Old Fort Lewis Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Old Fort Lewis Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	4 4 2 4 4 4 5% 4 6% 7% 57%	157468
	Night	4 5% 2 5% 3 4 4 3 5% 7% 58%	151879
Transparency	Day	23% 6% 9% 21% 26% 15%	152372
	Night	25% 5% 8% 21% 26% 16%	147982
Seeing	Day	9% 12% 29% 30% 16% 3	17747
	Night	8% 5% 23% 40% 21% 3	27888

By Month

Percentage Cloud Cover SW (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	3 7% 4 7% 4 4 4 4 5% 6% 53%	5318
	11th to 20th	Day	2 3 1 3 3 3 3 3 5% 5% 70%	5340
	21st to end	Day	3 8% 2 5% 3 3 4 3 4 5% 61%	5844
February	1st to 10th	Day	3 7% 3 6% 4 4 4 3 5% 6% 56%	1890
	11th to 20th	Day	5% 9% 3 7% 4 5% 4 3 5% 6% 49%	1887
	21st to end	Day	4 10% 2 6% 4 3 4 2 5% 5% 54%	1768
March	1st to 10th	Day	3 8% 2 5% 4 4 4 4 6% 7% 52%	5064
	11th to 20th	Day	5% 10% 3 7% 5% 5% 5% 4 6% 6% 42%	5110
	21st to end	Day	4 8% 3 7% 5% 5% 5% 4 6% 8% 46%	5607
April	1st to 10th	Day	4 5% 2 5% 5% 4 5% 4 5% 6% 55%	2502
	11th to 20th	Day	3 5% 2 5% 3 4 5% 4 6% 7% 53%	2491
	21st to end	Day	4 4 2 5% 4 4 4 4 6% 8% 56%	2520
May	1st to 10th	Day	6% 4 1 5% 4 4 5% 4 6% 7% 54%	7452
	11th to 20th	Day	3 3 1 4 3 4 4 4 6% 7% 61%	7596
	21st to end	Day	2 2 1 3 3 4 4 3 5% 6% 66%	8783
June	1st to 10th	Day	4 3 1 3 3 3 4 3 6% 7% 64%	2940
	11th to 20th	Day	3 2 1 3 2 3 4 3 4 4 72%	2940
	21st to end	Day	2 2 1 4 3 3 5% 3 6% 7% 64%	2940
July	1st to 10th	Day	1 2 1 3 3 3 4 4 5% 6% 68%	7779
	11th to 20th	Day	2 2 1 5% 4 6% 8% 7% 9% 10% 45%	7650
	21st to end	Day	2 3 1 4 5% 5% 7% 6% 8% 9% 50%	8304
August	1st to 10th	Day	3 2 1 4 4 5% 6% 6% 8% 9% 52%	7549
	11th to 20th	Day	2 2 1 4 5% 6% 6% 5% 8% 10% 51%	7339
	21st to end	Day	4 4 2 6% 5% 6% 7% 7% 9% 11% 39%	7751
September	1st to 10th	Day	6% 4 2 5% 4 4 5% 4 6% 7% 53%	2520
	11th to 20th	Day	7% 3 1 5% 4 4 5% 4 6% 7% 55%	2473
	21st to end	Day	7% 4 1 4 3 3 3 4 4 5% 64%	2232
October	1st to 10th	Day	5% 3 1 2 2 3 3 2 4 6% 71%	3370
	11th to 20th	Day	11% 4 2 5% 4 4 3 3 4 6% 54%	3280
	21st to end	Day	4 3 1 3 2 2 2 2 3 6% 71%	3548
November	1st to 10th	Day	4 4 1 2 2 3 3 3 4 7% 68%	2059
	11th to 20th	Day	3 4 1 3 2 2 3 3 5% 5% 69%	1932
	21st to end	Day	7% 7% 2 3 3 3 3 2 4 4 63%	1680
December	1st to 10th	Day	4 4 1 2 2 1 2 2 4 8% 69%	2700
	11th to 20th	Day	5% 5% 1 4 4 4 4 4 5% 7% 59%	3106
	21st to end	Day	4 7% 2 5% 3 3 4 3 4 5% 58%	4204
January	1st to 10th	Night	2 9% 3 9% 6% 5% 5% 3 5% 6% 48%	7605
	11th to 20th	Night	2 5% 2 3 2 3 3 2 3 5% 70%	7650
	21st to end	Night	4 8% 3 5% 3 3 3 2 4 5% 60%	8370
February	1st to 10th	Night	4 7% 2 4 3 2 2 2 4 7% 62%	2730
	11th to 20th	Night	6% 7% 2 5% 4 3 4 2 4 6% 56%	2619
	21st to end	Night	4 10% 2 5% 3 3 3 3 3 5% 60%	2034
March	1st to 10th	Night	4 10% 2 6% 4 4 4 3 5% 6% 52%	5050
	11th to 20th	Night	7% 9% 3 6% 4 5% 5% 3 5% 6% 48%	5110
	21st to end	Night	4 5% 2 6% 4 5% 5% 4 6% 7% 53%	5610
April	1st to 10th	Night	5% 6% 2 4 3 4 4 3 5% 6% 56%	2116
	11th to 20th	Night	6% 5% 2 6% 4 3 3 4 5% 5% 56%	2077
	21st to end	Night	5% 5% 2 4 3 3 3 3 5% 6% 60%	2100
May	1st to 10th	Night	5% 4 2 6% 4 3 4 3 5% 7% 57%	5479
	11th to 20th	Night	4 4 2 5% 3 2 3 3 5% 7% 61%	5296
	21st to end	Night	4 3 1 4 3 3 3 2 4 6% 67%	5419
June	1st to 10th	Night	2 4 1 4 3 3 3 2 4 7% 66%	1680
	11th to 20th	Night	3 3 1 4 3 2 3 2 5% 4 71%	1680
	21st to end	Night	3 2 1 3 3 4 5% 4 7% 7% 61%	1680
July	1st to 10th	Night	2 2 1 4 3 3 3 4 6% 5% 66%	5176
	11th to 20th	Night	3 3 2 6% 5% 5% 6% 5% 9% 11% 45%	5340
	21st to end	Night	4 4 2 6% 4 5% 6% 6% 8% 12% 43%	5805
August	1st to 10th	Night	3 3 1 5% 4 5% 5% 5% 8% 9% 52%	5273
	11th to 20th	Night	3 3 2 6% 5% 5% 5% 4 7% 10% 50%	5515
	21st to end	Night	5% 5% 2 8% 6% 6% 7% 5% 8% 12% 35%	6465
September	1st to 10th	Night	5% 4 2 6% 4 5% 5% 5% 7% 8% 50%	2100
	11th to 20th	Night	7% 5% 2 6% 4 4 4 3 5% 5% 57%	2142
	21st to end	Night	9% 4 1 3 3 3 2 2 4 4 65%	2386
October	1st to 10th	Night	7% 3 1 2 1 2 2 2 3 4 73%	4040
	11th to 20th	Night	11% 5% 2 6% 3 4 4 3 4 5% 53%	3934
	21st to end	Night	6% 3 1 3 2 2 2 2 3 5% 71%	4252
November	1st to 10th	Night	5% 3 1 3 2 2 3 2 5% 6% 69%	2460
	11th to 20th	Night	3 3 2 3 2 2 3 2 4 6% 72%	2658
	21st to end	Night	7% 6% 1 4 2 2 2 2 4 5% 63%	2940
December	1st to 10th	Night	4 4 1 3 2 2 2 2 4 6% 71%	4710
	11th to 20th	Night	5% 6% 2 4 3 3 3 3 4 7% 60%	5321
	21st to end	Night	6% 7% 2 5% 3 3 3 2 4 4 61%	7057

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	31% 5% 13% 31% 16% 4	5321
	11th to 20th	Day	16% 4 8% 26% 31% 15%	5340
	21st to end	Day	26% 4 5% 25% 29% 10%	5844
February	1st to 10th	Day	30% 4 11% 25% 22% 8%	1890
	11th to 20th	Day	35% 4 8% 24% 25% 5%	1896
	21st to end	Day	32% 4 6% 19% 30% 10%	1768
March	1st to 10th	Day	25% 6% 6% 20% 31% 11%	5064
	11th to 20th	Day	35% 8% 8% 23% 21% 6%	5110
	21st to end	Day	32% 6% 8% 25% 25% 3	5607
April	1st to 10th	Day	28% 5% 5% 20% 29% 13%	2502
	11th to 20th	Day	26% 5% 7% 19% 28% 15%	2494
	21st to end	Day	24% 5% 6% 21% 32% 12%	2520
May	1st to 10th	Day	25% 6% 9% 18% 28% 13%	7452
	11th to 20th	Day	18% 6% 6% 16% 38% 16%	7599
	21st to end	Day	15% 5% 5% 14% 40% 21%	8784
June	1st to 10th	Day	17% 5% 6% 17% 30% 24%	2940
	11th to 20th	Day	14% 4 4 11% 25% 42%	2940
	21st to end	Day	17% 5% 5% 14% 23% 36%	2940
July	1st to 10th	Day	14% 4 7% 8% 15% 52%	7716
	11th to 20th	Day	24% 10% 15% 26% 14% 11%	7581
	21st to end	Day	22% 10% 19% 21% 17% 11%	8120
August	1st to 10th	Day	26% 10% 17% 14% 16% 17%	5978
	11th to 20th	Day	25% 9% 16% 20% 17% 13%	5786
	21st to end	Day	29% 10% 18% 23% 15% 6%	6078
September	1st to 10th	Day	26% 7% 8% 18% 24% 17%	2520
	11th to 20th	Day	25% 7% 8% 15% 24% 21%	2473
	21st to end	Day	22% 4 4 17% 34% 18%	2232
October	1st to 10th	Day	16% 3 5% 15% 31% 30%	3370
	11th to 20th	Day	29% 5% 4 19% 30% 13%	3280
	21st to end	Day	16% 3 5% 24% 43% 10%	3548
November	1st to 10th	Day	18% 3 6% 29% 36% 9%	2059
	11th to 20th	Day	17% 2 7% 28% 37% 8%	1932
	21st to end	Day	27% 2 7% 26% 28% 11%	1680
December	1st to 10th	Day	17% 2 9% 34% 30% 8%	2700
	11th to 20th	Day	25% 4 7% 38% 24% 2	3104
	21st to end	Day	27% 4 12% 24% 25% 8%	4204
January	1st to 10th	Night	37% 4 9% 27% 20% 4	7605
	11th to 20th	Night	18% 3 8% 27% 29% 16%	7650
	21st to end	Night	28% 3 5% 25% 24% 15%	8370
February	1st to 10th	Night	24% 3 11% 27% 25% 11%	2730
	11th to 20th	Night	31% 3 8% 23% 27% 9%	2620
	21st to end	Night	28% 2 7% 19% 32% 12%	2034
March	1st to 10th	Night	30% 5% 5% 18% 32% 10%	5050
	11th to 20th	Night	34% 6% 6% 19% 25% 10%	5110
	21st to end	Night	27% 6% 7% 26% 28% 5%	5610
April	1st to 10th	Night	28% 4 5% 20% 28% 15%	2116
	11th to 20th	Night	28% 4 6% 17% 27% 18%	2077
	21st to end	Night	23% 3 6% 19% 35% 14%	2100
May	1st to 10th	Night	24% 6% 9% 20% 28% 13%	5477
	11th to 20th	Night	22% 5% 6% 16% 33% 19%	5297
	21st to end	Night	19% 4 5% 11% 36% 26%	5420
June	1st to 10th	Night	18% 3 6% 14% 28% 30%	1680
	11th to 20th	Night	16% 3 5% 11% 20% 45%	1680
	21st to end	Night	19% 6% 6% 12% 20% 37%	1680
July	1st to 10th	Night	18% 4 7% 7% 10% 53%	5131
	11th to 20th	Night	27% 8% 16% 23% 12% 13%	5286
	21st to end	Night	29% 8% 18% 21% 12% 12%	5677
August	1st to 10th	Night	27% 8% 14% 12% 14% 25%	4168
	11th to 20th	Night	31% 8% 14% 18% 13% 15%	4344
	21st to end	Night	34% 8% 13% 24% 13% 8%	5069
September	1st to 10th	Night	28% 7% 11% 17% 18% 20%	2100
	11th to 20th	Night	29% 5% 6% 15% 20% 26%	2142
	21st to end	Night	23% 3 4 13% 32% 24%	2386
October	1st to 10th	Night	16% 4 5% 10% 31% 35%	4040
	11th to 20th	Night	32% 5% 5% 11% 31% 17%	3934
	21st to end	Night	18% 2 3 18% 44% 14%	4252
November	1st to 10th	Night	18% 2 5% 25% 38% 13%	2460
	11th to 20th	Night	16% 2 5% 28% 38% 10%	2658
	21st to end	Night	25% 2 6% 23% 33% 11%	2940
December	1st to 10th	Night	16% 2 6% 37% 30% 8%	4710
	11th to 20th	Night	25% 3 6% 30% 30% 6%	5322
	21st to end	Night	28% 3 8% 25% 28% 9%	7057

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	15% 8% 38% 29% 10% 1	330
	11th to 20th	Day	15% 14% 34% 24% 12% 1	330
	21st to end	Day	8% 10% 35% 32% 12% 2	363
February	1st to 10th	Day	9% 16% 41% 27% 7% 1	330
	11th to 20th	Day	15% 17% 38% 23% 7%	330
	21st to end	Day	16% 18% 38% 22% 6%	311
March	1st to 10th	Day	12% 20% 37% 23% 8% 1	540
	11th to 20th	Day	13% 22% 32% 21% 10% 2	540
	21st to end	Day	15% 26% 35% 15% 7% 2	594
April	1st to 10th	Day	13% 23% 35% 18% 9% 2	540
	11th to 20th	Day	16% 28% 31% 19% 6%	532
	21st to end	Day	12% 25% 37% 19% 6%	540
May	1st to 10th	Day	11% 24% 36% 22% 6%	543
	11th to 20th	Day	10% 16% 36% 25% 12% 2	568
	21st to end	Day	7% 15% 33% 27% 15% 2	757
June	1st to 10th	Day	5% 7% 25% 39% 20% 3	780
	11th to 20th	Day	4 9% 31% 35% 18% 3	780
	21st to end	Day	4 8% 27% 38% 18% 4	780
July	1st to 10th	Day	4 4 23% 39% 27% 3	618
	11th to 20th	Day	4 3 16% 37% 33% 8%	564
	21st to end	Day	6% 3 13% 40% 31% 7%	619
August	1st to 10th	Day	5% 6% 17% 39% 26% 6%	567
	11th to 20th	Day	4 3 22% 41% 24% 6%	557
	21st to end	Day	6% 3 20% 41% 25% 6%	627
September	1st to 10th	Day	7% 3 24% 36% 25% 6%	570
	11th to 20th	Day	8% 5% 28% 41% 15% 4	570
	21st to end	Day	10% 10% 28% 29% 18% 4	492
October	1st to 10th	Day	9% 11% 20% 29% 23% 8%	360
	11th to 20th	Day	10% 18% 25% 30% 14% 4	339
	21st to end	Day	10% 11% 30% 30% 17% 2	363
November	1st to 10th	Day	9% 16% 28% 31% 15% 2	330
	11th to 20th	Day	7% 11% 36% 30% 14% 2	330
	21st to end	Day	12% 8% 33% 29% 15% 2	330
December	1st to 10th	Day	14% 10% 33% 28% 13% 2	330
	11th to 20th	Day	10% 10% 33% 31% 14% 2	330
	21st to end	Day	17% 13% 34% 29% 7%	363
January	1st to 10th	Night	8% 4 29% 39% 16% 4	840
	11th to 20th	Night	9% 4 29% 38% 18% 3	840
	21st to end	Night	10% 5% 28% 38% 17% 3	924
February	1st to 10th	Night	9% 5% 34% 39% 11% 2	840
	11th to 20th	Night	11% 6% 35% 37% 10% 1	824
	21st to end	Night	10% 5% 40% 39% 7%	692
March	1st to 10th	Night	9% 7% 31% 38% 13% 2	840
	11th to 20th	Night	10% 6% 28% 35% 20% 1	840
	21st to end	Night	9% 8% 31% 37% 13% 1	924
April	1st to 10th	Night	10% 10% 32% 39% 9% 1	646
	11th to 20th	Night	9% 13% 27% 37% 12% 1	623
	21st to end	Night	7% 10% 34% 37% 11% 1	630
May	1st to 10th	Night	8% 9% 28% 39% 15% 1	630
	11th to 20th	Night	10% 5% 24% 40% 19% 2	621
	21st to end	Night	10% 6% 25% 40% 17% 2	683
June	1st to 10th	Night	4 5% 17% 49% 24% 1	630
	11th to 20th	Night	5% 3 20% 47% 22% 2	630
	21st to end	Night	4 3 18% 43% 29% 2	630
July	1st to 10th	Night	4 1 10% 44% 37% 3	615
	11th to 20th	Night	6% 1 6% 37% 42% 7%	612
	21st to end	Night	7% 1 7% 35% 42% 7%	690
August	1st to 10th	Night	4 3 6% 34% 45% 8%	627
	11th to 20th	Night	5% 2 7% 43% 37% 5%	624
	21st to end	Night	7% 2 7% 39% 40% 5%	693
September	1st to 10th	Night	6% 2 8% 44% 35% 4	630
	11th to 20th	Night	7% 3 11% 46% 28% 4	672
	21st to end	Night	10% 3 14% 41% 27% 4	840
October	1st to 10th	Night	11% 4 19% 41% 24% 2	840
	11th to 20th	Night	8% 3 21% 44% 21% 3	812
	21st to end	Night	6% 4 21% 43% 22% 5%	880
November	1st to 10th	Night	7% 3 20% 46% 22% 3	820
	11th to 20th	Night	5% 5% 28% 41% 17% 4	973
	21st to end	Night	11% 7% 22% 37% 19% 4	1050
December	1st to 10th	Night	10% 6% 27% 41% 14% 2	1050
	11th to 20th	Night	13% 6% 27% 36% 17% 1	1050
	21st to end	Night	13% 7% 29% 39% 11% 1	1123

Page generated at 2026-10-11 03:06:27. Version ac07

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