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 2025-06-10.

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 5% 2 5% 4 4 5% 4 6% 7% 56%	123396
	Night	5% 6% 2 5% 3 3 4 3 5% 6% 58%	123785
Transparency	Day	24% 6% 9% 21% 26% 14%	123162
	Night	26% 4 8% 21% 26% 15%	123606
Seeing	Day	9% 12% 29% 30% 16% 3	15892
	Night	9% 5% 23% 40% 21% 3	26151

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	2 6% 3 7% 4 4 4 4 5% 6% 55%	4098
	11th to 20th	Day	2 4 2 4 4 4 5% 4 6% 6% 61%	4100
	21st to end	Day	3 9% 2 5% 3 3 4 3 3 5% 60%	4510
February	1st to 10th	Day	3 8% 3 6% 5% 4 4 3 5% 5% 55%	1800
	11th to 20th	Day	5% 8% 3 7% 4 5% 4 3 5% 6% 50%	1795
	21st to end	Day	4 10% 2 7% 5% 3 4 2 5% 5% 52%	1686
March	1st to 10th	Day	3 8% 2 5% 4 4 4 4 6% 7% 52%	4954
	11th to 20th	Day	5% 10% 3 7% 6% 5% 5% 4 6% 6% 41%	5000
	21st to end	Day	4 8% 3 7% 5% 5% 5% 4 6% 8% 46%	5486
April	1st to 10th	Day	4 5% 2 5% 5% 4 5% 3 5% 6% 55%	2383
	11th to 20th	Day	3 6% 2 5% 3 4 5% 4 6% 8% 53%	2371
	21st to end	Day	4 4 2 5% 4 4 4 4 6% 8% 56%	2400
May	1st to 10th	Day	7% 4 2 4 4 4 6% 4 6% 7% 52%	5745
	11th to 20th	Day	3 3 1 4 3 4 5% 4 6% 8% 58%	5841
	21st to end	Day	2 2 1 3 2 4 4 3 5% 7% 67%	6738
June	1st to 10th	Day	4 3 1 3 3 3 3 3 6% 7% 64%	2788
	11th to 20th	Day	3 2 1 3 3 3 4 3 4 4 71%	2660
	21st to end	Day	2 2 1 4 3 3 5% 4 6% 7% 62%	2660
July	1st to 10th	Day	2 3 1 3 3 4 4 4 6% 6% 63%	4184
	11th to 20th	Day	2 1 1 4 4 7% 9% 8% 10% 11% 42%	4110
	21st to end	Day	4 3 2 4 5% 5% 6% 5% 7% 8% 49%	4454
August	1st to 10th	Day	5% 3 2 6% 6% 7% 9% 8% 11% 10% 33%	4092
	11th to 20th	Day	3 3 1 5% 5% 5% 6% 4 8% 11% 48%	3906
	21st to end	Day	4 3 2 5% 5% 6% 6% 6% 9% 11% 43%	4180
September	1st to 10th	Day	7% 4 2 5% 4 5% 5% 4 6% 7% 52%	2280
	11th to 20th	Day	6% 3 1 5% 4 5% 5% 4 6% 7% 55%	2237
	21st to end	Day	6% 3 1 3 2 2 2 4 4 6% 67%	2020
October	1st to 10th	Day	5% 3 1 2 2 3 3 2 4 6% 71%	3170
	11th to 20th	Day	11% 4 2 5% 4 4 3 3 4 6% 54%	3150
	21st to end	Day	4 3 1 3 2 2 2 2 3 6% 71%	3438
November	1st to 10th	Day	4 4 1 3 3 3 2 3 4 7% 68%	1959
	11th to 20th	Day	3 3 1 3 2 2 3 3 4 5% 70%	1841
	21st to end	Day	6% 7% 2 3 3 3 3 2 4 4 63%	1600
December	1st to 10th	Day	4 4 1 2 2 1 2 2 4 8% 69%	2620
	11th to 20th	Day	5% 5% 2 4 4 4 4 4 5% 7% 58%	3026
	21st to end	Day	4 7% 2 5% 3 3 4 3 4 5% 58%	4114
January	1st to 10th	Night	2 8% 3 9% 5% 4 4 3 4 5% 53%	5867
	11th to 20th	Night	2 6% 2 3 3 3 3 3 3 5% 65%	5880
	21st to end	Night	4 9% 4 6% 4 3 3 2 4 5% 57%	6468
February	1st to 10th	Night	4 8% 2 4 3 2 2 2 5% 7% 61%	2600
	11th to 20th	Night	6% 7% 2 5% 3 3 4 2 4 6% 57%	2491
	21st to end	Night	4 10% 2 5% 3 3 3 3 3 5% 59%	1940
March	1st to 10th	Night	4 10% 2 6% 4 4 4 3 5% 6% 52%	4940
	11th to 20th	Night	7% 9% 3 6% 4 5% 5% 3 5% 6% 47%	5000
	21st to end	Night	4 5% 2 6% 4 5% 5% 4 6% 6% 52%	5489
April	1st to 10th	Night	5% 6% 2 4 4 5% 4 3 5% 7% 55%	2015
	11th to 20th	Night	6% 5% 2 6% 4 4 3 4 5% 5% 56%	1977
	21st to end	Night	5% 5% 2 4 3 3 3 3 6% 7% 60%	2000
May	1st to 10th	Night	6% 4 2 5% 3 3 4 3 5% 8% 57%	4219
	11th to 20th	Night	4 4 2 5% 3 3 4 3 5% 7% 61%	4074
	21st to end	Night	4 3 1 3 2 2 3 2 3 6% 70%	4154
June	1st to 10th	Night	2 4 1 4 3 3 3 2 4 7% 66%	1596
	11th to 20th	Night	4 3 1 4 3 2 3 2 5% 4 70%	1520
	21st to end	Night	3 2 1 3 3 4 5% 4 7% 8% 59%	1520
July	1st to 10th	Night	3 3 1 5% 4 4 4 4 5% 5% 62%	2780
	11th to 20th	Night	2 3 1 4 4 5% 7% 5% 9% 11% 47%	2860
	21st to end	Night	4 4 2 6% 4 5% 6% 5% 7% 9% 47%	3109
August	1st to 10th	Night	5% 5% 2 7% 6% 7% 8% 7% 11% 12% 32%	2847
	11th to 20th	Night	3 4 2 6% 5% 6% 6% 5% 8% 10% 45%	2931
	21st to end	Night	4 5% 2 7% 6% 5% 6% 5% 7% 11% 41%	3487
September	1st to 10th	Night	5% 4 2 6% 4 5% 5% 5% 7% 8% 49%	1900
	11th to 20th	Night	6% 4 2 6% 4 4 4 3 5% 5% 58%	1938
	21st to end	Night	7% 3 1 3 3 3 2 2 3 4 69%	2158
October	1st to 10th	Night	6% 3 1 2 1 2 2 2 3 4 74%	3800
	11th to 20th	Night	10% 5% 2 6% 3 4 4 3 4 5% 54%	3778
	21st to end	Night	6% 3 1 3 3 2 2 2 3 5% 71%	4120
November	1st to 10th	Night	5% 4 1 3 2 2 3 2 5% 6% 67%	2340
	11th to 20th	Night	2 3 2 3 2 2 3 2 4 6% 73%	2531
	21st to end	Night	7% 6% 1 4 2 2 2 2 4 5% 64%	2800
December	1st to 10th	Night	4 4 1 3 2 2 2 2 4 6% 71%	4570
	11th to 20th	Night	5% 6% 2 4 3 3 3 3 4 7% 60%	5181
	21st to end	Night	6% 7% 2 5% 3 3 3 2 4 4 61%	6905

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	29% 3 12% 30% 20% 5%	4099
	11th to 20th	Day	21% 5% 10% 25% 23% 16%	4100
	21st to end	Day	27% 5% 6% 25% 26% 10%	4510
February	1st to 10th	Day	31% 4 12% 25% 20% 9%	1800
	11th to 20th	Day	34% 3 8% 24% 26% 5%	1804
	21st to end	Day	34% 4 5% 17% 30% 10%	1686
March	1st to 10th	Day	26% 6% 6% 20% 31% 10%	4954
	11th to 20th	Day	35% 8% 8% 23% 20% 6%	5000
	21st to end	Day	32% 6% 8% 25% 25% 3	5486
April	1st to 10th	Day	28% 5% 5% 20% 30% 13%	2383
	11th to 20th	Day	26% 5% 7% 19% 28% 15%	2374
	21st to end	Day	24% 5% 6% 22% 31% 12%	2400
May	1st to 10th	Day	27% 7% 9% 18% 24% 15%	5745
	11th to 20th	Day	20% 7% 7% 18% 34% 14%	5844
	21st to end	Day	13% 5% 5% 13% 41% 22%	6739
June	1st to 10th	Day	17% 5% 7% 17% 30% 24%	2788
	11th to 20th	Day	14% 4 4 11% 25% 41%	2660
	21st to end	Day	18% 5% 5% 15% 24% 33%	2660
July	1st to 10th	Day	18% 6% 9% 9% 19% 40%	4120
	11th to 20th	Day	24% 11% 13% 27% 17% 8%	4041
	21st to end	Day	24% 10% 16% 21% 15% 14%	4314
August	1st to 10th	Day	34% 12% 22% 12% 10% 10%	4078
	11th to 20th	Day	26% 8% 15% 14% 18% 18%	3945
	21st to end	Day	28% 9% 14% 20% 21% 9%	4179
September	1st to 10th	Day	27% 7% 8% 17% 23% 18%	2280
	11th to 20th	Day	24% 7% 8% 15% 24% 21%	2237
	21st to end	Day	19% 4 4 18% 37% 19%	2020
October	1st to 10th	Day	16% 3 5% 15% 30% 31%	3170
	11th to 20th	Day	29% 5% 4 19% 30% 13%	3150
	21st to end	Day	16% 3 5% 23% 43% 10%	3438
November	1st to 10th	Day	18% 3 6% 27% 36% 9%	1959
	11th to 20th	Day	17% 2 7% 28% 38% 8%	1841
	21st to end	Day	27% 2 6% 25% 29% 12%	1600
December	1st to 10th	Day	17% 2 9% 34% 30% 8%	2620
	11th to 20th	Day	26% 4 8% 38% 23% 2	3024
	21st to end	Day	27% 4 12% 24% 24% 8%	4114
January	1st to 10th	Night	33% 3 7% 29% 24% 4	5867
	11th to 20th	Night	22% 3 10% 24% 23% 17%	5880
	21st to end	Night	30% 4 6% 27% 20% 12%	6468
February	1st to 10th	Night	24% 3 11% 27% 23% 11%	2600
	11th to 20th	Night	29% 3 8% 23% 28% 9%	2492
	21st to end	Night	29% 3 6% 18% 32% 13%	1940
March	1st to 10th	Night	30% 5% 5% 19% 32% 10%	4940
	11th to 20th	Night	35% 6% 6% 19% 24% 10%	5000
	21st to end	Night	28% 6% 7% 26% 28% 5%	5489
April	1st to 10th	Night	28% 4 5% 19% 28% 14%	2015
	11th to 20th	Night	28% 4 6% 17% 27% 18%	1977
	21st to end	Night	23% 3 6% 19% 34% 14%	2000
May	1st to 10th	Night	24% 5% 10% 21% 28% 11%	4216
	11th to 20th	Night	20% 5% 7% 19% 33% 16%	4075
	21st to end	Night	15% 4 5% 11% 36% 29%	4154
June	1st to 10th	Night	19% 3 6% 15% 28% 29%	1596
	11th to 20th	Night	17% 3 5% 11% 20% 44%	1520
	21st to end	Night	19% 6% 7% 12% 22% 34%	1520
July	1st to 10th	Night	23% 5% 8% 8% 10% 46%	2735
	11th to 20th	Night	24% 9% 14% 26% 17% 11%	2806
	21st to end	Night	28% 9% 15% 21% 11% 17%	3011
August	1st to 10th	Night	38% 11% 19% 11% 9% 13%	2838
	11th to 20th	Night	32% 8% 14% 14% 11% 21%	2958
	21st to end	Night	33% 8% 11% 20% 17% 11%	3487
September	1st to 10th	Night	29% 7% 10% 17% 17% 21%	1900
	11th to 20th	Night	27% 5% 6% 15% 20% 26%	1938
	21st to end	Night	20% 3 4 13% 33% 26%	2158
October	1st to 10th	Night	15% 4 5% 9% 30% 37%	3800
	11th to 20th	Night	32% 4 5% 11% 31% 17%	3778
	21st to end	Night	18% 2 3 18% 45% 14%	4120
November	1st to 10th	Night	19% 2 5% 24% 37% 13%	2340
	11th to 20th	Night	15% 2 5% 27% 39% 11%	2531
	21st to end	Night	25% 2 5% 22% 34% 12%	2800
December	1st to 10th	Night	16% 2 7% 37% 30% 9%	4570
	11th to 20th	Night	26% 3 6% 29% 30% 5%	5182
	21st to end	Night	28% 3 8% 25% 28% 9%	6905

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	14% 7% 39% 28% 11% 1	300
	11th to 20th	Day	16% 15% 34% 22% 12% 1	300
	21st to end	Day	8% 10% 35% 32% 13% 2	330
February	1st to 10th	Day	10% 17% 42% 26% 6%	300
	11th to 20th	Day	14% 16% 40% 23% 7%	300
	21st to end	Day	15% 19% 39% 22% 6%	285
March	1st to 10th	Day	13% 20% 37% 22% 8% 1	500
	11th to 20th	Day	13% 21% 32% 21% 11% 2	500
	21st to end	Day	17% 26% 33% 15% 7% 1	550
April	1st to 10th	Day	13% 23% 36% 17% 8% 2	500
	11th to 20th	Day	17% 27% 30% 19% 7%	492
	21st to end	Day	11% 23% 38% 20% 6%	500
May	1st to 10th	Day	11% 25% 36% 22% 6%	503
	11th to 20th	Day	10% 16% 35% 25% 12% 2	528
	21st to end	Day	7% 15% 32% 27% 16% 2	706
June	1st to 10th	Day	6% 6% 25% 40% 20% 3	726
	11th to 20th	Day	4 9% 29% 35% 19% 4	680
	21st to end	Day	4 6% 26% 40% 20% 4	680
July	1st to 10th	Day	5% 4 19% 39% 28% 4	534
	11th to 20th	Day	4 3 14% 36% 34% 8%	484
	21st to end	Day	7% 4 11% 40% 32% 6%	531
August	1st to 10th	Day	6% 5% 14% 39% 28% 7%	487
	11th to 20th	Day	5% 3 23% 39% 23% 7%	477
	21st to end	Day	6% 3 19% 42% 24% 6%	539
September	1st to 10th	Day	8% 3 20% 37% 26% 7%	490
	11th to 20th	Day	6% 4 27% 42% 16% 4	490
	21st to end	Day	8% 8% 29% 30% 20% 5%	420
October	1st to 10th	Day	9% 13% 19% 26% 24% 8%	300
	11th to 20th	Day	9% 14% 25% 32% 16% 4	300
	21st to end	Day	10% 12% 29% 30% 17% 2	330
November	1st to 10th	Day	10% 16% 28% 32% 14% 1	300
	11th to 20th	Day	6% 11% 37% 31% 13% 2	300
	21st to end	Day	12% 8% 33% 29% 14% 3	300
December	1st to 10th	Day	14% 9% 32% 28% 14% 3	300
	11th to 20th	Day	11% 9% 34% 32% 12% 2	300
	21st to end	Day	18% 13% 33% 28% 7%	330
January	1st to 10th	Night	8% 4 29% 39% 16% 4	800
	11th to 20th	Night	9% 4 29% 38% 17% 3	800
	21st to end	Night	10% 5% 28% 37% 17% 3	880
February	1st to 10th	Night	10% 5% 35% 38% 11% 2	800
	11th to 20th	Night	11% 6% 35% 37% 10% 1	784
	21st to end	Night	11% 5% 38% 39% 7%	660
March	1st to 10th	Night	10% 7% 30% 38% 14% 2	800
	11th to 20th	Night	10% 6% 27% 35% 20% 2	800
	21st to end	Night	10% 9% 32% 35% 13% 1	880
April	1st to 10th	Night	10% 10% 32% 39% 9% 1	615
	11th to 20th	Night	10% 13% 27% 37% 12% 1	593
	21st to end	Night	7% 10% 33% 38% 12% 1	600
May	1st to 10th	Night	9% 9% 28% 38% 16% 1	600
	11th to 20th	Night	10% 4 24% 40% 19% 2	591
	21st to end	Night	10% 6% 25% 39% 18% 2	650
June	1st to 10th	Night	4 5% 17% 49% 24% 2	599
	11th to 20th	Night	5% 4 20% 46% 23% 2	570
	21st to end	Night	5% 2 16% 43% 32% 2	570
July	1st to 10th	Night	5% 1 10% 42% 39% 4	555
	11th to 20th	Night	7% 1 6% 37% 42% 7%	552
	21st to end	Night	7% 1 6% 35% 43% 8%	624
August	1st to 10th	Night	5% 3 4 32% 48% 9%	567
	11th to 20th	Night	6% 2 7% 41% 38% 5%	564
	21st to end	Night	6% 2 7% 38% 41% 6%	627
September	1st to 10th	Night	6% 2 7% 41% 38% 5%	570
	11th to 20th	Night	7% 2 10% 46% 30% 4	608
	21st to end	Night	8% 2 14% 42% 29% 5%	760
October	1st to 10th	Night	11% 4 19% 41% 23% 3	760
	11th to 20th	Night	6% 2 20% 46% 22% 4	760
	21st to end	Night	7% 4 21% 42% 22% 5%	836
November	1st to 10th	Night	7% 3 19% 46% 22% 3	780
	11th to 20th	Night	5% 5% 28% 41% 17% 5%	926
	21st to end	Night	11% 7% 21% 37% 20% 4	1000
December	1st to 10th	Night	10% 5% 26% 42% 14% 2	1000
	11th to 20th	Night	13% 6% 27% 36% 16% 1	1000
	21st to end	Night	13% 7% 28% 39% 12% 1	1070

Page generated at 2025-06-07 08:28:17. Version ac07

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