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 2024-12-02.

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% 42%	101171
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	98376
Transparency	Day	38% 5% 12% 24% 17% 4	104821
	Night	39% 4 11% 23% 18% 5%	102865
Seeing	Day	19% 9% 25% 25% 15% 6%	15127
	Night	17% 7% 24% 34% 16% 2	24950

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	17% 5% 2 6% 4 4 4 3 4 5% 48%	2836
	11th to 20th	Day	12% 8% 3 7% 4 4 5% 4 4 5% 43%	2840
	21st to end	Day	17% 9% 3 6% 4 4 4 3 4 6% 40%	3124
February	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 5% 6% 43%	1710
	11th to 20th	Day	17% 6% 3 6% 3 3 4 4 5% 6% 43%	1663
	21st to end	Day	17% 7% 3 6% 4 4 4 4 5% 6% 39%	1643
March	1st to 10th	Day	21% 7% 2 6% 4 4 4 3 4 7% 38%	3435
	11th to 20th	Day	9% 5% 2 5% 3 4 4 3 6% 6% 52%	3480
	21st to end	Day	18% 8% 2 7% 4 4 5% 3 5% 5% 38%	3815
April	1st to 10th	Day	10% 5% 2 5% 5% 4 4 3 6% 7% 49%	2232
	11th to 20th	Day	12% 5% 2 5% 5% 4 3 4 6% 7% 48%	2254
	21st to end	Day	16% 6% 2 5% 5% 4 4 4 5% 6% 42%	2315
May	1st to 10th	Day	16% 6% 3 7% 5% 6% 6% 5% 6% 8% 32%	4082
	11th to 20th	Day	21% 6% 2 5% 4 4 4 5% 6% 8% 34%	4081
	21st to end	Day	9% 4 2 5% 4 4 5% 4 6% 10% 45%	4635
June	1st to 10th	Day	10% 5% 1 5% 6% 4 5% 5% 7% 9% 44%	2660
	11th to 20th	Day	10% 5% 2 6% 5% 5% 6% 5% 7% 11% 39%	2660
	21st to end	Day	7% 4 2 5% 5% 5% 5% 5% 8% 12% 42%	2660
July	1st to 10th	Day	6% 4 2 6% 4 5% 6% 5% 9% 11% 42%	4155
	11th to 20th	Day	9% 5% 2 6% 4 5% 6% 5% 8% 13% 36%	4120
	21st to end	Day	11% 7% 3 8% 6% 6% 6% 6% 8% 10% 30%	4466
August	1st to 10th	Day	14% 6% 2 7% 5% 5% 5% 5% 8% 10% 32%	4104
	11th to 20th	Day	8% 3 2 6% 5% 5% 6% 5% 8% 10% 40%	3966
	21st to end	Day	9% 4 2 4 4 4 4 5% 7% 10% 48%	4180
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 3 5% 7% 48%	2280
	11th to 20th	Day	13% 5% 3 4 4 4 4 4 6% 8% 46%	2185
	21st to end	Day	24% 6% 2 6% 4 3 4 4 4 5% 38%	2033
October	1st to 10th	Day	15% 4 2 5% 3 3 4 3 5% 7% 48%	1900
	11th to 20th	Day	15% 4 1 5% 3 2 4 3 4 6% 53%	1900
	21st to end	Day	13% 5% 2 5% 3 3 3 3 3 5% 55%	2090
November	1st to 10th	Day	12% 5% 1 4 3 3 4 3 4 5% 56%	1957
	11th to 20th	Day	15% 5% 2 5% 4 3 3 3 5% 5% 52%	1800
	21st to end	Day	13% 4 2 5% 4 3 3 3 5% 7% 51%	1600
December	1st to 10th	Day	15% 5% 2 6% 4 4 5% 4 5% 8% 41%	1528
	11th to 20th	Day	16% 5% 1 4 3 3 3 3 4 5% 53%	1910
	21st to end	Day	20% 5% 2 6% 5% 4 4 3 5% 7% 38%	2872
January	1st to 10th	Night	12% 6% 2 6% 3 4 4 3 5% 6% 50%	4109
	11th to 20th	Night	16% 8% 2 4 3 2 3 3 4 5% 49%	4120
	21st to end	Night	20% 8% 3 6% 3 3 3 2 4 5% 43%	4532
February	1st to 10th	Night	13% 6% 2 6% 4 5% 4 4 5% 7% 44%	2470
	11th to 20th	Night	20% 6% 2 5% 4 3 3 3 5% 5% 44%	2403
	21st to end	Night	14% 7% 2 5% 4 4 4 3 5% 7% 45%	1807
March	1st to 10th	Night	20% 7% 2 6% 4 5% 4 3 5% 6% 37%	3418
	11th to 20th	Night	11% 4 1 4 3 4 4 3 5% 8% 53%	3480
	21st to end	Night	15% 6% 2 5% 4 3 4 2 4 6% 49%	3817
April	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 5% 6% 46%	1946
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 5% 9% 49%	1875
	21st to end	Night	15% 5% 2 5% 3 4 5% 3 5% 8% 44%	1880
May	1st to 10th	Night	19% 8% 3 7% 4 4 4 4 5% 8% 35%	2813
	11th to 20th	Night	24% 6% 2 5% 4 5% 4 3 6% 7% 34%	2812
	21st to end	Night	11% 4 2 6% 4 5% 5% 4 7% 10% 44%	2902
June	1st to 10th	Night	10% 4 2 5% 4 4 6% 4 7% 9% 44%	1520
	11th to 20th	Night	10% 4 3 6% 4 5% 6% 3 6% 9% 44%	1520
	21st to end	Night	9% 4 2 5% 4 5% 6% 4 6% 12% 45%	1520
July	1st to 10th	Night	7% 4 2 6% 4 5% 6% 6% 8% 11% 40%	2798
	11th to 20th	Night	8% 7% 3 8% 5% 6% 5% 5% 7% 10% 36%	2840
	21st to end	Night	10% 8% 4 9% 7% 6% 5% 5% 7% 9% 30%	3087
August	1st to 10th	Night	16% 6% 2 7% 5% 6% 5% 4 7% 10% 32%	2825
	11th to 20th	Night	9% 6% 3 8% 5% 6% 6% 5% 8% 9% 36%	2817
	21st to end	Night	11% 6% 2 6% 4 4 5% 4 6% 9% 43%	3476
September	1st to 10th	Night	13% 6% 2 5% 4 5% 4 3 5% 7% 47%	1900
	11th to 20th	Night	13% 6% 2 6% 4 5% 4 4 5% 7% 45%	1990
	21st to end	Night	25% 6% 2 5% 3 3 3 3 5% 5% 40%	2147
October	1st to 10th	Night	17% 5% 2 5% 3 3 4 3 5% 6% 46%	2280
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 4 6% 51%	2280
	21st to end	Night	14% 4 1 4 3 4 3 3 5% 6% 54%	2508
November	1st to 10th	Night	12% 7% 2 5% 4 3 4 2 3 4 54%	2341
	11th to 20th	Night	14% 5% 2 5% 4 3 3 3 5% 7% 50%	2593
	21st to end	Night	13% 4 2 5% 4 3 4 3 5% 6% 51%	2800
December	1st to 10th	Night	15% 8% 3 6% 5% 4 4 3 5% 6% 43%	2686
	11th to 20th	Night	15% 7% 2 5% 4 4 3 3 4 6% 48%	3285
	21st to end	Night	16% 5% 2 5% 4 4 5% 4 6% 7% 42%	4779

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	39% 4 6% 30% 15% 6%	2837
	11th to 20th	Day	42% 4 7% 19% 20% 8%	2840
	21st to end	Day	46% 3 8% 23% 16% 3	3124
February	1st to 10th	Day	42% 4 10% 25% 15% 4	1710
	11th to 20th	Day	42% 4 7% 21% 20% 5%	1672
	21st to end	Day	47% 2 10% 17% 18% 6%	1643
March	1st to 10th	Day	47% 4 9% 19% 14% 7%	3435
	11th to 20th	Day	32% 4 7% 26% 22% 9%	3480
	21st to end	Day	48% 4 10% 19% 16% 4	3815
April	1st to 10th	Day	34% 4 11% 23% 19% 9%	2232
	11th to 20th	Day	36% 3 11% 23% 21% 6%	2254
	21st to end	Day	42% 4 11% 23% 14% 5%	2315
May	1st to 10th	Day	48% 6% 13% 19% 12% 2	4120
	11th to 20th	Day	47% 7% 12% 22% 11% 1	4085
	21st to end	Day	33% 5% 17% 23% 20% 2	4649
June	1st to 10th	Day	34% 6% 16% 27% 15% 1	2660
	11th to 20th	Day	37% 7% 17% 23% 15% 2	2660
	21st to end	Day	32% 7% 16% 27% 15% 2	2660
July	1st to 10th	Day	33% 8% 23% 20% 11% 6%	4090
	11th to 20th	Day	36% 7% 20% 26% 9% 1	4051
	21st to end	Day	47% 7% 13% 21% 11% 1	4324
August	1st to 10th	Day	44% 7% 17% 26% 6%	4091
	11th to 20th	Day	35% 7% 16% 28% 13%	4005
	21st to end	Day	31% 6% 19% 29% 15% 1	4180
September	1st to 10th	Day	36% 4 14% 18% 22% 4	2280
	11th to 20th	Day	37% 5% 7% 29% 19% 4	2185
	21st to end	Day	48% 4 9% 19% 17% 3	2033
October	1st to 10th	Day	34% 3 7% 19% 31% 6%	3160
	11th to 20th	Day	24% 3 8% 23% 31% 12%	3140
	21st to end	Day	21% 2 10% 31% 24% 12%	3428
November	1st to 10th	Day	31% 3 9% 25% 28% 5%	1957
	11th to 20th	Day	35% 2 10% 22% 23% 8%	1800
	21st to end	Day	33% 3 10% 26% 25% 4	1600
December	1st to 10th	Day	42% 3 11% 23% 15% 5%	1528
	11th to 20th	Day	35% 2 8% 30% 17% 8%	1906
	21st to end	Day	44% 3 11% 26% 13% 2	2872
January	1st to 10th	Night	36% 3 8% 24% 24% 5%	4109
	11th to 20th	Night	39% 3 7% 22% 23% 6%	4120
	21st to end	Night	45% 2 7% 24% 17% 5%	4532
February	1st to 10th	Night	40% 4 10% 25% 18% 3	2470
	11th to 20th	Night	43% 3 7% 20% 19% 8%	2405
	21st to end	Night	41% 3 10% 22% 18% 6%	1807
March	1st to 10th	Night	48% 4 8% 21% 15% 4	3418
	11th to 20th	Night	30% 3 6% 23% 30% 8%	3480
	21st to end	Night	38% 3 9% 24% 20% 5%	3817
April	1st to 10th	Night	38% 4 11% 17% 23% 7%	1946
	11th to 20th	Night	34% 3 13% 21% 23% 6%	1875
	21st to end	Night	38% 3 11% 25% 17% 5%	1880
May	1st to 10th	Night	48% 4 14% 20% 11% 3	2840
	11th to 20th	Night	48% 6% 10% 24% 10% 1	2813
	21st to end	Night	36% 5% 17% 25% 14% 3	2909
June	1st to 10th	Night	34% 7% 14% 28% 16% 1	1520
	11th to 20th	Night	36% 6% 18% 23% 14% 2	1520
	21st to end	Night	32% 7% 15% 24% 18% 3	1520
July	1st to 10th	Night	36% 9% 23% 15% 13% 3	2753
	11th to 20th	Night	41% 8% 19% 23% 8% 1	2786
	21st to end	Night	48% 7% 15% 19% 10%	2988
August	1st to 10th	Night	46% 6% 16% 24% 7% 1	2814
	11th to 20th	Night	41% 7% 15% 25% 12%	2844
	21st to end	Night	37% 5% 16% 32% 9% 2	3476
September	1st to 10th	Night	38% 5% 12% 21% 18% 7%	1900
	11th to 20th	Night	39% 4 10% 26% 18% 3	1990
	21st to end	Night	47% 3 9% 21% 16% 4	2147
October	1st to 10th	Night	41% 3 9% 19% 22% 6%	3800
	11th to 20th	Night	25% 3 7% 22% 30% 12%	3779
	21st to end	Night	20% 2 7% 32% 28% 11%	4120
November	1st to 10th	Night	35% 3 8% 22% 25% 7%	2341
	11th to 20th	Night	36% 3 6% 22% 24% 8%	2593
	21st to end	Night	34% 3 9% 25% 23% 6%	2800
December	1st to 10th	Night	43% 3 8% 25% 17% 4	2686
	11th to 20th	Night	39% 3 9% 22% 21% 7%	3288
	21st to end	Night	40% 4 11% 26% 16% 2	4779

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	23% 22% 32% 19% 4	270
	11th to 20th	Day	32% 19% 33% 13% 3	270
	21st to end	Day	27% 20% 37% 12% 4 1	297
February	1st to 10th	Day	24% 21% 39% 13% 2	270
	11th to 20th	Day	36% 22% 32% 8% 1	270
	21st to end	Day	31% 18% 28% 20% 4	367
March	1st to 10th	Day	25% 17% 30% 22% 6% 1	460
	11th to 20th	Day	22% 18% 32% 20% 6% 1	460
	21st to end	Day	25% 13% 32% 24% 5% 1	506
April	1st to 10th	Day	24% 16% 31% 19% 8% 2	460
	11th to 20th	Day	20% 12% 29% 23% 12% 4	452
	21st to end	Day	25% 10% 33% 20% 11% 2	460
May	1st to 10th	Day	22% 13% 29% 21% 10% 4	463
	11th to 20th	Day	23% 6% 25% 26% 15% 5%	487
	21st to end	Day	18% 4 22% 30% 20% 6%	529
June	1st to 10th	Day	12% 6% 25% 29% 21% 8%	490
	11th to 20th	Day	12% 5% 24% 29% 19% 11%	490
	21st to end	Day	9% 4 26% 33% 21% 7%	490
July	1st to 10th	Day	12% 2 14% 31% 30% 12%	485
	11th to 20th	Day	9% 2 15% 35% 25% 14%	484
	21st to end	Day	12% 2 18% 33% 23% 12%	532
August	1st to 10th	Day	14% 5% 17% 27% 24% 13%	486
	11th to 20th	Day	13% 1 18% 29% 26% 14%	479
	21st to end	Day	11% 2 15% 28% 27% 17%	539
September	1st to 10th	Day	16% 3 14% 28% 24% 16%	490
	11th to 20th	Day	18% 3 14% 27% 29% 9%	490
	21st to end	Day	32% 4 18% 24% 16% 6%	490
October	1st to 10th	Day	21% 4 19% 33% 16% 7%	490
	11th to 20th	Day	15% 13% 25% 26% 18% 3	490
	21st to end	Day	17% 8% 31% 31% 11% 3	539
November	1st to 10th	Day	18% 7% 28% 29% 14% 4	495
	11th to 20th	Day	20% 11% 33% 20% 13% 2	358
	21st to end	Day	17% 18% 34% 21% 10% 2	200
December	1st to 10th	Day	20% 13% 39% 22% 6%	182
	11th to 20th	Day	25% 10% 32% 23% 9% 1	180
	21st to end	Day	29% 14% 30% 19% 8% 1	227
January	1st to 10th	Night	17% 12% 36% 31% 4	760
	11th to 20th	Night	21% 11% 32% 32% 4	760
	21st to end	Night	18% 11% 38% 30% 4	836
February	1st to 10th	Night	18% 11% 36% 32% 3	760
	11th to 20th	Night	25% 13% 36% 25% 1	743
	21st to end	Night	22% 12% 34% 28% 4	628
March	1st to 10th	Night	17% 13% 33% 32% 4	760
	11th to 20th	Night	23% 13% 30% 26% 7% 1	760
	21st to end	Night	20% 10% 34% 31% 6%	836
April	1st to 10th	Night	17% 13% 37% 26% 6%	760
	11th to 20th	Night	15% 10% 29% 33% 12% 1	751
	21st to end	Night	19% 6% 29% 37% 9%	740
May	1st to 10th	Night	20% 6% 24% 35% 15% 1	570
	11th to 20th	Night	20% 3 20% 37% 18% 2	561
	21st to end	Night	16% 13% 38% 29% 3	517
June	1st to 10th	Night	9% 1 14% 45% 28% 2	380
	11th to 20th	Night	12% 1 13% 43% 30% 2	380
	21st to end	Night	12% 1 11% 49% 25% 2	380
July	1st to 10th	Night	12% 2 6% 36% 38% 7%	526
	11th to 20th	Night	6% 1 5% 38% 42% 7%	552
	21st to end	Night	10% 2 11% 38% 35% 5%	624
August	1st to 10th	Night	14% 1 6% 39% 35% 5%	567
	11th to 20th	Night	14% 1 7% 35% 38% 5%	583
	21st to end	Night	13% 2 8% 35% 37% 6%	836
September	1st to 10th	Night	16% 2 9% 34% 31% 8%	760
	11th to 20th	Night	16% 3 9% 36% 30% 6%	760
	21st to end	Night	29% 2 14% 31% 21% 2	760
October	1st to 10th	Night	21% 3 16% 31% 25% 4	760
	11th to 20th	Night	17% 6% 23% 35% 17% 2	760
	21st to end	Night	16% 5% 26% 38% 14% 1	836
November	1st to 10th	Night	16% 5% 26% 37% 13% 2	780
	11th to 20th	Night	17% 8% 27% 36% 11% 1	800
	21st to end	Night	15% 8% 31% 36% 9% 1	800
December	1st to 10th	Night	20% 8% 31% 34% 7% 1	768
	11th to 20th	Night	16% 8% 31% 39% 6%	760
	21st to end	Night	19% 8% 29% 40% 4	836

Page generated at 2024-11-29 10:43:11. Version ac07

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