Valley View Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Valley View Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	16% 8% 3 7% 5% 5% 5% 4 6% 7% 34%	127125
	Night	17% 8% 3 7% 5% 4 4 4 5% 7% 36%	123232
Transparency	Day	46% 6% 12% 22% 12% 2	130699
	Night	46% 6% 12% 21% 13% 2	127744
Seeing	Day	22% 15% 27% 21% 11% 4	16155
	Night	20% 12% 27% 29% 11% 1	25462

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% 11% 5% 9% 6% 4 5% 4 6% 5% 32%	3597
	11th to 20th	Day	14% 13% 6% 14% 8% 5% 4 3 5% 5% 22%	3780
	21st to end	Day	18% 13% 4 9% 5% 4 4 3 5% 6% 30%	4466
February	1st to 10th	Day	17% 10% 4 9% 7% 7% 5% 3 5% 6% 28%	1800
	11th to 20th	Day	15% 9% 4 9% 6% 5% 6% 3 5% 7% 31%	1803
	21st to end	Day	19% 9% 3 9% 6% 4 4 4 4 6% 31%	1650
March	1st to 10th	Day	20% 9% 2 8% 4 4 4 3 4 4 37%	4696
	11th to 20th	Day	13% 10% 3 8% 5% 4 4 3 5% 7% 39%	4980
	21st to end	Day	15% 8% 2 6% 5% 5% 5% 3 5% 6% 38%	5646
April	1st to 10th	Day	18% 10% 3 7% 6% 4 5% 4 4 7% 33%	2400
	11th to 20th	Day	18% 10% 2 6% 5% 4 4 3 5% 6% 36%	2374
	21st to end	Day	17% 6% 3 6% 5% 4 4 3 6% 7% 37%	2400
May	1st to 10th	Day	27% 7% 2 6% 4 5% 5% 4 5% 7% 28%	6016
	11th to 20th	Day	23% 8% 3 8% 6% 5% 4 3 5% 7% 28%	6318
	21st to end	Day	20% 5% 2 6% 4 5% 5% 5% 7% 9% 33%	6949
June	1st to 10th	Day	13% 6% 3 7% 6% 5% 6% 4 6% 9% 35%	2800
	11th to 20th	Day	15% 7% 2 6% 5% 5% 4 5% 6% 8% 37%	2800
	21st to end	Day	12% 6% 2 5% 5% 5% 5% 5% 7% 9% 41%	2800
July	1st to 10th	Day	7% 5% 3 8% 6% 6% 6% 5% 7% 9% 39%	6360
	11th to 20th	Day	8% 6% 3 8% 5% 7% 6% 6% 8% 11% 32%	6360
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 4 7% 8% 38%	6660
August	1st to 10th	Day	20% 6% 2 8% 5% 5% 5% 4 6% 8% 31%	4234
	11th to 20th	Day	16% 7% 2 6% 4 5% 6% 5% 7% 8% 34%	4019
	21st to end	Day	14% 5% 2 5% 3 4 4 4 6% 9% 44%	4312
September	1st to 10th	Day	15% 5% 3 5% 4 5% 4 4 5% 6% 45%	2400
	11th to 20th	Day	12% 5% 2 5% 3 4 4 3 5% 7% 50%	2400
	21st to end	Day	22% 8% 2 6% 4 4 4 3 5% 6% 36%	2122
October	1st to 10th	Day	17% 6% 2 6% 4 4 5% 3 4 7% 42%	2000
	11th to 20th	Day	17% 6% 3 6% 4 5% 4 4 6% 8% 37%	2000
	21st to end	Day	20% 6% 3 7% 4 5% 5% 4 4 6% 36%	2189
November	1st to 10th	Day	12% 8% 2 6% 5% 4 3 3 5% 7% 45%	1851
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 4 5% 6% 36%	1797
	21st to end	Day	17% 9% 3 5% 5% 4 4 4 6% 6% 36%	1600
December	1st to 10th	Day	16% 12% 4 9% 6% 5% 5% 4 5% 7% 28%	2600
	11th to 20th	Day	23% 12% 2 6% 4 4 4 3 4 7% 32%	2986
	21st to end	Day	18% 11% 4 10% 6% 6% 5% 4 6% 6% 24%	3960
January	1st to 10th	Night	12% 9% 4 11% 6% 6% 6% 4 5% 6% 31%	6349
	11th to 20th	Night	13% 9% 4 11% 6% 5% 6% 4 6% 7% 29%	6091
	21st to end	Night	17% 10% 3 9% 5% 5% 5% 4 5% 5% 31%	6490
February	1st to 10th	Night	17% 11% 3 7% 6% 5% 5% 4 6% 5% 33%	2600
	11th to 20th	Night	16% 11% 3 8% 6% 5% 5% 4 5% 7% 30%	2483
	21st to end	Night	18% 10% 3 9% 5% 5% 4 4 6% 6% 32%	1980
March	1st to 10th	Night	19% 12% 4 8% 4 3 3 3 4 6% 35%	5143
	11th to 20th	Night	11% 10% 3 6% 4 4 3 3 5% 7% 44%	4979
	21st to end	Night	16% 9% 3 7% 4 5% 4 3 5% 5% 39%	5284
April	1st to 10th	Night	20% 9% 3 8% 5% 5% 4 3 4 6% 32%	2000
	11th to 20th	Night	17% 7% 3 7% 4 4 4 4 6% 7% 37%	1975
	21st to end	Night	19% 5% 2 6% 4 3 4 5% 5% 6% 42%	2000
May	1st to 10th	Night	30% 7% 3 7% 4 4 4 3 5% 7% 27%	3940
	11th to 20th	Night	22% 8% 3 7% 5% 5% 4 4 5% 6% 32%	3575
	21st to end	Night	21% 6% 2 6% 4 4 4 3 6% 8% 36%	3926
June	1st to 10th	Night	14% 7% 2 6% 4 4 4 4 5% 8% 41%	1600
	11th to 20th	Night	14% 6% 3 6% 5% 5% 4 4 7% 9% 38%	1600
	21st to end	Night	12% 5% 2 5% 4 3 6% 5% 7% 11% 40%	1600
July	1st to 10th	Night	10% 6% 3 7% 6% 5% 5% 4 6% 9% 39%	3600
	11th to 20th	Night	11% 6% 3 7% 6% 5% 6% 6% 8% 10% 32%	3600
	21st to end	Night	12% 6% 3 6% 4 4 5% 4 6% 7% 45%	4189
August	1st to 10th	Night	20% 7% 2 7% 4 5% 5% 4 6% 7% 34%	2915
	11th to 20th	Night	15% 8% 3 7% 4 4 4 4 5% 6% 41%	2997
	21st to end	Night	14% 5% 1 5% 4 4 5% 4 6% 8% 45%	3586
September	1st to 10th	Night	14% 5% 2 5% 4 4 3 3 5% 6% 46%	2000
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 5% 8% 51%	2000
	21st to end	Night	19% 7% 2 6% 4 3 4 3 5% 7% 39%	2273
October	1st to 10th	Night	20% 6% 2 6% 3 3 4 3 5% 6% 43%	2400
	11th to 20th	Night	16% 7% 2 6% 4 4 4 3 4 6% 43%	2400
	21st to end	Night	19% 8% 2 7% 4 3 4 2 5% 6% 39%	2650
November	1st to 10th	Night	12% 7% 2 6% 4 3 4 3 5% 6% 49%	2667
	11th to 20th	Night	16% 9% 3 7% 4 4 4 3 5% 6% 38%	2756
	21st to end	Night	15% 10% 3 7% 5% 4 4 3 7% 8% 34%	2800
December	1st to 10th	Night	15% 11% 4 8% 4 4 4 3 5% 8% 34%	4580
	11th to 20th	Night	24% 9% 3 6% 4 4 4 3 5% 5% 33%	5208
	21st to end	Night	20% 13% 4 9% 5% 4 4 3 5% 6% 27%	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	52% 5% 11% 21% 10% 1	3598
	11th to 20th	Day	62% 5% 8% 15% 8% 2	3812
	21st to end	Day	55% 4 10% 19% 10%	4466
February	1st to 10th	Day	56% 5% 11% 17% 10% 1	1800
	11th to 20th	Day	53% 5% 9% 24% 8% 1	1812
	21st to end	Day	54% 4 10% 22% 7% 2	1650
March	1st to 10th	Day	50% 4 10% 21% 9% 5%	4699
	11th to 20th	Day	44% 5% 15% 24% 9% 4	4980
	21st to end	Day	45% 6% 11% 16% 19% 3	5646
April	1st to 10th	Day	52% 4 10% 19% 12% 3	2400
	11th to 20th	Day	46% 5% 11% 20% 15% 3	2374
	21st to end	Day	44% 5% 10% 22% 15% 4	2400
May	1st to 10th	Day	54% 7% 12% 17% 9% 1	6025
	11th to 20th	Day	54% 7% 10% 19% 10% 1	6322
	21st to end	Day	45% 7% 12% 22% 12% 2	6949
June	1st to 10th	Day	44% 7% 12% 22% 14% 2	2800
	11th to 20th	Day	43% 7% 13% 22% 13% 2	2800
	21st to end	Day	38% 8% 13% 26% 14% 1	2800
July	1st to 10th	Day	41% 8% 12% 25% 13% 1	6290
	11th to 20th	Day	41% 9% 16% 22% 11% 1	6285
	21st to end	Day	42% 6% 11% 20% 20% 1	6469
August	1st to 10th	Day	50% 7% 14% 19% 10%	4221
	11th to 20th	Day	44% 7% 14% 26% 8% 1	4058
	21st to end	Day	36% 5% 12% 33% 12% 1	4312
September	1st to 10th	Day	39% 6% 11% 24% 18% 2	2400
	11th to 20th	Day	33% 5% 12% 26% 22% 2	2400
	21st to end	Day	49% 5% 11% 17% 17% 2	2122
October	1st to 10th	Day	39% 5% 9% 24% 18% 4	3260
	11th to 20th	Day	36% 4 10% 28% 20% 2	3240
	21st to end	Day	32% 4 15% 31% 17% 1	3515
November	1st to 10th	Day	39% 4 14% 24% 17% 2	1851
	11th to 20th	Day	49% 4 11% 22% 12% 2	1797
	21st to end	Day	47% 6% 13% 26% 8% 1	1600
December	1st to 10th	Day	55% 5% 13% 21% 6% 1	2600
	11th to 20th	Day	53% 4 16% 20% 7%	2986
	21st to end	Day	58% 6% 13% 14% 9%	3960
January	1st to 10th	Night	52% 4 11% 20% 11% 2	6349
	11th to 20th	Night	53% 6% 14% 15% 9% 4	6134
	21st to end	Night	53% 5% 10% 17% 13% 2	6490
February	1st to 10th	Night	51% 5% 13% 22% 8% 1	2600
	11th to 20th	Night	52% 4 11% 20% 11% 2	2485
	21st to end	Night	51% 5% 11% 19% 12% 2	1980
March	1st to 10th	Night	50% 4 10% 20% 11% 4	5153
	11th to 20th	Night	38% 5% 18% 24% 9% 4	4980
	21st to end	Night	47% 4 7% 17% 20% 4	5284
April	1st to 10th	Night	53% 5% 9% 19% 12% 3	2000
	11th to 20th	Night	44% 6% 10% 21% 17% 2	1975
	21st to end	Night	41% 6% 11% 22% 17% 3	2000
May	1st to 10th	Night	57% 6% 9% 18% 8% 2	3947
	11th to 20th	Night	50% 9% 12% 15% 12% 1	3576
	21st to end	Night	46% 7% 12% 21% 12% 3	3927
June	1st to 10th	Night	40% 7% 10% 25% 17% 1	1600
	11th to 20th	Night	39% 8% 14% 23% 14% 1	1600
	21st to end	Night	36% 8% 14% 28% 13% 1	1600
July	1st to 10th	Night	42% 7% 13% 22% 15% 2	3560
	11th to 20th	Night	44% 9% 14% 21% 11% 1	3552
	21st to end	Night	37% 7% 12% 22% 21% 1	4065
August	1st to 10th	Night	46% 9% 15% 21% 9% 1	2904
	11th to 20th	Night	42% 6% 13% 24% 14%	3024
	21st to end	Night	35% 7% 11% 32% 14% 1	3586
September	1st to 10th	Night	38% 5% 11% 26% 17% 3	2000
	11th to 20th	Night	31% 5% 11% 30% 21% 2	2000
	21st to end	Night	43% 5% 10% 22% 19% 1	2273
October	1st to 10th	Night	43% 5% 10% 20% 21% 1	3920
	11th to 20th	Night	33% 4 11% 28% 21% 3	3899
	21st to end	Night	32% 4 14% 29% 19% 1	4274
November	1st to 10th	Night	37% 5% 14% 24% 18% 3	2667
	11th to 20th	Night	47% 5% 10% 22% 13% 3	2756
	21st to end	Night	47% 6% 14% 23% 9% 1	2800
December	1st to 10th	Night	48% 6% 14% 26% 5%	4580
	11th to 20th	Night	53% 5% 14% 18% 9% 1	5208
	21st to end	Night	57% 5% 11% 17% 9% 1	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	30% 27% 28% 12% 3 2	200
	11th to 20th	Day	31% 26% 32% 10% 1	247
	21st to end	Day	24% 28% 29% 14% 4 1	330
February	1st to 10th	Day	27% 28% 30% 13% 2	300
	11th to 20th	Day	30% 24% 37% 9% 1	350
	21st to end	Day	31% 20% 32% 16% 2	414
March	1st to 10th	Day	29% 21% 29% 17% 3 1	500
	11th to 20th	Day	27% 24% 27% 17% 3 2	500
	21st to end	Day	30% 23% 25% 15% 5% 2	550
April	1st to 10th	Day	29% 23% 26% 15% 5% 2	500
	11th to 20th	Day	26% 25% 24% 14% 8% 3	492
	21st to end	Day	25% 18% 32% 16% 8% 2	500
May	1st to 10th	Day	31% 15% 29% 15% 8% 3	503
	11th to 20th	Day	21% 17% 29% 21% 9% 3	527
	21st to end	Day	23% 9% 26% 22% 15% 5%	573
June	1st to 10th	Day	17% 14% 28% 26% 13% 3	530
	11th to 20th	Day	20% 13% 26% 25% 13% 3	530
	21st to end	Day	15% 11% 32% 27% 13% 2	530
July	1st to 10th	Day	12% 7% 26% 31% 18% 6%	525
	11th to 20th	Day	9% 6% 25% 32% 23% 5%	524
	21st to end	Day	13% 8% 27% 30% 19% 5%	576
August	1st to 10th	Day	13% 8% 23% 25% 22% 9%	526
	11th to 20th	Day	15% 7% 21% 30% 19% 8%	519
	21st to end	Day	14% 7% 25% 27% 16% 12%	583
September	1st to 10th	Day	18% 10% 20% 21% 20% 10%	530
	11th to 20th	Day	14% 5% 21% 25% 23% 12%	530
	21st to end	Day	25% 11% 21% 23% 14% 5%	530
October	1st to 10th	Day	22% 9% 22% 22% 15% 9%	530
	11th to 20th	Day	21% 16% 26% 23% 12% 2	530
	21st to end	Day	23% 16% 28% 21% 10% 1	576
November	1st to 10th	Day	22% 15% 26% 23% 11% 2	425
	11th to 20th	Day	27% 17% 28% 19% 8% 1	355
	21st to end	Day	27% 24% 29% 16% 4	200
December	1st to 10th	Day	28% 28% 29% 10% 6% 1	200
	11th to 20th	Day	33% 22% 27% 15% 4	200
	21st to end	Day	30% 20% 30% 16% 5%	220
January	1st to 10th	Night	20% 21% 36% 21% 3	800
	11th to 20th	Night	24% 18% 31% 25% 3	800
	21st to end	Night	18% 18% 39% 23% 2	880
February	1st to 10th	Night	23% 15% 33% 27% 2	800
	11th to 20th	Night	24% 21% 36% 17% 1	783
	21st to end	Night	23% 18% 37% 19% 3	660
March	1st to 10th	Night	24% 17% 32% 24% 4	800
	11th to 20th	Night	24% 17% 31% 23% 4	800
	21st to end	Night	22% 16% 36% 20% 5% 1	880
April	1st to 10th	Night	25% 18% 32% 21% 5%	800
	11th to 20th	Night	19% 17% 33% 24% 7%	791
	21st to end	Night	21% 14% 30% 29% 6%	800
May	1st to 10th	Night	27% 12% 25% 26% 10%	565
	11th to 20th	Night	18% 7% 32% 34% 8% 1	394
	21st to end	Night	17% 5% 23% 38% 16%	433
June	1st to 10th	Night	15% 7% 21% 40% 18%	400
	11th to 20th	Night	17% 5% 25% 41% 13%	400
	21st to end	Night	12% 3 25% 48% 13% 1	400
July	1st to 10th	Night	13% 4 21% 40% 20% 3	390
	11th to 20th	Night	9% 3 18% 46% 23% 2	388
	21st to end	Night	14% 3 17% 40% 25% 1	548
August	1st to 10th	Night	14% 2 11% 43% 27% 2	597
	11th to 20th	Night	15% 3 13% 37% 29% 2	664
	21st to end	Night	14% 5% 16% 36% 28% 2	880
September	1st to 10th	Night	17% 4 21% 35% 22% 2	800
	11th to 20th	Night	14% 4 16% 34% 29% 4	800
	21st to end	Night	21% 5% 20% 35% 17% 2	800
October	1st to 10th	Night	22% 8% 20% 28% 20% 3	800
	11th to 20th	Night	18% 12% 26% 32% 11% 1	800
	21st to end	Night	24% 10% 28% 31% 7%	880
November	1st to 10th	Night	16% 12% 29% 33% 10% 1	820
	11th to 20th	Night	23% 15% 27% 27% 7% 1	829
	21st to end	Night	20% 16% 30% 28% 6% 1	800
December	1st to 10th	Night	25% 17% 30% 23% 5%	800
	11th to 20th	Night	23% 15% 32% 26% 5%	800
	21st to end	Night	24% 14% 31% 28% 3	880

Page generated at 2025-11-15 00:55:59. Version ac07

Copyright © 2025 Attilla Danko.