Antelope Hills Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Antelope Hills Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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 5% 2 5% 3 4 4 4 6% 8% 56%	129733
	Night	5% 6% 2 5% 4 4 4 3 5% 7% 54%	127270
Transparency	Day	26% 5% 10% 24% 24% 12%	129429
	Night	28% 5% 10% 24% 23% 9%	127091
Seeing	Day	7% 6% 24% 32% 22% 8%	16663
	Night	7% 4 31% 43% 14% 1	26619

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	2 8% 3 7% 3 3 3 3 5% 8% 56%	3638
	11th to 20th	Day	2 5% 2 8% 3 4 4 3 5% 8% 55%	3658
	21st to end	Day	2 5% 2 6% 4 4 3 3 4 6% 61%	4510
February	1st to 10th	Day	3 7% 3 6% 4 3 4 3 4 8% 53%	1822
	11th to 20th	Day	1 5% 3 7% 5% 4 5% 3 5% 7% 55%	1949
	21st to end	Day	3 6% 2 6% 4 4 3 3 5% 6% 58%	1650
March	1st to 10th	Day	3 9% 3 4 3 3 3 3 5% 6% 59%	4500
	11th to 20th	Day	5% 8% 2 6% 4 4 4 4 5% 7% 53%	4816
	21st to end	Day	4 6% 2 5% 3 4 4 4 6% 9% 54%	5894
April	1st to 10th	Day	6% 7% 2 4 3 4 3 3 6% 8% 53%	2400
	11th to 20th	Day	9% 9% 3 6% 4 3 4 4 5% 7% 47%	2371
	21st to end	Day	10% 6% 2 5% 4 4 4 5% 6% 9% 47%	2400
May	1st to 10th	Day	8% 7% 2 6% 4 4 5% 3 5% 7% 48%	5843
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 5% 6% 8% 49%	6158
	21st to end	Day	8% 5% 2 5% 4 4 5% 4 7% 9% 48%	6925
June	1st to 10th	Day	5% 3 2 4 4 4 5% 4 8% 9% 52%	2800
	11th to 20th	Day	6% 3 1 4 4 4 4 5% 7% 9% 54%	2800
	21st to end	Day	3 3 2 4 3 5% 5% 5% 8% 9% 54%	2800
July	1st to 10th	Day	4 2 1 3 3 3 4 4 6% 8% 61%	6339
	11th to 20th	Day	2 2 1 4 3 4 5% 5% 8% 11% 53%	6340
	21st to end	Day	4 4 2 6% 5% 4 6% 5% 8% 8% 49%	6903
August	1st to 10th	Day	7% 4 1 5% 4 5% 6% 5% 8% 11% 44%	4072
	11th to 20th	Day	4 2 1 4 4 4 5% 4 7% 9% 56%	3726
	21st to end	Day	3 3 1 5% 4 4 5% 5% 7% 10% 54%	4180
September	1st to 10th	Day	5% 3 1 3 3 3 4 3 4 7% 64%	2280
	11th to 20th	Day	6% 3 1 3 3 4 3 4 6% 8% 62%	2228
	21st to end	Day	5% 4 1 3 3 2 3 2 3 5% 68%	2090
October	1st to 10th	Day	4 2 1 2 2 2 2 2 2 4 78%	3273
	11th to 20th	Day	4 4 2 4 3 3 3 3 4 7% 65%	3150
	21st to end	Day	5% 6% 1 3 2 3 2 2 4 5% 66%	3255
November	1st to 10th	Day	2 6% 1 4 3 2 3 3 5% 7% 64%	1763
	11th to 20th	Day	2 5% 1 5% 3 4 4 2 4 7% 62%	1800
	21st to end	Day	3 6% 2 5% 3 3 2 3 5% 7% 61%	1750
December	1st to 10th	Day	1 6% 2 4 3 3 4 3 4 5% 66%	2620
	11th to 20th	Day	2 4 1 4 3 3 4 2 5% 6% 65%	3026
	21st to end	Day	4 9% 3 6% 4 4 4 3 5% 7% 52%	4004
January	1st to 10th	Night	2 10% 3 7% 3 3 3 3 6% 7% 52%	6327
	11th to 20th	Night	1 6% 3 7% 5% 5% 4 3 5% 6% 55%	6340
	21st to end	Night	2 6% 3 7% 4 4 3 3 4 6% 59%	6492
February	1st to 10th	Night	3 8% 3 6% 3 4 4 3 5% 7% 53%	2576
	11th to 20th	Night	3 9% 4 7% 5% 4 5% 3 6% 7% 48%	2338
	21st to end	Night	3 8% 2 6% 3 3 3 3 5% 6% 58%	1980
March	1st to 10th	Night	4 9% 3 5% 3 4 3 3 4 5% 57%	5374
	11th to 20th	Night	6% 9% 2 5% 4 4 3 3 5% 6% 52%	5160
	21st to end	Night	7% 7% 2 5% 4 4 4 5% 6% 8% 47%	5060
April	1st to 10th	Night	7% 9% 3 5% 5% 4 4 2 5% 6% 51%	2000
	11th to 20th	Night	12% 8% 2 6% 4 4 4 3 6% 7% 46%	1977
	21st to end	Night	13% 8% 2 6% 4 3 3 3 5% 7% 46%	2000
May	1st to 10th	Night	11% 7% 3 6% 4 4 4 3 5% 7% 46%	4136
	11th to 20th	Night	8% 5% 2 6% 5% 5% 5% 4 6% 9% 45%	3753
	21st to end	Night	10% 6% 2 5% 4 4 4 4 6% 7% 47%	3972
June	1st to 10th	Night	6% 4 1 3 3 4 4 4 6% 11% 54%	1600
	11th to 20th	Night	5% 3 1 4 4 3 5% 3 5% 8% 59%	1600
	21st to end	Night	4 3 2 5% 4 4 5% 5% 7% 9% 52%	1600
July	1st to 10th	Night	3 3 1 4 4 5% 5% 4 8% 8% 55%	3640
	11th to 20th	Night	4 3 1 4 5% 5% 5% 5% 7% 11% 49%	3640
	21st to end	Night	4 4 2 6% 5% 5% 5% 5% 7% 10% 46%	3970
August	1st to 10th	Night	8% 5% 2 7% 5% 6% 7% 6% 8% 9% 38%	2949
	11th to 20th	Night	4 4 1 5% 5% 5% 5% 5% 8% 10% 47%	3113
	21st to end	Night	3 3 1 4 4 4 5% 6% 8% 11% 50%	3487
September	1st to 10th	Night	6% 4 1 4 3 4 4 5% 6% 8% 56%	1900
	11th to 20th	Night	6% 3 1 3 3 3 4 3 6% 7% 61%	1952
	21st to end	Night	6% 3 1 4 3 3 3 3 4 7% 63%	2090
October	1st to 10th	Night	6% 3 1 3 2 2 3 3 4 5% 68%	3690
	11th to 20th	Night	6% 4 2 4 3 3 4 3 5% 7% 59%	3778
	21st to end	Night	5% 7% 2 3 2 2 3 2 4 6% 64%	4271
November	1st to 10th	Night	3 4 1 5% 3 3 3 3 4 8% 63%	2535
	11th to 20th	Night	3 6% 1 4 3 3 3 3 4 6% 64%	2600
	21st to end	Night	4 7% 2 5% 3 3 3 3 6% 6% 59%	2645
December	1st to 10th	Night	1 5% 2 4 2 2 2 2 3 6% 70%	4570
	11th to 20th	Night	2 5% 2 5% 4 3 3 4 5% 8% 60%	5181
	21st to end	Night	5% 9% 3 7% 4 4 4 3 5% 6% 51%	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	26% 3 8% 34% 19% 10%	3639
	11th to 20th	Day	26% 4 10% 21% 29% 9%	3658
	21st to end	Day	26% 4 8% 28% 24% 11%	4510
February	1st to 10th	Day	29% 4 11% 28% 20% 8%	1822
	11th to 20th	Day	27% 5% 8% 29% 23% 7%	1959
	21st to end	Day	26% 4 7% 23% 26% 14%	1650
March	1st to 10th	Day	25% 4 6% 17% 30% 18%	4500
	11th to 20th	Day	28% 6% 9% 22% 27% 7%	4816
	21st to end	Day	25% 6% 11% 35% 19% 3	5894
April	1st to 10th	Day	28% 6% 12% 24% 22% 9%	2400
	11th to 20th	Day	34% 6% 11% 22% 18% 9%	2374
	21st to end	Day	31% 7% 12% 26% 21% 3	2400
May	1st to 10th	Day	30% 6% 11% 28% 20% 5%	5843
	11th to 20th	Day	30% 6% 9% 21% 23% 11%	6162
	21st to end	Day	29% 7% 15% 24% 20% 6%	6926
June	1st to 10th	Day	25% 8% 16% 24% 18% 9%	2800
	11th to 20th	Day	24% 7% 11% 19% 24% 16%	2800
	21st to end	Day	22% 6% 11% 22% 25% 13%	2800
July	1st to 10th	Day	21% 3 7% 17% 29% 22%	6271
	11th to 20th	Day	24% 3 13% 27% 21% 12%	6265
	21st to end	Day	30% 4 12% 19% 17% 17%	6705
August	1st to 10th	Day	33% 6% 16% 25% 15% 6%	4057
	11th to 20th	Day	25% 4 13% 20% 23% 15%	3762
	21st to end	Day	26% 4 10% 20% 26% 14%	4179
September	1st to 10th	Day	20% 4 8% 23% 26% 20%	2280
	11th to 20th	Day	19% 6% 9% 17% 30% 19%	2228
	21st to end	Day	20% 3 7% 20% 35% 14%	2090
October	1st to 10th	Day	14% 3 5% 15% 36% 28%	3273
	11th to 20th	Day	22% 2 6% 27% 33% 10%	3150
	21st to end	Day	22% 2 4 22% 37% 13%	3255
November	1st to 10th	Day	20% 3 9% 26% 27% 15%	1763
	11th to 20th	Day	22% 4 7% 23% 35% 10%	1800
	21st to end	Day	24% 3 7% 22% 30% 15%	1750
December	1st to 10th	Day	21% 4 7% 33% 26% 10%	2620
	11th to 20th	Day	20% 4 7% 30% 32% 8%	3024
	21st to end	Day	31% 3 7% 27% 22% 9%	4004
January	1st to 10th	Night	30% 4 7% 29% 23% 7%	6327
	11th to 20th	Night	29% 5% 10% 22% 21% 13%	6340
	21st to end	Night	27% 4 9% 24% 22% 13%	6492
February	1st to 10th	Night	28% 6% 10% 25% 22% 8%	2576
	11th to 20th	Night	34% 6% 9% 25% 18% 7%	2338
	21st to end	Night	25% 4 7% 25% 26% 13%	1980
March	1st to 10th	Night	29% 3 6% 16% 33% 13%	5374
	11th to 20th	Night	31% 5% 11% 13% 28% 13%	5160
	21st to end	Night	33% 7% 10% 32% 16% 2	5060
April	1st to 10th	Night	33% 5% 10% 22% 21% 9%	2000
	11th to 20th	Night	36% 5% 12% 22% 17% 7%	1977
	21st to end	Night	37% 7% 10% 25% 18% 4	2000
May	1st to 10th	Night	35% 8% 14% 29% 10% 5%	4135
	11th to 20th	Night	32% 8% 13% 24% 14% 8%	3754
	21st to end	Night	31% 9% 17% 24% 14% 4	3972
June	1st to 10th	Night	22% 9% 21% 28% 14% 6%	1600
	11th to 20th	Night	21% 7% 15% 24% 20% 13%	1600
	21st to end	Night	24% 8% 12% 24% 23% 8%	1600
July	1st to 10th	Night	25% 4 12% 21% 31% 8%	3600
	11th to 20th	Night	29% 4 18% 24% 19% 5%	3592
	21st to end	Night	32% 5% 16% 17% 22% 8%	3856
August	1st to 10th	Night	39% 7% 17% 21% 13% 3	2941
	11th to 20th	Night	30% 6% 20% 20% 16% 8%	3143
	21st to end	Night	25% 5% 13% 29% 23% 6%	3487
September	1st to 10th	Night	24% 5% 11% 22% 24% 14%	1900
	11th to 20th	Night	20% 5% 11% 24% 25% 15%	1952
	21st to end	Night	21% 4 10% 21% 31% 13%	2090
October	1st to 10th	Night	19% 4 5% 19% 31% 22%	3690
	11th to 20th	Night	26% 3 7% 27% 28% 10%	3778
	21st to end	Night	24% 3 6% 24% 30% 14%	4271
November	1st to 10th	Night	20% 3 8% 24% 28% 15%	2535
	11th to 20th	Night	22% 4 8% 25% 30% 12%	2600
	21st to end	Night	26% 4 7% 22% 30% 11%	2645
December	1st to 10th	Night	18% 3 7% 33% 30% 9%	4570
	11th to 20th	Night	22% 3 6% 30% 33% 6%	5182
	21st to end	Night	33% 4 7% 24% 25% 8%	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	4 6% 24% 43% 20% 3	300
	11th to 20th	Day	7% 8% 27% 41% 16% 1	300
	21st to end	Day	6% 5% 34% 38% 15% 2	330
February	1st to 10th	Day	4 9% 40% 38% 6% 2	300
	11th to 20th	Day	3 5% 41% 39% 10% 2	300
	21st to end	Day	3 8% 43% 35% 11% 1	249
March	1st to 10th	Day	6% 6% 29% 37% 17% 6%	300
	11th to 20th	Day	8% 11% 31% 31% 16% 4	420
	21st to end	Day	11% 14% 33% 27% 13% 3	550
April	1st to 10th	Day	9% 13% 32% 30% 13% 3	500
	11th to 20th	Day	14% 17% 29% 26% 12% 2	492
	21st to end	Day	14% 13% 31% 25% 14% 4	500
May	1st to 10th	Day	14% 12% 29% 27% 15% 4	672
	11th to 20th	Day	13% 6% 22% 31% 21% 6%	725
	21st to end	Day	9% 10% 25% 29% 19% 8%	789
June	1st to 10th	Day	7% 5% 19% 30% 26% 13%	730
	11th to 20th	Day	6% 5% 26% 32% 24% 6%	730
	21st to end	Day	5% 4 21% 33% 28% 8%	730
July	1st to 10th	Day	6% 2 16% 31% 33% 12%	721
	11th to 20th	Day	5% 1 11% 34% 34% 15%	718
	21st to end	Day	5% 2 11% 29% 36% 17%	794
August	1st to 10th	Day	6% 3 18% 27% 30% 17%	509
	11th to 20th	Day	5% 3 15% 31% 31% 14%	477
	21st to end	Day	4 2 16% 35% 29% 13%	539
September	1st to 10th	Day	7% 4 15% 30% 27% 17%	490
	11th to 20th	Day	5% 4 19% 29% 30% 13%	438
	21st to end	Day	6% 3 18% 30% 25% 18%	300
October	1st to 10th	Day	10% 4 19% 22% 24% 21%	300
	11th to 20th	Day	3 4 23% 39% 23% 8%	300
	21st to end	Day	8% 6% 26% 34% 19% 6%	330
November	1st to 10th	Day	7% 6% 24% 35% 20% 8%	300
	11th to 20th	Day	5% 7% 27% 38% 19% 4	300
	21st to end	Day	7% 4 29% 43% 13% 5%	300
December	1st to 10th	Day	5% 7% 36% 29% 21% 3	300
	11th to 20th	Day	7% 4 35% 40% 12% 1	300
	21st to end	Day	7% 6% 34% 40% 11% 2	330
January	1st to 10th	Night	5% 4 33% 43% 13% 2	1000
	11th to 20th	Night	5% 6% 37% 42% 10% 1	1000
	21st to end	Night	6% 4 35% 44% 12% 1	891
February	1st to 10th	Night	6% 3 43% 40% 7% 1	800
	11th to 20th	Night	6% 5% 41% 40% 8%	784
	21st to end	Night	6% 7% 41% 40% 6%	660
March	1st to 10th	Night	6% 5% 39% 42% 8%	800
	11th to 20th	Night	9% 7% 33% 40% 11%	800
	21st to end	Night	10% 6% 38% 37% 8%	697
April	1st to 10th	Night	11% 7% 42% 35% 5%	600
	11th to 20th	Night	15% 9% 35% 34% 6%	593
	21st to end	Night	14% 8% 31% 40% 8%	600
May	1st to 10th	Night	12% 8% 31% 38% 10% 1	600
	11th to 20th	Night	15% 4 24% 42% 14% 1	591
	21st to end	Night	11% 4 31% 38% 14% 1	650
June	1st to 10th	Night	7% 2 25% 47% 18% 1	600
	11th to 20th	Night	5% 3 29% 47% 16% 1	600
	21st to end	Night	5% 3 23% 50% 19% 1	600
July	1st to 10th	Night	5% 2 21% 46% 24% 1	585
	11th to 20th	Night	4 2 15% 48% 29% 2	582
	21st to end	Night	6% 1 15% 46% 30% 1	657
August	1st to 10th	Night	7% 2 15% 49% 26% 1	579
	11th to 20th	Night	7% 1 19% 52% 20% 1	564
	21st to end	Night	6% 2 20% 51% 20% 3	627
September	1st to 10th	Night	7% 1 18% 52% 20% 3	570
	11th to 20th	Night	6% 3 23% 46% 20% 2	570
	21st to end	Night	7% 3 24% 46% 19% 1	570
October	1st to 10th	Night	11% 4 26% 39% 19% 2	689
	11th to 20th	Night	5% 2 31% 43% 17% 2	760
	21st to end	Night	9% 5% 28% 40% 17% 2	925
November	1st to 10th	Night	5% 5% 30% 46% 14% 1	975
	11th to 20th	Night	5% 5% 35% 45% 10% 1	1000
	21st to end	Night	7% 6% 30% 42% 15% 1	1000
December	1st to 10th	Night	5% 3 35% 44% 12%	1000
	11th to 20th	Night	5% 4 37% 42% 11% 1	1000
	21st to end	Night	8% 5% 36% 42% 8% 1	1100

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