AASO/Ridgemont Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the AASO/Ridgemont Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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	13% 7% 2 6% 4 4 4 4 5% 7% 42%	125432
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 45%	122781
Transparency	Day	39% 6% 11% 24% 17% 3	129063
	Night	38% 5% 10% 24% 19% 4	127227
Seeing	Day	15% 8% 22% 26% 20% 10%	16471
	Night	16% 7% 25% 32% 18% 2	26229

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	16% 8% 4 9% 4 4 4 4 5% 5% 37%	3617
	11th to 20th	Day	14% 8% 3 6% 4 4 3 3 4 5% 45%	3781
	21st to end	Day	20% 9% 3 6% 4 4 4 3 4 5% 38%	4488
February	1st to 10th	Day	18% 11% 3 6% 4 4 3 3 6% 6% 37%	1980
	11th to 20th	Day	19% 10% 3 7% 4 4 4 3 4 5% 38%	1948
	21st to end	Day	18% 9% 4 7% 5% 4 3 3 4 6% 37%	1650
March	1st to 10th	Day	17% 12% 2 6% 4 5% 4 3 4 5% 39%	4480
	11th to 20th	Day	13% 7% 3 5% 3 3 3 2 4 6% 51%	4739
	21st to end	Day	12% 8% 2 7% 5% 4 4 3 5% 6% 43%	5963
April	1st to 10th	Day	14% 6% 2 5% 4 4 4 3 5% 8% 45%	2400
	11th to 20th	Day	14% 8% 2 7% 5% 3 4 4 4 6% 42%	2374
	21st to end	Day	17% 7% 2 7% 4 4 4 3 5% 8% 39%	2400
May	1st to 10th	Day	17% 9% 3 8% 5% 5% 5% 5% 7% 8% 27%	5716
	11th to 20th	Day	17% 6% 3 7% 5% 5% 6% 4 6% 8% 33%	5861
	21st to end	Day	15% 6% 3 7% 5% 5% 4 4 6% 7% 39%	6725
June	1st to 10th	Day	10% 5% 2 5% 4 5% 5% 4 7% 9% 45%	2800
	11th to 20th	Day	6% 4 1 5% 5% 4 5% 5% 6% 9% 49%	2800
	21st to end	Day	5% 5% 2 5% 5% 5% 5% 5% 7% 10% 46%	2800
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 5% 7% 9% 43%	6360
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 8% 12% 43%	6360
	21st to end	Day	7% 6% 3 7% 6% 6% 5% 5% 8% 11% 35%	6724
August	1st to 10th	Day	6% 4 1 4 3 4 4 4 6% 9% 56%	3833
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 6% 10% 44%	3729
	21st to end	Day	5% 3 1 5% 4 4 4 3 6% 9% 56%	4180
September	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 6% 7% 48%	2280
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 5% 6% 49%	2164
	21st to end	Day	13% 5% 2 5% 4 3 4 3 4 8% 51%	2090
October	1st to 10th	Day	9% 4 1 5% 4 3 3 3 4 6% 59%	2062
	11th to 20th	Day	14% 5% 2 5% 4 3 3 2 3 6% 53%	1900
	21st to end	Day	20% 6% 2 4 3 3 3 3 4 5% 48%	1995
November	1st to 10th	Day	11% 6% 2 5% 3 4 4 3 5% 6% 52%	1762
	11th to 20th	Day	15% 7% 2 6% 4 3 3 3 4 6% 48%	1800
	21st to end	Day	19% 8% 2 6% 4 3 3 3 4 6% 43%	1800
December	1st to 10th	Day	15% 9% 2 6% 3 3 4 3 4 6% 46%	2881
	11th to 20th	Day	15% 9% 3 7% 4 5% 4 4 5% 5% 39%	3008
	21st to end	Day	24% 12% 4 8% 5% 4 4 3 4 5% 28%	3982
January	1st to 10th	Night	18% 10% 4 8% 3 3 4 3 5% 6% 36%	6348
	11th to 20th	Night	15% 9% 3 7% 4 4 3 3 4 5% 44%	6194
	21st to end	Night	21% 9% 3 5% 3 2 3 3 5% 5% 40%	6490
February	1st to 10th	Night	21% 10% 4 7% 4 3 4 3 4 5% 34%	2417
	11th to 20th	Night	19% 8% 3 7% 4 4 3 3 4 5% 40%	2338
	21st to end	Night	17% 9% 3 7% 5% 4 4 3 4 4 39%	1980
March	1st to 10th	Night	20% 9% 4 7% 4 4 4 3 4 5% 37%	5373
	11th to 20th	Night	13% 6% 2 5% 3 4 4 4 5% 6% 49%	5265
	21st to end	Night	15% 7% 3 7% 5% 4 5% 3 5% 7% 40%	4964
April	1st to 10th	Night	14% 5% 2 5% 4 3 5% 3 6% 6% 48%	2000
	11th to 20th	Night	15% 5% 2 6% 4 3 4 3 5% 7% 48%	1975
	21st to end	Night	16% 7% 3 5% 5% 4 3 4 6% 7% 40%	2000
May	1st to 10th	Night	18% 8% 3 7% 4 4 4 4 6% 9% 33%	4178
	11th to 20th	Night	15% 7% 3 8% 6% 5% 4 3 5% 8% 35%	4032
	21st to end	Night	14% 6% 2 7% 5% 4 5% 4 6% 8% 40%	4125
June	1st to 10th	Night	10% 5% 3 6% 4 3 5% 4 6% 8% 47%	1600
	11th to 20th	Night	5% 3 2 5% 4 4 3 5% 8% 8% 54%	1600
	21st to end	Night	3 2 2 4 4 4 5% 4 9% 13% 51%	1600
July	1st to 10th	Night	8% 5% 2 6% 4 5% 4 5% 6% 7% 48%	3618
	11th to 20th	Night	5% 3 2 5% 4 5% 5% 4 6% 12% 49%	3617
	21st to end	Night	6% 5% 2 6% 4 4 5% 5% 7% 13% 42%	4113
August	1st to 10th	Night	4 3 1 4 4 3 4 4 5% 7% 61%	3184
	11th to 20th	Night	5% 4 1 5% 4 4 4 4 6% 8% 54%	3100
	21st to end	Night	5% 3 1 3 3 3 3 3 6% 8% 62%	3476
September	1st to 10th	Night	9% 5% 2 5% 4 3 3 3 4 5% 57%	1900
	11th to 20th	Night	9% 6% 2 5% 3 3 4 3 5% 7% 53%	1996
	21st to end	Night	11% 5% 1 4 3 3 4 3 4 6% 55%	2090
October	1st to 10th	Night	8% 4 2 4 3 3 4 2 4 7% 61%	2114
	11th to 20th	Night	14% 4 1 4 3 4 3 2 4 5% 56%	2280
	21st to end	Night	18% 6% 2 6% 3 3 3 3 4 5% 47%	2600
November	1st to 10th	Night	13% 5% 2 5% 3 3 3 2 4 6% 55%	2536
	11th to 20th	Night	14% 6% 2 5% 3 4 3 3 5% 6% 51%	2600
	21st to end	Night	17% 6% 2 5% 4 3 3 3 4 5% 49%	2600
December	1st to 10th	Night	18% 7% 2 7% 4 3 4 2 4 4 46%	4283
	11th to 20th	Night	19% 8% 2 5% 4 3 3 3 4 6% 44%	5199
	21st to end	Night	23% 11% 3 7% 4 4 4 3 4 5% 31%	6996

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	48% 5% 11% 24% 11% 2	3618
	11th to 20th	Day	41% 4 7% 25% 21% 2	3781
	21st to end	Day	48% 4 9% 18% 15% 6%	4488
February	1st to 10th	Day	48% 4 11% 23% 13% 2	1980
	11th to 20th	Day	49% 3 10% 20% 13% 5%	1958
	21st to end	Day	49% 4 9% 18% 14% 6%	1650
March	1st to 10th	Day	48% 3 5% 21% 17% 6%	4480
	11th to 20th	Day	36% 4 7% 25% 22% 5%	4739
	21st to end	Day	41% 5% 10% 26% 17%	5965
April	1st to 10th	Day	38% 6% 11% 25% 18% 2	2400
	11th to 20th	Day	42% 5% 9% 22% 18% 4	2374
	21st to end	Day	44% 6% 10% 25% 14% 2	2400
May	1st to 10th	Day	51% 10% 12% 17% 10%	5752
	11th to 20th	Day	47% 9% 12% 21% 10% 1	5865
	21st to end	Day	42% 8% 10% 21% 17% 3	6739
June	1st to 10th	Day	33% 9% 14% 23% 20% 2	2800
	11th to 20th	Day	29% 8% 13% 27% 21% 3	2800
	21st to end	Day	31% 9% 16% 23% 16% 5%	2800
July	1st to 10th	Day	35% 7% 12% 28% 16% 2	6291
	11th to 20th	Day	31% 10% 17% 32% 9% 2	6285
	21st to end	Day	40% 9% 19% 24% 8% 1	6531
August	1st to 10th	Day	24% 7% 15% 30% 23% 1	3821
	11th to 20th	Day	35% 7% 13% 26% 18% 1	3765
	21st to end	Day	24% 6% 13% 35% 20% 3	4179
September	1st to 10th	Day	34% 7% 10% 25% 20% 4	2280
	11th to 20th	Day	35% 6% 9% 23% 24% 2	2164
	21st to end	Day	34% 5% 8% 24% 25% 5%	2090
October	1st to 10th	Day	17% 3 6% 18% 45% 10%	3426
	11th to 20th	Day	24% 3 6% 25% 30% 13%	3140
	21st to end	Day	30% 3 6% 27% 29% 5%	3272
November	1st to 10th	Day	34% 4 9% 24% 24% 5%	1762
	11th to 20th	Day	38% 4 6% 22% 23% 6%	1800
	21st to end	Day	44% 3 7% 25% 17% 3	1800
December	1st to 10th	Day	41% 4 8% 29% 17% 1	2881
	11th to 20th	Day	46% 5% 9% 24% 13% 3	3005
	21st to end	Day	58% 5% 12% 17% 8% 1	3982
January	1st to 10th	Night	49% 4 9% 25% 12% 1	6348
	11th to 20th	Night	43% 5% 7% 23% 20% 2	6194
	21st to end	Night	46% 5% 7% 23% 15% 4	6490
February	1st to 10th	Night	53% 4 9% 19% 13% 1	2417
	11th to 20th	Night	48% 4 7% 21% 17% 3	2339
	21st to end	Night	47% 5% 8% 18% 18% 4	1980
March	1st to 10th	Night	50% 5% 5% 15% 19% 6%	5373
	11th to 20th	Night	36% 5% 9% 21% 18% 11%	5265
	21st to end	Night	44% 5% 9% 27% 14% 2	4964
April	1st to 10th	Night	35% 5% 11% 26% 19% 4	2000
	11th to 20th	Night	36% 5% 10% 23% 21% 6%	1975
	21st to end	Night	42% 6% 10% 22% 17% 2	2000
May	1st to 10th	Night	47% 8% 14% 21% 9% 1	4207
	11th to 20th	Night	48% 7% 13% 20% 11% 1	4033
	21st to end	Night	39% 9% 11% 24% 15% 2	4133
June	1st to 10th	Night	32% 8% 12% 22% 23% 3	1600
	11th to 20th	Night	25% 8% 15% 25% 22% 5%	1600
	21st to end	Night	22% 9% 17% 26% 18% 7%	1600
July	1st to 10th	Night	33% 7% 12% 25% 20% 3	3580
	11th to 20th	Night	27% 9% 20% 29% 13% 2	3572
	21st to end	Night	33% 8% 21% 28% 9%	3995
August	1st to 10th	Night	21% 7% 15% 36% 21% 1	3172
	11th to 20th	Night	26% 7% 17% 28% 19% 3	3130
	21st to end	Night	20% 5% 11% 36% 25% 3	3476
September	1st to 10th	Night	29% 6% 9% 28% 22% 5%	1900
	11th to 20th	Night	31% 6% 8% 27% 24% 5%	1996
	21st to end	Night	30% 4 10% 25% 23% 7%	2090
October	1st to 10th	Night	17% 3 5% 14% 46% 14%	3530
	11th to 20th	Night	21% 3 5% 20% 40% 13%	3779
	21st to end	Night	29% 3 6% 24% 27% 11%	4273
November	1st to 10th	Night	32% 3 8% 25% 26% 6%	2536
	11th to 20th	Night	35% 3 7% 24% 23% 8%	2600
	21st to end	Night	40% 3 7% 25% 22% 3	2600
December	1st to 10th	Night	43% 4 8% 25% 19% 1	4283
	11th to 20th	Night	43% 4 10% 23% 19% 2	5201
	21st to end	Night	55% 6% 9% 20% 9% 2	6996

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% 14% 35% 21% 5% 1	300
	11th to 20th	Day	24% 18% 35% 19% 3	300
	21st to end	Day	18% 17% 41% 19% 5%	330
February	1st to 10th	Day	27% 14% 36% 18% 4 2	300
	11th to 20th	Day	28% 17% 38% 14% 2 1	300
	21st to end	Day	27% 13% 36% 19% 5%	249
March	1st to 10th	Day	20% 13% 41% 19% 6% 1	300
	11th to 20th	Day	23% 18% 34% 20% 3 1	300
	21st to end	Day	23% 17% 33% 20% 7% 1	546
April	1st to 10th	Day	18% 13% 33% 25% 10% 1	500
	11th to 20th	Day	18% 15% 28% 25% 12% 2	492
	21st to end	Day	18% 11% 30% 27% 10% 3	500
May	1st to 10th	Day	17% 14% 25% 28% 12% 4	503
	11th to 20th	Day	19% 7% 21% 26% 19% 8%	527
	21st to end	Day	11% 4 23% 29% 20% 12%	696
June	1st to 10th	Day	10% 5% 16% 27% 25% 16%	730
	11th to 20th	Day	8% 2 14% 32% 30% 13%	730
	21st to end	Day	7% 2 15% 35% 27% 14%	730
July	1st to 10th	Day	9% 1 11% 27% 27% 24%	721
	11th to 20th	Day	6% 2 11% 28% 36% 16%	718
	21st to end	Day	7% 2 11% 27% 34% 19%	787
August	1st to 10th	Day	10% 1 10% 29% 30% 21%	502
	11th to 20th	Day	12% 2 12% 27% 30% 16%	479
	21st to end	Day	10% 1 5% 23% 37% 25%	539
September	1st to 10th	Day	8% 3 11% 25% 34% 19%	490
	11th to 20th	Day	11% 1 8% 24% 35% 21%	490
	21st to end	Day	15% 5% 18% 25% 24% 13%	490
October	1st to 10th	Day	10% 2 16% 35% 29% 9%	462
	11th to 20th	Day	14% 8% 31% 27% 14% 7%	300
	21st to end	Day	24% 13% 26% 23% 12% 2	330
November	1st to 10th	Day	16% 9% 28% 29% 13% 5%	300
	11th to 20th	Day	12% 16% 29% 28% 13% 2	300
	21st to end	Day	20% 14% 27% 26% 11% 2	300
December	1st to 10th	Day	19% 14% 33% 21% 11% 2	300
	11th to 20th	Day	25% 13% 27% 24% 11%	300
	21st to end	Day	28% 15% 33% 21% 4	330
January	1st to 10th	Night	23% 11% 32% 29% 5%	1000
	11th to 20th	Night	24% 12% 37% 22% 5%	1000
	21st to end	Night	20% 9% 40% 29% 2	1100
February	1st to 10th	Night	25% 9% 36% 26% 3	817
	11th to 20th	Night	22% 13% 39% 24% 2	783
	21st to end	Night	22% 14% 37% 26% 2	660
March	1st to 10th	Night	22% 13% 30% 30% 6%	800
	11th to 20th	Night	21% 9% 35% 27% 7%	729
	21st to end	Night	20% 11% 30% 31% 7%	660
April	1st to 10th	Night	16% 8% 35% 33% 8% 1	600
	11th to 20th	Night	15% 10% 29% 32% 12% 1	593
	21st to end	Night	17% 6% 33% 31% 12% 1	600
May	1st to 10th	Night	17% 7% 28% 34% 13% 2	600
	11th to 20th	Night	16% 5% 18% 37% 22% 2	591
	21st to end	Night	8% 3 18% 40% 28% 4	650
June	1st to 10th	Night	11% 3 14% 39% 29% 4	600
	11th to 20th	Night	6% 1 11% 45% 36% 2	600
	21st to end	Night	4 1 11% 44% 34% 5%	600
July	1st to 10th	Night	7% 2 7% 29% 45% 10%	585
	11th to 20th	Night	6% 3 9% 28% 46% 8%	582
	21st to end	Night	7% 1 10% 32% 42% 9%	657
August	1st to 10th	Night	7% 1 9% 33% 44% 7%	579
	11th to 20th	Night	9% 1 8% 38% 39% 6%	564
	21st to end	Night	8% 1 6% 27% 49% 9%	627
September	1st to 10th	Night	11% 1 10% 31% 41% 6%	570
	11th to 20th	Night	10% 1 10% 32% 40% 7%	666
	21st to end	Night	12% 2 17% 34% 30% 5%	760
October	1st to 10th	Night	8% 3 18% 36% 29% 5%	760
	11th to 20th	Night	16% 5% 25% 34% 18% 2	760
	21st to end	Night	21% 8% 26% 31% 13% 1	836
November	1st to 10th	Night	14% 7% 26% 37% 15% 1	780
	11th to 20th	Night	16% 7% 34% 32% 12%	800
	21st to end	Night	19% 10% 29% 30% 11% 1	800
December	1st to 10th	Night	23% 10% 28% 31% 7%	820
	11th to 20th	Night	23% 6% 31% 32% 7%	1000
	21st to end	Night	28% 9% 30% 28% 4	1100

Page generated at 2025-08-02 00:32:29. Version ac07

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