Hale Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hale Mountain Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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% 7% 3 6% 4 4 4 4 5% 7% 45%	127543
	Night	10% 7% 3 6% 4 3 4 3 5% 6% 50%	124410
Transparency	Day	37% 6% 10% 24% 18% 5%	131155
	Night	34% 5% 10% 24% 21% 6%	128874
Seeing	Day	17% 14% 30% 25% 12% 2	17019
	Night	16% 15% 31% 26% 11% 1	26922

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	10% 12% 5% 6% 3 3 3 3 4 5% 44%	4167
	11th to 20th	Day	6% 6% 2 7% 4 4 4 3 4 4 55%	4134
	21st to end	Day	19% 9% 3 4 3 2 3 3 3 4 47%	4488
February	1st to 10th	Day	17% 10% 3 7% 6% 4 3 3 3 3 40%	1800
	11th to 20th	Day	14% 11% 3 6% 3 4 3 3 4 6% 42%	1787
	21st to end	Day	15% 10% 3 6% 4 4 3 3 4 5% 41%	1790
March	1st to 10th	Day	13% 8% 2 6% 4 4 4 3 5% 5% 46%	4936
	11th to 20th	Day	9% 6% 2 5% 4 4 4 3 5% 6% 53%	4980
	21st to end	Day	13% 9% 3 7% 5% 5% 4 4 5% 7% 37%	5462
April	1st to 10th	Day	14% 6% 3 7% 5% 4 4 4 5% 6% 43%	2292
	11th to 20th	Day	16% 8% 2 7% 5% 4 4 3 5% 6% 41%	2374
	21st to end	Day	17% 11% 3 7% 5% 4 3 3 4 5% 38%	2407
May	1st to 10th	Day	17% 9% 3 8% 5% 4 5% 3 5% 5% 36%	5862
	11th to 20th	Day	10% 7% 3 8% 5% 5% 6% 4 5% 7% 39%	5862
	21st to end	Day	17% 8% 3 8% 5% 5% 5% 4 6% 7% 34%	6434
June	1st to 10th	Day	9% 6% 3 6% 5% 5% 5% 4 7% 9% 43%	2600
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 6% 8% 44%	2600
	21st to end	Day	4 4 2 4 5% 5% 5% 5% 6% 9% 52%	2600
July	1st to 10th	Day	7% 4 1 6% 5% 5% 6% 5% 8% 12% 41%	5900
	11th to 20th	Day	5% 5% 2 6% 4 4 5% 4 6% 10% 50%	5900
	21st to end	Day	5% 3 2 5% 4 5% 6% 5% 8% 10% 45%	6422
August	1st to 10th	Day	4 3 1 4 4 4 5% 4 6% 9% 55%	4234
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 6% 10% 47%	4122
	21st to end	Day	4 4 2 5% 4 5% 5% 4 6% 8% 53%	4312
September	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 5% 7% 53%	2400
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 8% 8% 45%	2204
	21st to end	Day	7% 4 2 5% 4 5% 4 4 5% 7% 53%	2185
October	1st to 10th	Day	8% 5% 2 5% 3 2 4 3 5% 6% 57%	2000
	11th to 20th	Day	12% 4 2 5% 3 2 2 2 4 5% 59%	2000
	21st to end	Day	18% 7% 3 4 3 3 3 2 4 4 50%	2200
November	1st to 10th	Day	10% 6% 2 4 3 3 2 2 4 6% 57%	2058
	11th to 20th	Day	10% 7% 2 5% 3 3 3 2 5% 6% 54%	2043
	21st to end	Day	14% 10% 3 6% 2 3 3 3 5% 5% 47%	1890
December	1st to 10th	Day	12% 9% 2 5% 4 3 3 3 3 5% 52%	3030
	11th to 20th	Day	10% 9% 3 7% 4 3 4 3 5% 6% 46%	3481
	21st to end	Day	16% 16% 7% 7% 4 3 3 3 4 5% 33%	4587
January	1st to 10th	Night	11% 8% 4 9% 6% 4 4 3 4 5% 43%	6018
	11th to 20th	Night	8% 6% 2 7% 4 3 4 2 4 5% 56%	5979
	21st to end	Night	17% 10% 4 6% 3 2 2 2 4 4 45%	6490
February	1st to 10th	Night	15% 10% 3 7% 4 3 3 3 5% 6% 43%	2600
	11th to 20th	Night	14% 8% 3 7% 4 4 3 3 4 5% 45%	2517
	21st to end	Night	13% 9% 3 6% 4 4 4 3 4 5% 46%	1838
March	1st to 10th	Night	15% 10% 3 5% 4 3 4 2 4 4 45%	4919
	11th to 20th	Night	9% 8% 2 5% 3 3 4 3 5% 5% 53%	4980
	21st to end	Night	10% 7% 3 7% 5% 4 4 3 5% 8% 43%	5467
April	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 5% 6% 48%	2106
	11th to 20th	Night	12% 8% 3 5% 3 4 3 3 5% 6% 49%	1975
	21st to end	Night	14% 8% 3 6% 4 4 5% 3 4 6% 43%	1990
May	1st to 10th	Night	14% 8% 3 6% 3 4 4 3 5% 7% 43%	4053
	11th to 20th	Night	11% 7% 2 7% 4 4 5% 4 6% 8% 42%	4053
	21st to end	Night	15% 8% 2 7% 5% 4 4 4 5% 7% 38%	4443
June	1st to 10th	Night	6% 4 2 5% 3 3 4 4 6% 8% 55%	1800
	11th to 20th	Night	5% 3 1 4 4 4 4 4 7% 9% 55%	1800
	21st to end	Night	3 2 1 3 3 3 4 3 5% 9% 63%	1800
July	1st to 10th	Night	7% 4 2 6% 4 4 5% 5% 7% 10% 46%	4078
	11th to 20th	Night	4 3 1 4 3 3 3 3 5% 9% 62%	4077
	21st to end	Night	3 3 1 5% 3 3 4 3 7% 10% 56%	4449
August	1st to 10th	Night	3 2 1 4 4 3 5% 4 6% 10% 58%	2926
	11th to 20th	Night	6% 4 2 4 4 3 4 4 6% 8% 54%	2920
	21st to end	Night	3 3 1 4 3 3 3 3 6% 8% 63%	3586
September	1st to 10th	Night	8% 4 2 5% 3 3 4 4 5% 7% 57%	2000
	11th to 20th	Night	9% 5% 2 5% 4 3 4 4 6% 8% 51%	2191
	21st to end	Night	7% 5% 2 5% 4 3 4 3 5% 6% 57%	2200
October	1st to 10th	Night	6% 3 1 4 3 3 4 3 5% 7% 62%	2393
	11th to 20th	Night	12% 4 1 4 3 3 3 3 4 5% 58%	2400
	21st to end	Night	17% 7% 3 4 2 2 3 2 4 5% 51%	2640
November	1st to 10th	Night	11% 6% 2 4 2 2 3 3 3 5% 59%	2460
	11th to 20th	Night	10% 6% 2 4 4 3 3 3 4 5% 57%	2573
	21st to end	Night	14% 9% 3 5% 3 3 3 2 4 5% 51%	2730
December	1st to 10th	Night	8% 11% 3 6% 3 3 3 2 4 5% 52%	4380
	11th to 20th	Night	13% 9% 3 7% 4 3 3 2 3 4 49%	4946
	21st to end	Night	16% 13% 7% 7% 4 3 3 2 3 4 38%	6633

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 7% 25% 20% 2	4168
	11th to 20th	Day	32% 4 10% 25% 24% 5%	4134
	21st to end	Day	42% 3 6% 20% 22% 8%	4488
February	1st to 10th	Day	49% 4 9% 20% 16% 3	1800
	11th to 20th	Day	44% 3 8% 22% 19% 3	1796
	21st to end	Day	46% 4 6% 18% 19% 8%	1790
March	1st to 10th	Day	40% 4 6% 19% 15% 16%	4936
	11th to 20th	Day	33% 5% 8% 19% 22% 14%	4980
	21st to end	Day	44% 6% 10% 28% 12% 1	5464
April	1st to 10th	Day	41% 5% 10% 20% 18% 5%	2292
	11th to 20th	Day	44% 5% 10% 20% 16% 5%	2374
	21st to end	Day	50% 5% 7% 21% 14% 2	2407
May	1st to 10th	Day	49% 6% 8% 20% 15% 1	5900
	11th to 20th	Day	42% 8% 11% 22% 16% 1	5866
	21st to end	Day	49% 7% 11% 16% 16% 1	6447
June	1st to 10th	Day	37% 9% 13% 25% 14% 2	2600
	11th to 20th	Day	36% 9% 13% 25% 16% 1	2600
	21st to end	Day	27% 8% 15% 29% 17% 4	2600
July	1st to 10th	Day	31% 8% 19% 29% 11% 1	5836
	11th to 20th	Day	28% 6% 15% 36% 14% 1	5831
	21st to end	Day	30% 9% 18% 32% 11% 1	6240
August	1st to 10th	Day	23% 7% 15% 28% 22% 4	4220
	11th to 20th	Day	33% 6% 16% 29% 14% 1	4161
	21st to end	Day	27% 8% 15% 35% 13% 3	4311
September	1st to 10th	Day	32% 5% 10% 26% 18% 9%	2400
	11th to 20th	Day	33% 7% 11% 23% 21% 5%	2204
	21st to end	Day	30% 6% 10% 21% 28% 6%	2185
October	1st to 10th	Day	18% 3 6% 14% 40% 19%	3260
	11th to 20th	Day	20% 2 4 19% 40% 16%	3240
	21st to end	Day	30% 3 10% 28% 23% 5%	3538
November	1st to 10th	Day	30% 5% 8% 26% 23% 8%	2058
	11th to 20th	Day	33% 3 7% 26% 22% 10%	2043
	21st to end	Day	40% 3 8% 22% 23% 4	1890
December	1st to 10th	Day	37% 3 7% 29% 22% 2	3030
	11th to 20th	Day	41% 3 6% 24% 23% 3	3479
	21st to end	Day	53% 4 7% 21% 13% 2	4587
January	1st to 10th	Night	42% 3 10% 24% 17% 3	6018
	11th to 20th	Night	28% 5% 11% 19% 32% 4	5979
	21st to end	Night	43% 3 8% 16% 25% 5%	6490
February	1st to 10th	Night	44% 4 8% 24% 15% 5%	2600
	11th to 20th	Night	41% 4 8% 23% 19% 4	2518
	21st to end	Night	41% 3 7% 17% 22% 9%	1838
March	1st to 10th	Night	43% 4 5% 18% 18% 12%	4919
	11th to 20th	Night	34% 5% 6% 22% 21% 12%	4980
	21st to end	Night	38% 5% 13% 26% 15% 3	5467
April	1st to 10th	Night	38% 5% 9% 21% 19% 7%	2106
	11th to 20th	Night	36% 5% 8% 23% 20% 8%	1975
	21st to end	Night	42% 4 9% 23% 19% 2	1990
May	1st to 10th	Night	41% 7% 11% 21% 17% 3	4080
	11th to 20th	Night	37% 8% 14% 23% 14% 4	4054
	21st to end	Night	43% 8% 14% 18% 13% 3	4452
June	1st to 10th	Night	24% 8% 14% 31% 19% 3	1800
	11th to 20th	Night	24% 6% 13% 32% 21% 3	1800
	21st to end	Night	19% 6% 16% 30% 24% 6%	1800
July	1st to 10th	Night	31% 8% 18% 26% 14% 3	4035
	11th to 20th	Night	20% 6% 15% 39% 18% 1	4026
	21st to end	Night	24% 7% 16% 37% 15% 1	4320
August	1st to 10th	Night	21% 7% 16% 33% 22% 2	2916
	11th to 20th	Night	28% 6% 19% 29% 15% 2	2947
	21st to end	Night	20% 6% 13% 38% 21% 3	3586
September	1st to 10th	Night	27% 5% 11% 28% 21% 10%	2000
	11th to 20th	Night	31% 6% 11% 23% 23% 5%	2191
	21st to end	Night	29% 5% 8% 22% 30% 6%	2200
October	1st to 10th	Night	14% 3 7% 14% 36% 26%	3913
	11th to 20th	Night	20% 3 4 17% 36% 19%	3899
	21st to end	Night	30% 3 6% 28% 25% 8%	4252
November	1st to 10th	Night	30% 4 7% 23% 26% 9%	2460
	11th to 20th	Night	30% 3 7% 25% 25% 9%	2573
	21st to end	Night	38% 3 7% 22% 24% 6%	2730
December	1st to 10th	Night	36% 4 7% 30% 21% 3	4380
	11th to 20th	Night	40% 3 5% 22% 21% 9%	4947
	21st to end	Night	51% 3 9% 20% 13% 4	6633

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% 27% 35% 10% 3	330
	11th to 20th	Day	25% 31% 36% 7% 2	318
	21st to end	Day	23% 32% 32% 11% 3	330
February	1st to 10th	Day	32% 25% 35% 7%	300
	11th to 20th	Day	35% 30% 27% 7% 1	300
	21st to end	Day	36% 25% 29% 8% 1	249
March	1st to 10th	Day	26% 27% 32% 14% 1	300
	11th to 20th	Day	34% 24% 33% 8% 1	300
	21st to end	Day	35% 29% 29% 6% 2	330
April	1st to 10th	Day	28% 25% 31% 11% 4 1	392
	11th to 20th	Day	27% 22% 36% 12% 3	492
	21st to end	Day	26% 20% 36% 14% 4 1	507
May	1st to 10th	Day	18% 20% 42% 16% 4	703
	11th to 20th	Day	17% 18% 34% 22% 8% 1	725
	21st to end	Day	14% 12% 40% 21% 11% 3	789
June	1st to 10th	Day	10% 8% 29% 34% 17% 2	730
	11th to 20th	Day	8% 7% 30% 37% 16% 2	730
	21st to end	Day	4 6% 34% 37% 17% 2	730
July	1st to 10th	Day	9% 3 22% 37% 22% 7%	721
	11th to 20th	Day	5% 3 23% 41% 24% 4	718
	21st to end	Day	7% 1 17% 40% 27% 7%	794
August	1st to 10th	Day	7% 3 21% 43% 22% 3	726
	11th to 20th	Day	11% 3 17% 41% 24% 3	692
	21st to end	Day	8% 2 18% 35% 29% 8%	583
September	1st to 10th	Day	11% 2 16% 40% 25% 6%	530
	11th to 20th	Day	6% 2 29% 37% 21% 4	334
	21st to end	Day	13% 6% 29% 33% 16% 2	330
October	1st to 10th	Day	8% 12% 32% 32% 12% 4	330
	11th to 20th	Day	17% 15% 41% 22% 4 1	330
	21st to end	Day	31% 17% 33% 17% 2 1	363
November	1st to 10th	Day	17% 19% 43% 16% 4 1	330
	11th to 20th	Day	20% 22% 35% 18% 5%	330
	21st to end	Day	28% 26% 28% 15% 3	330
December	1st to 10th	Day	25% 25% 35% 11% 2 2	330
	11th to 20th	Day	25% 27% 38% 8% 2 1	330
	21st to end	Day	25% 27% 38% 9% 2	363
January	1st to 10th	Night	20% 22% 37% 18% 4	1050
	11th to 20th	Night	21% 24% 36% 16% 3	1031
	21st to end	Night	19% 24% 33% 21% 3	1100
February	1st to 10th	Night	24% 23% 34% 17% 3	1000
	11th to 20th	Night	22% 27% 35% 13% 2	963
	21st to end	Night	25% 24% 31% 19% 2	518
March	1st to 10th	Night	21% 21% 34% 22% 2	600
	11th to 20th	Night	24% 19% 33% 21% 3	600
	21st to end	Night	18% 25% 38% 14% 4	660
April	1st to 10th	Night	19% 23% 35% 20% 4	600
	11th to 20th	Night	17% 22% 37% 19% 5%	593
	21st to end	Night	19% 17% 40% 17% 7%	600
May	1st to 10th	Night	14% 13% 41% 26% 7%	600
	11th to 20th	Night	15% 12% 32% 29% 11% 1	591
	21st to end	Night	11% 11% 34% 27% 15% 2	650
June	1st to 10th	Night	7% 6% 24% 39% 22% 1	600
	11th to 20th	Night	6% 2 34% 39% 17% 2	600
	21st to end	Night	3 2 29% 44% 20% 3	600
July	1st to 10th	Night	5% 1 18% 41% 31% 3	585
	11th to 20th	Night	5% 1 19% 43% 27% 4	582
	21st to end	Night	4 1 16% 41% 33% 4	657
August	1st to 10th	Night	4 1 20% 44% 29% 3	597
	11th to 20th	Night	8% 2 19% 42% 26% 4	594
	21st to end	Night	7% 2 19% 39% 28% 4	660
September	1st to 10th	Night	10% 2 17% 45% 24% 2	600
	11th to 20th	Night	12% 3 19% 39% 22% 5%	600
	21st to end	Night	10% 6% 26% 34% 21% 3	600
October	1st to 10th	Night	7% 7% 33% 31% 21% 2	793
	11th to 20th	Night	14% 14% 36% 27% 9%	800
	21st to end	Night	23% 14% 32% 22% 8% 1	880
November	1st to 10th	Night	14% 16% 37% 24% 8%	820
	11th to 20th	Night	16% 16% 37% 24% 6%	893
	21st to end	Night	23% 20% 32% 20% 6%	1050
December	1st to 10th	Night	20% 19% 37% 19% 4	1050
	11th to 20th	Night	21% 21% 33% 21% 4	1050
	21st to end	Night	21% 24% 34% 17% 4	1155

Page generated at 2026-01-14 00:14:29. Version ac07

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