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 2024-10-20.

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% 5% 5% 5% 5% 7% 8% 45%	101431
	Night	6% 8% 3 7% 5% 5% 5% 4 6% 9% 43%	97421
Transparency	Day	36% 5% 15% 25% 14% 6%	101165
	Night	37% 5% 17% 25% 12% 4	97277
Seeing	Day	12% 12% 27% 30% 16% 4	15334
	Night	11% 8% 26% 39% 15% 1	24666

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 9% 6% 6% 6% 7% 8% 12% 30%	2219
	11th to 20th	Day	3 10% 4 9% 7% 8% 8% 5% 7% 8% 31%	2476
	21st to end	Day	3 9% 3 7% 5% 5% 8% 7% 8% 13% 32%	2890
February	1st to 10th	Day	5% 15% 4 9% 7% 6% 6% 5% 6% 9% 26%	1710
	11th to 20th	Day	3 12% 4 13% 6% 5% 6% 4 6% 8% 33%	1665
	21st to end	Day	3 11% 4 11% 7% 6% 5% 5% 8% 9% 31%	1522
March	1st to 10th	Day	2 5% 2 7% 5% 5% 5% 5% 6% 11% 46%	3258
	11th to 20th	Day	3 7% 3 6% 5% 4 5% 5% 5% 9% 49%	3490
	21st to end	Day	5% 12% 5% 10% 7% 7% 7% 5% 7% 9% 26%	3825
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	11% 8% 2 6% 4 3 5% 3 5% 8% 44%	1969
	11th to 20th	Day	7% 7% 2 5% 4 4 5% 5% 6% 8% 47%	1890
	21st to end	Day	6% 7% 2 7% 5% 5% 6% 5% 7% 10% 40%	1818
November	1st to 10th	Day	6% 11% 3 7% 5% 6% 6% 5% 7% 8% 36%	1613
	11th to 20th	Day	5% 9% 3 7% 6% 5% 7% 5% 6% 10% 37%	1520
	21st to end	Day	4 10% 2 7% 4 5% 5% 3 8% 9% 41%	1520
December	1st to 10th	Day	5% 8% 3 9% 6% 5% 5% 5% 8% 9% 35%	1406
	11th to 20th	Day	2 8% 3 8% 6% 6% 8% 6% 7% 10% 36%	1685
	21st to end	Day	6% 8% 4 7% 4 5% 6% 5% 8% 10% 38%	2442
January	1st to 10th	Night	3 9% 3 7% 5% 5% 6% 6% 8% 14% 33%	4726
	11th to 20th	Night	3 9% 4 11% 8% 8% 7% 5% 6% 9% 31%	4512
	21st to end	Night	3 9% 4 8% 4 5% 6% 5% 9% 16% 31%	4766
February	1st to 10th	Night	5% 15% 6% 12% 7% 6% 4 4 6% 6% 30%	2470
	11th to 20th	Night	4 12% 4 10% 6% 6% 5% 4 7% 8% 33%	2402
	21st to end	Night	3 12% 4 10% 6% 6% 6% 5% 7% 8% 34%	1928
March	1st to 10th	Night	2 7% 3 6% 5% 5% 5% 5% 7% 10% 46%	3615
	11th to 20th	Night	3 9% 3 5% 4 5% 4 4 7% 8% 48%	3490
	21st to end	Night	6% 15% 5% 11% 6% 6% 7% 5% 7% 8% 25%	3828
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	12% 9% 2 5% 3 4 3 3 5% 7% 49%	2210
	11th to 20th	Night	7% 7% 2 5% 5% 4 3 4 5% 8% 51%	2274
	21st to end	Night	6% 10% 3 7% 5% 4 5% 4 7% 7% 43%	2536
November	1st to 10th	Night	8% 10% 3 8% 4 4 5% 4 5% 10% 39%	2447
	11th to 20th	Night	6% 8% 3 7% 5% 4 5% 6% 8% 9% 39%	2660
	21st to end	Night	5% 8% 3 6% 4 5% 5% 4 6% 11% 43%	2660
December	1st to 10th	Night	3 10% 4 8% 5% 5% 6% 5% 7% 10% 36%	2769
	11th to 20th	Night	2 9% 3 10% 6% 5% 6% 4 7% 11% 37%	3506
	21st to end	Night	3 8% 4 8% 5% 5% 5% 4 7% 11% 41%	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	50% 6% 19% 18% 6%	1674
	21st to end	Day	48% 6% 15% 20% 9% 2	1522
March	1st to 10th	Day	32% 5% 14% 34% 14%	3258
	11th to 20th	Day	34% 4 24% 32% 6%	3490
	21st to end	Day	55% 5% 17% 20% 3	3825
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	40% 3 14% 29% 13% 1	1969
	11th to 20th	Day	35% 6% 17% 29% 11% 2	1890
	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	47% 7% 21% 20% 5%	2403
	21st to end	Night	45% 8% 16% 20% 10% 1	1928
March	1st to 10th	Night	33% 6% 14% 27% 19%	3615
	11th to 20th	Night	34% 4 24% 32% 6%	3490
	21st to end	Night	57% 5% 17% 19% 2	3828
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	37% 4 15% 28% 15% 2	2210
	11th to 20th	Night	34% 3 20% 28% 13% 2	2274
	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	12% 12% 36% 32% 7% 1	270
	21st to end	Day	12% 17% 35% 29% 6% 1	334
March	1st to 10th	Day	10% 15% 37% 29% 8% 1	460
	11th to 20th	Day	11% 15% 30% 26% 17% 2	460
	21st to end	Day	15% 20% 29% 23% 12% 1	506
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% 8%	297
	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	744
	21st to end	Night	9% 13% 31% 34% 11% 2	628
March	1st to 10th	Night	7% 6% 37% 41% 8% 1	760
	11th to 20th	Night	9% 9% 30% 38% 13% 1	760
	21st to end	Night	11% 9% 30% 36% 13% 1	836
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	758
	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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