Iron Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Iron Mountain clear sky chart from 2005-11-06 to 2026-04-02.

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	13% 10% 3 8% 5% 5% 5% 4 5% 7% 35%	131501
	Night	13% 10% 4 9% 5% 5% 4 4 5% 7% 34%	130607
Transparency	Day	32% 36% 7% 16% 8% 1	134957
	Night	33% 36% 7% 15% 8% 1	135237
Seeing	Day	22% 14% 32% 20% 9% 3	17902
	Night	21% 11% 31% 27% 10% 1	27946

By Month

Percentage Cloud Cover NE (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	9% 13% 5% 14% 7% 5% 5% 4 6% 7% 24%	4691
	11th to 20th	Day	9% 15% 5% 10% 9% 5% 8% 5% 6% 6% 22%	4679
	21st to end	Day	7% 12% 6% 12% 6% 7% 6% 5% 6% 6% 27%	5156
February	1st to 10th	Day	9% 12% 5% 11% 5% 6% 5% 4 6% 6% 31%	1797
	11th to 20th	Day	9% 13% 4 11% 6% 4 5% 3 5% 6% 32%	1980
	21st to end	Day	12% 14% 5% 11% 5% 5% 5% 4 6% 7% 27%	1730
March	1st to 10th	Day	12% 10% 2 6% 4 4 4 3 5% 7% 41%	4578
	11th to 20th	Day	11% 11% 3 10% 8% 5% 6% 4 6% 6% 31%	4770
	21st to end	Day	23% 12% 5% 12% 5% 5% 4 3 3 4 26%	6101
April	1st to 10th	Day	16% 16% 4 9% 5% 5% 4 3 5% 6% 27%	2412
	11th to 20th	Day	19% 11% 4 8% 5% 5% 5% 3 5% 5% 31%	2374
	21st to end	Day	18% 10% 2 8% 5% 4 5% 4 6% 7% 31%	2600
May	1st to 10th	Day	14% 7% 3 6% 4 4 3 3 4 6% 46%	6154
	11th to 20th	Day	12% 8% 3 7% 4 4 5% 4 5% 6% 41%	6319
	21st to end	Day	13% 9% 3 8% 5% 5% 5% 4 5% 8% 35%	6949
June	1st to 10th	Day	13% 6% 2 6% 6% 4 4 4 6% 8% 40%	2800
	11th to 20th	Day	16% 7% 3 7% 5% 5% 5% 4 6% 8% 35%	2800
	21st to end	Day	14% 7% 2 7% 5% 5% 6% 5% 6% 9% 35%	2800
July	1st to 10th	Day	13% 4 2 5% 5% 5% 5% 3 6% 8% 42%	6359
	11th to 20th	Day	10% 4 1 5% 4 4 4 4 6% 8% 50%	6360
	21st to end	Day	10% 5% 2 6% 4 4 4 4 7% 8% 46%	6926
August	1st to 10th	Day	10% 6% 2 7% 5% 6% 5% 4 7% 8% 39%	4563
	11th to 20th	Day	13% 6% 2 5% 3 4 4 3 6% 7% 47%	4193
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 6% 43%	4312
September	1st to 10th	Day	12% 6% 3 7% 5% 5% 5% 4 5% 7% 42%	2400
	11th to 20th	Day	13% 7% 2 7% 4 5% 5% 4 6% 8% 41%	2289
	21st to end	Day	13% 8% 3 7% 5% 5% 5% 4 5% 9% 35%	2200
October	1st to 10th	Day	16% 8% 3 7% 5% 4 4 3 6% 8% 36%	2101
	11th to 20th	Day	21% 13% 3 8% 6% 5% 5% 3 5% 7% 25%	1920
	21st to end	Day	21% 13% 4 7% 5% 4 5% 3 5% 6% 27%	1980
November	1st to 10th	Day	15% 11% 5% 6% 5% 5% 5% 4 6% 7% 31%	1852
	11th to 20th	Day	15% 14% 6% 10% 5% 5% 5% 3 5% 6% 25%	1832
	21st to end	Day	16% 13% 6% 12% 6% 4 5% 5% 5% 6% 22%	1680
December	1st to 10th	Day	9% 13% 5% 16% 8% 7% 6% 4 6% 6% 20%	2690
	11th to 20th	Day	12% 19% 6% 13% 6% 5% 5% 3 4 5% 23%	3084
	21st to end	Day	23% 22% 6% 9% 5% 3 3 3 4 5% 19%	4070
January	1st to 10th	Night	11% 12% 5% 13% 7% 5% 5% 4 6% 7% 25%	8233
	11th to 20th	Night	8% 15% 7% 14% 9% 7% 5% 4 5% 6% 20%	8236
	21st to end	Night	7% 11% 6% 12% 7% 6% 4 5% 6% 7% 30%	9058
February	1st to 10th	Night	10% 13% 5% 11% 6% 5% 5% 4 5% 7% 30%	2819
	11th to 20th	Night	9% 12% 3 11% 7% 6% 4 4 6% 6% 32%	2525
	21st to end	Night	12% 15% 5% 8% 6% 5% 5% 4 6% 7% 29%	2076
March	1st to 10th	Night	13% 8% 3 8% 4 4 4 3 5% 8% 39%	5482
	11th to 20th	Night	12% 11% 5% 9% 5% 5% 5% 4 6% 8% 32%	5438
	21st to end	Night	20% 11% 4 8% 7% 4 4 3 5% 6% 27%	5072
April	1st to 10th	Night	19% 13% 3 7% 5% 4 4 4 5% 7% 30%	2010
	11th to 20th	Night	22% 9% 3 7% 5% 4 3 3 5% 5% 36%	1975
	21st to end	Night	16% 9% 3 7% 5% 4 5% 4 5% 7% 35%	1800
May	1st to 10th	Night	13% 9% 2 6% 4 3 4 3 4 6% 45%	3796
	11th to 20th	Night	13% 9% 3 7% 4 4 4 4 5% 6% 42%	3596
	21st to end	Night	17% 7% 2 6% 5% 4 4 3 5% 7% 39%	3949
June	1st to 10th	Night	11% 6% 3 6% 4 4 4 3 6% 8% 45%	1600
	11th to 20th	Night	15% 8% 3 7% 4 6% 5% 3 6% 7% 38%	1600
	21st to end	Night	15% 7% 2 5% 4 4 5% 4 6% 9% 41%	1600
July	1st to 10th	Night	13% 4 2 5% 3 3 4 4 5% 9% 48%	3620
	11th to 20th	Night	9% 4 1 5% 3 4 3 3 5% 6% 56%	3620
	21st to end	Night	9% 4 2 6% 4 4 3 3 5% 8% 52%	3948
August	1st to 10th	Night	12% 5% 2 7% 4 4 4 4 5% 7% 45%	2597
	11th to 20th	Night	13% 5% 1 5% 3 4 4 3 5% 7% 49%	2842
	21st to end	Night	15% 5% 2 6% 4 4 4 3 4 5% 47%	3586
September	1st to 10th	Night	10% 6% 2 5% 4 3 4 4 5% 7% 51%	2000
	11th to 20th	Night	12% 6% 2 5% 4 4 4 3 4 6% 49%	2090
	21st to end	Night	15% 7% 2 6% 4 4 4 4 5% 6% 43%	2200
October	1st to 10th	Night	15% 7% 2 5% 4 3 5% 4 5% 6% 43%	2295
	11th to 20th	Night	17% 12% 3 7% 5% 4 4 3 4 6% 34%	2480
	21st to end	Night	19% 12% 3 7% 5% 4 4 4 5% 8% 29%	2860
November	1st to 10th	Night	15% 10% 3 7% 5% 5% 4 4 6% 6% 34%	2666
	11th to 20th	Night	15% 12% 4 9% 6% 5% 5% 4 5% 6% 30%	2785
	21st to end	Night	15% 13% 5% 10% 5% 5% 4 5% 5% 6% 26%	2940
December	1st to 10th	Night	12% 12% 4 12% 6% 5% 5% 4 8% 8% 25%	4720
	11th to 20th	Night	12% 15% 7% 13% 7% 5% 4 3 4 4 24%	5343
	21st to end	Night	22% 17% 6% 9% 5% 4 4 3 4 5% 22%	7150

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	46% 21% 12% 14% 7%	4692
	11th to 20th	Day	47% 22% 7% 18% 5% 1	4710
	21st to end	Day	43% 22% 8% 16% 11%	5156
February	1st to 10th	Day	21% 59% 5% 9% 6%	1797
	11th to 20th	Day	24% 58% 3 10% 5%	1989
	21st to end	Day	23% 59% 4 8% 6% 1	1730
March	1st to 10th	Day	28% 28% 10% 15% 15% 3	4580
	11th to 20th	Day	40% 28% 8% 15% 8% 1	4770
	21st to end	Day	48% 27% 4 11% 9%	6101
April	1st to 10th	Day	24% 62% 3 7% 4	2412
	11th to 20th	Day	21% 62% 3 8% 4 1	2374
	21st to end	Day	20% 62% 3 9% 5% 1	2600
May	1st to 10th	Day	27% 28% 6% 22% 16% 2	6163
	11th to 20th	Day	32% 28% 7% 23% 9% 2	6323
	21st to end	Day	37% 29% 5% 14% 14% 1	6949
June	1st to 10th	Day	16% 62% 4 12% 6% 1	2800
	11th to 20th	Day	20% 62% 5% 7% 6% 1	2800
	21st to end	Day	18% 61% 5% 10% 5%	2800
July	1st to 10th	Day	31% 26% 11% 24% 7% 1	6291
	11th to 20th	Day	24% 26% 11% 29% 9% 1	6285
	21st to end	Day	27% 25% 9% 25% 14%	6728
August	1st to 10th	Day	28% 37% 10% 17% 8%	4548
	11th to 20th	Day	25% 37% 11% 21% 6%	4234
	21st to end	Day	31% 36% 10% 14% 9%	4311
September	1st to 10th	Day	21% 57% 6% 12% 4	2400
	11th to 20th	Day	18% 57% 7% 13% 5%	2289
	21st to end	Day	20% 57% 6% 13% 5%	2200
October	1st to 10th	Day	19% 36% 7% 21% 17%	3426
	11th to 20th	Day	32% 36% 5% 16% 11%	3110
	21st to end	Day	36% 36% 10% 14% 4	3180
November	1st to 10th	Day	27% 54% 5% 9% 5% 1	1852
	11th to 20th	Day	28% 54% 6% 8% 4	1832
	21st to end	Day	31% 55% 5% 6% 3	1680
December	1st to 10th	Day	42% 36% 7% 11% 3	2690
	11th to 20th	Day	46% 31% 4 8% 9% 1	3085
	21st to end	Day	53% 26% 2 14% 6%	4070
January	1st to 10th	Night	46% 21% 6% 20% 7%	8233
	11th to 20th	Night	52% 21% 7% 11% 9%	8280
	21st to end	Night	40% 22% 8% 18% 11%	9058
February	1st to 10th	Night	24% 58% 4 8% 5% 1	2819
	11th to 20th	Night	21% 58% 3 11% 6% 1	2526
	21st to end	Night	21% 59% 5% 9% 6% 1	2076
March	1st to 10th	Night	29% 29% 15% 14% 12% 1	5493
	11th to 20th	Night	38% 29% 12% 14% 7%	5439
	21st to end	Night	45% 28% 7% 12% 9% 1	5072
April	1st to 10th	Night	23% 61% 3 6% 7% 1	2010
	11th to 20th	Night	21% 62% 3 7% 5% 1	1975
	21st to end	Night	20% 61% 3 8% 7% 1	1800
May	1st to 10th	Night	28% 28% 8% 21% 13% 2	3803
	11th to 20th	Night	29% 30% 6% 20% 14% 1	3597
	21st to end	Night	35% 28% 8% 18% 9% 3	3950
June	1st to 10th	Night	15% 61% 5% 11% 8%	1600
	11th to 20th	Night	21% 62% 4 7% 5% 1	1600
	21st to end	Night	18% 60% 4 12% 6% 1	1600
July	1st to 10th	Night	28% 26% 17% 23% 5% 2	3580
	11th to 20th	Night	22% 26% 12% 30% 10% 1	3572
	21st to end	Night	25% 26% 11% 20% 17% 1	3832
August	1st to 10th	Night	26% 37% 10% 14% 13% 1	2588
	11th to 20th	Night	26% 37% 11% 19% 7% 1	2867
	21st to end	Night	28% 37% 8% 18% 9%	3586
September	1st to 10th	Night	17% 57% 8% 13% 4	2000
	11th to 20th	Night	17% 57% 6% 14% 5%	2090
	21st to end	Night	19% 57% 6% 12% 5% 1	2200
October	1st to 10th	Night	21% 36% 5% 26% 11% 1	3750
	11th to 20th	Night	24% 36% 8% 20% 12%	4029
	21st to end	Night	34% 36% 6% 17% 6%	4610
November	1st to 10th	Night	26% 54% 5% 11% 4	2666
	11th to 20th	Night	24% 55% 6% 11% 4 1	2785
	21st to end	Night	29% 54% 5% 8% 3 1	2940
December	1st to 10th	Night	36% 36% 14% 12% 2	4720
	11th to 20th	Night	46% 31% 4 11% 7%	5341
	21st to end	Night	50% 26% 3 13% 8%	7150

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	22% 14% 35% 22% 7% 1	330
	11th to 20th	Day	28% 16% 33% 19% 4 1	330
	21st to end	Day	24% 15% 40% 17% 4	363
February	1st to 10th	Day	21% 25% 37% 13% 3 1	330
	11th to 20th	Day	24% 25% 37% 11% 3	330
	21st to end	Day	29% 22% 39% 8% 1	273
March	1st to 10th	Day	26% 21% 40% 12% 2	330
	11th to 20th	Day	29% 17% 36% 13% 4	356
	21st to end	Day	30% 21% 28% 15% 6% 1	594
April	1st to 10th	Day	32% 16% 30% 17% 5%	504
	11th to 20th	Day	32% 17% 26% 18% 7% 1	492
	21st to end	Day	35% 13% 26% 17% 7% 2	500
May	1st to 10th	Day	26% 16% 31% 16% 9% 2	613
	11th to 20th	Day	24% 15% 34% 17% 6% 3	725
	21st to end	Day	21% 14% 33% 20% 7% 5%	789
June	1st to 10th	Day	18% 15% 33% 22% 8% 4	730
	11th to 20th	Day	23% 11% 36% 18% 10% 2	730
	21st to end	Day	19% 12% 32% 25% 9% 4	730
July	1st to 10th	Day	10% 9% 34% 28% 15% 4	721
	11th to 20th	Day	12% 9% 36% 23% 15% 5%	718
	21st to end	Day	10% 8% 35% 27% 13% 8%	794
August	1st to 10th	Day	12% 9% 29% 29% 16% 6%	726
	11th to 20th	Day	16% 6% 24% 29% 17% 8%	584
	21st to end	Day	21% 8% 26% 22% 18% 5%	583
September	1st to 10th	Day	19% 9% 26% 27% 13% 6%	530
	11th to 20th	Day	13% 7% 30% 32% 14% 4	530
	21st to end	Day	21% 12% 24% 27% 13% 4	530
October	1st to 10th	Day	25% 13% 29% 20% 12% 2	431
	11th to 20th	Day	28% 19% 35% 14% 4	330
	21st to end	Day	31% 16% 36% 13% 3	363
November	1st to 10th	Day	27% 16% 35% 15% 5% 1	330
	11th to 20th	Day	28% 22% 32% 12% 4 2	330
	21st to end	Day	29% 19% 36% 10% 5% 1	330
December	1st to 10th	Day	20% 18% 42% 13% 7%	330
	11th to 20th	Day	30% 18% 34% 15% 3	330
	21st to end	Day	37% 16% 31% 11% 3 1	363
January	1st to 10th	Night	20% 15% 38% 22% 5%	1050
	11th to 20th	Night	22% 15% 36% 19% 6% 1	1050
	21st to end	Night	18% 11% 40% 22% 8% 1	1155
February	1st to 10th	Night	19% 14% 38% 21% 7% 1	1050
	11th to 20th	Night	22% 17% 33% 21% 7%	890
	21st to end	Night	23% 15% 36% 21% 4	692
March	1st to 10th	Night	20% 13% 33% 25% 7% 1	840
	11th to 20th	Night	21% 16% 36% 21% 5% 1	824
	21st to end	Night	21% 11% 32% 27% 8% 1	693
April	1st to 10th	Night	27% 12% 31% 24% 7%	603
	11th to 20th	Night	28% 11% 28% 26% 7% 1	593
	21st to end	Night	23% 11% 31% 27% 7% 1	600
May	1st to 10th	Night	22% 10% 30% 29% 8% 1	600
	11th to 20th	Night	22% 8% 33% 28% 7% 1	591
	21st to end	Night	16% 6% 29% 36% 12%	650
June	1st to 10th	Night	15% 8% 28% 37% 11%	600
	11th to 20th	Night	21% 6% 25% 39% 9% 1	600
	21st to end	Night	19% 5% 27% 35% 14% 1	600
July	1st to 10th	Night	11% 4 26% 40% 17% 1	585
	11th to 20th	Night	12% 3 27% 37% 19%	582
	21st to end	Night	12% 2 28% 40% 17% 2	657
August	1st to 10th	Night	11% 3 25% 40% 19% 2	597
	11th to 20th	Night	13% 3 22% 35% 24% 3	594
	21st to end	Night	14% 3 24% 37% 21% 1	660
September	1st to 10th	Night	14% 6% 26% 30% 21% 3	600
	11th to 20th	Night	14% 6% 28% 33% 17% 2	690
	21st to end	Night	19% 9% 26% 32% 14% 1	800
October	1st to 10th	Night	21% 9% 29% 26% 14% 1	800
	11th to 20th	Night	25% 13% 31% 24% 7%	800
	21st to end	Night	29% 10% 28% 26% 7% 1	880
November	1st to 10th	Night	23% 12% 35% 23% 7% 1	820
	11th to 20th	Night	25% 16% 31% 20% 7%	895
	21st to end	Night	25% 17% 30% 24% 5%	1050
December	1st to 10th	Night	22% 15% 37% 20% 5% 1	1050
	11th to 20th	Night	22% 15% 31% 23% 7% 2	1050
	21st to end	Night	31% 12% 31% 20% 5%	1155

Page generated at 2026-03-30 01:18:41. Version ac07

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