Garvey Ranch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Garvey Ranch Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	4 4 2 4 3 2 3 2 4 6% 67%	140647
	Night	5% 5% 2 4 3 3 3 2 4 6% 63%	139370
Transparency	Day	22% 2 4 18% 28% 27%	135736
	Night	25% 2 5% 19% 26% 23%	135254
Seeing	Day	7% 7% 24% 35% 19% 8%	17636
	Night	7% 3 10% 38% 35% 8%	27561

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	10% 3 1 3 2 3 3 2 4 5% 64%	4722
	11th to 20th	Day	5% 2 1 3 1 2 2 3 4 6% 71%	5355
	21st to end	Day	9% 4 2 5% 4 3 4 3 5% 8% 54%	6500
February	1st to 10th	Day	9% 5% 1 4 3 3 2 3 4 5% 61%	2100
	11th to 20th	Day	10% 3 1 3 3 2 3 3 5% 7% 59%	2050
	21st to end	Day	7% 5% 2 4 3 3 3 3 5% 8% 59%	1730
March	1st to 10th	Day	10% 6% 2 5% 4 4 4 4 5% 7% 51%	4618
	11th to 20th	Day	8% 4 2 4 3 3 3 3 5% 7% 57%	4800
	21st to end	Day	9% 4 2 4 3 3 3 3 5% 9% 54%	6101
April	1st to 10th	Day	5% 4 1 4 3 3 2 4 4 8% 61%	2520
	11th to 20th	Day	4 4 2 4 2 2 4 3 4 6% 64%	2489
	21st to end	Day	4 4 1 4 3 2 2 2 4 7% 66%	2520
May	1st to 10th	Day	4 6% 2 6% 2 3 3 2 3 5% 64%	5601
	11th to 20th	Day	3 7% 3 7% 3 3 3 2 4 6% 59%	5841
	21st to end	Day	3 6% 3 7% 3 3 3 2 4 6% 63%	6423
June	1st to 10th	Day	4 8% 2 5% 2 3 2 2 4 8% 60%	2600
	11th to 20th	Day	4 5% 1 3 2 2 2 1 3 5% 72%	2798
	21st to end	Day	2 3 2 4 2 2 2 2 3 5% 74%	2800
July	1st to 10th	Day	1 2 1 3 2 2 2 1 2 4 80%	6319
	11th to 20th	Day	1 3 2 4 3 3 2 2 3 5% 72%	6215
	21st to end	Day	1 2 1 3 3 2 2 2 3 5% 77%	5976
August	1st to 10th	Day	1 1 2 2 2 2 1 2 5% 82%	5346
	11th to 20th	Day	1 2 1 2 3 1 1 1 1 3 83%	5364
	21st to end	Day	1 1 1 3 3 2 2 2 3 5% 78%	5984
September	1st to 10th	Day	3 2 1 2 1 2 2 2 3 5% 78%	2400
	11th to 20th	Day	2 3 1 2 2 2 2 2 3 5% 77%	2250
	21st to end	Day	1 2 1 3 2 2 2 2 5% 5% 75%	2200
October	1st to 10th	Day	2 2 1 2 2 2 2 2 2 4 78%	3600
	11th to 20th	Day	3 4 1 3 2 2 2 2 3 6% 73%	3372
	21st to end	Day	2 2 1 4 2 2 3 4 5% 7% 68%	3477
November	1st to 10th	Day	2 2 1 3 2 2 3 2 4 6% 72%	1854
	11th to 20th	Day	6% 3 1 3 2 2 3 2 4 6% 68%	1890
	21st to end	Day	5% 3 1 3 3 4 3 3 4 7% 63%	1890
December	1st to 10th	Day	4 2 1 4 3 3 3 3 5% 7% 65%	3040
	11th to 20th	Day	9% 4 2 4 3 3 4 4 6% 9% 52%	3499
	21st to end	Day	10% 5% 2 5% 4 3 4 3 6% 8% 48%	4403
January	1st to 10th	Night	11% 4 2 4 3 3 3 2 3 4 62%	8172
	11th to 20th	Night	4 3 1 4 2 2 2 2 3 6% 70%	7611
	21st to end	Night	10% 6% 2 5% 3 3 4 3 5% 5% 53%	7747
February	1st to 10th	Night	9% 5% 2 5% 4 2 3 2 4 4 61%	2520
	11th to 20th	Night	10% 5% 2 4 3 2 3 3 4 6% 58%	2458
	21st to end	Night	9% 4 1 4 3 3 3 3 4 6% 59%	2076
March	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 5% 8% 52%	5496
	11th to 20th	Night	7% 4 2 5% 4 3 4 3 5% 7% 56%	5403
	21st to end	Night	9% 7% 2 7% 3 3 3 3 5% 7% 52%	5116
April	1st to 10th	Night	6% 6% 2 5% 3 3 3 2 4 6% 60%	2100
	11th to 20th	Night	5% 6% 2 4 3 2 3 2 3 6% 64%	2079
	21st to end	Night	4 7% 3 4 3 2 3 2 4 7% 61%	2100
May	1st to 10th	Night	4 12% 2 7% 3 2 2 2 3 5% 57%	4414
	11th to 20th	Night	3 9% 4 8% 3 2 2 3 4 6% 57%	4074
	21st to end	Night	3 9% 2 5% 2 2 2 2 3 6% 65%	4474
June	1st to 10th	Night	2 9% 3 6% 3 2 3 3 5% 7% 59%	1794
	11th to 20th	Night	2 9% 1 3 1 2 2 2 3 6% 68%	1600
	21st to end	Night	2 6% 3 6% 2 2 2 2 4 7% 65%	1600
July	1st to 10th	Night	1 3 1 3 4 2 1 1 2 5% 75%	3640
	11th to 20th	Night	1 3 3 5% 2 2 3 2 4 6% 68%	3732
	21st to end	Night	1 3 1 3 5% 4 2 2 3 5% 71%	4901
August	1st to 10th	Night	1 1 1 3 4 3 2 2 3 6% 74%	4486
	11th to 20th	Night	1 5% 2 3 4 2 2 1 3 5% 72%	4506
	21st to end	Night	1 1 1 4 4 3 2 2 4 6% 72%	5015
September	1st to 10th	Night	4 5% 1 4 2 1 2 2 3 6% 70%	2000
	11th to 20th	Night	3 4 2 3 1 1 2 1 2 5% 75%	2149
	21st to end	Night	1 3 1 4 2 2 2 2 4 7% 72%	2200
October	1st to 10th	Night	2 2 1 3 3 3 2 2 3 4 75%	3600
	11th to 20th	Night	3 10% 2 4 3 2 2 2 4 7% 62%	3769
	21st to end	Night	2 3 1 3 2 2 3 2 3 4 74%	4328
November	1st to 10th	Night	3 4 1 3 3 2 2 2 4 7% 69%	2665
	11th to 20th	Night	4 3 2 5% 2 2 2 2 4 5% 68%	2730
	21st to end	Night	5% 4 2 4 3 3 4 4 6% 7% 59%	2730
December	1st to 10th	Night	5% 3 1 4 4 3 3 3 4 6% 65%	4370
	11th to 20th	Night	10% 4 2 5% 3 3 4 3 6% 7% 53%	4941
	21st to end	Night	10% 5% 2 6% 3 4 4 4 7% 9% 46%	6774

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	26% 1 5% 17% 27% 24%	4723
	11th to 20th	Day	17% 2 8% 14% 18% 42%	5355
	21st to end	Day	31% 1 2 17% 22% 26%	6500
February	1st to 10th	Day	28% 2 4 14% 26% 26%	2100
	11th to 20th	Day	26% 2 4 21% 25% 22%	2060
	21st to end	Day	25% 3 5% 22% 24% 21%	1730
March	1st to 10th	Day	32% 2 6% 20% 24% 16%	4618
	11th to 20th	Day	27% 3 8% 29% 20% 13%	4800
	21st to end	Day	29% 1 9% 32% 19% 10%	6101
April	1st to 10th	Day	23% 3 10% 25% 26% 14%	2520
	11th to 20th	Day	22% 2 5% 25% 29% 17%	2492
	21st to end	Day	20% 2 6% 30% 30% 12%	2520
May	1st to 10th	Day	25% 1 6% 21% 31% 16%	5601
	11th to 20th	Day	28% 1 5% 22% 36% 8%	5844
	21st to end	Day	26% 1 3 27% 34% 9%	6424
June	1st to 10th	Day	27% 2 6% 22% 29% 13%	2600
	11th to 20th	Day	19% 2 3 16% 33% 28%	2798
	21st to end	Day	18% 2 2 9% 32% 38%	2800
July	1st to 10th	Day	13% 1 1 6% 19% 59%	6251
	11th to 20th	Day	18% 2 2 10% 37% 30%	6140
	21st to end	Day	13% 2 3 16% 35% 31%	5804
August	1st to 10th	Day	11% 1 1 16% 39% 33%	3890
	11th to 20th	Day	9% 1 12% 31% 46%	3880
	21st to end	Day	12% 2 1 8% 30% 47%	4311
September	1st to 10th	Day	13% 2 2 10% 35% 39%	2400
	11th to 20th	Day	14% 2 1 13% 36% 34%	2250
	21st to end	Day	14% 1 2 18% 32% 33%	2200
October	1st to 10th	Day	14% 1 2 11% 20% 52%	3600
	11th to 20th	Day	17% 1 2 16% 34% 29%	3372
	21st to end	Day	17% 2 2 10% 28% 40%	3477
November	1st to 10th	Day	15% 2 4 17% 26% 35%	1854
	11th to 20th	Day	19% 2 5% 17% 20% 37%	1890
	21st to end	Day	23% 3 7% 13% 24% 30%	1890
December	1st to 10th	Day	22% 2 4 18% 22% 33%	3040
	11th to 20th	Day	30% 2 6% 16% 27% 19%	3498
	21st to end	Day	35% 1 9% 26% 19% 9%	4403
January	1st to 10th	Night	29% 1 5% 16% 28% 21%	8172
	11th to 20th	Night	19% 1 7% 15% 17% 41%	7611
	21st to end	Night	35% 2 2 16% 21% 24%	7747
February	1st to 10th	Night	29% 2 5% 16% 22% 27%	2520
	11th to 20th	Night	29% 2 6% 20% 22% 22%	2459
	21st to end	Night	29% 2 5% 19% 23% 22%	2076
March	1st to 10th	Night	31% 3 7% 21% 25% 12%	5496
	11th to 20th	Night	30% 2 10% 29% 16% 13%	5403
	21st to end	Night	33% 2 13% 26% 20% 6%	5116
April	1st to 10th	Night	28% 2 11% 25% 21% 13%	2100
	11th to 20th	Night	25% 2 7% 26% 25% 15%	2079
	21st to end	Night	27% 2 8% 29% 24% 10%	2100
May	1st to 10th	Night	32% 2 7% 24% 29% 6%	4411
	11th to 20th	Night	31% 2 7% 29% 26% 5%	4075
	21st to end	Night	23% 1 5% 38% 26% 7%	4474
June	1st to 10th	Night	29% 2 11% 23% 25% 9%	1794
	11th to 20th	Night	21% 1 5% 19% 36% 18%	1600
	21st to end	Night	24% 2 2 10% 34% 28%	1600
July	1st to 10th	Night	16% 2 1 11% 19% 52%	3600
	11th to 20th	Night	18% 3 3 14% 40% 22%	3684
	21st to end	Night	18% 2 3 23% 33% 20%	4761
August	1st to 10th	Night	14% 1 2 21% 40% 22%	3256
	11th to 20th	Night	14% 2 1 17% 34% 32%	3256
	21st to end	Night	18% 2 1 8% 41% 31%	3608
September	1st to 10th	Night	21% 2 2 9% 34% 32%	2000
	11th to 20th	Night	17% 2 2 14% 35% 30%	2149
	21st to end	Night	17% 1 2 18% 35% 26%	2200
October	1st to 10th	Night	17% 1 2 13% 23% 44%	3600
	11th to 20th	Night	25% 1 2 16% 30% 26%	3769
	21st to end	Night	16% 2 2 11% 32% 37%	4328
November	1st to 10th	Night	19% 2 4 18% 24% 33%	2665
	11th to 20th	Night	21% 2 5% 15% 20% 36%	2730
	21st to end	Night	25% 2 7% 18% 19% 29%	2730
December	1st to 10th	Night	23% 2 5% 15% 24% 31%	4370
	11th to 20th	Night	32% 1 8% 15% 25% 19%	4941
	21st to end	Night	36% 1 12% 27% 15% 9%	6774

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	14% 4 20% 39% 19% 5%	220
	11th to 20th	Day	11% 6% 27% 31% 20% 6%	410
	21st to end	Day	9% 9% 27% 35% 16% 3	594
February	1st to 10th	Day	13% 8% 26% 31% 18% 5%	540
	11th to 20th	Day	13% 12% 30% 30% 13% 2	535
	21st to end	Day	9% 12% 30% 33% 14% 2	446
March	1st to 10th	Day	13% 14% 32% 26% 13% 2	540
	11th to 20th	Day	11% 11% 30% 31% 13% 4	540
	21st to end	Day	11% 16% 31% 27% 11% 5%	594
April	1st to 10th	Day	9% 13% 31% 26% 14% 7%	540
	11th to 20th	Day	8% 15% 34% 29% 9% 6%	532
	21st to end	Day	6% 14% 32% 28% 14% 6%	540
May	1st to 10th	Day	11% 12% 30% 29% 13% 5%	508
	11th to 20th	Day	9% 12% 29% 31% 12% 6%	528
	21st to end	Day	7% 9% 33% 31% 15% 4	573
June	1st to 10th	Day	6% 8% 25% 37% 18% 6%	530
	11th to 20th	Day	6% 5% 25% 38% 18% 8%	530
	21st to end	Day	3 5% 22% 41% 20% 10%	530
July	1st to 10th	Day	2 2 16% 44% 25% 11%	526
	11th to 20th	Day	2 2 9% 46% 25% 16%	524
	21st to end	Day	2 1 13% 44% 26% 15%	575
August	1st to 10th	Day	4 1 14% 48% 20% 13%	527
	11th to 20th	Day	4 1 15% 43% 24% 14%	517
	21st to end	Day	3 2 17% 43% 23% 11%	583
September	1st to 10th	Day	5% 2 23% 41% 18% 11%	530
	11th to 20th	Day	4 4 30% 36% 17% 9%	530
	21st to end	Day	3 7% 27% 32% 21% 10%	530
October	1st to 10th	Day	3 8% 25% 33% 22% 8%	530
	11th to 20th	Day	6% 8% 25% 32% 21% 8%	530
	21st to end	Day	5% 7% 22% 34% 22% 11%	538
November	1st to 10th	Day	5% 4 15% 33% 32% 12%	330
	11th to 20th	Day	9% 3 15% 35% 28% 8%	330
	21st to end	Day	6% 4 19% 38% 24% 9%	330
December	1st to 10th	Day	10% 1 13% 41% 26% 8%	330
	11th to 20th	Day	5% 2 17% 38% 26% 12%	330
	21st to end	Day	18% 4 20% 37% 19% 2	316
January	1st to 10th	Night	11% 1 14% 45% 23% 5%	840
	11th to 20th	Night	9% 1 11% 46% 28% 4	840
	21st to end	Night	13% 2 15% 44% 23% 3	924
February	1st to 10th	Night	10% 3 13% 43% 25% 6%	840
	11th to 20th	Night	12% 3 16% 42% 23% 5%	824
	21st to end	Night	11% 2 16% 48% 20% 3	692
March	1st to 10th	Night	9% 4 18% 47% 18% 3	840
	11th to 20th	Night	10% 3 13% 42% 26% 7%	840
	21st to end	Night	9% 4 14% 45% 25% 4	924
April	1st to 10th	Night	9% 4 11% 45% 27% 4	840
	11th to 20th	Night	8% 5% 13% 41% 29% 4	831
	21st to end	Night	5% 6% 13% 40% 29% 6%	840
May	1st to 10th	Night	9% 5% 13% 39% 29% 4	739
	11th to 20th	Night	7% 4 14% 40% 30% 5%	591
	21st to end	Night	6% 4 16% 38% 32% 4	650
June	1st to 10th	Night	4 5% 14% 36% 36% 5%	594
	11th to 20th	Night	5% 4 2 28% 55% 7%	400
	21st to end	Night	6% 4 1 28% 54% 7%	400
July	1st to 10th	Night	4 2 1 11% 69% 13%	390
	11th to 20th	Night	3 2 1 15% 67% 14%	428
	21st to end	Night	4 2 3 21% 52% 18%	842
August	1st to 10th	Night	3 2 3 25% 52% 14%	796
	11th to 20th	Night	4 2 3 20% 53% 19%	792
	21st to end	Night	3 2 3 21% 51% 19%	880
September	1st to 10th	Night	8% 2 3 25% 46% 17%	800
	11th to 20th	Night	5% 2 4 33% 44% 11%	800
	21st to end	Night	4 2 4 32% 48% 10%	800
October	1st to 10th	Night	5% 2 5% 41% 42% 6%	800
	11th to 20th	Night	7% 3 7% 36% 39% 9%	800
	21st to end	Night	5% 2 8% 36% 38% 11%	880
November	1st to 10th	Night	5% 1 8% 41% 34% 11%	820
	11th to 20th	Night	6% 1 6% 44% 33% 9%	840
	21st to end	Night	6% 1 12% 45% 30% 6%	840
December	1st to 10th	Night	8% 3 10% 44% 29% 5%	840
	11th to 20th	Night	12% 4 10% 44% 26% 4	840
	21st to end	Night	12% 3 16% 44% 20% 5%	924

Page generated at 2026-04-28 23:21:44. Version ac07

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