Timberline Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Timberline Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	12% 8% 3 8% 5% 4 4 4 5% 7% 41%	132024
	Night	11% 8% 3 8% 4 4 4 3 5% 6% 44%	130762
Transparency	Day	42% 5% 12% 25% 15% 2	135625
	Night	40% 4 12% 25% 16% 2	135238
Seeing	Day	16% 9% 29% 28% 14% 4	17790
	Night	16% 9% 32% 32% 11% 1	27772

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	9% 12% 5% 17% 6% 5% 5% 3 4 5% 28%	4847
	11th to 20th	Day	5% 9% 4 11% 6% 6% 8% 4 6% 7% 35%	5310
	21st to end	Day	8% 11% 6% 12% 6% 6% 6% 4 5% 6% 32%	5812
February	1st to 10th	Day	10% 14% 5% 11% 6% 5% 4 3 5% 7% 30%	1890
	11th to 20th	Day	11% 11% 4 9% 5% 4 4 3 5% 7% 37%	1844
	21st to end	Day	13% 8% 3 10% 6% 5% 5% 4 4 5% 38%	1688
March	1st to 10th	Day	18% 9% 3 7% 3 3 3 2 3 4 45%	5046
	11th to 20th	Day	12% 7% 2 6% 4 4 4 3 5% 7% 46%	5090
	21st to end	Day	19% 14% 5% 8% 5% 4 4 4 5% 6% 28%	5583
April	1st to 10th	Day	17% 10% 3 6% 5% 3 4 4 6% 7% 35%	2483
	11th to 20th	Day	16% 9% 3 6% 5% 4 4 3 5% 6% 38%	2494
	21st to end	Day	20% 8% 3 6% 4 4 4 4 5% 6% 35%	2692
May	1st to 10th	Day	13% 8% 2 7% 5% 4 4 4 6% 7% 39%	5876
	11th to 20th	Day	16% 6% 2 5% 4 4 4 3 4 7% 46%	6089
	21st to end	Day	15% 8% 2 7% 4 4 5% 3 5% 7% 38%	6935
June	1st to 10th	Day	9% 7% 3 6% 4 5% 4 4 6% 9% 45%	2800
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 4 7% 11% 45%	2800
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 4 6% 8% 44%	2800
July	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 5% 7% 9% 43%	6360
	11th to 20th	Day	7% 6% 2 6% 5% 4 5% 4 5% 7% 49%	6140
	21st to end	Day	7% 5% 2 6% 4 5% 5% 5% 7% 10% 44%	6422
August	1st to 10th	Day	7% 5% 2 5% 4 4 5% 4 7% 10% 45%	4234
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 6% 8% 41%	4171
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 6% 9% 47%	4421
September	1st to 10th	Day	13% 4 2 4 3 3 4 3 4 6% 55%	2400
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 6% 9% 50%	2298
	21st to end	Day	10% 5% 2 4 3 3 4 4 5% 8% 52%	2145
October	1st to 10th	Day	14% 6% 2 5% 3 3 3 2 5% 6% 51%	2000
	11th to 20th	Day	15% 7% 3 6% 3 4 3 3 4 6% 48%	2000
	21st to end	Day	13% 9% 3 6% 4 3 4 3 6% 6% 43%	2200
November	1st to 10th	Day	11% 5% 2 6% 4 4 3 4 5% 8% 48%	1946
	11th to 20th	Day	13% 8% 3 7% 4 4 4 4 5% 8% 41%	1680
	21st to end	Day	15% 10% 4 7% 4 3 3 3 6% 7% 39%	1680
December	1st to 10th	Day	6% 7% 2 7% 5% 4 4 3 4 7% 51%	2690
	11th to 20th	Day	11% 9% 4 12% 6% 5% 5% 4 6% 6% 33%	3088
	21st to end	Day	20% 16% 6% 14% 5% 4 4 3 5% 5% 21%	4070
January	1st to 10th	Night	10% 12% 4 17% 7% 5% 4 3 5% 6% 27%	8073
	11th to 20th	Night	7% 8% 3 9% 7% 5% 6% 3 7% 6% 38%	7680
	21st to end	Night	9% 12% 5% 11% 5% 5% 4 3 4 5% 37%	8402
February	1st to 10th	Night	11% 10% 4 11% 6% 5% 4 3 5% 6% 35%	2730
	11th to 20th	Night	11% 10% 4 7% 5% 4 4 4 5% 6% 39%	2663
	21st to end	Night	10% 9% 4 9% 5% 4 4 3 4 5% 44%	2134
March	1st to 10th	Night	16% 9% 2 5% 2 3 3 2 3 4 50%	5029
	11th to 20th	Night	10% 6% 2 5% 4 4 3 3 5% 7% 52%	5090
	21st to end	Night	16% 11% 4 11% 4 4 4 3 5% 6% 31%	5588
April	1st to 10th	Night	14% 8% 2 7% 4 3 4 3 5% 6% 45%	2137
	11th to 20th	Night	13% 8% 2 7% 4 3 4 4 5% 6% 44%	2075
	21st to end	Night	18% 8% 2 6% 4 4 4 3 5% 6% 41%	1925
May	1st to 10th	Night	15% 6% 2 5% 3 3 4 4 5% 6% 47%	4062
	11th to 20th	Night	13% 6% 2 4 3 3 3 3 5% 6% 51%	3809
	21st to end	Night	17% 7% 2 5% 4 4 4 4 5% 6% 42%	3942
June	1st to 10th	Night	6% 5% 2 5% 4 3 5% 3 6% 7% 53%	1600
	11th to 20th	Night	7% 5% 1 5% 3 4 4 5% 6% 10% 50%	1600
	21st to end	Night	7% 4 2 4 4 4 4 4 6% 8% 53%	1600
July	1st to 10th	Night	7% 4 2 5% 3 4 5% 4 6% 10% 50%	3618
	11th to 20th	Night	6% 5% 2 5% 3 3 4 3 5% 9% 55%	3833
	21st to end	Night	7% 4 2 5% 3 4 5% 4 6% 9% 48%	4449
August	1st to 10th	Night	5% 4 2 5% 4 4 4 3 6% 9% 56%	2926
	11th to 20th	Night	9% 7% 2 5% 4 3 3 3 5% 6% 54%	2888
	21st to end	Night	8% 4 1 4 3 3 4 4 5% 8% 57%	3480
September	1st to 10th	Night	9% 4 1 4 3 3 3 3 4 7% 59%	2000
	11th to 20th	Night	10% 4 2 4 4 3 3 3 5% 7% 54%	2099
	21st to end	Night	10% 4 1 4 3 3 3 3 5% 6% 59%	2235
October	1st to 10th	Night	13% 7% 2 5% 2 3 3 3 4 6% 53%	2400
	11th to 20th	Night	13% 6% 2 6% 3 3 4 3 4 6% 50%	2400
	21st to end	Night	16% 7% 2 6% 4 3 3 2 5% 5% 48%	2640
November	1st to 10th	Night	9% 5% 3 5% 3 3 4 3 4 6% 54%	2566
	11th to 20th	Night	11% 7% 2 7% 4 4 4 3 6% 6% 47%	2940
	21st to end	Night	14% 9% 3 7% 4 4 3 3 4 5% 43%	2940
December	1st to 10th	Night	6% 9% 3 8% 5% 4 4 3 5% 6% 48%	4720
	11th to 20th	Night	11% 9% 4 10% 5% 4 4 3 6% 6% 37%	5339
	21st to end	Night	19% 15% 6% 12% 5% 4 4 3 4 4 24%	7150

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	59% 4 11% 16% 10% 1	4848
	11th to 20th	Day	49% 3 8% 22% 15% 2	5310
	21st to end	Day	54% 3 12% 23% 7%	5812
February	1st to 10th	Day	55% 4 12% 18% 10% 1	1890
	11th to 20th	Day	47% 5% 12% 22% 12% 2	1853
	21st to end	Day	48% 5% 11% 21% 14% 2	1688
March	1st to 10th	Day	44% 4 7% 25% 18% 2	5046
	11th to 20th	Day	37% 5% 9% 26% 20% 3	5090
	21st to end	Day	57% 5% 17% 15% 4 2	5585
April	1st to 10th	Day	47% 6% 11% 25% 10% 1	2483
	11th to 20th	Day	46% 6% 12% 22% 13% 2	2494
	21st to end	Day	48% 6% 11% 22% 12% 1	2692
May	1st to 10th	Day	43% 6% 11% 23% 16% 1	5914
	11th to 20th	Day	38% 5% 8% 29% 16% 3	6093
	21st to end	Day	45% 5% 11% 23% 15% 1	6949
June	1st to 10th	Day	35% 7% 16% 25% 13% 3	2800
	11th to 20th	Day	31% 7% 15% 25% 20% 2	2800
	21st to end	Day	36% 8% 14% 25% 15% 3	2800
July	1st to 10th	Day	37% 4 14% 32% 12% 1	6291
	11th to 20th	Day	35% 4 13% 29% 18% 2	6065
	21st to end	Day	34% 6% 19% 27% 12% 1	6240
August	1st to 10th	Day	32% 6% 15% 29% 16% 1	4220
	11th to 20th	Day	40% 5% 12% 27% 15% 1	4210
	21st to end	Day	35% 4 15% 28% 16% 3	4420
September	1st to 10th	Day	31% 5% 12% 28% 21% 4	2400
	11th to 20th	Day	30% 7% 12% 32% 18% 1	2298
	21st to end	Day	30% 4 11% 31% 23% 1	2145
October	1st to 10th	Day	21% 3 6% 24% 43% 2	3260
	11th to 20th	Day	25% 3 9% 32% 25% 7%	3240
	21st to end	Day	36% 4 10% 28% 19% 3	3538
November	1st to 10th	Day	34% 5% 15% 24% 19% 2	1946
	11th to 20th	Day	42% 5% 12% 23% 17% 1	1680
	21st to end	Day	46% 4 13% 24% 13%	1680
December	1st to 10th	Day	35% 3 17% 32% 12% 1	2690
	11th to 20th	Day	50% 5% 14% 22% 9%	3085
	21st to end	Day	67% 4 10% 14% 5% 1	4070
January	1st to 10th	Night	59% 4 12% 16% 8% 1	8074
	11th to 20th	Night	45% 3 10% 21% 18% 3	7680
	21st to end	Night	51% 3 12% 20% 13% 2	8402
February	1st to 10th	Night	51% 4 11% 21% 10% 2	2730
	11th to 20th	Night	45% 5% 10% 24% 14% 3	2664
	21st to end	Night	43% 3 10% 22% 18% 4	2134
March	1st to 10th	Night	39% 3 7% 28% 21% 2	5029
	11th to 20th	Night	34% 5% 7% 30% 19% 6%	5090
	21st to end	Night	54% 6% 16% 16% 6% 2	5588
April	1st to 10th	Night	41% 5% 11% 27% 15% 2	2137
	11th to 20th	Night	41% 5% 11% 24% 15% 3	2075
	21st to end	Night	44% 6% 13% 22% 14% 1	1925
May	1st to 10th	Night	37% 5% 10% 28% 19% 1	4089
	11th to 20th	Night	34% 5% 11% 27% 19% 4	3810
	21st to end	Night	41% 6% 12% 23% 16% 2	3950
June	1st to 10th	Night	29% 7% 16% 30% 15% 4	1600
	11th to 20th	Night	28% 7% 14% 25% 22% 4	1600
	21st to end	Night	28% 8% 14% 27% 20% 4	1600
July	1st to 10th	Night	31% 4 15% 33% 16% 2	3580
	11th to 20th	Night	28% 3 17% 28% 22% 1	3788
	21st to end	Night	32% 5% 20% 29% 13% 1	4320
August	1st to 10th	Night	26% 6% 15% 35% 18% 1	2916
	11th to 20th	Night	32% 5% 12% 33% 16% 2	2915
	21st to end	Night	26% 4 16% 29% 17% 6%	3480
September	1st to 10th	Night	26% 6% 10% 28% 25% 5%	2000
	11th to 20th	Night	29% 5% 12% 33% 19% 2	2099
	21st to end	Night	27% 4 9% 30% 28% 3	2235
October	1st to 10th	Night	22% 3 6% 21% 45% 3	3920
	11th to 20th	Night	28% 3 7% 27% 28% 7%	3899
	21st to end	Night	41% 3 9% 24% 21% 2	4252
November	1st to 10th	Night	31% 4 16% 25% 20% 4	2566
	11th to 20th	Night	38% 4 13% 27% 16% 2	2940
	21st to end	Night	44% 3 11% 27% 13% 1	2940
December	1st to 10th	Night	38% 3 11% 37% 11% 1	4720
	11th to 20th	Night	47% 5% 13% 25% 10%	5341
	21st to end	Night	64% 4 11% 15% 6%	7150

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	21% 9% 33% 27% 7% 2	330
	11th to 20th	Day	18% 13% 42% 22% 5%	330
	21st to end	Day	20% 9% 43% 23% 5%	363
February	1st to 10th	Day	17% 15% 42% 21% 4 1	330
	11th to 20th	Day	18% 15% 37% 23% 5% 1	330
	21st to end	Day	20% 14% 43% 18% 4 1	273
March	1st to 10th	Day	19% 15% 43% 18% 5%	330
	11th to 20th	Day	27% 11% 35% 18% 9%	330
	21st to end	Day	35% 13% 31% 17% 4	363
April	1st to 10th	Day	27% 13% 36% 18% 6% 1	503
	11th to 20th	Day	23% 16% 30% 23% 7% 1	532
	21st to end	Day	26% 13% 31% 23% 7% 1	712
May	1st to 10th	Day	21% 11% 33% 24% 10% 1	709
	11th to 20th	Day	23% 9% 29% 24% 12% 3	725
	21st to end	Day	19% 8% 28% 28% 14% 4	789
June	1st to 10th	Day	10% 6% 22% 33% 24% 5%	730
	11th to 20th	Day	10% 4 29% 36% 16% 5%	730
	21st to end	Day	9% 8% 32% 34% 14% 3	730
July	1st to 10th	Day	9% 2 17% 34% 27% 11%	721
	11th to 20th	Day	8% 3 24% 36% 22% 8%	718
	21st to end	Day	9% 1 18% 40% 26% 6%	794
August	1st to 10th	Day	8% 3 22% 35% 26% 5%	726
	11th to 20th	Day	12% 2 17% 36% 28% 5%	715
	21st to end	Day	11% 3 20% 36% 24% 7%	653
September	1st to 10th	Day	13% 5% 21% 30% 23% 8%	530
	11th to 20th	Day	9% 2 25% 35% 23% 7%	428
	21st to end	Day	15% 6% 23% 33% 18% 4	330
October	1st to 10th	Day	18% 7% 28% 24% 16% 6%	330
	11th to 20th	Day	17% 18% 37% 18% 8% 2	330
	21st to end	Day	19% 15% 35% 22% 6% 4	363
November	1st to 10th	Day	18% 11% 41% 19% 8% 2	330
	11th to 20th	Day	20% 17% 35% 19% 8% 1	330
	21st to end	Day	22% 12% 39% 21% 5% 1	330
December	1st to 10th	Day	13% 14% 40% 23% 9% 1	330
	11th to 20th	Day	19% 14% 35% 23% 7% 2	330
	21st to end	Day	26% 12% 40% 16% 5%	363
January	1st to 10th	Night	15% 12% 37% 29% 7%	1050
	11th to 20th	Night	16% 15% 37% 24% 7% 1	1050
	21st to end	Night	19% 11% 35% 29% 5% 1	1155
February	1st to 10th	Night	18% 10% 39% 28% 4 1	1050
	11th to 20th	Night	18% 11% 39% 28% 4	1029
	21st to end	Night	16% 14% 41% 23% 4 1	750
March	1st to 10th	Night	20% 12% 37% 23% 7% 1	630
	11th to 20th	Night	21% 8% 37% 26% 8%	630
	21st to end	Night	25% 11% 37% 22% 5% 1	693
April	1st to 10th	Night	18% 14% 39% 25% 4	630
	11th to 20th	Night	19% 13% 35% 27% 5% 1	623
	21st to end	Night	22% 9% 33% 28% 7% 1	630
May	1st to 10th	Night	18% 7% 33% 34% 8%	603
	11th to 20th	Night	19% 6% 30% 34% 12%	591
	21st to end	Night	18% 6% 26% 35% 14% 1	650
June	1st to 10th	Night	8% 4 27% 40% 20% 1	600
	11th to 20th	Night	9% 4 26% 47% 14%	600
	21st to end	Night	9% 5% 30% 43% 13%	600
July	1st to 10th	Night	7% 1 15% 46% 28% 3	585
	11th to 20th	Night	5% 1 18% 47% 27% 1	582
	21st to end	Night	6% 1 14% 53% 25% 1	657
August	1st to 10th	Night	9% 2 15% 50% 23% 2	597
	11th to 20th	Night	9% 3 13% 47% 26% 3	594
	21st to end	Night	9% 2 16% 44% 27% 3	660
September	1st to 10th	Night	11% 4 19% 40% 24% 3	600
	11th to 20th	Night	14% 2 21% 38% 23% 3	600
	21st to end	Night	11% 5% 26% 37% 20% 1	635
October	1st to 10th	Night	16% 6% 29% 31% 17% 1	800
	11th to 20th	Night	16% 8% 35% 33% 8%	800
	21st to end	Night	20% 10% 33% 29% 8%	880
November	1st to 10th	Night	13% 11% 36% 32% 8% 1	863
	11th to 20th	Night	15% 15% 40% 24% 6%	1050
	21st to end	Night	20% 11% 35% 27% 7%	1050
December	1st to 10th	Night	15% 13% 38% 27% 7% 1	1050
	11th to 20th	Night	19% 10% 36% 30% 6%	1050
	21st to end	Night	22% 12% 35% 25% 5%	1155

Page generated at 2026-04-29 01:21:44. Version ac07

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