Mount Washington Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Washington Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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	15% 11% 4 10% 6% 6% 6% 4 6% 6% 26%	149499
	Night	14% 12% 5% 10% 6% 6% 6% 5% 6% 6% 24%	142572
Transparency	Day	51% 7% 13% 17% 9% 2	152995
	Night	51% 8% 15% 16% 8% 2	147165
Seeing	Day	28% 21% 28% 15% 6% 2	17368
	Night	25% 18% 26% 22% 8% 1	26155

By Month

Percentage Cloud Cover NE (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	7% 11% 7% 16% 7% 7% 7% 5% 8% 5% 19%	4661
	11th to 20th	Day	7% 13% 5% 15% 8% 6% 6% 6% 10% 5% 18%	4650
	21st to end	Day	7% 10% 5% 13% 8% 7% 6% 5% 8% 7% 24%	5124
February	1st to 10th	Day	12% 12% 6% 12% 9% 6% 5% 5% 5% 5% 23%	1825
	11th to 20th	Day	7% 13% 5% 13% 8% 7% 5% 7% 7% 7% 21%	2048
	21st to end	Day	11% 14% 5% 12% 6% 6% 6% 5% 5% 5% 24%	1730
March	1st to 10th	Day	15% 15% 5% 14% 8% 5% 4 5% 5% 4 21%	4576
	11th to 20th	Day	12% 15% 7% 11% 5% 4 4 3 5% 5% 29%	4871
	21st to end	Day	19% 14% 6% 10% 6% 4 5% 4 5% 6% 23%	5911
April	1st to 10th	Day	16% 17% 6% 10% 5% 5% 4 3 4 4 26%	2520
	11th to 20th	Day	15% 15% 5% 9% 6% 5% 4 4 5% 5% 28%	2547
	21st to end	Day	19% 14% 4 9% 6% 5% 4 3 5% 4 26%	2730
May	1st to 10th	Day	25% 17% 5% 9% 6% 5% 5% 3 4 4 17%	7639
	11th to 20th	Day	16% 12% 4 11% 6% 6% 6% 4 5% 6% 23%	8042
	21st to end	Day	19% 12% 3 8% 6% 5% 5% 4 6% 6% 26%	9060
June	1st to 10th	Day	25% 10% 4 8% 6% 5% 5% 4 6% 6% 22%	2940
	11th to 20th	Day	19% 8% 3 7% 6% 5% 6% 5% 6% 7% 30%	2940
	21st to end	Day	21% 11% 3 9% 5% 5% 5% 5% 6% 6% 23%	2940
July	1st to 10th	Day	14% 8% 3 9% 6% 6% 6% 5% 8% 10% 25%	8280
	11th to 20th	Day	10% 6% 2 7% 6% 6% 6% 5% 8% 10% 33%	8280
	21st to end	Day	16% 8% 2 7% 5% 5% 5% 5% 7% 8% 32%	8993
August	1st to 10th	Day	15% 8% 4 11% 7% 7% 6% 4 6% 6% 25%	6252
	11th to 20th	Day	16% 8% 3 9% 6% 8% 7% 6% 8% 9% 21%	5953
	21st to end	Day	11% 7% 3 8% 6% 7% 6% 5% 7% 7% 32%	6136
September	1st to 10th	Day	13% 7% 2 7% 5% 5% 5% 4 6% 7% 39%	2520
	11th to 20th	Day	14% 8% 2 5% 3 4 4 3 5% 6% 46%	2316
	21st to end	Day	15% 9% 3 7% 4 5% 4 4 7% 8% 35%	2310
October	1st to 10th	Day	18% 12% 4 7% 5% 4 4 3 4 6% 32%	2310
	11th to 20th	Day	16% 15% 3 7% 4 5% 4 3 4 6% 31%	1992
	21st to end	Day	17% 17% 4 9% 6% 5% 5% 3 4 4 28%	2052
November	1st to 10th	Day	13% 14% 4 10% 5% 5% 6% 4 5% 5% 30%	1851
	11th to 20th	Day	13% 13% 6% 11% 6% 5% 8% 5% 5% 5% 24%	1890
	21st to end	Day	16% 13% 5% 11% 6% 6% 6% 5% 5% 5% 22%	1816
December	1st to 10th	Day	10% 16% 7% 18% 7% 5% 6% 6% 7% 4 14%	2680
	11th to 20th	Day	14% 14% 5% 12% 7% 5% 6% 4 6% 4 23%	3066
	21st to end	Day	15% 13% 5% 8% 6% 5% 5% 4 6% 6% 26%	4048
January	1st to 10th	Night	8% 14% 6% 14% 6% 8% 7% 5% 10% 6% 16%	8234
	11th to 20th	Night	8% 14% 7% 17% 10% 8% 7% 6% 7% 4 10%	8235
	21st to end	Night	8% 9% 4 12% 7% 8% 8% 8% 10% 7% 18%	9057
February	1st to 10th	Night	10% 12% 5% 13% 9% 6% 6% 6% 7% 7% 19%	2790
	11th to 20th	Night	8% 11% 5% 13% 7% 8% 8% 6% 9% 5% 19%	2458
	21st to end	Night	10% 13% 5% 11% 8% 5% 5% 5% 7% 7% 25%	2076
March	1st to 10th	Night	16% 13% 6% 12% 6% 6% 6% 6% 6% 5% 18%	5484
	11th to 20th	Night	13% 14% 6% 13% 6% 6% 5% 3 5% 5% 23%	5307
	21st to end	Night	17% 16% 6% 10% 7% 4 4 4 5% 5% 22%	5260
April	1st to 10th	Night	17% 17% 4 9% 6% 5% 5% 3 5% 4 24%	2100
	11th to 20th	Night	16% 15% 5% 8% 6% 6% 4 4 5% 5% 27%	2021
	21st to end	Night	17% 15% 3 9% 5% 6% 4 3 4 5% 31%	1890
May	1st to 10th	Night	27% 15% 4 9% 5% 5% 4 2 4 5% 20%	5259
	11th to 20th	Night	16% 12% 4 8% 6% 6% 6% 4 5% 5% 27%	4861
	21st to end	Night	19% 12% 3 7% 4 4 5% 4 5% 7% 30%	5114
June	1st to 10th	Night	23% 12% 4 7% 5% 5% 4 3 4 7% 28%	1680
	11th to 20th	Night	18% 9% 2 7% 6% 5% 5% 4 5% 6% 33%	1680
	21st to end	Night	20% 9% 3 8% 4 6% 5% 4 6% 6% 28%	1680
July	1st to 10th	Night	14% 8% 4 9% 6% 8% 6% 4 5% 7% 29%	4680
	11th to 20th	Night	14% 10% 4 8% 5% 5% 5% 3 4 6% 36%	4680
	21st to end	Night	14% 8% 3 7% 6% 6% 5% 3 5% 7% 37%	5084
August	1st to 10th	Night	18% 10% 3 9% 6% 7% 7% 4 5% 5% 28%	3850
	11th to 20th	Night	16% 10% 3 8% 7% 8% 5% 3 5% 6% 28%	4092
	21st to end	Night	11% 7% 2 7% 5% 8% 7% 4 6% 7% 35%	4974
September	1st to 10th	Night	14% 8% 2 6% 4 5% 4 3 6% 6% 43%	2100
	11th to 20th	Night	14% 7% 3 5% 4 4 4 3 5% 6% 45%	2303
	21st to end	Night	16% 10% 3 7% 4 4 4 4 5% 7% 36%	2310
October	1st to 10th	Night	18% 10% 3 7% 5% 5% 5% 4 4 6% 32%	2310
	11th to 20th	Night	16% 14% 3 8% 4 6% 5% 4 4 6% 30%	2624
	21st to end	Night	18% 14% 4 9% 6% 6% 5% 3 5% 5% 25%	2965
November	1st to 10th	Night	15% 12% 5% 9% 6% 5% 5% 4 5% 6% 28%	2667
	11th to 20th	Night	13% 12% 5% 12% 6% 6% 7% 5% 5% 5% 24%	2730
	21st to end	Night	14% 11% 6% 12% 7% 6% 6% 5% 5% 6% 21%	2799
December	1st to 10th	Night	12% 15% 7% 14% 10% 7% 8% 8% 6% 3 11%	4720
	11th to 20th	Night	16% 15% 7% 11% 6% 6% 6% 5% 6% 4 16%	5348
	21st to end	Night	13% 14% 5% 12% 5% 5% 5% 4 6% 7% 24%	7150

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	55% 6% 19% 14% 6%	4662
	11th to 20th	Day	52% 6% 21% 11% 6% 2	4680
	21st to end	Day	51% 5% 17% 15% 10% 2	5124
February	1st to 10th	Day	58% 5% 13% 13% 8% 3	1825
	11th to 20th	Day	53% 6% 16% 15% 8% 2	2057
	21st to end	Day	56% 5% 13% 14% 10% 3	1730
March	1st to 10th	Day	57% 8% 11% 11% 8% 6%	4579
	11th to 20th	Day	50% 5% 17% 17% 9% 3	4871
	21st to end	Day	54% 5% 9% 17% 13% 2	5911
April	1st to 10th	Day	58% 4 10% 15% 10% 2	2520
	11th to 20th	Day	52% 6% 10% 19% 11% 1	2547
	21st to end	Day	56% 4 9% 18% 10% 3	2730
May	1st to 10th	Day	65% 8% 9% 13% 5% 1	7651
	11th to 20th	Day	54% 11% 13% 14% 6% 1	8046
	21st to end	Day	53% 8% 11% 20% 7%	9061
June	1st to 10th	Day	59% 5% 11% 18% 7%	2940
	11th to 20th	Day	48% 7% 11% 19% 12% 2	2940
	21st to end	Day	56% 8% 10% 17% 8% 1	2940
July	1st to 10th	Day	48% 10% 16% 19% 8%	8210
	11th to 20th	Day	39% 9% 15% 26% 9%	8205
	21st to end	Day	44% 8% 13% 26% 8%	8795
August	1st to 10th	Day	53% 9% 15% 15% 8%	6238
	11th to 20th	Day	50% 11% 14% 17% 7%	5992
	21st to end	Day	42% 10% 15% 17% 14% 2	6136
September	1st to 10th	Day	41% 7% 12% 26% 13% 1	2520
	11th to 20th	Day	38% 4 10% 30% 15% 2	2316
	21st to end	Day	43% 6% 13% 22% 14% 2	2310
October	1st to 10th	Day	48% 6% 12% 20% 13% 1	3700
	11th to 20th	Day	44% 6% 14% 20% 12% 4	3156
	21st to end	Day	45% 5% 15% 18% 14% 4	3252
November	1st to 10th	Day	50% 6% 14% 17% 12% 2	1851
	11th to 20th	Day	52% 6% 14% 14% 11% 3	1890
	21st to end	Day	55% 8% 16% 16% 4 1	1816
December	1st to 10th	Day	65% 5% 15% 11% 4 1	2680
	11th to 20th	Day	56% 6% 13% 10% 14% 2	3066
	21st to end	Day	53% 7% 14% 9% 10% 7%	4048
January	1st to 10th	Night	55% 6% 22% 13% 4	8234
	11th to 20th	Night	62% 7% 18% 9% 3 1	8280
	21st to end	Night	52% 7% 19% 11% 8% 3	9057
February	1st to 10th	Night	56% 7% 15% 12% 9% 2	2790
	11th to 20th	Night	53% 6% 16% 16% 7% 1	2460
	21st to end	Night	51% 5% 15% 17% 9% 3	2076
March	1st to 10th	Night	56% 7% 14% 14% 7% 2	5494
	11th to 20th	Night	53% 6% 16% 14% 8% 1	5308
	21st to end	Night	54% 6% 11% 13% 14% 2	5260
April	1st to 10th	Night	56% 4 10% 17% 10% 2	2100
	11th to 20th	Night	53% 8% 9% 19% 10% 1	2021
	21st to end	Night	53% 5% 10% 17% 12% 2	1890
May	1st to 10th	Night	61% 8% 10% 13% 8%	5265
	11th to 20th	Night	49% 13% 11% 21% 6% 1	4862
	21st to end	Night	48% 8% 13% 22% 8%	5115
June	1st to 10th	Night	53% 7% 11% 19% 9%	1680
	11th to 20th	Night	47% 8% 13% 21% 9% 1	1680
	21st to end	Night	50% 9% 12% 20% 8% 1	1680
July	1st to 10th	Night	49% 10% 14% 19% 8% 1	4640
	11th to 20th	Night	44% 8% 14% 25% 9%	4632
	21st to end	Night	41% 9% 17% 26% 7%	4967
August	1st to 10th	Night	52% 10% 16% 15% 7% 1	3840
	11th to 20th	Night	46% 14% 16% 18% 7%	4119
	21st to end	Night	35% 12% 23% 19% 11% 1	4974
September	1st to 10th	Night	38% 6% 16% 28% 12%	2100
	11th to 20th	Night	37% 5% 11% 31% 15% 1	2303
	21st to end	Night	42% 7% 14% 21% 14% 2	2310
October	1st to 10th	Night	46% 10% 14% 18% 11% 1	3700
	11th to 20th	Night	45% 6% 16% 16% 12% 5%	4199
	21st to end	Night	49% 5% 15% 17% 12% 2	4715
November	1st to 10th	Night	52% 6% 13% 18% 9% 2	2667
	11th to 20th	Night	52% 6% 15% 15% 9% 3	2730
	21st to end	Night	54% 7% 17% 16% 5%	2799
December	1st to 10th	Night	64% 9% 15% 9% 3	4720
	11th to 20th	Night	59% 5% 16% 9% 9% 1	5348
	21st to end	Night	54% 6% 14% 11% 10% 5%	7150

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	29% 27% 29% 14% 2	220
	11th to 20th	Day	35% 25% 25% 12% 1 1	220
	21st to end	Day	26% 27% 32% 11% 5%	242
February	1st to 10th	Day	34% 27% 27% 11% 2	335
	11th to 20th	Day	34% 26% 29% 10% 2	536
	21st to end	Day	31% 26% 26% 14% 2	446
March	1st to 10th	Day	32% 25% 29% 11% 3	540
	11th to 20th	Day	34% 28% 25% 10% 3 1	540
	21st to end	Day	36% 24% 22% 13% 4 2	594
April	1st to 10th	Day	39% 24% 21% 10% 4 1	540
	11th to 20th	Day	34% 27% 23% 10% 5% 1	532
	21st to end	Day	35% 23% 26% 10% 4 2	540
May	1st to 10th	Day	42% 20% 22% 11% 2 2	543
	11th to 20th	Day	29% 21% 31% 12% 6% 1	567
	21st to end	Day	26% 19% 31% 18% 5% 1	617
June	1st to 10th	Day	30% 19% 31% 12% 6% 1	570
	11th to 20th	Day	24% 20% 30% 17% 8% 1	570
	21st to end	Day	28% 21% 29% 15% 5% 2	570
July	1st to 10th	Day	21% 19% 32% 20% 7% 1	565
	11th to 20th	Day	19% 16% 36% 20% 8% 1	564
	21st to end	Day	21% 15% 34% 20% 7% 2	620
August	1st to 10th	Day	16% 14% 31% 24% 12% 3	566
	11th to 20th	Day	21% 16% 28% 19% 11% 4	559
	21st to end	Day	16% 17% 32% 21% 9% 4	627
September	1st to 10th	Day	21% 18% 25% 20% 12% 5%	570
	11th to 20th	Day	21% 14% 26% 19% 14% 7%	570
	21st to end	Day	21% 20% 27% 18% 11% 4	570
October	1st to 10th	Day	29% 20% 21% 17% 11% 3	570
	11th to 20th	Day	31% 18% 23% 18% 8% 2	486
	21st to end	Day	31% 22% 25% 16% 6%	486
November	1st to 10th	Day	33% 18% 23% 20% 5%	425
	11th to 20th	Day	32% 20% 26% 16% 5% 1	430
	21st to end	Day	35% 20% 26% 13% 5% 1	356
December	1st to 10th	Day	33% 25% 25% 11% 6%	220
	11th to 20th	Day	40% 23% 26% 8% 2 1	220
	21st to end	Day	34% 20% 32% 11% 3	242
January	1st to 10th	Night	27% 24% 28% 18% 4	840
	11th to 20th	Night	27% 21% 31% 16% 3	840
	21st to end	Night	20% 25% 33% 17% 4	924
February	1st to 10th	Night	23% 25% 33% 16% 3	840
	11th to 20th	Night	24% 25% 33% 15% 3	823
	21st to end	Night	24% 25% 32% 17% 2	692
March	1st to 10th	Night	29% 22% 29% 17% 2	840
	11th to 20th	Night	29% 20% 27% 18% 5% 1	840
	21st to end	Night	29% 21% 26% 18% 5% 1	924
April	1st to 10th	Night	33% 20% 25% 18% 4	840
	11th to 20th	Night	29% 20% 25% 21% 5% 1	777
	21st to end	Night	29% 20% 29% 16% 7%	630
May	1st to 10th	Night	31% 17% 25% 19% 7% 1	630
	11th to 20th	Night	21% 17% 30% 22% 9%	495
	21st to end	Night	22% 13% 24% 30% 11% 1	455
June	1st to 10th	Night	26% 11% 27% 26% 9% 1	420
	11th to 20th	Night	22% 13% 23% 28% 15%	420
	21st to end	Night	23% 11% 21% 35% 10%	420
July	1st to 10th	Night	16% 10% 25% 34% 15%	410
	11th to 20th	Night	18% 7% 22% 35% 16% 1	408
	21st to end	Night	17% 7% 22% 37% 16% 2	460
August	1st to 10th	Night	15% 7% 21% 38% 18% 1	498
	11th to 20th	Night	18% 12% 22% 30% 17% 2	622
	21st to end	Night	17% 11% 22% 29% 19% 3	890
September	1st to 10th	Night	17% 10% 24% 32% 15% 2	840
	11th to 20th	Night	19% 13% 19% 26% 20% 3	840
	21st to end	Night	20% 14% 22% 25% 17% 2	840
October	1st to 10th	Night	27% 15% 20% 24% 12% 2	840
	11th to 20th	Night	26% 18% 23% 23% 10% 2	840
	21st to end	Night	30% 18% 26% 20% 5%	913
November	1st to 10th	Night	27% 18% 24% 22% 8% 1	820
	11th to 20th	Night	26% 20% 25% 20% 8% 1	840
	21st to end	Night	27% 24% 24% 18% 5% 2	840
December	1st to 10th	Night	28% 24% 26% 17% 4	840
	11th to 20th	Night	31% 22% 26% 17% 4	840
	21st to end	Night	29% 22% 29% 18% 2	924

Page generated at 2026-10-26 04:37:52. Version ac07

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