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 2024-03-23.

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% 11% 5% 8% 7% 4 5% 5% 5% 6% 27%	78552
	Night	15% 11% 5% 9% 8% 4 6% 5% 5% 6% 26%	81325
Transparency	Day	53% 6% 12% 18% 9% 2	78257
	Night	53% 6% 14% 18% 8% 1	81154
Seeing	Day	26% 19% 28% 17% 7% 2	13960
	Night	23% 17% 26% 23% 9% 1	22987

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% 13% 7% 15% 6% 4 5% 4 5% 6% 24%	2517
	11th to 20th	Day	10% 13% 5% 13% 10% 5% 5% 7% 5% 6% 20%	2520
	21st to end	Day	11% 12% 7% 12% 9% 4 6% 5% 4 6% 24%	2772
February	1st to 10th	Day	13% 13% 7% 10% 8% 4 6% 5% 5% 5% 23%	1650
	11th to 20th	Day	8% 12% 6% 12% 10% 6% 5% 7% 7% 7% 21%	1848
	21st to end	Day	12% 13% 6% 10% 8% 4 6% 5% 5% 6% 24%	1570
March	1st to 10th	Day	11% 12% 6% 11% 9% 5% 6% 6% 5% 5% 25%	1897
	11th to 20th	Day	12% 14% 6% 9% 7% 3 5% 4 6% 5% 28%	1998
	21st to end	Day	16% 14% 6% 9% 7% 3 4 4 4 4 29%	2323
April	1st to 10th	Day	16% 15% 7% 9% 6% 4 5% 4 3 4 26%	2160
	11th to 20th	Day	14% 13% 5% 8% 7% 3 4 5% 5% 5% 30%	2178
	21st to end	Day	19% 14% 5% 9% 7% 4 5% 4 5% 4 24%	2340
May	1st to 10th	Day	23% 14% 5% 7% 5% 4 3 3 4 5% 26%	2340
	11th to 20th	Day	20% 12% 4 7% 5% 3 4 4 5% 5% 29%	2424
	21st to end	Day	15% 10% 4 7% 6% 4 6% 5% 6% 8% 30%	2725
June	1st to 10th	Day	27% 10% 4 7% 7% 3 5% 5% 6% 6% 21%	2520
	11th to 20th	Day	19% 7% 3 6% 6% 3 5% 6% 6% 7% 31%	2520
	21st to end	Day	22% 10% 4 9% 6% 4 5% 5% 5% 7% 22%	2520
July	1st to 10th	Day	18% 7% 3 6% 6% 4 6% 6% 6% 9% 29%	2520
	11th to 20th	Day	15% 9% 3 7% 8% 5% 7% 5% 7% 8% 26%	2520
	21st to end	Day	17% 7% 3 7% 6% 5% 6% 7% 7% 8% 26%	2744
August	1st to 10th	Day	16% 8% 4 7% 8% 4 6% 5% 7% 7% 28%	2419
	11th to 20th	Day	15% 8% 4 8% 7% 5% 6% 6% 7% 9% 25%	2304
	21st to end	Day	14% 7% 3 6% 6% 4 6% 6% 7% 9% 34%	2404
September	1st to 10th	Day	14% 6% 3 6% 6% 4 6% 5% 5% 7% 38%	2160
	11th to 20th	Day	16% 8% 3 4 4 3 3 3 4 6% 45%	1986
	21st to end	Day	14% 8% 3 6% 5% 4 5% 4 7% 8% 35%	1980
October	1st to 10th	Day	21% 12% 4 6% 7% 3 4 4 4 6% 30%	1980
	11th to 20th	Day	16% 15% 4 7% 5% 4 5% 4 5% 6% 30%	1708
	21st to end	Day	19% 17% 5% 8% 7% 4 5% 4 4 4 24%	1782
November	1st to 10th	Day	14% 13% 4 8% 6% 3 6% 5% 6% 5% 31%	1671
	11th to 20th	Day	15% 11% 7% 10% 8% 4 6% 7% 6% 5% 23%	1710
	21st to end	Day	17% 11% 5% 9% 7% 5% 5% 6% 5% 6% 24%	1644
December	1st to 10th	Day	16% 16% 8% 11% 8% 4 5% 5% 4 5% 19%	1520
	11th to 20th	Day	18% 15% 6% 9% 6% 3 5% 3 4 5% 25%	1906
	21st to end	Day	17% 15% 5% 8% 8% 4 6% 6% 6% 6% 19%	2772
January	1st to 10th	Night	11% 12% 6% 13% 9% 4 5% 5% 6% 7% 22%	4429
	11th to 20th	Night	10% 13% 8% 14% 16% 6% 6% 5% 5% 5% 13%	4440
	21st to end	Night	13% 14% 6% 12% 9% 5% 6% 6% 7% 6% 16%	4884
February	1st to 10th	Night	11% 12% 5% 11% 10% 5% 7% 6% 7% 7% 19%	2525
	11th to 20th	Night	8% 10% 6% 12% 8% 5% 9% 8% 8% 5% 20%	2218
	21st to end	Night	11% 12% 6% 10% 9% 4 5% 5% 7% 7% 25%	1884
March	1st to 10th	Night	12% 12% 7% 9% 8% 5% 7% 6% 5% 6% 22%	2270
	11th to 20th	Night	14% 12% 5% 9% 8% 5% 6% 4 6% 5% 25%	2180
	21st to end	Night	14% 14% 6% 9% 7% 3 5% 5% 4 4 28%	2075
April	1st to 10th	Night	17% 16% 4 8% 7% 4 6% 4 5% 5% 25%	1800
	11th to 20th	Night	15% 12% 5% 7% 6% 4 6% 5% 5% 6% 29%	1730
	21st to end	Night	19% 14% 4 8% 6% 4 4 4 4 5% 28%	1620
May	1st to 10th	Night	20% 15% 4 6% 5% 4 5% 3 5% 6% 27%	1620
	11th to 20th	Night	18% 11% 4 7% 5% 4 5% 5% 5% 6% 31%	1485
	21st to end	Night	16% 9% 4 7% 6% 3 7% 4 5% 8% 31%	1550
June	1st to 10th	Night	24% 10% 4 7% 5% 4 5% 4 3 7% 27%	1440
	11th to 20th	Night	18% 7% 3 6% 7% 3 5% 5% 5% 7% 33%	1440
	21st to end	Night	18% 10% 3 7% 5% 4 6% 5% 7% 7% 28%	1440
July	1st to 10th	Night	18% 6% 4 6% 5% 4 5% 4 6% 8% 34%	1440
	11th to 20th	Night	15% 8% 3 5% 6% 4 4 5% 6% 9% 35%	1440
	21st to end	Night	14% 7% 3 7% 6% 4 6% 5% 6% 9% 34%	1574
August	1st to 10th	Night	14% 6% 3 6% 6% 3 4 5% 5% 8% 39%	1500
	11th to 20th	Night	15% 9% 4 7% 6% 4 5% 4 5% 9% 34%	1596
	21st to end	Night	16% 7% 3 4 6% 3 4 4 6% 8% 39%	1950
September	1st to 10th	Night	14% 7% 2 5% 5% 4 4 4 6% 7% 44%	1800
	11th to 20th	Night	16% 8% 3 4 5% 3 4 4 5% 6% 44%	1973
	21st to end	Night	16% 9% 3 5% 5% 2 5% 5% 5% 7% 38%	1980
October	1st to 10th	Night	20% 11% 4 6% 5% 4 5% 5% 4 6% 30%	1980
	11th to 20th	Night	16% 14% 3 7% 6% 4 5% 5% 4 6% 29%	2248
	21st to end	Night	20% 14% 5% 8% 7% 4 5% 4 4 5% 22%	2574
November	1st to 10th	Night	15% 11% 5% 8% 7% 4 5% 5% 5% 6% 29%	2407
	11th to 20th	Night	13% 11% 6% 10% 7% 4 7% 5% 5% 6% 24%	2470
	21st to end	Night	15% 10% 6% 10% 8% 5% 6% 7% 5% 6% 22%	2531
December	1st to 10th	Night	16% 14% 7% 12% 10% 5% 6% 5% 5% 5% 15%	2660
	11th to 20th	Night	17% 14% 6% 11% 7% 4 6% 6% 6% 5% 18%	3288
	21st to end	Night	17% 13% 5% 11% 7% 4 6% 5% 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	56% 6% 11% 15% 8% 3	1900
	11th to 20th	Day	52% 4 14% 16% 12% 3	1998
	21st to end	Day	55% 4 9% 18% 12% 3	2323
April	1st to 10th	Day	58% 4 10% 15% 10% 3	2160
	11th to 20th	Day	52% 6% 10% 20% 11% 1	2178
	21st to end	Day	59% 5% 9% 17% 8% 3	2340
May	1st to 10th	Day	59% 3 7% 19% 9% 2	2340
	11th to 20th	Day	55% 5% 8% 18% 12% 2	2428
	21st to end	Day	50% 7% 13% 20% 9% 1	2725
June	1st to 10th	Day	61% 5% 11% 17% 6%	2520
	11th to 20th	Day	49% 7% 10% 20% 13% 2	2520
	21st to end	Day	58% 8% 10% 15% 8% 1	2520
July	1st to 10th	Day	48% 8% 12% 20% 11%	2450
	11th to 20th	Day	52% 8% 12% 22% 7%	2445
	21st to end	Day	50% 8% 13% 20% 9% 1	2591
August	1st to 10th	Day	50% 8% 14% 20% 8%	2405
	11th to 20th	Day	51% 8% 14% 19% 8%	2304
	21st to end	Day	42% 8% 16% 21% 12% 1	2404
September	1st to 10th	Day	42% 7% 12% 25% 13% 1	2160
	11th to 20th	Day	40% 4 10% 29% 15% 2	1986
	21st to end	Day	43% 6% 14% 22% 13% 1	1980
October	1st to 10th	Day	55% 4 10% 19% 10% 1	1980
	11th to 20th	Day	51% 5% 14% 22% 7% 1	1708
	21st to end	Day	61% 6% 11% 12% 8% 2	1782
November	1st to 10th	Day	49% 6% 14% 17% 11% 2	1671
	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	55% 6% 11% 18% 8% 2	2280
	11th to 20th	Night	55% 5% 12% 16% 9% 2	2181
	21st to end	Night	52% 5% 10% 17% 12% 3	2075
April	1st to 10th	Night	56% 4 10% 18% 10% 2	1800
	11th to 20th	Night	51% 7% 9% 21% 11% 1	1730
	21st to end	Night	56% 5% 11% 16% 11% 2	1620
May	1st to 10th	Night	56% 4 8% 18% 12% 1	1620
	11th to 20th	Night	50% 6% 9% 20% 13% 3	1486
	21st to end	Night	48% 9% 15% 20% 8% 1	1551
June	1st to 10th	Night	55% 7% 11% 18% 8%	1440
	11th to 20th	Night	47% 8% 12% 21% 10% 2	1440
	21st to end	Night	50% 10% 13% 19% 8%	1440
July	1st to 10th	Night	45% 9% 15% 19% 12% 1	1400
	11th to 20th	Night	41% 8% 18% 25% 9%	1392
	21st to end	Night	42% 7% 16% 25% 10%	1486
August	1st to 10th	Night	41% 8% 17% 27% 8%	1490
	11th to 20th	Night	44% 8% 16% 21% 10%	1596
	21st to end	Night	39% 9% 15% 24% 13%	1950
September	1st to 10th	Night	37% 6% 16% 29% 12%	1800
	11th to 20th	Night	39% 5% 10% 31% 14% 1	1973
	21st to end	Night	41% 6% 15% 23% 13% 2	1980
October	1st to 10th	Night	52% 7% 12% 17% 12% 1	1980
	11th to 20th	Night	51% 7% 13% 21% 6% 1	2248
	21st to end	Night	60% 6% 11% 13% 8% 2	2574
November	1st to 10th	Night	52% 6% 13% 18% 9% 2	2407
	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	30% 23% 24% 16% 5% 2	470
April	1st to 10th	Day	36% 23% 22% 13% 5% 1	420
	11th to 20th	Day	28% 26% 24% 12% 7% 2	412
	21st to end	Day	34% 22% 26% 11% 4 2	420
May	1st to 10th	Day	40% 20% 23% 12% 3 3	423
	11th to 20th	Day	27% 21% 33% 13% 6% 1	447
	21st to end	Day	22% 18% 33% 20% 6% 1	485
June	1st to 10th	Day	32% 20% 29% 11% 7% 1	450
	11th to 20th	Day	21% 17% 32% 20% 9% 1	450
	21st to end	Day	25% 18% 31% 18% 6% 2	450
July	1st to 10th	Day	22% 13% 30% 25% 9% 2	445
	11th to 20th	Day	16% 12% 37% 23% 10% 2	444
	21st to end	Day	18% 13% 36% 23% 8% 2	488
August	1st to 10th	Day	16% 12% 30% 25% 14% 2	446
	11th to 20th	Day	18% 14% 30% 20% 14% 4	439
	21st to end	Day	15% 16% 30% 23% 11% 5%	495
September	1st to 10th	Day	17% 16% 25% 23% 13% 5%	450
	11th to 20th	Day	22% 14% 24% 18% 14% 7%	450
	21st to end	Day	18% 18% 27% 21% 12% 4	450
October	1st to 10th	Day	28% 18% 21% 18% 12% 3	450
	11th to 20th	Day	25% 19% 25% 19% 10% 2	387
	21st to end	Day	31% 21% 25% 17% 6%	396
November	1st to 10th	Day	27% 18% 25% 24% 6% 1	365
	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% 27% 19% 6% 1	760
	21st to end	Night	27% 19% 27% 20% 6% 1	801
April	1st to 10th	Night	31% 21% 24% 20% 4	720
	11th to 20th	Night	25% 18% 26% 24% 6% 1	666
	21st to end	Night	29% 18% 29% 16% 7%	540
May	1st to 10th	Night	29% 16% 26% 20% 8% 1	540
	11th to 20th	Night	21% 16% 31% 23% 9%	424
	21st to end	Night	20% 12% 23% 32% 13% 1	389
June	1st to 10th	Night	27% 11% 26% 26% 9% 1	360
	11th to 20th	Night	19% 11% 23% 31% 16%	360
	21st to end	Night	21% 8% 22% 38% 11% 1	360
July	1st to 10th	Night	17% 7% 25% 34% 18%	350
	11th to 20th	Night	16% 5% 21% 38% 18% 2	348
	21st to end	Night	14% 6% 21% 39% 17% 2	394
August	1st to 10th	Night	14% 6% 22% 39% 19% 1	426
	11th to 20th	Night	17% 10% 22% 32% 17% 2	532
	21st to end	Night	18% 7% 22% 30% 20% 3	762
September	1st to 10th	Night	15% 7% 25% 34% 16% 2	720
	11th to 20th	Night	19% 12% 19% 26% 19% 4	720
	21st to end	Night	18% 13% 21% 27% 18% 3	720
October	1st to 10th	Night	27% 14% 20% 25% 12% 2	720
	11th to 20th	Night	23% 18% 23% 23% 10% 2	720
	21st to end	Night	30% 19% 24% 21% 6%	792
November	1st to 10th	Night	24% 17% 24% 25% 9% 1	740
	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 2024-03-19 23:31:07. Version ac07

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