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 2025-05-11.

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 6% 2 7% 5% 5% 5% 4 6% 8% 49%	118076
	Night	3 6% 3 7% 5% 4 5% 4 6% 8% 49%	119832
Transparency	Day	30% 8% 13% 21% 19% 10%	117832
	Night	29% 8% 14% 24% 17% 7%	119666
Seeing	Day	7% 10% 25% 30% 20% 8%	16190
	Night	6% 7% 25% 42% 19% 1	26451

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	1 6% 3 12% 7% 7% 8% 7% 9% 10% 31%	3799
	11th to 20th	Day	1 5% 3 11% 7% 7% 7% 7% 8% 9% 36%	4100
	21st to end	Day	1 7% 4 15% 8% 7% 5% 6% 9% 7% 30%	4510
February	1st to 10th	Day	2 8% 4 8% 6% 6% 5% 5% 6% 9% 41%	1800
	11th to 20th	Day	2 8% 4 10% 6% 6% 6% 4 7% 9% 38%	1755
	21st to end	Day	1 7% 3 9% 5% 5% 5% 6% 7% 7% 43%	1592
March	1st to 10th	Day	2 10% 4 8% 6% 5% 5% 5% 7% 10% 37%	4954
	11th to 20th	Day	1 8% 3 10% 7% 6% 7% 6% 7% 9% 36%	5000
	21st to end	Day	3 11% 4 9% 5% 5% 6% 5% 7% 8% 36%	5486
April	1st to 10th	Day	5% 10% 2 6% 5% 5% 4 4 7% 10% 43%	2350
	11th to 20th	Day	5% 9% 3 8% 5% 5% 5% 5% 6% 7% 42%	2371
	21st to end	Day	5% 8% 2 8% 6% 5% 4 4 6% 9% 43%	2596
May	1st to 10th	Day	5% 12% 3 8% 5% 5% 5% 4 6% 8% 39%	4240
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 4 7% 8% 50%	4199
	21st to end	Day	6% 6% 2 5% 5% 5% 5% 5% 6% 7% 49%	4825
June	1st to 10th	Day	5% 4 1 5% 4 4 4 5% 7% 8% 52%	2660
	11th to 20th	Day	5% 4 2 5% 4 3 4 3 6% 7% 58%	2660
	21st to end	Day	2 3 1 4 3 4 4 4 6% 7% 63%	2660
July	1st to 10th	Day	2 2 1 3 2 3 4 3 4 5% 72%	4430
	11th to 20th	Day	2 2 1 4 4 4 5% 4 6% 8% 58%	4243
	21st to end	Day	3 3 1 4 3 4 4 4 7% 7% 59%	4454
August	1st to 10th	Day	3 3 1 4 3 5% 5% 5% 7% 8% 56%	4092
	11th to 20th	Day	3 3 1 4 4 5% 6% 5% 6% 8% 56%	4028
	21st to end	Day	2 2 1 3 3 3 4 3 6% 8% 65%	4314
September	1st to 10th	Day	3 4 1 3 3 3 4 3 4 6% 65%	2280
	11th to 20th	Day	3 2 1 5% 4 3 3 4 6% 7% 61%	2157
	21st to end	Day	5% 5% 1 3 2 2 3 2 3 5% 68%	2071
October	1st to 10th	Day	3 3 1 3 3 2 3 2 4 5% 71%	3170
	11th to 20th	Day	6% 6% 2 5% 3 3 3 3 6% 7% 56%	3150
	21st to end	Day	5% 7% 2 5% 4 3 3 3 5% 7% 55%	3438
November	1st to 10th	Day	3 5% 2 5% 5% 5% 4 4 8% 8% 50%	1842
	11th to 20th	Day	4 6% 3 6% 4 4 5% 3 7% 9% 51%	1600
	21st to end	Day	2 6% 3 7% 5% 6% 5% 4 6% 9% 49%	1600
December	1st to 10th	Day	2 5% 2 6% 4 3 3 4 7% 10% 54%	2620
	11th to 20th	Day	2 6% 2 7% 5% 5% 7% 5% 7% 11% 42%	3026
	21st to end	Day	2 9% 4 11% 8% 7% 8% 6% 7% 8% 29%	4004
January	1st to 10th	Night	1 6% 3 10% 6% 6% 7% 6% 9% 10% 34%	6210
	11th to 20th	Night	1 6% 4 12% 8% 6% 5% 5% 7% 9% 38%	5880
	21st to end	Night	2 7% 4 14% 9% 5% 5% 5% 7% 7% 36%	6468
February	1st to 10th	Night	2 6% 3 8% 6% 6% 6% 6% 7% 10% 42%	2600
	11th to 20th	Night	2 8% 3 9% 5% 5% 5% 5% 6% 9% 43%	2532
	21st to end	Night	2 7% 4 8% 6% 5% 5% 5% 7% 9% 43%	2035
March	1st to 10th	Night	2 9% 4 8% 6% 5% 6% 5% 8% 8% 39%	4940
	11th to 20th	Night	2 7% 3 6% 5% 6% 7% 5% 6% 8% 45%	5000
	21st to end	Night	3 10% 4 9% 6% 5% 5% 4 6% 8% 41%	5489
April	1st to 10th	Night	4 9% 3 5% 4 3 6% 5% 6% 7% 48%	2050
	11th to 20th	Night	6% 9% 3 7% 5% 4 5% 5% 6% 6% 44%	1977
	21st to end	Night	5% 7% 3 6% 4 5% 5% 5% 7% 8% 45%	1800
May	1st to 10th	Night	6% 11% 3 8% 5% 5% 4 4 5% 7% 43%	2950
	11th to 20th	Night	5% 5% 2 6% 5% 4 4 4 7% 8% 52%	2712
	21st to end	Night	5% 6% 2 6% 4 4 4 3 6% 7% 54%	2762
June	1st to 10th	Night	4 5% 2 3 3 4 4 6% 7% 8% 54%	1520
	11th to 20th	Night	5% 3 2 4 3 3 4 3 5% 7% 61%	1520
	21st to end	Night	3 2 1 3 2 3 3 4 5% 7% 67%	1520
July	1st to 10th	Night	3 2 1 3 3 3 4 3 5% 7% 65%	2540
	11th to 20th	Night	2 2 1 5% 5% 4 5% 5% 7% 9% 56%	2723
	21st to end	Night	2 2 1 4 3 4 5% 4 8% 9% 58%	3109
August	1st to 10th	Night	2 2 1 4 4 3 4 3 7% 8% 62%	2847
	11th to 20th	Night	3 3 1 5% 4 5% 4 3 5% 6% 60%	2811
	21st to end	Night	2 2 1 3 3 3 3 3 5% 7% 70%	3353
September	1st to 10th	Night	4 2 1 2 2 2 3 3 5% 6% 69%	1900
	11th to 20th	Night	3 3 1 3 3 3 3 3 5% 7% 66%	2021
	21st to end	Night	4 3 1 3 2 3 3 3 4 6% 68%	2108
October	1st to 10th	Night	3 3 1 3 2 2 3 2 4 5% 71%	3800
	11th to 20th	Night	5% 5% 2 6% 4 3 3 3 4 6% 58%	3778
	21st to end	Night	5% 7% 2 5% 3 3 3 4 5% 9% 54%	4120
November	1st to 10th	Night	3 6% 2 6% 3 4 4 4 6% 8% 53%	2432
	11th to 20th	Night	3 6% 2 7% 4 4 4 4 6% 8% 51%	2800
	21st to end	Night	2 7% 3 6% 5% 4 3 3 5% 8% 51%	2800
December	1st to 10th	Night	1 5% 2 5% 4 4 4 4 7% 8% 57%	4570
	11th to 20th	Night	2 7% 3 7% 5% 5% 5% 5% 7% 10% 45%	5181
	21st to end	Night	2 8% 3 10% 7% 7% 7% 5% 8% 9% 34%	6974

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	39% 15% 20% 17% 9% 1	3800
	11th to 20th	Day	36% 13% 15% 23% 12% 1	4100
	21st to end	Day	46% 11% 13% 17% 12% 1	4510
February	1st to 10th	Day	39% 11% 18% 21% 11% 1	1800
	11th to 20th	Day	38% 10% 18% 24% 9% 1	1764
	21st to end	Day	33% 10% 20% 21% 14% 3	1592
March	1st to 10th	Day	35% 11% 14% 26% 9% 6%	4954
	11th to 20th	Day	37% 10% 16% 22% 14%	5000
	21st to end	Day	41% 9% 20% 22% 8%	5486
April	1st to 10th	Day	35% 8% 17% 22% 15% 3	2350
	11th to 20th	Day	37% 9% 15% 23% 15% 2	2374
	21st to end	Day	35% 9% 13% 24% 18% 1	2596
May	1st to 10th	Day	36% 11% 12% 20% 17% 6%	4240
	11th to 20th	Day	25% 9% 13% 30% 18% 5%	4203
	21st to end	Day	27% 10% 11% 25% 24% 3	4825
June	1st to 10th	Day	26% 8% 14% 23% 22% 7%	2660
	11th to 20th	Day	24% 7% 9% 19% 26% 15%	2660
	21st to end	Day	18% 5% 7% 18% 28% 24%	2660
July	1st to 10th	Day	15% 4 5% 11% 21% 43%	4361
	11th to 20th	Day	22% 5% 4 13% 23% 33%	4169
	21st to end	Day	22% 5% 6% 13% 25% 30%	4314
August	1st to 10th	Day	23% 6% 8% 18% 25% 21%	4078
	11th to 20th	Day	24% 5% 8% 18% 26% 18%	4067
	21st to end	Day	18% 4 8% 16% 30% 24%	4313
September	1st to 10th	Day	21% 4 8% 15% 28% 24%	2280
	11th to 20th	Day	21% 6% 8% 21% 28% 16%	2157
	21st to end	Day	20% 4 6% 24% 33% 13%	2071
October	1st to 10th	Day	17% 4 7% 20% 33% 20%	3170
	11th to 20th	Day	27% 4 9% 30% 24% 6%	3150
	21st to end	Day	29% 4 11% 35% 18% 2	3438
November	1st to 10th	Day	29% 8% 17% 27% 18% 2	1842
	11th to 20th	Day	28% 8% 22% 26% 15% 1	1600
	21st to end	Day	31% 9% 17% 27% 16% 1	1600
December	1st to 10th	Day	24% 7% 20% 35% 13% 1	2620
	11th to 20th	Day	33% 9% 25% 26% 7%	3024
	21st to end	Day	47% 10% 14% 17% 10% 3	4004
January	1st to 10th	Night	38% 11% 19% 23% 8% 1	6210
	11th to 20th	Night	38% 11% 15% 24% 10% 1	5880
	21st to end	Night	42% 12% 14% 19% 13% 1	6468
February	1st to 10th	Night	35% 12% 21% 22% 9% 2	2600
	11th to 20th	Night	34% 10% 17% 25% 12% 2	2533
	21st to end	Night	34% 10% 21% 19% 14% 2	2035
March	1st to 10th	Night	34% 10% 15% 28% 7% 6%	4940
	11th to 20th	Night	31% 12% 17% 29% 12%	5000
	21st to end	Night	38% 10% 20% 25% 7% 1	5489
April	1st to 10th	Night	33% 8% 17% 24% 15% 4	2050
	11th to 20th	Night	36% 7% 15% 24% 15% 1	1977
	21st to end	Night	32% 10% 15% 29% 13% 1	1800
May	1st to 10th	Night	35% 10% 12% 21% 20% 3	2950
	11th to 20th	Night	24% 10% 15% 29% 16% 5%	2713
	21st to end	Night	25% 7% 13% 33% 20% 2	2762
June	1st to 10th	Night	22% 7% 18% 25% 22% 6%	1520
	11th to 20th	Night	22% 5% 11% 20% 27% 16%	1520
	21st to end	Night	14% 5% 7% 17% 31% 26%	1520
July	1st to 10th	Night	16% 5% 6% 11% 20% 42%	2500
	11th to 20th	Night	20% 5% 6% 18% 24% 27%	2674
	21st to end	Night	17% 6% 8% 16% 31% 22%	3011
August	1st to 10th	Night	18% 5% 7% 22% 24% 24%	2838
	11th to 20th	Night	25% 4 7% 17% 30% 18%	2838
	21st to end	Night	14% 3 10% 20% 28% 25%	3353
September	1st to 10th	Night	15% 4 9% 19% 28% 25%	1900
	11th to 20th	Night	17% 5% 12% 21% 29% 16%	2021
	21st to end	Night	17% 4 9% 26% 35% 10%	2108
October	1st to 10th	Night	15% 4 6% 19% 39% 16%	3800
	11th to 20th	Night	29% 3 7% 32% 22% 8%	3778
	21st to end	Night	28% 5% 11% 35% 19% 3	4120
November	1st to 10th	Night	27% 9% 14% 28% 19% 3	2432
	11th to 20th	Night	28% 7% 19% 32% 12% 2	2800
	21st to end	Night	30% 6% 16% 29% 17% 2	2800
December	1st to 10th	Night	24% 7% 16% 37% 15% 1	4570
	11th to 20th	Night	32% 9% 23% 29% 7%	5182
	21st to end	Night	42% 11% 15% 20% 11% 1	6974

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% 12% 15% 38% 20% 7%	300
	11th to 20th	Day	6% 11% 24% 35% 20% 4	300
	21st to end	Day	6% 7% 25% 35% 21% 6%	330
February	1st to 10th	Day	9% 18% 30% 33% 8% 2	300
	11th to 20th	Day	6% 13% 31% 30% 17% 4	300
	21st to end	Day	6% 13% 26% 39% 14% 2	302
March	1st to 10th	Day	8% 10% 31% 32% 14% 6%	500
	11th to 20th	Day	7% 13% 26% 30% 21% 3	500
	21st to end	Day	12% 13% 26% 33% 14% 2	550
April	1st to 10th	Day	12% 12% 28% 27% 18% 4	500
	11th to 20th	Day	13% 18% 31% 24% 11% 3	492
	21st to end	Day	11% 20% 31% 17% 16% 5%	500
May	1st to 10th	Day	11% 19% 27% 20% 17% 6%	503
	11th to 20th	Day	10% 12% 25% 28% 19% 6%	563
	21st to end	Day	10% 10% 24% 25% 21% 10%	734
June	1st to 10th	Day	6% 9% 22% 27% 25% 11%	680
	11th to 20th	Day	7% 12% 28% 31% 19% 3	680
	21st to end	Day	3 7% 27% 31% 27% 5%	680
July	1st to 10th	Day	4 3 20% 33% 32% 8%	671
	11th to 20th	Day	3 6% 25% 30% 28% 9%	560
	21st to end	Day	5% 3 22% 30% 26% 15%	531
August	1st to 10th	Day	6% 5% 22% 29% 26% 13%	487
	11th to 20th	Day	3 5% 25% 30% 25% 12%	477
	21st to end	Day	3 6% 25% 29% 19% 17%	539
September	1st to 10th	Day	5% 6% 25% 25% 24% 15%	490
	11th to 20th	Day	6% 8% 30% 24% 18% 14%	490
	21st to end	Day	7% 10% 24% 22% 24% 13%	471
October	1st to 10th	Day	7% 8% 19% 24% 24% 17%	300
	11th to 20th	Day	9% 12% 21% 25% 23% 10%	300
	21st to end	Day	8% 16% 26% 30% 16% 4	330
November	1st to 10th	Day	8% 14% 24% 33% 14% 8%	300
	11th to 20th	Day	8% 10% 23% 37% 18% 4	300
	21st to end	Day	6% 8% 24% 37% 20% 6%	300
December	1st to 10th	Day	5% 8% 22% 39% 19% 6%	300
	11th to 20th	Day	6% 11% 19% 47% 16% 2	300
	21st to end	Day	8% 10% 30% 31% 13% 8%	330
January	1st to 10th	Night	5% 9% 26% 40% 18% 1	948
	11th to 20th	Night	5% 10% 29% 39% 16% 2	800
	21st to end	Night	4 7% 29% 44% 14% 1	880
February	1st to 10th	Night	4 11% 37% 37% 10% 1	800
	11th to 20th	Night	4 10% 33% 42% 11% 2	784
	21st to end	Night	5% 8% 34% 44% 9%	660
March	1st to 10th	Night	6% 7% 29% 44% 14% 1	800
	11th to 20th	Night	6% 6% 27% 46% 15% 1	800
	21st to end	Night	8% 7% 33% 40% 12%	880
April	1st to 10th	Night	9% 10% 31% 40% 11%	650
	11th to 20th	Night	10% 10% 29% 40% 9% 1	593
	21st to end	Night	7% 11% 30% 37% 14% 1	600
May	1st to 10th	Night	9% 10% 29% 35% 16% 2	600
	11th to 20th	Night	10% 6% 22% 40% 19% 2	563
	21st to end	Night	7% 8% 23% 39% 21% 2	617
June	1st to 10th	Night	6% 5% 24% 41% 23% 2	570
	11th to 20th	Night	6% 8% 26% 43% 16%	570
	21st to end	Night	3 2 21% 48% 25% 1	570
July	1st to 10th	Night	3 2 15% 45% 34% 1	555
	11th to 20th	Night	3 2 15% 46% 32% 1	552
	21st to end	Night	3 1 11% 47% 36% 2	624
August	1st to 10th	Night	3 2 12% 46% 36% 1	567
	11th to 20th	Night	2 2 13% 47% 34% 1	564
	21st to end	Night	3 3 13% 41% 36% 4	627
September	1st to 10th	Night	5% 4 16% 40% 32% 3	570
	11th to 20th	Night	3 3 19% 44% 27% 4	691
	21st to end	Night	4 5% 18% 37% 32% 4	760
October	1st to 10th	Night	7% 5% 25% 36% 24% 3	760
	11th to 20th	Night	7% 7% 22% 40% 23% 2	760
	21st to end	Night	6% 6% 25% 42% 18% 2	836
November	1st to 10th	Night	6% 8% 25% 42% 19% 1	800
	11th to 20th	Night	6% 9% 29% 42% 13% 1	1000
	21st to end	Night	6% 9% 24% 44% 16% 1	1000
December	1st to 10th	Night	5% 10% 28% 43% 13% 1	1000
	11th to 20th	Night	8% 8% 28% 44% 12%	1000
	21st to end	Night	6% 13% 30% 38% 11% 2	1100

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