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 2025-03-27.

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% 7% 2 6% 4 5% 5% 4 6% 8% 46%	112459
	Night	5% 8% 3 7% 4 4 5% 4 6% 9% 44%	115613
Transparency	Day	35% 5% 15% 25% 15% 5%	112193
	Night	37% 5% 18% 25% 12% 3	115469
Seeing	Day	12% 11% 27% 30% 16% 4	15787
	Night	11% 8% 26% 39% 15% 1	25364

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	4 12% 5% 11% 7% 6% 6% 6% 7% 10% 27%	3179
	11th to 20th	Day	2 11% 4 11% 6% 6% 7% 5% 7% 7% 33%	3548
	21st to end	Day	3 7% 3 8% 4 5% 7% 6% 7% 11% 38%	4156
February	1st to 10th	Day	5% 14% 5% 10% 7% 7% 6% 5% 6% 8% 26%	1800
	11th to 20th	Day	4 12% 5% 13% 6% 5% 6% 4 6% 9% 32%	1755
	21st to end	Day	3 11% 4 10% 7% 6% 5% 5% 8% 10% 32%	1600
March	1st to 10th	Day	2 5% 2 7% 5% 5% 5% 5% 6% 11% 46%	3362
	11th to 20th	Day	3 7% 3 6% 5% 4 5% 5% 5% 8% 49%	3600
	21st to end	Day	5% 12% 4 10% 7% 7% 7% 5% 8% 9% 26%	3892
April	1st to 10th	Day	6% 11% 3 8% 6% 6% 6% 5% 7% 8% 34%	2241
	11th to 20th	Day	8% 12% 4 9% 7% 4 6% 5% 6% 8% 32%	2437
	21st to end	Day	11% 12% 3 8% 5% 5% 5% 4 6% 8% 34%	2470
May	1st to 10th	Day	15% 13% 3 7% 4 4 5% 4 6% 8% 30%	4110
	11th to 20th	Day	8% 5% 1 5% 5% 5% 6% 5% 9% 10% 39%	4311
	21st to end	Day	13% 8% 2 7% 4 5% 6% 4 7% 8% 35%	5088
June	1st to 10th	Day	11% 6% 2 7% 5% 5% 6% 5% 8% 10% 35%	2850
	11th to 20th	Day	13% 6% 2 5% 5% 5% 6% 5% 7% 9% 38%	2850
	21st to end	Day	5% 3 1 4 3 5% 5% 5% 7% 8% 55%	2850
July	1st to 10th	Day	2 2 1 3 3 3 4 4 6% 7% 65%	4740
	11th to 20th	Day	2 2 1 2 2 3 3 3 4 5% 72%	4740
	21st to end	Day	3 1 1 2 3 4 4 3 5% 6% 67%	4720
August	1st to 10th	Day	5% 3 1 4 4 5% 5% 5% 7% 9% 52%	4092
	11th to 20th	Day	3 2 1 3 4 5% 4 5% 7% 7% 60%	4028
	21st to end	Day	4 2 1 2 2 3 4 4 6% 8% 64%	4521
September	1st to 10th	Day	7% 3 1 3 3 4 4 3 4 7% 63%	2285
	11th to 20th	Day	7% 5% 2 5% 5% 4 4 4 6% 8% 51%	2090
	21st to end	Day	9% 5% 1 3 3 3 3 3 6% 7% 56%	2090
October	1st to 10th	Day	7% 5% 2 4 3 3 3 2 4 7% 59%	3278
	11th to 20th	Day	5% 6% 2 3 3 4 4 4 5% 7% 58%	3150
	21st to end	Day	4 6% 2 6% 5% 5% 6% 5% 6% 8% 45%	3162
November	1st to 10th	Day	6% 11% 3 7% 5% 6% 6% 5% 7% 8% 36%	1699
	11th to 20th	Day	5% 9% 3 7% 6% 5% 7% 5% 7% 10% 37%	1600
	21st to end	Day	5% 10% 2 7% 4 6% 5% 3 8% 9% 40%	1600
December	1st to 10th	Day	3 7% 3 7% 5% 3 4 4 7% 8% 50%	2422
	11th to 20th	Day	3 8% 3 7% 5% 6% 7% 6% 7% 10% 39%	2645
	21st to end	Day	4 9% 4 9% 6% 6% 7% 5% 7% 9% 37%	3498
January	1st to 10th	Night	3 12% 4 9% 6% 5% 6% 5% 7% 11% 32%	6766
	11th to 20th	Night	3 11% 4 10% 7% 7% 6% 4 6% 8% 34%	6440
	21st to end	Night	2 8% 3 8% 4 4 5% 4 8% 13% 40%	6804
February	1st to 10th	Night	5% 15% 6% 13% 7% 5% 5% 4 6% 6% 29%	2600
	11th to 20th	Night	4 12% 4 10% 6% 6% 5% 4 7% 8% 33%	2532
	21st to end	Night	3 12% 4 9% 6% 5% 6% 5% 7% 8% 34%	2026
March	1st to 10th	Night	2 7% 3 6% 5% 5% 5% 5% 6% 10% 46%	3731
	11th to 20th	Night	3 8% 3 6% 5% 5% 4 5% 7% 8% 47%	3600
	21st to end	Night	5% 15% 5% 11% 6% 6% 7% 5% 7% 8% 25%	3899
April	1st to 10th	Night	7% 11% 3 7% 5% 5% 5% 5% 6% 9% 38%	1937
	11th to 20th	Night	8% 13% 4 8% 6% 5% 6% 4 7% 7% 32%	1691
	21st to end	Night	12% 11% 3 5% 4 5% 5% 5% 6% 8% 37%	1710
May	1st to 10th	Night	17% 11% 4 7% 4 4 5% 3 5% 6% 34%	2860
	11th to 20th	Night	8% 4 2 5% 4 4 4 4 6% 9% 50%	2591
	21st to end	Night	13% 7% 2 6% 5% 5% 5% 4 7% 9% 38%	2486
June	1st to 10th	Night	13% 6% 3 6% 5% 4 4 5% 6% 8% 40%	1330
	11th to 20th	Night	13% 6% 2 5% 5% 4 4 3 5% 7% 46%	1330
	21st to end	Night	4 3 1 5% 4 3 4 4 7% 7% 59%	1330
July	1st to 10th	Night	2 2 1 2 2 2 3 3 6% 8% 69%	2220
	11th to 20th	Night	2 1 1 2 2 2 3 3 4 6% 75%	2220
	21st to end	Night	3 1 2 2 4 4 3 5% 7% 68%	2827
August	1st to 10th	Night	6% 3 1 4 4 4 5% 5% 7% 8% 53%	2847
	11th to 20th	Night	4 2 1 4 4 4 5% 4 6% 9% 56%	2811
	21st to end	Night	5% 3 1 3 3 3 3 4 6% 7% 63%	3146
September	1st to 10th	Night	7% 4 1 4 3 3 3 4 5% 7% 58%	1892
	11th to 20th	Night	9% 4 1 4 4 3 4 4 7% 8% 50%	2090
	21st to end	Night	13% 5% 1 3 2 3 3 3 4 6% 56%	2090
October	1st to 10th	Night	7% 5% 1 3 2 3 3 2 4 7% 61%	3694
	11th to 20th	Night	7% 5% 2 5% 4 3 3 3 4 6% 57%	3778
	21st to end	Night	5% 8% 2 6% 4 4 5% 4 6% 7% 50%	4394
November	1st to 10th	Night	8% 9% 3 8% 4 4 5% 4 5% 9% 40%	2580
	11th to 20th	Night	6% 8% 3 7% 5% 4 6% 6% 8% 9% 39%	2800
	21st to end	Night	5% 8% 3 6% 4 5% 6% 4 6% 11% 42%	2800
December	1st to 10th	Night	2 8% 3 6% 4 4 5% 4 6% 9% 49%	4757
	11th to 20th	Night	4 9% 4 9% 6% 5% 6% 4 7% 10% 35%	5546
	21st to end	Night	3 8% 4 9% 5% 5% 6% 4 7% 11% 38%	7458

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	53% 6% 18% 21% 2	3179
	11th to 20th	Day	49% 5% 21% 17% 8% 1	3548
	21st to end	Day	36% 4 25% 27% 7%	4156
February	1st to 10th	Day	54% 8% 17% 15% 5% 1	1800
	11th to 20th	Day	50% 6% 18% 18% 7%	1764
	21st to end	Day	47% 6% 15% 21% 9% 2	1600
March	1st to 10th	Day	32% 5% 14% 33% 15%	3362
	11th to 20th	Day	34% 4 24% 32% 6%	3600
	21st to end	Day	54% 5% 17% 20% 3	3892
April	1st to 10th	Day	48% 6% 16% 21% 8% 1	2241
	11th to 20th	Day	50% 5% 13% 24% 8% 1	2440
	21st to end	Day	50% 4 15% 22% 8% 1	2470
May	1st to 10th	Day	52% 3 11% 26% 7% 1	4110
	11th to 20th	Day	37% 5% 14% 27% 13% 3	4315
	21st to end	Day	46% 4 15% 25% 10%	5088
June	1st to 10th	Day	43% 6% 20% 21% 9% 1	2850
	11th to 20th	Day	41% 6% 15% 19% 13% 6%	2850
	21st to end	Day	25% 5% 12% 32% 22% 4	2850
July	1st to 10th	Day	17% 4 8% 29% 29% 13%	4666
	11th to 20th	Day	15% 2 5% 16% 25% 37%	4659
	21st to end	Day	16% 3 5% 20% 33% 23%	4570
August	1st to 10th	Day	26% 6% 11% 23% 22% 13%	4078
	11th to 20th	Day	22% 5% 11% 34% 23% 5%	4067
	21st to end	Day	18% 4 10% 30% 29% 9%	4520
September	1st to 10th	Day	24% 3 9% 28% 25% 11%	2285
	11th to 20th	Day	31% 3 12% 30% 20% 4	2090
	21st to end	Day	29% 3 15% 30% 19% 4	2090
October	1st to 10th	Day	28% 2 9% 35% 25% 1	3278
	11th to 20th	Day	26% 4 19% 33% 16% 1	3150
	21st to end	Day	35% 6% 16% 28% 12% 3	3162
November	1st to 10th	Day	45% 6% 15% 25% 8% 1	1699
	11th to 20th	Day	42% 7% 21% 22% 7%	1600
	21st to end	Day	39% 6% 19% 28% 7% 1	1600
December	1st to 10th	Day	32% 6% 27% 27% 8%	2422
	11th to 20th	Day	40% 7% 29% 19% 4	2644
	21st to end	Day	42% 5% 19% 22% 11%	3498
January	1st to 10th	Night	46% 6% 20% 25% 2	6766
	11th to 20th	Night	48% 5% 20% 19% 8% 1	6440
	21st to end	Night	34% 6% 27% 25% 8%	6804
February	1st to 10th	Night	55% 5% 15% 18% 7%	2600
	11th to 20th	Night	47% 7% 20% 20% 6%	2533
	21st to end	Night	45% 8% 17% 20% 10% 1	2026
March	1st to 10th	Night	33% 6% 14% 27% 19%	3731
	11th to 20th	Night	34% 4 24% 32% 6%	3600
	21st to end	Night	57% 5% 17% 19% 3	3899
April	1st to 10th	Night	43% 7% 17% 24% 8% 1	1937
	11th to 20th	Night	50% 6% 15% 22% 6% 1	1691
	21st to end	Night	46% 5% 16% 26% 7%	1710
May	1st to 10th	Night	52% 4 12% 23% 7% 1	2860
	11th to 20th	Night	32% 4 14% 29% 20% 1	2592
	21st to end	Night	43% 4 19% 26% 7% 1	2486
June	1st to 10th	Night	42% 6% 19% 23% 9%	1330
	11th to 20th	Night	39% 4 17% 22% 13% 5%	1330
	21st to end	Night	23% 4 17% 33% 21% 2	1330
July	1st to 10th	Night	14% 3 9% 32% 29% 13%	2185
	11th to 20th	Night	11% 2 6% 19% 25% 37%	2178
	21st to end	Night	16% 3 7% 24% 26% 25%	2739
August	1st to 10th	Night	27% 5% 11% 23% 23% 12%	2838
	11th to 20th	Night	24% 6% 12% 31% 21% 6%	2838
	21st to end	Night	19% 4 10% 36% 24% 6%	3146
September	1st to 10th	Night	25% 3 10% 26% 21% 14%	1892
	11th to 20th	Night	32% 4 12% 29% 19% 5%	2090
	21st to end	Night	30% 3 13% 29% 21% 4	2090
October	1st to 10th	Night	26% 2 9% 33% 26% 3	3694
	11th to 20th	Night	30% 3 20% 30% 16% 1	3778
	21st to end	Night	32% 5% 16% 27% 19% 2	4394
November	1st to 10th	Night	41% 7% 15% 26% 8% 2	2580
	11th to 20th	Night	40% 7% 20% 26% 7%	2800
	21st to end	Night	37% 6% 22% 26% 9% 1	2800
December	1st to 10th	Night	32% 7% 25% 28% 7% 1	4757
	11th to 20th	Night	45% 6% 27% 18% 4	5547
	21st to end	Night	40% 6% 21% 21% 11%	7458

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%	200
	11th to 20th	Day	12% 7% 30% 36% 13% 4	200
	21st to end	Day	10% 11% 29% 37% 11% 4	251
February	1st to 10th	Day	16% 14% 41% 24% 5%	300
	11th to 20th	Day	12% 11% 34% 33% 9% 1	300
	21st to end	Day	11% 17% 37% 28% 6% 1	364
March	1st to 10th	Day	11% 14% 36% 29% 8% 1	500
	11th to 20th	Day	10% 15% 29% 28% 16% 2	500
	21st to end	Day	15% 21% 29% 23% 12% 1	531
April	1st to 10th	Day	14% 17% 37% 24% 7% 1	460
	11th to 20th	Day	17% 22% 29% 23% 8% 2	452
	21st to end	Day	18% 16% 30% 22% 12% 1	460
May	1st to 10th	Day	15% 14% 24% 29% 14% 4	463
	11th to 20th	Day	18% 8% 21% 30% 18% 5%	624
	21st to end	Day	20% 10% 18% 25% 21% 6%	734
June	1st to 10th	Day	13% 11% 23% 28% 21% 4	680
	11th to 20th	Day	18% 14% 26% 27% 12% 2	680
	21st to end	Day	8% 9% 22% 34% 23% 4	680
July	1st to 10th	Day	5% 6% 23% 34% 28% 4	671
	11th to 20th	Day	5% 7% 25% 37% 23% 3	668
	21st to end	Day	4 5% 24% 39% 22% 6%	537
August	1st to 10th	Day	6% 8% 27% 32% 24% 4	487
	11th to 20th	Day	5% 6% 23% 37% 22% 7%	477
	21st to end	Day	6% 9% 24% 32% 22% 6%	539
September	1st to 10th	Day	8% 8% 29% 28% 20% 7%	490
	11th to 20th	Day	9% 12% 27% 28% 19% 6%	490
	21st to end	Day	14% 11% 24% 27% 17% 6%	490
October	1st to 10th	Day	13% 9% 25% 29% 16% 7%	369
	11th to 20th	Day	12% 12% 24% 27% 16% 9%	300
	21st to end	Day	12% 15% 31% 29% 13% 1	330
November	1st to 10th	Day	17% 10% 28% 29% 15% 1	300
	11th to 20th	Day	10% 13% 38% 28% 9% 1	300
	21st to end	Day	12% 11% 28% 39% 9% 1	300
December	1st to 10th	Day	8% 11% 33% 34% 13% 1	240
	11th to 20th	Day	13% 12% 30% 30% 15% 1	200
	21st to end	Day	13% 12% 27% 32% 10% 5%	220
January	1st to 10th	Night	10% 10% 32% 38% 10% 1	1000
	11th to 20th	Night	11% 12% 33% 33% 9% 2	855
	21st to end	Night	9% 10% 31% 40% 9% 1	880
February	1st to 10th	Night	13% 13% 32% 34% 7% 1	800
	11th to 20th	Night	10% 12% 32% 34% 10% 1	784
	21st to end	Night	9% 13% 32% 34% 10% 2	660
March	1st to 10th	Night	7% 6% 37% 41% 9% 1	800
	11th to 20th	Night	9% 9% 31% 38% 13% 1	800
	21st to end	Night	11% 9% 31% 36% 12% 1	862
April	1st to 10th	Night	15% 8% 25% 40% 11% 1	684
	11th to 20th	Night	15% 9% 28% 37% 11% 1	563
	21st to end	Night	18% 8% 22% 38% 13% 1	570
May	1st to 10th	Night	14% 8% 23% 35% 18% 1	570
	11th to 20th	Night	17% 6% 17% 34% 24% 2	561
	21st to end	Night	21% 4 13% 33% 25% 3	458
June	1st to 10th	Night	17% 5% 15% 37% 25% 2	380
	11th to 20th	Night	14% 5% 30% 38% 12% 1	380
	21st to end	Night	6% 4 14% 50% 26%	380
July	1st to 10th	Night	4 2 12% 52% 29% 1	370
	11th to 20th	Night	2 3 12% 64% 18%	368
	21st to end	Night	4 1 10% 57% 27% 2	456
August	1st to 10th	Night	6% 3 14% 52% 25%	567
	11th to 20th	Night	5% 3 16% 47% 28% 2	564
	21st to end	Night	8% 3 17% 43% 27% 1	627
September	1st to 10th	Night	8% 3 19% 44% 22% 4	624
	11th to 20th	Night	10% 3 15% 41% 29% 3	760
	21st to end	Night	15% 4 19% 38% 21% 3	760
October	1st to 10th	Night	16% 4 19% 41% 18% 2	760
	11th to 20th	Night	10% 7% 26% 37% 18% 2	760
	21st to end	Night	12% 10% 28% 36% 13% 1	836
November	1st to 10th	Night	13% 8% 31% 38% 9% 1	825
	11th to 20th	Night	11% 13% 34% 35% 7%	1000
	21st to end	Night	9% 9% 33% 41% 8%	1000
December	1st to 10th	Night	10% 11% 32% 37% 10% 1	1000
	11th to 20th	Night	9% 9% 35% 38% 9% 1	1000
	21st to end	Night	9% 11% 36% 34% 10% 1	1100

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