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 2024-11-02.

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	6% 7% 2 3 2 2 2 2 4 5% 64%	101091
	Night	6% 7% 2 4 2 2 3 2 4 5% 63%	98068
Transparency	Day	23% 4 7% 20% 25% 20%	100839
	Night	24% 4 7% 21% 25% 18%	97910
Seeing	Day	12% 14% 35% 26% 10% 3	15194
	Night	10% 6% 23% 34% 22% 6%	24631

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	9% 17% 4 4 3 2 3 2 4 5% 47%	2539
	11th to 20th	Day	15% 11% 1 5% 5% 5% 5% 3 4 7% 40%	2681
	21st to end	Day	11% 13% 3 5% 3 4 3 3 5% 8% 41%	3146
February	1st to 10th	Day	11% 11% 3 5% 2 2 3 4 4 6% 49%	1776
	11th to 20th	Day	7% 12% 4 4 3 3 3 3 4 5% 52%	1850
	21st to end	Day	9% 14% 5% 7% 3 3 3 3 3 4 46%	1570
March	1st to 10th	Day	7% 18% 8% 9% 3 4 4 3 5% 6% 32%	3138
	11th to 20th	Day	8% 12% 2 4 2 2 3 2 4 6% 55%	3418
	21st to end	Day	8% 23% 7% 5% 3 3 3 2 3 6% 36%	4092
April	1st to 10th	Day	9% 11% 3 6% 3 3 4 3 5% 6% 48%	2280
	11th to 20th	Day	9% 13% 2 4 3 3 3 2 4 6% 51%	2249
	21st to end	Day	9% 10% 1 3 3 3 3 4 5% 6% 54%	2280
May	1st to 10th	Day	7% 6% 1 2 2 2 1 1 3 5% 70%	3941
	11th to 20th	Day	5% 4 1 2 2 2 3 3 4 6% 68%	4150
	21st to end	Day	6% 6% 1 3 2 2 2 2 3 5% 69%	4813
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	5% 5% 1 3 2 2 2 2 4 4 69%	1970
	11th to 20th	Day	4 2 1 3 2 3 2 3 4 5% 70%	1900
	21st to end	Day	6% 4 2 1 2 2 3 4 4 70%	2060
November	1st to 10th	Day	8% 7% 2 4 3 2 4 2 4 6% 58%	1683
	11th to 20th	Day	8% 4 1 3 3 2 3 3 4 6% 62%	1710
	21st to end	Day	7% 7% 3 3 3 3 3 2 5% 6% 58%	1710
December	1st to 10th	Day	11% 8% 1 4 4 4 3 3 5% 5% 52%	1558
	11th to 20th	Day	17% 13% 3 6% 4 4 4 3 4 5% 37%	1928
	21st to end	Day	9% 9% 4 6% 4 4 4 5% 7% 8% 40%	2794
January	1st to 10th	Night	7% 11% 3 6% 3 3 3 3 4 5% 53%	4406
	11th to 20th	Night	15% 11% 3 6% 3 4 4 4 5% 7% 38%	4269
	21st to end	Night	8% 14% 3 5% 4 4 4 4 5% 7% 41%	4521
February	1st to 10th	Night	10% 10% 3 5% 4 2 3 3 4 6% 50%	2401
	11th to 20th	Night	7% 12% 3 4 2 3 3 3 4 7% 52%	2218
	21st to end	Night	9% 12% 2 8% 3 3 3 2 3 5% 51%	1884
March	1st to 10th	Night	7% 17% 6% 8% 3 3 3 4 5% 6% 39%	3736
	11th to 20th	Night	8% 11% 3 4 2 3 2 2 3 5% 58%	3558
	21st to end	Night	8% 18% 4 6% 3 3 3 2 4 6% 42%	3560
April	1st to 10th	Night	7% 10% 2 5% 3 3 3 2 5% 7% 52%	1900
	11th to 20th	Night	5% 9% 2 5% 3 2 3 2 4 5% 59%	1879
	21st to end	Night	6% 8% 1 3 3 2 3 3 5% 5% 62%	1900
May	1st to 10th	Night	3 4 1 3 1 2 2 2 3 5% 74%	3034
	11th to 20th	Night	3 3 1 2 2 1 2 2 3 5% 76%	2747
	21st to end	Night	3 3 1 3 1 1 2 2 3 4 76%	2762
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	5% 4 1 2 2 2 2 1 3 4 74%	2207
	11th to 20th	Night	5% 1 1 2 1 2 2 2 4 5% 75%	2280
	21st to end	Night	4 2 2 2 2 2 1 3 5% 77%	2536
November	1st to 10th	Night	7% 7% 2 3 2 2 2 2 5% 7% 61%	2418
	11th to 20th	Night	7% 4 1 3 2 2 3 3 6% 7% 61%	2470
	21st to end	Night	7% 7% 2 4 3 3 3 3 5% 5% 59%	2470
December	1st to 10th	Night	12% 7% 2 4 3 3 3 2 4 6% 54%	2617
	11th to 20th	Night	17% 13% 2 5% 3 3 4 3 6% 6% 39%	3276
	21st to end	Night	10% 9% 3 7% 5% 4 5% 4 6% 7% 41%	4862

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	34% 9% 10% 21% 20% 6%	2539
	11th to 20th	Day	42% 9% 8% 19% 13% 9%	2681
	21st to end	Day	41% 7% 10% 22% 14% 6%	3146
February	1st to 10th	Day	37% 4 8% 21% 17% 13%	1776
	11th to 20th	Day	35% 5% 8% 23% 22% 7%	1860
	21st to end	Day	44% 5% 7% 20% 19% 5%	1570
March	1st to 10th	Day	49% 9% 9% 14% 13% 6%	3138
	11th to 20th	Day	29% 7% 13% 29% 14% 8%	3418
	21st to end	Day	49% 7% 11% 20% 11% 2	4092
April	1st to 10th	Day	36% 6% 14% 21% 18% 5%	2280
	11th to 20th	Day	35% 5% 12% 26% 17% 5%	2252
	21st to end	Day	30% 6% 14% 31% 16% 3	2280
May	1st to 10th	Day	19% 3 10% 32% 26% 10%	3941
	11th to 20th	Day	18% 4 9% 32% 33% 5%	4153
	21st to end	Day	19% 3 9% 37% 31% 2	4814
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	20% 3 6% 17% 32% 23%	1970
	11th to 20th	Day	17% 2 7% 22% 30% 21%	1900
	21st to end	Day	19% 4 5% 21% 33% 18%	2060
November	1st to 10th	Day	29% 4 8% 21% 24% 13%	1683
	11th to 20th	Day	24% 5% 8% 21% 29% 13%	1710
	21st to end	Day	29% 4 7% 17% 29% 15%	1710
December	1st to 10th	Day	34% 4 8% 20% 21% 13%	1558
	11th to 20th	Day	52% 4 6% 14% 14% 10%	1928
	21st to end	Day	38% 8% 10% 18% 17% 9%	2794
January	1st to 10th	Night	31% 6% 10% 26% 20% 7%	4406
	11th to 20th	Night	41% 9% 7% 18% 17% 9%	4269
	21st to end	Night	39% 9% 9% 19% 16% 7%	4521
February	1st to 10th	Night	36% 5% 6% 19% 21% 13%	2401
	11th to 20th	Night	32% 5% 8% 22% 23% 11%	2219
	21st to end	Night	38% 3 10% 20% 22% 6%	1884
March	1st to 10th	Night	43% 8% 7% 18% 17% 6%	3736
	11th to 20th	Night	30% 4 9% 29% 18% 9%	3558
	21st to end	Night	40% 9% 11% 24% 14% 3	3560
April	1st to 10th	Night	33% 5% 13% 26% 17% 7%	1900
	11th to 20th	Night	28% 7% 12% 25% 21% 7%	1879
	21st to end	Night	24% 5% 13% 35% 21% 2	1900
May	1st to 10th	Night	15% 4 9% 35% 30% 7%	3034
	11th to 20th	Night	13% 4 10% 32% 34% 7%	2748
	21st to end	Night	13% 3 9% 40% 32% 3	2762
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	17% 3 6% 19% 33% 23%	2207
	11th to 20th	Night	14% 3 6% 20% 34% 23%	2280
	21st to end	Night	13% 2 6% 22% 37% 19%	2536
November	1st to 10th	Night	24% 4 8% 22% 27% 16%	2418
	11th to 20th	Night	23% 4 8% 20% 31% 14%	2470
	21st to end	Night	28% 4 7% 18% 28% 15%	2470
December	1st to 10th	Night	32% 5% 7% 19% 22% 14%	2617
	11th to 20th	Night	46% 4 8% 14% 18% 9%	3276
	21st to end	Night	38% 8% 8% 17% 20% 9%	4862

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% 14% 24% 20% 8% 1	180
	11th to 20th	Day	22% 12% 27% 26% 9% 4	269
	21st to end	Day	16% 15% 31% 21% 13% 3	407
February	1st to 10th	Day	22% 18% 27% 20% 9% 3	401
	11th to 20th	Day	18% 21% 31% 21% 7% 1	455
	21st to end	Day	21% 20% 34% 21% 3 1	382
March	1st to 10th	Day	23% 22% 34% 15% 5%	460
	11th to 20th	Day	22% 22% 25% 22% 8% 1	460
	21st to end	Day	22% 25% 33% 16% 4	506
April	1st to 10th	Day	18% 20% 35% 20% 7% 1	460
	11th to 20th	Day	21% 20% 31% 21% 5% 1	452
	21st to end	Day	13% 17% 34% 23% 11% 2	460
May	1st to 10th	Day	18% 20% 32% 21% 8% 2	463
	11th to 20th	Day	19% 13% 30% 23% 12% 3	488
	21st to end	Day	16% 14% 25% 29% 11% 4	529
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% 14% 33% 31% 10% 2	509
November	1st to 10th	Day	13% 11% 32% 23% 19% 3	273
	11th to 20th	Day	11% 10% 27% 30% 15% 6%	270
	21st to end	Day	13% 11% 30% 30% 13% 3	270
December	1st to 10th	Day	20% 11% 27% 23% 17% 3	198
	11th to 20th	Day	14% 16% 33% 20% 14% 3	180
	21st to end	Day	23% 13% 30% 19% 11% 4	198
January	1st to 10th	Night	17% 9% 28% 23% 15% 8%	760
	11th to 20th	Night	13% 10% 28% 31% 13% 5%	760
	21st to end	Night	15% 10% 31% 24% 15% 5%	836
February	1st to 10th	Night	17% 8% 27% 30% 14% 4	760
	11th to 20th	Night	13% 12% 26% 26% 17% 7%	744
	21st to end	Night	16% 8% 29% 32% 12% 2	628
March	1st to 10th	Night	18% 10% 27% 29% 13% 4	760
	11th to 20th	Night	16% 9% 22% 28% 19% 6%	760
	21st to end	Night	15% 10% 30% 29% 15% 2	836
April	1st to 10th	Night	12% 8% 27% 34% 16% 4	760
	11th to 20th	Night	10% 9% 28% 30% 17% 5%	751
	21st to end	Night	10% 5% 27% 33% 19% 5%	760
May	1st to 10th	Night	6% 7% 27% 34% 20% 5%	679
	11th to 20th	Night	8% 5% 21% 40% 22% 3	514
	21st to end	Night	6% 3 22% 34% 28% 7%	411
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% 22% 32% 24% 11%	836
November	1st to 10th	Night	10% 5% 17% 36% 24% 8%	744
	11th to 20th	Night	9% 6% 25% 37% 18% 6%	760
	21st to end	Night	11% 5% 29% 34% 17% 5%	760
December	1st to 10th	Night	18% 7% 26% 34% 12% 4	760
	11th to 20th	Night	20% 9% 26% 27% 15% 3	760
	21st to end	Night	18% 11% 30% 26% 12% 3	836

Page generated at 2024-10-29 22:22:53. Version ac07

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