Horsetail Fall
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Horsetail Fall clear sky chart from 2005-11-06 to 2026-09-29.

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	5% 6% 2 3 2 2 2 2 3 5% 67%	157393
	Night	5% 6% 2 3 2 2 3 2 4 5% 66%	151544
Transparency	Day	22% 3 7% 20% 27% 21%	152405
	Night	22% 4 7% 20% 27% 20%	147513
Seeing	Day	12% 15% 35% 25% 10% 3	17694
	Night	9% 7% 25% 33% 21% 5%	27095

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	8% 16% 2 4 2 2 2 2 4 6% 51%	4722
	11th to 20th	Day	8% 6% 1 3 3 3 3 2 2 5% 64%	5021
	21st to end	Day	7% 8% 2 5% 3 4 3 3 5% 8% 50%	5843
February	1st to 10th	Day	11% 11% 3 5% 2 2 3 4 4 6% 49%	1964
	11th to 20th	Day	7% 13% 5% 4 3 3 3 3 4 5% 50%	2050
	21st to end	Day	9% 13% 4 7% 4 3 3 3 4 4 45%	1730
March	1st to 10th	Day	6% 17% 8% 8% 3 3 3 3 5% 6% 38%	4618
	11th to 20th	Day	8% 14% 4 5% 2 3 3 2 4 5% 50%	5009
	21st to end	Day	8% 20% 5% 5% 3 3 3 2 4 6% 42%	5999
April	1st to 10th	Day	8% 11% 3 5% 2 3 3 3 5% 6% 49%	2520
	11th to 20th	Day	8% 13% 2 4 4 3 3 2 4 6% 51%	2489
	21st to end	Day	9% 11% 2 3 3 3 3 4 4 6% 52%	2520
May	1st to 10th	Day	9% 6% 1 3 2 2 2 2 3 5% 66%	7301
	11th to 20th	Day	3 5% 1 2 2 2 2 2 3 4 73%	7740
	21st to end	Day	4 6% 1 3 2 2 2 2 3 5% 70%	8992
June	1st to 10th	Day	5% 4 1 3 3 3 3 3 5% 6% 63%	2940
	11th to 20th	Day	2 2 1 2 2 2 2 2 2 5% 79%	2940
	21st to end	Day	2 2 1 2 2 2 2 1 3 4 80%	2940
July	1st to 10th	Day	1 1 1 1 2 1 2 2 90%	8219
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 4 80%	8220
	21st to end	Day	1 1 1 1 1 1 2 3 4 85%	8567
August	1st to 10th	Day	1 1 1 1 1 1 1 2 2 88%	7268
	11th to 20th	Day	1 1 2 1 1 1 2 3 4 84%	7001
	21st to end	Day	4 2 1 3 2 2 3 2 3 5% 73%	7750
September	1st to 10th	Day	2 1 2 1 1 2 2 3 3 83%	2520
	11th to 20th	Day	4 2 1 3 3 2 3 2 3 4 74%	2467
	21st to end	Day	3 2 1 3 1 2 2 3 4 4 75%	2289
October	1st to 10th	Day	3 4 1 2 2 2 2 2 4 4 75%	3389
	11th to 20th	Day	4 2 1 3 2 2 2 2 3 4 76%	3261
	21st to end	Day	4 5% 1 2 2 3 3 3 4 5% 67%	3502
November	1st to 10th	Day	7% 6% 2 4 2 2 4 2 4 6% 59%	1854
	11th to 20th	Day	9% 5% 1 3 3 3 3 3 4 5% 60%	1890
	21st to end	Day	8% 7% 2 3 3 3 3 2 5% 6% 59%	1890
December	1st to 10th	Day	6% 5% 1 3 3 3 2 3 6% 7% 61%	2768
	11th to 20th	Day	15% 11% 2 5% 3 3 4 3 4 6% 42%	3108
	21st to end	Day	14% 11% 3 5% 3 4 4 4 6% 8% 40%	4092
January	1st to 10th	Night	9% 11% 3 5% 2 3 3 3 4 6% 53%	8172
	11th to 20th	Night	8% 6% 1 4 2 3 3 3 4 6% 61%	7939
	21st to end	Night	5% 9% 3 5% 4 4 4 3 4 7% 52%	8371
February	1st to 10th	Night	11% 10% 3 5% 4 2 3 3 4 6% 50%	2653
	11th to 20th	Night	7% 12% 4 5% 3 3 3 3 4 7% 50%	2458
	21st to end	Night	9% 11% 2 8% 3 3 3 3 3 5% 51%	2076
March	1st to 10th	Night	6% 15% 6% 8% 3 3 3 3 4 6% 42%	5496
	11th to 20th	Night	7% 13% 4 6% 3 3 3 2 3 5% 51%	5206
	21st to end	Night	8% 17% 4 6% 3 3 3 2 4 6% 45%	5217
April	1st to 10th	Night	7% 10% 2 5% 3 3 3 2 4 7% 54%	2100
	11th to 20th	Night	5% 9% 2 5% 4 3 3 2 4 5% 59%	2079
	21st to end	Night	6% 9% 2 3 3 2 3 3 4 5% 61%	2100
May	1st to 10th	Night	4 5% 1 3 1 2 2 2 3 5% 71%	5663
	11th to 20th	Night	2 3 1 2 2 2 2 2 3 4 79%	5125
	21st to end	Night	2 3 1 2 1 1 2 2 3 4 78%	5181
June	1st to 10th	Night	4 3 1 3 2 2 3 3 4 6% 69%	1680
	11th to 20th	Night	1 1 1 1 1 2 2 4 4 84%	1680
	21st to end	Night	2 1 1 1 1 1 1 2 3 86%	1680
July	1st to 10th	Night	1 1 1 1 1 1 2 93%	4740
	11th to 20th	Night	3 3 1 3 2 2 2 1 3 3 79%	4740
	21st to end	Night	1 1 1 1 1 1 1 2 3 86%	5510
August	1st to 10th	Night	1 1 1 1 1 2 1 2 3 87%	5568
	11th to 20th	Night	1 1 1 1 2 2 1 2 4 85%	5886
	21st to end	Night	3 2 1 2 2 2 2 2 3 5% 75%	6499
September	1st to 10th	Night	1 1 2 1 1 1 1 2 2 89%	2100
	11th to 20th	Night	2 2 1 2 2 2 2 1 2 3 81%	2152
	21st to end	Night	3 2 1 2 1 2 2 2 3 4 78%	2288
October	1st to 10th	Night	3 2 1 1 1 2 2 1 2 3 81%	3807
	11th to 20th	Night	3 2 1 2 1 2 2 2 3 4 79%	3897
	21st to end	Night	4 3 1 3 2 2 2 2 3 5% 74%	4305
November	1st to 10th	Night	6% 6% 2 3 2 2 2 2 5% 7% 63%	2665
	11th to 20th	Night	8% 5% 1 4 3 3 3 3 5% 7% 59%	2730
	21st to end	Night	8% 7% 2 4 3 3 3 3 4 6% 58%	2730
December	1st to 10th	Night	7% 4 1 3 3 3 3 2 4 6% 65%	4629
	11th to 20th	Night	14% 12% 2 4 3 3 3 3 6% 9% 41%	5316
	21st to end	Night	11% 9% 3 6% 5% 4 5% 4 6% 7% 39%	7106

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	33% 6% 11% 20% 21% 8%	4723
	11th to 20th	Day	24% 5% 5% 12% 26% 29%	5021
	21st to end	Day	32% 5% 7% 18% 24% 15%	5843
February	1st to 10th	Day	38% 4 8% 19% 17% 14%	1964
	11th to 20th	Day	36% 4 8% 22% 22% 7%	2060
	21st to end	Day	43% 5% 7% 21% 18% 6%	1730
March	1st to 10th	Day	45% 6% 7% 14% 21% 7%	4618
	11th to 20th	Day	36% 6% 13% 26% 12% 7%	5009
	21st to end	Day	44% 6% 11% 23% 13% 3	5999
April	1st to 10th	Day	35% 6% 13% 21% 20% 5%	2520
	11th to 20th	Day	35% 5% 11% 26% 17% 5%	2492
	21st to end	Day	31% 6% 13% 30% 17% 2	2520
May	1st to 10th	Day	23% 3 8% 30% 28% 9%	7301
	11th to 20th	Day	16% 3 7% 31% 37% 7%	7743
	21st to end	Day	18% 2 7% 36% 34% 3	8994
June	1st to 10th	Day	21% 6% 11% 27% 27% 8%	2940
	11th to 20th	Day	11% 2 7% 25% 38% 17%	2940
	21st to end	Day	11% 2 3 16% 40% 28%	2940
July	1st to 10th	Day	5% 1 1 6% 23% 63%	8151
	11th to 20th	Day	10% 1 5% 10% 22% 52%	8145
	21st to end	Day	6% 1 4 14% 32% 43%	8376
August	1st to 10th	Day	7% 1 3 13% 33% 42%	5752
	11th to 20th	Day	9% 2 4 22% 26% 38%	5517
	21st to end	Day	13% 1 3 24% 31% 28%	6077
September	1st to 10th	Day	8% 2 4 15% 37% 34%	2520
	11th to 20th	Day	14% 3 6% 22% 35% 20%	2467
	21st to end	Day	14% 2 6% 22% 36% 20%	2289
October	1st to 10th	Day	15% 2 4 11% 37% 31%	3389
	11th to 20th	Day	15% 1 4 18% 40% 23%	3261
	21st to end	Day	20% 3 5% 20% 38% 14%	3502
November	1st to 10th	Day	28% 4 8% 22% 25% 13%	1854
	11th to 20th	Day	26% 5% 8% 21% 28% 12%	1890
	21st to end	Day	29% 4 6% 17% 29% 15%	1890
December	1st to 10th	Day	22% 3 6% 25% 27% 16%	2768
	11th to 20th	Day	44% 3 9% 16% 19% 9%	3108
	21st to end	Day	40% 7% 13% 17% 16% 8%	4092
January	1st to 10th	Night	31% 5% 13% 21% 21% 9%	8172
	11th to 20th	Night	24% 5% 4 12% 28% 27%	7939
	21st to end	Night	31% 5% 6% 15% 26% 16%	8371
February	1st to 10th	Night	36% 4 6% 19% 20% 14%	2653
	11th to 20th	Night	34% 4 8% 21% 22% 10%	2459
	21st to end	Night	37% 4 10% 20% 21% 8%	2076
March	1st to 10th	Night	40% 6% 7% 16% 21% 10%	5496
	11th to 20th	Night	36% 5% 9% 27% 15% 7%	5206
	21st to end	Night	37% 8% 16% 22% 14% 3	5217
April	1st to 10th	Night	32% 4 13% 25% 19% 7%	2100
	11th to 20th	Night	28% 6% 12% 26% 20% 7%	2079
	21st to end	Night	25% 5% 12% 35% 20% 2	2100
May	1st to 10th	Night	17% 4 10% 35% 27% 8%	5661
	11th to 20th	Night	11% 3 10% 30% 37% 9%	5126
	21st to end	Night	12% 2 7% 39% 36% 4	5182
June	1st to 10th	Night	17% 4 11% 28% 31% 8%	1680
	11th to 20th	Night	7% 2 8% 25% 39% 18%	1680
	21st to end	Night	7% 2 3 16% 40% 32%	1680
July	1st to 10th	Night	3 1 2 7% 23% 65%	4700
	11th to 20th	Night	13% 1 4 9% 25% 48%	4692
	21st to end	Night	7% 1 3 16% 27% 46%	5389
August	1st to 10th	Night	8% 1 2 16% 31% 41%	4402
	11th to 20th	Night	8% 2 3 23% 24% 40%	4636
	21st to end	Night	13% 1 2 21% 35% 27%	5092
September	1st to 10th	Night	6% 1 1 17% 39% 35%	2100
	11th to 20th	Night	12% 2 5% 23% 38% 21%	2152
	21st to end	Night	12% 2 5% 25% 35% 22%	2288
October	1st to 10th	Night	11% 2 4 12% 39% 32%	3807
	11th to 20th	Night	11% 2 4 15% 43% 24%	3897
	21st to end	Night	16% 2 5% 19% 43% 16%	4305
November	1st to 10th	Night	22% 4 8% 23% 27% 16%	2665
	11th to 20th	Night	25% 4 8% 20% 30% 13%	2730
	21st to end	Night	28% 4 6% 19% 29% 14%	2730
December	1st to 10th	Night	22% 3 6% 23% 26% 19%	4629
	11th to 20th	Night	40% 4 9% 17% 22% 8%	5316
	21st to end	Night	40% 8% 10% 16% 19% 7%	7106

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	32% 15% 25% 19% 8% 1	220
	11th to 20th	Day	19% 11% 30% 27% 10% 3	319
	21st to end	Day	14% 17% 33% 21% 12% 3	473
February	1st to 10th	Day	24% 18% 26% 20% 9% 3	469
	11th to 20th	Day	21% 22% 31% 19% 7% 1	535
	21st to end	Day	20% 20% 38% 19% 2 1	446
March	1st to 10th	Day	21% 23% 36% 14% 5% 1	540
	11th to 20th	Day	22% 22% 27% 21% 7% 1	540
	21st to end	Day	21% 24% 32% 18% 4	594
April	1st to 10th	Day	17% 19% 36% 20% 7% 1	540
	11th to 20th	Day	21% 20% 33% 20% 5% 1	532
	21st to end	Day	14% 19% 33% 22% 10% 2	540
May	1st to 10th	Day	19% 20% 31% 21% 8% 2	543
	11th to 20th	Day	21% 14% 29% 22% 11% 3	568
	21st to end	Day	16% 14% 27% 27% 11% 5%	617
June	1st to 10th	Day	7% 14% 38% 24% 12% 5%	570
	11th to 20th	Day	7% 16% 36% 24% 12% 5%	570
	21st to end	Day	4 13% 39% 28% 10% 6%	570
July	1st to 10th	Day	3 10% 41% 30% 12% 4	566
	11th to 20th	Day	2 7% 37% 34% 13% 7%	564
	21st to end	Day	2 8% 42% 30% 10% 7%	619
August	1st to 10th	Day	2 10% 48% 25% 10% 5%	567
	11th to 20th	Day	3 10% 45% 26% 11% 5%	557
	21st to end	Day	4 8% 41% 30% 14% 3	627
September	1st to 10th	Day	3 11% 43% 29% 12% 2	570
	11th to 20th	Day	8% 16% 42% 25% 9% 1	570
	21st to end	Day	7% 15% 41% 29% 8% 1	563
October	1st to 10th	Day	9% 18% 33% 29% 11%	530
	11th to 20th	Day	8% 13% 37% 32% 9% 1	530
	21st to end	Day	11% 14% 34% 31% 9% 1	551
November	1st to 10th	Day	13% 11% 34% 22% 18% 2	330
	11th to 20th	Day	16% 13% 28% 26% 12% 5%	330
	21st to end	Day	14% 12% 33% 28% 12% 2	330
December	1st to 10th	Day	17% 10% 30% 24% 17% 3	242
	11th to 20th	Day	15% 15% 34% 20% 13% 4	220
	21st to end	Day	26% 13% 31% 17% 9% 3	242
January	1st to 10th	Night	16% 11% 30% 21% 14% 7%	840
	11th to 20th	Night	12% 11% 31% 29% 12% 4	840
	21st to end	Night	14% 11% 33% 23% 14% 4	924
February	1st to 10th	Night	18% 9% 30% 27% 12% 4	840
	11th to 20th	Night	15% 13% 28% 23% 16% 6%	824
	21st to end	Night	15% 9% 34% 30% 11% 2	692
March	1st to 10th	Night	17% 11% 30% 27% 12% 3	840
	11th to 20th	Night	15% 10% 25% 27% 18% 5%	840
	21st to end	Night	14% 9% 31% 30% 13% 2	924
April	1st to 10th	Night	12% 8% 28% 34% 14% 3	840
	11th to 20th	Night	10% 9% 31% 30% 16% 5%	831
	21st to end	Night	10% 7% 27% 33% 18% 5%	840
May	1st to 10th	Night	7% 7% 28% 34% 19% 5%	751
	11th to 20th	Night	8% 6% 24% 39% 20% 3	568
	21st to end	Night	5% 3 25% 35% 26% 6%	455
June	1st to 10th	Night	5% 3 18% 40% 30% 4	420
	11th to 20th	Night	1 2 22% 42% 28% 4	420
	21st to end	Night	2 24% 39% 31% 4	420
July	1st to 10th	Night	18% 39% 39% 3	410
	11th to 20th	Night	3 1 14% 42% 36% 4	408
	21st to end	Night	3 1 15% 35% 36% 10%	586
August	1st to 10th	Night	2 3 16% 41% 32% 6%	730
	11th to 20th	Night	1 16% 46% 31% 6%	832
	21st to end	Night	2 1 11% 43% 37% 5%	924
September	1st to 10th	Night	2 1 13% 44% 32% 8%	840
	11th to 20th	Night	4 2 19% 40% 29% 7%	840
	21st to end	Night	3 3 21% 37% 28% 8%	832
October	1st to 10th	Night	6% 3 24% 37% 24% 6%	800
	11th to 20th	Night	5% 4 24% 36% 24% 7%	800
	21st to end	Night	6% 5% 24% 33% 23% 10%	880
November	1st to 10th	Night	10% 6% 18% 36% 23% 7%	820
	11th to 20th	Night	9% 8% 27% 35% 16% 5%	840
	21st to end	Night	11% 7% 32% 31% 15% 4	840
December	1st to 10th	Night	16% 8% 28% 33% 11% 4	840
	11th to 20th	Night	20% 10% 29% 26% 13% 3	840
	21st to end	Night	19% 14% 30% 24% 11% 2	924

Page generated at 2026-09-26 01:18:45. Version ac07

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