Christmas Valley
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Christmas Valley clear sky chart from 2005-11-06 to 2025-06-10.

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	5% 7% 2 6% 5% 4 4 4 5% 7% 50%	123758
	Night	5% 8% 3 7% 5% 5% 5% 4 6% 7% 46%	123268
Transparency	Day	32% 6% 10% 21% 19% 11%	123502
	Night	34% 7% 12% 22% 18% 8%	123111
Seeing	Day	9% 12% 21% 26% 23% 10%	15406
	Night	9% 8% 23% 37% 21% 2	24595

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	8% 15% 5% 11% 9% 7% 6% 6% 8% 7% 18%	3639
	11th to 20th	Day	7% 11% 4 10% 6% 6% 5% 4 5% 7% 36%	3640
	21st to end	Day	5% 9% 3 9% 5% 5% 5% 3 5% 6% 44%	4322
February	1st to 10th	Day	6% 11% 4 10% 7% 6% 5% 6% 8% 9% 30%	1800
	11th to 20th	Day	6% 12% 4 8% 6% 6% 5% 5% 7% 7% 34%	1811
	21st to end	Day	3 10% 4 9% 7% 6% 7% 5% 8% 9% 32%	1650
March	1st to 10th	Day	5% 12% 4 10% 6% 6% 7% 5% 7% 9% 29%	4518
	11th to 20th	Day	3 11% 3 8% 6% 6% 5% 4 6% 8% 39%	4988
	21st to end	Day	5% 10% 4 10% 7% 7% 6% 5% 7% 7% 31%	5783
April	1st to 10th	Day	7% 13% 4 10% 5% 5% 5% 4 5% 7% 34%	2400
	11th to 20th	Day	7% 10% 4 7% 5% 5% 4 4 6% 7% 41%	2369
	21st to end	Day	6% 10% 3 6% 5% 5% 4 3 5% 8% 46%	2400
May	1st to 10th	Day	7% 8% 2 6% 4 4 4 4 5% 6% 51%	6293
	11th to 20th	Day	5% 6% 2 6% 4 4 4 4 4 6% 55%	6277
	21st to end	Day	5% 6% 2 4 4 3 4 3 5% 8% 56%	6902
June	1st to 10th	Day	9% 7% 3 6% 5% 5% 5% 4 6% 6% 46%	2789
	11th to 20th	Day	6% 4 2 4 4 3 4 3 5% 6% 58%	2660
	21st to end	Day	3 3 1 3 3 3 3 3 4 5% 69%	2660
July	1st to 10th	Day	1 1 1 1 1 2 2 2 3 3 84%	4420
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 5% 76%	4420
	21st to end	Day	1 1 1 2 2 2 3 2 3 4 80%	4790
August	1st to 10th	Day	3 3 1 4 2 3 3 3 5% 7% 65%	4196
	11th to 20th	Day	1 2 1 3 2 2 3 2 4 5% 74%	3914
	21st to end	Day	2 2 1 3 2 3 3 2 4 5% 75%	4180
September	1st to 10th	Day	2 2 1 3 2 2 3 2 3 5% 76%	2280
	11th to 20th	Day	4 4 2 5% 3 3 4 3 5% 6% 61%	2280
	21st to end	Day	5% 5% 2 5% 2 3 3 3 4 7% 62%	2052
October	1st to 10th	Day	5% 4 2 3 3 2 3 2 4 5% 68%	3180
	11th to 20th	Day	4 6% 2 6% 4 4 5% 4 7% 9% 49%	3161
	21st to end	Day	6% 7% 2 8% 6% 6% 6% 5% 6% 8% 41%	3273
November	1st to 10th	Day	8% 9% 3 8% 5% 5% 5% 4 8% 8% 38%	1764
	11th to 20th	Day	6% 8% 3 10% 6% 7% 6% 6% 6% 8% 32%	1695
	21st to end	Day	6% 10% 3 8% 7% 6% 6% 5% 6% 11% 34%	1600
December	1st to 10th	Day	3 6% 2 8% 5% 5% 4 4 8% 12% 42%	2620
	11th to 20th	Day	10% 15% 4 12% 8% 6% 7% 6% 7% 8% 18%	3028
	21st to end	Day	7% 12% 6% 13% 9% 8% 8% 5% 7% 8% 17%	4004
January	1st to 10th	Night	6% 13% 5% 12% 9% 8% 7% 6% 8% 9% 18%	6306
	11th to 20th	Night	6% 9% 3 8% 7% 5% 4 4 6% 10% 39%	6320
	21st to end	Night	6% 10% 4 8% 5% 5% 5% 4 5% 6% 42%	6644
February	1st to 10th	Night	6% 12% 4 9% 5% 5% 6% 5% 7% 8% 34%	2600
	11th to 20th	Night	5% 11% 5% 9% 6% 5% 5% 4 6% 7% 37%	2475
	21st to end	Night	4 7% 5% 9% 7% 6% 6% 5% 8% 8% 35%	1980
March	1st to 10th	Night	4 12% 4 8% 6% 6% 7% 6% 9% 10% 27%	5376
	11th to 20th	Night	4 9% 2 7% 4 5% 6% 5% 8% 9% 42%	5010
	21st to end	Night	3 9% 4 10% 6% 7% 7% 5% 8% 8% 33%	5191
April	1st to 10th	Night	7% 12% 3 8% 5% 5% 4 4 6% 8% 39%	2000
	11th to 20th	Night	6% 9% 3 6% 6% 6% 4 4 5% 7% 45%	1979
	21st to end	Night	7% 10% 3 5% 3 3 4 4 6% 9% 46%	1997
May	1st to 10th	Night	5% 6% 2 5% 3 4 4 3 5% 6% 57%	3650
	11th to 20th	Night	6% 6% 3 6% 3 4 4 3 5% 6% 56%	3616
	21st to end	Night	5% 5% 2 4 3 3 3 3 4 6% 62%	3972
June	1st to 10th	Night	8% 7% 2 5% 4 4 4 3 5% 7% 50%	1595
	11th to 20th	Night	7% 5% 2 3 3 4 3 3 5% 6% 59%	1520
	21st to end	Night	3 3 1 3 2 3 2 2 3 5% 73%	1520
July	1st to 10th	Night	1 1 1 2 2 2 2 3 3 83%	2540
	11th to 20th	Night	2 1 1 2 2 2 2 3 3 5% 77%	2540
	21st to end	Night	1 1 1 2 1 2 3 2 3 4 79%	2761
August	1st to 10th	Night	2 2 1 4 3 3 3 3 6% 6% 66%	2737
	11th to 20th	Night	1 2 1 3 3 2 3 2 4 4 74%	2926
	21st to end	Night	2 2 1 3 2 2 2 2 3 5% 77%	3498
September	1st to 10th	Night	2 1 1 2 2 2 2 2 3 5% 78%	1900
	11th to 20th	Night	2 3 2 3 3 3 4 2 5% 5% 69%	1900
	21st to end	Night	4 5% 1 3 3 3 3 2 4 6% 66%	2127
October	1st to 10th	Night	3 3 1 3 3 3 3 2 3 4 73%	3800
	11th to 20th	Night	5% 4 2 5% 3 4 4 3 5% 7% 58%	3777
	21st to end	Night	4 7% 2 7% 4 5% 5% 3 6% 8% 49%	4288
November	1st to 10th	Night	9% 10% 3 6% 5% 5% 5% 4 6% 7% 40%	2535
	11th to 20th	Night	8% 9% 3 9% 5% 7% 6% 5% 7% 10% 32%	2700
	21st to end	Night	7% 9% 3 6% 5% 5% 7% 5% 7% 10% 34%	2800
December	1st to 10th	Night	4 5% 2 7% 4 5% 5% 5% 8% 16% 38%	4560
	11th to 20th	Night	7% 14% 4 11% 9% 7% 7% 6% 8% 8% 18%	5176
	21st to end	Night	9% 12% 4 12% 8% 7% 7% 5% 8% 8% 21%	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	56% 11% 13% 15% 5%	3639
	11th to 20th	Day	46% 9% 9% 23% 13% 2	3640
	21st to end	Day	37% 7% 12% 15% 21% 8%	4322
February	1st to 10th	Day	47% 10% 16% 19% 9%	1800
	11th to 20th	Day	44% 9% 17% 19% 10% 2	1820
	21st to end	Day	43% 12% 15% 19% 10% 3	1650
March	1st to 10th	Day	43% 9% 12% 23% 11% 3	4518
	11th to 20th	Day	39% 7% 13% 31% 10%	4988
	21st to end	Day	46% 10% 15% 20% 9% 1	5783
April	1st to 10th	Day	47% 8% 13% 20% 10% 3	2400
	11th to 20th	Day	41% 6% 14% 20% 15% 3	2372
	21st to end	Day	36% 7% 15% 25% 15% 2	2400
May	1st to 10th	Day	30% 5% 10% 26% 25% 4	6293
	11th to 20th	Day	29% 4 11% 25% 26% 5%	6280
	21st to end	Day	25% 6% 14% 32% 22% 1	6904
June	1st to 10th	Day	35% 8% 11% 28% 16% 1	2789
	11th to 20th	Day	26% 5% 10% 29% 24% 6%	2660
	21st to end	Day	18% 3 8% 25% 32% 14%	2660
July	1st to 10th	Day	8% 2 5% 11% 17% 57%	4351
	11th to 20th	Day	13% 2 4 10% 20% 51%	4345
	21st to end	Day	10% 2 3 12% 35% 37%	4639
August	1st to 10th	Day	19% 3 5% 13% 27% 33%	4180
	11th to 20th	Day	13% 3 5% 20% 37% 23%	3953
	21st to end	Day	14% 2 5% 25% 29% 25%	4179
September	1st to 10th	Day	13% 3 5% 18% 34% 28%	2280
	11th to 20th	Day	22% 5% 11% 21% 26% 15%	2280
	21st to end	Day	24% 4 9% 24% 25% 13%	2052
October	1st to 10th	Day	21% 3 9% 18% 28% 21%	3180
	11th to 20th	Day	30% 5% 13% 30% 18% 4	3161
	21st to end	Day	38% 6% 10% 24% 19% 3	3273
November	1st to 10th	Day	40% 6% 14% 23% 16% 1	1764
	11th to 20th	Day	46% 9% 19% 15% 9% 1	1695
	21st to end	Day	41% 8% 16% 18% 15% 2	1600
December	1st to 10th	Day	32% 7% 10% 25% 24% 2	2620
	11th to 20th	Day	57% 9% 14% 15% 5%	3028
	21st to end	Day	57% 12% 11% 15% 5%	4004
January	1st to 10th	Night	55% 10% 14% 15% 5%	6306
	11th to 20th	Night	39% 7% 10% 25% 16% 2	6320
	21st to end	Night	41% 7% 11% 14% 17% 10%	6644
February	1st to 10th	Night	44% 9% 16% 18% 11% 1	2600
	11th to 20th	Night	42% 9% 15% 21% 11% 3	2477
	21st to end	Night	41% 11% 16% 21% 10% 2	1980
March	1st to 10th	Night	42% 9% 13% 24% 11% 3	5376
	11th to 20th	Night	34% 6% 13% 32% 16%	5010
	21st to end	Night	43% 12% 14% 21% 9%	5191
April	1st to 10th	Night	43% 8% 18% 20% 11% 1	2000
	11th to 20th	Night	38% 7% 15% 22% 16% 2	1979
	21st to end	Night	34% 8% 17% 26% 13% 2	1997
May	1st to 10th	Night	26% 6% 12% 25% 28% 3	3647
	11th to 20th	Night	30% 4 14% 27% 24% 2	3616
	21st to end	Night	24% 5% 19% 35% 16%	3972
June	1st to 10th	Night	33% 8% 15% 27% 17% 1	1595
	11th to 20th	Night	26% 6% 10% 33% 19% 5%	1520
	21st to end	Night	16% 4 11% 29% 31% 10%	1520
July	1st to 10th	Night	8% 3 5% 13% 19% 52%	2500
	11th to 20th	Night	11% 2 4 12% 26% 44%	2492
	21st to end	Night	10% 2 4 14% 34% 36%	2674
August	1st to 10th	Night	18% 3 5% 17% 34% 23%	2729
	11th to 20th	Night	14% 2 5% 22% 39% 18%	2953
	21st to end	Night	12% 2 7% 28% 25% 25%	3498
September	1st to 10th	Night	10% 2 5% 21% 34% 28%	1900
	11th to 20th	Night	18% 4 10% 24% 27% 16%	1900
	21st to end	Night	21% 4 9% 22% 26% 17%	2127
October	1st to 10th	Night	19% 3 7% 20% 26% 25%	3800
	11th to 20th	Night	27% 4 15% 28% 21% 5%	3777
	21st to end	Night	31% 5% 12% 29% 20% 3	4288
November	1st to 10th	Night	42% 7% 12% 20% 16% 3	2535
	11th to 20th	Night	44% 7% 16% 22% 10% 1	2700
	21st to end	Night	37% 10% 15% 18% 17% 3	2800
December	1st to 10th	Night	28% 8% 9% 26% 26% 2	4560
	11th to 20th	Night	55% 9% 15% 16% 5%	5176
	21st to end	Night	52% 10% 13% 17% 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	18% 19% 22% 26% 14% 3	200
	11th to 20th	Day	15% 13% 21% 29% 16% 7%	200
	21st to end	Day	10% 13% 24% 25% 22% 7%	356
February	1st to 10th	Day	10% 19% 25% 26% 16% 4	400
	11th to 20th	Day	15% 17% 25% 26% 13% 3	423
	21st to end	Day	8% 19% 27% 26% 16% 4	414
March	1st to 10th	Day	12% 19% 30% 27% 12% 1	500
	11th to 20th	Day	15% 16% 22% 28% 15% 4	500
	21st to end	Day	13% 18% 27% 27% 13% 2	550
April	1st to 10th	Day	12% 18% 24% 27% 13% 7%	500
	11th to 20th	Day	11% 18% 27% 24% 13% 7%	492
	21st to end	Day	8% 16% 26% 26% 17% 7%	500
May	1st to 10th	Day	10% 16% 23% 26% 18% 7%	503
	11th to 20th	Day	16% 14% 21% 22% 17% 9%	528
	21st to end	Day	11% 9% 19% 25% 23% 13%	573
June	1st to 10th	Day	9% 11% 20% 23% 23% 15%	527
	11th to 20th	Day	7% 17% 23% 24% 23% 6%	490
	21st to end	Day	3 5% 19% 28% 28% 17%	490
July	1st to 10th	Day	3 3 16% 33% 32% 13%	486
	11th to 20th	Day	2 3 19% 30% 32% 15%	484
	21st to end	Day	2 2 12% 30% 37% 17%	531
August	1st to 10th	Day	3 3 17% 26% 34% 18%	487
	11th to 20th	Day	1 4 15% 25% 31% 25%	477
	21st to end	Day	2 5% 13% 25% 35% 20%	539
September	1st to 10th	Day	2 3 13% 28% 35% 19%	490
	11th to 20th	Day	6% 7% 17% 26% 28% 17%	490
	21st to end	Day	7% 7% 17% 24% 31% 13%	490
October	1st to 10th	Day	11% 8% 19% 26% 21% 15%	490
	11th to 20th	Day	10% 11% 19% 21% 26% 13%	490
	21st to end	Day	10% 15% 23% 25% 18% 10%	439
November	1st to 10th	Day	14% 14% 18% 30% 15% 9%	300
	11th to 20th	Day	9% 13% 23% 28% 20% 6%	247
	21st to end	Day	14% 14% 15% 23% 29% 7%	200
December	1st to 10th	Day	9% 9% 24% 24% 23% 12%	200
	11th to 20th	Day	13% 16% 27% 30% 15% 1	200
	21st to end	Day	12% 21% 21% 26% 17% 3	220
January	1st to 10th	Night	14% 12% 26% 32% 15% 2	800
	11th to 20th	Night	9% 13% 23% 30% 22% 3	800
	21st to end	Night	12% 7% 23% 35% 21% 3	880
February	1st to 10th	Night	11% 9% 29% 34% 16% 1	800
	11th to 20th	Night	10% 11% 26% 34% 17% 1	784
	21st to end	Night	7% 11% 28% 38% 15% 1	660
March	1st to 10th	Night	11% 12% 29% 32% 14% 2	800
	11th to 20th	Night	11% 11% 23% 35% 17% 2	800
	21st to end	Night	9% 13% 31% 34% 14%	880
April	1st to 10th	Night	12% 10% 25% 36% 15% 1	800
	11th to 20th	Night	8% 10% 31% 31% 17% 3	791
	21st to end	Night	10% 10% 27% 36% 16% 1	797
May	1st to 10th	Night	9% 7% 29% 37% 17% 1	410
	11th to 20th	Night	10% 6% 23% 37% 23% 1	394
	21st to end	Night	10% 7% 23% 39% 19% 2	433
June	1st to 10th	Night	10% 4 23% 43% 19% 1	399
	11th to 20th	Night	8% 7% 31% 41% 13% 1	380
	21st to end	Night	3 2 25% 50% 20% 1	380
July	1st to 10th	Night	2 2 23% 51% 23%	370
	11th to 20th	Night	2 1 27% 53% 16%	368
	21st to end	Night	1 1 25% 52% 21%	416
August	1st to 10th	Night	3 1 15% 50% 28% 3	495
	11th to 20th	Night	2 1 13% 45% 36% 3	626
	21st to end	Night	3 3 14% 47% 31% 2	836
September	1st to 10th	Night	2 2 13% 46% 35% 3	760
	11th to 20th	Night	3 3 16% 36% 37% 5%	760
	21st to end	Night	6% 4 16% 35% 34% 5%	760
October	1st to 10th	Night	7% 3 20% 39% 29% 3	760
	11th to 20th	Night	8% 6% 18% 38% 28% 3	760
	21st to end	Night	10% 7% 20% 37% 21% 6%	836
November	1st to 10th	Night	12% 9% 22% 38% 18% 2	780
	11th to 20th	Night	12% 14% 23% 32% 18% 2	800
	21st to end	Night	12% 9% 20% 34% 21% 4	800
December	1st to 10th	Night	11% 9% 21% 34% 20% 5%	800
	11th to 20th	Night	12% 11% 24% 34% 19% 1	800
	21st to end	Night	13% 14% 30% 31% 12% 1	880

Page generated at 2025-06-07 07:28:18. Version ac07

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