Hat Ranch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hat Ranch Observatory clear sky chart from 2005-11-06 to 2026-08-30.

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 3 3 3 4 4 6% 8% 63%	151780
	Night	3 3 2 4 3 4 4 4 6% 8% 60%	147977
Transparency	Day	16% 6% 10% 22% 28% 17%	146794
	Night	20% 6% 10% 21% 27% 16%	143971
Seeing	Day	5% 10% 27% 30% 21% 7%	17575
	Night	5% 6% 29% 42% 17% 1	27626

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 6% 3 5% 3 3 4 3 5% 8% 60%	5218
	11th to 20th	Day	1 3 1 2 2 2 2 2 3 5% 77%	5340
	21st to end	Day	2 4 1 4 3 3 3 4 6% 6% 62%	5844
February	1st to 10th	Day	2 4 2 5% 4 2 3 3 5% 7% 64%	1922
	11th to 20th	Day	3 4 2 4 4 3 4 2 6% 7% 61%	2049
	21st to end	Day	2 4 1 3 3 2 3 3 4 7% 67%	1730
March	1st to 10th	Day	2 4 1 3 2 3 3 3 5% 7% 67%	4833
	11th to 20th	Day	3 4 2 4 3 4 4 4 6% 8% 59%	5110
	21st to end	Day	2 3 1 4 2 3 4 4 5% 8% 64%	5786
April	1st to 10th	Day	3 2 1 3 3 3 3 3 5% 8% 66%	2520
	11th to 20th	Day	2 3 1 4 3 3 4 3 4 7% 64%	2491
	21st to end	Day	2 3 1 2 3 3 3 3 5% 7% 69%	2520
May	1st to 10th	Day	3 2 1 4 3 4 4 4 6% 8% 61%	7057
	11th to 20th	Day	2 2 1 2 2 3 3 3 5% 6% 72%	7315
	21st to end	Day	2 1 1 2 2 2 3 3 4 7% 72%	9026
June	1st to 10th	Day	1 1 1 2 2 3 4 4 7% 9% 64%	2940
	11th to 20th	Day	1 1 2 3 2 3 3 5% 6% 73%	2940
	21st to end	Day	4 3 1 4 3 3 4 4 6% 7% 60%	2940
July	1st to 10th	Day	1 2 1 3 4 4 5% 4 7% 8% 62%	8134
	11th to 20th	Day	1 1 1 3 4 5% 7% 7% 10% 12% 49%	7650
	21st to end	Day	2 2 1 4 5% 5% 7% 6% 9% 13% 46%	8304
August	1st to 10th	Day	2 2 1 3 3 4 5% 5% 8% 12% 56%	5743
	11th to 20th	Day	2 2 1 3 3 4 5% 5% 7% 10% 59%	5486
	21st to end	Day	2 3 1 5% 4 5% 6% 6% 9% 11% 47%	6093
September	1st to 10th	Day	5% 3 1 3 4 4 4 4 7% 8% 56%	2400
	11th to 20th	Day	4 3 1 3 3 4 4 4 6% 7% 61%	2400
	21st to end	Day	3 2 1 3 2 2 3 3 4 6% 71%	2176
October	1st to 10th	Day	4 2 1 3 2 2 2 2 3 5% 75%	3270
	11th to 20th	Day	3 2 1 4 3 3 4 3 5% 6% 66%	3250
	21st to end	Day	2 2 1 2 1 2 1 2 3 5% 78%	3548
November	1st to 10th	Day	2 1 1 2 2 2 2 2 5% 6% 73%	1996
	11th to 20th	Day	2 2 1 2 2 2 3 3 4 7% 72%	1890
	21st to end	Day	3 5% 1 3 3 3 3 2 5% 6% 66%	1890
December	1st to 10th	Day	1 2 1 2 2 2 2 2 3 5% 78%	2771
	11th to 20th	Day	2 2 1 4 4 3 3 2 5% 7% 68%	3106
	21st to end	Day	2 3 1 3 3 3 2 3 4 6% 70%	4092
January	1st to 10th	Night	1 7% 3 7% 4 4 5% 4 5% 7% 53%	7745
	11th to 20th	Night	1 3 1 3 2 2 3 2 3 5% 74%	7650
	21st to end	Night	3 5% 2 4 3 3 4 4 5% 8% 60%	8370
February	1st to 10th	Night	2 5% 2 4 2 2 3 3 5% 6% 66%	2697
	11th to 20th	Night	4 6% 2 4 3 4 3 3 5% 8% 59%	2458
	21st to end	Night	3 4 2 3 3 3 4 2 4 6% 67%	2076
March	1st to 10th	Night	2 4 1 4 3 3 3 3 4 5% 66%	5255
	11th to 20th	Night	4 5% 2 5% 4 3 4 3 5% 6% 59%	5110
	21st to end	Night	2 3 1 4 3 3 4 3 5% 7% 67%	5408
April	1st to 10th	Night	3 3 1 4 3 4 3 3 5% 7% 64%	2100
	11th to 20th	Night	3 4 2 4 3 3 4 3 4 7% 62%	2077
	21st to end	Night	3 3 1 3 3 2 3 3 5% 7% 67%	2100
May	1st to 10th	Night	4 4 2 5% 4 5% 5% 4 7% 7% 54%	5890
	11th to 20th	Night	3 3 1 4 3 3 3 3 5% 7% 65%	5558
	21st to end	Night	2 2 1 4 3 3 4 3 6% 8% 64%	5181
June	1st to 10th	Night	2 3 1 5% 4 4 5% 4 7% 11% 55%	1680
	11th to 20th	Night	2 1 1 3 3 2 3 3 6% 7% 68%	1680
	21st to end	Night	4 3 1 5% 4 4 5% 5% 7% 9% 51%	1680
July	1st to 10th	Night	2 3 1 5% 4 5% 5% 5% 7% 9% 54%	4835
	11th to 20th	Night	2 3 2 5% 5% 6% 7% 6% 11% 15% 39%	5340
	21st to end	Night	4 4 2 6% 5% 7% 9% 7% 10% 12% 33%	5805
August	1st to 10th	Night	2 2 2 8% 6% 7% 8% 6% 9% 10% 40%	4287
	11th to 20th	Night	2 3 2 6% 5% 6% 7% 7% 10% 12% 41%	4583
	21st to end	Night	3 4 2 8% 7% 7% 7% 6% 10% 13% 33%	5083
September	1st to 10th	Night	4 3 2 5% 4 5% 5% 6% 7% 10% 50%	2000
	11th to 20th	Night	4 3 1 5% 4 4 4 4 6% 7% 58%	2000
	21st to end	Night	3 3 1 3 3 2 3 3 4 6% 67%	2217
October	1st to 10th	Night	4 2 1 2 2 2 3 2 4 5% 73%	3920
	11th to 20th	Night	4 3 2 4 3 3 3 3 5% 7% 63%	3898
	21st to end	Night	2 1 1 2 2 2 2 2 3 6% 77%	4252
November	1st to 10th	Night	2 2 1 2 1 2 2 3 4 6% 76%	2499
	11th to 20th	Night	2 2 1 3 2 2 2 2 5% 6% 74%	2730
	21st to end	Night	5% 4 2 4 3 3 3 3 4 5% 65%	2730
December	1st to 10th	Night	2 3 1 2 2 1 2 2 4 7% 75%	4634
	11th to 20th	Night	2 2 1 2 2 2 3 3 5% 9% 70%	5321
	21st to end	Night	3 4 1 4 3 3 3 2 4 6% 67%	7128

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	20% 5% 9% 36% 26% 3	5221
	11th to 20th	Day	11% 3 8% 28% 36% 14%	5340
	21st to end	Day	20% 6% 7% 18% 40% 11%	5844
February	1st to 10th	Day	20% 4 10% 26% 28% 11%	1922
	11th to 20th	Day	20% 5% 7% 27% 30% 11%	2059
	21st to end	Day	17% 4 5% 22% 33% 20%	1730
March	1st to 10th	Day	14% 4 5% 16% 36% 25%	4833
	11th to 20th	Day	18% 6% 9% 22% 31% 13%	5110
	21st to end	Day	15% 4 7% 30% 33% 10%	5786
April	1st to 10th	Day	15% 4 7% 20% 34% 19%	2520
	11th to 20th	Day	18% 4 7% 19% 30% 22%	2494
	21st to end	Day	14% 4 7% 21% 36% 19%	2520
May	1st to 10th	Day	16% 7% 11% 19% 30% 17%	7057
	11th to 20th	Day	11% 4 8% 15% 30% 32%	7318
	21st to end	Day	11% 5% 9% 16% 20% 39%	9027
June	1st to 10th	Day	13% 6% 11% 22% 25% 24%	2940
	11th to 20th	Day	11% 4 5% 15% 23% 42%	2940
	21st to end	Day	20% 5% 9% 19% 22% 25%	2940
July	1st to 10th	Day	17% 8% 9% 15% 20% 31%	8067
	11th to 20th	Day	18% 11% 21% 29% 14% 7%	7581
	21st to end	Day	23% 11% 21% 30% 13% 3	8120
August	1st to 10th	Day	21% 10% 21% 24% 16% 7%	4216
	11th to 20th	Day	16% 7% 13% 26% 23% 14%	4002
	21st to end	Day	25% 11% 20% 24% 17% 3	4420
September	1st to 10th	Day	23% 7% 12% 23% 24% 11%	2400
	11th to 20th	Day	21% 6% 10% 19% 26% 18%	2400
	21st to end	Day	15% 4 6% 22% 34% 19%	2176
October	1st to 10th	Day	14% 4 5% 14% 33% 32%	3270
	11th to 20th	Day	16% 6% 8% 16% 31% 23%	3250
	21st to end	Day	10% 2 5% 20% 40% 23%	3548
November	1st to 10th	Day	12% 3 6% 23% 41% 16%	1996
	11th to 20th	Day	13% 4 7% 26% 39% 11%	1890
	21st to end	Day	18% 3 7% 24% 33% 14%	1890
December	1st to 10th	Day	10% 4 7% 31% 39% 10%	2771
	11th to 20th	Day	17% 4 7% 31% 35% 6%	3104
	21st to end	Day	16% 4 10% 28% 34% 9%	4092
January	1st to 10th	Night	25% 7% 9% 30% 24% 5%	7746
	11th to 20th	Night	12% 5% 7% 28% 36% 12%	7650
	21st to end	Night	19% 5% 7% 18% 41% 9%	8370
February	1st to 10th	Night	18% 4 10% 26% 29% 13%	2697
	11th to 20th	Night	23% 5% 8% 26% 26% 12%	2458
	21st to end	Night	18% 3 5% 16% 37% 20%	2076
March	1st to 10th	Night	17% 4 5% 12% 39% 23%	5255
	11th to 20th	Night	22% 7% 7% 18% 31% 15%	5110
	21st to end	Night	14% 5% 8% 26% 36% 11%	5408
April	1st to 10th	Night	18% 4 6% 20% 31% 21%	2100
	11th to 20th	Night	20% 4 7% 18% 26% 25%	2077
	21st to end	Night	17% 4 9% 20% 34% 17%	2100
May	1st to 10th	Night	22% 8% 12% 19% 21% 18%	5888
	11th to 20th	Night	17% 6% 8% 16% 23% 30%	5559
	21st to end	Night	16% 6% 13% 12% 13% 40%	5182
June	1st to 10th	Night	22% 5% 15% 17% 19% 22%	1680
	11th to 20th	Night	15% 4 6% 15% 21% 39%	1680
	21st to end	Night	25% 7% 12% 15% 16% 24%	1680
July	1st to 10th	Night	23% 9% 11% 18% 15% 24%	4794
	11th to 20th	Night	25% 12% 27% 21% 8% 6%	5286
	21st to end	Night	34% 12% 22% 21% 9% 1	5677
August	1st to 10th	Night	35% 13% 16% 17% 13% 6%	3138
	11th to 20th	Night	27% 12% 16% 18% 17% 10%	3343
	21st to end	Night	35% 12% 18% 22% 9% 4	3687
September	1st to 10th	Night	25% 9% 16% 20% 20% 9%	2000
	11th to 20th	Night	22% 9% 12% 17% 23% 19%	2000
	21st to end	Night	18% 5% 10% 18% 30% 20%	2217
October	1st to 10th	Night	14% 4 6% 15% 31% 30%	3920
	11th to 20th	Night	18% 7% 7% 13% 28% 26%	3898
	21st to end	Night	10% 2 4 24% 37% 22%	4252
November	1st to 10th	Night	10% 3 6% 22% 42% 17%	2499
	11th to 20th	Night	12% 3 6% 24% 41% 15%	2730
	21st to end	Night	21% 3 6% 21% 32% 16%	2730
December	1st to 10th	Night	11% 3 6% 35% 32% 12%	4634
	11th to 20th	Night	14% 4 6% 31% 36% 9%	5322
	21st to end	Night	17% 5% 8% 28% 36% 7%	7128

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	5% 13% 39% 32% 10%	330
	11th to 20th	Day	8% 12% 37% 30% 11% 2	330
	21st to end	Day	4 7% 39% 35% 13% 2	363
February	1st to 10th	Day	4 14% 41% 29% 11% 2	330
	11th to 20th	Day	7% 18% 40% 29% 5%	330
	21st to end	Day	11% 18% 41% 23% 6% 1	273
March	1st to 10th	Day	6% 20% 34% 29% 11%	433
	11th to 20th	Day	11% 21% 34% 24% 8% 2	540
	21st to end	Day	10% 26% 32% 22% 10% 1	594
April	1st to 10th	Day	9% 20% 35% 25% 8% 3	540
	11th to 20th	Day	10% 23% 39% 18% 8% 1	532
	21st to end	Day	8% 22% 39% 23% 7% 1	540
May	1st to 10th	Day	7% 19% 38% 25% 8% 3	543
	11th to 20th	Day	6% 13% 33% 30% 14% 4	604
	21st to end	Day	5% 11% 30% 36% 16% 2	844
June	1st to 10th	Day	2 5% 23% 40% 25% 6%	780
	11th to 20th	Day	2 6% 24% 36% 27% 4	780
	21st to end	Day	5% 5% 20% 35% 28% 6%	780
July	1st to 10th	Day	3 2 13% 36% 33% 13%	735
	11th to 20th	Day	2 1 7% 29% 37% 24%	564
	21st to end	Day	3 1 6% 24% 40% 25%	619
August	1st to 10th	Day	3 3 9% 26% 38% 21%	567
	11th to 20th	Day	3 2 12% 32% 34% 16%	557
	21st to end	Day	4 2 11% 31% 37% 16%	620
September	1st to 10th	Day	5% 3 13% 29% 34% 15%	530
	11th to 20th	Day	3 3 24% 36% 26% 8%	530
	21st to end	Day	5% 5% 21% 32% 30% 7%	351
October	1st to 10th	Day	6% 8% 23% 27% 27% 9%	330
	11th to 20th	Day	6% 11% 31% 25% 21% 6%	330
	21st to end	Day	8% 9% 28% 29% 20% 6%	363
November	1st to 10th	Day	5% 13% 32% 32% 16% 2	330
	11th to 20th	Day	4 7% 25% 43% 20% 2	330
	21st to end	Day	8% 12% 34% 31% 14% 1	330
December	1st to 10th	Day	4 11% 37% 32% 15% 2	330
	11th to 20th	Day	6% 8% 35% 35% 13% 2	330
	21st to end	Day	8% 13% 38% 33% 7% 1	363
January	1st to 10th	Night	4 5% 33% 43% 14% 1	892
	11th to 20th	Night	5% 3 34% 44% 13% 1	840
	21st to end	Night	6% 6% 33% 42% 12% 2	924
February	1st to 10th	Night	5% 7% 36% 42% 10%	840
	11th to 20th	Night	7% 9% 41% 35% 8%	824
	21st to end	Night	6% 8% 47% 36% 4	692
March	1st to 10th	Night	6% 7% 41% 38% 7%	840
	11th to 20th	Night	7% 10% 35% 37% 10%	840
	21st to end	Night	4 10% 37% 41% 8%	838
April	1st to 10th	Night	4 13% 40% 38% 5%	630
	11th to 20th	Night	6% 14% 38% 36% 6%	623
	21st to end	Night	5% 11% 43% 35% 6%	630
May	1st to 10th	Night	5% 8% 36% 40% 10%	630
	11th to 20th	Night	7% 8% 31% 40% 13% 1	621
	21st to end	Night	4 7% 35% 43% 12%	683
June	1st to 10th	Night	3 6% 26% 48% 17% 1	630
	11th to 20th	Night	2 3 28% 52% 14% 1	630
	21st to end	Night	4 4 20% 49% 23% 1	630
July	1st to 10th	Night	3 2 16% 48% 29% 2	615
	11th to 20th	Night	3 1 10% 37% 45% 3	612
	21st to end	Night	6% 2 7% 34% 46% 5%	690
August	1st to 10th	Night	4 2 8% 34% 46% 6%	627
	11th to 20th	Night	3 3 12% 45% 35% 3	624
	21st to end	Night	3 1 10% 43% 39% 4	687
September	1st to 10th	Night	4 2 14% 50% 28% 2	600
	11th to 20th	Night	5% 3 15% 49% 28% 1	600
	21st to end	Night	4 3 23% 48% 20% 2	640
October	1st to 10th	Night	6% 4 22% 48% 20% 1	800
	11th to 20th	Night	4 5% 27% 47% 16% 2	800
	21st to end	Night	5% 4 25% 43% 19% 3	880
November	1st to 10th	Night	3 4 27% 46% 19% 1	859
	11th to 20th	Night	3 5% 27% 46% 17% 2	1050
	21st to end	Night	6% 7% 30% 39% 16% 2	1050
December	1st to 10th	Night	5% 7% 32% 44% 12% 1	1050
	11th to 20th	Night	6% 6% 32% 43% 12%	1050
	21st to end	Night	7% 6% 37% 43% 7%	1155

Page generated at 2026-08-27 02:07:16. Version ac07

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