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 2025-07-06.

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	16% 12% 4 10% 6% 5% 5% 4 6% 6% 25%	119701
	Night	15% 12% 5% 10% 6% 6% 5% 5% 6% 6% 24%	119087
Transparency	Day	52% 7% 13% 17% 9% 2	123240
	Night	52% 7% 15% 16% 8% 2	123709
Seeing	Day	27% 20% 28% 16% 7% 2	15652
	Night	24% 18% 26% 23% 8% 1	24524

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	9% 12% 8% 16% 5% 6% 7% 4 8% 5% 21%	3597
	11th to 20th	Day	8% 12% 5% 14% 9% 6% 5% 6% 9% 6% 21%	3570
	21st to end	Day	8% 11% 6% 14% 7% 5% 6% 5% 7% 6% 25%	3960
February	1st to 10th	Day	12% 13% 7% 11% 9% 6% 5% 5% 4 5% 22%	1738
	11th to 20th	Day	8% 13% 5% 13% 8% 7% 5% 6% 7% 7% 21%	1948
	21st to end	Day	12% 14% 5% 11% 6% 6% 6% 5% 5% 5% 24%	1650
March	1st to 10th	Day	15% 15% 5% 14% 8% 5% 4 5% 5% 4 21%	4476
	11th to 20th	Day	12% 15% 7% 11% 5% 4 4 3 5% 5% 29%	4766
	21st to end	Day	18% 14% 6% 10% 6% 4 5% 3 5% 6% 23%	5783
April	1st to 10th	Day	16% 17% 5% 10% 5% 5% 4 3 4 4 26%	2400
	11th to 20th	Day	15% 15% 5% 9% 6% 4 4 4 5% 5% 29%	2424
	21st to end	Day	19% 15% 4 10% 6% 4 4 3 5% 4 26%	2600
May	1st to 10th	Day	28% 17% 4 8% 5% 5% 4 3 4 4 18%	5886
	11th to 20th	Day	17% 12% 4 12% 6% 6% 6% 3 5% 6% 22%	6180
	21st to end	Day	19% 11% 3 8% 5% 5% 5% 4 6% 7% 26%	6949
June	1st to 10th	Day	26% 10% 4 7% 6% 5% 5% 4 5% 6% 22%	2800
	11th to 20th	Day	18% 8% 3 7% 6% 4 6% 4 6% 7% 31%	2800
	21st to end	Day	22% 11% 3 9% 5% 5% 5% 5% 6% 6% 23%	2800
July	1st to 10th	Day	15% 8% 3 8% 6% 6% 7% 5% 7% 10% 26%	4510
	11th to 20th	Day	12% 8% 2 8% 7% 6% 7% 5% 8% 10% 28%	4440
	21st to end	Day	18% 9% 3 8% 5% 5% 5% 4 6% 7% 29%	4814
August	1st to 10th	Day	16% 8% 4 11% 7% 6% 6% 5% 6% 7% 25%	4267
	11th to 20th	Day	16% 10% 4 10% 7% 8% 7% 5% 7% 8% 20%	4045
	21st to end	Day	13% 7% 3 8% 5% 6% 5% 5% 7% 8% 33%	4222
September	1st to 10th	Day	13% 6% 2 7% 6% 5% 5% 5% 5% 7% 39%	2280
	11th to 20th	Day	15% 8% 2 5% 3 4 3 3 4 6% 45%	2096
	21st to end	Day	15% 9% 3 6% 4 5% 4 4 7% 8% 34%	2090
October	1st to 10th	Day	20% 13% 4 7% 6% 4 4 3 4 5% 30%	2090
	11th to 20th	Day	15% 16% 3 8% 4 5% 4 3 5% 6% 31%	1802
	21st to end	Day	18% 17% 4 9% 6% 5% 5% 3 4 4 26%	1881
November	1st to 10th	Day	13% 13% 4 9% 5% 5% 5% 4 6% 5% 31%	1761
	11th to 20th	Day	14% 13% 6% 10% 6% 5% 7% 5% 6% 5% 24%	1800
	21st to end	Day	16% 13% 5% 10% 6% 5% 6% 5% 5% 6% 23%	1730
December	1st to 10th	Day	10% 16% 7% 18% 8% 5% 6% 6% 7% 4 14%	2600
	11th to 20th	Day	14% 15% 5% 12% 6% 5% 6% 3 5% 4 23%	2986
	21st to end	Day	15% 14% 5% 8% 6% 5% 5% 4 6% 6% 26%	3960
January	1st to 10th	Night	8% 14% 7% 14% 6% 7% 6% 5% 10% 6% 16%	6349
	11th to 20th	Night	8% 13% 7% 17% 11% 9% 7% 6% 7% 4 12%	6315
	21st to end	Night	9% 11% 5% 13% 7% 7% 6% 7% 9% 6% 20%	6996
February	1st to 10th	Night	11% 13% 5% 13% 9% 6% 6% 6% 6% 7% 19%	2657
	11th to 20th	Night	8% 11% 5% 13% 7% 8% 7% 6% 9% 5% 20%	2338
	21st to end	Night	10% 13% 5% 11% 8% 5% 5% 5% 7% 7% 24%	1980
March	1st to 10th	Night	16% 13% 6% 12% 6% 6% 6% 6% 6% 5% 17%	5364
	11th to 20th	Night	13% 14% 6% 13% 6% 6% 5% 3 5% 5% 24%	5192
	21st to end	Night	17% 16% 6% 10% 7% 4 4 4 5% 5% 22%	5146
April	1st to 10th	Night	17% 17% 4 9% 6% 5% 5% 3 5% 5% 24%	2000
	11th to 20th	Night	15% 15% 5% 8% 6% 6% 4 4 5% 5% 27%	1924
	21st to end	Night	18% 15% 4 9% 5% 6% 4 3 4 5% 30%	1800
May	1st to 10th	Night	30% 15% 4 8% 5% 6% 4 3 4 5% 18%	4054
	11th to 20th	Night	16% 14% 5% 9% 6% 6% 5% 4 5% 5% 26%	3743
	21st to end	Night	18% 12% 3 7% 5% 4 4 4 5% 7% 30%	3926
June	1st to 10th	Night	23% 12% 4 7% 5% 5% 4 3 4 8% 27%	1600
	11th to 20th	Night	17% 9% 2 8% 6% 5% 5% 4 5% 6% 33%	1600
	21st to end	Night	20% 10% 3 8% 4 5% 6% 4 7% 6% 28%	1600
July	1st to 10th	Night	13% 7% 3 8% 6% 9% 6% 4 6% 8% 31%	2560
	11th to 20th	Night	17% 8% 3 7% 5% 5% 5% 4 6% 6% 32%	2520
	21st to end	Night	15% 10% 3 8% 6% 6% 5% 3 6% 7% 33%	2739
August	1st to 10th	Night	20% 10% 3 9% 5% 6% 6% 4 4 5% 28%	2630
	11th to 20th	Night	16% 10% 4 8% 7% 8% 5% 3 5% 7% 27%	2782
	21st to end	Night	11% 6% 2 6% 6% 8% 6% 4 7% 7% 38%	3432
September	1st to 10th	Night	14% 8% 2 5% 4 4 4 3 6% 6% 44%	1900
	11th to 20th	Night	15% 8% 3 5% 4 3 4 3 5% 5% 45%	2083
	21st to end	Night	16% 10% 3 7% 4 4 4 4 5% 7% 37%	2090
October	1st to 10th	Night	20% 11% 3 7% 5% 6% 5% 4 4 5% 30%	2090
	11th to 20th	Night	16% 14% 3 9% 4 6% 5% 3 4 6% 30%	2374
	21st to end	Night	19% 15% 5% 9% 6% 6% 4 3 4 5% 24%	2717
November	1st to 10th	Night	15% 12% 5% 9% 6% 5% 5% 4 5% 6% 29%	2537
	11th to 20th	Night	13% 12% 5% 11% 7% 6% 6% 4 5% 5% 25%	2600
	21st to end	Night	14% 11% 6% 11% 7% 6% 6% 5% 5% 6% 22%	2665
December	1st to 10th	Night	12% 15% 7% 15% 10% 6% 8% 8% 5% 3 10%	4580
	11th to 20th	Night	17% 15% 7% 11% 6% 6% 6% 5% 6% 4 16%	5208
	21st to end	Night	13% 14% 5% 12% 5% 5% 5% 4 5% 7% 24%	6996

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% 5% 19% 16% 6%	3598
	11th to 20th	Day	50% 7% 21% 13% 7% 3	3600
	21st to end	Day	52% 5% 17% 14% 10% 3	3960
February	1st to 10th	Day	59% 5% 13% 13% 7% 3	1738
	11th to 20th	Day	54% 6% 16% 15% 7% 2	1957
	21st to end	Day	56% 4 13% 14% 10% 3	1650
March	1st to 10th	Day	57% 8% 11% 10% 8% 6%	4479
	11th to 20th	Day	49% 5% 17% 17% 9% 4	4766
	21st to end	Day	54% 5% 9% 17% 13% 2	5783
April	1st to 10th	Day	58% 4 10% 15% 10% 3	2400
	11th to 20th	Day	52% 6% 10% 19% 11% 1	2424
	21st to end	Day	57% 4 9% 17% 10% 3	2600
May	1st to 10th	Day	67% 8% 6% 14% 5% 1	5897
	11th to 20th	Day	56% 11% 12% 13% 6% 2	6184
	21st to end	Day	51% 9% 12% 19% 8%	6949
June	1st to 10th	Day	59% 5% 11% 17% 6%	2800
	11th to 20th	Day	48% 7% 11% 20% 12% 2	2800
	21st to end	Day	56% 8% 10% 16% 8% 1	2800
July	1st to 10th	Day	47% 10% 14% 19% 9%	4440
	11th to 20th	Day	45% 10% 15% 25% 6%	4365
	21st to end	Day	49% 7% 10% 24% 9%	4661
August	1st to 10th	Day	53% 9% 14% 17% 7%	4253
	11th to 20th	Day	55% 10% 14% 16% 5%	4084
	21st to end	Day	41% 9% 16% 19% 13% 3	4222
September	1st to 10th	Day	41% 7% 12% 26% 13% 1	2280
	11th to 20th	Day	39% 4 10% 31% 14% 2	2096
	21st to end	Day	43% 7% 13% 22% 14% 1	2090
October	1st to 10th	Day	50% 6% 12% 19% 12% 1	3480
	11th to 20th	Day	44% 6% 14% 20% 12% 4	2966
	21st to end	Day	45% 5% 15% 19% 13% 3	3081
November	1st to 10th	Day	49% 6% 14% 17% 12% 2	1761
	11th to 20th	Day	53% 5% 14% 14% 11% 3	1800
	21st to end	Day	55% 8% 16% 16% 4 1	1730
December	1st to 10th	Day	66% 5% 15% 11% 3 1	2600
	11th to 20th	Day	56% 6% 12% 10% 13% 2	2986
	21st to end	Day	53% 7% 14% 9% 10% 8%	3960
January	1st to 10th	Night	55% 5% 22% 13% 3	6349
	11th to 20th	Night	60% 7% 18% 10% 3 1	6360
	21st to end	Night	51% 7% 18% 12% 9% 3	6996
February	1st to 10th	Night	57% 7% 15% 11% 8% 2	2657
	11th to 20th	Night	53% 6% 16% 16% 7% 1	2340
	21st to end	Night	51% 5% 16% 18% 8% 3	1980
March	1st to 10th	Night	57% 7% 14% 13% 7% 2	5374
	11th to 20th	Night	53% 6% 16% 15% 9% 1	5193
	21st to end	Night	54% 5% 11% 13% 15% 2	5146
April	1st to 10th	Night	56% 4 10% 18% 10% 2	2000
	11th to 20th	Night	53% 8% 9% 20% 11% 1	1924
	21st to end	Night	54% 5% 10% 17% 12% 2	1800
May	1st to 10th	Night	64% 8% 9% 12% 6% 1	4060
	11th to 20th	Night	52% 13% 9% 18% 6% 1	3744
	21st to end	Night	48% 9% 14% 20% 9% 1	3927
June	1st to 10th	Night	54% 7% 11% 18% 9%	1600
	11th to 20th	Night	46% 9% 13% 22% 9% 2	1600
	21st to end	Night	50% 9% 13% 19% 8% 1	1600
July	1st to 10th	Night	44% 11% 15% 19% 10%	2520
	11th to 20th	Night	45% 9% 15% 25% 6%	2472
	21st to end	Night	46% 8% 12% 25% 9%	2651
August	1st to 10th	Night	53% 9% 13% 19% 7%	2620
	11th to 20th	Night	47% 13% 15% 17% 8%	2809
	21st to end	Night	33% 11% 23% 20% 13% 1	3432
September	1st to 10th	Night	37% 6% 16% 29% 12%	1900
	11th to 20th	Night	38% 5% 11% 31% 14% 1	2083
	21st to end	Night	42% 6% 15% 22% 13% 2	2090
October	1st to 10th	Night	47% 11% 14% 17% 10% 1	3480
	11th to 20th	Night	45% 5% 17% 16% 11% 6%	3949
	21st to end	Night	49% 5% 15% 18% 11% 2	4467
November	1st to 10th	Night	52% 6% 14% 18% 9% 2	2537
	11th to 20th	Night	53% 6% 14% 15% 10% 3	2600
	21st to end	Night	53% 7% 17% 17% 5%	2665
December	1st to 10th	Night	65% 9% 15% 9% 2	4580
	11th to 20th	Night	59% 5% 15% 9% 9% 1	5208
	21st to end	Night	54% 7% 14% 11% 10% 5%	6996

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	28% 28% 28% 15% 2	200
	11th to 20th	Day	33% 24% 28% 13% 2 1	200
	21st to end	Day	22% 28% 33% 12% 5%	220
February	1st to 10th	Day	35% 26% 26% 11% 2	308
	11th to 20th	Day	34% 26% 28% 10% 2	496
	21st to end	Day	31% 26% 26% 15% 2	414
March	1st to 10th	Day	30% 24% 31% 12% 3	500
	11th to 20th	Day	31% 28% 26% 11% 3 1	500
	21st to end	Day	33% 24% 23% 14% 4 2	550
April	1st to 10th	Day	40% 23% 21% 11% 5% 1	500
	11th to 20th	Day	32% 27% 23% 11% 6% 1	492
	21st to end	Day	34% 23% 27% 10% 4 1	500
May	1st to 10th	Day	41% 20% 23% 12% 2 2	503
	11th to 20th	Day	26% 20% 33% 13% 7% 1	527
	21st to end	Day	24% 18% 33% 19% 5% 1	573
June	1st to 10th	Day	31% 19% 30% 12% 6% 1	530
	11th to 20th	Day	22% 19% 32% 18% 8% 1	530
	21st to end	Day	27% 21% 29% 16% 5% 2	530
July	1st to 10th	Day	21% 16% 31% 22% 8% 2	505
	11th to 20th	Day	17% 15% 37% 21% 10% 1	484
	21st to end	Day	20% 14% 35% 21% 8% 2	532
August	1st to 10th	Day	17% 14% 30% 24% 13% 2	486
	11th to 20th	Day	20% 14% 29% 20% 13% 4	479
	21st to end	Day	15% 16% 31% 22% 11% 5%	539
September	1st to 10th	Day	18% 17% 25% 22% 13% 5%	490
	11th to 20th	Day	21% 13% 25% 19% 15% 7%	490
	21st to end	Day	19% 18% 27% 20% 12% 4	490
October	1st to 10th	Day	28% 19% 21% 18% 11% 3	490
	11th to 20th	Day	26% 19% 24% 19% 9% 2	420
	21st to end	Day	31% 22% 25% 16% 5%	429
November	1st to 10th	Day	29% 18% 24% 22% 6% 1	395
	11th to 20th	Day	32% 19% 26% 17% 5% 2	400
	21st to end	Day	34% 19% 26% 14% 5% 1	330
December	1st to 10th	Day	30% 27% 26% 12% 7%	200
	11th to 20th	Day	39% 23% 27% 9% 3 1	200
	21st to end	Day	33% 18% 34% 12% 3	220
January	1st to 10th	Night	26% 23% 28% 18% 4	800
	11th to 20th	Night	26% 21% 32% 17% 4	800
	21st to end	Night	19% 25% 34% 18% 4	880
February	1st to 10th	Night	24% 24% 33% 16% 3	800
	11th to 20th	Night	24% 25% 33% 15% 3	783
	21st to end	Night	24% 24% 32% 18% 3	660
March	1st to 10th	Night	29% 21% 30% 18% 2	800
	11th to 20th	Night	27% 21% 27% 19% 5% 1	800
	21st to end	Night	28% 20% 27% 19% 5% 1	880
April	1st to 10th	Night	33% 20% 25% 18% 4	800
	11th to 20th	Night	27% 20% 26% 22% 5% 1	740
	21st to end	Night	30% 20% 28% 16% 7%	600
May	1st to 10th	Night	30% 16% 26% 19% 8% 1	600
	11th to 20th	Night	21% 15% 31% 23% 10%	471
	21st to end	Night	20% 12% 24% 31% 12% 1	433
June	1st to 10th	Night	27% 11% 26% 26% 9% 1	400
	11th to 20th	Night	20% 13% 23% 29% 15%	400
	21st to end	Night	22% 11% 21% 35% 10% 1	400
July	1st to 10th	Night	17% 7% 24% 35% 16%	380
	11th to 20th	Night	17% 6% 21% 37% 17% 2	368
	21st to end	Night	16% 6% 22% 38% 16% 2	416
August	1st to 10th	Night	15% 6% 22% 38% 18% 1	450
	11th to 20th	Night	18% 11% 22% 31% 17% 2	562
	21st to end	Night	17% 8% 21% 30% 20% 3	805
September	1st to 10th	Night	16% 8% 24% 33% 16% 2	760
	11th to 20th	Night	19% 12% 18% 26% 21% 4	760
	21st to end	Night	18% 13% 22% 27% 17% 3	760
October	1st to 10th	Night	26% 14% 21% 25% 12% 2	760
	11th to 20th	Night	24% 18% 23% 23% 10% 2	760
	21st to end	Night	30% 19% 25% 20% 5%	836
November	1st to 10th	Night	25% 18% 24% 23% 8% 1	780
	11th to 20th	Night	26% 19% 26% 20% 8% 1	800
	21st to end	Night	27% 24% 24% 19% 5% 2	800
December	1st to 10th	Night	27% 24% 27% 18% 4 1	800
	11th to 20th	Night	30% 21% 27% 18% 4	800
	21st to end	Night	27% 22% 30% 19% 2	880

Page generated at 2025-07-03 01:30:16. Version ac07

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