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 2026-10-14.

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	4 6% 2 6% 4 4 4 4 5% 7% 53%	158651
	Night	5% 7% 3 6% 5% 4 5% 4 6% 7% 49%	150466
Transparency	Day	30% 5% 9% 21% 21% 13%	153494
	Night	32% 6% 11% 22% 20% 9%	146629
Seeing	Day	9% 11% 20% 26% 23% 11%	17186
	Night	8% 8% 23% 38% 21% 2	26230

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% 14% 4 10% 8% 7% 7% 6% 7% 8% 21%	4722
	11th to 20th	Day	6% 8% 3 7% 5% 4 4 4 5% 9% 45%	4740
	21st to end	Day	4 8% 3 9% 5% 6% 6% 4 6% 8% 41%	5599
February	1st to 10th	Day	6% 10% 4 9% 6% 6% 5% 6% 8% 9% 31%	1890
	11th to 20th	Day	5% 12% 4 9% 6% 6% 5% 5% 7% 8% 34%	1904
	21st to end	Day	4 11% 4 9% 6% 6% 7% 5% 8% 9% 32%	1730
March	1st to 10th	Day	5% 11% 4 10% 6% 6% 7% 5% 7% 9% 30%	4618
	11th to 20th	Day	3 11% 3 8% 6% 6% 5% 4 6% 8% 39%	5097
	21st to end	Day	5% 10% 4 10% 7% 7% 6% 5% 7% 7% 31%	5911
April	1st to 10th	Day	7% 12% 4 10% 5% 5% 5% 4 5% 8% 35%	2520
	11th to 20th	Day	7% 10% 4 7% 5% 5% 4 4 6% 7% 41%	2489
	21st to end	Day	6% 10% 3 6% 5% 5% 4 3 5% 7% 46%	2520
May	1st to 10th	Day	6% 6% 2 5% 4 4 4 4 5% 6% 54%	8155
	11th to 20th	Day	4 5% 2 5% 4 4 4 4 5% 7% 58%	8160
	21st to end	Day	6% 5% 2 5% 4 3 4 3 5% 8% 54%	8992
June	1st to 10th	Day	9% 7% 3 6% 4 5% 5% 4 6% 6% 46%	2940
	11th to 20th	Day	5% 4 2 4 4 3 4 3 5% 6% 59%	2940
	21st to end	Day	3 2 1 3 3 3 3 3 4 6% 68%	2940
July	1st to 10th	Day	1 1 1 2 2 2 3 2 3 4 78%	8219
	11th to 20th	Day	2 3 1 3 2 3 3 2 3 4 73%	8220
	21st to end	Day	1 2 1 2 2 3 3 2 3 5% 76%	8922
August	1st to 10th	Day	2 2 1 3 2 2 3 3 4 6% 72%	7742
	11th to 20th	Day	2 2 1 2 2 2 2 2 4 6% 75%	7360
	21st to end	Day	3 3 1 4 3 3 4 3 4 6% 67%	7750
September	1st to 10th	Day	3 2 1 3 2 2 3 2 3 5% 74%	2520
	11th to 20th	Day	5% 4 2 5% 3 3 4 3 5% 6% 60%	2520
	21st to end	Day	5% 4 2 5% 3 4 3 3 3 6% 62%	2267
October	1st to 10th	Day	4 4 2 3 3 2 2 3 4 5% 67%	3380
	11th to 20th	Day	4 6% 2 6% 4 4 5% 4 7% 9% 50%	3292
	21st to end	Day	6% 7% 2 8% 6% 6% 6% 5% 6% 7% 41%	3378
November	1st to 10th	Day	7% 9% 3 8% 5% 5% 4 4 8% 8% 38%	1854
	11th to 20th	Day	7% 8% 3 10% 6% 7% 7% 7% 6% 8% 32%	1780
	21st to end	Day	5% 10% 3 8% 7% 6% 6% 5% 6% 11% 34%	1680
December	1st to 10th	Day	3 6% 2 8% 5% 5% 5% 4 9% 11% 41%	2700
	11th to 20th	Day	9% 14% 4 12% 8% 6% 7% 6% 8% 8% 19%	3108
	21st to end	Day	7% 12% 5% 13% 9% 8% 8% 5% 8% 8% 17%	4092
January	1st to 10th	Night	5% 12% 4 12% 8% 7% 8% 6% 8% 9% 20%	8172
	11th to 20th	Night	4 7% 2 6% 6% 4 3 3 5% 11% 48%	8220
	21st to end	Night	5% 9% 4 9% 6% 6% 5% 5% 6% 7% 39%	8599
February	1st to 10th	Night	6% 12% 4 9% 5% 5% 6% 5% 7% 8% 35%	2730
	11th to 20th	Night	5% 11% 5% 9% 6% 5% 5% 4 6% 8% 37%	2602
	21st to end	Night	4 7% 5% 9% 7% 7% 7% 5% 8% 8% 34%	2076
March	1st to 10th	Night	4 12% 4 8% 6% 6% 7% 6% 9% 10% 28%	5496
	11th to 20th	Night	4 9% 2 7% 4 5% 6% 5% 8% 9% 42%	5121
	21st to end	Night	3 9% 4 10% 6% 7% 7% 5% 8% 8% 33%	5305
April	1st to 10th	Night	7% 11% 3 8% 5% 5% 4 4 6% 8% 39%	2100
	11th to 20th	Night	6% 9% 3 7% 6% 6% 4 4 5% 7% 44%	2079
	21st to end	Night	7% 9% 3 5% 3 3 4 4 6% 8% 47%	2097
May	1st to 10th	Night	5% 5% 2 4 3 3 4 3 5% 6% 60%	4747
	11th to 20th	Night	5% 6% 2 5% 3 4 4 3 5% 6% 59%	4700
	21st to end	Night	8% 5% 2 4 3 3 3 3 5% 6% 58%	5181
June	1st to 10th	Night	8% 7% 2 5% 4 4 4 3 5% 7% 50%	1680
	11th to 20th	Night	7% 5% 2 4 3 4 3 3 4 6% 60%	1680
	21st to end	Night	3 3 1 3 2 3 3 2 4 5% 71%	1680
July	1st to 10th	Night	1 2 1 3 2 2 2 2 3 3 79%	4740
	11th to 20th	Night	2 2 1 3 2 2 2 2 3 4 77%	4740
	21st to end	Night	3 2 1 3 2 3 3 2 3 5% 74%	5153
August	1st to 10th	Night	2 2 1 3 2 3 3 3 5% 6% 71%	5054
	11th to 20th	Night	2 2 1 2 2 2 3 2 4 5% 75%	5504
	21st to end	Night	3 3 1 5% 3 3 3 3 4 5% 68%	6499
September	1st to 10th	Night	2 2 1 3 2 3 2 2 3 5% 75%	2100
	11th to 20th	Night	3 3 2 3 3 3 4 2 5% 5% 68%	2100
	21st to end	Night	4 5% 1 4 3 3 3 2 4 6% 66%	2351
October	1st to 10th	Night	3 3 1 3 3 3 3 2 3 4 72%	4040
	11th to 20th	Night	5% 5% 2 5% 3 4 4 3 5% 7% 58%	3933
	21st to end	Night	4 6% 2 6% 4 4 5% 3 6% 8% 50%	4425
November	1st to 10th	Night	9% 10% 3 6% 5% 5% 5% 4 6% 7% 40%	2665
	11th to 20th	Night	8% 10% 3 9% 6% 7% 6% 5% 6% 9% 31%	2835
	21st to end	Night	7% 9% 3 6% 5% 5% 7% 5% 7% 11% 35%	2940
December	1st to 10th	Night	4 5% 2 7% 4 5% 5% 5% 8% 16% 38%	4700
	11th to 20th	Night	7% 14% 4 11% 8% 7% 7% 6% 8% 8% 19%	5316
	21st to end	Night	9% 12% 4 12% 8% 7% 7% 5% 8% 8% 21%	7106

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	54% 9% 15% 17% 4	4723
	11th to 20th	Day	35% 7% 7% 19% 23% 9%	4740
	21st to end	Day	39% 6% 11% 15% 21% 8%	5599
February	1st to 10th	Day	45% 9% 15% 19% 11%	1890
	11th to 20th	Day	44% 8% 17% 19% 10% 2	1913
	21st to end	Day	44% 11% 15% 19% 10% 2	1730
March	1st to 10th	Day	43% 9% 12% 23% 11% 2	4618
	11th to 20th	Day	39% 7% 13% 30% 11%	5097
	21st to end	Day	46% 9% 15% 20% 9% 1	5911
April	1st to 10th	Day	46% 7% 13% 21% 10% 3	2520
	11th to 20th	Day	41% 6% 14% 20% 15% 3	2492
	21st to end	Day	36% 7% 14% 25% 16% 2	2520
May	1st to 10th	Day	28% 5% 8% 26% 29% 5%	8155
	11th to 20th	Day	25% 3 11% 26% 29% 5%	8163
	21st to end	Day	28% 5% 12% 31% 22% 2	8994
June	1st to 10th	Day	35% 8% 11% 27% 17% 2	2940
	11th to 20th	Day	25% 5% 10% 28% 26% 7%	2940
	21st to end	Day	19% 3 8% 24% 32% 14%	2940
July	1st to 10th	Day	12% 2 4 16% 26% 40%	8151
	11th to 20th	Day	17% 2 3 8% 22% 48%	8145
	21st to end	Day	13% 2 3 17% 32% 32%	8723
August	1st to 10th	Day	13% 2 4 10% 23% 48%	6135
	11th to 20th	Day	13% 3 4 24% 38% 18%	5807
	21st to end	Day	15% 2 4 25% 34% 20%	6077
September	1st to 10th	Day	15% 3 5% 19% 33% 26%	2520
	11th to 20th	Day	22% 5% 10% 22% 27% 15%	2520
	21st to end	Day	24% 4 8% 24% 25% 14%	2267
October	1st to 10th	Day	21% 3 8% 18% 29% 21%	3380
	11th to 20th	Day	29% 5% 13% 30% 19% 5%	3292
	21st to end	Day	38% 6% 10% 25% 19% 3	3378
November	1st to 10th	Day	40% 6% 15% 23% 15% 1	1854
	11th to 20th	Day	46% 9% 19% 16% 9% 1	1780
	21st to end	Day	40% 8% 17% 19% 14% 2	1680
December	1st to 10th	Day	32% 7% 10% 24% 24% 2	2700
	11th to 20th	Day	56% 9% 13% 15% 6% 1	3108
	21st to end	Day	56% 12% 11% 16% 5%	4092
January	1st to 10th	Night	53% 9% 17% 17% 4	8172
	11th to 20th	Night	31% 6% 8% 21% 24% 10%	8220
	21st to end	Night	42% 6% 10% 14% 19% 9%	8599
February	1st to 10th	Night	43% 9% 16% 19% 12% 2	2730
	11th to 20th	Night	43% 8% 15% 21% 11% 2	2604
	21st to end	Night	41% 11% 16% 20% 11% 1	2076
March	1st to 10th	Night	42% 9% 13% 24% 11% 3	5496
	11th to 20th	Night	33% 6% 13% 32% 16% 1	5121
	21st to end	Night	43% 12% 14% 21% 9% 1	5305
April	1st to 10th	Night	43% 7% 17% 21% 11% 1	2100
	11th to 20th	Night	38% 7% 15% 21% 16% 2	2079
	21st to end	Night	34% 8% 17% 26% 14% 2	2097
May	1st to 10th	Night	24% 5% 10% 27% 32% 3	4745
	11th to 20th	Night	27% 4 15% 28% 25% 1	4700
	21st to end	Night	27% 5% 16% 34% 19%	5182
June	1st to 10th	Night	33% 7% 14% 27% 18% 1	1680
	11th to 20th	Night	26% 5% 9% 32% 22% 6%	1680
	21st to end	Night	17% 3 11% 29% 31% 10%	1680
July	1st to 10th	Night	13% 2 3 20% 31% 31%	4700
	11th to 20th	Night	14% 1 3 12% 25% 45%	4692
	21st to end	Night	16% 2 4 20% 29% 29%	5040
August	1st to 10th	Night	12% 2 4 13% 31% 38%	3999
	11th to 20th	Night	13% 2 6% 30% 36% 13%	4329
	21st to end	Night	16% 2 6% 25% 33% 19%	5092
September	1st to 10th	Night	14% 3 5% 20% 32% 26%	2100
	11th to 20th	Night	19% 3 10% 24% 28% 16%	2100
	21st to end	Night	21% 4 9% 24% 26% 17%	2351
October	1st to 10th	Night	19% 3 7% 20% 26% 24%	4040
	11th to 20th	Night	27% 4 15% 28% 21% 5%	3933
	21st to end	Night	31% 5% 12% 30% 20% 3	4425
November	1st to 10th	Night	42% 7% 12% 20% 15% 3	2665
	11th to 20th	Night	45% 7% 16% 21% 10% 1	2835
	21st to end	Night	37% 9% 16% 18% 17% 3	2940
December	1st to 10th	Night	29% 8% 10% 26% 25% 2	4700
	11th to 20th	Night	55% 9% 14% 16% 6%	5316
	21st to end	Night	52% 10% 13% 17% 7%	7106

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% 20% 22% 24% 14% 3	220
	11th to 20th	Day	14% 11% 19% 27% 19% 10%	220
	21st to end	Day	10% 12% 23% 26% 23% 6%	385
February	1st to 10th	Day	10% 18% 25% 26% 17% 5%	430
	11th to 20th	Day	14% 17% 25% 27% 13% 4	456
	21st to end	Day	10% 19% 26% 26% 15% 3	446
March	1st to 10th	Day	12% 19% 30% 27% 11% 1	540
	11th to 20th	Day	15% 17% 22% 28% 14% 4	540
	21st to end	Day	13% 18% 27% 27% 13% 2	594
April	1st to 10th	Day	12% 17% 23% 27% 14% 8%	540
	11th to 20th	Day	11% 18% 27% 24% 13% 6%	532
	21st to end	Day	9% 16% 25% 28% 17% 6%	540
May	1st to 10th	Day	9% 15% 23% 27% 18% 7%	543
	11th to 20th	Day	16% 15% 21% 22% 18% 9%	568
	21st to end	Day	12% 9% 18% 24% 23% 13%	617
June	1st to 10th	Day	8% 11% 21% 22% 23% 14%	570
	11th to 20th	Day	6% 15% 21% 24% 26% 7%	570
	21st to end	Day	3 6% 20% 26% 29% 17%	570
July	1st to 10th	Day	3 3 17% 31% 32% 15%	566
	11th to 20th	Day	2 3 19% 30% 32% 15%	564
	21st to end	Day	2 2 13% 29% 37% 18%	619
August	1st to 10th	Day	2 3 16% 25% 34% 20%	567
	11th to 20th	Day	2 3 14% 25% 32% 24%	557
	21st to end	Day	2 5% 14% 24% 35% 20%	627
September	1st to 10th	Day	4 3 14% 28% 33% 18%	570
	11th to 20th	Day	5% 6% 17% 25% 27% 19%	570
	21st to end	Day	7% 7% 16% 24% 31% 15%	570
October	1st to 10th	Day	10% 8% 18% 25% 22% 17%	570
	11th to 20th	Day	10% 11% 19% 23% 24% 12%	543
	21st to end	Day	9% 15% 21% 24% 20% 10%	478
November	1st to 10th	Day	14% 14% 18% 30% 15% 9%	330
	11th to 20th	Day	9% 13% 23% 29% 19% 7%	272
	21st to end	Day	13% 12% 15% 24% 29% 7%	220
December	1st to 10th	Day	9% 10% 26% 23% 22% 10%	220
	11th to 20th	Day	14% 16% 25% 28% 15% 2	220
	21st to end	Day	12% 22% 20% 25% 17% 4	242
January	1st to 10th	Night	13% 12% 26% 31% 15% 2	840
	11th to 20th	Night	9% 12% 22% 31% 23% 3	840
	21st to end	Night	11% 6% 22% 36% 21% 3	924
February	1st to 10th	Night	11% 9% 29% 34% 16% 1	840
	11th to 20th	Night	10% 11% 27% 35% 16% 1	824
	21st to end	Night	7% 12% 29% 38% 14% 1	692
March	1st to 10th	Night	11% 12% 29% 33% 14% 1	840
	11th to 20th	Night	11% 11% 24% 36% 16% 2	840
	21st to end	Night	8% 13% 31% 34% 13%	924
April	1st to 10th	Night	12% 10% 25% 36% 16% 1	840
	11th to 20th	Night	8% 11% 30% 32% 16% 3	831
	21st to end	Night	10% 10% 26% 37% 16% 1	837
May	1st to 10th	Night	9% 7% 28% 38% 17% 1	431
	11th to 20th	Night	10% 6% 24% 37% 22% 1	414
	21st to end	Night	11% 7% 23% 39% 19% 2	455
June	1st to 10th	Night	10% 5% 24% 42% 19% 1	420
	11th to 20th	Night	7% 7% 29% 43% 13% 1	420
	21st to end	Night	3 2 24% 50% 19% 1	420
July	1st to 10th	Night	2 2 23% 52% 22%	410
	11th to 20th	Night	2 1 26% 55% 15%	408
	21st to end	Night	1 1 25% 53% 20%	460
August	1st to 10th	Night	3 1 15% 51% 29% 2	547
	11th to 20th	Night	2 1 13% 46% 35% 3	693
	21st to end	Night	3 3 15% 47% 30% 2	924
September	1st to 10th	Night	3 2 13% 46% 34% 3	840
	11th to 20th	Night	3 3 17% 36% 36% 4	840
	21st to end	Night	6% 4 16% 35% 35% 5%	840
October	1st to 10th	Night	7% 3 19% 38% 30% 3	840
	11th to 20th	Night	8% 6% 19% 38% 26% 3	812
	21st to end	Night	10% 7% 19% 37% 21% 6%	880
November	1st to 10th	Night	12% 10% 21% 37% 18% 2	820
	11th to 20th	Night	12% 13% 23% 32% 18% 2	840
	21st to end	Night	11% 9% 20% 35% 21% 4	840
December	1st to 10th	Night	11% 9% 22% 34% 20% 4	840
	11th to 20th	Night	12% 11% 23% 33% 19% 1	840
	21st to end	Night	13% 15% 29% 30% 12% 1	924

Page generated at 2026-10-11 02:06:27. Version ac07

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