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 2026-08-15.

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% 65%	152468
	Night	2 4 2 4 3 3 4 3 5% 7% 62%	146807
Transparency	Day	18% 6% 10% 18% 26% 22%	147430
	Night	20% 7% 11% 21% 25% 17%	142829
Seeing	Day	8% 17% 28% 27% 15% 4	17293
	Night	5% 11% 24% 36% 20% 4	26551

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 6% 5% 5% 6% 5% 7% 9% 48%	4722
	11th to 20th	Day	2 4 2 5% 3 3 4 3 5% 6% 62%	4818
	21st to end	Day	3 7% 3 7% 5% 5% 5% 5% 7% 10% 43%	5843
February	1st to 10th	Day	3 5% 2 6% 5% 4 5% 5% 6% 10% 49%	1890
	11th to 20th	Day	2 7% 3 6% 5% 4 5% 4 6% 10% 48%	2036
	21st to end	Day	1 5% 2 4 3 4 5% 5% 7% 10% 53%	1730
March	1st to 10th	Day	2 10% 4 8% 5% 6% 5% 5% 7% 10% 38%	4618
	11th to 20th	Day	1 5% 1 4 3 3 4 4 6% 9% 59%	5020
	21st to end	Day	2 8% 3 7% 4 4 4 4 6% 8% 50%	5961
April	1st to 10th	Day	2 4 2 6% 5% 4 4 4 7% 8% 52%	2520
	11th to 20th	Day	2 5% 2 4 4 4 5% 3 6% 9% 57%	2489
	21st to end	Day	3 6% 2 5% 3 4 4 3 5% 7% 60%	2520
May	1st to 10th	Day	4 4 2 4 3 2 3 3 4 5% 67%	7447
	11th to 20th	Day	1 2 1 3 3 2 3 2 3 5% 74%	8101
	21st to end	Day	2 3 1 4 3 3 3 2 5% 6% 67%	8992
June	1st to 10th	Day	2 3 1 4 4 4 3 4 5% 9% 61%	2940
	11th to 20th	Day	1 2 1 2 2 3 2 2 4 6% 76%	2940
	21st to end	Day	1 2 1 2 2 2 2 2 4 4 77%	2940
July	1st to 10th	Day	1 2 1 2 2 2 2 3 85%	8219
	11th to 20th	Day	2 2 1 3 3 2 3 2 3 5% 74%	8220
	21st to end	Day	1 1 1 2 2 3 3 3 4 5% 74%	8795
August	1st to 10th	Day	1 1 1 2 1 2 2 2 3 4 80%	5874
	11th to 20th	Day	1 1 1 1 2 1 1 2 3 86%	5413
	21st to end	Day	2 2 1 3 3 3 3 2 3 3 75%	5984
September	1st to 10th	Day	1 1 1 2 2 2 2 2 3 4 82%	2400
	11th to 20th	Day	2 2 1 3 2 2 2 2 4 5% 74%	2370
	21st to end	Day	3 2 1 3 1 2 2 2 3 5% 75%	2200
October	1st to 10th	Day	3 2 1 3 2 2 2 3 4 7% 71%	3320
	11th to 20th	Day	3 3 1 3 2 2 2 2 3 5% 73%	3261
	21st to end	Day	1 2 1 3 3 3 3 3 6% 9% 66%	3446
November	1st to 10th	Day	3 4 1 3 3 4 3 4 6% 8% 59%	1854
	11th to 20th	Day	3 4 2 4 3 4 5% 5% 7% 10% 53%	1890
	21st to end	Day	2 4 2 4 4 4 5% 5% 5% 9% 56%	1795
December	1st to 10th	Day	2 3 1 4 3 3 3 4 6% 9% 62%	2700
	11th to 20th	Day	5% 9% 3 8% 4 5% 6% 5% 9% 11% 34%	3108
	21st to end	Day	2 7% 3 8% 4 5% 6% 6% 8% 13% 36%	4092
January	1st to 10th	Night	2 5% 2 5% 4 4 5% 5% 8% 11% 47%	8172
	11th to 20th	Night	1 4 2 6% 3 3 4 4 5% 7% 61%	8132
	21st to end	Night	3 8% 3 7% 5% 5% 6% 4 6% 7% 47%	8371
February	1st to 10th	Night	2 5% 3 5% 4 5% 4 4 6% 9% 53%	2730
	11th to 20th	Night	1 6% 2 6% 4 4 5% 4 8% 11% 49%	2469
	21st to end	Night	2 4 2 5% 4 4 3 4 6% 9% 56%	2076
March	1st to 10th	Night	2 8% 4 8% 5% 5% 5% 4 6% 9% 44%	5496
	11th to 20th	Night	1 5% 2 4 3 4 4 4 7% 8% 59%	5183
	21st to end	Night	3 7% 2 7% 5% 5% 5% 5% 7% 9% 45%	5254
April	1st to 10th	Night	2 4 2 4 4 4 4 4 8% 10% 54%	2100
	11th to 20th	Night	3 4 2 4 4 4 4 4 5% 7% 60%	2079
	21st to end	Night	4 4 1 4 3 4 3 3 6% 7% 61%	2100
May	1st to 10th	Night	4 3 1 3 3 3 2 2 4 5% 69%	5506
	11th to 20th	Night	1 2 1 3 2 3 3 2 4 5% 74%	4745
	21st to end	Night	2 2 1 3 2 3 3 3 5% 6% 71%	5181
June	1st to 10th	Night	4 4 1 4 3 3 3 2 5% 7% 64%	1680
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 5% 79%	1680
	21st to end	Night	2 2 1 2 2 3 3 3 3 3 78%	1680
July	1st to 10th	Night	1 1 1 1 1 1 2 3 88%	4740
	11th to 20th	Night	3 3 1 3 2 3 3 2 3 3 75%	4740
	21st to end	Night	4 2 1 2 2 2 3 2 4 5% 73%	5271
August	1st to 10th	Night	2 2 1 2 1 2 2 2 3 4 80%	4160
	11th to 20th	Night	1 1 1 1 1 2 2 3 3 85%	4546
	21st to end	Night	4 3 1 3 2 2 2 2 2 3 75%	5015
September	1st to 10th	Night	2 1 1 2 1 1 2 1 3 4 82%	2000
	11th to 20th	Night	3 3 1 2 2 2 2 2 3 5% 75%	2030
	21st to end	Night	1 2 1 3 2 2 2 2 3 5% 77%	2200
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 78%	3878
	11th to 20th	Night	2 2 1 2 1 2 2 2 3 5% 79%	3897
	21st to end	Night	3 3 1 4 2 2 3 3 5% 8% 66%	4359
November	1st to 10th	Night	3 4 1 5% 4 3 3 4 6% 8% 61%	2665
	11th to 20th	Night	2 3 2 4 3 3 4 4 7% 9% 59%	2730
	21st to end	Night	2 4 2 5% 4 3 4 4 6% 8% 59%	2820
December	1st to 10th	Night	2 3 2 4 2 3 3 3 5% 7% 66%	4700
	11th to 20th	Night	5% 6% 2 7% 5% 4 6% 5% 8% 14% 38%	5316
	21st to end	Night	3 7% 3 8% 6% 6% 6% 5% 8% 11% 36%	7106

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	26% 12% 22% 28% 11%	4723
	11th to 20th	Day	21% 7% 11% 20% 30% 12%	4818
	21st to end	Day	32% 9% 10% 18% 23% 8%	5843
February	1st to 10th	Day	29% 8% 16% 25% 16% 6%	1890
	11th to 20th	Day	28% 7% 17% 28% 17% 3	2046
	21st to end	Day	22% 9% 21% 27% 16% 5%	1730
March	1st to 10th	Day	34% 11% 15% 19% 16% 4	4618
	11th to 20th	Day	19% 7% 15% 32% 24% 3	5020
	21st to end	Day	27% 10% 17% 23% 20% 3	5961
April	1st to 10th	Day	27% 8% 17% 25% 19% 5%	2520
	11th to 20th	Day	23% 9% 13% 27% 21% 7%	2492
	21st to end	Day	22% 7% 14% 29% 24% 3	2520
May	1st to 10th	Day	18% 5% 8% 20% 40% 9%	7447
	11th to 20th	Day	12% 4 8% 20% 41% 16%	8104
	21st to end	Day	16% 4 8% 22% 41% 9%	8994
June	1st to 10th	Day	18% 6% 11% 21% 32% 12%	2940
	11th to 20th	Day	11% 4 6% 18% 34% 27%	2940
	21st to end	Day	11% 3 4 13% 32% 38%	2940
July	1st to 10th	Day	7% 1 2 5% 19% 65%	8151
	11th to 20th	Day	13% 2 4 6% 14% 61%	8145
	21st to end	Day	11% 3 4 11% 25% 47%	8598
August	1st to 10th	Day	13% 2 4 7% 21% 53%	4314
	11th to 20th	Day	7% 2 3 5% 25% 58%	3929
	21st to end	Day	8% 1 3 8% 37% 43%	4311
September	1st to 10th	Day	8% 2 3 11% 29% 48%	2400
	11th to 20th	Day	14% 4 6% 17% 34% 26%	2370
	21st to end	Day	13% 3 6% 17% 35% 26%	2200
October	1st to 10th	Day	13% 3 6% 14% 35% 29%	3320
	11th to 20th	Day	14% 3 7% 20% 44% 12%	3261
	21st to end	Day	14% 4 9% 30% 31% 12%	3446
November	1st to 10th	Day	21% 8% 14% 25% 24% 8%	1854
	11th to 20th	Day	21% 10% 18% 27% 22% 3	1890
	21st to end	Day	24% 9% 10% 28% 25% 5%	1795
December	1st to 10th	Day	19% 7% 10% 31% 27% 6%	2700
	11th to 20th	Day	36% 13% 13% 23% 11% 3	3108
	21st to end	Day	31% 13% 24% 23% 8% 1	4092
January	1st to 10th	Night	24% 12% 23% 28% 12% 1	8172
	11th to 20th	Night	20% 8% 11% 18% 27% 15%	8132
	21st to end	Night	33% 9% 9% 16% 27% 6%	8371
February	1st to 10th	Night	26% 10% 15% 26% 18% 5%	2730
	11th to 20th	Night	25% 9% 16% 30% 19% 2	2470
	21st to end	Night	23% 10% 18% 29% 14% 6%	2076
March	1st to 10th	Night	31% 12% 13% 22% 17% 5%	5496
	11th to 20th	Night	19% 8% 13% 41% 16% 2	5183
	21st to end	Night	28% 13% 21% 24% 12% 2	5254
April	1st to 10th	Night	21% 11% 17% 29% 18% 4	2100
	11th to 20th	Night	22% 8% 16% 29% 19% 6%	2079
	21st to end	Night	20% 9% 15% 31% 21% 4	2100
May	1st to 10th	Night	17% 5% 10% 24% 36% 8%	5504
	11th to 20th	Night	12% 4 8% 21% 42% 12%	4745
	21st to end	Night	12% 5% 8% 28% 39% 7%	5182
June	1st to 10th	Night	20% 6% 10% 23% 29% 11%	1680
	11th to 20th	Night	10% 3 8% 19% 34% 26%	1680
	21st to end	Night	11% 3 5% 14% 32% 35%	1680
July	1st to 10th	Night	5% 1 2 6% 23% 63%	4700
	11th to 20th	Night	15% 3 3 5% 15% 59%	4692
	21st to end	Night	14% 3 4 11% 23% 46%	5156
August	1st to 10th	Night	13% 3 5% 7% 24% 49%	3042
	11th to 20th	Night	6% 2 3 6% 26% 57%	3296
	21st to end	Night	9% 1 2 10% 35% 43%	3608
September	1st to 10th	Night	7% 3 4 8% 32% 46%	2000
	11th to 20th	Night	13% 4 7% 17% 36% 24%	2030
	21st to end	Night	12% 3 6% 19% 34% 27%	2200
October	1st to 10th	Night	11% 3 6% 13% 34% 34%	3878
	11th to 20th	Night	10% 3 7% 21% 46% 13%	3897
	21st to end	Night	16% 4 7% 29% 33% 11%	4359
November	1st to 10th	Night	20% 8% 15% 27% 20% 10%	2665
	11th to 20th	Night	20% 7% 16% 28% 24% 4	2730
	21st to end	Night	23% 6% 13% 27% 24% 7%	2820
December	1st to 10th	Night	19% 7% 10% 27% 27% 10%	4700
	11th to 20th	Night	32% 12% 14% 24% 16% 2	5316
	21st to end	Night	37% 13% 18% 21% 10% 1	7106

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% 24% 26% 23% 10% 2	220
	11th to 20th	Day	7% 24% 24% 25% 17% 4	246
	21st to end	Day	6% 14% 34% 28% 14% 3	473
February	1st to 10th	Day	12% 23% 27% 24% 12% 1	430
	11th to 20th	Day	10% 26% 29% 26% 9% 1	529
	21st to end	Day	11% 20% 31% 31% 7%	446
March	1st to 10th	Day	13% 28% 31% 20% 7% 1	540
	11th to 20th	Day	14% 26% 26% 20% 12% 1	540
	21st to end	Day	13% 28% 28% 23% 7% 1	594
April	1st to 10th	Day	12% 24% 27% 24% 9% 3	540
	11th to 20th	Day	12% 28% 28% 20% 10% 2	532
	21st to end	Day	15% 21% 26% 25% 10% 3	540
May	1st to 10th	Day	12% 22% 29% 23% 11% 3	543
	11th to 20th	Day	13% 21% 30% 20% 10% 6%	568
	21st to end	Day	11% 17% 25% 24% 16% 7%	617
June	1st to 10th	Day	6% 17% 30% 23% 18% 6%	570
	11th to 20th	Day	8% 18% 32% 26% 12% 4	570
	21st to end	Day	4 12% 31% 30% 17% 6%	570
July	1st to 10th	Day	2 10% 35% 30% 17% 6%	566
	11th to 20th	Day	2 8% 33% 33% 20% 4	564
	21st to end	Day	2 8% 29% 34% 21% 7%	619
August	1st to 10th	Day	3 9% 30% 29% 22% 7%	567
	11th to 20th	Day	3 11% 28% 31% 19% 8%	534
	21st to end	Day	3 8% 31% 34% 19% 5%	583
September	1st to 10th	Day	2 10% 32% 30% 21% 5%	530
	11th to 20th	Day	7% 12% 28% 31% 20% 2	530
	21st to end	Day	4 14% 28% 36% 16% 2	530
October	1st to 10th	Day	6% 17% 24% 29% 20% 4	530
	11th to 20th	Day	8% 16% 21% 30% 20% 5%	530
	21st to end	Day	9% 16% 24% 29% 18% 3	520
November	1st to 10th	Day	12% 14% 21% 27% 21% 6%	330
	11th to 20th	Day	8% 15% 25% 30% 15% 6%	330
	21st to end	Day	9% 18% 24% 26% 19% 5%	280
December	1st to 10th	Day	10% 16% 23% 25% 18% 8%	220
	11th to 20th	Day	11% 17% 30% 18% 16% 7%	220
	21st to end	Day	20% 17% 22% 28% 12% 1	242
January	1st to 10th	Night	5% 17% 28% 28% 16% 6%	840
	11th to 20th	Night	4 15% 22% 37% 18% 3	840
	21st to end	Night	6% 9% 28% 35% 18% 4	924
February	1st to 10th	Night	7% 13% 26% 35% 17% 3	840
	11th to 20th	Night	5% 15% 28% 33% 16% 4	824
	21st to end	Night	6% 18% 24% 38% 12% 2	692
March	1st to 10th	Night	6% 18% 32% 27% 14% 3	840
	11th to 20th	Night	6% 18% 25% 29% 19% 3	840
	21st to end	Night	5% 21% 29% 30% 13% 1	924
April	1st to 10th	Night	5% 17% 27% 34% 16% 1	840
	11th to 20th	Night	7% 17% 27% 31% 15% 3	831
	21st to end	Night	6% 15% 26% 32% 18% 3	840
May	1st to 10th	Night	8% 14% 26% 34% 16% 2	698
	11th to 20th	Night	4 9% 28% 38% 20% 2	434
	21st to end	Night	5% 7% 23% 38% 24% 3	455
June	1st to 10th	Night	5% 7% 24% 40% 22% 2	420
	11th to 20th	Night	1 7% 30% 38% 24% 1	420
	21st to end	Night	2 3 26% 44% 23% 2	420
July	1st to 10th	Night	1 3 24% 45% 25% 2	410
	11th to 20th	Night	1 2 25% 48% 23%	408
	21st to end	Night	3 2 16% 48% 28% 3	502
August	1st to 10th	Night	4 16% 47% 29% 3	637
	11th to 20th	Night	2 3 24% 39% 28% 4	808
	21st to end	Night	3 3 22% 45% 25% 3	880
September	1st to 10th	Night	1 4 20% 40% 30% 4	800
	11th to 20th	Night	4 6% 19% 35% 31% 6%	800
	21st to end	Night	2 7% 21% 38% 25% 8%	800
October	1st to 10th	Night	5% 7% 20% 39% 24% 6%	800
	11th to 20th	Night	5% 8% 17% 38% 27% 5%	800
	21st to end	Night	6% 9% 18% 32% 24% 10%	880
November	1st to 10th	Night	5% 13% 21% 29% 25% 6%	820
	11th to 20th	Night	6% 11% 26% 35% 17% 5%	840
	21st to end	Night	5% 13% 24% 32% 23% 4	840
December	1st to 10th	Night	6% 12% 24% 36% 17% 4	840
	11th to 20th	Night	8% 15% 24% 33% 15% 5%	840
	21st to end	Night	10% 18% 29% 31% 12% 1	924

Page generated at 2026-08-12 00:37:20. Version ac07

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