Waltman Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Waltman 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	11% 6% 2 6% 4 4 5% 4 6% 8% 43%	126280
	Night	13% 6% 2 5% 4 4 4 3 5% 7% 46%	125849
Transparency	Day	37% 7% 12% 20% 18% 6%	129917
	Night	35% 6% 11% 19% 20% 8%	130288
Seeing	Day	13% 8% 21% 25% 23% 11%	16672
	Night	14% 6% 23% 32% 21% 4	26484

By Month

Percentage Cloud Cover SE (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	14% 10% 4 8% 3 3 3 3 5% 5% 43%	4167
	11th to 20th	Day	12% 6% 2 5% 3 3 4 3 5% 5% 52%	4170
	21st to end	Day	21% 10% 3 6% 4 3 3 2 3 5% 40%	4615
February	1st to 10th	Day	18% 9% 3 6% 4 3 4 3 5% 6% 40%	2000
	11th to 20th	Day	19% 10% 3 6% 5% 3 3 3 3 5% 39%	1948
	21st to end	Day	18% 8% 3 7% 4 5% 4 3 5% 5% 38%	1650
March	1st to 10th	Day	19% 9% 3 9% 5% 5% 4 4 4 6% 32%	4706
	11th to 20th	Day	13% 6% 2 6% 4 3 5% 4 6% 7% 45%	4980
	21st to end	Day	12% 9% 3 9% 6% 5% 4 3 5% 5% 40%	5709
April	1st to 10th	Day	12% 6% 2 6% 4 4 5% 5% 6% 7% 43%	2400
	11th to 20th	Day	15% 6% 3 6% 4 4 4 3 5% 6% 44%	2374
	21st to end	Day	9% 5% 2 6% 5% 4 5% 4 6% 8% 47%	2400
May	1st to 10th	Day	14% 7% 3 6% 5% 4 5% 4 6% 9% 37%	5405
	11th to 20th	Day	11% 6% 3 7% 5% 6% 6% 4 7% 8% 38%	5404
	21st to end	Day	9% 5% 2 6% 4 5% 5% 5% 7% 10% 42%	6189
June	1st to 10th	Day	9% 5% 2 6% 5% 6% 7% 5% 7% 10% 39%	2800
	11th to 20th	Day	4 5% 2 5% 6% 6% 7% 5% 7% 9% 43%	2800
	21st to end	Day	4 5% 2 6% 6% 6% 7% 5% 7% 10% 41%	2745
July	1st to 10th	Day	5% 5% 3 8% 6% 7% 8% 6% 9% 10% 34%	5900
	11th to 20th	Day	6% 4 2 6% 4 6% 7% 6% 9% 12% 40%	5900
	21st to end	Day	9% 7% 3 7% 5% 5% 5% 4 7% 11% 35%	6269
August	1st to 10th	Day	3 2 1 3 3 4 4 3 5% 8% 64%	3905
	11th to 20th	Day	5% 3 2 5% 4 5% 6% 5% 7% 10% 47%	3849
	21st to end	Day	9% 5% 2 6% 5% 4 5% 4 7% 8% 46%	4312
September	1st to 10th	Day	7% 5% 2 6% 6% 6% 6% 5% 6% 8% 43%	2400
	11th to 20th	Day	11% 6% 3 6% 4 5% 5% 3 5% 8% 44%	2361
	21st to end	Day	11% 5% 1 4 3 3 4 3 5% 8% 54%	2200
October	1st to 10th	Day	6% 3 2 4 4 4 4 3 5% 6% 59%	2050
	11th to 20th	Day	7% 5% 2 5% 4 4 3 3 5% 6% 58%	2000
	21st to end	Day	13% 5% 2 4 3 3 3 3 4 5% 56%	2200
November	1st to 10th	Day	11% 6% 2 4 3 3 4 3 4 6% 55%	2058
	11th to 20th	Day	12% 6% 2 5% 4 3 3 2 4 6% 54%	1909
	21st to end	Day	15% 6% 2 6% 4 4 3 3 5% 6% 46%	1890
December	1st to 10th	Day	18% 8% 3 6% 3 3 4 2 5% 6% 43%	3020
	11th to 20th	Day	14% 10% 3 8% 4 4 4 3 4 5% 41%	3269
	21st to end	Day	15% 6% 2 6% 3 3 2 3 4 6% 49%	4326
January	1st to 10th	Night	18% 7% 4 5% 3 3 3 2 3 4 47%	6018
	11th to 20th	Night	14% 7% 3 6% 3 3 3 3 5% 6% 48%	6030
	21st to end	Night	24% 9% 2 5% 4 3 3 3 3 4 40%	6588
February	1st to 10th	Night	17% 8% 3 6% 4 3 4 3 4 6% 43%	2401
	11th to 20th	Night	19% 8% 2 5% 3 3 3 3 3 5% 44%	2338
	21st to end	Night	21% 8% 3 5% 4 3 3 3 4 5% 42%	1980
March	1st to 10th	Night	17% 9% 3 7% 4 4 4 3 5% 5% 39%	5189
	11th to 20th	Night	11% 6% 2 5% 4 3 4 3 5% 8% 49%	4980
	21st to end	Night	16% 8% 2 6% 5% 4 4 3 5% 6% 42%	5206
April	1st to 10th	Night	12% 5% 2 5% 5% 4 4 4 5% 7% 47%	2000
	11th to 20th	Night	13% 5% 2 5% 3 4 4 4 5% 6% 49%	1975
	21st to end	Night	8% 4 2 5% 4 4 4 3 6% 8% 51%	2000
May	1st to 10th	Night	13% 5% 2 6% 5% 4 4 4 6% 9% 41%	4490
	11th to 20th	Night	15% 6% 2 7% 4 4 4 4 7% 9% 36%	4489
	21st to end	Night	8% 5% 2 6% 5% 4 6% 5% 7% 10% 42%	4656
June	1st to 10th	Night	6% 5% 1 6% 6% 5% 5% 3 8% 10% 46%	1600
	11th to 20th	Night	4 4 2 3 4 4 5% 5% 8% 11% 51%	1600
	21st to end	Night	4 4 2 5% 5% 4 4 4 8% 13% 47%	1647
July	1st to 10th	Night	4 5% 3 7% 5% 5% 6% 5% 8% 12% 41%	4078
	11th to 20th	Night	5% 4 1 5% 4 5% 6% 6% 9% 14% 40%	4077
	21st to end	Night	7% 6% 3 7% 5% 5% 5% 5% 8% 12% 37%	4579
August	1st to 10th	Night	2 2 1 3 3 3 4 3 6% 10% 64%	3244
	11th to 20th	Night	4 2 1 4 4 4 5% 5% 8% 12% 49%	3200
	21st to end	Night	6% 4 2 6% 5% 5% 4 4 6% 8% 49%	3586
September	1st to 10th	Night	7% 5% 2 5% 5% 4 6% 5% 6% 9% 46%	2000
	11th to 20th	Night	9% 6% 2 5% 3 4 5% 4 6% 8% 48%	2016
	21st to end	Night	8% 4 2 5% 3 4 4 3 5% 6% 57%	2200
October	1st to 10th	Night	6% 3 1 3 3 3 3 3 5% 6% 64%	2345
	11th to 20th	Night	11% 4 2 4 3 3 3 2 5% 5% 58%	2400
	21st to end	Night	13% 5% 2 5% 3 3 3 2 3 3 57%	2640
November	1st to 10th	Night	12% 5% 1 4 3 3 3 2 4 5% 59%	2460
	11th to 20th	Night	12% 5% 1 4 2 3 3 3 5% 6% 56%	2708
	21st to end	Night	13% 5% 2 5% 4 4 4 3 5% 6% 49%	2730
December	1st to 10th	Night	21% 6% 3 5% 3 3 4 3 4 5% 44%	4380
	11th to 20th	Night	15% 9% 2 6% 4 4 3 3 4 5% 45%	5132
	21st to end	Night	17% 9% 3 5% 3 2 3 3 4 5% 46%	6887

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	43% 3 6% 23% 21% 3	4168
	11th to 20th	Day	33% 4 8% 21% 26% 7%	4170
	21st to end	Day	49% 3 5% 13% 20% 11%	4615
February	1st to 10th	Day	45% 5% 8% 16% 17% 9%	2000
	11th to 20th	Day	47% 4 6% 17% 18% 8%	1958
	21st to end	Day	48% 5% 6% 14% 18% 9%	1650
March	1st to 10th	Day	52% 5% 6% 15% 15% 7%	4706
	11th to 20th	Day	37% 7% 7% 15% 20% 15%	4980
	21st to end	Day	46% 5% 8% 19% 16% 6%	5711
April	1st to 10th	Day	38% 7% 10% 21% 17% 7%	2400
	11th to 20th	Day	40% 5% 8% 16% 24% 7%	2374
	21st to end	Day	33% 7% 11% 24% 22% 3	2400
May	1st to 10th	Day	40% 9% 14% 22% 13% 2	5440
	11th to 20th	Day	40% 8% 17% 19% 16% 1	5408
	21st to end	Day	33% 9% 18% 24% 14% 1	6203
June	1st to 10th	Day	37% 10% 15% 23% 13% 1	2800
	11th to 20th	Day	34% 9% 17% 23% 15% 3	2800
	21st to end	Day	34% 10% 17% 23% 13% 2	2745
July	1st to 10th	Day	39% 12% 18% 20% 10% 1	5836
	11th to 20th	Day	32% 11% 29% 25% 2 1	5831
	21st to end	Day	41% 9% 18% 28% 4	6089
August	1st to 10th	Day	19% 7% 12% 32% 29% 2	3893
	11th to 20th	Day	29% 9% 19% 28% 14% 1	3885
	21st to end	Day	35% 8% 14% 21% 20% 3	4311
September	1st to 10th	Day	37% 9% 15% 17% 17% 7%	2400
	11th to 20th	Day	38% 7% 10% 18% 20% 7%	2361
	21st to end	Day	28% 6% 12% 24% 19% 11%	2200
October	1st to 10th	Day	17% 4 5% 15% 27% 32%	3336
	11th to 20th	Day	17% 3 5% 14% 34% 29%	3240
	21st to end	Day	23% 3 4 17% 37% 16%	3538
November	1st to 10th	Day	31% 4 8% 17% 29% 11%	2058
	11th to 20th	Day	32% 4 7% 17% 27% 13%	1909
	21st to end	Day	39% 4 7% 18% 23% 9%	1890
December	1st to 10th	Day	43% 4 7% 18% 23% 5%	3020
	11th to 20th	Day	46% 5% 6% 16% 19% 8%	3266
	21st to end	Day	36% 4 7% 23% 27% 2	4326
January	1st to 10th	Night	41% 3 6% 21% 23% 5%	6018
	11th to 20th	Night	37% 4 5% 17% 27% 10%	6030
	21st to end	Night	50% 3 5% 14% 20% 9%	6588
February	1st to 10th	Night	43% 4 6% 16% 21% 9%	2401
	11th to 20th	Night	43% 4 7% 17% 19% 10%	2339
	21st to end	Night	46% 5% 7% 14% 17% 12%	1980
March	1st to 10th	Night	44% 6% 8% 12% 19% 11%	5189
	11th to 20th	Night	33% 6% 7% 17% 19% 17%	4980
	21st to end	Night	42% 6% 10% 17% 19% 7%	5206
April	1st to 10th	Night	36% 7% 10% 19% 19% 9%	2000
	11th to 20th	Night	34% 5% 9% 19% 23% 10%	1975
	21st to end	Night	30% 6% 11% 24% 25% 5%	2000
May	1st to 10th	Night	38% 8% 13% 22% 16% 3	4520
	11th to 20th	Night	42% 7% 16% 22% 12% 1	4490
	21st to end	Night	32% 11% 18% 22% 16% 2	4664
June	1st to 10th	Night	32% 10% 15% 24% 17% 2	1600
	11th to 20th	Night	24% 8% 21% 27% 16% 5%	1600
	21st to end	Night	27% 9% 21% 24% 16% 3	1647
July	1st to 10th	Night	33% 12% 22% 22% 9% 1	4035
	11th to 20th	Night	29% 12% 32% 23% 3 1	4026
	21st to end	Night	37% 10% 19% 27% 7%	4447
August	1st to 10th	Night	16% 7% 18% 30% 28% 2	3232
	11th to 20th	Night	23% 10% 22% 32% 12% 2	3230
	21st to end	Night	32% 8% 13% 19% 22% 5%	3586
September	1st to 10th	Night	31% 10% 15% 20% 17% 7%	2000
	11th to 20th	Night	32% 8% 14% 16% 22% 8%	2016
	21st to end	Night	27% 7% 10% 25% 20% 11%	2200
October	1st to 10th	Night	15% 3 6% 16% 24% 35%	3839
	11th to 20th	Night	19% 3 5% 14% 27% 32%	3899
	21st to end	Night	22% 3 6% 14% 35% 21%	4252
November	1st to 10th	Night	30% 3 7% 18% 29% 13%	2460
	11th to 20th	Night	29% 4 8% 17% 26% 15%	2708
	21st to end	Night	36% 4 8% 20% 20% 12%	2730
December	1st to 10th	Night	43% 4 6% 17% 24% 5%	4380
	11th to 20th	Night	42% 4 7% 16% 24% 6%	5134
	21st to end	Night	40% 4 7% 21% 25% 3	6887

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	24% 18% 38% 15% 5% 1	330
	11th to 20th	Day	24% 17% 34% 18% 7% 1	330
	21st to end	Day	25% 21% 33% 16% 4	363
February	1st to 10th	Day	24% 17% 34% 18% 6% 1	300
	11th to 20th	Day	33% 13% 37% 14% 2	300
	21st to end	Day	26% 22% 35% 15% 2	249
March	1st to 10th	Day	25% 15% 36% 20% 5%	300
	11th to 20th	Day	23% 16% 36% 21% 4	300
	21st to end	Day	18% 16% 34% 24% 9%	435
April	1st to 10th	Day	17% 15% 33% 24% 11% 1	500
	11th to 20th	Day	21% 12% 30% 25% 11% 2	492
	21st to end	Day	14% 9% 26% 32% 18% 1	500
May	1st to 10th	Day	14% 8% 29% 31% 15% 3	503
	11th to 20th	Day	13% 6% 24% 28% 21% 8%	527
	21st to end	Day	7% 4 19% 32% 25% 12%	581
June	1st to 10th	Day	10% 4 12% 32% 28% 14%	730
	11th to 20th	Day	6% 2 12% 31% 36% 13%	730
	21st to end	Day	6% 2 10% 27% 37% 18%	730
July	1st to 10th	Day	5% 2 9% 25% 38% 21%	721
	11th to 20th	Day	6% 1 4 21% 40% 28%	718
	21st to end	Day	8% 1 7% 21% 41% 23%	729
August	1st to 10th	Day	5% 1 6% 21% 38% 29%	526
	11th to 20th	Day	6% 1 5% 19% 41% 29%	519
	21st to end	Day	13% 1 8% 19% 33% 26%	583
September	1st to 10th	Day	8% 2 8% 29% 37% 17%	530
	11th to 20th	Day	12% 2 10% 19% 34% 23%	530
	21st to end	Day	12% 4 11% 30% 32% 11%	530
October	1st to 10th	Day	7% 2 23% 36% 23% 9%	380
	11th to 20th	Day	8% 6% 25% 36% 20% 6%	330
	21st to end	Day	19% 10% 23% 26% 17% 6%	363
November	1st to 10th	Day	16% 7% 25% 32% 16% 4	330
	11th to 20th	Day	13% 12% 36% 27% 11% 2	330
	21st to end	Day	15% 15% 37% 20% 10% 3	330
December	1st to 10th	Day	21% 18% 36% 18% 6% 1	330
	11th to 20th	Day	18% 12% 37% 23% 8% 1	330
	21st to end	Day	18% 17% 36% 20% 8% 1	363
January	1st to 10th	Night	21% 11% 33% 29% 5%	1050
	11th to 20th	Night	19% 13% 33% 28% 7%	1050
	21st to end	Night	22% 12% 35% 29% 3	1136
February	1st to 10th	Night	20% 10% 34% 30% 5%	801
	11th to 20th	Night	22% 14% 31% 28% 4	783
	21st to end	Night	23% 14% 37% 23% 2	660
March	1st to 10th	Night	18% 12% 32% 33% 5%	720
	11th to 20th	Night	16% 10% 37% 31% 6% 1	600
	21st to end	Night	16% 9% 36% 30% 9%	660
April	1st to 10th	Night	15% 9% 35% 31% 10% 1	600
	11th to 20th	Night	14% 8% 34% 31% 12% 1	593
	21st to end	Night	8% 5% 37% 36% 14%	600
May	1st to 10th	Night	10% 5% 26% 42% 15% 2	600
	11th to 20th	Night	11% 3 22% 40% 21% 3	591
	21st to end	Night	6% 3 17% 37% 31% 6%	650
June	1st to 10th	Night	7% 2 11% 39% 37% 5%	600
	11th to 20th	Night	4 2 7% 33% 47% 8%	600
	21st to end	Night	5% 2 8% 28% 46% 12%	600
July	1st to 10th	Night	5% 1 6% 28% 51% 9%	585
	11th to 20th	Night	5% 1 3 26% 47% 17%	582
	21st to end	Night	5% 1 4 28% 51% 12%	657
August	1st to 10th	Night	3 4 26% 50% 16%	597
	11th to 20th	Night	5% 1 5% 22% 52% 14%	594
	21st to end	Night	9% 2 6% 27% 41% 15%	660
September	1st to 10th	Night	8% 2 7% 34% 43% 7%	600
	11th to 20th	Night	9% 2 8% 29% 44% 7%	616
	21st to end	Night	10% 3 10% 37% 37% 5%	800
October	1st to 10th	Night	8% 2 14% 38% 32% 6%	800
	11th to 20th	Night	12% 5% 20% 37% 24% 2	800
	21st to end	Night	16% 5% 21% 35% 21% 3	880
November	1st to 10th	Night	14% 5% 20% 39% 20% 3	820
	11th to 20th	Night	13% 7% 27% 38% 14% 1	840
	21st to end	Night	15% 7% 30% 34% 13% 2	840
December	1st to 10th	Night	20% 7% 32% 32% 9%	840
	11th to 20th	Night	19% 7% 31% 34% 9%	924
	21st to end	Night	23% 11% 29% 30% 7%	1155

Page generated at 2026-01-29 00:21:42. Version ac07

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