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 2024-01-17.

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% 3 6% 5% 3 5% 5% 5% 7% 40%	74711
	Night	12% 7% 3 6% 5% 3 4 4 5% 7% 44%	75143
Transparency	Day	42% 6% 13% 23% 14% 2	74423
	Night	40% 6% 12% 25% 15% 2	74951
Seeing	Day	17% 8% 28% 28% 15% 4	14664
	Night	16% 8% 31% 32% 11% 1	24668

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% 11% 5% 11% 7% 3 5% 5% 4 6% 33%	1563
	11th to 20th	Day	10% 12% 6% 10% 9% 4 6% 3 4 6% 29%	1674
	21st to end	Day	14% 12% 5% 10% 7% 3 6% 5% 4 7% 26%	1782
February	1st to 10th	Day	12% 13% 5% 10% 8% 4 4 3 4 7% 31%	1620
	11th to 20th	Day	12% 10% 5% 8% 5% 3 5% 4 5% 7% 36%	1574
	21st to end	Day	15% 8% 3 9% 8% 4 5% 4 4 6% 34%	1443
March	1st to 10th	Day	14% 10% 4 9% 6% 3 5% 4 4 5% 36%	1980
	11th to 20th	Day	15% 11% 4 8% 6% 3 4 4 4 7% 34%	1980
	21st to end	Day	20% 13% 4 7% 6% 3 4 4 4 4 30%	2178
April	1st to 10th	Day	16% 9% 4 6% 5% 3 4 4 6% 7% 35%	2128
	11th to 20th	Day	16% 9% 4 6% 6% 3 4 5% 4 6% 37%	2134
	21st to end	Day	21% 8% 3 5% 4 3 4 5% 5% 6% 36%	2307
May	1st to 10th	Day	16% 8% 2 6% 7% 4 6% 6% 6% 7% 32%	2340
	11th to 20th	Day	18% 7% 3 6% 5% 3 5% 5% 5% 9% 34%	2389
	21st to end	Day	16% 6% 3 7% 6% 3 5% 4 6% 7% 37%	2725
June	1st to 10th	Day	9% 6% 3 5% 5% 3 5% 4 5% 9% 46%	2520
	11th to 20th	Day	8% 4 2 4 5% 3 5% 4 7% 11% 46%	2520
	21st to end	Day	8% 5% 2 6% 6% 4 5% 6% 7% 8% 44%	2520
July	1st to 10th	Day	8% 4 1 4 5% 3 5% 5% 6% 9% 50%	2520
	11th to 20th	Day	6% 4 2 5% 5% 3 5% 6% 6% 8% 51%	2440
	21st to end	Day	6% 3 2 4 5% 3 6% 5% 7% 11% 48%	2546
August	1st to 10th	Day	8% 5% 3 5% 6% 4 6% 6% 7% 10% 41%	2328
	11th to 20th	Day	10% 5% 3 5% 5% 3 5% 5% 6% 9% 47%	2301
	21st to end	Day	9% 5% 2 5% 4 3 4 4 5% 7% 51%	2440
September	1st to 10th	Day	13% 4 2 4 4 2 4 4 4 6% 54%	2160
	11th to 20th	Day	9% 4 2 4 4 3 4 4 5% 9% 52%	2069
	21st to end	Day	11% 5% 2 4 4 2 4 5% 5% 8% 52%	1931
October	1st to 10th	Day	15% 6% 3 4 4 3 3 3 4 7% 48%	1800
	11th to 20th	Day	16% 6% 3 5% 4 3 3 4 4 6% 47%	1800
	21st to end	Day	14% 9% 3 5% 5% 2 3 4 5% 6% 43%	1980
November	1st to 10th	Day	11% 4 2 5% 4 3 4 5% 5% 8% 49%	1758
	11th to 20th	Day	13% 7% 3 6% 5% 3 4 4 5% 8% 41%	1520
	21st to end	Day	15% 9% 4 5% 5% 2 3 4 5% 7% 40%	1520
December	1st to 10th	Day	10% 11% 4 7% 6% 4 4 5% 5% 7% 38%	1520
	11th to 20th	Day	10% 9% 4 8% 7% 4 6% 5% 6% 8% 33%	1918
	21st to end	Day	20% 14% 6% 13% 6% 3 4 3 5% 5% 21%	2783
January	1st to 10th	Night	9% 9% 5% 8% 6% 4 4 4 5% 7% 39%	2599
	11th to 20th	Night	11% 11% 5% 10% 7% 4 6% 4 6% 7% 30%	2431
	21st to end	Night	13% 12% 4 8% 6% 3 5% 4 4 7% 34%	2574
February	1st to 10th	Night	13% 10% 4 9% 8% 4 5% 3 4 6% 35%	2340
	11th to 20th	Night	12% 10% 4 6% 5% 4 4 5% 5% 6% 39%	2273
	21st to end	Night	11% 8% 5% 8% 7% 3 4 3 4 5% 41%	1827
March	1st to 10th	Night	12% 9% 3 6% 5% 2 3 4 3 6% 46%	1980
	11th to 20th	Night	16% 8% 3 6% 5% 2 4 4 5% 7% 41%	1980
	21st to end	Night	20% 12% 4 9% 6% 3 5% 5% 5% 5% 27%	2178
April	1st to 10th	Night	13% 7% 3 6% 5% 2 4 3 5% 6% 46%	1832
	11th to 20th	Night	15% 9% 3 6% 5% 3 4 4 5% 5% 42%	1775
	21st to end	Night	18% 7% 2 5% 5% 3 4 3 5% 6% 42%	1651
May	1st to 10th	Night	16% 5% 2 5% 4 3 6% 6% 7% 9% 38%	1620
	11th to 20th	Night	17% 5% 2 4 5% 3 4 4 5% 7% 44%	1503
	21st to end	Night	13% 5% 2 4 6% 3 4 6% 6% 7% 44%	1551
June	1st to 10th	Night	6% 4 2 4 5% 2 5% 4 6% 8% 53%	1440
	11th to 20th	Night	7% 4 2 4 4 3 4 5% 5% 10% 52%	1440
	21st to end	Night	7% 3 2 4 4 3 5% 5% 6% 9% 53%	1440
July	1st to 10th	Night	5% 3 1 4 3 2 4 4 6% 10% 58%	1440
	11th to 20th	Night	4 2 1 2 3 2 3 4 5% 9% 63%	1516
	21st to end	Night	4 2 2 3 4 3 5% 5% 6% 10% 57%	1772
August	1st to 10th	Night	7% 4 2 5% 5% 4 4 5% 6% 8% 51%	1611
	11th to 20th	Night	7% 4 2 5% 4 2 4 3 5% 6% 58%	1600
	21st to end	Night	8% 3 1 3 4 2 3 5% 6% 7% 59%	1916
September	1st to 10th	Night	9% 4 2 4 4 3 3 4 4 7% 57%	1800
	11th to 20th	Night	11% 4 2 3 4 2 3 3 5% 7% 56%	1889
	21st to end	Night	10% 4 1 3 3 2 3 3 4 7% 60%	2011
October	1st to 10th	Night	13% 7% 3 4 3 2 4 4 4 6% 52%	2160
	11th to 20th	Night	14% 6% 2 4 4 3 3 4 4 6% 49%	2160
	21st to end	Night	16% 6% 2 5% 4 2 3 3 5% 5% 48%	2376
November	1st to 10th	Night	8% 5% 2 4 4 2 4 4 4 7% 55%	2315
	11th to 20th	Night	12% 6% 3 6% 4 2 4 3 5% 6% 47%	2660
	21st to end	Night	14% 8% 4 6% 5% 3 4 4 3 6% 45%	2660
December	1st to 10th	Night	10% 10% 5% 8% 6% 3 5% 4 4 6% 39%	2660
	11th to 20th	Night	11% 9% 4 8% 6% 3 4 4 5% 6% 40%	3279
	21st to end	Night	18% 16% 8% 12% 6% 3 4 4 3 4 23%	4884

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	52% 5% 13% 18% 11% 1	1564
	11th to 20th	Day	57% 4 11% 20% 8%	1674
	21st to end	Day	58% 5% 13% 15% 8% 1	1782
February	1st to 10th	Day	55% 4 11% 19% 10% 1	1620
	11th to 20th	Day	48% 5% 13% 21% 11% 2	1583
	21st to end	Day	51% 5% 10% 19% 12% 1	1443
March	1st to 10th	Day	50% 6% 10% 21% 11% 2	1980
	11th to 20th	Day	51% 6% 11% 20% 10% 2	1980
	21st to end	Day	57% 5% 12% 17% 7% 2	2178
April	1st to 10th	Day	47% 7% 10% 24% 11% 1	2128
	11th to 20th	Day	48% 6% 11% 22% 12% 1	2134
	21st to end	Day	48% 7% 11% 23% 11% 1	2307
May	1st to 10th	Day	50% 8% 12% 18% 11% 1	2340
	11th to 20th	Day	47% 7% 11% 23% 11% 1	2393
	21st to end	Day	47% 7% 14% 19% 10% 2	2725
June	1st to 10th	Day	35% 8% 17% 24% 14% 2	2520
	11th to 20th	Day	30% 8% 15% 25% 21% 2	2520
	21st to end	Day	36% 9% 14% 24% 15% 3	2520
July	1st to 10th	Day	30% 6% 15% 29% 15% 4	2451
	11th to 20th	Day	31% 7% 13% 26% 18% 4	2365
	21st to end	Day	28% 9% 15% 28% 18% 1	2405
August	1st to 10th	Day	35% 10% 17% 25% 12% 2	2314
	11th to 20th	Day	33% 8% 14% 29% 15% 1	2301
	21st to end	Day	32% 6% 12% 27% 19% 4	2440
September	1st to 10th	Day	32% 5% 12% 26% 21% 4	2160
	11th to 20th	Day	29% 7% 12% 32% 19% 1	2069
	21st to end	Day	31% 5% 11% 31% 22% 1	1931
October	1st to 10th	Day	37% 5% 11% 27% 19% 1	1800
	11th to 20th	Day	39% 4 8% 27% 19% 4	1800
	21st to end	Day	41% 6% 10% 22% 17% 3	1980
November	1st to 10th	Day	32% 5% 15% 25% 20% 3	1758
	11th to 20th	Day	42% 5% 12% 23% 18% 1	1520
	21st to end	Day	45% 4 13% 24% 13% 1	1520
December	1st to 10th	Day	47% 5% 15% 21% 11% 1	1520
	11th to 20th	Day	48% 6% 15% 24% 6%	1915
	21st to end	Day	66% 6% 9% 14% 5% 1	2783
January	1st to 10th	Night	44% 5% 13% 22% 15% 1	2599
	11th to 20th	Night	53% 6% 13% 19% 8%	2431
	21st to end	Night	50% 6% 13% 19% 11% 1	2574
February	1st to 10th	Night	53% 4 11% 20% 10% 2	2340
	11th to 20th	Night	45% 5% 10% 24% 13% 3	2274
	21st to end	Night	46% 4 10% 20% 16% 3	1827
March	1st to 10th	Night	41% 4 12% 27% 14% 2	1980
	11th to 20th	Night	45% 6% 9% 22% 15% 3	1980
	21st to end	Night	59% 6% 12% 14% 8% 1	2178
April	1st to 10th	Night	39% 5% 11% 27% 16% 2	1832
	11th to 20th	Night	43% 5% 10% 25% 14% 3	1775
	21st to end	Night	43% 6% 13% 21% 15% 1	1651
May	1st to 10th	Night	40% 10% 15% 20% 14% 1	1620
	11th to 20th	Night	40% 7% 13% 25% 13% 1	1504
	21st to end	Night	38% 8% 15% 25% 11% 3	1552
June	1st to 10th	Night	28% 7% 16% 30% 16% 3	1440
	11th to 20th	Night	28% 8% 14% 24% 22% 5%	1440
	21st to end	Night	27% 8% 15% 27% 19% 4	1440
July	1st to 10th	Night	22% 6% 18% 33% 17% 3	1400
	11th to 20th	Night	18% 6% 17% 37% 19% 4	1468
	21st to end	Night	22% 6% 17% 34% 19% 3	1673
August	1st to 10th	Night	30% 9% 15% 29% 15% 2	1601
	11th to 20th	Night	26% 7% 11% 36% 17% 3	1600
	21st to end	Night	24% 6% 12% 30% 20% 9%	1916
September	1st to 10th	Night	28% 7% 10% 26% 26% 5%	1800
	11th to 20th	Night	28% 6% 11% 33% 20% 2	1889
	21st to end	Night	26% 4 9% 32% 26% 2	2011
October	1st to 10th	Night	35% 5% 9% 26% 23% 3	2160
	11th to 20th	Night	36% 5% 9% 24% 21% 5%	2160
	21st to end	Night	39% 4 11% 21% 22% 2	2376
November	1st to 10th	Night	29% 5% 17% 24% 21% 4	2315
	11th to 20th	Night	37% 4 13% 28% 16% 2	2660
	21st to end	Night	43% 4 11% 27% 14% 2	2660
December	1st to 10th	Night	46% 5% 12% 25% 11% 1	2660
	11th to 20th	Night	44% 5% 13% 28% 9% 1	3281
	21st to end	Night	65% 5% 8% 14% 7% 1	4884

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	22% 10% 32% 26% 8% 3	270
	11th to 20th	Day	22% 11% 41% 20% 5%	258
	21st to end	Day	22% 9% 41% 24% 3	264
February	1st to 10th	Day	19% 15% 43% 20% 2 1	240
	11th to 20th	Day	20% 14% 40% 20% 5% 1	240
	21st to end	Day	24% 13% 43% 16% 3 1	198
March	1st to 10th	Day	18% 16% 41% 18% 6%	240
	11th to 20th	Day	28% 9% 35% 19% 10%	240
	21st to end	Day	40% 13% 27% 16% 4	264
April	1st to 10th	Day	26% 11% 36% 21% 6% 1	388
	11th to 20th	Day	22% 14% 28% 26% 8% 1	412
	21st to end	Day	26% 12% 30% 22% 8% 2	567
May	1st to 10th	Day	22% 12% 33% 23% 9% 1	603
	11th to 20th	Day	24% 10% 30% 23% 11% 3	625
	21st to end	Day	19% 7% 26% 29% 15% 5%	679
June	1st to 10th	Day	9% 6% 22% 33% 25% 6%	630
	11th to 20th	Day	10% 3 29% 37% 16% 5%	630
	21st to end	Day	10% 8% 31% 33% 15% 3	630
July	1st to 10th	Day	9% 1 16% 33% 29% 11%	621
	11th to 20th	Day	7% 2 23% 37% 22% 7%	618
	21st to end	Day	8% 1 17% 42% 26% 6%	684
August	1st to 10th	Day	9% 2 20% 35% 29% 5%	626
	11th to 20th	Day	11% 3 15% 36% 29% 6%	615
	21st to end	Day	11% 3 19% 36% 24% 8%	559
September	1st to 10th	Day	15% 4 21% 30% 22% 8%	450
	11th to 20th	Day	9% 1 27% 34% 22% 7%	359
	21st to end	Day	15% 7% 26% 33% 17% 2	270
October	1st to 10th	Day	20% 8% 27% 24% 16% 5%	270
	11th to 20th	Day	18% 19% 37% 16% 9% 2	270
	21st to end	Day	19% 16% 31% 23% 7% 4	297
November	1st to 10th	Day	17% 10% 42% 19% 9% 3	270
	11th to 20th	Day	20% 16% 35% 20% 8% 1	270
	21st to end	Day	23% 11% 39% 22% 5% 1	270
December	1st to 10th	Day	13% 14% 38% 25% 10%	270
	11th to 20th	Day	19% 12% 36% 24% 8% 2	270
	21st to end	Day	27% 13% 41% 14% 4	297
January	1st to 10th	Night	14% 12% 36% 29% 8%	950
	11th to 20th	Night	17% 13% 36% 25% 8% 1	935
	21st to end	Night	20% 11% 35% 29% 5% 1	990
February	1st to 10th	Night	19% 11% 37% 28% 4 1	900
	11th to 20th	Night	19% 11% 38% 28% 5%	879
	21st to end	Night	17% 14% 40% 23% 5% 1	643
March	1st to 10th	Night	19% 11% 38% 23% 8% 1	540
	11th to 20th	Night	22% 6% 35% 28% 9%	540
	21st to end	Night	27% 10% 35% 22% 6% 1	594
April	1st to 10th	Night	16% 14% 39% 27% 5%	540
	11th to 20th	Night	20% 12% 33% 29% 6% 1	533
	21st to end	Night	21% 8% 34% 28% 8% 1	540
May	1st to 10th	Night	18% 7% 33% 34% 8%	540
	11th to 20th	Night	19% 5% 31% 34% 11%	531
	21st to end	Night	17% 6% 26% 35% 15% 1	584
June	1st to 10th	Night	8% 3 27% 40% 21% 1	540
	11th to 20th	Night	9% 3 27% 46% 15%	540
	21st to end	Night	9% 4 29% 43% 14%	540
July	1st to 10th	Night	7% 1 15% 45% 30% 3	525
	11th to 20th	Night	5% 1 18% 48% 26% 1	522
	21st to end	Night	4 1 14% 54% 26% 1	591
August	1st to 10th	Night	9% 1 15% 49% 24% 2	537
	11th to 20th	Night	8% 3 13% 46% 27% 3	534
	21st to end	Night	8% 2 16% 44% 26% 3	594
September	1st to 10th	Night	11% 4 19% 39% 24% 3	540
	11th to 20th	Night	14% 2 21% 37% 23% 3	540
	21st to end	Night	12% 5% 28% 36% 19% 1	571
October	1st to 10th	Night	16% 6% 29% 32% 16% 1	720
	11th to 20th	Night	18% 8% 34% 33% 8%	720
	21st to end	Night	20% 10% 33% 29% 8%	792
November	1st to 10th	Night	12% 12% 36% 32% 8% 1	778
	11th to 20th	Night	16% 15% 40% 23% 7%	950
	21st to end	Night	19% 11% 36% 27% 7%	950
December	1st to 10th	Night	15% 12% 37% 27% 8% 1	950
	11th to 20th	Night	18% 9% 35% 31% 6%	950
	21st to end	Night	21% 13% 36% 25% 5%	1045

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