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 2024-11-02.

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	4 5% 2 5% 4 4 5% 4 6% 7% 54%	100994
	Night	5% 6% 2 5% 4 4 4 3 5% 6% 56%	98217
Transparency	Day	25% 6% 10% 20% 25% 14%	100759
	Night	27% 5% 9% 21% 24% 15%	98041
Seeing	Day	9% 12% 29% 31% 17% 3	15112
	Night	9% 5% 22% 40% 22% 3	25296

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 6% 3 6% 3 3 4 4 5% 6% 55%	2858
	11th to 20th	Day	3 5% 2 5% 4 5% 5% 4 6% 6% 55%	2860
	21st to end	Day	5% 10% 3 6% 3 4 4 3 4 6% 52%	3146
February	1st to 10th	Day	3 8% 3 6% 5% 4 4 3 4 5% 55%	1710
	11th to 20th	Day	5% 8% 3 6% 4 5% 4 3 5% 6% 50%	1703
	21st to end	Day	5% 11% 3 7% 5% 4 4 2 5% 5% 50%	1604
March	1st to 10th	Day	4 9% 3 6% 4 4 4 4 6% 8% 49%	3444
	11th to 20th	Day	7% 11% 3 7% 4 5% 5% 4 6% 7% 41%	3490
	21st to end	Day	5% 11% 4 8% 5% 5% 5% 4 5% 7% 41%	3825
April	1st to 10th	Day	4 5% 2 5% 5% 4 5% 3 5% 6% 55%	2264
	11th to 20th	Day	3 5% 2 5% 4 4 5% 4 6% 7% 54%	2251
	21st to end	Day	5% 4 2 5% 4 4 4 4 6% 7% 55%	2280
May	1st to 10th	Day	6% 4 1 4 3 4 4 3 4 6% 60%	4014
	11th to 20th	Day	3 3 1 5% 4 5% 6% 5% 8% 9% 51%	4071
	21st to end	Day	3 2 1 3 3 3 4 3 5% 7% 66%	4694
June	1st to 10th	Day	3 3 1 3 3 3 3 3 6% 8% 65%	2660
	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	8% 4 1 3 3 4 4 3 4 7% 58%	1900
	11th to 20th	Day	6% 2 1 3 3 3 3 2 3 5% 71%	1900
	21st to end	Day	4 4 1 4 3 2 3 3 3 6% 69%	2090
November	1st to 10th	Day	4 3 2 2 3 2 3 3 6% 70%	1869
	11th to 20th	Day	3 4 1 3 2 3 3 3 5% 5% 69%	1750
	21st to end	Day	6% 6% 2 3 4 3 3 3 4 4 63%	1520
December	1st to 10th	Day	7% 7% 2 4 4 2 3 3 4 7% 57%	1520
	11th to 20th	Day	7% 8% 2 6% 4 4 4 4 5% 7% 48%	1926
	21st to end	Day	5% 9% 3 5% 3 3 4 3 5% 5% 54%	2862
January	1st to 10th	Night	3 8% 3 9% 5% 4 4 3 4 5% 52%	4097
	11th to 20th	Night	3 9% 3 4 3 3 4 3 4 5% 58%	4110
	21st to end	Night	5% 9% 3 6% 4 3 3 3 5% 5% 54%	4521
February	1st to 10th	Night	4 8% 2 4 3 2 2 2 4 6% 61%	2470
	11th to 20th	Night	6% 7% 2 5% 3 3 3 2 4 6% 58%	2364
	21st to end	Night	4 11% 2 5% 3 3 3 3 3 5% 57%	1846
March	1st to 10th	Night	3 10% 2 5% 4 3 4 3 5% 7% 52%	3430
	11th to 20th	Night	9% 11% 3 7% 4 4 4 4 5% 6% 43%	3490
	21st to end	Night	5% 7% 3 7% 5% 5% 5% 4 5% 6% 47%	3828
April	1st to 10th	Night	6% 6% 2 4 3 5% 4 3 5% 7% 55%	1914
	11th to 20th	Night	6% 5% 2 6% 3 3 3 4 5% 6% 58%	1877
	21st to end	Night	5% 5% 2 4 3 3 3 3 6% 6% 60%	1900
May	1st to 10th	Night	5% 4 1 5% 2 2 3 3 5% 7% 64%	2940
	11th to 20th	Night	5% 5% 2 5% 3 3 4 3 6% 7% 56%	2834
	21st to end	Night	5% 3 1 4 3 2 3 2 3 5% 68%	2888
June	1st to 10th	Night	2 4 1 3 2 3 3 2 4 8% 67%	1520
	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	11% 4 1 3 2 3 3 3 5% 5% 61%	2280
	11th to 20th	Night	5% 2 1 3 2 3 3 3 3 4 71%	2280
	21st to end	Night	5% 3 1 3 2 2 2 2 4 6% 70%	2508
November	1st to 10th	Night	5% 3 1 2 2 2 3 2 5% 6% 69%	2244
	11th to 20th	Night	2 3 2 3 2 2 3 2 4 6% 72%	2403
	21st to end	Night	7% 6% 2 4 2 2 2 2 4 5% 64%	2660
December	1st to 10th	Night	6% 6% 2 5% 3 3 2 3 4 5% 61%	2660
	11th to 20th	Night	7% 9% 2 5% 4 4 4 4 5% 7% 50%	3271
	21st to end	Night	7% 9% 2 5% 3 3 3 2 3 4 58%	4832

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 16% 28% 18% 6%	2859
	11th to 20th	Day	27% 6% 14% 34% 16% 4	2860
	21st to end	Day	31% 6% 9% 31% 20% 3	3146
February	1st to 10th	Day	31% 4 12% 24% 20% 8%	1710
	11th to 20th	Day	34% 3 8% 24% 26% 4	1712
	21st to end	Day	35% 4 6% 18% 29% 8%	1604
March	1st to 10th	Day	26% 7% 8% 24% 29% 5%	3444
	11th to 20th	Day	36% 7% 9% 27% 17% 4	3490
	21st to end	Day	37% 8% 8% 19% 24% 4	3825
April	1st to 10th	Day	28% 5% 6% 20% 29% 13%	2264
	11th to 20th	Day	26% 5% 7% 20% 28% 15%	2254
	21st to end	Day	25% 5% 6% 22% 30% 12%	2280
May	1st to 10th	Day	22% 5% 6% 15% 31% 21%	4014
	11th to 20th	Day	23% 8% 9% 21% 28% 11%	4074
	21st to end	Day	15% 5% 6% 10% 43% 21%	4694
June	1st to 10th	Day	17% 4 6% 17% 31% 25%	2660
	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	25% 5% 8% 19% 23% 20%	1900
	11th to 20th	Day	17% 3 3 18% 38% 20%	1900
	21st to end	Day	19% 3 7% 24% 37% 11%	2090
November	1st to 10th	Day	17% 3 6% 27% 37% 10%	1869
	11th to 20th	Day	18% 2 7% 28% 38% 7%	1750
	21st to end	Day	27% 2 6% 26% 28% 11%	1520
December	1st to 10th	Day	28% 3 14% 27% 19% 8%	1520
	11th to 20th	Day	34% 5% 10% 28% 19% 3	1924
	21st to end	Day	31% 4 12% 16% 25% 11%	2862
January	1st to 10th	Night	34% 4 9% 28% 23% 3	4097
	11th to 20th	Night	28% 4 13% 31% 21% 4	4110
	21st to end	Night	32% 5% 8% 32% 18% 4	4521
February	1st to 10th	Night	25% 3 11% 27% 23% 10%	2470
	11th to 20th	Night	29% 3 8% 23% 28% 9%	2365
	21st to end	Night	30% 3 6% 19% 31% 11%	1846
March	1st to 10th	Night	28% 6% 6% 24% 29% 7%	3430
	11th to 20th	Night	38% 6% 8% 22% 19% 7%	3490
	21st to end	Night	33% 7% 8% 22% 24% 6%	3828
April	1st to 10th	Night	28% 4 5% 20% 28% 15%	1914
	11th to 20th	Night	27% 4 6% 17% 27% 18%	1877
	21st to end	Night	24% 3 6% 20% 32% 14%	1900
May	1st to 10th	Night	19% 4 6% 16% 39% 16%	2940
	11th to 20th	Night	24% 6% 9% 22% 26% 14%	2835
	21st to end	Night	18% 4 6% 10% 34% 28%	2888
June	1st to 10th	Night	18% 3 6% 15% 28% 30%	1520
	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	25% 5% 7% 15% 22% 26%	2280
	11th to 20th	Night	18% 3 4 14% 36% 25%	2280
	21st to end	Night	18% 2 4 22% 38% 16%	2508
November	1st to 10th	Night	17% 2 5% 25% 37% 14%	2244
	11th to 20th	Night	16% 2 5% 28% 38% 10%	2403
	21st to end	Night	25% 2 5% 23% 34% 12%	2660
December	1st to 10th	Night	26% 3 10% 28% 23% 8%	2660
	11th to 20th	Night	34% 4 9% 24% 24% 5%	3272
	21st to end	Night	32% 3 8% 19% 26% 13%	4832

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	16% 7% 37% 28% 11% 1	270
	11th to 20th	Day	17% 14% 35% 21% 12% 1	270
	21st to end	Day	8% 9% 33% 34% 14% 2	297
February	1st to 10th	Day	9% 16% 41% 27% 6%	270
	11th to 20th	Day	13% 15% 40% 24% 8%	270
	21st to end	Day	15% 19% 39% 22% 5%	259
March	1st to 10th	Day	12% 20% 37% 22% 9%	460
	11th to 20th	Day	12% 20% 32% 22% 11% 2	460
	21st to end	Day	17% 27% 34% 15% 6% 1	506
April	1st to 10th	Day	13% 23% 36% 17% 8% 2	460
	11th to 20th	Day	16% 27% 30% 19% 7%	452
	21st to end	Day	12% 23% 38% 20% 6%	460
May	1st to 10th	Day	11% 26% 36% 21% 6%	463
	11th to 20th	Day	9% 15% 35% 26% 13% 2	488
	21st to end	Day	8% 16% 32% 26% 15% 3	655
June	1st to 10th	Day	5% 6% 25% 41% 20% 3	680
	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% 31% 17% 2	330
November	1st to 10th	Day	9% 15% 28% 32% 14% 1	273
	11th to 20th	Day	6% 10% 38% 30% 14% 3	270
	21st to end	Day	12% 8% 33% 29% 15% 3	270
December	1st to 10th	Day	15% 9% 34% 28% 12% 3	270
	11th to 20th	Day	11% 9% 35% 30% 12% 3	270
	21st to end	Day	17% 13% 34% 28% 7%	297
January	1st to 10th	Night	8% 4 28% 39% 17% 4	760
	11th to 20th	Night	10% 4 28% 38% 18% 3	760
	21st to end	Night	10% 5% 25% 38% 18% 3	836
February	1st to 10th	Night	10% 5% 32% 39% 11% 2	760
	11th to 20th	Night	11% 5% 34% 38% 11% 1	744
	21st to end	Night	11% 5% 38% 38% 7%	628
March	1st to 10th	Night	9% 7% 30% 38% 14% 2	760
	11th to 20th	Night	10% 6% 26% 36% 21% 2	760
	21st to end	Night	10% 9% 32% 35% 12% 1	836
April	1st to 10th	Night	10% 10% 32% 38% 9% 1	584
	11th to 20th	Night	10% 13% 26% 37% 13% 1	563
	21st to end	Night	7% 10% 33% 37% 12% 1	570
May	1st to 10th	Night	8% 8% 28% 38% 16% 1	570
	11th to 20th	Night	10% 4 23% 41% 20% 2	561
	21st to end	Night	11% 6% 25% 39% 17% 2	617
June	1st to 10th	Night	4 4 17% 49% 24% 2	570
	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 18% 46% 23% 3	748
	11th to 20th	Night	5% 5% 26% 42% 18% 5%	879
	21st to end	Night	11% 7% 21% 36% 21% 5%	950
December	1st to 10th	Night	10% 6% 27% 42% 13% 2	950
	11th to 20th	Night	14% 6% 26% 36% 17% 1	950
	21st to end	Night	14% 6% 27% 40% 12% 1	1017

Page generated at 2024-10-30 11:19:23. Version ac07

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