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 2025-01-07.

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% 40%	104997
	Night	12% 8% 3 8% 4 4 4 3 5% 6% 43%	104154
Transparency	Day	42% 6% 12% 24% 15% 2	108637
	Night	40% 5% 12% 24% 16% 2	108654
Seeing	Day	16% 8% 29% 28% 15% 4	16022
	Night	16% 8% 31% 32% 11% 1	25974

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% 12% 6% 22% 7% 5% 5% 4 4 5% 22%	2647
	11th to 20th	Day	8% 13% 6% 12% 7% 5% 8% 3 5% 7% 26%	2850
	21st to end	Day	13% 13% 8% 17% 6% 5% 5% 3 4 5% 22%	3135
February	1st to 10th	Day	11% 14% 5% 11% 6% 5% 4 3 4 7% 31%	1710
	11th to 20th	Day	12% 10% 4 9% 4 4 4 4 5% 7% 37%	1664
	21st to end	Day	14% 9% 3 10% 6% 5% 5% 4 4 5% 36%	1531
March	1st to 10th	Day	18% 10% 3 7% 4 3 4 2 4 4 42%	3436
	11th to 20th	Day	11% 8% 3 7% 4 4 4 4 5% 6% 45%	3480
	21st to end	Day	22% 15% 5% 8% 5% 4 4 3 4 5% 23%	3814
April	1st to 10th	Day	16% 10% 3 6% 5% 3 4 4 6% 7% 36%	2247
	11th to 20th	Day	15% 9% 3 6% 5% 4 4 3 5% 6% 38%	2254
	21st to end	Day	21% 8% 2 6% 4 4 4 4 5% 6% 35%	2436
May	1st to 10th	Day	16% 8% 2 8% 6% 5% 5% 5% 7% 7% 31%	4082
	11th to 20th	Day	15% 7% 2 7% 5% 4 5% 4 5% 8% 38%	4239
	21st to end	Day	13% 6% 2 6% 4 4 4 3 5% 8% 43%	4824
June	1st to 10th	Day	9% 7% 2 6% 4 5% 4 4 6% 9% 45%	2660
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 4 7% 11% 45%	2660
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 4 6% 8% 44%	2660
July	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 9% 39%	4440
	11th to 20th	Day	5% 4 1 5% 4 3 4 4 5% 7% 57%	4290
	21st to end	Day	4 3 2 6% 4 5% 6% 5% 8% 12% 46%	4465
August	1st to 10th	Day	7% 5% 2 5% 4 4 5% 4 7% 10% 46%	4104
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 6% 8% 41%	4041
	21st to end	Day	7% 5% 2 7% 4 5% 5% 4 6% 8% 46%	4286
September	1st to 10th	Day	13% 4 2 4 3 2 4 3 4 6% 55%	2280
	11th to 20th	Day	9% 5% 2 5% 3 4 3 4 5% 9% 51%	2183
	21st to end	Day	10% 5% 2 4 3 3 4 4 5% 8% 52%	2038
October	1st to 10th	Day	14% 6% 2 5% 3 3 3 2 4 6% 50%	1900
	11th to 20th	Day	15% 6% 3 5% 3 4 3 3 4 6% 48%	1900
	21st to end	Day	13% 9% 3 6% 4 3 4 3 6% 6% 43%	2090
November	1st to 10th	Day	12% 5% 2 6% 4 4 3 4 5% 8% 48%	1851
	11th to 20th	Day	13% 8% 3 7% 4 5% 4 3 5% 8% 41%	1600
	21st to end	Day	14% 10% 4 6% 4 3 3 3 6% 7% 39%	1600
December	1st to 10th	Day	6% 7% 2 7% 4 4 4 4 4 7% 52%	2610
	11th to 20th	Day	11% 9% 4 11% 6% 5% 6% 4 6% 7% 32%	3008
	21st to end	Day	20% 16% 6% 14% 4 4 4 3 5% 4 21%	3982
January	1st to 10th	Night	10% 11% 5% 20% 7% 5% 3 3 4 5% 26%	4438
	11th to 20th	Night	10% 13% 5% 10% 6% 5% 7% 3 9% 6% 26%	4120
	21st to end	Night	13% 16% 5% 14% 5% 4 4 3 4 6% 27%	4532
February	1st to 10th	Night	12% 11% 4 11% 6% 5% 4 3 5% 6% 35%	2470
	11th to 20th	Night	12% 10% 4 7% 4 4 4 4 5% 6% 40%	2403
	21st to end	Night	11% 9% 4 9% 5% 4 4 3 4 5% 43%	1938
March	1st to 10th	Night	13% 10% 3 6% 3 3 3 3 3 5% 49%	3419
	11th to 20th	Night	12% 7% 2 5% 3 3 3 2 4 6% 53%	3480
	21st to end	Night	19% 11% 4 13% 5% 5% 5% 3 5% 5% 24%	3817
April	1st to 10th	Night	13% 8% 2 7% 4 3 4 3 5% 6% 46%	1933
	11th to 20th	Night	14% 9% 3 7% 4 4 3 3 5% 6% 43%	1875
	21st to end	Night	18% 8% 2 6% 4 4 3 3 5% 6% 42%	1742
May	1st to 10th	Night	19% 6% 2 5% 4 3 5% 4 5% 7% 40%	2823
	11th to 20th	Night	13% 7% 2 5% 4 3 4 3 5% 5% 48%	2649
	21st to end	Night	13% 6% 2 5% 4 4 4 4 6% 7% 46%	2743
June	1st to 10th	Night	6% 5% 2 5% 5% 3 5% 3 6% 7% 53%	1520
	11th to 20th	Night	7% 5% 2 5% 3 4 4 4 6% 9% 51%	1520
	21st to end	Night	7% 4 2 4 4 4 5% 4 6% 9% 52%	1520
July	1st to 10th	Night	8% 4 1 6% 4 4 5% 4 6% 10% 48%	2530
	11th to 20th	Night	4 3 1 3 3 3 3 3 5% 9% 63%	2676
	21st to end	Night	5% 3 2 5% 3 4 5% 4 7% 10% 51%	3098
August	1st to 10th	Night	5% 4 2 5% 4 4 4 3 6% 9% 56%	2836
	11th to 20th	Night	10% 7% 2 5% 4 3 3 3 4 6% 54%	2798
	21st to end	Night	8% 4 1 4 3 3 3 4 5% 8% 57%	3373
September	1st to 10th	Night	9% 4 1 4 3 3 3 3 4 7% 59%	1900
	11th to 20th	Night	10% 4 2 4 3 3 3 3 5% 7% 55%	1994
	21st to end	Night	10% 4 1 4 3 2 3 2 5% 6% 59%	2123
October	1st to 10th	Night	12% 7% 2 5% 2 3 3 3 5% 6% 53%	2280
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 4 6% 51%	2280
	21st to end	Night	15% 6% 2 6% 4 4 3 2 5% 5% 48%	2508
November	1st to 10th	Night	9% 5% 2 5% 3 3 4 3 4 7% 54%	2441
	11th to 20th	Night	12% 7% 2 7% 4 4 3 3 6% 6% 47%	2800
	21st to end	Night	14% 9% 3 7% 4 4 3 3 4 5% 44%	2800
December	1st to 10th	Night	6% 9% 3 7% 4 4 4 3 5% 6% 48%	4580
	11th to 20th	Night	11% 9% 4 10% 5% 4 4 3 6% 6% 37%	5199
	21st to end	Night	19% 15% 6% 12% 5% 4 4 3 4 4 23%	6996

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	64% 5% 11% 13% 7% 1	2648
	11th to 20th	Day	59% 4 8% 22% 7%	2850
	21st to end	Day	65% 4 11% 14% 6%	3135
February	1st to 10th	Day	55% 4 10% 19% 10% 1	1710
	11th to 20th	Day	47% 5% 13% 22% 12% 2	1673
	21st to end	Day	49% 5% 10% 20% 13% 2	1531
March	1st to 10th	Day	46% 5% 8% 22% 17% 1	3436
	11th to 20th	Day	39% 7% 13% 23% 16% 2	3480
	21st to end	Day	61% 7% 13% 13% 5% 1	3814
April	1st to 10th	Day	46% 6% 10% 25% 11% 1	2247
	11th to 20th	Day	46% 6% 11% 21% 13% 2	2254
	21st to end	Day	47% 7% 11% 22% 12% 1	2436
May	1st to 10th	Day	48% 8% 13% 18% 12% 1	4120
	11th to 20th	Day	43% 7% 9% 29% 11%	4243
	21st to end	Day	39% 7% 12% 24% 17% 2	4837
June	1st to 10th	Day	35% 8% 17% 25% 14% 3	2660
	11th to 20th	Day	31% 7% 15% 25% 21% 2	2660
	21st to end	Day	36% 8% 14% 25% 15% 3	2660
July	1st to 10th	Day	41% 5% 15% 28% 10% 2	4371
	11th to 20th	Day	27% 5% 13% 33% 20% 2	4215
	21st to end	Day	28% 8% 19% 26% 17% 2	4325
August	1st to 10th	Day	32% 6% 15% 29% 16% 1	4090
	11th to 20th	Day	41% 5% 12% 27% 15% 1	4080
	21st to end	Day	35% 4 15% 28% 16% 3	4285
September	1st to 10th	Day	31% 5% 11% 26% 22% 4	2280
	11th to 20th	Day	29% 7% 12% 32% 19% 1	2183
	21st to end	Day	30% 5% 11% 31% 22% 1	2038
October	1st to 10th	Day	21% 3 6% 24% 44% 2	3160
	11th to 20th	Day	24% 2 9% 32% 25% 7%	3140
	21st to end	Day	36% 4 9% 28% 19% 3	3428
November	1st to 10th	Day	34% 5% 16% 24% 19% 2	1851
	11th to 20th	Day	42% 5% 12% 23% 18% 1	1600
	21st to end	Day	45% 4 14% 24% 13% 1	1600
December	1st to 10th	Day	34% 3 18% 32% 12% 1	2610
	11th to 20th	Day	50% 5% 14% 21% 9%	3005
	21st to end	Day	67% 4 9% 14% 5% 1	3982
January	1st to 10th	Night	59% 5% 10% 16% 9% 1	4438
	11th to 20th	Night	55% 4 12% 16% 11% 1	4120
	21st to end	Night	59% 4 12% 15% 8% 1	4532
February	1st to 10th	Night	51% 4 11% 20% 10% 2	2470
	11th to 20th	Night	44% 5% 10% 25% 14% 2	2404
	21st to end	Night	45% 4 10% 20% 17% 4	1938
March	1st to 10th	Night	39% 4 9% 25% 21% 2	3419
	11th to 20th	Night	34% 6% 9% 30% 17% 5%	3480
	21st to end	Night	60% 8% 16% 11% 4 1	3817
April	1st to 10th	Night	39% 5% 11% 28% 15% 2	1933
	11th to 20th	Night	43% 5% 10% 25% 15% 3	1875
	21st to end	Night	43% 6% 13% 22% 15% 1	1742
May	1st to 10th	Night	42% 7% 12% 20% 18% 1	2850
	11th to 20th	Night	37% 7% 11% 26% 17% 2	2650
	21st to end	Night	36% 8% 12% 22% 19% 2	2751
June	1st to 10th	Night	29% 7% 16% 30% 15% 4	1520
	11th to 20th	Night	28% 7% 13% 25% 22% 5%	1520
	21st to end	Night	28% 8% 14% 27% 20% 3	1520
July	1st to 10th	Night	32% 4 15% 32% 14% 2	2490
	11th to 20th	Night	20% 4 18% 34% 22% 2	2628
	21st to end	Night	27% 6% 20% 28% 17% 2	2999
August	1st to 10th	Night	25% 6% 15% 35% 18% 1	2826
	11th to 20th	Night	33% 5% 12% 33% 16% 2	2825
	21st to end	Night	26% 4 17% 29% 17% 7%	3373
September	1st to 10th	Night	26% 6% 9% 27% 26% 5%	1900
	11th to 20th	Night	28% 5% 12% 33% 19% 2	1994
	21st to end	Night	27% 4 9% 31% 26% 3	2123
October	1st to 10th	Night	22% 3 6% 21% 46% 3	3800
	11th to 20th	Night	27% 3 7% 28% 28% 7%	3779
	21st to end	Night	41% 3 9% 24% 21% 2	4120
November	1st to 10th	Night	31% 5% 17% 24% 20% 4	2441
	11th to 20th	Night	38% 4 13% 27% 16% 2	2800
	21st to end	Night	44% 3 11% 27% 14% 2	2800
December	1st to 10th	Night	37% 3 11% 38% 11% 1	4580
	11th to 20th	Night	47% 5% 14% 25% 10%	5201
	21st to end	Night	65% 4 10% 15% 6%	6996

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% 9% 33% 25% 7% 2	288
	11th to 20th	Day	21% 11% 40% 23% 5%	270
	21st to end	Day	21% 9% 40% 25% 5%	297
February	1st to 10th	Day	18% 14% 43% 20% 3 1	270
	11th to 20th	Day	18% 14% 38% 23% 6% 1	270
	21st to end	Day	23% 12% 44% 16% 4 1	225
March	1st to 10th	Day	19% 15% 40% 19% 6%	270
	11th to 20th	Day	26% 10% 36% 19% 9%	270
	21st to end	Day	38% 13% 28% 16% 4	297
April	1st to 10th	Day	26% 11% 36% 20% 6% 1	427
	11th to 20th	Day	23% 15% 29% 25% 8% 1	452
	21st to end	Day	27% 12% 29% 22% 8% 2	616
May	1st to 10th	Day	22% 11% 33% 23% 9% 1	653
	11th to 20th	Day	23% 9% 30% 25% 11% 3	675
	21st to end	Day	19% 7% 27% 28% 15% 4	734
June	1st to 10th	Day	9% 6% 22% 33% 24% 6%	680
	11th to 20th	Day	10% 4 29% 36% 15% 4	680
	21st to end	Day	9% 8% 32% 34% 15% 3	680
July	1st to 10th	Day	9% 2 17% 33% 28% 11%	671
	11th to 20th	Day	7% 2 24% 37% 22% 7%	668
	21st to end	Day	7% 1 18% 41% 26% 6%	739
August	1st to 10th	Day	9% 3 22% 35% 27% 5%	676
	11th to 20th	Day	12% 3 17% 36% 27% 5%	665
	21st to end	Day	11% 3 20% 36% 23% 7%	606
September	1st to 10th	Day	14% 4 20% 30% 23% 8%	490
	11th to 20th	Day	9% 2 26% 34% 23% 7%	393
	21st to end	Day	15% 7% 25% 34% 17% 3	300
October	1st to 10th	Day	19% 7% 28% 24% 16% 6%	300
	11th to 20th	Day	17% 17% 37% 18% 9% 2	300
	21st to end	Day	18% 16% 33% 23% 6% 4	330
November	1st to 10th	Day	18% 11% 39% 20% 9% 2	300
	11th to 20th	Day	21% 17% 34% 18% 8% 1	300
	21st to end	Day	22% 12% 38% 22% 5% 1	300
December	1st to 10th	Day	12% 14% 40% 24% 9%	300
	11th to 20th	Day	19% 14% 36% 22% 7% 2	300
	21st to end	Day	27% 13% 39% 15% 5%	330
January	1st to 10th	Night	15% 13% 36% 29% 8%	981
	11th to 20th	Night	17% 13% 36% 25% 8% 1	950
	21st to end	Night	20% 11% 34% 29% 6% 1	1045
February	1st to 10th	Night	19% 10% 38% 28% 4 1	950
	11th to 20th	Night	18% 10% 38% 28% 5%	929
	21st to end	Night	17% 14% 40% 23% 5% 1	682
March	1st to 10th	Night	19% 11% 38% 23% 8% 1	570
	11th to 20th	Night	21% 7% 36% 27% 8%	570
	21st to end	Night	27% 10% 35% 22% 6% 1	627
April	1st to 10th	Night	17% 14% 39% 26% 4	570
	11th to 20th	Night	20% 12% 33% 28% 6% 1	563
	21st to end	Night	21% 8% 35% 28% 8% 1	570
May	1st to 10th	Night	18% 8% 33% 34% 7%	570
	11th to 20th	Night	19% 5% 30% 34% 11%	561
	21st to end	Night	17% 6% 27% 35% 15% 1	617
June	1st to 10th	Night	8% 4 27% 39% 20% 1	570
	11th to 20th	Night	9% 4 27% 46% 14%	570
	21st to end	Night	9% 5% 30% 42% 13%	570
July	1st to 10th	Night	7% 1 16% 45% 28% 3	555
	11th to 20th	Night	5% 1 18% 48% 26% 1	552
	21st to end	Night	4 1 14% 53% 26% 1	624
August	1st to 10th	Night	9% 2 15% 50% 23% 2	567
	11th to 20th	Night	9% 3 14% 46% 25% 3	564
	21st to end	Night	9% 2 16% 44% 26% 3	627
September	1st to 10th	Night	11% 4 19% 40% 24% 3	570
	11th to 20th	Night	14% 2 21% 38% 23% 3	570
	21st to end	Night	11% 5% 27% 37% 19% 1	603
October	1st to 10th	Night	16% 6% 28% 32% 17% 1	760
	11th to 20th	Night	17% 8% 34% 33% 8%	760
	21st to end	Night	20% 10% 33% 28% 8%	836
November	1st to 10th	Night	13% 11% 35% 32% 8% 1	821
	11th to 20th	Night	16% 15% 40% 23% 6%	1000
	21st to end	Night	19% 11% 36% 27% 7%	1000
December	1st to 10th	Night	15% 13% 38% 27% 7% 1	1000
	11th to 20th	Night	19% 9% 36% 30% 6%	1000
	21st to end	Night	22% 12% 35% 26% 5%	1100

Page generated at 2025-01-04 02:24:03. Version ac07

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