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 2024-11-03.

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% 26%	101348
	Night	15% 12% 4 10% 6% 6% 5% 4 5% 6% 25%	97640
Transparency	Day	52% 7% 13% 18% 9% 1	101106
	Night	52% 7% 14% 17% 8% 1	97502
Seeing	Day	26% 20% 28% 17% 7% 2	14840
	Night	24% 17% 26% 23% 9% 1	23676

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	12% 14% 7% 16% 5% 4 5% 3 5% 6% 23%	2517
	11th to 20th	Day	10% 14% 5% 15% 9% 6% 6% 5% 5% 6% 20%	2520
	21st to end	Day	11% 13% 7% 13% 7% 6% 5% 4 4 6% 24%	2772
February	1st to 10th	Day	13% 13% 7% 11% 9% 6% 5% 5% 4 5% 23%	1650
	11th to 20th	Day	8% 13% 6% 13% 8% 7% 5% 6% 7% 7% 21%	1848
	21st to end	Day	12% 15% 5% 11% 6% 6% 5% 5% 5% 5% 24%	1570
March	1st to 10th	Day	15% 16% 6% 14% 7% 5% 5% 3 4 4 21%	3116
	11th to 20th	Day	12% 16% 8% 12% 6% 5% 4 3 5% 4 24%	3336
	21st to end	Day	17% 13% 6% 11% 7% 4 4 3 4 6% 27%	4035
April	1st to 10th	Day	16% 18% 5% 10% 5% 5% 4 3 4 4 26%	2280
	11th to 20th	Day	14% 15% 4 9% 6% 4 4 4 5% 5% 29%	2301
	21st to end	Day	19% 15% 4 10% 6% 5% 4 3 5% 4 25%	2470
May	1st to 10th	Day	24% 17% 4 7% 5% 5% 4 3 4 4 23%	4106
	11th to 20th	Day	19% 13% 4 10% 6% 6% 5% 4 5% 5% 24%	4302
	21st to end	Day	12% 9% 3 7% 5% 5% 5% 5% 7% 9% 33%	4837
June	1st to 10th	Day	26% 10% 4 7% 6% 5% 5% 4 5% 6% 22%	2660
	11th to 20th	Day	19% 8% 3 7% 5% 4 5% 5% 6% 6% 31%	2660
	21st to end	Day	22% 11% 3 10% 5% 5% 5% 5% 6% 6% 22%	2660
July	1st to 10th	Day	15% 8% 3 8% 6% 6% 7% 5% 7% 10% 26%	4440
	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%	1689
	11th to 20th	Day	14% 12% 6% 11% 7% 5% 7% 5% 6% 5% 23%	1710
	21st to end	Day	16% 13% 5% 10% 6% 5% 6% 5% 6% 6% 24%	1644
December	1st to 10th	Day	16% 17% 7% 13% 6% 5% 5% 3 4 5% 19%	1520
	11th to 20th	Day	17% 16% 5% 11% 5% 4 4 3 4 5% 25%	1906
	21st to end	Day	17% 16% 5% 9% 7% 5% 6% 4 6% 6% 19%	2772
January	1st to 10th	Night	10% 13% 6% 15% 7% 6% 5% 4 6% 6% 22%	4429
	11th to 20th	Night	10% 13% 7% 18% 12% 7% 5% 4 5% 5% 13%	4440
	21st to end	Night	13% 14% 5% 14% 7% 6% 6% 5% 7% 6% 16%	4884
February	1st to 10th	Night	11% 13% 5% 12% 9% 6% 6% 6% 6% 7% 19%	2525
	11th to 20th	Night	8% 11% 5% 14% 7% 7% 7% 6% 8% 5% 20%	2218
	21st to end	Night	11% 13% 5% 11% 8% 5% 5% 5% 6% 7% 25%	1884
March	1st to 10th	Night	18% 14% 7% 9% 6% 7% 6% 4 5% 5% 21%	3724
	11th to 20th	Night	12% 15% 8% 14% 7% 6% 5% 3 5% 4 21%	3622
	21st to end	Night	17% 17% 6% 10% 8% 4 3 3 4 5% 23%	3594
April	1st to 10th	Night	17% 17% 4 9% 6% 6% 5% 3 5% 5% 25%	1900
	11th to 20th	Night	15% 15% 5% 8% 6% 6% 4 4 5% 5% 28%	1827
	21st to end	Night	18% 15% 4 9% 5% 5% 4 3 4 5% 29%	1710
May	1st to 10th	Night	25% 15% 3 8% 5% 5% 4 3 5% 5% 22%	2834
	11th to 20th	Night	17% 14% 5% 8% 5% 6% 5% 5% 5% 5% 26%	2607
	21st to end	Night	15% 8% 2 7% 4 5% 5% 4 6% 9% 35%	2738
June	1st to 10th	Night	23% 12% 4 7% 5% 5% 4 3 4 8% 26%	1520
	11th to 20th	Night	18% 8% 2 8% 7% 5% 5% 4 5% 6% 33%	1520
	21st to end	Night	20% 10% 3 8% 4 5% 5% 4 7% 6% 27%	1520
July	1st to 10th	Night	13% 7% 3 8% 6% 9% 6% 4 6% 8% 31%	2520
	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% 3 5% 6% 29%	2434
	11th to 20th	Night	13% 12% 5% 11% 7% 6% 6% 4 6% 5% 24%	2470
	21st to end	Night	15% 11% 6% 11% 7% 6% 6% 5% 5% 6% 22%	2531
December	1st to 10th	Night	16% 15% 6% 13% 8% 7% 6% 4 5% 4 15%	2660
	11th to 20th	Night	17% 15% 5% 12% 6% 6% 5% 5% 6% 5% 18%	3288
	21st to end	Night	17% 14% 4 13% 6% 5% 5% 4 5% 6% 21%	4884

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	56% 4 16% 19% 4	2518
	11th to 20th	Day	54% 8% 18% 14% 5% 1	2520
	21st to end	Day	56% 6% 16% 13% 7% 2	2772
February	1st to 10th	Day	59% 5% 13% 13% 7% 3	1650
	11th to 20th	Day	55% 6% 15% 15% 7% 2	1857
	21st to end	Day	56% 4 13% 14% 10% 3	1570
March	1st to 10th	Day	59% 10% 8% 10% 9% 3	3119
	11th to 20th	Day	51% 6% 15% 15% 9% 4	3336
	21st to end	Day	52% 5% 7% 19% 14% 3	4035
April	1st to 10th	Day	58% 4 10% 15% 10% 3	2280
	11th to 20th	Day	52% 7% 10% 20% 11% 1	2301
	21st to end	Day	58% 5% 9% 17% 9% 3	2470
May	1st to 10th	Day	61% 8% 7% 17% 5% 1	4120
	11th to 20th	Day	55% 11% 13% 14% 7% 1	4306
	21st to end	Day	42% 9% 14% 23% 12% 1	4837
June	1st to 10th	Day	59% 5% 11% 18% 6%	2660
	11th to 20th	Day	48% 7% 11% 20% 12% 2	2660
	21st to end	Day	58% 8% 10% 16% 8% 1	2660
July	1st to 10th	Day	48% 10% 14% 19% 9%	4370
	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	54% 4 11% 19% 11% 1	2090
	11th to 20th	Day	50% 5% 14% 21% 8% 1	1802
	21st to end	Day	59% 5% 11% 13% 9% 2	1881
November	1st to 10th	Day	49% 6% 14% 17% 12% 2	1689
	11th to 20th	Day	54% 5% 14% 15% 10% 2	1710
	21st to end	Day	55% 7% 16% 17% 5% 1	1644
December	1st to 10th	Day	64% 5% 13% 13% 5% 1	1520
	11th to 20th	Day	57% 5% 10% 13% 12% 3	1906
	21st to end	Day	59% 7% 15% 9% 8% 2	2772
January	1st to 10th	Night	55% 6% 17% 18% 4	4429
	11th to 20th	Night	61% 6% 16% 14% 3 1	4440
	21st to end	Night	60% 8% 13% 12% 5% 1	4884
February	1st to 10th	Night	57% 7% 15% 11% 7% 2	2525
	11th to 20th	Night	53% 6% 15% 16% 8% 1	2220
	21st to end	Night	51% 5% 15% 18% 8% 3	1884
March	1st to 10th	Night	56% 9% 10% 16% 7% 2	3734
	11th to 20th	Night	55% 6% 14% 14% 9% 2	3623
	21st to end	Night	54% 5% 9% 16% 14% 2	3594
April	1st to 10th	Night	56% 4 10% 18% 10% 2	1900
	11th to 20th	Night	52% 7% 9% 20% 11% 1	1827
	21st to end	Night	55% 5% 10% 16% 12% 2	1710
May	1st to 10th	Night	58% 8% 10% 16% 7% 1	2840
	11th to 20th	Night	52% 12% 10% 17% 7% 2	2608
	21st to end	Night	40% 10% 18% 19% 12% 1	2739
June	1st to 10th	Night	54% 7% 12% 19% 8%	1520
	11th to 20th	Night	46% 9% 13% 21% 10% 2	1520
	21st to end	Night	51% 9% 13% 18% 8% 1	1520
July	1st to 10th	Night	45% 11% 15% 18% 10%	2480
	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	51% 8% 12% 17% 11% 1	2090
	11th to 20th	Night	51% 7% 14% 21% 6% 2	2374
	21st to end	Night	58% 6% 12% 14% 9% 2	2717
November	1st to 10th	Night	52% 6% 13% 17% 9% 2	2434
	11th to 20th	Night	54% 6% 14% 15% 9% 2	2470
	21st to end	Night	53% 6% 18% 17% 5%	2531
December	1st to 10th	Night	65% 6% 13% 12% 3 1	2660
	11th to 20th	Night	57% 6% 14% 14% 7% 2	3288
	21st to end	Night	57% 7% 15% 13% 8%	4884

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	25% 28% 29% 16% 2	180
	11th to 20th	Day	33% 23% 28% 13% 1 1	180
	21st to end	Day	16% 29% 36% 13% 6%	198
February	1st to 10th	Day	33% 25% 28% 13% 2	280
	11th to 20th	Day	32% 27% 29% 11% 2	456
	21st to end	Day	29% 25% 27% 16% 3	382
March	1st to 10th	Day	28% 24% 32% 13% 3	460
	11th to 20th	Day	31% 27% 27% 11% 3 1	460
	21st to end	Day	32% 24% 23% 15% 4 2	506
April	1st to 10th	Day	37% 23% 22% 12% 5% 1	460
	11th to 20th	Day	29% 27% 24% 12% 6% 2	452
	21st to end	Day	33% 23% 27% 11% 4 2	460
May	1st to 10th	Day	39% 20% 24% 12% 3 2	463
	11th to 20th	Day	25% 20% 33% 14% 7% 1	487
	21st to end	Day	22% 17% 33% 20% 6% 1	529
June	1st to 10th	Day	31% 20% 29% 12% 7% 1	490
	11th to 20th	Day	21% 18% 32% 19% 9% 1	490
	21st to end	Day	28% 19% 29% 17% 6% 2	490
July	1st to 10th	Day	20% 15% 31% 23% 8% 2	485
	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% 21% 26% 16% 5%	429
November	1st to 10th	Day	28% 18% 25% 23% 6% 1	371
	11th to 20th	Day	31% 17% 27% 18% 6% 2	370
	21st to end	Day	31% 19% 28% 15% 6% 1	304
December	1st to 10th	Day	30% 26% 24% 13% 7%	180
	11th to 20th	Day	38% 22% 28% 8% 3 1	180
	21st to end	Day	34% 18% 34% 11% 3 1	198
January	1st to 10th	Night	25% 21% 29% 19% 4	760
	11th to 20th	Night	26% 20% 32% 18% 3	760
	21st to end	Night	18% 25% 35% 19% 4	836
February	1st to 10th	Night	22% 24% 35% 17% 3	760
	11th to 20th	Night	22% 25% 34% 16% 3	743
	21st to end	Night	23% 24% 32% 18% 3	628
March	1st to 10th	Night	28% 20% 30% 19% 3	760
	11th to 20th	Night	27% 21% 28% 19% 6% 1	760
	21st to end	Night	27% 19% 27% 20% 5% 1	836
April	1st to 10th	Night	31% 20% 25% 19% 4	760
	11th to 20th	Night	26% 19% 26% 23% 6% 1	703
	21st to end	Night	29% 19% 29% 16% 7%	570
May	1st to 10th	Night	29% 16% 26% 20% 8% 1	570
	11th to 20th	Night	20% 15% 31% 24% 10%	447
	21st to end	Night	20% 11% 24% 32% 12% 1	411
June	1st to 10th	Night	26% 11% 26% 26% 9% 1	380
	11th to 20th	Night	19% 13% 23% 30% 16%	380
	21st to end	Night	23% 9% 21% 36% 11% 1	380
July	1st to 10th	Night	17% 7% 25% 34% 17%	370
	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	24% 18% 24% 24% 9% 1	749
	11th to 20th	Night	24% 19% 26% 21% 8% 1	760
	21st to end	Night	26% 23% 24% 19% 6% 2	760
December	1st to 10th	Night	27% 24% 26% 19% 4 1	760
	11th to 20th	Night	28% 22% 27% 19% 4	760
	21st to end	Night	28% 22% 30% 18% 2	836

Page generated at 2024-10-31 03:39:21. Version ac07

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