Horseappleacres Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Horseappleacres 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	3 5% 2 5% 4 4 5% 4 6% 8% 53%	140460
	Night	3 5% 2 5% 4 4 4 4 5% 7% 57%	139534
Transparency	Day	26% 6% 10% 23% 24% 11%	135435
	Night	24% 5% 11% 25% 25% 10%	135567
Seeing	Day	7% 12% 31% 29% 17% 5%	17500
	Night	6% 6% 34% 39% 14% 2	27636

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 7% 3 6% 3 3 4 3 4 8% 60%	4720
	11th to 20th	Day	1 2 1 4 4 4 4 4 6% 8% 63%	4920
	21st to end	Day	1 3 2 9% 4 4 3 2 5% 5% 63%	5844
February	1st to 10th	Day	1 7% 3 6% 3 4 4 3 6% 7% 56%	1905
	11th to 20th	Day	1 4 2 5% 4 5% 5% 4 7% 8% 56%	2049
	21st to end	Day	1 5% 3 7% 3 4 4 3 5% 5% 60%	1730
March	1st to 10th	Day	1 5% 2 5% 3 4 4 3 5% 6% 62%	4600
	11th to 20th	Day	3 9% 3 7% 4 5% 4 3 5% 8% 48%	5007
	21st to end	Day	2 6% 2 6% 4 4 5% 4 7% 8% 51%	5980
April	1st to 10th	Day	4 7% 2 6% 4 4 4 5% 6% 7% 51%	2520
	11th to 20th	Day	5% 7% 3 7% 4 5% 5% 4 7% 7% 46%	2491
	21st to end	Day	6% 8% 2 5% 4 5% 5% 5% 6% 8% 45%	2520
May	1st to 10th	Day	10% 8% 2 6% 4 4 4 4 6% 7% 44%	5743
	11th to 20th	Day	6% 5% 2 6% 5% 5% 5% 4 6% 9% 47%	6160
	21st to end	Day	5% 5% 2 5% 4 5% 6% 6% 8% 9% 46%	6925
June	1st to 10th	Day	4 4 2 5% 5% 5% 6% 7% 8% 9% 46%	2800
	11th to 20th	Day	4 3 1 5% 4 5% 5% 5% 8% 9% 50%	2800
	21st to end	Day	3 3 2 5% 5% 6% 6% 5% 9% 9% 46%	2800
July	1st to 10th	Day	3 2 1 4 4 4 5% 5% 8% 11% 52%	6339
	11th to 20th	Day	3 3 2 6% 5% 6% 6% 6% 9% 11% 43%	6340
	21st to end	Day	5% 4 2 7% 5% 6% 7% 6% 7% 8% 43%	6768
August	1st to 10th	Day	5% 5% 2 6% 5% 6% 7% 6% 9% 10% 39%	5694
	11th to 20th	Day	3 3 1 5% 4 5% 6% 5% 8% 10% 49%	5366
	21st to end	Day	5% 4 2 6% 5% 6% 6% 5% 8% 10% 43%	5984
September	1st to 10th	Day	5% 4 2 4 4 4 4 4 6% 7% 56%	2400
	11th to 20th	Day	5% 3 1 5% 4 4 5% 4 5% 8% 57%	2360
	21st to end	Day	4 3 2 4 4 4 4 3 4 6% 63%	2200
October	1st to 10th	Day	3 2 1 3 2 2 3 2 3 4 72%	3340
	11th to 20th	Day	4 4 2 5% 4 3 4 3 5% 7% 59%	3250
	21st to end	Day	3 4 1 3 2 2 2 2 4 6% 70%	3423
November	1st to 10th	Day	1 3 2 3 2 3 3 3 6% 8% 64%	1853
	11th to 20th	Day	1 4 2 3 3 3 3 3 5% 8% 65%	1890
	21st to end	Day	1 5% 2 4 3 3 4 3 5% 6% 62%	1841
December	1st to 10th	Day	1 4 2 5% 3 2 2 3 4 6% 69%	2700
	11th to 20th	Day	1 3 2 4 3 2 3 2 5% 7% 68%	3106
	21st to end	Day	1 4 2 6% 4 4 4 3 5% 7% 60%	4092
January	1st to 10th	Night	6% 3 7% 4 4 4 3 5% 7% 56%	8201
	11th to 20th	Night	1 3 1 4 4 4 4 4 5% 7% 64%	8115
	21st to end	Night	1 3 2 6% 5% 4 3 3 4 6% 64%	8370
February	1st to 10th	Night	1 5% 3 6% 3 3 5% 3 5% 8% 58%	2714
	11th to 20th	Night	1 6% 3 7% 5% 4 4 5% 6% 7% 53%	2458
	21st to end	Night	1 5% 2 6% 3 4 4 3 4 5% 64%	2076
March	1st to 10th	Night	1 4 2 7% 4 4 5% 4 5% 6% 58%	5494
	11th to 20th	Night	3 10% 3 5% 3 3 4 3 4 6% 55%	5205
	21st to end	Night	3 7% 2 7% 4 4 4 4 6% 7% 53%	5219
April	1st to 10th	Night	3 6% 2 5% 4 4 3 3 5% 7% 57%	2100
	11th to 20th	Night	6% 9% 2 6% 4 4 4 3 5% 7% 49%	2077
	21st to end	Night	7% 9% 2 5% 4 4 4 4 5% 7% 49%	2100
May	1st to 10th	Night	8% 8% 2 7% 5% 4 4 4 5% 7% 46%	4256
	11th to 20th	Night	6% 6% 2 6% 4 4 4 4 6% 9% 49%	3751
	21st to end	Night	6% 5% 2 6% 5% 5% 4 4 6% 7% 50%	3972
June	1st to 10th	Night	4 5% 2 4 3 4 4 5% 7% 11% 54%	1600
	11th to 20th	Night	2 2 1 4 4 3 4 4 6% 10% 60%	1600
	21st to end	Night	3 4 2 4 4 5% 5% 6% 7% 8% 52%	1600
July	1st to 10th	Night	4 3 2 5% 5% 4 5% 5% 7% 8% 53%	3640
	11th to 20th	Night	2 3 2 5% 5% 5% 6% 5% 8% 12% 46%	3640
	21st to end	Night	4 3 2 6% 4 5% 6% 5% 8% 10% 47%	4101
August	1st to 10th	Night	5% 5% 2 7% 6% 5% 5% 5% 7% 10% 44%	4116
	11th to 20th	Night	3 3 2 5% 5% 5% 6% 5% 8% 10% 47%	4483
	21st to end	Night	6% 5% 3 7% 5% 5% 6% 5% 8% 10% 42%	4993
September	1st to 10th	Night	4 5% 2 5% 4 4 6% 5% 6% 9% 51%	2000
	11th to 20th	Night	5% 2 1 5% 3 4 4 4 5% 7% 60%	2038
	21st to end	Night	4 3 2 4 3 3 4 3 5% 6% 62%	2200
October	1st to 10th	Night	4 3 1 3 3 3 3 3 4 5% 68%	3844
	11th to 20th	Night	7% 4 2 5% 3 3 4 3 5% 6% 58%	3898
	21st to end	Night	3 3 2 4 3 3 3 3 4 5% 67%	4343
November	1st to 10th	Night	1 3 1 4 3 3 3 3 5% 7% 66%	2665
	11th to 20th	Night	1 3 2 5% 3 2 3 3 4 6% 68%	2730
	21st to end	Night	2 5% 2 5% 4 4 4 3 5% 6% 61%	2776
December	1st to 10th	Night	1 2 1 4 2 3 2 2 4 6% 72%	4710
	11th to 20th	Night	1 3 2 4 3 3 3 2 5% 8% 65%	5321
	21st to end	Night	2 6% 2 5% 4 4 4 3 5% 8% 58%	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	23% 3 7% 32% 26% 9%	4723
	11th to 20th	Day	19% 4 10% 25% 31% 12%	4920
	21st to end	Day	24% 3 6% 23% 30% 13%	5844
February	1st to 10th	Day	24% 5% 12% 27% 22% 11%	1905
	11th to 20th	Day	22% 6% 9% 27% 27% 8%	2059
	21st to end	Day	24% 5% 7% 21% 25% 19%	1730
March	1st to 10th	Day	18% 5% 7% 15% 33% 21%	4600
	11th to 20th	Day	30% 5% 9% 22% 26% 7%	5007
	21st to end	Day	25% 6% 10% 37% 18% 3	5980
April	1st to 10th	Day	27% 6% 11% 24% 21% 10%	2520
	11th to 20th	Day	32% 7% 11% 21% 19% 11%	2494
	21st to end	Day	32% 7% 12% 25% 19% 5%	2520
May	1st to 10th	Day	34% 7% 11% 24% 16% 8%	5743
	11th to 20th	Day	29% 8% 11% 19% 22% 11%	6164
	21st to end	Day	28% 9% 14% 22% 20% 7%	6926
June	1st to 10th	Day	28% 9% 14% 23% 16% 9%	2800
	11th to 20th	Day	24% 7% 12% 18% 23% 17%	2800
	21st to end	Day	27% 8% 12% 20% 22% 12%	2800
July	1st to 10th	Day	23% 7% 9% 19% 25% 17%	6271
	11th to 20th	Day	31% 7% 12% 24% 16% 9%	6265
	21st to end	Day	35% 6% 13% 15% 16% 15%	6573
August	1st to 10th	Day	39% 7% 15% 21% 12% 4	4145
	11th to 20th	Day	29% 8% 13% 18% 19% 14%	3882
	21st to end	Day	29% 6% 14% 19% 25% 6%	4311
September	1st to 10th	Day	25% 5% 10% 22% 25% 14%	2400
	11th to 20th	Day	23% 6% 10% 17% 29% 15%	2360
	21st to end	Day	22% 4 8% 21% 33% 13%	2200
October	1st to 10th	Day	15% 4 7% 15% 39% 20%	3340
	11th to 20th	Day	25% 4 6% 24% 31% 10%	3250
	21st to end	Day	16% 2 5% 23% 38% 15%	3423
November	1st to 10th	Day	15% 4 11% 23% 29% 18%	1853
	11th to 20th	Day	16% 4 7% 29% 31% 13%	1890
	21st to end	Day	21% 3 10% 23% 28% 14%	1841
December	1st to 10th	Day	18% 3 8% 29% 32% 10%	2700
	11th to 20th	Day	17% 3 7% 30% 31% 11%	3104
	21st to end	Day	22% 5% 8% 28% 25% 11%	4092
January	1st to 10th	Night	26% 3 7% 31% 27% 6%	8202
	11th to 20th	Night	18% 5% 10% 25% 26% 15%	8115
	21st to end	Night	21% 4 9% 22% 30% 13%	8370
February	1st to 10th	Night	23% 5% 10% 28% 25% 10%	2714
	11th to 20th	Night	28% 5% 10% 28% 20% 9%	2458
	21st to end	Night	20% 4 7% 26% 27% 16%	2076
March	1st to 10th	Night	21% 6% 7% 15% 37% 14%	5494
	11th to 20th	Night	29% 5% 9% 18% 28% 11%	5205
	21st to end	Night	27% 6% 11% 33% 20% 3	5219
April	1st to 10th	Night	25% 5% 13% 24% 24% 10%	2100
	11th to 20th	Night	30% 5% 12% 25% 20% 8%	2077
	21st to end	Night	32% 7% 13% 27% 17% 5%	2100
May	1st to 10th	Night	33% 7% 14% 29% 10% 7%	4253
	11th to 20th	Night	28% 7% 15% 22% 18% 9%	3752
	21st to end	Night	29% 10% 15% 24% 17% 6%	3972
June	1st to 10th	Night	24% 8% 21% 25% 14% 9%	1600
	11th to 20th	Night	18% 7% 15% 23% 20% 16%	1600
	21st to end	Night	25% 9% 10% 23% 22% 11%	1600
July	1st to 10th	Night	28% 6% 11% 20% 29% 7%	3600
	11th to 20th	Night	29% 6% 19% 27% 15% 4	3592
	21st to end	Night	30% 7% 18% 20% 15% 10%	3985
August	1st to 10th	Night	39% 9% 18% 19% 11% 4	2989
	11th to 20th	Night	30% 7% 19% 21% 19% 5%	3243
	21st to end	Night	28% 7% 19% 25% 17% 4	3597
September	1st to 10th	Night	28% 7% 10% 22% 22% 11%	2000
	11th to 20th	Night	22% 4 11% 23% 25% 14%	2038
	21st to end	Night	23% 5% 8% 24% 30% 10%	2200
October	1st to 10th	Night	18% 3 7% 21% 33% 17%	3844
	11th to 20th	Night	26% 4 7% 27% 26% 9%	3898
	21st to end	Night	17% 3 6% 27% 32% 14%	4343
November	1st to 10th	Night	17% 3 11% 24% 29% 16%	2665
	11th to 20th	Night	16% 4 8% 29% 31% 12%	2730
	21st to end	Night	22% 4 11% 22% 30% 11%	2776
December	1st to 10th	Night	15% 3 8% 33% 32% 9%	4710
	11th to 20th	Night	17% 3 7% 29% 35% 9%	5322
	21st to end	Night	24% 5% 10% 23% 26% 12%	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	7% 11% 34% 34% 12% 3	330
	11th to 20th	Day	6% 14% 34% 32% 12% 2	330
	21st to end	Day	2 13% 40% 32% 10% 2	363
February	1st to 10th	Day	5% 16% 45% 25% 7% 2	330
	11th to 20th	Day	3 17% 45% 26% 8% 1	330
	21st to end	Day	2 21% 42% 27% 7% 1	273
March	1st to 10th	Day	5% 15% 40% 27% 11% 3	330
	11th to 20th	Day	11% 18% 37% 22% 9% 2	489
	21st to end	Day	11% 23% 37% 21% 6% 1	594
April	1st to 10th	Day	10% 20% 39% 21% 8% 2	540
	11th to 20th	Day	13% 26% 36% 18% 6% 2	532
	21st to end	Day	14% 20% 34% 24% 6% 1	540
May	1st to 10th	Day	14% 19% 34% 22% 9% 2	629
	11th to 20th	Day	12% 13% 30% 29% 13% 3	725
	21st to end	Day	9% 13% 31% 26% 17% 4	789
June	1st to 10th	Day	6% 10% 25% 31% 22% 6%	730
	11th to 20th	Day	5% 11% 30% 32% 18% 5%	730
	21st to end	Day	5% 8% 28% 31% 22% 6%	730
July	1st to 10th	Day	7% 4 21% 29% 31% 8%	721
	11th to 20th	Day	6% 1 17% 34% 32% 9%	718
	21st to end	Day	8% 2 16% 29% 34% 10%	734
August	1st to 10th	Day	9% 3 22% 35% 24% 6%	527
	11th to 20th	Day	7% 4 23% 32% 25% 9%	517
	21st to end	Day	7% 3 25% 38% 19% 7%	583
September	1st to 10th	Day	6% 5% 24% 37% 17% 10%	530
	11th to 20th	Day	5% 7% 31% 28% 22% 7%	490
	21st to end	Day	8% 8% 21% 27% 28% 9%	330
October	1st to 10th	Day	8% 13% 22% 28% 20% 9%	330
	11th to 20th	Day	3 12% 35% 31% 15% 5%	330
	21st to end	Day	6% 13% 36% 24% 18% 2	363
November	1st to 10th	Day	5% 15% 34% 27% 15% 4	330
	11th to 20th	Day	3 11% 35% 37% 11% 3	330
	21st to end	Day	5% 14% 37% 28% 12% 4	330
December	1st to 10th	Day	5% 14% 41% 25% 14% 2	330
	11th to 20th	Day	3 12% 43% 29% 12% 2	330
	21st to end	Day	4 14% 45% 26% 9% 1	363
January	1st to 10th	Night	3 6% 39% 37% 11% 3	1050
	11th to 20th	Night	3 7% 42% 37% 9% 2	1006
	21st to end	Night	2 5% 40% 43% 10% 1	924
February	1st to 10th	Night	3 6% 52% 32% 6% 1	840
	11th to 20th	Night	4 6% 52% 32% 7%	824
	21st to end	Night	4 8% 52% 32% 4	692
March	1st to 10th	Night	4 7% 47% 34% 8% 1	840
	11th to 20th	Night	6% 8% 37% 41% 7% 1	840
	21st to end	Night	5% 10% 44% 34% 6%	753
April	1st to 10th	Night	7% 11% 46% 32% 4	630
	11th to 20th	Night	13% 11% 38% 32% 6% 1	623
	21st to end	Night	13% 11% 36% 32% 7% 1	630
May	1st to 10th	Night	12% 10% 30% 36% 10% 1	603
	11th to 20th	Night	13% 7% 30% 37% 11% 2	591
	21st to end	Night	11% 6% 34% 33% 14% 2	650
June	1st to 10th	Night	6% 6% 23% 43% 19% 3	600
	11th to 20th	Night	4 6% 28% 49% 14%	600
	21st to end	Night	5% 3 23% 47% 21% 1	600
July	1st to 10th	Night	6% 3 16% 45% 27% 2	585
	11th to 20th	Night	5% 1 13% 45% 33% 3	582
	21st to end	Night	8% 2 10% 40% 37% 3	657
August	1st to 10th	Night	8% 3 13% 42% 32% 3	597
	11th to 20th	Night	6% 2 12% 50% 27% 2	594
	21st to end	Night	5% 2 13% 47% 29% 4	660
September	1st to 10th	Night	6% 3 16% 44% 28% 4	600
	11th to 20th	Night	8% 2 19% 48% 20% 3	600
	21st to end	Night	5% 4 25% 43% 20% 3	600
October	1st to 10th	Night	9% 5% 28% 38% 17% 3	750
	11th to 20th	Night	5% 4 36% 37% 15% 3	800
	21st to end	Night	6% 6% 33% 36% 16% 3	935
November	1st to 10th	Night	3 5% 36% 40% 13% 2	1025
	11th to 20th	Night	3 6% 38% 42% 9% 2	1050
	21st to end	Night	5% 7% 37% 38% 11% 1	1050
December	1st to 10th	Night	2 6% 43% 38% 10% 1	1050
	11th to 20th	Night	3 6% 44% 34% 11% 1	1050
	21st to end	Night	4 10% 44% 34% 7% 1	1155

Page generated at 2026-04-29 00:21:41. Version ac07

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