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 2024-02-15.

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	7% 7% 3 5% 6% 4 6% 6% 7% 9% 42%	77979
	Night	6% 8% 3 6% 5% 3 5% 6% 6% 10% 42%	81107
Transparency	Day	38% 5% 16% 24% 13% 4	77673
	Night	39% 5% 18% 24% 11% 3	80935
Seeing	Day	12% 12% 27% 29% 16% 3	14298
	Night	11% 8% 26% 39% 15% 1	23895

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	3 9% 4 8% 7% 4 6% 8% 8% 12% 30%	2219
	11th to 20th	Day	3 9% 4 7% 8% 6% 8% 7% 7% 8% 31%	2476
	21st to end	Day	3 9% 3 6% 6% 4 7% 8% 8% 13% 32%	2890
February	1st to 10th	Day	6% 13% 6% 8% 9% 5% 6% 7% 7% 9% 27%	1710
	11th to 20th	Day	4 12% 5% 11% 7% 4 5% 5% 6% 8% 33%	1611
	21st to end	Day	3 10% 5% 9% 8% 5% 7% 6% 8% 9% 31%	1435
March	1st to 10th	Day	4 6% 3 8% 8% 4 6% 6% 6% 12% 36%	1868
	11th to 20th	Day	5% 9% 4 6% 7% 4 6% 8% 6% 9% 38%	1980
	21st to end	Day	5% 10% 4 7% 7% 4 8% 7% 7% 10% 31%	2178
April	1st to 10th	Day	6% 10% 3 7% 7% 5% 6% 7% 7% 8% 33%	2123
	11th to 20th	Day	9% 12% 4 8% 7% 4 6% 6% 6% 8% 31%	2307
	21st to end	Day	11% 11% 3 7% 5% 4 5% 6% 5% 8% 34%	2340
May	1st to 10th	Day	8% 7% 3 7% 6% 4 5% 6% 7% 9% 39%	2340
	11th to 20th	Day	13% 6% 2 5% 7% 4 7% 6% 8% 8% 35%	2429
	21st to end	Day	17% 8% 3 7% 6% 4 7% 6% 7% 9% 28%	2870
June	1st to 10th	Day	12% 5% 3 6% 6% 4 6% 7% 8% 11% 34%	2700
	11th to 20th	Day	14% 5% 2 4 6% 4 6% 7% 7% 9% 37%	2700
	21st to end	Day	5% 3 1 3 4 3 5% 6% 7% 7% 55%	2700
July	1st to 10th	Day	3 3 1 2 4 3 5% 5% 7% 9% 57%	2700
	11th to 20th	Day	4 2 1 3 3 3 5% 5% 5% 6% 64%	2700
	21st to end	Day	3 2 1 2 3 3 5% 5% 6% 8% 63%	2707
August	1st to 10th	Day	4 3 1 3 4 4 5% 5% 6% 8% 56%	2328
	11th to 20th	Day	4 2 1 3 4 3 4 5% 6% 7% 62%	2299
	21st to end	Day	6% 3 1 2 3 2 4 5% 6% 8% 59%	2574
September	1st to 10th	Day	7% 3 1 3 4 3 4 4 4 7% 62%	2165
	11th to 20th	Day	7% 4 2 4 5% 3 4 4 6% 9% 53%	1980
	21st to end	Day	10% 5% 2 3 4 2 3 4 6% 7% 54%	1980
October	1st to 10th	Day	12% 8% 3 5% 5% 3 5% 4 5% 9% 42%	1866
	11th to 20th	Day	7% 7% 2 5% 5% 4 5% 6% 6% 8% 46%	1800
	21st to end	Day	6% 7% 2 6% 6% 4 6% 7% 7% 10% 41%	1818
November	1st to 10th	Day	6% 10% 4 6% 6% 5% 6% 6% 7% 8% 36%	1613
	11th to 20th	Day	5% 8% 4 6% 7% 3 7% 6% 6% 10% 37%	1520
	21st to end	Day	5% 9% 3 6% 4 4 6% 4 8% 9% 41%	1520
December	1st to 10th	Day	5% 8% 3 8% 7% 4 5% 7% 8% 9% 36%	1406
	11th to 20th	Day	2 8% 4 7% 7% 4 8% 8% 7% 10% 36%	1685
	21st to end	Day	6% 8% 4 6% 5% 3 6% 6% 7% 11% 38%	2442
January	1st to 10th	Night	3 9% 3 6% 6% 4 5% 7% 8% 15% 33%	4726
	11th to 20th	Night	3 8% 4 9% 10% 6% 7% 6% 6% 9% 31%	4512
	21st to end	Night	3 8% 4 7% 5% 4 6% 7% 9% 16% 31%	4766
February	1st to 10th	Night	5% 14% 7% 10% 8% 4 5% 4 6% 7% 30%	2470
	11th to 20th	Night	4 11% 5% 9% 7% 5% 5% 6% 7% 9% 33%	2330
	21st to end	Night	3 11% 5% 9% 7% 4 7% 7% 6% 8% 33%	1817
March	1st to 10th	Night	3 9% 4 7% 7% 4 5% 6% 6% 9% 40%	2091
	11th to 20th	Night	5% 9% 4 5% 5% 3 5% 6% 7% 10% 40%	1980
	21st to end	Night	7% 12% 4 8% 6% 4 6% 7% 6% 9% 31%	2178
April	1st to 10th	Night	8% 9% 3 6% 5% 4 5% 6% 6% 9% 38%	1836
	11th to 20th	Night	9% 13% 4 7% 6% 4 6% 5% 6% 7% 33%	1601
	21st to end	Night	12% 11% 3 5% 5% 3 5% 5% 6% 8% 38%	1620
May	1st to 10th	Night	9% 7% 2 6% 4 4 5% 4 7% 8% 44%	1620
	11th to 20th	Night	14% 6% 2 4 5% 3 4 6% 6% 11% 39%	1463
	21st to end	Night	18% 7% 3 5% 6% 3 4 5% 6% 10% 33%	1402
June	1st to 10th	Night	13% 6% 3 5% 6% 3 4 6% 6% 8% 39%	1260
	11th to 20th	Night	14% 5% 2 5% 5% 3 4 4 4 8% 45%	1260
	21st to end	Night	4 3 1 4 4 3 3 5% 7% 8% 58%	1260
July	1st to 10th	Night	4 2 1 3 2 2 3 3 6% 10% 64%	1260
	11th to 20th	Night	2 2 1 2 3 2 3 4 5% 7% 68%	1260
	21st to end	Night	4 1 1 2 3 2 4 4 5% 9% 67%	1603
August	1st to 10th	Night	5% 3 1 3 4 3 4 5% 7% 6% 59%	1611
	11th to 20th	Night	5% 2 1 2 3 2 4 4 6% 8% 63%	1602
	21st to end	Night	7% 4 1 3 3 3 3 5% 7% 9% 56%	1782
September	1st to 10th	Night	7% 5% 1 3 4 2 4 5% 5% 7% 58%	1792
	11th to 20th	Night	8% 4 2 3 5% 2 4 5% 6% 8% 52%	1980
	21st to end	Night	14% 5% 1 3 3 2 3 4 4 7% 55%	1980
October	1st to 10th	Night	12% 9% 2 4 4 3 3 4 5% 7% 47%	2093
	11th to 20th	Night	7% 6% 3 5% 6% 3 3 4 5% 8% 50%	2160
	21st to end	Night	6% 9% 3 6% 5% 3 5% 5% 7% 7% 43%	2536
November	1st to 10th	Night	8% 9% 4 7% 5% 3 5% 5% 5% 10% 40%	2447
	11th to 20th	Night	6% 7% 3 6% 5% 3 5% 6% 8% 10% 39%	2660
	21st to end	Night	5% 7% 4 5% 5% 3 5% 6% 6% 11% 43%	2660
December	1st to 10th	Night	4 9% 4 7% 6% 4 6% 7% 6% 10% 37%	2769
	11th to 20th	Night	2 9% 4 9% 7% 4 6% 6% 7% 11% 37%	3506
	21st to end	Night	3 7% 4 7% 6% 4 6% 5% 7% 11% 42%	5214

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	47% 4 20% 25% 2	2219
	11th to 20th	Day	51% 6% 19% 14% 9% 1	2476
	21st to end	Day	40% 4 30% 22% 4	2890
February	1st to 10th	Day	54% 8% 18% 16% 4 1	1710
	11th to 20th	Day	51% 6% 19% 18% 6%	1620
	21st to end	Day	49% 6% 15% 19% 9% 2	1435
March	1st to 10th	Day	41% 7% 21% 23% 8%	1868
	11th to 20th	Day	42% 7% 18% 26% 7%	1980
	21st to end	Day	49% 8% 18% 20% 5%	2178
April	1st to 10th	Day	48% 7% 16% 21% 9% 1	2123
	11th to 20th	Day	51% 5% 13% 23% 7% 1	2310
	21st to end	Day	50% 4 15% 22% 8% 1	2340
May	1st to 10th	Day	41% 5% 14% 30% 9% 1	2340
	11th to 20th	Day	45% 7% 18% 21% 8% 2	2433
	21st to end	Day	53% 6% 19% 17% 5%	2870
June	1st to 10th	Day	44% 7% 20% 21% 8%	2700
	11th to 20th	Day	42% 6% 16% 19% 13% 5%	2700
	21st to end	Day	26% 5% 13% 32% 22% 3	2700
July	1st to 10th	Day	22% 5% 10% 31% 25% 7%	2626
	11th to 20th	Day	21% 4 7% 23% 25% 20%	2619
	21st to end	Day	19% 4 5% 24% 31% 18%	2556
August	1st to 10th	Day	26% 4 9% 28% 23% 11%	2313
	11th to 20th	Day	21% 4 10% 28% 29% 8%	2299
	21st to end	Day	22% 4 10% 27% 25% 10%	2574
September	1st to 10th	Day	24% 3 9% 28% 25% 11%	2165
	11th to 20th	Day	29% 3 12% 30% 21% 5%	1980
	21st to end	Day	30% 3 16% 30% 18% 4	1980
October	1st to 10th	Day	42% 4 15% 28% 11% 1	1866
	11th to 20th	Day	36% 6% 18% 28% 10% 2	1800
	21st to end	Day	38% 7% 20% 28% 6% 1	1818
November	1st to 10th	Day	45% 6% 15% 25% 8% 1	1613
	11th to 20th	Day	43% 7% 21% 23% 6%	1520
	21st to end	Day	39% 6% 19% 28% 7% 1	1520
December	1st to 10th	Day	43% 7% 21% 22% 6% 1	1406
	11th to 20th	Day	44% 7% 30% 17% 3	1684
	21st to end	Day	38% 6% 23% 20% 13%	2442
January	1st to 10th	Night	41% 5% 20% 30% 3	4726
	11th to 20th	Night	51% 5% 18% 19% 7% 1	4512
	21st to end	Night	39% 6% 28% 21% 4 1	4766
February	1st to 10th	Night	55% 6% 15% 19% 6%	2470
	11th to 20th	Night	48% 7% 21% 19% 5%	2331
	21st to end	Night	47% 8% 15% 19% 10% 1	1817
March	1st to 10th	Night	41% 6% 20% 23% 9%	2091
	11th to 20th	Night	38% 6% 22% 25% 8%	1980
	21st to end	Night	48% 7% 19% 21% 4	2178
April	1st to 10th	Night	43% 7% 17% 25% 8% 1	1836
	11th to 20th	Night	50% 6% 15% 21% 7% 1	1601
	21st to end	Night	45% 5% 17% 26% 6%	1620
May	1st to 10th	Night	38% 6% 17% 29% 8% 1	1620
	11th to 20th	Night	41% 5% 19% 23% 11% 1	1464
	21st to end	Night	47% 5% 23% 19% 5%	1402
June	1st to 10th	Night	43% 7% 19% 23% 8%	1260
	11th to 20th	Night	40% 4 18% 22% 12% 5%	1260
	21st to end	Night	24% 4 17% 33% 21% 1	1260
July	1st to 10th	Night	18% 3 14% 33% 22% 10%	1225
	11th to 20th	Night	15% 3 9% 28% 27% 17%	1218
	21st to end	Night	16% 3 7% 27% 28% 18%	1514
August	1st to 10th	Night	24% 4 9% 27% 25% 11%	1602
	11th to 20th	Night	20% 5% 12% 26% 28% 10%	1602
	21st to end	Night	25% 4 10% 28% 23% 9%	1782
September	1st to 10th	Night	26% 4 10% 25% 21% 14%	1792
	11th to 20th	Night	31% 4 11% 29% 20% 5%	1980
	21st to end	Night	32% 3 13% 29% 20% 4	1980
October	1st to 10th	Night	38% 4 16% 28% 13% 1	2093
	11th to 20th	Night	35% 3 20% 29% 11% 2	2160
	21st to end	Night	40% 5% 19% 23% 12% 1	2536
November	1st to 10th	Night	42% 7% 15% 26% 8% 2	2447
	11th to 20th	Night	40% 7% 20% 26% 7%	2660
	21st to end	Night	36% 5% 21% 26% 9% 1	2660
December	1st to 10th	Night	43% 8% 19% 22% 6% 1	2769
	11th to 20th	Night	42% 7% 27% 21% 4	3507
	21st to end	Night	37% 5% 22% 22% 13%	5214

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% 12% 26% 32% 17% 4	180
	11th to 20th	Day	12% 7% 26% 37% 14% 4	180
	21st to end	Day	10% 11% 30% 35% 11% 4	225
February	1st to 10th	Day	17% 14% 39% 24% 5%	270
	11th to 20th	Day	14% 13% 36% 31% 6%	252
	21st to end	Day	11% 17% 35% 31% 6% 1	301
March	1st to 10th	Day	11% 15% 39% 29% 5%	420
	11th to 20th	Day	12% 15% 31% 25% 15% 2	420
	21st to end	Day	16% 21% 28% 23% 11% 1	462
April	1st to 10th	Day	15% 17% 36% 24% 8% 1	420
	11th to 20th	Day	18% 23% 30% 21% 7% 1	412
	21st to end	Day	20% 15% 30% 21% 12% 1	420
May	1st to 10th	Day	13% 15% 26% 29% 14% 4	423
	11th to 20th	Day	19% 8% 19% 30% 19% 5%	576
	21st to end	Day	21% 10% 17% 25% 21% 7%	679
June	1st to 10th	Day	13% 11% 22% 28% 22% 5%	630
	11th to 20th	Day	19% 13% 26% 27% 12% 2	630
	21st to end	Day	8% 9% 23% 33% 24% 4	630
July	1st to 10th	Day	5% 7% 22% 34% 28% 4	621
	11th to 20th	Day	6% 7% 26% 36% 23% 3	618
	21st to end	Day	4 5% 25% 39% 23% 5%	493
August	1st to 10th	Day	6% 8% 28% 32% 23% 4	447
	11th to 20th	Day	5% 7% 24% 37% 21% 7%	437
	21st to end	Day	7% 9% 25% 32% 21% 6%	495
September	1st to 10th	Day	8% 8% 30% 29% 19% 5%	450
	11th to 20th	Day	9% 12% 28% 26% 19% 6%	450
	21st to end	Day	15% 12% 24% 27% 16% 6%	450
October	1st to 10th	Day	14% 9% 25% 28% 16% 7%	336
	11th to 20th	Day	13% 13% 26% 27% 14% 7%	270
	21st to end	Day	13% 16% 29% 28% 13% 1	297
November	1st to 10th	Day	18% 11% 30% 29% 12%	270
	11th to 20th	Day	11% 13% 38% 28% 10% 1	270
	21st to end	Day	12% 12% 27% 39% 10% 1	270
December	1st to 10th	Day	8% 12% 34% 32% 13% 1	216
	11th to 20th	Day	13% 12% 31% 28% 15% 1	180
	21st to end	Day	14% 12% 27% 31% 11% 6%	198
January	1st to 10th	Night	10% 10% 32% 37% 10% 1	950
	11th to 20th	Night	11% 12% 33% 32% 10% 2	813
	21st to end	Night	9% 10% 31% 40% 9% 1	836
February	1st to 10th	Night	13% 14% 32% 33% 7% 1	760
	11th to 20th	Night	10% 12% 32% 34% 10% 1	722
	21st to end	Night	9% 13% 30% 35% 11% 2	592
March	1st to 10th	Night	7% 6% 37% 41% 8% 1	720
	11th to 20th	Night	10% 9% 30% 37% 13% 1	720
	21st to end	Night	11% 9% 31% 35% 13% 1	792
April	1st to 10th	Night	15% 8% 24% 40% 12% 1	648
	11th to 20th	Night	15% 9% 27% 36% 11% 1	533
	21st to end	Night	18% 9% 21% 37% 14% 1	540
May	1st to 10th	Night	12% 8% 23% 36% 19% 1	540
	11th to 20th	Night	18% 6% 16% 34% 24% 2	531
	21st to end	Night	22% 4 13% 32% 26% 3	434
June	1st to 10th	Night	18% 4 14% 37% 26% 2	360
	11th to 20th	Night	15% 5% 28% 38% 13% 1	360
	21st to end	Night	6% 4 14% 49% 28%	360
July	1st to 10th	Night	4 1 12% 52% 29% 1	350
	11th to 20th	Night	3 3 12% 63% 18%	348
	21st to end	Night	3 1 10% 56% 28% 2	431
August	1st to 10th	Night	6% 3 13% 53% 25%	537
	11th to 20th	Night	5% 3 16% 46% 28% 2	534
	21st to end	Night	8% 3 16% 43% 28% 1	594
September	1st to 10th	Night	8% 3 19% 44% 21% 4	591
	11th to 20th	Night	9% 3 15% 40% 30% 3	720
	21st to end	Night	15% 4 18% 38% 21% 3	720
October	1st to 10th	Night	17% 4 19% 40% 18% 3	720
	11th to 20th	Night	10% 8% 27% 37% 17% 2	720
	21st to end	Night	13% 10% 28% 35% 13% 1	792
November	1st to 10th	Night	14% 8% 32% 37% 9% 1	782
	11th to 20th	Night	11% 13% 33% 36% 7%	950
	21st to end	Night	9% 9% 32% 42% 8%	950
December	1st to 10th	Night	10% 11% 31% 37% 11% 1	950
	11th to 20th	Night	8% 9% 35% 38% 9% 1	950
	21st to end	Night	9% 11% 35% 33% 10% 1	1045

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