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 2025-05-26.

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	6% 7% 2 3 2 2 3 2 4 5% 64%	117941
	Night	6% 7% 2 4 2 2 3 2 4 5% 63%	120195
Transparency	Day	24% 4 8% 19% 26% 20%	117689
	Night	24% 4 7% 20% 27% 18%	120037
Seeing	Day	13% 15% 35% 25% 10% 3	15874
	Night	10% 6% 24% 33% 22% 5%	25425

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	7% 12% 3 4 2 2 2 2 4 6% 55%	3639
	11th to 20th	Day	10% 8% 1 4 3 3 3 2 3 5% 58%	3851
	21st to end	Day	9% 10% 3 5% 3 3 3 2 4 7% 50%	4510
February	1st to 10th	Day	11% 12% 3 5% 2 2 3 4 4 6% 49%	1870
	11th to 20th	Day	7% 12% 4 4 3 3 3 3 4 5% 51%	1950
	21st to end	Day	9% 14% 5% 7% 3 3 3 3 3 4 46%	1650
March	1st to 10th	Day	6% 17% 8% 8% 3 3 3 3 5% 6% 37%	4518
	11th to 20th	Day	8% 15% 4 5% 2 3 3 2 4 5% 49%	4902
	21st to end	Day	8% 20% 5% 5% 3 3 3 2 3 6% 42%	5870
April	1st to 10th	Day	8% 11% 3 6% 3 3 4 3 5% 6% 49%	2400
	11th to 20th	Day	8% 13% 2 4 4 3 3 2 4 6% 51%	2369
	21st to end	Day	9% 10% 1 3 3 3 3 4 5% 6% 53%	2400
May	1st to 10th	Day	7% 5% 1 2 2 2 1 1 3 6% 70%	4065
	11th to 20th	Day	5% 5% 1 2 2 2 3 3 4 6% 68%	4283
	21st to end	Day	6% 6% 1 3 2 2 2 2 3 5% 69%	4886
June	1st to 10th	Day	6% 4 1 3 3 3 3 3 5% 6% 62%	2660
	11th to 20th	Day	3 2 1 2 2 2 2 2 2 5% 78%	2660
	21st to end	Day	2 2 1 2 2 2 2 2 3 4 79%	2660
July	1st to 10th	Day	1 1 1 2 1 2 2 89%	4420
	11th to 20th	Day	2 1 1 2 2 2 2 2 4 5% 77%	4420
	21st to end	Day	1 1 1 1 2 2 2 2 3 4 82%	4591
August	1st to 10th	Day	2 2 2 1 1 2 2 2 3 82%	3937
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	3724
	21st to end	Day	2 1 1 1 1 1 1 1 2 2 86%	4180
September	1st to 10th	Day	2 1 2 1 1 2 2 3 3 83%	2280
	11th to 20th	Day	4 1 1 3 3 2 3 2 3 4 74%	2233
	21st to end	Day	3 2 1 2 1 2 2 2 4 4 75%	2090
October	1st to 10th	Day	3 4 1 3 2 2 2 2 4 4 74%	3286
	11th to 20th	Day	4 2 1 3 2 2 2 2 3 4 76%	3161
	21st to end	Day	4 5% 1 2 2 4 3 3 4 5% 67%	3394
November	1st to 10th	Day	7% 6% 2 4 2 2 4 2 4 6% 58%	1764
	11th to 20th	Day	8% 4 1 3 3 3 3 4 4 6% 62%	1800
	21st to end	Day	9% 7% 2 3 3 3 3 2 5% 6% 57%	1800
December	1st to 10th	Day	7% 5% 1 3 3 3 3 3 6% 7% 60%	2686
	11th to 20th	Day	15% 12% 2 5% 3 3 4 3 4 6% 42%	3028
	21st to end	Day	14% 10% 3 5% 3 3 4 4 6% 8% 41%	4004
January	1st to 10th	Night	5% 8% 2 4 2 3 3 3 4 6% 58%	6306
	11th to 20th	Night	10% 7% 2 5% 2 3 3 3 4 5% 55%	6104
	21st to end	Night	6% 11% 3 4 3 3 4 3 4 6% 52%	6468
February	1st to 10th	Night	11% 10% 3 5% 4 2 3 3 4 6% 49%	2527
	11th to 20th	Night	7% 12% 3 4 3 3 3 3 4 7% 51%	2338
	21st to end	Night	8% 11% 2 8% 3 3 3 3 3 5% 51%	1980
March	1st to 10th	Night	6% 16% 6% 8% 3 3 3 3 5% 6% 41%	5376
	11th to 20th	Night	7% 14% 4 6% 3 3 3 2 3 5% 50%	5094
	21st to end	Night	9% 17% 4 6% 3 3 3 2 4 6% 45%	5104
April	1st to 10th	Night	7% 10% 2 5% 3 3 3 2 5% 7% 54%	2000
	11th to 20th	Night	5% 9% 2 5% 4 2 3 2 4 5% 59%	1979
	21st to end	Night	6% 8% 1 3 3 2 3 3 4 5% 61%	2000
May	1st to 10th	Night	3 4 1 3 1 2 2 2 3 5% 74%	3130
	11th to 20th	Night	3 3 1 2 2 1 2 2 3 5% 76%	2834
	21st to end	Night	3 3 1 3 1 1 2 2 3 4 77%	2805
June	1st to 10th	Night	4 3 1 3 2 2 3 3 5% 6% 68%	1520
	11th to 20th	Night	1 1 2 1 1 2 2 4 4 83%	1520
	21st to end	Night	2 1 1 1 1 1 1 2 3 85%	1520
July	1st to 10th	Night	1 1 1 2 2 92%	2540
	11th to 20th	Night	3 2 1 3 2 2 3 2 3 3 76%	2540
	21st to end	Night	2 2 1 2 2 2 2 2 3 4 81%	2950
August	1st to 10th	Night	2 1 1 2 1 2 2 2 3 4 79%	3006
	11th to 20th	Night	1 1 1 2 1 1 2 2 89%	3126
	21st to end	Night	2 1 1 2 1 1 1 1 1 2 88%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 2 90%	1900
	11th to 20th	Night	2 2 1 2 2 2 2 1 2 3 82%	1947
	21st to end	Night	3 2 1 2 1 2 2 2 3 4 80%	2090
October	1st to 10th	Night	3 2 1 1 1 2 2 1 2 3 81%	3691
	11th to 20th	Night	3 1 1 2 1 2 2 2 3 4 79%	3777
	21st to end	Night	4 3 1 3 2 2 2 2 3 5% 73%	4171
November	1st to 10th	Night	7% 7% 2 3 2 2 2 2 5% 7% 62%	2535
	11th to 20th	Night	7% 4 1 4 2 2 3 3 5% 7% 61%	2600
	21st to end	Night	8% 7% 2 4 3 3 3 3 5% 5% 57%	2600
December	1st to 10th	Night	7% 4 1 3 3 3 3 2 4 6% 64%	4491
	11th to 20th	Night	14% 12% 2 4 3 3 3 3 6% 9% 40%	5176
	21st to end	Night	11% 8% 3 7% 5% 4 5% 4 6% 7% 39%	6952

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	29% 7% 14% 21% 20% 10%	3639
	11th to 20th	Day	29% 7% 6% 14% 25% 20%	3851
	21st to end	Day	34% 5% 8% 16% 21% 16%	4510
February	1st to 10th	Day	38% 4 8% 20% 17% 14%	1870
	11th to 20th	Day	35% 5% 8% 23% 22% 7%	1960
	21st to end	Day	43% 5% 7% 20% 18% 7%	1650
March	1st to 10th	Day	46% 6% 7% 13% 21% 7%	4518
	11th to 20th	Day	37% 6% 14% 26% 12% 6%	4902
	21st to end	Day	44% 6% 11% 23% 12% 3	5870
April	1st to 10th	Day	35% 6% 14% 21% 19% 5%	2400
	11th to 20th	Day	35% 5% 11% 26% 18% 5%	2372
	21st to end	Day	30% 6% 13% 31% 17% 2	2400
May	1st to 10th	Day	20% 3 10% 32% 26% 10%	4065
	11th to 20th	Day	18% 4 9% 32% 33% 5%	4286
	21st to end	Day	18% 3 9% 37% 31% 2	4887
June	1st to 10th	Day	22% 6% 12% 27% 27% 7%	2660
	11th to 20th	Day	12% 3 8% 26% 38% 14%	2660
	21st to end	Day	11% 2 3 17% 38% 29%	2660
July	1st to 10th	Day	4 1 2 7% 17% 68%	4351
	11th to 20th	Day	11% 2 6% 10% 22% 51%	4345
	21st to end	Day	8% 2 5% 13% 28% 44%	4446
August	1st to 10th	Day	10% 1 3 10% 34% 41%	3922
	11th to 20th	Day	5% 1 3 7% 28% 55%	3760
	21st to end	Day	8% 1 2 11% 38% 39%	4179
September	1st to 10th	Day	8% 2 4 13% 36% 37%	2280
	11th to 20th	Day	14% 3 6% 24% 33% 20%	2233
	21st to end	Day	13% 2 5% 22% 36% 21%	2090
October	1st to 10th	Day	16% 2 4 10% 37% 31%	3286
	11th to 20th	Day	14% 1 4 17% 40% 22%	3161
	21st to end	Day	20% 3 6% 20% 37% 14%	3394
November	1st to 10th	Day	28% 4 8% 22% 24% 13%	1764
	11th to 20th	Day	25% 4 8% 22% 29% 13%	1800
	21st to end	Day	30% 4 6% 17% 28% 15%	1800
December	1st to 10th	Day	23% 4 6% 25% 26% 16%	2686
	11th to 20th	Day	44% 3 9% 15% 20% 8%	3028
	21st to end	Day	40% 7% 13% 17% 16% 8%	4004
January	1st to 10th	Night	25% 5% 14% 24% 22% 10%	6306
	11th to 20th	Night	29% 6% 5% 15% 29% 16%	6104
	21st to end	Night	31% 6% 7% 14% 22% 18%	6468
February	1st to 10th	Night	37% 5% 6% 19% 20% 14%	2527
	11th to 20th	Night	33% 4 8% 22% 23% 10%	2339
	21st to end	Night	37% 3 10% 20% 21% 8%	1980
March	1st to 10th	Night	41% 6% 7% 15% 21% 10%	5376
	11th to 20th	Night	37% 5% 9% 28% 15% 7%	5094
	21st to end	Night	38% 8% 16% 22% 14% 3	5104
April	1st to 10th	Night	32% 4 14% 26% 18% 7%	2000
	11th to 20th	Night	28% 6% 12% 25% 21% 8%	1979
	21st to end	Night	25% 5% 13% 35% 21% 2	2000
May	1st to 10th	Night	14% 4 9% 35% 30% 7%	3130
	11th to 20th	Night	13% 4 10% 33% 34% 7%	2835
	21st to end	Night	13% 3 8% 39% 33% 3	2805
June	1st to 10th	Night	18% 5% 12% 27% 31% 7%	1520
	11th to 20th	Night	8% 3 9% 27% 39% 15%	1520
	21st to end	Night	7% 2 3 16% 38% 33%	1520
July	1st to 10th	Night	2 1 2 7% 19% 68%	2500
	11th to 20th	Night	15% 1 4 8% 24% 48%	2492
	21st to end	Night	10% 2 3 15% 26% 45%	2857
August	1st to 10th	Night	10% 1 3 12% 29% 44%	2997
	11th to 20th	Night	5% 1 3 10% 24% 57%	3156
	21st to end	Night	9% 1 1 10% 41% 38%	3498
September	1st to 10th	Night	6% 1 1 14% 40% 38%	1900
	11th to 20th	Night	12% 2 5% 24% 37% 21%	1947
	21st to end	Night	11% 2 5% 25% 36% 22%	2090
October	1st to 10th	Night	11% 2 4 12% 39% 33%	3691
	11th to 20th	Night	11% 2 4 15% 43% 24%	3777
	21st to end	Night	16% 2 5% 19% 43% 16%	4171
November	1st to 10th	Night	23% 4 8% 22% 27% 16%	2535
	11th to 20th	Night	23% 4 8% 20% 31% 14%	2600
	21st to end	Night	29% 4 7% 18% 28% 15%	2600
December	1st to 10th	Night	23% 3 6% 23% 26% 19%	4491
	11th to 20th	Night	41% 4 8% 17% 22% 7%	5176
	21st to end	Night	40% 8% 10% 17% 19% 7%	6952

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	31% 16% 25% 20% 8% 1	200
	11th to 20th	Day	21% 12% 30% 25% 9% 3	294
	21st to end	Day	15% 16% 32% 21% 13% 3	440
February	1st to 10th	Day	25% 19% 26% 19% 9% 3	435
	11th to 20th	Day	19% 22% 32% 20% 7% 1	495
	21st to end	Day	20% 20% 37% 20% 3 1	414
March	1st to 10th	Day	22% 23% 35% 15% 5% 1	500
	11th to 20th	Day	24% 22% 25% 20% 7% 1	500
	21st to end	Day	22% 25% 31% 17% 4	550
April	1st to 10th	Day	18% 20% 36% 20% 6% 1	500
	11th to 20th	Day	21% 20% 33% 21% 5% 1	492
	21st to end	Day	13% 17% 35% 23% 10% 2	500
May	1st to 10th	Day	18% 20% 31% 21% 8% 2	503
	11th to 20th	Day	20% 14% 30% 22% 12% 3	528
	21st to end	Day	16% 14% 26% 28% 11% 4	550
June	1st to 10th	Day	8% 15% 35% 26% 12% 4	490
	11th to 20th	Day	7% 15% 36% 24% 12% 5%	490
	21st to end	Day	2 11% 39% 30% 10% 6%	490
July	1st to 10th	Day	2 5% 43% 33% 13% 4	486
	11th to 20th	Day	2 5% 35% 39% 13% 7%	484
	21st to end	Day	2 7% 40% 34% 9% 7%	531
August	1st to 10th	Day	2 10% 49% 25% 10% 5%	487
	11th to 20th	Day	3 9% 44% 28% 11% 5%	477
	21st to end	Day	4 7% 40% 31% 14% 3	539
September	1st to 10th	Day	3 9% 44% 30% 13% 2	490
	11th to 20th	Day	8% 14% 43% 25% 9% 1	490
	21st to end	Day	8% 12% 41% 30% 8% 1	490
October	1st to 10th	Day	8% 19% 32% 29% 11%	490
	11th to 20th	Day	7% 13% 37% 33% 9% 1	490
	21st to end	Day	10% 15% 33% 31% 10% 2	509
November	1st to 10th	Day	13% 12% 33% 22% 17% 2	300
	11th to 20th	Day	13% 11% 28% 28% 13% 6%	300
	21st to end	Day	15% 11% 30% 28% 12% 3	300
December	1st to 10th	Day	18% 10% 28% 24% 18% 3	220
	11th to 20th	Day	15% 16% 33% 21% 13% 4	200
	21st to end	Day	24% 13% 32% 18% 10% 4	220
January	1st to 10th	Night	16% 10% 30% 22% 14% 7%	800
	11th to 20th	Night	13% 11% 30% 30% 12% 5%	800
	21st to end	Night	14% 11% 32% 24% 15% 4	880
February	1st to 10th	Night	18% 10% 27% 29% 13% 4	800
	11th to 20th	Night	14% 12% 27% 25% 16% 6%	784
	21st to end	Night	16% 8% 32% 31% 11% 2	660
March	1st to 10th	Night	18% 11% 28% 28% 12% 3	800
	11th to 20th	Night	16% 10% 23% 27% 18% 6%	800
	21st to end	Night	15% 10% 30% 29% 14% 2	880
April	1st to 10th	Night	12% 8% 28% 33% 15% 4	800
	11th to 20th	Night	9% 9% 30% 30% 16% 5%	791
	21st to end	Night	10% 6% 27% 33% 19% 5%	800
May	1st to 10th	Night	6% 8% 27% 34% 20% 5%	715
	11th to 20th	Night	9% 5% 23% 39% 21% 3	541
	21st to end	Night	5% 3 24% 34% 27% 6%	422
June	1st to 10th	Night	6% 4 15% 39% 33% 4	380
	11th to 20th	Night	2 2 21% 40% 31% 4	380
	21st to end	Night	2 21% 39% 34% 4	380
July	1st to 10th	Night	1 14% 39% 43% 4	370
	11th to 20th	Night	3 1 13% 40% 39% 4	368
	21st to end	Night	3 1 13% 32% 40% 11%	530
August	1st to 10th	Night	2 3 15% 39% 35% 7%	660
	11th to 20th	Night	2 14% 44% 33% 7%	752
	21st to end	Night	2 1 9% 41% 41% 6%	836
September	1st to 10th	Night	2 1 10% 44% 35% 8%	760
	11th to 20th	Night	4 1 17% 39% 31% 8%	760
	21st to end	Night	3 2 18% 38% 29% 9%	760
October	1st to 10th	Night	5% 3 24% 37% 25% 6%	760
	11th to 20th	Night	5% 4 23% 36% 26% 7%	760
	21st to end	Night	6% 5% 23% 32% 24% 11%	836
November	1st to 10th	Night	10% 6% 17% 36% 23% 7%	780
	11th to 20th	Night	9% 7% 27% 35% 17% 5%	800
	21st to end	Night	11% 6% 30% 32% 16% 5%	800
December	1st to 10th	Night	17% 7% 27% 33% 12% 4	800
	11th to 20th	Night	20% 10% 28% 26% 14% 3	800
	21st to end	Night	18% 13% 30% 25% 11% 3	880

Page generated at 2025-05-23 06:20:57. Version ac07

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