Deer Lodge Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Deer Lodge Site clear sky chart from 2005-11-06 to 2024-10-24.

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	2 4 2 4 3 3 3 3 5% 7% 63%	101148
	Night	3 4 2 5% 3 3 4 3 5% 7% 60%	97780
Transparency	Day	19% 7% 11% 19% 23% 22%	100892
	Night	21% 8% 13% 21% 21% 16%	97625
Seeing	Day	8% 18% 28% 27% 15% 4	14973
	Night	5% 11% 23% 36% 22% 4	24266

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 5% 3 6% 6% 6% 6% 6% 8% 9% 43%	2539
	11th to 20th	Day	3 8% 3 8% 5% 5% 7% 5% 7% 8% 40%	2576
	21st to end	Day	5% 10% 4 9% 7% 6% 6% 5% 7% 9% 31%	3146
February	1st to 10th	Day	3 5% 2 6% 5% 4 5% 5% 6% 10% 48%	1710
	11th to 20th	Day	2 6% 3 6% 5% 4 5% 4 6% 10% 49%	1836
	21st to end	Day	1 6% 2 4 4 4 5% 5% 7% 10% 52%	1570
March	1st to 10th	Day	2 9% 5% 9% 6% 6% 6% 6% 7% 10% 33%	3138
	11th to 20th	Day	2 4 1 4 3 3 3 3 5% 8% 64%	3428
	21st to end	Day	2 8% 2 7% 4 4 4 4 7% 9% 48%	4067
April	1st to 10th	Day	3 5% 2 6% 5% 4 4 4 7% 8% 52%	2280
	11th to 20th	Day	2 5% 2 4 4 4 5% 3 6% 9% 57%	2249
	21st to end	Day	2 5% 2 5% 3 3 4 3 5% 7% 61%	2280
May	1st to 10th	Day	2 3 1 4 3 3 3 3 4 6% 66%	4009
	11th to 20th	Day	2 2 1 3 3 2 3 2 3 5% 72%	4343
	21st to end	Day	2 3 2 5% 3 3 4 3 5% 6% 64%	4813
June	1st to 10th	Day	3 3 1 4 4 4 3 4 5% 9% 59%	2660
	11th to 20th	Day	1 2 1 2 2 3 3 2 4 5% 75%	2660
	21st to end	Day	2 2 1 2 2 3 2 2 4 4 75%	2660
July	1st to 10th	Day	1 1 1 1 1 2 2 3 87%	4420
	11th to 20th	Day	2 2 1 3 3 2 3 3 3 5% 73%	4420
	21st to end	Day	1 1 1 2 2 3 3 3 4 4 77%	4715
August	1st to 10th	Day	2 2 1 3 2 3 2 2 4 5% 74%	4070
	11th to 20th	Day	1 1 1 1 2 2 1 2 3 85%	3724
	21st to end	Day	1 1 1 1 1 2 2 2 3 85%	4180
September	1st to 10th	Day	1 1 1 2 1 1 2 2 3 4 83%	2280
	11th to 20th	Day	2 2 1 3 2 2 2 2 4 5% 74%	2252
	21st to end	Day	3 2 1 2 1 2 2 2 3 5% 76%	2090
October	1st to 10th	Day	4 3 2 4 2 3 2 2 4 6% 68%	1927
	11th to 20th	Day	2 2 1 3 2 2 2 3 4 6% 72%	1900
	21st to end	Day	2 3 1 4 3 3 3 2 6% 8% 66%	1955
November	1st to 10th	Day	3 5% 2 3 4 4 4 4 6% 8% 58%	1674
	11th to 20th	Day	3 4 2 3 3 3 5% 5% 7% 10% 55%	1710
	21st to end	Day	2 4 1 3 4 4 5% 5% 6% 10% 55%	1625
December	1st to 10th	Day	4 5% 2 6% 4 4 4 4 7% 9% 50%	1520
	11th to 20th	Day	6% 11% 3 10% 5% 5% 5% 4 8% 11% 33%	1928
	21st to end	Day	2 7% 3 9% 4 6% 6% 5% 8% 13% 37%	2794
January	1st to 10th	Night	2 5% 2 6% 6% 5% 6% 5% 8% 11% 44%	4406
	11th to 20th	Night	2 7% 3 10% 6% 5% 6% 6% 8% 9% 38%	4384
	21st to end	Night	5% 10% 3 9% 6% 5% 6% 5% 7% 8% 35%	4521
February	1st to 10th	Night	2 5% 3 5% 4 5% 4 4 7% 9% 53%	2470
	11th to 20th	Night	1 6% 2 5% 4 4 5% 4 8% 11% 51%	2229
	21st to end	Night	2 5% 2 6% 4 4 3 4 6% 9% 55%	1884
March	1st to 10th	Night	1 8% 4 8% 5% 5% 6% 5% 7% 10% 40%	3736
	11th to 20th	Night	1 5% 2 4 3 3 4 4 6% 7% 62%	3535
	21st to end	Night	3 8% 3 8% 5% 5% 5% 5% 8% 10% 41%	3584
April	1st to 10th	Night	2 5% 2 4 4 4 5% 4 8% 10% 54%	1900
	11th to 20th	Night	3 4 2 4 3 3 4 4 5% 7% 61%	1879
	21st to end	Night	4 4 1 4 3 3 3 3 6% 7% 63%	1900
May	1st to 10th	Night	3 3 1 3 3 3 3 2 4 6% 70%	2961
	11th to 20th	Night	2 3 1 2 2 3 2 2 5% 5% 72%	2534
	21st to end	Night	3 3 1 4 2 3 3 3 5% 6% 68%	2762
June	1st to 10th	Night	4 4 1 4 3 3 3 3 6% 7% 63%	1520
	11th to 20th	Night	1 3 1 2 2 2 2 2 3 5% 78%	1520
	21st to end	Night	2 2 1 2 2 3 3 3 3 3 77%	1520
July	1st to 10th	Night	1 1 1 1 1 2 3 89%	2540
	11th to 20th	Night	3 3 1 3 3 2 3 2 2 3 76%	2540
	21st to end	Night	3 2 1 2 1 1 2 1 4 4 79%	2823
August	1st to 10th	Night	3 2 1 3 2 2 3 2 3 5% 73%	2870
	11th to 20th	Night	1 1 1 1 1 2 2 2 3 85%	3126
	21st to end	Night	3 2 1 2 1 1 1 1 1 2 86%	3498
September	1st to 10th	Night	2 1 1 2 1 1 2 1 3 4 84%	1900
	11th to 20th	Night	2 3 1 2 2 2 2 2 3 5% 76%	1928
	21st to end	Night	1 2 1 3 2 2 2 2 3 5% 77%	2090
October	1st to 10th	Night	3 3 1 3 3 2 3 3 3 5% 71%	2251
	11th to 20th	Night	2 2 1 2 1 2 2 2 4 5% 76%	2280
	21st to end	Night	2 3 1 3 2 2 3 3 4 8% 70%	2466
November	1st to 10th	Night	3 4 1 5% 4 3 3 4 6% 8% 60%	2405
	11th to 20th	Night	2 3 2 4 3 3 4 4 7% 9% 60%	2470
	21st to end	Night	2 4 2 5% 4 3 4 4 6% 8% 58%	2550
December	1st to 10th	Night	4 5% 3 6% 3 3 4 4 7% 10% 52%	2660
	11th to 20th	Night	7% 8% 3 8% 5% 4 5% 5% 7% 13% 36%	3276
	21st to end	Night	3 7% 3 8% 6% 6% 7% 6% 8% 10% 37%	4862

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% 14% 21% 25% 8% 1	2539
	11th to 20th	Day	36% 12% 18% 22% 9% 3	2576
	21st to end	Day	43% 13% 14% 20% 9% 1	3146
February	1st to 10th	Day	29% 9% 17% 25% 15% 5%	1710
	11th to 20th	Day	28% 7% 18% 28% 17% 4	1846
	21st to end	Day	23% 9% 21% 27% 16% 3	1570
March	1st to 10th	Day	35% 15% 20% 18% 11% 1	3138
	11th to 20th	Day	19% 8% 15% 33% 23% 2	3428
	21st to end	Day	28% 12% 20% 23% 15% 2	4067
April	1st to 10th	Day	28% 8% 17% 26% 17% 5%	2280
	11th to 20th	Day	23% 10% 14% 28% 21% 6%	2252
	21st to end	Day	21% 7% 15% 29% 24% 3	2280
May	1st to 10th	Day	17% 7% 10% 27% 33% 6%	4009
	11th to 20th	Day	14% 5% 8% 20% 38% 15%	4346
	21st to end	Day	19% 7% 10% 27% 34% 4	4814
June	1st to 10th	Day	20% 6% 11% 22% 30% 11%	2660
	11th to 20th	Day	12% 4 7% 20% 35% 23%	2660
	21st to end	Day	12% 3 4 13% 30% 38%	2660
July	1st to 10th	Day	5% 2 3 5% 14% 72%	4351
	11th to 20th	Day	13% 2 4 6% 17% 58%	4345
	21st to end	Day	9% 3 4 7% 25% 51%	4566
August	1st to 10th	Day	14% 3 4 7% 20% 53%	4055
	11th to 20th	Day	7% 2 3 5% 25% 59%	3760
	21st to end	Day	8% 1 3 8% 37% 44%	4179
September	1st to 10th	Day	7% 2 3 10% 28% 50%	2280
	11th to 20th	Day	13% 4 7% 16% 33% 26%	2252
	21st to end	Day	12% 3 6% 17% 34% 27%	2090
October	1st to 10th	Day	17% 4 8% 19% 31% 21%	1927
	11th to 20th	Day	13% 4 10% 24% 32% 17%	1900
	21st to end	Day	15% 5% 11% 28% 27% 13%	1955
November	1st to 10th	Day	22% 8% 14% 24% 24% 8%	1674
	11th to 20th	Day	20% 10% 18% 27% 23% 3	1710
	21st to end	Day	24% 9% 11% 28% 25% 4	1625
December	1st to 10th	Day	28% 10% 15% 27% 16% 4	1520
	11th to 20th	Day	40% 14% 13% 22% 10% 2	1928
	21st to end	Day	33% 13% 22% 22% 10% 1	2794
January	1st to 10th	Night	28% 13% 24% 25% 9% 1	4406
	11th to 20th	Night	36% 15% 20% 18% 8% 4	4384
	21st to end	Night	41% 13% 14% 19% 11% 1	4521
February	1st to 10th	Night	26% 10% 15% 27% 17% 5%	2470
	11th to 20th	Night	24% 9% 16% 30% 19% 2	2230
	21st to end	Night	24% 10% 17% 30% 15% 4	1884
March	1st to 10th	Night	31% 16% 17% 20% 13% 2	3736
	11th to 20th	Night	18% 10% 14% 39% 17% 2	3535
	21st to end	Night	30% 15% 24% 20% 10% 2	3584
April	1st to 10th	Night	23% 11% 17% 30% 15% 4	1900
	11th to 20th	Night	21% 9% 17% 30% 18% 6%	1879
	21st to end	Night	20% 9% 15% 31% 21% 4	1900
May	1st to 10th	Night	14% 6% 14% 30% 30% 6%	2961
	11th to 20th	Night	12% 6% 10% 20% 38% 14%	2534
	21st to end	Night	15% 8% 11% 33% 29% 4	2762
June	1st to 10th	Night	21% 7% 11% 24% 27% 11%	1520
	11th to 20th	Night	10% 3 9% 21% 34% 23%	1520
	21st to end	Night	11% 4 5% 14% 30% 36%	1520
July	1st to 10th	Night	4 1 3 5% 16% 71%	2500
	11th to 20th	Night	14% 3 3 6% 16% 59%	2492
	21st to end	Night	10% 3 4 5% 28% 50%	2734
August	1st to 10th	Night	13% 3 5% 7% 23% 49%	2861
	11th to 20th	Night	6% 2 3 6% 25% 58%	3156
	21st to end	Night	8% 1 2 10% 35% 44%	3498
September	1st to 10th	Night	7% 3 4 7% 31% 49%	1900
	11th to 20th	Night	12% 4 7% 17% 36% 25%	1928
	21st to end	Night	12% 3 6% 18% 33% 27%	2090
October	1st to 10th	Night	17% 4 8% 19% 28% 23%	2251
	11th to 20th	Night	12% 4 9% 23% 34% 18%	2280
	21st to end	Night	14% 4 10% 27% 31% 13%	2466
November	1st to 10th	Night	21% 8% 14% 25% 21% 10%	2405
	11th to 20th	Night	19% 8% 16% 28% 25% 4	2470
	21st to end	Night	23% 7% 14% 26% 23% 6%	2550
December	1st to 10th	Night	28% 10% 14% 25% 20% 4	2660
	11th to 20th	Night	36% 14% 13% 21% 13% 2	3276
	21st to end	Night	35% 14% 19% 19% 12% 2	4862

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	13% 27% 27% 23% 9% 2	180
	11th to 20th	Day	8% 27% 24% 24% 15% 2	203
	21st to end	Day	7% 15% 35% 26% 13% 3	407
February	1st to 10th	Day	8% 26% 29% 23% 12% 1	370
	11th to 20th	Day	8% 27% 27% 27% 10% 1	449
	21st to end	Day	11% 22% 30% 30% 7% 1	382
March	1st to 10th	Day	13% 30% 30% 19% 7% 1	460
	11th to 20th	Day	13% 28% 27% 19% 12% 2	460
	21st to end	Day	12% 29% 29% 23% 6%	506
April	1st to 10th	Day	12% 27% 28% 22% 8% 3	460
	11th to 20th	Day	13% 28% 27% 20% 10% 2	452
	21st to end	Day	14% 23% 25% 25% 11% 3	460
May	1st to 10th	Day	13% 24% 30% 23% 10% 2	463
	11th to 20th	Day	13% 22% 29% 20% 11% 6%	488
	21st to end	Day	11% 18% 25% 24% 16% 7%	529
June	1st to 10th	Day	7% 18% 29% 22% 18% 6%	490
	11th to 20th	Day	9% 19% 33% 25% 12% 3	490
	21st to end	Day	3 12% 32% 30% 17% 6%	490
July	1st to 10th	Day	1 9% 36% 30% 17% 6%	486
	11th to 20th	Day	2 8% 33% 32% 20% 5%	484
	21st to end	Day	2 7% 29% 34% 21% 7%	531
August	1st to 10th	Day	3 10% 31% 28% 21% 7%	487
	11th to 20th	Day	3 12% 28% 30% 18% 8%	477
	21st to end	Day	3 9% 32% 33% 19% 4	539
September	1st to 10th	Day	2 9% 32% 30% 21% 5%	490
	11th to 20th	Day	7% 13% 29% 30% 20% 2	490
	21st to end	Day	4 14% 28% 36% 15% 2	490
October	1st to 10th	Day	6% 17% 24% 29% 20% 4	490
	11th to 20th	Day	8% 16% 20% 30% 20% 5%	490
	21st to end	Day	8% 17% 24% 30% 18% 3	452
November	1st to 10th	Day	11% 14% 21% 27% 20% 7%	270
	11th to 20th	Day	4 16% 23% 32% 17% 8%	270
	21st to end	Day	6% 19% 25% 25% 20% 5%	230
December	1st to 10th	Day	11% 19% 23% 24% 16% 6%	180
	11th to 20th	Day	7% 18% 35% 17% 16% 7%	180
	21st to end	Day	14% 19% 23% 30% 12% 2	198
January	1st to 10th	Night	5% 16% 27% 28% 18% 7%	760
	11th to 20th	Night	5% 16% 22% 36% 19% 3	760
	21st to end	Night	6% 9% 28% 34% 19% 5%	836
February	1st to 10th	Night	5% 13% 26% 34% 18% 3	760
	11th to 20th	Night	4 14% 26% 34% 17% 4	744
	21st to end	Night	6% 19% 21% 38% 13% 2	628
March	1st to 10th	Night	6% 19% 29% 27% 15% 3	760
	11th to 20th	Night	5% 18% 24% 29% 20% 3	760
	21st to end	Night	5% 23% 28% 30% 14% 1	836
April	1st to 10th	Night	5% 17% 26% 33% 18% 1	760
	11th to 20th	Night	7% 18% 25% 30% 16% 4	751
	21st to end	Night	6% 15% 25% 32% 19% 3	760
May	1st to 10th	Night	8% 14% 26% 33% 16% 2	632
	11th to 20th	Night	3 9% 24% 41% 22% 2	392
	21st to end	Night	6% 7% 20% 38% 26% 3	411
June	1st to 10th	Night	5% 6% 22% 41% 24% 3	380
	11th to 20th	Night	2 7% 28% 37% 26% 1	380
	21st to end	Night	2 2 24% 44% 26% 2	380
July	1st to 10th	Night	1 2 20% 47% 27% 2	370
	11th to 20th	Night	1 2 20% 51% 26%	368
	21st to end	Night	2 1 12% 50% 31% 4	454
August	1st to 10th	Night	4 14% 46% 32% 3	576
	11th to 20th	Night	2 3 22% 40% 29% 4	752
	21st to end	Night	3 3 21% 44% 26% 3	836
September	1st to 10th	Night	1 4 19% 41% 31% 4	760
	11th to 20th	Night	4 6% 18% 34% 33% 6%	760
	21st to end	Night	2 7% 20% 37% 26% 9%	760
October	1st to 10th	Night	5% 7% 18% 40% 24% 6%	760
	11th to 20th	Night	4 8% 16% 38% 28% 5%	760
	21st to end	Night	5% 9% 18% 32% 25% 11%	804
November	1st to 10th	Night	5% 13% 20% 29% 26% 7%	740
	11th to 20th	Night	5% 11% 25% 36% 19% 5%	760
	21st to end	Night	4 13% 24% 31% 24% 4	760
December	1st to 10th	Night	7% 13% 23% 36% 16% 5%	760
	11th to 20th	Night	7% 15% 24% 33% 16% 5%	760
	21st to end	Night	8% 17% 28% 33% 13% 2	836

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