Rachel
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rachel 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	2 3 1 3 3 3 3 3 4 6% 69%	140631
	Night	2 3 1 4 3 3 3 3 5% 7% 67%	139331
Transparency	Day	16% 4 6% 16% 28% 30%	135571
	Night	17% 5% 7% 18% 28% 25%	135376
Seeing	Day	5% 9% 20% 30% 24% 13%	17061
	Night	3 5% 18% 41% 29% 3	26845

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	4 5% 2 5% 3 4 4 3 4 8% 57%	4722
	11th to 20th	Day	1 3 1 4 2 2 2 2 5% 6% 72%	5054
	21st to end	Day	3 7% 2 5% 3 4 4 3 6% 8% 55%	5943
February	1st to 10th	Day	2 5% 1 5% 4 3 4 3 5% 10% 59%	2100
	11th to 20th	Day	4 5% 2 4 4 4 4 4 6% 8% 56%	2050
	21st to end	Day	3 5% 2 4 4 4 4 4 4 8% 60%	1730
March	1st to 10th	Day	2 6% 2 7% 4 4 4 3 5% 8% 55%	4618
	11th to 20th	Day	2 6% 2 6% 5% 5% 4 3 6% 7% 54%	5014
	21st to end	Day	2 5% 2 5% 4 4 5% 4 6% 9% 54%	5690
April	1st to 10th	Day	2 4 1 5% 4 4 4 3 5% 7% 62%	2520
	11th to 20th	Day	2 4 2 3 3 3 3 3 5% 7% 65%	2489
	21st to end	Day	3 4 1 3 2 3 3 2 5% 6% 67%	2676
May	1st to 10th	Day	3 4 1 4 3 3 3 3 4 6% 66%	5894
	11th to 20th	Day	2 3 1 3 2 2 3 3 4 6% 71%	5841
	21st to end	Day	1 3 1 3 2 2 3 3 4 6% 72%	6423
June	1st to 10th	Day	1 2 1 3 3 3 3 3 6% 7% 68%	2615
	11th to 20th	Day	1 2 1 2 2 2 2 3 3 5% 78%	2800
	21st to end	Day	1 1 1 2 1 2 1 2 3 4 81%	2800
July	1st to 10th	Day	2 1 1 2 2 2 2 2 2 3 83%	6319
	11th to 20th	Day	2 2 1 3 3 2 3 3 4 6% 72%	6157
	21st to end	Day	1 2 1 3 2 3 3 3 4 5% 73%	6401
August	1st to 10th	Day	2 1 1 2 2 2 2 2 3 4 79%	5779
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 88%	5452
	21st to end	Day	2 2 1 3 2 3 3 2 4 6% 72%	5984
September	1st to 10th	Day	1 2 1 4 3 3 3 3 3 5% 72%	2346
	11th to 20th	Day	2 2 2 2 2 2 2 2 4 82%	2200
	21st to end	Day	1 1 1 2 2 2 2 2 3 5% 80%	2200
October	1st to 10th	Day	1 1 1 2 2 2 2 2 4 5% 78%	3600
	11th to 20th	Day	1 2 1 2 1 2 2 2 3 5% 80%	3486
	21st to end	Day	1 3 1 2 2 1 1 1 2 5% 80%	3205
November	1st to 10th	Day	2 3 1 3 2 3 2 2 6% 6% 71%	1854
	11th to 20th	Day	2 3 1 3 3 2 2 2 4 7% 70%	1890
	21st to end	Day	3 4 1 3 3 3 3 4 5% 8% 64%	1890
December	1st to 10th	Day	2 3 1 4 3 3 4 3 4 6% 68%	3040
	11th to 20th	Day	3 6% 3 5% 3 3 4 4 6% 10% 53%	3499
	21st to end	Day	2 5% 2 5% 3 4 5% 4 6% 9% 55%	4350
January	1st to 10th	Night	3 8% 2 6% 4 3 3 3 5% 7% 56%	8172
	11th to 20th	Night	1 3 1 3 2 3 3 3 4 7% 72%	7903
	21st to end	Night	3 6% 2 5% 3 4 4 4 6% 9% 55%	8273
February	1st to 10th	Night	3 5% 1 4 3 4 5% 4 6% 9% 55%	2520
	11th to 20th	Night	2 4 2 4 3 4 5% 4 7% 9% 55%	2458
	21st to end	Night	3 6% 2 5% 3 3 3 3 5% 10% 57%	2076
March	1st to 10th	Night	1 4 1 5% 4 3 4 4 5% 8% 60%	5496
	11th to 20th	Night	2 6% 2 7% 4 5% 5% 4 6% 7% 52%	5205
	21st to end	Night	1 6% 3 5% 3 3 3 3 5% 7% 59%	5524
April	1st to 10th	Night	2 3 1 3 3 3 4 4 6% 7% 64%	2100
	11th to 20th	Night	2 3 1 3 3 3 3 2 5% 5% 69%	2079
	21st to end	Night	3 4 1 3 3 3 3 3 5% 7% 65%	1943
May	1st to 10th	Night	2 3 1 5% 3 3 3 2 4 5% 68%	4109
	11th to 20th	Night	2 3 1 4 2 3 3 3 4 7% 68%	4074
	21st to end	Night	2 2 1 3 3 3 4 4 5% 6% 68%	4474
June	1st to 10th	Night	1 2 1 3 2 2 3 3 4 7% 72%	1778
	11th to 20th	Night	2 2 1 3 2 2 2 2 2 4 79%	1600
	21st to end	Night	1 1 2 2 2 2 2 3 5% 81%	1600
July	1st to 10th	Night	1 1 1 3 2 2 2 2 2 3 82%	3640
	11th to 20th	Night	1 2 1 3 2 3 3 3 4 6% 72%	3794
	21st to end	Night	1 2 1 4 3 3 3 2 3 5% 71%	4472
August	1st to 10th	Night	1 1 1 2 3 2 2 2 4 5% 76%	4033
	11th to 20th	Night	1 1 1 1 1 1 2 3 88%	4410
	21st to end	Night	1 2 1 3 2 2 3 3 4 6% 73%	5015
September	1st to 10th	Night	1 2 1 3 2 3 3 2 4 5% 75%	2052
	11th to 20th	Night	1 2 1 3 2 2 2 2 3 5% 78%	2200
	21st to end	Night	1 1 1 2 2 2 2 2 2 5% 83%	2200
October	1st to 10th	Night	1 2 2 2 2 2 2 3 4 80%	3600
	11th to 20th	Night	2 2 1 2 2 2 3 3 3 5% 76%	3669
	21st to end	Night	2 2 1 2 2 3 3 2 3 4 76%	4590
November	1st to 10th	Night	2 3 1 3 2 2 2 2 3 6% 74%	2665
	11th to 20th	Night	1 3 1 3 3 2 3 3 4 8% 68%	2730
	21st to end	Night	2 3 1 3 3 3 3 3 4 7% 67%	2730
December	1st to 10th	Night	2 3 1 4 3 2 3 3 6% 6% 67%	4370
	11th to 20th	Night	3 4 1 4 3 3 4 3 6% 12% 57%	4941
	21st to end	Night	2 6% 3 6% 4 5% 6% 4 7% 10% 47%	6836

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	24% 5% 16% 28% 23% 4	4723
	11th to 20th	Day	13% 4 8% 19% 31% 24%	5054
	21st to end	Day	25% 5% 9% 21% 29% 11%	5943
February	1st to 10th	Day	22% 5% 10% 26% 26% 11%	2100
	11th to 20th	Day	23% 6% 8% 23% 29% 10%	2060
	21st to end	Day	23% 6% 10% 26% 25% 9%	1730
March	1st to 10th	Day	24% 7% 7% 19% 31% 13%	4618
	11th to 20th	Day	27% 7% 7% 25% 25% 9%	5014
	21st to end	Day	23% 6% 11% 24% 25% 11%	5690
April	1st to 10th	Day	22% 5% 8% 20% 28% 18%	2520
	11th to 20th	Day	19% 4 7% 20% 29% 22%	2492
	21st to end	Day	18% 4 6% 21% 36% 14%	2676
May	1st to 10th	Day	18% 5% 6% 17% 39% 16%	5894
	11th to 20th	Day	14% 4 5% 12% 43% 23%	5844
	21st to end	Day	14% 4 4 10% 38% 31%	6424
June	1st to 10th	Day	13% 4 6% 12% 31% 33%	2615
	11th to 20th	Day	10% 3 4 7% 24% 53%	2800
	21st to end	Day	8% 2 3 5% 21% 61%	2800
July	1st to 10th	Day	10% 2 2 3 10% 74%	6251
	11th to 20th	Day	14% 5% 3 6% 15% 59%	6082
	21st to end	Day	13% 5% 4 8% 15% 55%	6217
August	1st to 10th	Day	14% 3 3 9% 30% 41%	4208
	11th to 20th	Day	5% 3 5% 6% 19% 62%	3946
	21st to end	Day	9% 3 4 7% 19% 58%	4311
September	1st to 10th	Day	15% 4 5% 7% 20% 49%	2346
	11th to 20th	Day	9% 3 5% 11% 28% 45%	2200
	21st to end	Day	9% 2 4 10% 36% 39%	2200
October	1st to 10th	Day	11% 3 3 8% 30% 46%	3600
	11th to 20th	Day	10% 2 3 13% 37% 36%	3486
	21st to end	Day	11% 2 2 18% 47% 20%	3205
November	1st to 10th	Day	14% 3 7% 25% 35% 17%	1854
	11th to 20th	Day	16% 3 7% 26% 38% 10%	1890
	21st to end	Day	20% 4 8% 24% 32% 14%	1890
December	1st to 10th	Day	19% 4 6% 21% 33% 17%	3040
	11th to 20th	Day	26% 6% 9% 24% 30% 5%	3498
	21st to end	Day	23% 7% 13% 31% 21% 4	4350
January	1st to 10th	Night	27% 4 13% 28% 25% 4	8172
	11th to 20th	Night	13% 4 8% 24% 30% 22%	7903
	21st to end	Night	25% 5% 8% 21% 27% 14%	8273
February	1st to 10th	Night	24% 5% 10% 25% 27% 9%	2520
	11th to 20th	Night	23% 5% 9% 21% 30% 12%	2459
	21st to end	Night	23% 5% 11% 24% 27% 9%	2076
March	1st to 10th	Night	19% 8% 9% 19% 31% 15%	5496
	11th to 20th	Night	27% 8% 7% 27% 23% 7%	5205
	21st to end	Night	21% 7% 9% 25% 29% 9%	5524
April	1st to 10th	Night	17% 6% 9% 26% 27% 15%	2100
	11th to 20th	Night	17% 4 7% 23% 32% 18%	2079
	21st to end	Night	20% 4 6% 20% 38% 13%	1943
May	1st to 10th	Night	18% 5% 6% 17% 39% 15%	4106
	11th to 20th	Night	12% 5% 6% 17% 39% 20%	4075
	21st to end	Night	13% 5% 5% 9% 41% 27%	4474
June	1st to 10th	Night	12% 5% 5% 13% 29% 36%	1778
	11th to 20th	Night	12% 2 3 9% 24% 51%	1600
	21st to end	Night	7% 3 3 7% 19% 61%	1600
July	1st to 10th	Night	10% 2 2 3 9% 74%	3600
	11th to 20th	Night	13% 4 3 7% 17% 56%	3746
	21st to end	Night	17% 4 5% 7% 10% 57%	4344
August	1st to 10th	Night	14% 4 3 10% 28% 41%	2928
	11th to 20th	Night	6% 3 3 7% 18% 64%	3184
	21st to end	Night	8% 2 2 7% 19% 62%	3608
September	1st to 10th	Night	13% 4 5% 7% 20% 51%	2052
	11th to 20th	Night	11% 3 4 10% 25% 46%	2200
	21st to end	Night	8% 2 3 13% 31% 43%	2200
October	1st to 10th	Night	10% 3 3 9% 25% 51%	3600
	11th to 20th	Night	13% 3 3 11% 35% 36%	3669
	21st to end	Night	14% 2 3 14% 43% 23%	4590
November	1st to 10th	Night	14% 3 7% 24% 36% 18%	2665
	11th to 20th	Night	16% 4 8% 21% 39% 13%	2730
	21st to end	Night	18% 4 6% 21% 36% 15%	2730
December	1st to 10th	Night	17% 4 6% 22% 33% 18%	4370
	11th to 20th	Night	20% 5% 11% 28% 30% 6%	4941
	21st to end	Night	28% 7% 14% 25% 22% 4	6836

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	10% 12% 20% 34% 20% 5%	220
	11th to 20th	Day	4 7% 19% 36% 26% 9%	276
	21st to end	Day	4 10% 15% 30% 32% 9%	397
February	1st to 10th	Day	6% 16% 21% 29% 21% 6%	540
	11th to 20th	Day	8% 15% 26% 32% 14% 4	535
	21st to end	Day	5% 11% 28% 34% 17% 5%	446
March	1st to 10th	Day	10% 17% 27% 25% 19% 3	540
	11th to 20th	Day	9% 15% 26% 27% 16% 7%	540
	21st to end	Day	8% 20% 27% 26% 15% 5%	594
April	1st to 10th	Day	7% 16% 22% 28% 19% 9%	540
	11th to 20th	Day	7% 19% 26% 26% 17% 6%	532
	21st to end	Day	7% 15% 26% 26% 18% 9%	540
May	1st to 10th	Day	4 13% 29% 28% 18% 8%	508
	11th to 20th	Day	5% 12% 26% 25% 21% 12%	528
	21st to end	Day	5% 9% 20% 28% 25% 13%	573
June	1st to 10th	Day	3 8% 18% 29% 26% 17%	530
	11th to 20th	Day	3 10% 23% 30% 22% 12%	530
	21st to end	Day	2 5% 19% 29% 28% 17%	530
July	1st to 10th	Day	3 2 18% 34% 27% 17%	526
	11th to 20th	Day	2 1 13% 33% 29% 22%	524
	21st to end	Day	3 2 11% 35% 29% 20%	575
August	1st to 10th	Day	3 3 20% 30% 27% 17%	527
	11th to 20th	Day	2 2 17% 29% 28% 22%	517
	21st to end	Day	3 4 18% 26% 30% 19%	583
September	1st to 10th	Day	2 5% 17% 26% 26% 24%	530
	11th to 20th	Day	2 8% 23% 26% 25% 15%	530
	21st to end	Day	2 6% 16% 29% 31% 17%	530
October	1st to 10th	Day	2 13% 17% 27% 24% 18%	530
	11th to 20th	Day	4 8% 19% 25% 29% 15%	475
	21st to end	Day	4 7% 14% 32% 31% 13%	363
November	1st to 10th	Day	6% 7% 15% 28% 27% 18%	330
	11th to 20th	Day	5% 7% 15% 38% 22% 13%	330
	21st to end	Day	5% 7% 15% 32% 28% 13%	330
December	1st to 10th	Day	5% 4 15% 40% 27% 10%	330
	11th to 20th	Day	4 5% 16% 39% 25% 11%	330
	21st to end	Day	10% 11% 19% 34% 23% 3	302
January	1st to 10th	Night	6% 5% 20% 43% 23% 3	840
	11th to 20th	Night	3 3 20% 48% 23% 3	840
	21st to end	Night	6% 3 21% 44% 23% 3	924
February	1st to 10th	Night	5% 6% 23% 42% 22% 3	840
	11th to 20th	Night	5% 6% 25% 44% 19% 2	824
	21st to end	Night	4 6% 25% 44% 18% 2	692
March	1st to 10th	Night	3 10% 24% 44% 18% 2	840
	11th to 20th	Night	4 8% 22% 39% 24% 3	840
	21st to end	Night	3 12% 26% 40% 17% 2	924
April	1st to 10th	Night	3 10% 24% 39% 23% 1	840
	11th to 20th	Night	4 11% 28% 37% 19% 2	831
	21st to end	Night	5% 8% 27% 37% 19% 4	683
May	1st to 10th	Night	4 9% 25% 44% 17% 1	603
	11th to 20th	Night	5% 6% 22% 37% 29% 2	591
	21st to end	Night	3 8% 20% 39% 27% 2	650
June	1st to 10th	Night	2 5% 16% 42% 34% 2	578
	11th to 20th	Night	3 3 18% 45% 30% 1	400
	21st to end	Night	2 12% 48% 35% 4	400
July	1st to 10th	Night	2 1 7% 41% 46% 4	390
	11th to 20th	Night	2 1 8% 40% 46% 4	451
	21st to end	Night	3 1 6% 40% 47% 3	657
August	1st to 10th	Night	3 2 12% 50% 32% 2	597
	11th to 20th	Night	1 1 8% 36% 49% 4	746
	21st to end	Night	2 1 8% 36% 48% 6%	880
September	1st to 10th	Night	2 2 11% 32% 46% 9%	800
	11th to 20th	Night	2 3 14% 37% 40% 6%	800
	21st to end	Night	1 2 14% 36% 40% 8%	800
October	1st to 10th	Night	3 4 17% 41% 31% 6%	800
	11th to 20th	Night	3 3 16% 38% 35% 6%	800
	21st to end	Night	2 4 17% 38% 35% 5%	880
November	1st to 10th	Night	3 5% 15% 38% 34% 6%	820
	11th to 20th	Night	2 4 15% 47% 28% 3	840
	21st to end	Night	4 3 18% 43% 27% 5%	840
December	1st to 10th	Night	5% 4 18% 47% 23% 4	840
	11th to 20th	Night	7% 3 20% 42% 25% 3	840
	21st to end	Night	5% 7% 25% 43% 18% 2	924

Page generated at 2026-04-28 23:21:48. Version ac07

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