Cave Mountain Observatory Retreat
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cave Mountain Observatory Retreat clear sky chart from 2005-11-06 to 2025-09-19.

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	12% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	125835
	Night	11% 7% 3 7% 5% 4 5% 4 6% 7% 40%	123208
Transparency	Day	44% 4 11% 23% 15% 3	129441
	Night	43% 4 12% 24% 14% 3	127671
Seeing	Day	24% 14% 26% 21% 11% 4	16045
	Night	21% 13% 26% 28% 11% 1	25791

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% 13% 5% 7% 4 4 4 3 4 5% 42%	4056
	11th to 20th	Day	8% 10% 5% 11% 6% 4 4 3 5% 6% 38%	4060
	21st to end	Day	10% 9% 4 9% 5% 4 4 3 5% 5% 42%	4466
February	1st to 10th	Day	9% 10% 4 8% 5% 6% 5% 4 5% 7% 37%	1800
	11th to 20th	Day	12% 10% 3 10% 5% 4 5% 3 5% 7% 36%	1753
	21st to end	Day	11% 9% 3 7% 5% 5% 5% 4 4 6% 41%	1700
March	1st to 10th	Day	12% 9% 3 6% 4 4 4 3 5% 7% 42%	4935
	11th to 20th	Day	10% 6% 2 5% 4 4 4 3 5% 7% 49%	4980
	21st to end	Day	11% 8% 3 8% 6% 5% 6% 4 6% 5% 38%	5465
April	1st to 10th	Day	11% 9% 3 8% 6% 5% 4 4 6% 6% 39%	2330
	11th to 20th	Day	12% 7% 3 6% 4 4 4 4 6% 7% 44%	2374
	21st to end	Day	15% 8% 2 7% 5% 4 5% 3 6% 7% 38%	2520
May	1st to 10th	Day	16% 7% 3 7% 6% 6% 6% 5% 7% 8% 30%	5862
	11th to 20th	Day	20% 8% 3 7% 5% 5% 5% 4 6% 8% 30%	5861
	21st to end	Day	15% 6% 2 7% 6% 6% 6% 5% 6% 8% 33%	6698
June	1st to 10th	Day	12% 5% 2 4 5% 5% 6% 5% 7% 8% 41%	2800
	11th to 20th	Day	11% 5% 3 6% 5% 7% 6% 5% 6% 10% 38%	2800
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 9% 42%	2800
July	1st to 10th	Day	6% 4 2 6% 6% 6% 5% 5% 7% 10% 43%	6000
	11th to 20th	Day	8% 6% 2 7% 6% 7% 7% 6% 8% 10% 33%	5900
	21st to end	Day	10% 6% 2 7% 7% 5% 6% 5% 7% 9% 36%	6424
August	1st to 10th	Day	14% 6% 2 7% 6% 6% 6% 5% 7% 9% 32%	4234
	11th to 20th	Day	8% 4 2 7% 7% 6% 6% 6% 7% 8% 38%	4174
	21st to end	Day	6% 4 2 5% 5% 5% 5% 5% 6% 10% 47%	4325
September	1st to 10th	Day	14% 5% 2 6% 5% 4 5% 4 5% 6% 45%	2400
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 6% 7% 45%	2233
	21st to end	Day	20% 6% 2 7% 5% 5% 4 4 5% 6% 38%	2057
October	1st to 10th	Day	14% 6% 3 6% 4 4 3 2 5% 6% 46%	1900
	11th to 20th	Day	15% 8% 2 6% 4 4 4 3 4 5% 45%	1900
	21st to end	Day	13% 7% 2 6% 4 4 4 3 4 7% 47%	2090
November	1st to 10th	Day	9% 7% 2 5% 4 4 4 3 5% 6% 53%	1957
	11th to 20th	Day	12% 8% 3 5% 4 3 4 4 6% 8% 42%	1720
	21st to end	Day	12% 9% 3 6% 5% 4 4 3 6% 5% 43%	1600
December	1st to 10th	Day	9% 7% 3 7% 5% 4 4 3 5% 6% 47%	2600
	11th to 20th	Day	16% 11% 3 6% 4 4 4 3 4 7% 38%	2990
	21st to end	Day	15% 9% 3 8% 5% 4 4 4 6% 8% 32%	4071
January	1st to 10th	Night	8% 12% 4 9% 5% 5% 4 4 5% 5% 38%	5889
	11th to 20th	Night	8% 9% 3 12% 6% 4 4 3 5% 6% 42%	5900
	21st to end	Night	11% 8% 3 8% 4 4 4 4 4 5% 45%	6490
February	1st to 10th	Night	10% 9% 3 7% 5% 5% 5% 4 6% 8% 39%	2600
	11th to 20th	Night	12% 8% 4 9% 6% 6% 5% 5% 5% 6% 33%	2533
	21st to end	Night	11% 9% 4 8% 4 4 5% 5% 6% 7% 37%	1925
March	1st to 10th	Night	14% 9% 3 6% 3 4 4 3 5% 6% 44%	4918
	11th to 20th	Night	8% 8% 5% 6% 4 3 3 3 5% 8% 48%	4980
	21st to end	Night	9% 7% 2 7% 5% 5% 5% 4 6% 6% 43%	5467
April	1st to 10th	Night	11% 9% 2 8% 5% 5% 5% 4 5% 7% 39%	2070
	11th to 20th	Night	10% 7% 2 6% 5% 4 4 4 6% 8% 44%	1975
	21st to end	Night	15% 6% 2 5% 4 5% 4 4 6% 7% 43%	1895
May	1st to 10th	Night	18% 8% 3 7% 5% 5% 5% 4 6% 8% 32%	4033
	11th to 20th	Night	22% 8% 3 6% 4 4 5% 3 6% 7% 31%	4032
	21st to end	Night	15% 8% 2 7% 6% 5% 6% 4 6% 8% 33%	4152
June	1st to 10th	Night	10% 4 1 5% 5% 5% 5% 4 6% 9% 47%	1600
	11th to 20th	Night	9% 5% 2 6% 4 5% 5% 4 6% 10% 44%	1600
	21st to end	Night	8% 4 2 5% 4 5% 6% 5% 7% 10% 45%	1600
July	1st to 10th	Night	6% 4 2 6% 5% 5% 5% 5% 7% 10% 44%	3952
	11th to 20th	Night	8% 6% 3 8% 5% 7% 6% 5% 8% 10% 33%	4057
	21st to end	Night	8% 7% 3 7% 6% 5% 6% 4 8% 11% 35%	4427
August	1st to 10th	Night	14% 4 2 6% 4 4 6% 4 6% 10% 38%	2915
	11th to 20th	Night	10% 6% 2 7% 4 5% 6% 4 7% 9% 40%	2875
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 7% 9% 48%	3538
September	1st to 10th	Night	13% 5% 2 5% 3 4 3 4 6% 6% 50%	2000
	11th to 20th	Night	12% 4 2 5% 5% 4 4 3 6% 8% 48%	2119
	21st to end	Night	19% 6% 2 6% 4 4 4 3 5% 5% 42%	2121
October	1st to 10th	Night	17% 5% 2 5% 3 3 4 3 5% 6% 47%	2280
	11th to 20th	Night	13% 7% 3 4 3 3 4 3 5% 7% 49%	2280
	21st to end	Night	13% 7% 2 5% 4 4 4 3 6% 6% 47%	2508
November	1st to 10th	Night	8% 8% 3 5% 4 3 4 4 6% 7% 48%	2341
	11th to 20th	Night	11% 7% 3 6% 4 4 4 4 7% 7% 42%	2671
	21st to end	Night	12% 8% 3 7% 5% 4 5% 4 6% 6% 40%	2800
December	1st to 10th	Night	7% 7% 4 7% 5% 4 5% 4 6% 8% 43%	4580
	11th to 20th	Night	13% 8% 4 9% 5% 5% 4 4 5% 6% 36%	5205
	21st to end	Night	12% 9% 4 8% 5% 5% 6% 5% 7% 8% 31%	6880

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	47% 3 5% 21% 19% 5%	4057
	11th to 20th	Day	48% 3 7% 17% 18% 8%	4060
	21st to end	Day	45% 2 7% 25% 16% 5%	4466
February	1st to 10th	Day	49% 5% 10% 22% 14% 2	1800
	11th to 20th	Day	50% 3 10% 20% 15% 2	1762
	21st to end	Day	47% 3 11% 20% 16% 4	1700
March	1st to 10th	Day	44% 3 7% 23% 16% 7%	4935
	11th to 20th	Day	37% 2 8% 29% 19% 5%	4980
	21st to end	Day	49% 3 8% 18% 18% 4	5465
April	1st to 10th	Day	46% 5% 11% 19% 15% 4	2330
	11th to 20th	Day	41% 4 10% 22% 19% 4	2374
	21st to end	Day	47% 4 10% 25% 12% 3	2520
May	1st to 10th	Day	52% 4 10% 22% 9% 2	5897
	11th to 20th	Day	53% 5% 10% 23% 8% 1	5865
	21st to end	Day	49% 6% 12% 20% 11% 2	6713
June	1st to 10th	Day	40% 7% 14% 23% 15% 1	2800
	11th to 20th	Day	43% 7% 15% 22% 12% 2	2800
	21st to end	Day	39% 6% 16% 25% 13% 2	2800
July	1st to 10th	Day	38% 7% 14% 25% 12% 3	5935
	11th to 20th	Day	45% 6% 16% 25% 7% 1	5831
	21st to end	Day	46% 4 11% 24% 14% 1	6240
August	1st to 10th	Day	48% 5% 16% 23% 8%	4221
	11th to 20th	Day	43% 4 12% 28% 12%	4213
	21st to end	Day	34% 5% 16% 29% 14% 1	4325
September	1st to 10th	Day	40% 5% 12% 19% 20% 3	2400
	11th to 20th	Day	40% 4 8% 26% 18% 3	2233
	21st to end	Day	48% 4 9% 21% 15% 2	2057
October	1st to 10th	Day	31% 2 9% 24% 28% 5%	3160
	11th to 20th	Day	36% 2 7% 26% 18% 11%	3140
	21st to end	Day	28% 2 9% 31% 23% 7%	3428
November	1st to 10th	Day	35% 3 10% 26% 21% 5%	1957
	11th to 20th	Day	41% 3 11% 19% 22% 4	1720
	21st to end	Day	43% 3 11% 25% 16% 2	1600
December	1st to 10th	Day	40% 3 11% 26% 17% 4	2600
	11th to 20th	Day	47% 4 13% 18% 15% 2	2986
	21st to end	Day	49% 5% 14% 19% 12% 1	4071
January	1st to 10th	Night	49% 4 5% 22% 18% 2	5889
	11th to 20th	Night	45% 3 8% 23% 13% 8%	5900
	21st to end	Night	43% 3 9% 28% 15% 3	6490
February	1st to 10th	Night	45% 5% 14% 24% 12% 1	2600
	11th to 20th	Night	52% 4 9% 20% 12% 2	2535
	21st to end	Night	47% 4 12% 22% 11% 4	1925
March	1st to 10th	Night	43% 4 8% 25% 14% 7%	4918
	11th to 20th	Night	37% 3 9% 31% 17% 4	4980
	21st to end	Night	42% 3 12% 22% 17% 3	5467
April	1st to 10th	Night	46% 5% 11% 21% 15% 2	2070
	11th to 20th	Night	39% 5% 13% 24% 15% 4	1975
	21st to end	Night	41% 5% 12% 27% 12% 2	1895
May	1st to 10th	Night	51% 6% 15% 17% 8% 2	4060
	11th to 20th	Night	54% 5% 10% 23% 7%	4033
	21st to end	Night	49% 5% 17% 21% 5% 2	4159
June	1st to 10th	Night	35% 8% 15% 27% 15% 1	1600
	11th to 20th	Night	38% 8% 17% 25% 11% 1	1600
	21st to end	Night	33% 7% 18% 25% 15% 1	1600
July	1st to 10th	Night	36% 8% 21% 24% 9% 2	3909
	11th to 20th	Night	46% 7% 19% 21% 6% 1	4006
	21st to end	Night	43% 6% 18% 23% 11%	4297
August	1st to 10th	Night	43% 6% 19% 25% 7%	2904
	11th to 20th	Night	41% 4 17% 24% 14% 1	2902
	21st to end	Night	33% 5% 17% 33% 11% 1	3538
September	1st to 10th	Night	35% 6% 16% 21% 19% 3	2000
	11th to 20th	Night	35% 5% 10% 31% 17% 2	2119
	21st to end	Night	45% 5% 11% 24% 15% 1	2121
October	1st to 10th	Night	36% 2 11% 27% 21% 3	3800
	11th to 20th	Night	32% 3 11% 22% 25% 8%	3779
	21st to end	Night	27% 2 11% 30% 24% 5%	4120
November	1st to 10th	Night	38% 3 12% 22% 20% 5%	2341
	11th to 20th	Night	41% 3 9% 25% 18% 4	2671
	21st to end	Night	45% 4 11% 22% 14% 3	2800
December	1st to 10th	Night	39% 4 13% 27% 16% 1	4580
	11th to 20th	Night	50% 4 14% 17% 14% 1	5208
	21st to end	Night	49% 6% 13% 20% 11%	6880

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	40% 25% 24% 8% 3	300
	11th to 20th	Day	39% 22% 30% 8% 2	300
	21st to end	Day	39% 22% 30% 7% 1	330
February	1st to 10th	Day	37% 26% 27% 9%	300
	11th to 20th	Day	50% 22% 24% 4	300
	21st to end	Day	41% 17% 29% 11% 1	349
March	1st to 10th	Day	34% 19% 28% 16% 3	500
	11th to 20th	Day	34% 23% 25% 13% 4 1	500
	21st to end	Day	31% 23% 29% 13% 4 1	550
April	1st to 10th	Day	31% 23% 26% 15% 5%	500
	11th to 20th	Day	29% 18% 27% 16% 7% 2	492
	21st to end	Day	27% 19% 30% 16% 6% 2	500
May	1st to 10th	Day	28% 16% 26% 17% 9% 3	503
	11th to 20th	Day	28% 13% 28% 20% 8% 3	527
	21st to end	Day	22% 10% 27% 22% 16% 5%	573
June	1st to 10th	Day	15% 12% 28% 26% 15% 5%	530
	11th to 20th	Day	16% 10% 29% 23% 18% 4	530
	21st to end	Day	11% 12% 27% 31% 15% 4	530
July	1st to 10th	Day	12% 6% 26% 27% 21% 8%	525
	11th to 20th	Day	7% 4 29% 32% 19% 9%	524
	21st to end	Day	13% 8% 22% 28% 21% 8%	576
August	1st to 10th	Day	15% 6% 20% 31% 19% 8%	526
	11th to 20th	Day	13% 3 24% 32% 19% 9%	519
	21st to end	Day	10% 6% 20% 32% 20% 11%	583
September	1st to 10th	Day	15% 9% 20% 28% 21% 7%	530
	11th to 20th	Day	14% 4 19% 33% 21% 8%	522
	21st to end	Day	29% 8% 25% 23% 12% 3	490
October	1st to 10th	Day	19% 8% 23% 29% 17% 4	490
	11th to 20th	Day	25% 14% 25% 23% 11% 3	490
	21st to end	Day	24% 17% 30% 19% 8% 2	539
November	1st to 10th	Day	18% 15% 31% 24% 9% 2	495
	11th to 20th	Day	30% 14% 30% 19% 6% 1	278
	21st to end	Day	32% 14% 37% 15% 3	200
December	1st to 10th	Day	36% 19% 31% 11% 4	200
	11th to 20th	Day	37% 17% 27% 16% 4	200
	21st to end	Day	36% 18% 29% 14% 3	244
January	1st to 10th	Night	32% 22% 30% 14% 2	800
	11th to 20th	Night	29% 20% 30% 20% 2	800
	21st to end	Night	26% 23% 35% 15% 2	880
February	1st to 10th	Night	27% 21% 30% 20% 2	800
	11th to 20th	Night	33% 25% 28% 13% 1	783
	21st to end	Night	28% 21% 34% 16% 1	660
March	1st to 10th	Night	25% 21% 32% 19% 2	800
	11th to 20th	Night	30% 20% 26% 19% 5% 1	800
	21st to end	Night	23% 18% 33% 20% 5%	880
April	1st to 10th	Night	24% 22% 30% 19% 5%	800
	11th to 20th	Night	22% 17% 28% 25% 8% 1	791
	21st to end	Night	24% 12% 30% 28% 6%	695
May	1st to 10th	Night	22% 11% 26% 28% 12% 1	600
	11th to 20th	Night	19% 8% 26% 33% 13%	591
	21st to end	Night	15% 4 26% 36% 17% 3	536
June	1st to 10th	Night	12% 5% 26% 38% 19% 1	400
	11th to 20th	Night	13% 5% 20% 44% 18% 2	400
	21st to end	Night	8% 4 25% 45% 18% 1	400
July	1st to 10th	Night	11% 2 15% 42% 26% 3	531
	11th to 20th	Night	7% 3 16% 40% 30% 3	582
	21st to end	Night	9% 2 17% 43% 26% 2	657
August	1st to 10th	Night	11% 2 13% 44% 27% 2	597
	11th to 20th	Night	12% 2 12% 47% 25% 3	594
	21st to end	Night	11% 2 16% 40% 28% 2	845
September	1st to 10th	Night	15% 4 16% 38% 24% 5%	800
	11th to 20th	Night	13% 4 15% 36% 28% 4	793
	21st to end	Night	21% 6% 20% 31% 20% 1	760
October	1st to 10th	Night	21% 8% 18% 31% 19% 3	760
	11th to 20th	Night	20% 14% 27% 27% 11% 1	760
	21st to end	Night	22% 12% 29% 28% 8% 1	836
November	1st to 10th	Night	17% 13% 26% 32% 11% 1	780
	11th to 20th	Night	24% 13% 31% 23% 8% 1	800
	21st to end	Night	25% 17% 30% 22% 5% 1	800
December	1st to 10th	Night	28% 18% 31% 20% 4	800
	11th to 20th	Night	24% 17% 32% 24% 3	800
	21st to end	Night	26% 15% 32% 25% 2	880

Page generated at 2025-09-16 01:49:13. Version ac07

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