Helena
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Helena clear sky chart from 2005-11-06 to 2026-07-31.

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	6% 6% 2 6% 4 4 5% 4 6% 8% 47%	147043
	Night	5% 8% 3 7% 4 4 5% 4 6% 9% 45%	143343
Transparency	Day	34% 5% 15% 26% 16% 5%	141731
	Night	36% 5% 18% 26% 13% 3	139663
Seeing	Day	11% 11% 26% 30% 17% 4	17691
	Night	11% 8% 27% 39% 15% 1	26988

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	5% 12% 4 10% 6% 6% 6% 6% 7% 9% 30%	4124
	11th to 20th	Day	2 11% 4 9% 5% 5% 6% 4 6% 8% 39%	4620
	21st to end	Day	3 9% 4 9% 5% 5% 6% 5% 7% 10% 38%	5381
February	1st to 10th	Day	5% 14% 4 10% 7% 6% 6% 5% 7% 9% 28%	1890
	11th to 20th	Day	3 12% 4 13% 6% 5% 6% 4 6% 9% 33%	1845
	21st to end	Day	3 10% 4 10% 6% 6% 5% 5% 8% 10% 34%	1678
March	1st to 10th	Day	5% 6% 2 6% 4 4 4 4 5% 9% 50%	4794
	11th to 20th	Day	3 7% 3 7% 5% 5% 7% 6% 6% 9% 42%	5110
	21st to end	Day	5% 11% 4 9% 6% 7% 7% 5% 8% 10% 27%	5607
April	1st to 10th	Day	6% 10% 3 7% 6% 6% 5% 5% 7% 8% 36%	2477
	11th to 20th	Day	8% 11% 3 8% 7% 5% 6% 5% 7% 8% 33%	2697
	21st to end	Day	10% 11% 3 7% 5% 5% 5% 4 6% 8% 35%	2730
May	1st to 10th	Day	10% 8% 2 5% 3 4 4 4 6% 9% 44%	7626
	11th to 20th	Day	11% 6% 2 6% 5% 5% 6% 5% 8% 9% 39%	8046
	21st to end	Day	11% 5% 2 5% 3 4 5% 4 7% 9% 47%	9510
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 6% 5% 8% 10% 36%	3150
	11th to 20th	Day	12% 5% 2 5% 5% 5% 6% 5% 7% 9% 39%	3150
	21st to end	Day	5% 3 1 4 4 5% 5% 5% 7% 8% 55%	3150
July	1st to 10th	Day	3 2 1 3 3 4 5% 4 6% 7% 63%	6929
	11th to 20th	Day	6% 2 1 2 2 3 3 3 4 5% 69%	6930
	21st to end	Day	2 1 1 3 3 4 5% 4 6% 9% 62%	6924
August	1st to 10th	Day	4 2 1 4 4 5% 6% 5% 7% 10% 50%	5779
	11th to 20th	Day	3 1 1 3 3 4 4 4 6% 7% 64%	5798
	21st to end	Day	5% 3 1 3 3 3 3 4 6% 8% 62%	6468
September	1st to 10th	Day	6% 3 1 3 3 3 4 3 4 7% 63%	2405
	11th to 20th	Day	7% 5% 1 5% 5% 4 4 4 6% 9% 51%	2200
	21st to end	Day	9% 5% 1 3 3 3 3 3 6% 7% 57%	2200
October	1st to 10th	Day	7% 5% 2 4 3 3 3 2 4 7% 59%	3381
	11th to 20th	Day	5% 6% 2 4 3 4 4 4 5% 7% 57%	3250
	21st to end	Day	4 6% 2 6% 5% 5% 6% 5% 6% 8% 46%	3263
November	1st to 10th	Day	6% 10% 3 7% 5% 6% 6% 5% 7% 8% 37%	1785
	11th to 20th	Day	5% 9% 3 7% 6% 6% 7% 5% 6% 10% 37%	1680
	21st to end	Day	4 10% 2 7% 4 6% 5% 3 8% 9% 41%	1680
December	1st to 10th	Day	4 7% 3 7% 5% 3 4 4 7% 8% 50%	2496
	11th to 20th	Day	3 8% 3 7% 5% 6% 6% 6% 7% 10% 39%	2715
	21st to end	Day	4 8% 4 9% 6% 6% 7% 5% 7% 9% 37%	3575
January	1st to 10th	Night	3 11% 4 9% 5% 5% 6% 5% 7% 12% 32%	8768
	11th to 20th	Night	3 10% 3 9% 6% 6% 6% 4 6% 8% 40%	8382
	21st to end	Night	2 10% 4 9% 4 4 5% 4 8% 12% 38%	8807
February	1st to 10th	Night	5% 14% 6% 12% 6% 5% 5% 4 6% 6% 30%	2730
	11th to 20th	Night	4 11% 4 10% 6% 6% 5% 4 8% 9% 33%	2662
	21st to end	Night	3 11% 4 9% 6% 6% 6% 5% 7% 8% 36%	2124
March	1st to 10th	Night	3 7% 2 6% 4 4 5% 4 7% 10% 46%	5319
	11th to 20th	Night	3 8% 3 7% 5% 7% 6% 6% 8% 8% 40%	5110
	21st to end	Night	7% 13% 4 10% 5% 6% 7% 5% 7% 8% 27%	5610
April	1st to 10th	Night	7% 11% 3 6% 4 5% 5% 5% 6% 8% 40%	2141
	11th to 20th	Night	8% 12% 4 7% 6% 5% 6% 4 7% 7% 33%	1871
	21st to end	Night	11% 11% 3 5% 4 5% 5% 5% 6% 8% 37%	1890
May	1st to 10th	Night	12% 8% 2 5% 3 4 4 3 5% 6% 48%	5321
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 6% 9% 45%	4843
	21st to end	Night	12% 5% 1 5% 4 4 4 3 6% 8% 49%	4667
June	1st to 10th	Night	12% 6% 3 6% 5% 4 4 5% 6% 8% 42%	1470
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 3 5% 7% 46%	1470
	21st to end	Night	5% 3 1 5% 4 3 4 4 7% 7% 58%	1470
July	1st to 10th	Night	3 1 1 2 2 2 3 3 6% 9% 68%	3250
	11th to 20th	Night	6% 1 1 2 2 2 3 3 4 6% 72%	3250
	21st to end	Night	4 2 1 3 3 4 5% 4 7% 9% 58%	4145
August	1st to 10th	Night	6% 2 1 4 4 5% 6% 5% 8% 10% 50%	4033
	11th to 20th	Night	3 2 1 3 3 3 4 3 6% 8% 65%	4051
	21st to end	Night	6% 3 1 4 3 3 4 4 6% 8% 59%	4509
September	1st to 10th	Night	6% 4 1 4 3 3 3 4 5% 7% 59%	1992
	11th to 20th	Night	9% 4 1 4 4 4 4 4 7% 8% 51%	2200
	21st to end	Night	12% 5% 1 3 2 3 3 3 4 6% 57%	2200
October	1st to 10th	Night	7% 5% 1 3 3 3 3 3 4 7% 61%	3811
	11th to 20th	Night	7% 6% 2 5% 4 3 3 3 4 6% 56%	3898
	21st to end	Night	5% 8% 2 6% 4 4 5% 4 6% 8% 50%	4535
November	1st to 10th	Night	7% 9% 3 8% 4 4 5% 4 6% 10% 40%	2712
	11th to 20th	Night	6% 8% 3 7% 5% 4 6% 6% 8% 9% 39%	2940
	21st to end	Night	5% 8% 3 6% 4 5% 5% 4 6% 11% 42%	2940
December	1st to 10th	Night	3 9% 3 6% 4 4 5% 4 6% 9% 48%	4903
	11th to 20th	Night	4 9% 4 9% 6% 5% 6% 5% 7% 10% 35%	5696
	21st to end	Night	3 8% 4 9% 5% 5% 6% 4 7% 11% 38%	7623

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	49% 6% 18% 22% 5%	4125
	11th to 20th	Day	43% 5% 24% 18% 9% 1	4620
	21st to end	Day	38% 5% 23% 26% 8%	5381
February	1st to 10th	Day	52% 7% 18% 17% 5% 1	1890
	11th to 20th	Day	49% 6% 19% 19% 7%	1854
	21st to end	Day	45% 5% 15% 23% 9% 1	1678
March	1st to 10th	Day	31% 5% 13% 31% 20% 1	4794
	11th to 20th	Day	35% 5% 20% 30% 9%	5110
	21st to end	Day	50% 6% 16% 23% 4	5607
April	1st to 10th	Day	45% 6% 16% 23% 9% 1	2477
	11th to 20th	Day	48% 5% 13% 25% 8% 1	2700
	21st to end	Day	48% 4 15% 23% 10% 1	2730
May	1st to 10th	Day	37% 4 13% 33% 13% 1	7626
	11th to 20th	Day	39% 5% 14% 23% 16% 3	8050
	21st to end	Day	34% 4 14% 33% 15% 1	9511
June	1st to 10th	Day	42% 6% 19% 22% 9% 1	3150
	11th to 20th	Day	40% 6% 15% 19% 14% 5%	3150
	21st to end	Day	26% 5% 12% 32% 21% 3	3150
July	1st to 10th	Day	19% 4 10% 29% 26% 12%	6856
	11th to 20th	Day	18% 3 5% 20% 28% 27%	6849
	21st to end	Day	18% 5% 12% 21% 28% 16%	6726
August	1st to 10th	Day	26% 6% 11% 23% 21% 13%	4208
	11th to 20th	Day	21% 5% 11% 34% 24% 5%	4197
	21st to end	Day	18% 4 10% 30% 29% 9%	4663
September	1st to 10th	Day	23% 3 10% 29% 25% 11%	2405
	11th to 20th	Day	30% 4 12% 30% 19% 4	2200
	21st to end	Day	28% 3 15% 30% 20% 5%	2200
October	1st to 10th	Day	28% 2 9% 35% 24% 1	3381
	11th to 20th	Day	27% 4 19% 33% 16% 1	3250
	21st to end	Day	34% 6% 16% 28% 13% 3	3263
November	1st to 10th	Day	43% 6% 17% 24% 9% 1	1785
	11th to 20th	Day	42% 8% 21% 22% 7%	1680
	21st to end	Day	39% 6% 20% 28% 7% 1	1680
December	1st to 10th	Day	32% 6% 27% 26% 7%	2496
	11th to 20th	Day	40% 7% 29% 19% 5%	2714
	21st to end	Day	42% 6% 19% 22% 11%	3575
January	1st to 10th	Night	44% 7% 22% 23% 4	8769
	11th to 20th	Night	43% 5% 23% 20% 7% 2	8382
	21st to end	Night	38% 5% 25% 24% 7%	8807
February	1st to 10th	Night	54% 5% 15% 18% 7%	2730
	11th to 20th	Night	46% 6% 20% 21% 6%	2663
	21st to end	Night	44% 7% 16% 22% 10% 1	2124
March	1st to 10th	Night	32% 4 14% 30% 18% 1	5319
	11th to 20th	Night	38% 5% 20% 30% 6%	5110
	21st to end	Night	54% 5% 15% 21% 5%	5610
April	1st to 10th	Night	42% 6% 17% 26% 8% 1	2141
	11th to 20th	Night	48% 6% 15% 23% 7% 1	1871
	21st to end	Night	45% 5% 17% 26% 8%	1890
May	1st to 10th	Night	37% 4 13% 33% 12% 1	5319
	11th to 20th	Night	35% 5% 14% 27% 17% 1	4844
	21st to end	Night	34% 4 16% 32% 13% 1	4668
June	1st to 10th	Night	41% 6% 18% 24% 10% 1	1470
	11th to 20th	Night	38% 4 18% 21% 14% 5%	1470
	21st to end	Night	25% 4 16% 33% 20% 2	1470
July	1st to 10th	Night	15% 3 11% 33% 26% 12%	3215
	11th to 20th	Night	14% 3 7% 26% 24% 26%	3208
	21st to end	Night	22% 6% 12% 20% 23% 17%	4031
August	1st to 10th	Night	27% 5% 11% 23% 23% 11%	2928
	11th to 20th	Night	23% 6% 12% 32% 21% 6%	2928
	21st to end	Night	20% 4 10% 36% 25% 6%	3245
September	1st to 10th	Night	24% 4 10% 27% 21% 14%	1992
	11th to 20th	Night	32% 4 12% 29% 19% 5%	2200
	21st to end	Night	30% 3 13% 28% 23% 4	2200
October	1st to 10th	Night	26% 2 9% 34% 25% 3	3811
	11th to 20th	Night	31% 3 20% 29% 16% 1	3898
	21st to end	Night	32% 4 16% 27% 19% 2	4535
November	1st to 10th	Night	41% 7% 15% 27% 9% 2	2712
	11th to 20th	Night	40% 7% 20% 25% 7%	2940
	21st to end	Night	37% 6% 22% 25% 8% 1	2940
December	1st to 10th	Night	33% 7% 25% 28% 7%	4903
	11th to 20th	Night	45% 6% 27% 18% 4	5697
	21st to end	Night	40% 6% 22% 21% 11%	7623

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	9% 11% 26% 35% 15% 5%	220
	11th to 20th	Day	10% 8% 30% 37% 12% 3	220
	21st to end	Day	10% 10% 29% 38% 10% 3	276
February	1st to 10th	Day	15% 13% 41% 26% 5% 1	330
	11th to 20th	Day	11% 11% 33% 34% 9% 1	330
	21st to end	Day	10% 19% 36% 27% 6% 1	394
March	1st to 10th	Day	11% 15% 36% 29% 8% 1	540
	11th to 20th	Day	11% 15% 30% 28% 15% 1	540
	21st to end	Day	14% 21% 29% 24% 11% 1	594
April	1st to 10th	Day	13% 17% 34% 24% 10% 3	540
	11th to 20th	Day	15% 21% 28% 25% 9% 1	532
	21st to end	Day	16% 15% 29% 24% 14% 2	540
May	1st to 10th	Day	13% 12% 24% 29% 16% 5%	543
	11th to 20th	Day	18% 10% 21% 30% 17% 5%	719
	21st to end	Day	18% 9% 17% 25% 23% 7%	844
June	1st to 10th	Day	13% 11% 23% 28% 21% 4	780
	11th to 20th	Day	16% 13% 27% 29% 13% 2	780
	21st to end	Day	8% 9% 22% 34% 23% 4	780
July	1st to 10th	Day	4 6% 22% 36% 28% 5%	771
	11th to 20th	Day	6% 6% 23% 37% 24% 4	768
	21st to end	Day	3 5% 23% 39% 24% 7%	622
August	1st to 10th	Day	5% 7% 27% 31% 24% 5%	527
	11th to 20th	Day	4 6% 23% 37% 23% 7%	517
	21st to end	Day	7% 8% 23% 32% 23% 7%	583
September	1st to 10th	Day	7% 7% 27% 27% 22% 10%	530
	11th to 20th	Day	8% 11% 25% 27% 20% 8%	530
	21st to end	Day	13% 11% 23% 28% 19% 7%	530
October	1st to 10th	Day	13% 9% 24% 29% 17% 8%	402
	11th to 20th	Day	12% 13% 23% 27% 16% 8%	330
	21st to end	Day	11% 14% 30% 31% 13% 1	363
November	1st to 10th	Day	16% 10% 28% 31% 13% 1	330
	11th to 20th	Day	10% 13% 36% 28% 11% 2	330
	21st to end	Day	13% 10% 26% 42% 8% 1	330
December	1st to 10th	Day	9% 11% 34% 33% 12% 1	264
	11th to 20th	Day	13% 14% 29% 30% 13% 1	220
	21st to end	Day	12% 12% 29% 33% 11% 5%	242
January	1st to 10th	Night	10% 10% 33% 38% 9%	1050
	11th to 20th	Night	11% 12% 34% 33% 9% 2	898
	21st to end	Night	8% 9% 32% 41% 9% 1	924
February	1st to 10th	Night	13% 13% 32% 35% 7% 1	840
	11th to 20th	Night	10% 11% 32% 36% 10% 1	824
	21st to end	Night	9% 12% 33% 34% 10% 2	692
March	1st to 10th	Night	7% 6% 38% 40% 8% 1	840
	11th to 20th	Night	9% 9% 32% 37% 12% 1	840
	21st to end	Night	11% 9% 32% 35% 12% 1	924
April	1st to 10th	Night	15% 7% 25% 41% 12% 1	756
	11th to 20th	Night	14% 8% 29% 38% 11% 1	623
	21st to end	Night	17% 8% 23% 38% 14% 1	630
May	1st to 10th	Night	13% 7% 23% 37% 18% 1	630
	11th to 20th	Night	17% 6% 18% 35% 23% 2	621
	21st to end	Night	21% 4 13% 34% 25% 3	507
June	1st to 10th	Night	16% 5% 17% 36% 23% 2	420
	11th to 20th	Night	14% 5% 30% 39% 12% 1	420
	21st to end	Night	7% 4 14% 50% 25%	420
July	1st to 10th	Night	4 1 12% 53% 29% 1	410
	11th to 20th	Night	3 3 12% 63% 19%	408
	21st to end	Night	4 1 10% 56% 27% 2	501
August	1st to 10th	Night	6% 3 14% 52% 24%	597
	11th to 20th	Night	5% 3 15% 47% 28% 2	594
	21st to end	Night	8% 3 17% 44% 27% 1	660
September	1st to 10th	Night	7% 3 18% 43% 25% 4	657
	11th to 20th	Night	10% 3 14% 41% 29% 3	800
	21st to end	Night	14% 4 18% 39% 22% 3	800
October	1st to 10th	Night	16% 4 18% 42% 18% 2	800
	11th to 20th	Night	10% 8% 27% 37% 18% 2	800
	21st to end	Night	12% 10% 29% 36% 12% 1	880
November	1st to 10th	Night	13% 7% 32% 39% 9% 1	867
	11th to 20th	Night	10% 13% 33% 35% 8%	1050
	21st to end	Night	9% 9% 33% 41% 8%	1050
December	1st to 10th	Night	10% 11% 33% 35% 10% 1	1050
	11th to 20th	Night	9% 9% 36% 37% 8% 1	1050
	21st to end	Night	9% 11% 36% 34% 10% 1	1155

Page generated at 2026-07-28 01:25:22. Version ac07

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