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 2025-01-07.

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 4 3 3 3 3 4 6% 68%	109253
	Night	2 3 1 4 3 3 3 3 5% 7% 65%	108540
Transparency	Day	16% 5% 7% 16% 27% 29%	109007
	Night	17% 5% 8% 18% 27% 24%	108375
Seeing	Day	4 9% 20% 30% 24% 13%	15296
	Night	3 5% 18% 41% 29% 4	25083

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	3 4 2 5% 4 4 4 4 5% 9% 57%	2587
	11th to 20th	Day	1 5% 2 7% 3 5% 4 4 7% 10% 52%	2700
	21st to end	Day	5% 8% 2 6% 3 5% 5% 4 7% 9% 45%	3189
February	1st to 10th	Day	2 5% 1 5% 4 3 4 3 5% 10% 58%	1900
	11th to 20th	Day	4 6% 2 4 4 4 4 4 6% 8% 56%	1850
	21st to end	Day	3 5% 2 4 4 4 4 4 4 7% 58%	1570
March	1st to 10th	Day	1 4 2 7% 4 4 5% 3 5% 7% 55%	3138
	11th to 20th	Day	3 6% 2 6% 4 4 4 3 6% 7% 54%	3422
	21st to end	Day	2 7% 3 6% 5% 5% 5% 4 7% 9% 48%	3880
April	1st to 10th	Day	2 4 2 5% 4 4 4 3 5% 6% 61%	2280
	11th to 20th	Day	2 4 2 4 3 3 3 3 5% 7% 65%	2249
	21st to end	Day	3 4 1 3 2 3 3 2 4 6% 68%	2421
May	1st to 10th	Day	1 4 1 3 3 3 3 3 4 5% 71%	4110
	11th to 20th	Day	1 2 1 3 2 3 3 3 5% 6% 71%	4071
	21st to end	Day	1 3 1 3 2 3 3 3 4 5% 72%	4477
June	1st to 10th	Day	1 2 1 3 3 3 3 3 5% 7% 69%	2484
	11th to 20th	Day	1 2 1 2 2 2 2 3 3 5% 77%	2660
	21st to end	Day	1 1 1 2 1 2 1 2 4 4 81%	2660
July	1st to 10th	Day	1 1 1 2 2 2 2 2 3 3 82%	4420
	11th to 20th	Day	2 2 1 3 3 3 3 3 5% 7% 68%	4309
	21st to end	Day	2 3 1 4 3 4 4 4 5% 7% 63%	4454
August	1st to 10th	Day	2 2 1 3 2 3 2 2 3 5% 75%	4092
	11th to 20th	Day	1 1 1 1 1 2 2 3 4 84%	3786
	21st to end	Day	1 1 2 2 2 2 2 3 4 80%	4180
September	1st to 10th	Day	1 2 1 3 3 3 3 3 3 5% 73%	2229
	11th to 20th	Day	1 2 2 2 1 2 2 2 4 82%	2090
	21st to end	Day	1 1 1 2 2 2 2 2 3 4 81%	2090
October	1st to 10th	Day	1 1 1 2 2 2 2 2 4 5% 78%	3490
	11th to 20th	Day	1 2 1 2 1 2 2 2 3 5% 80%	3379
	21st to end	Day	1 3 1 2 2 1 1 1 2 5% 80%	3106
November	1st to 10th	Day	2 3 1 3 2 3 2 2 6% 6% 70%	1764
	11th to 20th	Day	1 2 1 3 3 2 2 2 4 7% 73%	1800
	21st to end	Day	3 4 1 4 4 3 3 4 4 8% 64%	1800
December	1st to 10th	Day	2 3 1 4 3 3 4 3 4 6% 67%	2950
	11th to 20th	Day	3 6% 3 5% 3 3 4 4 6% 10% 53%	3409
	21st to end	Day	2 5% 2 5% 3 4 5% 4 6% 9% 55%	4257
January	1st to 10th	Night	2 6% 3 6% 4 3 3 3 5% 9% 56%	4490
	11th to 20th	Night	1 4 2 5% 4 4 5% 4 5% 11% 54%	4259
	21st to end	Night	5% 7% 2 5% 4 4 4 4 7% 11% 46%	4476
February	1st to 10th	Night	3 5% 1 4 3 4 5% 4 6% 9% 54%	2280
	11th to 20th	Night	3 5% 2 4 3 4 5% 3 7% 9% 57%	2218
	21st to end	Night	3 6% 2 6% 3 3 3 3 5% 8% 56%	1884
March	1st to 10th	Night	1 3 2 6% 4 4 4 4 5% 9% 57%	3736
	11th to 20th	Night	3 6% 3 7% 4 4 5% 4 6% 7% 50%	3557
	21st to end	Night	2 7% 4 6% 4 4 4 3 5% 7% 53%	3770
April	1st to 10th	Night	2 3 1 3 3 3 4 4 7% 7% 64%	1900
	11th to 20th	Night	3 3 1 3 3 3 3 2 5% 5% 69%	1879
	21st to end	Night	3 4 1 3 3 3 3 3 5% 6% 65%	1758
May	1st to 10th	Night	2 2 1 4 3 2 2 2 5% 6% 71%	2860
	11th to 20th	Night	2 3 1 3 2 3 3 3 4 7% 68%	2834
	21st to end	Night	2 2 1 4 3 3 3 3 5% 6% 67%	3110
June	1st to 10th	Night	1 1 1 3 2 2 3 2 4 7% 74%	1689
	11th to 20th	Night	2 2 1 3 2 2 2 2 3 4 78%	1520
	21st to end	Night	1 1 2 2 1 2 2 3 5% 80%	1520
July	1st to 10th	Night	1 1 1 2 1 1 2 2 2 4 82%	2540
	11th to 20th	Night	1 2 1 3 2 3 3 3 5% 6% 70%	2652
	21st to end	Night	2 3 1 6% 5% 5% 5% 3 4 6% 60%	3109
August	1st to 10th	Night	2 2 1 3 3 3 3 3 4 6% 71%	2847
	11th to 20th	Night	1 1 2 1 1 2 1 3 3 85%	3058
	21st to end	Night	1 1 1 1 1 2 2 2 3 85%	3498
September	1st to 10th	Night	1 1 1 3 2 3 3 2 4 5% 75%	1949
	11th to 20th	Night	1 2 1 3 2 2 2 2 3 5% 78%	2090
	21st to end	Night	1 1 2 1 2 2 2 2 5% 82%	2090
October	1st to 10th	Night	1 2 2 2 2 2 2 3 4 80%	3490
	11th to 20th	Night	1 2 1 2 1 2 3 3 3 5% 77%	3556
	21st to end	Night	2 2 1 2 2 3 3 2 3 4 76%	4447
November	1st to 10th	Night	2 3 1 4 2 2 2 2 3 6% 73%	2535
	11th to 20th	Night	1 2 1 3 3 2 3 3 4 8% 70%	2600
	21st to end	Night	2 3 1 3 3 3 3 3 4 7% 66%	2600
December	1st to 10th	Night	2 3 1 4 3 2 3 3 6% 6% 66%	4240
	11th to 20th	Night	3 4 1 4 3 3 4 3 6% 13% 57%	4811
	21st to end	Night	2 6% 3 7% 4 5% 6% 4 7% 10% 47%	6688

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	21% 7% 15% 34% 21% 2	2587
	11th to 20th	Day	23% 7% 15% 31% 17% 6%	2700
	21st to end	Day	30% 7% 15% 26% 19% 3	3189
February	1st to 10th	Day	23% 5% 11% 26% 26% 9%	1900
	11th to 20th	Day	23% 6% 9% 24% 29% 10%	1860
	21st to end	Day	24% 7% 10% 25% 27% 8%	1570
March	1st to 10th	Day	23% 8% 10% 23% 27% 9%	3138
	11th to 20th	Day	24% 7% 9% 24% 27% 8%	3422
	21st to end	Day	26% 8% 13% 25% 22% 6%	3880
April	1st to 10th	Day	23% 5% 8% 21% 26% 17%	2280
	11th to 20th	Day	19% 4 7% 20% 28% 22%	2252
	21st to end	Day	17% 4 7% 21% 36% 15%	2421
May	1st to 10th	Day	14% 4 6% 16% 39% 20%	4110
	11th to 20th	Day	11% 4 7% 14% 38% 27%	4074
	21st to end	Day	14% 5% 5% 12% 38% 26%	4477
June	1st to 10th	Day	13% 4 6% 11% 30% 35%	2484
	11th to 20th	Day	10% 3 4 7% 25% 51%	2660
	21st to end	Day	8% 2 3 6% 21% 61%	2660
July	1st to 10th	Day	9% 2 3 4 7% 75%	4351
	11th to 20th	Day	15% 5% 3 7% 18% 51%	4234
	21st to end	Day	18% 6% 5% 12% 18% 40%	4314
August	1st to 10th	Day	14% 3 3 9% 31% 41%	4078
	11th to 20th	Day	6% 3 5% 6% 19% 62%	3824
	21st to end	Day	9% 3 4 6% 18% 60%	4179
September	1st to 10th	Day	15% 4 5% 6% 19% 51%	2229
	11th to 20th	Day	9% 3 5% 11% 27% 45%	2090
	21st to end	Day	9% 2 4 9% 36% 39%	2090
October	1st to 10th	Day	11% 3 3 7% 29% 46%	3490
	11th to 20th	Day	10% 2 3 13% 37% 36%	3379
	21st to end	Day	11% 2 2 18% 47% 20%	3106
November	1st to 10th	Day	15% 3 7% 24% 34% 16%	1764
	11th to 20th	Day	13% 3 7% 26% 40% 11%	1800
	21st to end	Day	20% 4 8% 23% 31% 14%	1800
December	1st to 10th	Day	19% 4 6% 20% 33% 18%	2950
	11th to 20th	Day	26% 6% 9% 24% 30% 4	3408
	21st to end	Day	23% 7% 14% 31% 21% 4	4257
January	1st to 10th	Night	23% 6% 12% 33% 24% 2	4490
	11th to 20th	Night	22% 6% 15% 34% 18% 6%	4259
	21st to end	Night	28% 7% 14% 28% 19% 4	4476
February	1st to 10th	Night	24% 5% 10% 26% 26% 8%	2280
	11th to 20th	Night	22% 5% 9% 21% 30% 12%	2219
	21st to end	Night	26% 5% 10% 23% 28% 8%	1884
March	1st to 10th	Night	20% 9% 12% 21% 28% 9%	3736
	11th to 20th	Night	27% 9% 10% 25% 20% 9%	3557
	21st to end	Night	24% 8% 11% 25% 26% 6%	3770
April	1st to 10th	Night	17% 6% 10% 28% 26% 14%	1900
	11th to 20th	Night	17% 4 7% 24% 31% 17%	1879
	21st to end	Night	20% 4 6% 19% 37% 14%	1758
May	1st to 10th	Night	13% 6% 7% 13% 41% 20%	2860
	11th to 20th	Night	10% 6% 9% 17% 33% 25%	2835
	21st to end	Night	14% 6% 7% 11% 38% 24%	3110
June	1st to 10th	Night	11% 4 5% 12% 30% 38%	1689
	11th to 20th	Night	12% 2 3 9% 25% 49%	1520
	21st to end	Night	7% 3 3 7% 20% 60%	1520
July	1st to 10th	Night	9% 2 3 4 7% 75%	2500
	11th to 20th	Night	14% 5% 4 8% 18% 52%	2604
	21st to end	Night	24% 5% 7% 10% 14% 40%	3011
August	1st to 10th	Night	15% 4 3 10% 28% 40%	2838
	11th to 20th	Night	6% 3 3 7% 18% 64%	3086
	21st to end	Night	7% 2 2 6% 20% 63%	3498
September	1st to 10th	Night	13% 4 5% 6% 19% 53%	1949
	11th to 20th	Night	11% 3 5% 10% 25% 46%	2090
	21st to end	Night	8% 2 3 12% 31% 44%	2090
October	1st to 10th	Night	10% 3 3 8% 24% 52%	3490
	11th to 20th	Night	12% 3 3 11% 35% 36%	3556
	21st to end	Night	14% 2 3 14% 42% 23%	4447
November	1st to 10th	Night	15% 3 7% 23% 35% 17%	2535
	11th to 20th	Night	13% 3 8% 22% 40% 14%	2600
	21st to end	Night	18% 4 6% 21% 36% 16%	2600
December	1st to 10th	Night	18% 4 7% 21% 32% 18%	4240
	11th to 20th	Night	20% 5% 12% 28% 30% 5%	4811
	21st to end	Night	28% 7% 14% 25% 22% 4	6688

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	8% 10% 20% 37% 19% 5%	192
	11th to 20th	Day	4 8% 21% 35% 23% 8%	225
	21st to end	Day	5% 12% 15% 29% 30% 10%	327
February	1st to 10th	Day	5% 18% 22% 28% 21% 7%	460
	11th to 20th	Day	8% 13% 25% 33% 16% 5%	455
	21st to end	Day	6% 12% 30% 32% 15% 4	382
March	1st to 10th	Day	10% 16% 28% 25% 19% 3	460
	11th to 20th	Day	9% 13% 27% 29% 16% 6%	460
	21st to end	Day	8% 21% 28% 26% 14% 3	506
April	1st to 10th	Day	8% 17% 22% 28% 18% 8%	460
	11th to 20th	Day	7% 21% 27% 25% 15% 6%	452
	21st to end	Day	6% 15% 26% 25% 18% 10%	460
May	1st to 10th	Day	4 14% 30% 27% 19% 6%	463
	11th to 20th	Day	4 11% 25% 26% 22% 13%	488
	21st to end	Day	5% 9% 21% 27% 24% 13%	529
June	1st to 10th	Day	3 9% 19% 29% 25% 16%	490
	11th to 20th	Day	3 10% 22% 30% 22% 13%	490
	21st to end	Day	2 4 20% 28% 29% 17%	490
July	1st to 10th	Day	3 1 16% 35% 28% 17%	486
	11th to 20th	Day	3 1 13% 33% 29% 22%	484
	21st to end	Day	3 2 9% 36% 30% 21%	531
August	1st to 10th	Day	3 3 19% 31% 28% 17%	487
	11th to 20th	Day	1 2 17% 29% 29% 22%	477
	21st to end	Day	3 4 19% 25% 30% 19%	539
September	1st to 10th	Day	2 4 17% 26% 26% 25%	490
	11th to 20th	Day	2 8% 24% 25% 26% 15%	490
	21st to end	Day	2 6% 16% 28% 32% 16%	490
October	1st to 10th	Day	2 12% 17% 27% 23% 19%	490
	11th to 20th	Day	3 7% 19% 25% 30% 16%	438
	21st to end	Day	4 7% 14% 32% 32% 12%	330
November	1st to 10th	Day	6% 7% 16% 28% 26% 17%	300
	11th to 20th	Day	4 6% 14% 40% 24% 13%	300
	21st to end	Day	5% 7% 16% 32% 27% 13%	300
December	1st to 10th	Day	5% 4 16% 39% 26% 9%	300
	11th to 20th	Day	4 6% 16% 40% 25% 9%	300
	21st to end	Day	8% 11% 19% 36% 23% 4	275
January	1st to 10th	Night	5% 4 19% 43% 24% 4	784
	11th to 20th	Night	3 4 19% 48% 23% 3	760
	21st to end	Night	7% 4 21% 43% 22% 3	836
February	1st to 10th	Night	5% 5% 23% 43% 22% 3	760
	11th to 20th	Night	4 5% 23% 44% 21% 2	744
	21st to end	Night	5% 7% 26% 43% 17% 2	628
March	1st to 10th	Night	3 11% 23% 43% 18% 2	760
	11th to 20th	Night	4 8% 21% 38% 25% 3	760
	21st to end	Night	4 13% 26% 40% 15% 2	836
April	1st to 10th	Night	3 11% 25% 38% 21% 1	760
	11th to 20th	Night	4 11% 28% 36% 20% 2	751
	21st to end	Night	5% 8% 27% 37% 20% 4	618
May	1st to 10th	Night	4 9% 25% 44% 16% 1	570
	11th to 20th	Night	5% 6% 21% 37% 30% 2	561
	21st to end	Night	3 9% 20% 38% 27% 2	617
June	1st to 10th	Night	1 5% 15% 42% 34% 2	549
	11th to 20th	Night	3 3 18% 44% 31% 1	380
	21st to end	Night	2 12% 49% 34% 4	380
July	1st to 10th	Night	2 1 6% 39% 48% 4	370
	11th to 20th	Night	2 8% 40% 46% 4	428
	21st to end	Night	3 1 6% 38% 48% 3	624
August	1st to 10th	Night	3 2 11% 49% 34% 2	567
	11th to 20th	Night	1 1 9% 35% 49% 4	708
	21st to end	Night	2 1 8% 35% 48% 6%	836
September	1st to 10th	Night	1 1 10% 32% 47% 9%	760
	11th to 20th	Night	1 3 14% 36% 40% 6%	760
	21st to end	Night	1 2 14% 35% 40% 8%	760
October	1st to 10th	Night	3 3 16% 41% 31% 6%	760
	11th to 20th	Night	2 3 15% 38% 36% 6%	760
	21st to end	Night	2 4 18% 37% 35% 5%	836
November	1st to 10th	Night	3 5% 16% 38% 34% 5%	780
	11th to 20th	Night	1 4 15% 48% 29% 4	800
	21st to end	Night	4 4 19% 42% 26% 5%	800
December	1st to 10th	Night	5% 4 19% 46% 23% 4	800
	11th to 20th	Night	8% 3 20% 42% 25% 3	800
	21st to end	Night	5% 6% 25% 43% 19% 2	880

Page generated at 2025-01-04 00:24:01. Version ac07

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