High Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the High Mountain Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	17% 7% 2 6% 4 4 4 4 5% 7% 40%	127552
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	124480
Transparency	Day	43% 4 10% 23% 17% 3	131066
	Night	43% 3 10% 23% 17% 3	129040
Seeing	Day	22% 14% 30% 22% 10% 3	16479
	Night	21% 11% 30% 28% 9% 1	25740

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	13% 6% 2 5% 4 3 4 3 5% 7% 48%	3677
	11th to 20th	Day	14% 8% 3 6% 5% 5% 4 3 4 5% 41%	3650
	21st to end	Day	18% 9% 3 7% 5% 3 4 3 4 4 40%	4420
February	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 5% 7% 39%	1800
	11th to 20th	Day	15% 7% 3 6% 4 3 4 3 5% 5% 44%	1946
	21st to end	Day	19% 8% 2 6% 4 4 4 4 6% 5% 39%	1650
March	1st to 10th	Day	21% 6% 2 5% 4 3 4 3 4 5% 42%	4476
	11th to 20th	Day	12% 7% 2 7% 4 4 4 4 6% 7% 42%	4791
	21st to end	Day	18% 6% 2 5% 4 4 4 3 5% 6% 42%	5915
April	1st to 10th	Day	18% 8% 3 7% 4 4 4 3 5% 6% 38%	2400
	11th to 20th	Day	18% 6% 2 6% 4 5% 3 3 5% 7% 41%	2374
	21st to end	Day	19% 6% 2 5% 4 4 4 3 5% 7% 41%	2417
May	1st to 10th	Day	30% 7% 2 5% 4 4 4 3 5% 7% 29%	5886
	11th to 20th	Day	24% 8% 3 7% 4 3 3 3 5% 6% 34%	6313
	21st to end	Day	21% 6% 2 6% 4 4 4 4 6% 7% 36%	6949
June	1st to 10th	Day	16% 6% 2 5% 5% 5% 5% 4 7% 9% 37%	2800
	11th to 20th	Day	17% 7% 2 6% 4 5% 6% 4 6% 8% 36%	2800
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 7% 10% 39%	2800
July	1st to 10th	Day	10% 7% 3 7% 5% 5% 6% 5% 6% 10% 36%	6360
	11th to 20th	Day	10% 7% 3 7% 5% 5% 6% 6% 9% 11% 32%	6360
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	6926
August	1st to 10th	Day	20% 5% 3 7% 4 5% 5% 4 6% 8% 32%	4312
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 8% 43%	3850
	21st to end	Day	11% 5% 2 5% 4 4 5% 3 6% 8% 47%	4312
September	1st to 10th	Day	15% 4 3 6% 5% 5% 5% 3 5% 7% 43%	2400
	11th to 20th	Day	13% 5% 2 4 4 3 3 3 5% 7% 50%	2320
	21st to end	Day	18% 6% 2 7% 5% 5% 4 4 5% 6% 39%	2200
October	1st to 10th	Day	19% 6% 2 5% 3 4 3 3 5% 6% 43%	2076
	11th to 20th	Day	16% 5% 2 5% 3 4 3 3 5% 7% 47%	2000
	21st to end	Day	20% 5% 1 4 3 4 3 4 4 7% 44%	2047
November	1st to 10th	Day	14% 6% 2 5% 3 3 3 2 5% 6% 52%	1851
	11th to 20th	Day	17% 6% 1 4 3 3 4 4 4 6% 47%	1890
	21st to end	Day	16% 7% 1 5% 4 3 4 4 6% 7% 43%	1790
December	1st to 10th	Day	16% 7% 2 5% 4 4 3 3 5% 6% 44%	2680
	11th to 20th	Day	22% 6% 2 4 3 3 3 3 5% 6% 45%	3066
	21st to end	Day	18% 7% 3 7% 4 4 3 3 5% 7% 38%	4048
January	1st to 10th	Night	13% 6% 2 6% 3 4 4 3 5% 5% 48%	6489
	11th to 20th	Night	13% 8% 2 7% 4 4 4 3 5% 5% 45%	6455
	21st to end	Night	18% 9% 2 6% 3 4 4 3 4 6% 41%	6774
February	1st to 10th	Night	17% 9% 3 6% 3 4 4 3 5% 5% 42%	2601
	11th to 20th	Night	17% 8% 2 6% 4 4 4 3 5% 5% 41%	2338
	21st to end	Night	18% 8% 2 6% 4 5% 3 3 4 5% 41%	1980
March	1st to 10th	Night	20% 8% 2 5% 3 3 3 2 4 6% 43%	5364
	11th to 20th	Night	14% 10% 2 5% 3 3 3 3 4 5% 47%	5163
	21st to end	Night	18% 9% 3 6% 4 3 3 3 4 5% 41%	5015
April	1st to 10th	Night	20% 7% 2 6% 4 4 5% 3 5% 6% 38%	2000
	11th to 20th	Night	20% 7% 2 5% 4 3 4 3 5% 7% 40%	1975
	21st to end	Night	18% 7% 2 6% 4 4 3 3 5% 6% 42%	1980
May	1st to 10th	Night	29% 9% 3 7% 4 4 3 3 4 6% 28%	4054
	11th to 20th	Night	24% 9% 3 6% 4 4 4 3 4 5% 35%	3623
	21st to end	Night	22% 7% 2 6% 4 3 3 3 5% 8% 36%	3926
June	1st to 10th	Night	16% 7% 2 5% 4 3 5% 4 5% 9% 41%	1600
	11th to 20th	Night	14% 7% 2 4 5% 4 4 4 6% 9% 41%	1600
	21st to end	Night	11% 5% 2 5% 4 4 5% 4 6% 9% 43%	1600
July	1st to 10th	Night	10% 6% 3 8% 5% 5% 5% 4 7% 9% 38%	3600
	11th to 20th	Night	15% 9% 3 8% 4 5% 5% 4 7% 8% 33%	3600
	21st to end	Night	13% 6% 2 5% 3 4 4 4 6% 8% 44%	3926
August	1st to 10th	Night	22% 7% 2 7% 4 4 4 4 5% 7% 34%	2801
	11th to 20th	Night	14% 6% 3 7% 5% 5% 5% 4 7% 6% 40%	3197
	21st to end	Night	11% 5% 2 5% 3 3 4 3 5% 7% 53%	3586
September	1st to 10th	Night	15% 6% 2 6% 4 4 3 4 5% 7% 44%	2000
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 6% 52%	2079
	21st to end	Night	19% 6% 3 5% 3 3 3 2 5% 7% 44%	2200
October	1st to 10th	Night	19% 7% 2 5% 3 4 3 3 4 7% 43%	2320
	11th to 20th	Night	17% 5% 2 5% 3 3 3 3 4 6% 49%	2400
	21st to end	Night	21% 6% 2 4 3 3 3 3 4 6% 45%	2791
November	1st to 10th	Night	13% 4 2 5% 3 3 4 3 5% 6% 52%	2667
	11th to 20th	Night	18% 6% 2 5% 3 3 3 2 4 5% 48%	2730
	21st to end	Night	14% 6% 2 5% 5% 3 4 3 5% 6% 46%	2828
December	1st to 10th	Night	14% 7% 2 6% 3 4 3 3 4 5% 48%	4720
	11th to 20th	Night	25% 7% 1 5% 3 3 3 2 4 4 43%	5348
	21st to end	Night	19% 9% 3 6% 4 4 3 3 4 5% 40%	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	36% 2 7% 33% 19% 3	3678
	11th to 20th	Day	46% 2 8% 22% 15% 6%	3680
	21st to end	Day	48% 2 9% 23% 16% 1	4420
February	1st to 10th	Day	45% 4 12% 23% 14% 2	1800
	11th to 20th	Day	43% 3 8% 25% 17% 3	1955
	21st to end	Day	46% 3 10% 22% 15% 3	1650
March	1st to 10th	Day	45% 2 9% 21% 14% 8%	4479
	11th to 20th	Day	41% 3 11% 29% 12% 4	4791
	21st to end	Day	44% 3 9% 14% 24% 6%	5915
April	1st to 10th	Day	47% 3 9% 21% 16% 5%	2400
	11th to 20th	Day	43% 3 11% 21% 18% 4	2374
	21st to end	Day	44% 3 8% 23% 18% 4	2417
May	1st to 10th	Day	55% 4 13% 16% 11% 1	5897
	11th to 20th	Day	52% 2 9% 21% 14% 2	6317
	21st to end	Day	47% 3 9% 25% 15% 1	6949
June	1st to 10th	Day	43% 5% 12% 22% 16% 2	2800
	11th to 20th	Day	46% 4 11% 25% 12% 2	2800
	21st to end	Day	40% 5% 14% 24% 16% 2	2800
July	1st to 10th	Day	42% 9% 11% 22% 15% 1	6290
	11th to 20th	Day	42% 8% 16% 22% 11% 1	6285
	21st to end	Day	41% 6% 10% 21% 21% 2	6728
August	1st to 10th	Day	49% 5% 13% 22% 10% 1	4298
	11th to 20th	Day	38% 4 14% 29% 13% 1	3886
	21st to end	Day	35% 5% 9% 30% 20% 2	4312
September	1st to 10th	Day	41% 4 9% 23% 19% 3	2400
	11th to 20th	Day	34% 3 9% 25% 25% 3	2320
	21st to end	Day	46% 4 9% 20% 18% 3	2200
October	1st to 10th	Day	38% 4 8% 20% 24% 6%	3375
	11th to 20th	Day	28% 3 9% 25% 31% 4	3240
	21st to end	Day	27% 3 13% 26% 26% 4	3285
November	1st to 10th	Day	35% 2 11% 25% 24% 3	1851
	11th to 20th	Day	38% 3 9% 25% 21% 4	1890
	21st to end	Day	41% 3 12% 30% 12% 2	1790
December	1st to 10th	Day	41% 3 14% 28% 12% 2	2680
	11th to 20th	Day	42% 3 14% 23% 15% 3	3066
	21st to end	Day	48% 3 7% 24% 16% 2	4048
January	1st to 10th	Night	38% 2 8% 27% 21% 4	6489
	11th to 20th	Night	42% 3 13% 21% 16% 6%	6500
	21st to end	Night	46% 1 8% 22% 17% 5%	6774
February	1st to 10th	Night	45% 3 12% 25% 13% 2	2601
	11th to 20th	Night	45% 3 8% 20% 18% 5%	2340
	21st to end	Night	47% 2 5% 25% 17% 4	1980
March	1st to 10th	Night	45% 2 9% 22% 13% 8%	5374
	11th to 20th	Night	41% 2 14% 25% 14% 4	5164
	21st to end	Night	46% 3 6% 16% 24% 6%	5015
April	1st to 10th	Night	47% 2 9% 21% 16% 4	2000
	11th to 20th	Night	45% 4 9% 21% 20% 2	1975
	21st to end	Night	44% 2 8% 22% 19% 4	1980
May	1st to 10th	Night	59% 3 11% 15% 11% 1	4060
	11th to 20th	Night	53% 3 12% 15% 15% 2	3624
	21st to end	Night	48% 3 13% 23% 11% 2	3927
June	1st to 10th	Night	41% 4 11% 22% 21% 2	1600
	11th to 20th	Night	41% 4 13% 29% 12% 1	1600
	21st to end	Night	36% 6% 14% 26% 16% 2	1600
July	1st to 10th	Night	42% 8% 12% 20% 15% 3	3560
	11th to 20th	Night	48% 6% 13% 21% 11% 1	3552
	21st to end	Night	37% 6% 12% 23% 21% 1	3809
August	1st to 10th	Night	50% 5% 15% 19% 10% 1	2791
	11th to 20th	Night	43% 6% 11% 29% 10% 1	3227
	21st to end	Night	32% 4 10% 33% 20% 1	3586
September	1st to 10th	Night	40% 4 9% 26% 19% 1	2000
	11th to 20th	Night	34% 4 9% 25% 26% 3	2079
	21st to end	Night	42% 3 9% 23% 22% 2	2200
October	1st to 10th	Night	38% 4 10% 24% 21% 3	3788
	11th to 20th	Night	27% 3 9% 29% 29% 4	3899
	21st to end	Night	31% 2 11% 32% 22% 2	4503
November	1st to 10th	Night	34% 4 10% 25% 22% 4	2667
	11th to 20th	Night	40% 2 8% 24% 21% 5%	2730
	21st to end	Night	39% 3 12% 29% 16% 1	2828
December	1st to 10th	Night	39% 3 12% 31% 12% 2	4720
	11th to 20th	Night	47% 2 10% 22% 16% 4	5348
	21st to end	Night	49% 2 9% 19% 17% 4	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	23% 25% 35% 14% 3	220
	11th to 20th	Day	30% 25% 34% 9% 2	220
	21st to end	Day	21% 21% 40% 12% 4 2	328
February	1st to 10th	Day	30% 18% 38% 12% 2	300
	11th to 20th	Day	27% 19% 36% 16% 2	494
	21st to end	Day	27% 18% 35% 16% 4	414
March	1st to 10th	Day	25% 22% 31% 17% 5%	500
	11th to 20th	Day	22% 21% 34% 16% 5% 1	500
	21st to end	Day	29% 20% 30% 14% 7% 1	550
April	1st to 10th	Day	27% 21% 31% 13% 6% 1	500
	11th to 20th	Day	26% 20% 32% 14% 6% 2	492
	21st to end	Day	29% 16% 31% 17% 4 2	500
May	1st to 10th	Day	34% 16% 27% 15% 6% 1	503
	11th to 20th	Day	25% 13% 33% 19% 8% 1	527
	21st to end	Day	22% 8% 28% 27% 13% 2	573
June	1st to 10th	Day	18% 11% 31% 24% 13% 3	530
	11th to 20th	Day	22% 10% 29% 27% 11% 1	530
	21st to end	Day	15% 11% 30% 27% 14% 2	530
July	1st to 10th	Day	16% 5% 24% 32% 18% 5%	525
	11th to 20th	Day	11% 6% 27% 33% 20% 3	524
	21st to end	Day	13% 8% 28% 34% 15% 2	576
August	1st to 10th	Day	14% 4 26% 34% 16% 7%	526
	11th to 20th	Day	16% 5% 22% 33% 19% 6%	519
	21st to end	Day	14% 7% 30% 27% 16% 7%	583
September	1st to 10th	Day	18% 9% 26% 25% 15% 7%	530
	11th to 20th	Day	15% 6% 27% 25% 16% 11%	530
	21st to end	Day	22% 8% 30% 24% 11% 5%	530
October	1st to 10th	Day	24% 9% 28% 20% 14% 6%	530
	11th to 20th	Day	20% 17% 29% 20% 11% 2	530
	21st to end	Day	25% 12% 32% 19% 10% 1	498
November	1st to 10th	Day	19% 14% 32% 24% 9% 3	425
	11th to 20th	Day	25% 17% 32% 21% 5% 1	430
	21st to end	Day	22% 20% 33% 18% 5% 2	330
December	1st to 10th	Day	25% 24% 30% 16% 6%	220
	11th to 20th	Day	30% 20% 32% 15% 3	220
	21st to end	Day	28% 16% 37% 13% 6%	242
January	1st to 10th	Night	20% 16% 38% 22% 3	840
	11th to 20th	Night	24% 15% 33% 24% 4	840
	21st to end	Night	20% 16% 38% 24% 2	924
February	1st to 10th	Night	23% 11% 37% 27% 2	801
	11th to 20th	Night	23% 18% 40% 19% 1	783
	21st to end	Night	24% 15% 35% 21% 4	660
March	1st to 10th	Night	23% 15% 35% 21% 5%	800
	11th to 20th	Night	23% 15% 36% 22% 5% 1	800
	21st to end	Night	23% 15% 37% 21% 5%	880
April	1st to 10th	Night	24% 17% 34% 21% 5%	800
	11th to 20th	Night	24% 14% 34% 23% 5%	791
	21st to end	Night	23% 13% 33% 25% 6%	780
May	1st to 10th	Night	25% 13% 33% 23% 7%	600
	11th to 20th	Night	24% 12% 27% 30% 7%	416
	21st to end	Night	21% 6% 20% 39% 13% 1	433
June	1st to 10th	Night	19% 4 27% 37% 12% 1	400
	11th to 20th	Night	17% 5% 29% 37% 13%	400
	21st to end	Night	13% 4 26% 46% 12%	400
July	1st to 10th	Night	13% 4 23% 39% 20% 1	390
	11th to 20th	Night	13% 4 20% 43% 20% 1	388
	21st to end	Night	15% 2 20% 46% 14% 1	438
August	1st to 10th	Night	13% 2 20% 43% 20% 2	522
	11th to 20th	Night	14% 4 19% 38% 24% 1	790
	21st to end	Night	15% 4 20% 37% 22% 2	880
September	1st to 10th	Night	17% 4 24% 36% 18% 2	800
	11th to 20th	Night	17% 4 19% 37% 20% 4	800
	21st to end	Night	22% 4 23% 34% 15% 2	800
October	1st to 10th	Night	23% 7% 27% 24% 16% 3	800
	11th to 20th	Night	19% 11% 28% 30% 11%	800
	21st to end	Night	24% 9% 34% 26% 8%	880
November	1st to 10th	Night	16% 11% 31% 28% 13% 1	820
	11th to 20th	Night	22% 13% 33% 24% 7% 1	840
	21st to end	Night	20% 15% 33% 26% 6% 1	840
December	1st to 10th	Night	24% 13% 32% 26% 5% 1	840
	11th to 20th	Night	24% 13% 34% 26% 3	840
	21st to end	Night	24% 13% 34% 27% 3	924

Page generated at 2026-01-29 01:21:39. Version ac07

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