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 2025-04-11.

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 4 3 5% 7% 63%	117373
	Night	2 4 2 5% 3 3 4 3 5% 7% 61%	119732
Transparency	Day	19% 6% 11% 19% 25% 20%	117117
	Night	21% 8% 12% 21% 23% 15%	119577
Seeing	Day	8% 18% 28% 27% 15% 4	15489
	Night	5% 11% 23% 35% 21% 4	24944

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	1 4 2 5% 5% 5% 6% 6% 7% 9% 50%	3639
	11th to 20th	Day	2 5% 2 6% 4 4 5% 3 5% 6% 58%	3704
	21st to end	Day	3 8% 4 8% 5% 5% 5% 4 6% 8% 44%	4510
February	1st to 10th	Day	4 5% 2 6% 5% 4 5% 5% 6% 10% 48%	1800
	11th to 20th	Day	2 6% 3 6% 5% 4 5% 4 6% 10% 49%	1936
	21st to end	Day	1 5% 2 4 3 4 5% 5% 7% 10% 54%	1650
March	1st to 10th	Day	2 10% 4 8% 5% 6% 5% 5% 7% 10% 37%	4518
	11th to 20th	Day	1 5% 1 5% 3 3 4 4 6% 9% 58%	4912
	21st to end	Day	2 8% 3 7% 4 4 4 4 6% 8% 50%	5832
April	1st to 10th	Day	2 5% 2 6% 5% 4 4 4 7% 8% 52%	2400
	11th to 20th	Day	2 5% 2 4 4 4 5% 3 6% 9% 57%	2251
	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	2 2 1 3 2 2 2 3 4 7% 71%	3219
	11th to 20th	Day	3 3 1 3 2 2 2 2 3 5% 74%	3161
	21st to end	Day	1 2 1 3 3 3 3 3 6% 9% 66%	3339
November	1st to 10th	Day	3 5% 1 3 4 4 3 4 6% 9% 59%	1764
	11th to 20th	Day	2 4 2 3 3 4 5% 5% 7% 10% 55%	1800
	21st to end	Day	2 4 2 4 4 4 5% 5% 6% 9% 55%	1710
December	1st to 10th	Day	2 3 1 4 3 3 3 4 6% 9% 62%	2620
	11th to 20th	Day	5% 10% 3 8% 4 5% 6% 5% 9% 11% 34%	3028
	21st to end	Day	2 7% 3 8% 4 6% 6% 6% 9% 14% 36%	4004
January	1st to 10th	Night	1 4 2 5% 4 4 5% 5% 9% 12% 48%	6306
	11th to 20th	Night	1 5% 2 8% 4 4 4 4 6% 7% 54%	6258
	21st to end	Night	4 9% 3 7% 4 4 5% 4 5% 7% 48%	6468
February	1st to 10th	Night	2 5% 3 6% 4 5% 4 4 7% 9% 53%	2600
	11th to 20th	Night	1 6% 2 6% 4 4 5% 4 8% 11% 50%	2349
	21st to end	Night	2 5% 2 5% 4 4 3 4 6% 9% 56%	1980
March	1st to 10th	Night	2 8% 4 8% 5% 5% 5% 4 6% 9% 44%	5376
	11th to 20th	Night	1 5% 2 5% 3 4 4 4 7% 8% 59%	5071
	21st to end	Night	3 7% 2 7% 5% 5% 5% 5% 7% 9% 45%	5141
April	1st to 10th	Night	2 4 2 4 4 4 4 4 8% 10% 54%	2000
	11th to 20th	Night	3 4 2 4 3 3 4 4 5% 7% 61%	1883
	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	2 2 1 2 2 2 2 2 3 5% 77%	3759
	11th to 20th	Night	2 2 1 2 1 2 2 2 3 5% 79%	3777
	21st to end	Night	3 3 1 4 2 2 3 3 5% 8% 66%	4224
November	1st to 10th	Night	3 4 1 5% 4 3 3 4 6% 8% 60%	2535
	11th to 20th	Night	2 3 2 4 3 3 4 4 7% 9% 60%	2600
	21st to end	Night	2 4 2 5% 4 3 4 4 6% 8% 59%	2685
December	1st to 10th	Night	2 3 2 4 2 3 3 3 5% 7% 65%	4560
	11th to 20th	Night	5% 6% 2 7% 5% 5% 6% 5% 8% 14% 37%	5176
	21st to end	Night	3 7% 3 8% 6% 6% 6% 5% 8% 11% 36%	6952

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	25% 13% 24% 25% 13%	3639
	11th to 20th	Day	25% 8% 13% 23% 25% 4	3704
	21st to end	Day	35% 10% 11% 16% 19% 8%	4510
February	1st to 10th	Day	30% 8% 16% 24% 15% 6%	1800
	11th to 20th	Day	28% 7% 17% 28% 17% 3	1946
	21st to end	Day	23% 8% 20% 27% 16% 5%	1650
March	1st to 10th	Day	35% 11% 15% 19% 16% 4	4518
	11th to 20th	Day	20% 7% 15% 32% 24% 3	4912
	21st to end	Day	27% 10% 18% 23% 19% 3	5832
April	1st to 10th	Day	27% 8% 17% 26% 17% 5%	2400
	11th to 20th	Day	23% 9% 14% 28% 21% 6%	2254
	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	13% 3 6% 13% 35% 29%	3219
	11th to 20th	Day	14% 3 7% 19% 44% 12%	3161
	21st to end	Day	14% 4 9% 30% 31% 12%	3339
November	1st to 10th	Day	21% 8% 14% 25% 24% 8%	1764
	11th to 20th	Day	19% 10% 18% 27% 23% 3	1800
	21st to end	Day	25% 9% 11% 26% 25% 5%	1710
December	1st to 10th	Day	19% 7% 10% 30% 27% 6%	2620
	11th to 20th	Day	37% 13% 13% 23% 11% 2	3028
	21st to end	Day	31% 13% 24% 22% 8% 1	4004
January	1st to 10th	Night	23% 13% 24% 25% 15% 1	6306
	11th to 20th	Night	26% 10% 15% 22% 22% 5%	6258
	21st to end	Night	34% 10% 10% 14% 27% 5%	6468
February	1st to 10th	Night	26% 10% 15% 26% 17% 6%	2600
	11th to 20th	Night	25% 9% 16% 29% 20% 2	2350
	21st to end	Night	23% 10% 16% 30% 14% 6%	1980
March	1st to 10th	Night	32% 12% 13% 22% 16% 5%	5376
	11th to 20th	Night	20% 8% 13% 41% 16% 2	5071
	21st to end	Night	29% 14% 21% 23% 11% 2	5141
April	1st to 10th	Night	22% 11% 17% 30% 17% 4	2000
	11th to 20th	Night	21% 9% 17% 30% 18% 6%	1883
	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	11% 3 6% 12% 34% 35%	3759
	11th to 20th	Night	10% 3 7% 21% 46% 13%	3777
	21st to end	Night	17% 4 8% 28% 33% 12%	4224
November	1st to 10th	Night	21% 8% 14% 26% 21% 10%	2535
	11th to 20th	Night	19% 8% 16% 28% 25% 4	2600
	21st to end	Night	23% 7% 14% 26% 23% 7%	2685
December	1st to 10th	Night	20% 7% 9% 26% 27% 11%	4560
	11th to 20th	Night	32% 12% 14% 24% 16% 1	5176
	21st to end	Night	37% 13% 18% 21% 10% 1	6952

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% 25% 27% 24% 10% 2	200
	11th to 20th	Day	8% 26% 25% 24% 14% 3	225
	21st to end	Day	7% 15% 34% 27% 13% 3	440
February	1st to 10th	Day	12% 25% 27% 23% 12% 1	400
	11th to 20th	Day	10% 26% 28% 26% 9% 1	489
	21st to end	Day	11% 21% 31% 30% 7%	414
March	1st to 10th	Day	13% 29% 30% 19% 7% 1	500
	11th to 20th	Day	15% 27% 27% 18% 11% 2	500
	21st to end	Day	14% 28% 28% 23% 7%	550
April	1st to 10th	Day	12% 26% 28% 24% 8% 3	500
	11th to 20th	Day	13% 28% 27% 20% 10% 2	453
	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	479
November	1st to 10th	Day	11% 15% 21% 28% 20% 6%	300
	11th to 20th	Day	6% 16% 24% 31% 16% 7%	300
	21st to end	Day	10% 18% 24% 24% 19% 5%	255
December	1st to 10th	Day	10% 18% 22% 25% 17% 9%	200
	11th to 20th	Day	9% 18% 33% 19% 16% 6%	200
	21st to end	Day	18% 18% 23% 29% 11% 1	220
January	1st to 10th	Night	5% 16% 28% 28% 17% 6%	800
	11th to 20th	Night	5% 16% 23% 36% 18% 3	800
	21st to end	Night	6% 9% 28% 35% 18% 4	880
February	1st to 10th	Night	7% 13% 26% 34% 17% 3	800
	11th to 20th	Night	5% 15% 27% 33% 16% 4	784
	21st to end	Night	6% 18% 22% 39% 12% 2	660
March	1st to 10th	Night	6% 19% 30% 27% 15% 3	800
	11th to 20th	Night	6% 19% 24% 28% 19% 3	800
	21st to end	Night	6% 22% 28% 29% 14% 1	880
April	1st to 10th	Night	5% 17% 27% 33% 17% 1	800
	11th to 20th	Night	7% 18% 25% 30% 16% 4	753
	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	6% 9% 18% 32% 24% 11%	836
November	1st to 10th	Night	5% 13% 21% 29% 25% 7%	780
	11th to 20th	Night	5% 11% 25% 35% 18% 5%	800
	21st to end	Night	5% 13% 24% 31% 24% 4	800
December	1st to 10th	Night	6% 13% 23% 36% 17% 5%	800
	11th to 20th	Night	7% 15% 24% 34% 15% 5%	800
	21st to end	Night	9% 17% 29% 31% 12% 1	880

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