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 2025-04-26.

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% 64%	117505
	Night	3 3 1 4 3 3 4 3 5% 7% 61%	119998
Transparency	Day	17% 6% 10% 23% 28% 17%	117264
	Night	19% 6% 9% 22% 28% 16%	119828
Seeing	Day	6% 11% 27% 31% 20% 7%	15674
	Night	5% 6% 28% 42% 17% 1	25920

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% 7% 61%	4024
	11th to 20th	Day	2 4 1 3 2 2 2 3 4 6% 71%	4100
	21st to end	Day	2 4 1 4 4 4 4 4 5% 7% 61%	4510
February	1st to 10th	Day	2 4 2 5% 4 2 3 3 5% 7% 63%	1830
	11th to 20th	Day	3 4 1 4 4 3 4 2 6% 7% 62%	1949
	21st to end	Day	2 4 1 4 3 3 3 3 5% 7% 65%	1650
March	1st to 10th	Day	2 4 1 3 3 3 3 3 5% 7% 67%	4728
	11th to 20th	Day	3 4 2 4 4 4 4 4 6% 8% 58%	5000
	21st to end	Day	2 3 1 4 2 3 4 4 5% 8% 64%	5661
April	1st to 10th	Day	3 2 1 3 3 3 3 3 5% 8% 67%	2400
	11th to 20th	Day	2 3 1 4 3 3 4 3 4 7% 65%	2371
	21st to end	Day	3 3 1 2 3 3 3 3 5% 7% 68%	2341
May	1st to 10th	Day	1 1 1 2 2 2 3 3 4 6% 74%	3800
	11th to 20th	Day	3 3 1 4 3 3 4 4 6% 8% 61%	3914
	21st to end	Day	2 1 1 2 2 2 2 2 3 5% 79%	4825
June	1st to 10th	Day	1 1 1 2 2 3 4 4 7% 9% 65%	2660
	11th to 20th	Day	2 2 2 3 2 3 2 5% 6% 73%	2660
	21st to end	Day	4 3 1 4 3 4 4 4 6% 7% 59%	2660
July	1st to 10th	Day	2 2 1 3 4 5% 5% 5% 8% 8% 57%	4371
	11th to 20th	Day	1 2 1 3 3 4 6% 7% 9% 12% 51%	4110
	21st to end	Day	3 3 2 4 5% 5% 7% 7% 10% 12% 43%	4454
August	1st to 10th	Day	3 2 1 4 4 4 6% 6% 9% 13% 47%	3978
	11th to 20th	Day	2 2 1 3 3 3 4 4 6% 9% 62%	3726
	21st to end	Day	2 3 2 6% 5% 6% 6% 6% 10% 11% 44%	4180
September	1st to 10th	Day	5% 3 1 3 4 4 4 4 7% 8% 56%	2280
	11th to 20th	Day	4 3 1 3 3 3 4 4 6% 7% 62%	2280
	21st to end	Day	2 2 1 2 2 2 3 2 4 6% 72%	2067
October	1st to 10th	Day	4 2 1 3 2 2 2 2 3 5% 75%	3170
	11th to 20th	Day	3 2 1 4 3 3 4 3 5% 6% 66%	3150
	21st to end	Day	2 2 1 2 1 2 1 2 3 5% 79%	3438
November	1st to 10th	Day	3 1 1 2 2 2 2 3 5% 6% 72%	1899
	11th to 20th	Day	2 2 1 2 2 2 3 3 4 7% 73%	1800
	21st to end	Day	3 5% 1 3 3 3 3 3 5% 6% 66%	1800
December	1st to 10th	Day	1 2 1 2 2 2 2 2 3 5% 78%	2689
	11th to 20th	Day	2 3 1 4 4 3 3 2 5% 7% 67%	3026
	21st to end	Day	2 3 1 3 3 3 2 3 4 6% 70%	4004
January	1st to 10th	Night	1 6% 4 6% 4 3 4 3 5% 7% 58%	5982
	11th to 20th	Night	2 4 1 3 3 3 3 2 4 6% 69%	5880
	21st to end	Night	3 4 2 5% 3 3 4 4 5% 8% 59%	6468
February	1st to 10th	Night	2 6% 2 4 3 3 3 3 5% 6% 65%	2569
	11th to 20th	Night	4 6% 2 4 3 4 3 3 5% 8% 59%	2338
	21st to end	Night	3 4 2 4 3 3 4 2 4 6% 66%	1980
March	1st to 10th	Night	2 4 1 4 3 3 4 3 4 5% 66%	5140
	11th to 20th	Night	4 5% 2 5% 4 3 5% 3 5% 6% 58%	5000
	21st to end	Night	2 3 1 4 3 3 4 3 5% 7% 66%	5291
April	1st to 10th	Night	3 3 1 3 3 4 3 3 5% 7% 64%	2000
	11th to 20th	Night	3 4 2 4 3 3 3 3 4 7% 63%	1977
	21st to end	Night	3 3 1 3 3 2 3 3 5% 7% 67%	1955
May	1st to 10th	Night	2 3 1 3 3 3 4 3 5% 6% 66%	3170
	11th to 20th	Night	5% 4 2 5% 4 4 5% 4 7% 8% 53%	2972
	21st to end	Night	3 2 1 3 2 2 3 3 4 6% 72%	2762
June	1st to 10th	Night	2 3 1 4 4 4 5% 4 7% 10% 56%	1520
	11th to 20th	Night	3 1 1 3 3 2 3 3 6% 7% 68%	1520
	21st to end	Night	4 3 2 5% 5% 4 5% 5% 7% 9% 51%	1520
July	1st to 10th	Night	2 3 2 6% 5% 5% 4 5% 8% 8% 52%	2593
	11th to 20th	Night	2 3 2 5% 5% 5% 6% 6% 11% 14% 40%	2860
	21st to end	Night	6% 5% 2 5% 5% 5% 6% 5% 9% 12% 40%	3109
August	1st to 10th	Night	2 3 2 9% 7% 8% 8% 7% 10% 12% 33%	2960
	11th to 20th	Night	2 3 1 5% 5% 6% 7% 7% 9% 12% 42%	3113
	21st to end	Night	2 3 2 8% 7% 7% 8% 6% 11% 11% 33%	3487
September	1st to 10th	Night	4 3 1 5% 4 5% 5% 6% 7% 10% 51%	1900
	11th to 20th	Night	4 3 1 5% 3 4 4 4 6% 7% 58%	1900
	21st to end	Night	4 3 2 3 3 2 4 3 4 6% 68%	2106
October	1st to 10th	Night	4 2 1 2 2 2 3 2 4 5% 74%	3800
	11th to 20th	Night	4 3 1 4 3 3 3 3 5% 7% 63%	3778
	21st to end	Night	3 1 1 2 2 2 2 1 3 6% 77%	4120
November	1st to 10th	Night	2 2 1 2 1 2 2 3 4 6% 75%	2377
	11th to 20th	Night	2 2 1 2 2 2 2 2 5% 5% 75%	2600
	21st to end	Night	5% 4 2 4 3 3 3 3 4 5% 66%	2600
December	1st to 10th	Night	2 3 1 2 1 1 2 2 4 7% 75%	4496
	11th to 20th	Night	2 2 1 2 2 2 3 3 5% 9% 69%	5181
	21st to end	Night	3 4 1 4 3 3 3 2 4 6% 67%	6974

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	21% 5% 7% 33% 30% 4	4025
	11th to 20th	Day	14% 4 9% 29% 27% 17%	4100
	21st to end	Day	20% 6% 8% 18% 35% 12%	4510
February	1st to 10th	Day	21% 5% 11% 26% 27% 11%	1830
	11th to 20th	Day	20% 5% 7% 27% 30% 11%	1959
	21st to end	Day	17% 4 6% 21% 32% 20%	1650
March	1st to 10th	Day	14% 4 5% 16% 36% 25%	4728
	11th to 20th	Day	18% 6% 10% 23% 31% 13%	5000
	21st to end	Day	16% 4 7% 30% 34% 10%	5661
April	1st to 10th	Day	15% 4 8% 19% 34% 19%	2400
	11th to 20th	Day	18% 4 7% 19% 30% 22%	2374
	21st to end	Day	15% 4 7% 22% 35% 17%	2341
May	1st to 10th	Day	9% 5% 9% 17% 34% 27%	3800
	11th to 20th	Day	17% 6% 11% 20% 29% 16%	3917
	21st to end	Day	9% 3 7% 13% 22% 46%	4825
June	1st to 10th	Day	13% 5% 11% 23% 25% 24%	2660
	11th to 20th	Day	12% 4 5% 14% 23% 42%	2660
	21st to end	Day	21% 5% 10% 19% 21% 23%	2660
July	1st to 10th	Day	20% 9% 12% 16% 18% 25%	4303
	11th to 20th	Day	18% 9% 19% 30% 18% 5%	4041
	21st to end	Day	25% 11% 19% 32% 12% 2	4314
August	1st to 10th	Day	22% 10% 23% 24% 14% 7%	3964
	11th to 20th	Day	16% 7% 13% 26% 23% 15%	3762
	21st to end	Day	26% 11% 19% 23% 18% 4	4179
September	1st to 10th	Day	23% 7% 12% 22% 24% 12%	2280
	11th to 20th	Day	21% 6% 10% 18% 26% 19%	2280
	21st to end	Day	14% 4 6% 22% 34% 20%	2067
October	1st to 10th	Day	14% 4 5% 14% 33% 32%	3170
	11th to 20th	Day	16% 6% 8% 16% 31% 23%	3150
	21st to end	Day	10% 2 5% 20% 40% 23%	3438
November	1st to 10th	Day	12% 3 6% 23% 40% 16%	1899
	11th to 20th	Day	12% 4 7% 26% 39% 12%	1800
	21st to end	Day	19% 3 7% 22% 34% 15%	1800
December	1st to 10th	Day	10% 3 7% 30% 39% 10%	2689
	11th to 20th	Day	18% 4 7% 32% 34% 6%	3024
	21st to end	Day	16% 4 10% 28% 34% 9%	4004
January	1st to 10th	Night	22% 6% 8% 27% 29% 7%	5982
	11th to 20th	Night	14% 6% 7% 29% 28% 14%	5880
	21st to end	Night	19% 7% 9% 19% 38% 9%	6468
February	1st to 10th	Night	19% 4 10% 26% 28% 13%	2569
	11th to 20th	Night	22% 5% 8% 26% 27% 13%	2338
	21st to end	Night	19% 4 5% 16% 36% 21%	1980
March	1st to 10th	Night	17% 4 5% 13% 39% 22%	5140
	11th to 20th	Night	23% 7% 7% 18% 30% 15%	5000
	21st to end	Night	14% 5% 8% 26% 36% 10%	5291
April	1st to 10th	Night	18% 4 6% 20% 31% 21%	2000
	11th to 20th	Night	20% 4 7% 18% 25% 25%	1977
	21st to end	Night	17% 4 9% 20% 35% 15%	1955
May	1st to 10th	Night	13% 7% 11% 14% 26% 28%	3170
	11th to 20th	Night	24% 9% 14% 22% 17% 15%	2973
	21st to end	Night	12% 4 9% 12% 18% 46%	2762
June	1st to 10th	Night	21% 6% 16% 17% 19% 22%	1520
	11th to 20th	Night	15% 4 6% 14% 21% 40%	1520
	21st to end	Night	26% 8% 13% 15% 17% 22%	1520
July	1st to 10th	Night	26% 9% 12% 18% 13% 22%	2552
	11th to 20th	Night	25% 12% 23% 27% 9% 4	2806
	21st to end	Night	33% 10% 20% 27% 10% 1	3011
August	1st to 10th	Night	36% 13% 17% 16% 12% 6%	2951
	11th to 20th	Night	27% 12% 16% 17% 17% 11%	3143
	21st to end	Night	35% 12% 17% 22% 9% 4	3487
September	1st to 10th	Night	25% 8% 17% 20% 20% 10%	1900
	11th to 20th	Night	22% 8% 12% 16% 22% 19%	1900
	21st to end	Night	18% 5% 9% 19% 30% 20%	2106
October	1st to 10th	Night	14% 4 6% 15% 32% 30%	3800
	11th to 20th	Night	18% 6% 7% 13% 29% 26%	3778
	21st to end	Night	10% 2 4 24% 37% 22%	4120
November	1st to 10th	Night	11% 3 6% 23% 40% 17%	2377
	11th to 20th	Night	12% 3 5% 24% 41% 15%	2600
	21st to end	Night	22% 3 6% 20% 32% 17%	2600
December	1st to 10th	Night	11% 3 6% 35% 32% 13%	4496
	11th to 20th	Night	14% 4 6% 32% 36% 9%	5182
	21st to end	Night	17% 5% 8% 28% 36% 7%	6974

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% 12% 39% 33% 11%	300
	11th to 20th	Day	9% 12% 37% 30% 10% 1	300
	21st to end	Day	4 7% 38% 36% 13% 2	330
February	1st to 10th	Day	5% 15% 41% 29% 10% 1	300
	11th to 20th	Day	6% 19% 41% 29% 5%	300
	21st to end	Day	11% 19% 42% 22% 5% 1	249
March	1st to 10th	Day	6% 21% 33% 29% 11% 1	398
	11th to 20th	Day	11% 22% 34% 23% 8% 2	500
	21st to end	Day	11% 27% 32% 21% 9%	550
April	1st to 10th	Day	10% 20% 36% 24% 8% 3	500
	11th to 20th	Day	10% 24% 39% 17% 8% 1	492
	21st to end	Day	8% 20% 39% 25% 7% 1	481
May	1st to 10th	Day	7% 19% 40% 25% 8% 1	463
	11th to 20th	Day	6% 12% 32% 31% 15% 4	520
	21st to end	Day	5% 11% 31% 36% 15% 1	734
June	1st to 10th	Day	1 6% 21% 41% 26% 6%	680
	11th to 20th	Day	3 7% 23% 37% 26% 4	680
	21st to end	Day	6% 5% 18% 36% 29% 6%	680
July	1st to 10th	Day	3 2 14% 35% 33% 13%	639
	11th to 20th	Day	3 1 7% 29% 37% 24%	484
	21st to end	Day	3 1 6% 25% 40% 25%	531
August	1st to 10th	Day	3 3 8% 27% 38% 21%	487
	11th to 20th	Day	3 2 14% 33% 32% 15%	477
	21st to end	Day	4 1 12% 32% 35% 15%	539
September	1st to 10th	Day	6% 3 13% 30% 33% 15%	490
	11th to 20th	Day	3 3 23% 37% 26% 9%	490
	21st to end	Day	4 5% 21% 32% 30% 8%	320
October	1st to 10th	Day	7% 8% 23% 27% 27% 9%	300
	11th to 20th	Day	6% 11% 27% 27% 22% 7%	300
	21st to end	Day	9% 10% 27% 28% 20% 6%	330
November	1st to 10th	Day	5% 14% 33% 32% 15% 1	300
	11th to 20th	Day	4 6% 24% 43% 20% 2	300
	21st to end	Day	8% 13% 33% 31% 14% 1	300
December	1st to 10th	Day	5% 11% 34% 32% 16% 2	300
	11th to 20th	Day	6% 9% 35% 36% 13% 1	300
	21st to end	Day	8% 14% 39% 32% 6% 1	330
January	1st to 10th	Night	4 5% 32% 43% 14% 1	850
	11th to 20th	Night	6% 3 35% 43% 13% 1	800
	21st to end	Night	5% 6% 33% 42% 12% 2	880
February	1st to 10th	Night	5% 7% 37% 40% 10%	800
	11th to 20th	Night	7% 9% 41% 35% 8%	784
	21st to end	Night	7% 8% 46% 36% 3	660
March	1st to 10th	Night	6% 8% 41% 37% 8%	800
	11th to 20th	Night	7% 10% 35% 37% 10%	800
	21st to end	Night	5% 10% 37% 41% 7%	798
April	1st to 10th	Night	5% 13% 40% 38% 4	600
	11th to 20th	Night	6% 14% 37% 36% 7%	593
	21st to end	Night	5% 10% 42% 35% 7%	587
May	1st to 10th	Night	5% 9% 36% 40% 11%	570
	11th to 20th	Night	7% 8% 29% 41% 14% 1	561
	21st to end	Night	4 7% 34% 42% 12%	617
June	1st to 10th	Night	2 6% 25% 47% 18% 1	570
	11th to 20th	Night	2 3 28% 51% 15% 1	570
	21st to end	Night	4 3 19% 48% 25% 1	570
July	1st to 10th	Night	3 2 15% 47% 30% 3	555
	11th to 20th	Night	4 1 10% 36% 46% 4	552
	21st to end	Night	6% 2 7% 33% 47% 5%	624
August	1st to 10th	Night	4 2 8% 32% 48% 7%	567
	11th to 20th	Night	4 2 11% 45% 34% 3	564
	21st to end	Night	3 1 10% 42% 40% 4	627
September	1st to 10th	Night	4 2 14% 50% 28% 2	570
	11th to 20th	Night	5% 3 14% 48% 29% 1	570
	21st to end	Night	4 3 23% 47% 21% 2	608
October	1st to 10th	Night	6% 3 21% 48% 20% 1	760
	11th to 20th	Night	4 4 27% 47% 17% 2	760
	21st to end	Night	5% 5% 26% 42% 20% 3	836
November	1st to 10th	Night	3 5% 26% 45% 19% 1	817
	11th to 20th	Night	3 5% 27% 47% 16% 2	1000
	21st to end	Night	6% 7% 29% 39% 17% 2	1000
December	1st to 10th	Night	5% 7% 30% 45% 12% 1	1000
	11th to 20th	Night	6% 7% 32% 43% 12%	1000
	21st to end	Night	7% 6% 37% 42% 8%	1100

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