Fan Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fan Mountain Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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% 5% 2 6% 4 4 5% 4 6% 8% 43%	130748
	Night	13% 6% 2 6% 4 4 4 4 5% 7% 45%	130391
Transparency	Day	38% 5% 12% 23% 17% 5%	134354
	Night	38% 4 11% 23% 19% 6%	134855
Seeing	Day	19% 10% 25% 25% 15% 6%	16703
	Night	17% 7% 25% 34% 15% 2	26623

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% 6% 2 4 4 4 4 3 5% 5% 48%	5258
	11th to 20th	Day	10% 7% 2 6% 3 3 3 3 4 4 54%	5280
	21st to end	Day	13% 6% 3 6% 4 4 5% 3 5% 5% 47%	5780
February	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 5% 6% 43%	1890
	11th to 20th	Day	18% 7% 3 6% 3 3 4 4 5% 6% 42%	1843
	21st to end	Day	17% 7% 3 6% 4 4 4 4 5% 6% 40%	1812
March	1st to 10th	Day	17% 5% 2 6% 3 4 5% 5% 5% 7% 40%	4957
	11th to 20th	Day	10% 6% 2 5% 3 4 3 3 6% 6% 52%	4980
	21st to end	Day	15% 7% 2 7% 5% 5% 5% 4 5% 6% 41%	5465
April	1st to 10th	Day	11% 6% 2 5% 5% 3 4 3 6% 7% 48%	2350
	11th to 20th	Day	12% 5% 2 5% 5% 4 3 4 6% 7% 48%	2374
	21st to end	Day	15% 6% 2 5% 5% 4 4 4 5% 7% 42%	2437
May	1st to 10th	Day	14% 6% 3 7% 5% 6% 6% 5% 8% 10% 32%	5862
	11th to 20th	Day	21% 6% 2 6% 4 4 5% 4 6% 8% 32%	5861
	21st to end	Day	15% 5% 2 5% 4 4 4 4 6% 9% 41%	6662
June	1st to 10th	Day	10% 4 1 5% 5% 4 5% 5% 7% 9% 44%	2800
	11th to 20th	Day	11% 5% 2 6% 5% 5% 6% 5% 7% 11% 39%	2800
	21st to end	Day	7% 4 2 5% 5% 5% 5% 5% 8% 12% 43%	2800
July	1st to 10th	Day	6% 4 2 6% 4 5% 6% 5% 8% 11% 42%	5949
	11th to 20th	Day	7% 5% 2 8% 5% 6% 7% 6% 10% 14% 29%	5900
	21st to end	Day	8% 5% 2 7% 5% 6% 6% 6% 9% 11% 35%	6424
August	1st to 10th	Day	15% 6% 2 7% 5% 5% 5% 5% 8% 10% 32%	4234
	11th to 20th	Day	9% 4 2 6% 5% 5% 6% 5% 8% 10% 39%	4094
	21st to end	Day	8% 4 2 4 4 4 4 5% 6% 10% 48%	4312
September	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 5% 7% 49%	2400
	11th to 20th	Day	13% 5% 2 4 4 4 4 4 6% 7% 46%	2300
	21st to end	Day	24% 6% 2 6% 4 4 4 4 4 5% 37%	2140
October	1st to 10th	Day	14% 4 2 5% 4 3 4 3 5% 7% 50%	2000
	11th to 20th	Day	15% 4 1 5% 3 2 4 3 4 6% 53%	2000
	21st to end	Day	14% 5% 2 5% 3 3 3 3 4 5% 55%	2200
November	1st to 10th	Day	12% 5% 1 4 3 3 4 3 5% 6% 56%	2057
	11th to 20th	Day	14% 5% 2 4 4 3 3 3 5% 5% 53%	1890
	21st to end	Day	13% 5% 2 5% 4 3 3 3 5% 7% 51%	1680
December	1st to 10th	Day	11% 4 2 6% 3 4 4 4 5% 6% 51%	2680
	11th to 20th	Day	19% 6% 2 4 3 3 3 3 4 6% 48%	3070
	21st to end	Day	19% 6% 3 6% 4 4 4 3 5% 7% 39%	4207
January	1st to 10th	Night	11% 6% 2 5% 3 3 4 4 6% 6% 51%	7635
	11th to 20th	Night	12% 7% 2 5% 2 2 3 2 4 4 57%	7680
	21st to end	Night	13% 6% 2 5% 3 3 3 3 4 5% 52%	8401
February	1st to 10th	Night	13% 6% 2 6% 4 5% 4 4 6% 7% 44%	2730
	11th to 20th	Night	20% 6% 2 5% 4 3 3 3 5% 5% 43%	2663
	21st to end	Night	13% 6% 3 6% 4 4 4 3 5% 6% 45%	1989
March	1st to 10th	Night	16% 5% 2 4 4 4 4 3 5% 6% 47%	4941
	11th to 20th	Night	10% 4 1 4 3 4 4 3 5% 8% 54%	4980
	21st to end	Night	12% 6% 2 5% 4 4 5% 4 6% 6% 45%	5467
April	1st to 10th	Night	14% 7% 2 5% 4 4 4 4 5% 6% 45%	2048
	11th to 20th	Night	11% 6% 3 5% 3 3 3 3 5% 9% 48%	1975
	21st to end	Night	15% 5% 2 6% 3 4 5% 3 5% 8% 45%	1979
May	1st to 10th	Night	17% 8% 3 7% 4 4 4 4 6% 8% 35%	4033
	11th to 20th	Night	23% 6% 2 6% 5% 5% 4 4 6% 8% 31%	4032
	21st to end	Night	15% 5% 2 6% 4 5% 5% 4 6% 9% 39%	4174
June	1st to 10th	Night	9% 4 2 6% 4 4 6% 4 7% 9% 44%	1600
	11th to 20th	Night	11% 4 3 6% 4 5% 6% 3 6% 9% 44%	1600
	21st to end	Night	9% 4 2 5% 4 5% 6% 4 7% 11% 45%	1600
July	1st to 10th	Night	10% 5% 2 6% 5% 5% 6% 5% 7% 10% 38%	4002
	11th to 20th	Night	11% 9% 4 9% 6% 6% 6% 6% 8% 10% 26%	4057
	21st to end	Night	7% 7% 3 9% 7% 6% 6% 6% 7% 10% 32%	4427
August	1st to 10th	Night	16% 6% 2 8% 5% 6% 5% 4 7% 10% 32%	2915
	11th to 20th	Night	9% 6% 3 8% 5% 6% 6% 5% 8% 9% 35%	2908
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 6% 9% 43%	3586
September	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 7% 48%	2000
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 5% 7% 45%	2095
	21st to end	Night	25% 6% 2 5% 4 3 4 3 5% 5% 38%	2260
October	1st to 10th	Night	17% 5% 2 4 3 3 4 3 5% 6% 47%	2400
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 5% 6% 51%	2400
	21st to end	Night	14% 4 1 4 3 4 3 3 5% 6% 54%	2640
November	1st to 10th	Night	12% 6% 2 5% 4 3 4 2 3 4 54%	2461
	11th to 20th	Night	13% 5% 2 5% 4 3 3 3 5% 7% 50%	2723
	21st to end	Night	13% 4 2 5% 4 4 4 3 5% 6% 51%	2940
December	1st to 10th	Night	9% 5% 2 4 3 3 4 3 5% 7% 54%	4720
	11th to 20th	Night	18% 8% 2 5% 4 4 3 3 4 6% 44%	5345
	21st to end	Night	17% 5% 2 5% 3 4 4 3 6% 7% 44%	6985

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	38% 4 9% 23% 18% 9%	5259
	11th to 20th	Day	34% 2 5% 18% 24% 17%	5280
	21st to end	Day	40% 2 6% 28% 19% 5%	5780
February	1st to 10th	Day	41% 4 10% 25% 16% 4	1890
	11th to 20th	Day	43% 4 7% 21% 20% 5%	1852
	21st to end	Day	46% 3 10% 18% 17% 6%	1812
March	1st to 10th	Day	41% 4 10% 18% 17% 10%	4957
	11th to 20th	Day	33% 3 7% 26% 22% 9%	4980
	21st to end	Day	44% 4 10% 18% 17% 8%	5465
April	1st to 10th	Day	36% 4 10% 22% 18% 9%	2350
	11th to 20th	Day	36% 4 11% 23% 21% 6%	2374
	21st to end	Day	42% 4 10% 24% 15% 5%	2437
May	1st to 10th	Day	44% 7% 15% 20% 12% 1	5897
	11th to 20th	Day	49% 6% 12% 20% 13% 1	5865
	21st to end	Day	39% 5% 16% 20% 18% 3	6677
June	1st to 10th	Day	34% 6% 16% 27% 16% 1	2800
	11th to 20th	Day	37% 7% 17% 22% 15% 2	2800
	21st to end	Day	32% 7% 16% 28% 16% 1	2800
July	1st to 10th	Day	34% 9% 21% 20% 12% 4	5884
	11th to 20th	Day	41% 11% 20% 21% 6% 1	5831
	21st to end	Day	39% 7% 15% 24% 14%	6240
August	1st to 10th	Day	44% 7% 17% 26% 6%	4221
	11th to 20th	Day	36% 7% 17% 28% 12%	4133
	21st to end	Day	30% 5% 19% 29% 15% 1	4312
September	1st to 10th	Day	35% 4 15% 19% 22% 4	2400
	11th to 20th	Day	37% 5% 7% 29% 20% 3	2300
	21st to end	Day	49% 5% 9% 18% 16% 2	2140
October	1st to 10th	Day	33% 3 7% 20% 31% 6%	3260
	11th to 20th	Day	24% 3 8% 23% 30% 12%	3240
	21st to end	Day	21% 2 10% 30% 25% 12%	3538
November	1st to 10th	Day	30% 3 9% 25% 27% 5%	2057
	11th to 20th	Day	34% 2 9% 23% 22% 8%	1890
	21st to end	Day	33% 3 9% 26% 25% 4	1680
December	1st to 10th	Day	34% 3 10% 26% 18% 9%	2680
	11th to 20th	Day	38% 2 13% 24% 16% 6%	3066
	21st to end	Day	46% 3 11% 23% 16% 2	4207
January	1st to 10th	Night	34% 3 9% 22% 23% 9%	7636
	11th to 20th	Night	32% 2 6% 19% 27% 14%	7680
	21st to end	Night	36% 2 6% 27% 22% 7%	8401
February	1st to 10th	Night	40% 3 11% 25% 18% 3	2730
	11th to 20th	Night	44% 3 7% 20% 19% 7%	2665
	21st to end	Night	40% 3 10% 21% 18% 7%	1989
March	1st to 10th	Night	38% 3 8% 21% 20% 9%	4941
	11th to 20th	Night	29% 3 7% 22% 30% 9%	4980
	21st to end	Night	39% 3 10% 21% 21% 6%	5467
April	1st to 10th	Night	39% 4 11% 17% 22% 7%	2048
	11th to 20th	Night	34% 3 13% 21% 23% 5%	1975
	21st to end	Night	38% 3 11% 25% 17% 5%	1979
May	1st to 10th	Night	46% 5% 16% 19% 10% 3	4060
	11th to 20th	Night	49% 6% 10% 23% 10% 1	4033
	21st to end	Night	41% 5% 16% 25% 11% 2	4181
June	1st to 10th	Night	34% 6% 14% 28% 17% 1	1600
	11th to 20th	Night	37% 6% 18% 23% 15% 2	1600
	21st to end	Night	32% 7% 15% 24% 18% 3	1600
July	1st to 10th	Night	39% 10% 19% 16% 14% 2	3959
	11th to 20th	Night	50% 10% 17% 17% 6% 1	4006
	21st to end	Night	44% 8% 15% 21% 11%	4297
August	1st to 10th	Night	46% 6% 16% 24% 7% 1	2904
	11th to 20th	Night	41% 7% 15% 24% 12%	2935
	21st to end	Night	37% 5% 16% 31% 9% 2	3586
September	1st to 10th	Night	37% 5% 12% 21% 19% 6%	2000
	11th to 20th	Night	38% 4 10% 26% 18% 3	2095
	21st to end	Night	48% 3 10% 20% 16% 3	2260
October	1st to 10th	Night	40% 3 9% 19% 23% 6%	3920
	11th to 20th	Night	25% 3 7% 22% 30% 12%	3899
	21st to end	Night	21% 2 7% 31% 28% 11%	4252
November	1st to 10th	Night	35% 3 7% 22% 26% 7%	2461
	11th to 20th	Night	36% 3 6% 22% 24% 8%	2723
	21st to end	Night	35% 3 9% 25% 23% 6%	2940
December	1st to 10th	Night	31% 3 10% 28% 24% 4	4720
	11th to 20th	Night	43% 3 11% 19% 17% 7%	5348
	21st to end	Night	39% 4 12% 26% 17% 2	6985

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% 24% 31% 19% 3	330
	11th to 20th	Day	30% 24% 31% 12% 3	330
	21st to end	Day	26% 22% 37% 12% 3 1	363
February	1st to 10th	Day	23% 23% 39% 12% 2	330
	11th to 20th	Day	36% 24% 31% 8% 2	330
	21st to end	Day	29% 18% 30% 19% 4	430
March	1st to 10th	Day	26% 16% 31% 21% 6% 1	508
	11th to 20th	Day	23% 18% 32% 21% 6% 1	500
	21st to end	Day	26% 14% 32% 22% 5% 1	550
April	1st to 10th	Day	26% 17% 30% 18% 8% 1	500
	11th to 20th	Day	20% 13% 29% 23% 12% 3	492
	21st to end	Day	24% 11% 33% 20% 11% 2	500
May	1st to 10th	Day	21% 14% 30% 21% 9% 4	503
	11th to 20th	Day	23% 8% 25% 25% 14% 4	527
	21st to end	Day	20% 5% 23% 28% 18% 6%	573
June	1st to 10th	Day	11% 6% 26% 29% 20% 8%	530
	11th to 20th	Day	13% 5% 24% 29% 19% 11%	530
	21st to end	Day	9% 4 24% 33% 22% 8%	530
July	1st to 10th	Day	11% 2 14% 32% 29% 11%	525
	11th to 20th	Day	8% 2 16% 34% 25% 15%	524
	21st to end	Day	11% 2 17% 32% 25% 13%	576
August	1st to 10th	Day	15% 5% 17% 27% 24% 13%	526
	11th to 20th	Day	13% 1 17% 30% 25% 14%	519
	21st to end	Day	10% 2 16% 30% 26% 16%	583
September	1st to 10th	Day	15% 3 16% 28% 23% 15%	530
	11th to 20th	Day	17% 3 14% 28% 29% 9%	530
	21st to end	Day	31% 5% 19% 24% 15% 5%	530
October	1st to 10th	Day	20% 5% 18% 33% 16% 8%	530
	11th to 20th	Day	15% 14% 26% 26% 17% 3	530
	21st to end	Day	18% 10% 30% 29% 10% 3	583
November	1st to 10th	Day	17% 9% 30% 29% 13% 3	535
	11th to 20th	Day	19% 13% 34% 19% 13% 2	388
	21st to end	Day	18% 19% 32% 19% 10% 1	220
December	1st to 10th	Day	22% 13% 39% 21% 5%	220
	11th to 20th	Day	25% 11% 32% 23% 8% 1	220
	21st to end	Day	28% 15% 28% 19% 8% 1	278
January	1st to 10th	Night	17% 12% 38% 29% 3	840
	11th to 20th	Night	21% 11% 34% 30% 4	840
	21st to end	Night	17% 11% 40% 28% 4	924
February	1st to 10th	Night	17% 11% 38% 31% 3	840
	11th to 20th	Night	25% 13% 36% 24% 1	823
	21st to end	Night	21% 13% 35% 27% 4	692
March	1st to 10th	Night	17% 14% 34% 31% 4	809
	11th to 20th	Night	22% 13% 30% 28% 7% 1	800
	21st to end	Night	19% 10% 35% 30% 6%	880
April	1st to 10th	Night	18% 13% 37% 26% 6%	800
	11th to 20th	Night	15% 10% 30% 32% 12% 1	791
	21st to end	Night	18% 7% 28% 38% 8%	779
May	1st to 10th	Night	20% 6% 24% 35% 14% 1	600
	11th to 20th	Night	21% 3 19% 38% 18% 2	591
	21st to end	Night	17% 1 14% 37% 28% 3	545
June	1st to 10th	Night	9% 2 15% 45% 28% 2	400
	11th to 20th	Night	13% 1 13% 43% 29% 2	400
	21st to end	Night	12% 1 11% 48% 26% 3	400
July	1st to 10th	Night	13% 1 6% 36% 37% 6%	555
	11th to 20th	Night	7% 1 7% 37% 42% 7%	582
	21st to end	Night	9% 2 10% 38% 36% 5%	657
August	1st to 10th	Night	14% 1 7% 39% 34% 5%	597
	11th to 20th	Night	14% 1 7% 34% 39% 5%	614
	21st to end	Night	12% 2 9% 36% 36% 5%	880
September	1st to 10th	Night	16% 2 10% 36% 30% 7%	800
	11th to 20th	Night	16% 2 9% 36% 30% 6%	800
	21st to end	Night	29% 2 15% 31% 21% 2	800
October	1st to 10th	Night	21% 3 15% 30% 26% 5%	800
	11th to 20th	Night	17% 7% 23% 35% 17% 2	800
	21st to end	Night	17% 6% 27% 37% 13% 1	880
November	1st to 10th	Night	16% 6% 27% 37% 13% 2	820
	11th to 20th	Night	18% 9% 28% 35% 10% 1	840
	21st to end	Night	15% 8% 32% 36% 9% 1	840
December	1st to 10th	Night	19% 8% 33% 33% 6% 1	840
	11th to 20th	Night	17% 8% 32% 38% 6%	840
	21st to end	Night	19% 8% 29% 39% 3	924

Page generated at 2026-02-28 01:40:20. Version ac07

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