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 2024-03-08.

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	7% 7% 2 3 3 2 3 3 4 5% 60%	78661
	Night	7% 7% 2 3 3 2 3 3 4 6% 60%	80919
Transparency	Day	27% 4 8% 20% 25% 16%	78372
	Night	26% 5% 8% 21% 25% 15%	80730
Seeing	Day	12% 14% 34% 26% 10% 3	14242
	Night	10% 6% 22% 33% 23% 6%	23838

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 1 2 2 4 5% 47%	2539
	11th to 20th	Day	15% 11% 2 5% 5% 4 5% 4 4 7% 40%	2681
	21st to end	Day	12% 12% 3 5% 4 3 3 4 5% 8% 41%	3146
February	1st to 10th	Day	11% 11% 3 4 3 2 3 4 3 7% 49%	1776
	11th to 20th	Day	7% 11% 5% 4 3 2 4 3 4 6% 52%	1850
	21st to end	Day	10% 13% 6% 7% 4 2 3 3 3 4 46%	1570
March	1st to 10th	Day	9% 15% 7% 7% 4 3 4 4 4 6% 37%	1871
	11th to 20th	Day	11% 15% 5% 5% 4 2 3 4 5% 6% 40%	1934
	21st to end	Day	9% 15% 5% 5% 4 2 3 3 3 6% 44%	2329
April	1st to 10th	Day	9% 11% 3 5% 3 2 3 4 5% 6% 47%	2160
	11th to 20th	Day	10% 12% 3 4 4 2 3 3 4 6% 51%	2129
	21st to end	Day	9% 9% 2 3 4 2 3 4 4 6% 54%	2160
May	1st to 10th	Day	7% 8% 2 3 3 2 2 3 4 8% 58%	2236
	11th to 20th	Day	8% 7% 2 4 3 2 3 4 4 5% 57%	2341
	21st to end	Day	8% 10% 2 3 3 2 3 3 4 6% 54%	2723
June	1st to 10th	Day	7% 3 2 3 3 2 3 4 4 7% 62%	2520
	11th to 20th	Day	3 2 1 2 2 1 2 2 2 5% 78%	2520
	21st to end	Day	2 2 1 2 2 1 2 2 3 4 79%	2520
July	1st to 10th	Day	1 1 1 1 2 1 3 3 3 4 81%	2520
	11th to 20th	Day	2 1 1 2 2 1 2 2 4 4 80%	2520
	21st to end	Day	2 2 1 2 3 2 3 3 3 6% 72%	2630
August	1st to 10th	Day	2 2 1 2 2 1 3 2 3 4 79%	2239
	11th to 20th	Day	1 1 2 2 1 2 3 3 5% 80%	2122
	21st to end	Day	2 1 1 1 2 1 2 2 2 3 83%	2376
September	1st to 10th	Day	2 1 2 2 1 2 3 3 3 82%	2160
	11th to 20th	Day	4 1 1 2 3 2 2 3 3 4 75%	2116
	21st to end	Day	3 2 1 2 2 2 2 3 5% 4 75%	1980
October	1st to 10th	Day	6% 5% 2 3 2 2 2 2 4 5% 69%	1867
	11th to 20th	Day	4 2 1 3 2 2 2 3 4 5% 70%	1800
	21st to end	Day	7% 4 2 2 2 2 3 4 4 71%	1952
November	1st to 10th	Day	8% 6% 3 4 3 2 4 3 4 6% 58%	1674
	11th to 20th	Day	8% 4 1 3 3 2 3 4 4 6% 62%	1710
	21st to end	Day	8% 7% 3 3 3 3 3 3 4 6% 58%	1710
December	1st to 10th	Day	12% 7% 1 4 4 3 4 4 4 6% 52%	1558
	11th to 20th	Day	17% 12% 3 6% 5% 3 4 4 3 5% 37%	1928
	21st to end	Day	9% 9% 4 6% 4 3 4 6% 7% 8% 41%	2794
January	1st to 10th	Night	7% 11% 3 5% 4 2 3 4 4 5% 53%	4406
	11th to 20th	Night	15% 10% 3 5% 4 3 4 5% 5% 7% 38%	4269
	21st to end	Night	8% 13% 3 4 5% 3 4 5% 5% 7% 41%	4521
February	1st to 10th	Night	10% 10% 3 4 4 1 3 4 4 6% 50%	2401
	11th to 20th	Night	7% 11% 4 4 3 2 3 3 5% 7% 52%	2218
	21st to end	Night	9% 10% 4 7% 4 2 4 3 2 5% 51%	1884
March	1st to 10th	Night	9% 14% 5% 7% 5% 2 3 4 4 5% 43%	2249
	11th to 20th	Night	9% 10% 6% 5% 4 3 3 4 4 7% 46%	2022
	21st to end	Night	9% 12% 4 4 3 2 3 2 4 6% 52%	2026
April	1st to 10th	Night	8% 9% 3 4 3 3 3 3 5% 7% 52%	1800
	11th to 20th	Night	6% 8% 3 4 4 1 3 3 4 5% 59%	1779
	21st to end	Night	6% 7% 2 3 3 2 3 3 4 6% 62%	1800
May	1st to 10th	Night	5% 4 2 3 2 2 3 4 4 7% 64%	1724
	11th to 20th	Night	5% 5% 2 2 3 2 2 3 3 6% 68%	1549
	21st to end	Night	5% 5% 2 3 2 1 3 4 4 6% 66%	1552
June	1st to 10th	Night	4 3 1 2 3 2 3 3 4 6% 68%	1440
	11th to 20th	Night	1 1 1 1 1 2 3 4 4 82%	1440
	21st to end	Night	2 1 1 1 1 1 2 2 2 4 85%	1440
July	1st to 10th	Night	1 1 1 2 2 3 3 87%	1440
	11th to 20th	Night	2 2 1 2 2 1 2 2 3 4 80%	1440
	21st to end	Night	3 2 2 2 3 2 3 3 4 5% 72%	1684
August	1st to 10th	Night	2 2 1 2 2 2 2 2 3 4 79%	1699
	11th to 20th	Night	1 1 1 1 2 2 1 3 4 82%	1780
	21st to end	Night	2 1 1 1 1 1 2 1 2 3 86%	1980
September	1st to 10th	Night	1 1 1 1 1 1 1 1 2 89%	1800
	11th to 20th	Night	2 2 1 2 2 2 2 1 1 3 83%	1844
	21st to end	Night	3 2 1 2 1 1 2 2 3 4 79%	1980
October	1st to 10th	Night	5% 4 1 2 2 1 2 2 3 4 73%	2090
	11th to 20th	Night	5% 1 1 2 2 2 2 3 3 5% 74%	2160
	21st to end	Night	5% 2 1 2 2 2 2 3 5% 77%	2402
November	1st to 10th	Night	7% 7% 2 2 2 2 2 3 4 7% 62%	2405
	11th to 20th	Night	7% 4 1 3 3 1 3 3 5% 7% 62%	2470
	21st to end	Night	7% 7% 3 4 3 2 3 4 5% 5% 59%	2470
December	1st to 10th	Night	12% 7% 2 4 4 2 3 3 4 6% 54%	2617
	11th to 20th	Night	17% 13% 2 5% 3 2 4 4 5% 6% 39%	3276
	21st to end	Night	10% 9% 3 6% 6% 3 4 5% 5% 7% 42%	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% 6% 9% 15% 15% 6%	1871
	11th to 20th	Day	45% 5% 10% 17% 17% 6%	1934
	21st to end	Day	43% 5% 14% 21% 13% 4	2329
April	1st to 10th	Day	37% 6% 14% 20% 18% 5%	2160
	11th to 20th	Day	36% 5% 11% 25% 17% 5%	2132
	21st to end	Day	31% 6% 14% 30% 17% 3	2160
May	1st to 10th	Day	28% 4 12% 33% 21% 3	2236
	11th to 20th	Day	29% 5% 10% 29% 22% 5%	2344
	21st to end	Day	31% 5% 13% 30% 18% 3	2724
June	1st to 10th	Day	23% 5% 12% 27% 26% 7%	2520
	11th to 20th	Day	12% 3 8% 26% 37% 14%	2520
	21st to end	Day	12% 2 3 18% 39% 27%	2520
July	1st to 10th	Day	8% 3 3 13% 31% 43%	2451
	11th to 20th	Day	10% 2 6% 15% 31% 36%	2445
	21st to end	Day	14% 3 7% 15% 27% 33%	2484
August	1st to 10th	Day	12% 2 4 14% 28% 40%	2223
	11th to 20th	Day	9% 2 6% 13% 39% 32%	2122
	21st to end	Day	9% 2 3 14% 39% 34%	2376
September	1st to 10th	Day	9% 2 4 14% 34% 38%	2160
	11th to 20th	Day	13% 3 6% 25% 33% 20%	2116
	21st to end	Day	14% 3 6% 22% 36% 19%	1980
October	1st to 10th	Day	20% 3 6% 17% 32% 22%	1867
	11th to 20th	Day	17% 3 7% 23% 29% 21%	1800
	21st to end	Day	18% 4 5% 21% 33% 19%	1952
November	1st to 10th	Day	29% 4 8% 21% 24% 13%	1674
	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% 5% 9% 18% 17% 7%	2249
	11th to 20th	Night	39% 5% 9% 20% 20% 7%	2022
	21st to end	Night	34% 6% 14% 26% 15% 5%	2026
April	1st to 10th	Night	33% 5% 14% 25% 17% 7%	1800
	11th to 20th	Night	28% 6% 12% 24% 22% 8%	1779
	21st to end	Night	24% 5% 14% 35% 20% 2	1800
May	1st to 10th	Night	23% 5% 11% 33% 24% 5%	1724
	11th to 20th	Night	19% 4 13% 34% 26% 4	1550
	21st to end	Night	20% 5% 14% 33% 23% 5%	1552
June	1st to 10th	Night	18% 5% 12% 27% 32% 7%	1440
	11th to 20th	Night	8% 3 9% 26% 38% 16%	1440
	21st to end	Night	8% 3 3 17% 39% 30%	1440
July	1st to 10th	Night	4 2 4 12% 35% 44%	1400
	11th to 20th	Night	12% 2 5% 13% 31% 38%	1392
	21st to end	Night	16% 3 5% 15% 29% 32%	1590
August	1st to 10th	Night	11% 2 4 16% 26% 40%	1690
	11th to 20th	Night	8% 3 4 17% 36% 31%	1780
	21st to end	Night	9% 1 2 14% 41% 33%	1980
September	1st to 10th	Night	6% 1 1 15% 38% 39%	1800
	11th to 20th	Night	11% 2 5% 24% 37% 20%	1844
	21st to end	Night	11% 2 5% 25% 36% 20%	1980
October	1st to 10th	Night	17% 3 6% 20% 32% 22%	2090
	11th to 20th	Night	14% 4 6% 20% 33% 23%	2160
	21st to end	Night	13% 2 6% 22% 37% 19%	2402
November	1st to 10th	Night	24% 4 8% 22% 27% 16%	2405
	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% 33% 16% 5%	449
	11th to 20th	Day	23% 21% 25% 21% 8% 1	420
	21st to end	Day	21% 25% 33% 16% 5%	462
April	1st to 10th	Day	19% 19% 35% 20% 7%	420
	11th to 20th	Day	23% 21% 31% 20% 4 1	412
	21st to end	Day	14% 17% 34% 23% 10% 2	420
May	1st to 10th	Day	18% 19% 30% 22% 8% 2	423
	11th to 20th	Day	21% 14% 31% 21% 11% 3	448
	21st to end	Day	18% 15% 24% 28% 11% 4	485
June	1st to 10th	Day	9% 16% 34% 25% 12% 5%	450
	11th to 20th	Day	7% 15% 35% 25% 13% 5%	450
	21st to end	Day	3 12% 38% 32% 10% 6%	450
July	1st to 10th	Day	2 4 43% 35% 12% 4	446
	11th to 20th	Day	2 4 34% 41% 12% 7%	444
	21st to end	Day	2 6% 39% 36% 10% 7%	487
August	1st to 10th	Day	2 10% 48% 25% 10% 5%	447
	11th to 20th	Day	3 7% 44% 29% 12% 5%	437
	21st to end	Day	3 6% 41% 33% 14% 4	495
September	1st to 10th	Day	3 8% 44% 30% 14% 2	450
	11th to 20th	Day	7% 13% 43% 26% 10% 1	450
	21st to end	Day	7% 12% 40% 31% 9% 1	450
October	1st to 10th	Day	8% 19% 32% 30% 11%	450
	11th to 20th	Day	6% 12% 36% 34% 9% 2	450
	21st to end	Day	10% 13% 32% 32% 11% 2	467
November	1st to 10th	Day	13% 11% 31% 23% 19% 3	270
	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	19% 10% 26% 29% 13% 4	750
	11th to 20th	Night	16% 9% 21% 28% 20% 6%	720
	21st to end	Night	14% 9% 29% 29% 16% 2	792
April	1st to 10th	Night	12% 7% 27% 34% 16% 4	720
	11th to 20th	Night	10% 9% 29% 29% 18% 6%	711
	21st to end	Night	11% 5% 26% 33% 20% 6%	720
May	1st to 10th	Night	6% 6% 26% 35% 21% 5%	644
	11th to 20th	Night	9% 5% 21% 39% 23% 3	487
	21st to end	Night	6% 3 20% 34% 30% 7%	389
June	1st to 10th	Night	6% 4 13% 38% 35% 4	360
	11th to 20th	Night	2 2 19% 41% 33% 4	360
	21st to end	Night	2 18% 39% 36% 4	360
July	1st to 10th	Night	1 13% 37% 45% 4	350
	11th to 20th	Night	3 1 11% 39% 41% 5%	348
	21st to end	Night	3 1 11% 31% 42% 12%	502
August	1st to 10th	Night	2 3 14% 38% 37% 8%	625
	11th to 20th	Night	2 11% 45% 35% 7%	712
	21st to end	Night	2 8% 41% 42% 6%	792
September	1st to 10th	Night	2 1 9% 43% 37% 9%	720
	11th to 20th	Night	4 1 15% 39% 33% 8%	720
	21st to end	Night	3 2 17% 37% 31% 10%	720
October	1st to 10th	Night	5% 3 24% 36% 25% 7%	720
	11th to 20th	Night	5% 3 21% 35% 27% 8%	720
	21st to end	Night	6% 4 20% 32% 25% 11%	792
November	1st to 10th	Night	10% 5% 17% 36% 24% 8%	740
	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 2024-03-05 04:23:59. Version ac07

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