Washoe Zephyr Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Washoe Zephyr Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	2 3 1 4 3 3 3 3 5% 6% 68%	140010
	Night	2 4 2 4 3 3 3 3 5% 7% 65%	139237
Transparency	Day	17% 5% 10% 18% 27% 22%	134983
	Night	19% 6% 12% 21% 25% 16%	135245
Seeing	Day	8% 16% 27% 27% 18% 5%	16788
	Night	5% 10% 23% 35% 22% 4	26253

By Month

Percentage Cloud Cover SW (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	1 4 3 5% 4 4 4 4 7% 9% 55%	4722
	11th to 20th	Day	1 3 1 4 3 3 4 3 4 6% 68%	4837
	21st to end	Day	2 6% 2 6% 4 4 5% 4 6% 10% 50%	5843
February	1st to 10th	Day	2 5% 2 4 4 4 4 4 6% 10% 54%	1901
	11th to 20th	Day	1 6% 3 6% 4 4 5% 5% 6% 8% 52%	2050
	21st to end	Day	1 5% 2 3 4 3 5% 4 7% 9% 56%	1730
March	1st to 10th	Day	2 8% 4 8% 4 4 5% 4 8% 10% 42%	4618
	11th to 20th	Day	1 3 2 4 3 3 4 3 5% 7% 65%	5010
	21st to end	Day	1 7% 2 5% 4 4 5% 4 6% 8% 54%	5982
April	1st to 10th	Day	2 5% 2 5% 4 5% 5% 4 6% 9% 54%	2413
	11th to 20th	Day	2 4 2 4 4 3 4 4 6% 9% 58%	2369
	21st to end	Day	3 6% 1 4 4 3 3 4 5% 6% 62%	2400
May	1st to 10th	Day	2 5% 1 3 2 2 2 2 4 5% 71%	5738
	11th to 20th	Day	2 3 1 3 3 2 3 3 4 6% 70%	6161
	21st to end	Day	3 2 1 3 2 2 2 2 4 5% 73%	6902
June	1st to 10th	Day	3 3 1 4 4 3 4 3 5% 8% 62%	2800
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 5% 77%	2800
	21st to end	Day	2 1 1 2 2 2 2 2 4 4 78%	2800
July	1st to 10th	Day	1 1 1 1 2 1 2 2 88%	6319
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 3 80%	6320
	21st to end	Day	1 1 1 2 2 2 3 3 5% 6% 73%	6761
August	1st to 10th	Day	2 1 1 2 1 2 2 2 2 3 82%	5697
	11th to 20th	Day	1 1 1 1 1 1 2 3 89%	5364
	21st to end	Day	2 2 1 3 2 2 2 2 3 4 77%	5984
September	1st to 10th	Day	1 1 1 2 1 2 2 2 3 5% 81%	2400
	11th to 20th	Day	2 2 1 3 3 3 2 2 3 5% 75%	2360
	21st to end	Day	2 1 1 3 2 2 3 2 2 5% 78%	2200
October	1st to 10th	Day	2 2 1 2 2 2 2 2 4 6% 75%	3349
	11th to 20th	Day	2 4 1 2 2 2 2 2 3 4 76%	3261
	21st to end	Day	1 2 1 3 2 3 3 3 5% 7% 69%	3445
November	1st to 10th	Day	2 4 2 4 4 3 4 4 6% 7% 61%	1854
	11th to 20th	Day	2 3 1 4 3 3 4 4 7% 9% 59%	1890
	21st to end	Day	2 4 1 5% 3 3 4 5% 5% 10% 58%	1830
December	1st to 10th	Day	2 3 1 3 3 3 3 3 6% 9% 65%	2700
	11th to 20th	Day	1 6% 2 7% 5% 5% 5% 6% 8% 10% 44%	3108
	21st to end	Day	2 6% 4 8% 4 4 5% 4 7% 12% 44%	4092
January	1st to 10th	Night	1 4 2 5% 3 3 4 4 7% 11% 56%	8172
	11th to 20th	Night	3 2 4 4 3 3 3 5% 6% 66%	8124
	21st to end	Night	2 6% 2 6% 4 4 4 4 6% 7% 53%	8371
February	1st to 10th	Night	2 5% 2 5% 4 4 4 4 6% 8% 56%	2718
	11th to 20th	Night	1 5% 2 5% 3 4 4 5% 8% 9% 54%	2458
	21st to end	Night	2 4 1 5% 4 4 4 4 6% 10% 57%	2076
March	1st to 10th	Night	1 7% 4 7% 5% 5% 5% 4 6% 8% 48%	5496
	11th to 20th	Night	1 4 2 5% 3 3 4 4 6% 8% 61%	5205
	21st to end	Night	2 8% 3 5% 4 4 5% 4 6% 8% 52%	5234
April	1st to 10th	Night	2 4 2 4 4 4 4 3 8% 9% 56%	2010
	11th to 20th	Night	2 4 2 3 3 4 4 4 7% 7% 61%	1979
	21st to end	Night	3 3 1 3 3 3 4 3 4 7% 65%	2000
May	1st to 10th	Night	2 3 1 3 2 2 2 2 4 5% 74%	4247
	11th to 20th	Night	1 2 1 3 2 2 3 2 4 6% 74%	3720
	21st to end	Night	3 2 1 2 2 2 2 2 3 6% 75%	3972
June	1st to 10th	Night	4 3 2 4 3 3 3 3 5% 7% 64%	1600
	11th to 20th	Night	1 1 1 2 2 2 2 2 4 5% 79%	1600
	21st to end	Night	2 2 1 2 2 1 2 2 2 4 80%	1600
July	1st to 10th	Night	1 1 1 1 1 1 2 2 90%	3640
	11th to 20th	Night	3 2 1 2 2 2 2 2 3 3 79%	3640
	21st to end	Night	3 1 1 2 2 3 3 3 5% 6% 70%	4091
August	1st to 10th	Night	2 1 2 1 2 2 2 3 4 81%	4116
	11th to 20th	Night	1 1 1 1 1 2 1 3 4 85%	4506
	21st to end	Night	3 3 1 3 2 2 2 2 3 3 76%	5015
September	1st to 10th	Night	2 1 2 1 1 1 2 2 4 84%	2000
	11th to 20th	Night	2 2 1 2 2 2 2 3 3 4 76%	2039
	21st to end	Night	2 2 1 3 2 2 3 2 3 5% 77%	2200
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 4 79%	3846
	11th to 20th	Night	2 2 1 2 1 1 2 2 3 4 81%	3897
	21st to end	Night	2 3 1 4 2 3 3 3 4 6% 70%	4360
November	1st to 10th	Night	2 3 1 4 3 3 4 3 6% 7% 65%	2665
	11th to 20th	Night	2 3 1 4 3 3 3 3 6% 10% 62%	2730
	21st to end	Night	1 4 2 4 3 3 3 4 7% 8% 60%	2788
December	1st to 10th	Night	1 3 1 4 2 3 3 3 5% 6% 67%	4700
	11th to 20th	Night	2 5% 2 5% 4 4 6% 5% 9% 13% 46%	5316
	21st to end	Night	1 6% 3 8% 5% 6% 6% 5% 8% 9% 42%	7106

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	23% 9% 24% 30% 14%	4723
	11th to 20th	Day	19% 6% 10% 20% 29% 16%	4837
	21st to end	Day	30% 7% 9% 22% 24% 8%	5843
February	1st to 10th	Day	27% 8% 14% 26% 18% 8%	1901
	11th to 20th	Day	29% 6% 16% 28% 18% 4	2060
	21st to end	Day	23% 9% 22% 24% 16% 6%	1730
March	1st to 10th	Day	34% 9% 16% 17% 20% 4	4618
	11th to 20th	Day	16% 7% 15% 32% 25% 4	5010
	21st to end	Day	25% 9% 18% 22% 23% 3	5982
April	1st to 10th	Day	26% 10% 17% 22% 20% 5%	2413
	11th to 20th	Day	23% 7% 14% 27% 22% 7%	2372
	21st to end	Day	22% 7% 14% 28% 25% 4	2400
May	1st to 10th	Day	17% 4 6% 25% 39% 8%	5738
	11th to 20th	Day	14% 5% 9% 20% 40% 12%	6164
	21st to end	Day	15% 4 7% 18% 43% 14%	6904
June	1st to 10th	Day	19% 5% 10% 21% 33% 11%	2800
	11th to 20th	Day	11% 3 6% 19% 34% 27%	2800
	21st to end	Day	11% 3 4 9% 32% 41%	2800
July	1st to 10th	Day	5% 1 2 4 17% 70%	6251
	11th to 20th	Day	10% 1 3 5% 12% 68%	6245
	21st to end	Day	10% 3 5% 11% 29% 43%	6564
August	1st to 10th	Day	12% 2 3 6% 22% 54%	4148
	11th to 20th	Day	6% 2 3 4 21% 64%	3880
	21st to end	Day	7% 1 2 8% 37% 46%	4311
September	1st to 10th	Day	8% 2 4 9% 30% 47%	2400
	11th to 20th	Day	14% 3 6% 16% 35% 25%	2360
	21st to end	Day	13% 2 6% 16% 37% 26%	2200
October	1st to 10th	Day	12% 3 6% 12% 37% 31%	3349
	11th to 20th	Day	13% 3 6% 19% 46% 13%	3261
	21st to end	Day	13% 5% 9% 28% 34% 12%	3445
November	1st to 10th	Day	22% 7% 13% 24% 26% 8%	1854
	11th to 20th	Day	20% 8% 17% 26% 24% 4	1890
	21st to end	Day	23% 8% 12% 26% 26% 5%	1830
December	1st to 10th	Day	18% 6% 11% 30% 29% 6%	2700
	11th to 20th	Day	32% 10% 17% 21% 15% 4	3108
	21st to end	Day	27% 13% 24% 24% 11% 1	4092
January	1st to 10th	Night	20% 8% 25% 30% 15% 1	8172
	11th to 20th	Night	20% 6% 11% 17% 29% 16%	8124
	21st to end	Night	30% 7% 10% 19% 27% 7%	8371
February	1st to 10th	Night	26% 9% 14% 24% 20% 7%	2718
	11th to 20th	Night	22% 9% 17% 29% 19% 3	2459
	21st to end	Night	25% 7% 19% 27% 15% 7%	2076
March	1st to 10th	Night	31% 8% 15% 21% 21% 5%	5496
	11th to 20th	Night	18% 9% 12% 41% 17% 3	5205
	21st to end	Night	26% 11% 24% 25% 11% 3	5234
April	1st to 10th	Night	22% 11% 19% 27% 17% 4	2010
	11th to 20th	Night	21% 8% 16% 29% 19% 7%	1979
	21st to end	Night	20% 8% 15% 32% 21% 3	2000
May	1st to 10th	Night	13% 5% 9% 30% 38% 5%	4244
	11th to 20th	Night	12% 5% 9% 26% 37% 10%	3720
	21st to end	Night	12% 4 8% 21% 44% 10%	3972
June	1st to 10th	Night	20% 7% 8% 23% 31% 11%	1600
	11th to 20th	Night	9% 4 6% 20% 37% 25%	1600
	21st to end	Night	11% 2 5% 12% 34% 35%	1600
July	1st to 10th	Night	4 1 2 4 20% 69%	3600
	11th to 20th	Night	12% 2 3 5% 13% 66%	3592
	21st to end	Night	14% 3 5% 12% 25% 41%	3976
August	1st to 10th	Night	13% 3 4 8% 26% 46%	2986
	11th to 20th	Night	7% 1 3 5% 24% 60%	3256
	21st to end	Night	9% 1 3 9% 37% 41%	3608
September	1st to 10th	Night	7% 2 4 8% 37% 43%	2000
	11th to 20th	Night	13% 3 7% 20% 35% 24%	2039
	21st to end	Night	12% 2 6% 18% 36% 25%	2200
October	1st to 10th	Night	11% 2 6% 13% 34% 34%	3846
	11th to 20th	Night	10% 3 6% 19% 47% 15%	3897
	21st to end	Night	13% 5% 8% 27% 35% 12%	4360
November	1st to 10th	Night	19% 6% 15% 25% 23% 11%	2665
	11th to 20th	Night	19% 7% 14% 28% 26% 5%	2730
	21st to end	Night	21% 8% 12% 28% 25% 7%	2788
December	1st to 10th	Night	19% 7% 11% 27% 27% 10%	4700
	11th to 20th	Night	25% 13% 17% 24% 19% 3	5316
	21st to end	Night	32% 13% 20% 21% 12% 2	7106

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	13% 21% 30% 21% 14% 1	220
	11th to 20th	Day	10% 12% 31% 26% 19% 3	252
	21st to end	Day	7% 12% 32% 29% 16% 4	473
February	1st to 10th	Day	15% 13% 28% 26% 15% 3	436
	11th to 20th	Day	9% 21% 30% 28% 11% 1	535
	21st to end	Day	9% 20% 28% 33% 10%	446
March	1st to 10th	Day	12% 24% 32% 21% 9% 2	540
	11th to 20th	Day	13% 22% 26% 23% 13% 3	540
	21st to end	Day	13% 25% 28% 24% 10% 1	594
April	1st to 10th	Day	13% 24% 28% 21% 10% 4	505
	11th to 20th	Day	13% 25% 26% 20% 13% 2	492
	21st to end	Day	12% 20% 28% 23% 13% 4	500
May	1st to 10th	Day	12% 23% 26% 24% 11% 4	503
	11th to 20th	Day	13% 22% 23% 22% 13% 6%	528
	21st to end	Day	10% 16% 21% 27% 19% 7%	573
June	1st to 10th	Day	8% 15% 26% 25% 21% 5%	530
	11th to 20th	Day	9% 18% 31% 24% 15% 4	530
	21st to end	Day	3 12% 28% 30% 22% 6%	530
July	1st to 10th	Day	2 12% 30% 29% 21% 5%	526
	11th to 20th	Day	2 8% 27% 33% 22% 7%	524
	21st to end	Day	3 7% 27% 30% 26% 8%	575
August	1st to 10th	Day	3 11% 27% 28% 24% 7%	527
	11th to 20th	Day	2 11% 24% 28% 23% 12%	517
	21st to end	Day	4 7% 27% 31% 24% 7%	583
September	1st to 10th	Day	3 9% 27% 32% 23% 7%	530
	11th to 20th	Day	6% 14% 25% 27% 22% 6%	530
	21st to end	Day	5% 12% 25% 37% 17% 5%	530
October	1st to 10th	Day	5% 14% 27% 26% 22% 5%	530
	11th to 20th	Day	8% 13% 21% 29% 22% 6%	530
	21st to end	Day	9% 13% 26% 26% 22% 5%	519
November	1st to 10th	Day	11% 13% 23% 23% 21% 9%	330
	11th to 20th	Day	7% 15% 24% 28% 19% 7%	330
	21st to end	Day	8% 13% 22% 30% 20% 6%	298
December	1st to 10th	Day	10% 10% 25% 24% 22% 9%	220
	11th to 20th	Day	9% 18% 29% 23% 14% 8%	220
	21st to end	Day	21% 14% 24% 26% 13% 2	242
January	1st to 10th	Night	6% 13% 31% 26% 16% 7%	840
	11th to 20th	Night	5% 11% 24% 38% 18% 5%	840
	21st to end	Night	6% 7% 29% 35% 19% 5%	924
February	1st to 10th	Night	8% 12% 25% 35% 18% 3	840
	11th to 20th	Night	5% 13% 28% 33% 17% 5%	824
	21st to end	Night	7% 15% 26% 37% 13% 2	692
March	1st to 10th	Night	6% 15% 35% 28% 13% 3	840
	11th to 20th	Night	5% 17% 26% 27% 22% 3	840
	21st to end	Night	6% 19% 29% 31% 13% 1	924
April	1st to 10th	Night	5% 15% 29% 31% 19% 1	804
	11th to 20th	Night	7% 16% 26% 32% 16% 4	791
	21st to end	Night	6% 13% 25% 33% 19% 4	800
May	1st to 10th	Night	6% 14% 24% 33% 20% 3	669
	11th to 20th	Night	4 9% 23% 39% 22% 2	446
	21st to end	Night	6% 6% 18% 40% 27% 2	433
June	1st to 10th	Night	5% 5% 21% 40% 26% 4	400
	11th to 20th	Night	1 6% 23% 43% 26% 2	400
	21st to end	Night	3 3 18% 45% 30% 2	400
July	1st to 10th	Night	1 2 14% 51% 30% 2	390
	11th to 20th	Night	1 1 17% 52% 28% 1	388
	21st to end	Night	4 1 10% 45% 36% 5%	484
August	1st to 10th	Night	3 1 14% 44% 35% 3	628
	11th to 20th	Night	2 3 17% 44% 29% 4	792
	21st to end	Night	3 3 17% 45% 29% 3	880
September	1st to 10th	Night	2 3 18% 38% 34% 5%	800
	11th to 20th	Night	3 5% 18% 34% 34% 6%	800
	21st to end	Night	2 5% 22% 36% 28% 7%	800
October	1st to 10th	Night	4 6% 21% 36% 27% 6%	800
	11th to 20th	Night	5% 8% 16% 38% 27% 6%	800
	21st to end	Night	4 8% 19% 32% 27% 10%	880
November	1st to 10th	Night	5% 12% 21% 28% 28% 6%	820
	11th to 20th	Night	5% 10% 28% 35% 18% 5%	840
	21st to end	Night	7% 9% 25% 31% 23% 4	840
December	1st to 10th	Night	7% 11% 25% 35% 18% 5%	840
	11th to 20th	Night	8% 13% 26% 31% 16% 5%	840
	21st to end	Night	9% 14% 31% 31% 13% 2	924

Page generated at 2026-03-29 23:18:43. Version ac07

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