White Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the White Mountain clear sky chart from 2005-11-06 to 2026-09-14.

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	3 5% 2 6% 5% 5% 5% 4 6% 8% 50%	158126
	Night	3 6% 2 7% 5% 4 5% 4 6% 8% 50%	150638
Transparency	Day	29% 7% 11% 22% 19% 11%	152925
	Night	29% 7% 13% 25% 18% 8%	146848
Seeing	Day	7% 11% 26% 29% 20% 8%	18123
	Night	6% 7% 25% 41% 19% 1	28183

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	2 7% 3 11% 7% 7% 7% 6% 8% 9% 32%	4937
	11th to 20th	Day	1 4 2 9% 5% 6% 6% 5% 7% 8% 47%	5340
	21st to end	Day	1 6% 4 14% 7% 7% 5% 5% 8% 7% 37%	5844
February	1st to 10th	Day	2 8% 4 8% 6% 6% 5% 4 6% 9% 42%	1890
	11th to 20th	Day	2 8% 4 9% 6% 6% 6% 4 7% 9% 38%	1845
	21st to end	Day	1 7% 3 9% 5% 5% 5% 5% 7% 7% 44%	1670
March	1st to 10th	Day	2 10% 4 9% 6% 5% 5% 5% 7% 9% 37%	5064
	11th to 20th	Day	1 8% 3 10% 7% 6% 7% 6% 7% 9% 36%	5110
	21st to end	Day	3 11% 4 9% 5% 5% 6% 5% 7% 9% 36%	5607
April	1st to 10th	Day	5% 10% 2 6% 5% 5% 4 4 7% 10% 44%	2467
	11th to 20th	Day	5% 9% 3 7% 5% 5% 5% 5% 6% 7% 42%	2491
	21st to end	Day	5% 8% 2 8% 6% 5% 5% 4 6% 9% 43%	2726
May	1st to 10th	Day	4 8% 2 6% 4 4 4 4 6% 7% 51%	7626
	11th to 20th	Day	6% 6% 2 5% 4 5% 5% 4 6% 8% 48%	7834
	21st to end	Day	6% 5% 1 6% 5% 5% 4 4 6% 8% 50%	9026
June	1st to 10th	Day	5% 4 1 5% 4 4 4 4 7% 8% 53%	2940
	11th to 20th	Day	5% 4 1 5% 4 4 4 3 5% 6% 59%	2940
	21st to end	Day	2 3 1 4 3 4 4 4 6% 7% 62%	2940
July	1st to 10th	Day	2 2 1 4 3 3 4 4 6% 6% 64%	8249
	11th to 20th	Day	2 2 1 4 4 4 5% 4 6% 8% 59%	7895
	21st to end	Day	3 3 2 5% 4 5% 5% 5% 7% 9% 53%	8304
August	1st to 10th	Day	3 3 1 4 3 4 4 4 6% 6% 63%	7549
	11th to 20th	Day	2 3 2 5% 4 5% 5% 5% 6% 8% 54%	7565
	21st to end	Day	3 4 2 5% 4 5% 6% 4 8% 9% 51%	7998
September	1st to 10th	Day	3 4 1 3 3 3 4 3 5% 6% 65%	2520
	11th to 20th	Day	3 2 2 5% 4 3 3 4 6% 8% 60%	2307
	21st to end	Day	5% 5% 1 3 2 3 3 2 4 5% 66%	2180
October	1st to 10th	Day	4 3 1 3 3 2 3 2 4 5% 70%	3270
	11th to 20th	Day	6% 6% 2 5% 3 3 3 3 6% 7% 57%	3250
	21st to end	Day	5% 7% 2 5% 4 3 3 3 5% 7% 55%	3548
November	1st to 10th	Day	3 5% 2 5% 5% 5% 4 4 7% 9% 51%	1936
	11th to 20th	Day	4 6% 3 6% 4 4 5% 3 7% 9% 50%	1680
	21st to end	Day	2 6% 3 7% 5% 5% 5% 4 6% 9% 49%	1680
December	1st to 10th	Day	2 5% 2 6% 4 4 3 4 7% 10% 53%	2700
	11th to 20th	Day	2 6% 2 7% 5% 5% 6% 5% 7% 11% 42%	3106
	21st to end	Day	2 9% 4 11% 8% 7% 8% 6% 7% 8% 29%	4092
January	1st to 10th	Night	2 7% 4 10% 6% 6% 6% 5% 8% 10% 36%	8042
	11th to 20th	Night	1 5% 3 9% 6% 4 4 4 6% 7% 51%	7650
	21st to end	Night	1 6% 3 12% 8% 5% 5% 5% 7% 7% 42%	8370
February	1st to 10th	Night	2 6% 3 8% 6% 6% 6% 6% 7% 10% 42%	2730
	11th to 20th	Night	2 8% 3 9% 5% 5% 5% 5% 6% 9% 42%	2662
	21st to end	Night	2 7% 4 7% 5% 5% 5% 5% 7% 9% 44%	2133
March	1st to 10th	Night	2 9% 4 8% 6% 5% 6% 5% 8% 8% 39%	5050
	11th to 20th	Night	2 7% 3 6% 5% 6% 7% 5% 6% 8% 45%	5110
	21st to end	Night	3 10% 4 9% 6% 5% 5% 4 6% 8% 41%	5610
April	1st to 10th	Night	4 9% 3 5% 4 3 6% 5% 6% 7% 48%	2153
	11th to 20th	Night	5% 9% 3 7% 5% 4 5% 5% 6% 6% 44%	2077
	21st to end	Night	5% 7% 3 7% 4 5% 6% 5% 7% 8% 45%	1890
May	1st to 10th	Night	4 8% 3 7% 5% 4 4 4 5% 7% 47%	5321
	11th to 20th	Night	5% 5% 2 5% 4 4 4 4 6% 8% 54%	5058
	21st to end	Night	5% 6% 2 6% 5% 5% 5% 4 6% 8% 47%	5181
June	1st to 10th	Night	4 4 2 3 3 3 4 5% 7% 8% 56%	1680
	11th to 20th	Night	4 4 2 4 3 3 4 3 5% 7% 62%	1680
	21st to end	Night	4 2 1 3 2 3 3 4 5% 7% 67%	1680
July	1st to 10th	Night	2 3 2 5% 4 4 5% 4 6% 7% 60%	4740
	11th to 20th	Night	2 2 1 5% 4 5% 5% 5% 6% 8% 56%	5091
	21st to end	Night	3 3 2 5% 4 5% 5% 4 8% 10% 51%	5805
August	1st to 10th	Night	1 1 1 3 3 3 3 3 6% 7% 68%	5273
	11th to 20th	Night	3 4 2 6% 4 6% 5% 3 5% 6% 57%	5291
	21st to end	Night	2 3 2 6% 4 5% 6% 4 7% 9% 51%	6218
September	1st to 10th	Night	4 3 1 3 2 2 3 3 5% 6% 68%	2100
	11th to 20th	Night	3 3 1 3 3 3 3 3 5% 7% 65%	2158
	21st to end	Night	4 3 1 3 2 3 3 3 4 7% 67%	2219
October	1st to 10th	Night	3 3 1 3 2 2 3 2 4 5% 71%	3920
	11th to 20th	Night	5% 5% 2 6% 4 3 3 3 4 6% 58%	3898
	21st to end	Night	5% 7% 2 5% 3 3 3 3 5% 8% 54%	4252
November	1st to 10th	Night	3 6% 2 5% 3 4 4 4 6% 8% 54%	2557
	11th to 20th	Night	4 6% 2 7% 4 4 4 4 6% 9% 51%	2940
	21st to end	Night	2 7% 3 6% 5% 4 3 4 5% 8% 52%	2940
December	1st to 10th	Night	1 5% 2 5% 4 4 4 4 6% 8% 56%	4710
	11th to 20th	Night	2 7% 3 7% 5% 5% 5% 5% 7% 10% 46%	5321
	21st to end	Night	2 8% 3 10% 7% 6% 7% 5% 8% 9% 34%	7128

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	40% 12% 22% 18% 7%	4940
	11th to 20th	Day	29% 10% 16% 29% 13% 3	5340
	21st to end	Day	41% 9% 14% 22% 14% 1	5844
February	1st to 10th	Day	38% 10% 18% 22% 11% 1	1890
	11th to 20th	Day	39% 9% 17% 24% 10% 1	1854
	21st to end	Day	32% 10% 19% 22% 15% 2	1670
March	1st to 10th	Day	35% 11% 14% 26% 9% 6%	5064
	11th to 20th	Day	37% 9% 16% 23% 15% 1	5110
	21st to end	Day	41% 9% 20% 22% 8% 1	5607
April	1st to 10th	Day	35% 8% 17% 23% 15% 3	2467
	11th to 20th	Day	37% 9% 14% 23% 16% 2	2494
	21st to end	Day	36% 9% 12% 24% 18% 1	2726
May	1st to 10th	Day	28% 7% 10% 23% 24% 8%	7626
	11th to 20th	Day	30% 6% 9% 23% 26% 6%	7838
	21st to end	Day	29% 7% 9% 26% 25% 5%	9027
June	1st to 10th	Day	26% 7% 13% 23% 24% 7%	2940
	11th to 20th	Day	23% 6% 9% 18% 27% 17%	2940
	21st to end	Day	19% 5% 7% 18% 29% 22%	2940
July	1st to 10th	Day	19% 5% 3 10% 24% 39%	8181
	11th to 20th	Day	22% 3 5% 17% 21% 31%	7821
	21st to end	Day	25% 5% 5% 18% 23% 24%	8120
August	1st to 10th	Day	20% 4 5% 14% 20% 35%	5978
	11th to 20th	Day	28% 5% 9% 23% 21% 13%	5964
	21st to end	Day	23% 4 12% 23% 22% 17%	6277
September	1st to 10th	Day	21% 4 8% 16% 29% 23%	2520
	11th to 20th	Day	21% 5% 8% 22% 28% 15%	2307
	21st to end	Day	21% 4 6% 23% 32% 13%	2180
October	1st to 10th	Day	17% 3 7% 20% 33% 20%	3270
	11th to 20th	Day	27% 4 9% 30% 24% 6%	3250
	21st to end	Day	29% 4 10% 35% 19% 2	3548
November	1st to 10th	Day	28% 8% 16% 28% 18% 2	1936
	11th to 20th	Day	28% 8% 22% 27% 14% 1	1680
	21st to end	Day	31% 8% 17% 27% 16% 1	1680
December	1st to 10th	Day	24% 7% 21% 34% 13% 1	2700
	11th to 20th	Day	32% 9% 25% 27% 7% 1	3104
	21st to end	Day	46% 10% 14% 18% 10% 3	4092
January	1st to 10th	Night	38% 9% 20% 23% 9%	8043
	11th to 20th	Night	30% 9% 18% 30% 12% 2	7650
	21st to end	Night	37% 10% 14% 22% 16% 1	8370
February	1st to 10th	Night	34% 12% 20% 23% 9% 2	2730
	11th to 20th	Night	35% 9% 16% 26% 12% 2	2663
	21st to end	Night	33% 10% 20% 21% 14% 2	2133
March	1st to 10th	Night	34% 10% 15% 28% 8% 6%	5050
	11th to 20th	Night	30% 12% 17% 29% 12% 1	5110
	21st to end	Night	38% 9% 20% 25% 8% 1	5610
April	1st to 10th	Night	32% 7% 16% 25% 15% 3	2153
	11th to 20th	Night	36% 7% 15% 24% 16% 1	2077
	21st to end	Night	32% 10% 14% 29% 13% 1	1890
May	1st to 10th	Night	32% 7% 9% 24% 24% 4	5319
	11th to 20th	Night	26% 7% 9% 25% 27% 7%	5059
	21st to end	Night	32% 5% 11% 29% 21% 1	5182
June	1st to 10th	Night	22% 6% 16% 25% 24% 8%	1680
	11th to 20th	Night	21% 5% 10% 20% 27% 18%	1680
	21st to end	Night	14% 4 7% 17% 32% 25%	1680
July	1st to 10th	Night	22% 4 4 11% 23% 36%	4700
	11th to 20th	Night	25% 3 6% 16% 22% 28%	5042
	21st to end	Night	24% 5% 6% 20% 26% 17%	5677
August	1st to 10th	Night	15% 4 5% 17% 21% 38%	4168
	11th to 20th	Night	30% 4 9% 22% 23% 13%	4168
	21st to end	Night	21% 3 14% 26% 20% 17%	4870
September	1st to 10th	Night	16% 4 9% 19% 28% 25%	2100
	11th to 20th	Night	18% 5% 11% 23% 29% 15%	2158
	21st to end	Night	17% 4 9% 26% 34% 10%	2219
October	1st to 10th	Night	16% 4 6% 19% 39% 16%	3920
	11th to 20th	Night	29% 3 7% 32% 22% 8%	3898
	21st to end	Night	28% 4 10% 35% 19% 3	4252
November	1st to 10th	Night	26% 8% 13% 30% 20% 3	2557
	11th to 20th	Night	28% 7% 19% 32% 12% 2	2940
	21st to end	Night	30% 6% 16% 30% 16% 2	2940
December	1st to 10th	Night	25% 7% 16% 37% 15% 1	4710
	11th to 20th	Night	32% 8% 23% 29% 7%	5322
	21st to end	Night	41% 10% 15% 20% 12% 1	7128

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	9% 13% 16% 38% 18% 6%	330
	11th to 20th	Day	6% 12% 26% 33% 19% 4	330
	21st to end	Day	6% 9% 27% 33% 20% 6%	363
February	1st to 10th	Day	8% 18% 29% 34% 8% 3	330
	11th to 20th	Day	6% 14% 31% 30% 16% 3	330
	21st to end	Day	9% 12% 26% 37% 14% 2	329
March	1st to 10th	Day	9% 12% 30% 31% 13% 5%	540
	11th to 20th	Day	10% 14% 26% 28% 19% 3	540
	21st to end	Day	12% 14% 27% 31% 14% 2	594
April	1st to 10th	Day	12% 12% 28% 27% 17% 4	540
	11th to 20th	Day	13% 18% 31% 24% 11% 3	532
	21st to end	Day	12% 21% 31% 17% 15% 5%	540
May	1st to 10th	Day	10% 18% 27% 21% 17% 6%	543
	11th to 20th	Day	11% 13% 26% 27% 17% 6%	650
	21st to end	Day	9% 11% 25% 26% 20% 9%	844
June	1st to 10th	Day	6% 10% 22% 28% 24% 10%	780
	11th to 20th	Day	7% 12% 29% 31% 18% 3	780
	21st to end	Day	4 8% 28% 31% 24% 5%	780
July	1st to 10th	Day	5% 3 20% 34% 30% 8%	771
	11th to 20th	Day	3 5% 24% 29% 28% 10%	648
	21st to end	Day	5% 3 22% 30% 25% 15%	619
August	1st to 10th	Day	6% 5% 24% 29% 25% 12%	567
	11th to 20th	Day	3 4 25% 30% 26% 11%	557
	21st to end	Day	4 7% 25% 28% 20% 16%	627
September	1st to 10th	Day	4 6% 26% 26% 24% 14%	570
	11th to 20th	Day	5% 9% 31% 24% 18% 13%	543
	21st to end	Day	7% 9% 24% 22% 24% 14%	510
October	1st to 10th	Day	8% 8% 19% 25% 23% 18%	330
	11th to 20th	Day	10% 15% 22% 23% 21% 9%	330
	21st to end	Day	7% 15% 26% 29% 17% 4	363
November	1st to 10th	Day	8% 14% 25% 31% 15% 7%	330
	11th to 20th	Day	7% 11% 24% 36% 18% 5%	330
	21st to end	Day	6% 9% 25% 35% 20% 5%	330
December	1st to 10th	Day	8% 13% 21% 35% 18% 6%	330
	11th to 20th	Day	7% 12% 20% 43% 15% 2	330
	21st to end	Day	9% 11% 30% 31% 12% 7%	363
January	1st to 10th	Night	6% 9% 27% 40% 18% 1	995
	11th to 20th	Night	5% 10% 31% 38% 15% 2	840
	21st to end	Night	4 7% 31% 44% 13% 1	924
February	1st to 10th	Night	4 10% 37% 37% 10% 2	840
	11th to 20th	Night	4 10% 32% 42% 10% 2	824
	21st to end	Night	5% 10% 33% 43% 9%	692
March	1st to 10th	Night	6% 7% 29% 43% 13% 1	840
	11th to 20th	Night	6% 8% 27% 44% 15%	840
	21st to end	Night	8% 7% 34% 40% 12%	924
April	1st to 10th	Night	9% 10% 30% 39% 11%	683
	11th to 20th	Night	10% 11% 30% 40% 9% 1	623
	21st to end	Night	7% 12% 30% 36% 14% 1	630
May	1st to 10th	Night	9% 10% 29% 35% 15% 2	630
	11th to 20th	Night	10% 7% 24% 39% 18% 2	621
	21st to end	Night	7% 8% 23% 40% 20% 2	683
June	1st to 10th	Night	6% 5% 24% 41% 22% 1	630
	11th to 20th	Night	6% 8% 27% 43% 16%	630
	21st to end	Night	3 3 23% 46% 23% 1	630
July	1st to 10th	Night	3 2 14% 47% 33% 1	615
	11th to 20th	Night	3 2 16% 46% 31% 1	612
	21st to end	Night	3 1 11% 48% 35% 2	690
August	1st to 10th	Night	3 2 13% 47% 34% 1	627
	11th to 20th	Night	3 2 13% 47% 34% 1	624
	21st to end	Night	4 3 13% 42% 34% 3	693
September	1st to 10th	Night	5% 4 16% 42% 30% 3	630
	11th to 20th	Night	4 3 19% 44% 26% 4	737
	21st to end	Night	5% 5% 18% 37% 32% 5%	800
October	1st to 10th	Night	7% 5% 25% 36% 24% 3	800
	11th to 20th	Night	7% 8% 22% 39% 22% 2	800
	21st to end	Night	6% 6% 26% 43% 18% 2	880
November	1st to 10th	Night	6% 8% 25% 43% 18% 1	841
	11th to 20th	Night	6% 9% 29% 42% 13% 1	1050
	21st to end	Night	6% 9% 24% 44% 16% 1	1050
December	1st to 10th	Night	6% 12% 28% 42% 12% 1	1050
	11th to 20th	Night	9% 8% 29% 43% 12%	1050
	21st to end	Night	6% 13% 30% 38% 11% 2	1155

Page generated at 2026-09-11 02:11:57. Version ac07

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