Helmer Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Helmer Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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	14% 11% 4 10% 5% 5% 4 3 5% 7% 32%	125408
	Night	14% 12% 5% 11% 6% 5% 5% 3 5% 6% 28%	122851
Transparency	Day	51% 6% 12% 20% 10% 1	128862
	Night	55% 6% 11% 18% 8% 1	127471
Seeing	Day	20% 16% 33% 20% 8% 2	16727
	Night	20% 12% 31% 26% 9% 1	26271

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% 14% 7% 31% 9% 6% 4 3 4 4 10%	3617
	11th to 20th	Day	11% 14% 5% 20% 10% 9% 6% 4 5% 5% 10%	3589
	21st to end	Day	9% 18% 8% 17% 9% 7% 6% 4 4 5% 14%	3982
February	1st to 10th	Day	8% 14% 5% 12% 11% 8% 6% 5% 7% 6% 18%	1645
	11th to 20th	Day	12% 16% 6% 15% 10% 7% 5% 4 5% 4 16%	1921
	21st to end	Day	11% 15% 8% 15% 8% 7% 5% 3 5% 5% 19%	1650
March	1st to 10th	Day	13% 11% 4 12% 8% 5% 4 3 5% 7% 28%	4478
	11th to 20th	Day	13% 13% 5% 14% 7% 5% 5% 3 5% 6% 24%	4812
	21st to end	Day	20% 13% 8% 14% 7% 4 4 2 4 4 18%	5731
April	1st to 10th	Day	17% 15% 3 11% 5% 4 4 3 4 6% 28%	2400
	11th to 20th	Day	18% 13% 5% 8% 4 3 3 4 5% 5% 31%	2487
	21st to end	Day	16% 12% 3 8% 5% 4 4 4 5% 7% 33%	2600
May	1st to 10th	Day	14% 9% 3 6% 4 3 4 3 4 6% 44%	5888
	11th to 20th	Day	14% 8% 2 6% 3 3 4 3 5% 7% 45%	6272
	21st to end	Day	12% 9% 3 7% 4 4 4 4 5% 8% 41%	7036
June	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 4 6% 8% 41%	3000
	11th to 20th	Day	14% 7% 2 7% 4 4 4 4 6% 8% 38%	3000
	21st to end	Day	16% 8% 2 6% 5% 4 5% 3 6% 7% 39%	3000
July	1st to 10th	Day	12% 7% 2 5% 4 4 4 3 6% 8% 44%	6437
	11th to 20th	Day	9% 6% 2 6% 3 4 4 3 6% 8% 48%	6360
	21st to end	Day	10% 6% 2 5% 4 4 4 4 6% 9% 47%	6926
August	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 4 7% 8% 42%	4416
	11th to 20th	Day	14% 6% 2 6% 3 4 4 3 5% 7% 46%	4044
	21st to end	Day	15% 7% 3 6% 4 4 3 3 5% 8% 42%	4373
September	1st to 10th	Day	16% 7% 2 6% 4 4 5% 3 5% 8% 39%	2275
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 9% 40%	2090
	21st to end	Day	16% 10% 3 7% 5% 4 4 4 6% 7% 35%	2090
October	1st to 10th	Day	19% 8% 3 9% 5% 4 5% 4 6% 6% 32%	2090
	11th to 20th	Day	22% 15% 4 10% 6% 4 4 4 6% 6% 20%	1754
	21st to end	Day	24% 17% 3 10% 6% 6% 4 4 5% 6% 15%	1881
November	1st to 10th	Day	19% 15% 5% 7% 5% 4 5% 4 5% 5% 25%	1762
	11th to 20th	Day	16% 19% 6% 13% 5% 5% 4 3 5% 6% 18%	1800
	21st to end	Day	18% 20% 7% 13% 8% 6% 5% 4 4 4 11%	1487
December	1st to 10th	Day	14% 21% 7% 18% 7% 5% 5% 3 4 4 10%	2280
	11th to 20th	Day	12% 21% 6% 17% 9% 7% 5% 3 5% 5% 10%	2621
	21st to end	Day	25% 23% 9% 12% 7% 4 3 2 3 3 9%	3614
January	1st to 10th	Night	10% 15% 6% 30% 8% 5% 4 2 3 3 14%	6348
	11th to 20th	Night	8% 17% 6% 20% 12% 8% 6% 4 4 4 12%	6316
	21st to end	Night	9% 14% 8% 17% 9% 8% 6% 4 5% 5% 14%	6996
February	1st to 10th	Night	8% 16% 5% 12% 10% 8% 5% 5% 5% 6% 20%	2753
	11th to 20th	Night	10% 15% 6% 14% 10% 6% 5% 3 4 5% 21%	2363
	21st to end	Night	11% 15% 6% 14% 7% 5% 5% 4 6% 6% 22%	1980
March	1st to 10th	Night	15% 10% 3 11% 6% 5% 4 3 6% 8% 29%	5362
	11th to 20th	Night	12% 12% 5% 11% 5% 5% 5% 4 6% 8% 27%	5146
	21st to end	Night	19% 13% 4 9% 6% 4 4 4 5% 6% 26%	5243
April	1st to 10th	Night	17% 14% 3 7% 5% 5% 4 4 7% 6% 28%	2000
	11th to 20th	Night	19% 10% 4 8% 4 3 4 4 5% 5% 35%	1862
	21st to end	Night	17% 10% 3 8% 5% 4 5% 3 5% 5% 37%	1800
May	1st to 10th	Night	15% 9% 3 6% 4 3 4 3 4 6% 44%	4053
	11th to 20th	Night	14% 8% 3 7% 4 4 4 3 5% 6% 41%	3626
	21st to end	Night	15% 9% 2 6% 4 3 4 4 5% 8% 40%	3857
June	1st to 10th	Night	13% 6% 3 6% 4 4 4 5% 7% 8% 41%	1400
	11th to 20th	Night	14% 7% 2 6% 4 4 5% 3 8% 8% 40%	1400
	21st to end	Night	16% 8% 2 6% 3 3 3 3 5% 7% 44%	1400
July	1st to 10th	Night	13% 5% 2 6% 4 4 4 4 6% 9% 42%	3523
	11th to 20th	Night	9% 5% 2 5% 3 4 4 3 5% 7% 52%	3620
	21st to end	Night	7% 5% 2 5% 3 3 4 3 4 8% 55%	3948
August	1st to 10th	Night	11% 7% 2 7% 5% 5% 5% 3 6% 7% 44%	2610
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 4 6% 52%	2785
	21st to end	Night	11% 6% 2 6% 4 3 4 3 5% 8% 48%	3279
September	1st to 10th	Night	12% 7% 2 6% 4 4 4 4 5% 6% 46%	1900
	11th to 20th	Night	14% 7% 2 6% 4 3 4 3 5% 8% 44%	2076
	21st to end	Night	16% 7% 2 6% 4 4 4 3 6% 7% 40%	2090
October	1st to 10th	Night	17% 9% 2 7% 4 4 4 4 5% 6% 37%	2090
	11th to 20th	Night	19% 13% 3 7% 5% 4 5% 4 6% 6% 27%	2421
	21st to end	Night	22% 14% 4 8% 5% 5% 5% 4 6% 6% 21%	2717
November	1st to 10th	Night	16% 15% 4 9% 5% 5% 5% 3 4 5% 28%	2536
	11th to 20th	Night	17% 16% 5% 12% 5% 4 5% 4 7% 5% 20%	2600
	21st to end	Night	16% 18% 7% 11% 7% 5% 5% 3 5% 5% 17%	2908
December	1st to 10th	Night	14% 15% 7% 17% 7% 6% 5% 4 5% 6% 13%	4910
	11th to 20th	Night	12% 17% 6% 19% 7% 5% 4 3 5% 5% 16%	5586
	21st to end	Night	22% 21% 12% 15% 6% 4 4 2 3 3 9%	7347

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	77% 6% 8% 7% 1	3618
	11th to 20th	Day	74% 8% 7% 10% 1	3620
	21st to end	Day	72% 6% 9% 7% 5%	3982
February	1st to 10th	Day	64% 8% 11% 11% 5%	1645
	11th to 20th	Day	69% 7% 10% 10% 5%	1931
	21st to end	Day	67% 6% 8% 13% 5%	1650
March	1st to 10th	Day	55% 7% 16% 12% 10% 1	4480
	11th to 20th	Day	61% 5% 13% 15% 6% 1	4812
	21st to end	Day	69% 5% 7% 12% 6% 1	5731
April	1st to 10th	Day	58% 6% 9% 19% 8% 1	2400
	11th to 20th	Day	54% 4 9% 21% 10% 1	2487
	21st to end	Day	51% 5% 10% 17% 13% 4	2600
May	1st to 10th	Day	41% 5% 9% 26% 17% 3	5897
	11th to 20th	Day	39% 6% 9% 26% 17% 2	6276
	21st to end	Day	40% 6% 11% 22% 16% 4	7036
June	1st to 10th	Day	40% 7% 12% 24% 15% 2	3000
	11th to 20th	Day	42% 7% 12% 23% 15% 1	3000
	21st to end	Day	43% 7% 14% 23% 11% 2	3000
July	1st to 10th	Day	37% 6% 15% 28% 15% 1	6369
	11th to 20th	Day	33% 7% 15% 33% 11% 1	6285
	21st to end	Day	33% 7% 14% 30% 16% 1	6728
August	1st to 10th	Day	38% 6% 13% 29% 12%	4401
	11th to 20th	Day	37% 6% 12% 33% 12%	4083
	21st to end	Day	41% 6% 15% 28% 10%	4372
September	1st to 10th	Day	43% 6% 12% 26% 12%	2275
	11th to 20th	Day	39% 8% 18% 28% 7%	2090
	21st to end	Day	46% 8% 16% 22% 9%	2090
October	1st to 10th	Day	39% 6% 14% 28% 12%	3480
	11th to 20th	Day	54% 5% 11% 22% 7%	2879
	21st to end	Day	63% 7% 14% 14% 3	3081
November	1st to 10th	Day	60% 6% 13% 16% 5%	1762
	11th to 20th	Day	66% 7% 13% 11% 4	1800
	21st to end	Day	76% 7% 10% 6% 1	1487
December	1st to 10th	Day	77% 6% 9% 6% 2	2280
	11th to 20th	Day	74% 7% 9% 8% 2	2621
	21st to end	Day	81% 5% 4 6% 4	3614
January	1st to 10th	Night	75% 5% 6% 10% 3	6348
	11th to 20th	Night	73% 8% 8% 8% 3	6360
	21st to end	Night	69% 8% 10% 10% 4	6996
February	1st to 10th	Night	63% 7% 9% 13% 7% 1	2753
	11th to 20th	Night	64% 6% 10% 15% 5%	2364
	21st to end	Night	62% 7% 9% 15% 6%	1980
March	1st to 10th	Night	52% 5% 18% 19% 6%	5373
	11th to 20th	Night	53% 6% 15% 19% 7% 1	5147
	21st to end	Night	58% 6% 11% 16% 8%	5243
April	1st to 10th	Night	54% 7% 9% 17% 11% 2	2000
	11th to 20th	Night	50% 5% 9% 24% 11% 2	1862
	21st to end	Night	50% 5% 10% 18% 15% 3	1800
May	1st to 10th	Night	41% 5% 9% 25% 17% 3	4060
	11th to 20th	Night	43% 7% 8% 25% 15% 2	3627
	21st to end	Night	43% 7% 10% 24% 10% 7%	3858
June	1st to 10th	Night	38% 7% 14% 24% 17% 2	1400
	11th to 20th	Night	40% 6% 14% 22% 14% 3	1400
	21st to end	Night	39% 6% 11% 28% 14% 2	1400
July	1st to 10th	Night	38% 6% 14% 25% 15% 1	3483
	11th to 20th	Night	30% 5% 15% 36% 12% 1	3572
	21st to end	Night	29% 5% 13% 30% 20% 2	3832
August	1st to 10th	Night	39% 6% 12% 28% 14% 1	2601
	11th to 20th	Night	34% 5% 15% 33% 12% 1	2812
	21st to end	Night	34% 7% 13% 31% 14% 1	3279
September	1st to 10th	Night	37% 7% 14% 27% 13% 1	1900
	11th to 20th	Night	38% 8% 17% 27% 10% 1	2076
	21st to end	Night	42% 7% 17% 26% 8%	2090
October	1st to 10th	Night	41% 5% 12% 32% 9% 1	3480
	11th to 20th	Night	48% 6% 14% 24% 8%	4022
	21st to end	Night	58% 6% 15% 18% 3	4467
November	1st to 10th	Night	58% 6% 12% 18% 6%	2536
	11th to 20th	Night	64% 8% 10% 12% 6%	2600
	21st to end	Night	67% 8% 14% 9% 2	2908
December	1st to 10th	Night	70% 8% 12% 9% 2	4910
	11th to 20th	Night	69% 6% 10% 10% 4	5585
	21st to end	Night	81% 4 6% 7% 2	7347

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	19% 13% 44% 20% 3	300
	11th to 20th	Day	22% 18% 41% 16% 3 1	300
	21st to end	Day	22% 19% 36% 18% 5%	330
February	1st to 10th	Day	19% 26% 38% 11% 5% 1	300
	11th to 20th	Day	22% 25% 38% 11% 4 1	300
	21st to end	Day	25% 29% 35% 9% 2	249
March	1st to 10th	Day	22% 25% 37% 12% 3	300
	11th to 20th	Day	24% 18% 35% 15% 7%	423
	21st to end	Day	29% 19% 29% 16% 6% 1	550
April	1st to 10th	Day	26% 18% 31% 20% 3 1	500
	11th to 20th	Day	26% 23% 25% 17% 8% 1	492
	21st to end	Day	24% 16% 32% 17% 9% 2	500
May	1st to 10th	Day	25% 18% 30% 16% 8% 4	503
	11th to 20th	Day	19% 17% 38% 17% 8% 2	704
	21st to end	Day	17% 17% 34% 21% 9% 2	789
June	1st to 10th	Day	17% 17% 37% 20% 8% 1	730
	11th to 20th	Day	18% 17% 35% 22% 5% 2	730
	21st to end	Day	22% 13% 33% 22% 7% 2	730
July	1st to 10th	Day	11% 11% 38% 27% 10% 3	721
	11th to 20th	Day	12% 12% 35% 26% 10% 5%	718
	21st to end	Day	12% 9% 36% 25% 13% 5%	794
August	1st to 10th	Day	12% 11% 33% 27% 12% 5%	659
	11th to 20th	Day	16% 5% 22% 30% 20% 7%	479
	21st to end	Day	21% 10% 28% 24% 12% 5%	539
September	1st to 10th	Day	21% 12% 27% 22% 13% 5%	490
	11th to 20th	Day	13% 11% 32% 27% 12% 5%	490
	21st to end	Day	22% 13% 22% 30% 11% 2	490
October	1st to 10th	Day	23% 17% 29% 19% 9% 3	490
	11th to 20th	Day	29% 17% 35% 16% 2 1	334
	21st to end	Day	35% 22% 30% 11% 2	330
November	1st to 10th	Day	24% 24% 31% 17% 3	300
	11th to 20th	Day	26% 25% 31% 13% 5%	300
	21st to end	Day	31% 26% 33% 9% 1	213
December	1st to 10th	Day	26% 19% 31% 17% 7% 2	200
	11th to 20th	Day	24% 20% 40% 12% 2 3	200
	21st to end	Day	34% 19% 29% 12% 5% 1	250
January	1st to 10th	Night	18% 19% 36% 22% 6%	1000
	11th to 20th	Night	21% 17% 34% 19% 8% 1	1000
	21st to end	Night	20% 14% 38% 22% 6% 1	1100
February	1st to 10th	Night	17% 17% 38% 22% 7%	953
	11th to 20th	Night	21% 17% 34% 23% 5% 1	783
	21st to end	Night	18% 16% 39% 22% 5%	660
March	1st to 10th	Night	17% 16% 36% 24% 8% 1	800
	11th to 20th	Night	20% 18% 34% 22% 6% 1	800
	21st to end	Night	21% 12% 32% 27% 8% 1	785
April	1st to 10th	Night	25% 12% 30% 26% 6% 1	600
	11th to 20th	Night	25% 10% 29% 26% 8% 1	593
	21st to end	Night	21% 11% 32% 28% 8% 1	600
May	1st to 10th	Night	17% 11% 33% 30% 8% 1	600
	11th to 20th	Night	20% 8% 32% 31% 7% 1	591
	21st to end	Night	16% 9% 29% 32% 14%	610
June	1st to 10th	Night	15% 8% 33% 35% 9%	400
	11th to 20th	Night	18% 9% 28% 35% 11%	400
	21st to end	Night	17% 7% 31% 35% 9% 1	400
July	1st to 10th	Night	11% 4 30% 40% 14% 1	540
	11th to 20th	Night	13% 4 26% 34% 21% 1	582
	21st to end	Night	11% 4 25% 39% 19% 2	657
August	1st to 10th	Night	10% 3 23% 43% 21%	579
	11th to 20th	Night	12% 4 19% 36% 26% 2	564
	21st to end	Night	15% 5% 26% 35% 17% 2	627
September	1st to 10th	Night	16% 5% 24% 34% 19% 3	570
	11th to 20th	Night	17% 9% 25% 29% 17% 3	746
	21st to end	Night	20% 8% 25% 32% 14% 2	760
October	1st to 10th	Night	24% 9% 29% 25% 12% 1	760
	11th to 20th	Night	25% 13% 31% 23% 9%	760
	21st to end	Night	32% 12% 25% 24% 5% 1	836
November	1st to 10th	Night	24% 15% 32% 22% 6% 1	780
	11th to 20th	Night	26% 18% 30% 20% 6%	800
	21st to end	Night	27% 17% 32% 19% 5%	935
December	1st to 10th	Night	23% 19% 32% 20% 5% 1	1000
	11th to 20th	Night	22% 19% 34% 17% 5% 2	1000
	21st to end	Night	29% 15% 33% 19% 4	1100

Page generated at 2025-08-02 01:32:25. Version ac07

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