Day	24% 21% 31% 17% 7% 1	355
December	1st to 10th	Day	23% 25% 32% 15% 6%	180
	11th to 20th	Day	32% 26% 31% 9% 2	180
	21st to end	Day	27% 25% 27% 16% 4 2	198
January	1st to 10th	Night	22% 21% 38% 16% 3	760
	11th to 20th	Night	29% 19% 32% 17% 2	760
	21st to end	Night	23% 23% 32% 19% 3	836
February	1st to 10th	Night	23% 19% 35% 20% 2	760
	11th to 20th	Night	24% 24% 37% 14% 1	743
	21st to end	Night	27% 22% 33% 16% 3	628
March	1st to 10th	Night	26% 19% 35% 16% 3	760
	11th to 20th	Night	23% 20% 35% 18% 3	760
	21st to end	Night	25% 22% 34% 16% 3	836
April	1st to 10th	Night	24% 21% 34% 19% 3	760
	11th to 20th	Night	23% 20% 34% 20% 3	751
	21st to end	Night	22% 18% 36% 18% 5%	641
May	1st to 10th	Night	25% 19% 30% 20% 5%	570
	11th to 20th	Night	20% 17% 38% 20% 4	483
	21st to end	Night	20% 8% 30% 34% 8%	411
June	1st to 10th	Night	22% 6% 32% 31% 9%	380
	11th to 20th	Night	20% 6% 32% 34% 8%	380
	21st to end	Night	15% 7% 33% 38% 7%	380
July	1st to 10th	Night	14% 4 32% 37% 14%	370
	11th to 20th	Night	14% 3 27% 42% 14% 1	368
	21st to end	Night	16% 7% 28% 39% 9%	416
August	1st to 10th	Night	11% 5% 32% 40% 11% 1	463
	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% 34% 24% 9% 1	755
	11th to 20th	Night	24% 18% 33% 19% 6% 1	760
	21st to end	Night	22% 19% 32% 21% 5% 1	760
December	1st to 10th	Night	28% 17% 30% 20% 5% 1	760
	11th to 20th	Night	24% 22% 31% 20% 3	760
	21st to end	Night	27% 18% 36% 16% 2	836

Page generated at 2024-11-01 22:47:42. Version ac07

Copyright © 2024 Attilla Danko.