High Flight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the High Flight Observatory clear sky chart from 2005-11-06 to 2025-02-21.

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	13% 6% 2 5% 4 4 4 3 5% 7% 47%	109202
	Night	14% 6% 2 5% 3 3 3 3 4 6% 51%	109618
Transparency	Day	37% 5% 9% 20% 19% 10%	113046
	Night	35% 4 10% 19% 19% 12%	113901
Seeing	Day	15% 9% 24% 31% 17% 4	15122
	Night	17% 8% 27% 35% 13% 1	25211

By Month

Percentage Cloud Cover SE (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	13% 9% 3 6% 3 3 2 2 3 4 51%	4077
	11th to 20th	Day	15% 6% 2 4 2 2 2 2 5% 5% 54%	4080
	21st to end	Day	23% 7% 2 5% 3 2 3 2 4 6% 44%	4944
February	1st to 10th	Day	17% 7% 3 6% 3 3 4 2 5% 4 47%	2000
	11th to 20th	Day	16% 7% 2 6% 4 3 5% 2 5% 4 47%	1949
	21st to end	Day	17% 6% 1 5% 3 3 3 3 4 5% 51%	1570
March	1st to 10th	Day	16% 8% 2 5% 4 3 4 3 4 6% 44%	3342
	11th to 20th	Day	22% 10% 3 8% 5% 4 4 3 4 4 34%	3480
	21st to end	Day	15% 7% 2 5% 3 3 4 3 5% 7% 46%	3814
April	1st to 10th	Day	14% 7% 3 6% 4 4 3 4 4 5% 46%	2090
	11th to 20th	Day	20% 6% 2 6% 4 3 3 4 4 5% 43%	2119
	21st to end	Day	15% 7% 2 6% 5% 4 4 3 4 6% 45%	2380
May	1st to 10th	Day	23% 9% 3 6% 4 5% 4 3 5% 6% 33%	4082
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 4 7% 44%	4082
	21st to end	Day	11% 6% 3 6% 5% 5% 5% 5% 8% 9% 37%	4477
June	1st to 10th	Day	6% 5% 2 5% 5% 4 6% 5% 6% 8% 48%	2470
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 5% 7% 8% 46%	2470
	21st to end	Day	9% 5% 2 7% 6% 6% 6% 5% 7% 9% 39%	2470
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 8% 46%	4120
	11th to 20th	Day	5% 4 2 4 4 4 5% 4 6% 11% 50%	4120
	21st to end	Day	9% 6% 2 6% 5% 5% 6% 5% 7% 10% 39%	4465
August	1st to 10th	Day	1 1 1 3 2 3 4 3 5% 9% 68%	3960
	11th to 20th	Day	4 2 1 4 3 3 3 3 6% 11% 60%	3726
	21st to end	Day	5% 3 1 6% 5% 5% 5% 5% 7% 10% 46%	4058
September	1st to 10th	Day	12% 7% 2 6% 5% 5% 5% 4 5% 6% 43%	2090
	11th to 20th	Day	12% 7% 2 5% 5% 5% 6% 4 5% 8% 40%	2090
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 6% 8% 48%	2090
October	1st to 10th	Day	7% 5% 2 4 4 4 5% 3 5% 6% 55%	2090
	11th to 20th	Day	13% 5% 2 4 3 3 4 3 4 6% 52%	2003
	21st to end	Day	12% 4 2 5% 4 2 2 3 3 6% 57%	2090
November	1st to 10th	Day	14% 8% 2 6% 3 5% 4 3 4 6% 45%	1958
	11th to 20th	Day	12% 5% 2 6% 3 3 4 3 5% 5% 50%	1827
	21st to end	Day	17% 8% 2 5% 3 3 3 3 4 6% 47%	1800
December	1st to 10th	Day	26% 8% 2 7% 3 3 2 2 3 5% 40%	2940
	11th to 20th	Day	16% 8% 2 6% 4 4 4 3 4 5% 45%	3391
	21st to end	Day	13% 5% 2 4 2 2 3 2 3 3 60%	4488
January	1st to 10th	Night	17% 6% 3 5% 2 3 2 2 3 5% 52%	5888
	11th to 20th	Night	11% 4 1 3 2 2 2 3 4 5% 64%	5900
	21st to end	Night	27% 6% 2 5% 3 3 3 2 4 4 41%	6076
February	1st to 10th	Night	16% 7% 2 5% 3 2 3 3 4 5% 50%	2400
	11th to 20th	Night	18% 7% 2 4 3 2 2 2 4 4 51%	2338
	21st to end	Night	18% 7% 2 5% 3 3 3 2 4 5% 48%	1884
March	1st to 10th	Night	16% 9% 3 5% 4 4 4 3 4 6% 42%	3514
	11th to 20th	Night	21% 12% 3 6% 4 4 3 2 4 5% 36%	3480
	21st to end	Night	17% 8% 2 5% 4 3 3 3 4 6% 45%	3817
April	1st to 10th	Night	12% 6% 2 6% 4 3 3 3 5% 7% 47%	2090
	11th to 20th	Night	19% 7% 2 6% 4 2 3 2 4 5% 46%	2005
	21st to end	Night	18% 7% 2 5% 4 3 3 3 3 5% 47%	1798
May	1st to 10th	Night	25% 10% 3 5% 3 3 2 2 4 5% 38%	2823
	11th to 20th	Night	13% 6% 2 5% 4 4 3 3 5% 6% 50%	2823
	21st to end	Night	14% 6% 2 5% 3 4 4 3 5% 9% 44%	3090
June	1st to 10th	Night	6% 4 1 3 3 3 3 3 5% 10% 59%	1710
	11th to 20th	Night	5% 4 1 4 4 4 4 3 5% 8% 59%	1710
	21st to end	Night	8% 4 1 5% 4 4 4 4 6% 10% 51%	1710
July	1st to 10th	Night	7% 4 1 3 3 3 5% 3 7% 9% 54%	2850
	11th to 20th	Night	3 3 1 5% 4 4 5% 4 7% 9% 55%	2850
	21st to end	Night	11% 7% 2 6% 4 4 4 4 7% 9% 42%	3098
August	1st to 10th	Night	1 2 1 2 2 2 3 3 5% 8% 73%	2961
	11th to 20th	Night	3 3 1 3 3 3 4 3 5% 12% 61%	3113
	21st to end	Night	5% 4 2 4 3 3 4 4 6% 10% 54%	3601
September	1st to 10th	Night	8% 6% 2 6% 4 4 5% 3 6% 7% 48%	2090
	11th to 20th	Night	10% 6% 3 5% 4 3 4 4 7% 8% 46%	2090
	21st to end	Night	9% 5% 2 5% 3 4 4 3 6% 8% 51%	2090
October	1st to 10th	Night	6% 4 1 5% 3 3 3 3 4 6% 62%	2090
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 5% 6% 54%	2156
	21st to end	Night	14% 5% 1 3 2 2 2 2 2 4 61%	2508
November	1st to 10th	Night	14% 6% 2 5% 3 3 3 2 4 5% 52%	2340
	11th to 20th	Night	10% 7% 2 4 3 3 3 3 5% 5% 55%	2569
	21st to end	Night	18% 6% 2 4 3 3 3 3 4 7% 49%	2600
December	1st to 10th	Night	23% 8% 3 6% 3 3 3 2 5% 5% 40%	4250
	11th to 20th	Night	19% 9% 2 6% 3 3 3 2 2 4 48%	4816
	21st to end	Night	16% 6% 2 3 2 2 2 2 3 4 58%	6490

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	40% 2 4 18% 23% 13%	4078
	11th to 20th	Day	34% 2 5% 15% 25% 20%	4080
	21st to end	Day	44% 2 5% 10% 25% 13%	4944
February	1st to 10th	Day	43% 3 5% 12% 20% 17%	2000
	11th to 20th	Day	42% 3 6% 18% 18% 14%	1959
	21st to end	Day	38% 4 6% 17% 20% 16%	1570
March	1st to 10th	Day	43% 4 6% 14% 22% 12%	3342
	11th to 20th	Day	55% 4 6% 9% 15% 10%	3480
	21st to end	Day	38% 4 10% 15% 19% 13%	3814
April	1st to 10th	Day	42% 4 8% 17% 17% 12%	2090
	11th to 20th	Day	44% 5% 7% 16% 18% 11%	2119
	21st to end	Day	43% 5% 5% 19% 18% 10%	2380
May	1st to 10th	Day	52% 5% 9% 13% 14% 6%	4120
	11th to 20th	Day	41% 6% 9% 18% 20% 7%	4086
	21st to end	Day	39% 9% 11% 29% 11% 2	4489
June	1st to 10th	Day	32% 8% 11% 25% 20% 3	2470
	11th to 20th	Day	35% 7% 10% 25% 19% 4	2470
	21st to end	Day	40% 9% 10% 23% 14% 4	2470
July	1st to 10th	Day	34% 6% 16% 37% 6% 1	4056
	11th to 20th	Day	29% 6% 18% 30% 15% 2	4051
	21st to end	Day	37% 6% 18% 29% 9%	4325
August	1st to 10th	Day	15% 4 11% 44% 26%	3946
	11th to 20th	Day	21% 4 24% 38% 11% 2	3762
	21st to end	Day	31% 7% 18% 24% 18% 2	4057
September	1st to 10th	Day	41% 7% 10% 18% 16% 8%	2090
	11th to 20th	Day	39% 9% 11% 18% 18% 5%	2090
	21st to end	Day	32% 7% 9% 22% 17% 13%	2090
October	1st to 10th	Day	19% 4 6% 12% 25% 35%	3480
	11th to 20th	Day	26% 3 8% 18% 22% 22%	3308
	21st to end	Day	27% 4 5% 17% 34% 13%	3428
November	1st to 10th	Day	41% 4 7% 18% 17% 12%	1958
	11th to 20th	Day	37% 4 7% 16% 21% 16%	1827
	21st to end	Day	41% 4 6% 15% 20% 14%	1800
December	1st to 10th	Day	51% 2 5% 19% 14% 10%	2940
	11th to 20th	Day	42% 3 8% 12% 20% 14%	3389
	21st to end	Day	32% 2 4 15% 29% 18%	4488
January	1st to 10th	Night	38% 2 4 17% 23% 15%	5888
	11th to 20th	Night	25% 2 4 18% 27% 23%	5900
	21st to end	Night	48% 2 3 9% 21% 16%	6076
February	1st to 10th	Night	39% 3 6% 12% 21% 19%	2400
	11th to 20th	Night	40% 2 6% 16% 19% 16%	2338
	21st to end	Night	40% 4 6% 14% 19% 17%	1884
March	1st to 10th	Night	45% 3 8% 14% 19% 11%	3514
	11th to 20th	Night	53% 4 8% 10% 14% 11%	3480
	21st to end	Night	40% 3 11% 15% 18% 12%	3817
April	1st to 10th	Night	38% 4 10% 18% 15% 15%	2090
	11th to 20th	Night	42% 4 8% 13% 17% 16%	2005
	21st to end	Night	42% 4 8% 19% 17% 10%	1798
May	1st to 10th	Night	50% 4 8% 16% 12% 9%	2850
	11th to 20th	Night	35% 7% 10% 21% 19% 8%	2824
	21st to end	Night	38% 6% 14% 27% 11% 4	3099
June	1st to 10th	Night	23% 6% 17% 31% 21% 3	1710
	11th to 20th	Night	25% 5% 14% 30% 21% 5%	1710
	21st to end	Night	30% 7% 17% 27% 16% 3	1710
July	1st to 10th	Night	25% 7% 25% 36% 5% 1	2805
	11th to 20th	Night	25% 7% 21% 30% 15% 1	2796
	21st to end	Night	38% 6% 22% 26% 7% 1	2999
August	1st to 10th	Night	12% 4 16% 46% 22%	2951
	11th to 20th	Night	19% 6% 31% 34% 9% 2	3143
	21st to end	Night	26% 11% 21% 25% 16% 2	3601
September	1st to 10th	Night	34% 7% 15% 20% 17% 8%	2090
	11th to 20th	Night	36% 8% 15% 21% 15% 5%	2090
	21st to end	Night	30% 7% 12% 21% 18% 12%	2090
October	1st to 10th	Night	17% 4 8% 14% 21% 37%	3480
	11th to 20th	Night	24% 3 9% 15% 30% 19%	3577
	21st to end	Night	26% 2 6% 18% 32% 15%	4120
November	1st to 10th	Night	37% 4 8% 18% 20% 14%	2340
	11th to 20th	Night	32% 4 8% 18% 20% 17%	2569
	21st to end	Night	38% 4 8% 16% 20% 15%	2600
December	1st to 10th	Night	49% 3 6% 15% 15% 12%	4250
	11th to 20th	Night	44% 2 5% 10% 21% 18%	4817
	21st to end	Night	34% 2 4 14% 28% 18%	6490

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	22% 19% 35% 20% 3	300
	11th to 20th	Day	25% 18% 32% 19% 6%	300
	21st to end	Day	24% 18% 36% 18% 5%	330
February	1st to 10th	Day	26% 18% 28% 24% 3	300
	11th to 20th	Day	26% 22% 38% 11% 2	300
	21st to end	Day	28% 18% 43% 10% 1	225
March	1st to 10th	Day	23% 17% 43% 16% 1	270
	11th to 20th	Day	21% 21% 40% 15% 3	270
	21st to end	Day	21% 18% 43% 16% 2 1	297
April	1st to 10th	Day	23% 19% 42% 16% 1	270
	11th to 20th	Day	23% 17% 35% 21% 3	317
	21st to end	Day	21% 15% 41% 20% 3	560
May	1st to 10th	Day	21% 12% 39% 23% 5%	653
	11th to 20th	Day	20% 9% 30% 33% 6% 2	675
	21st to end	Day	19% 7% 32% 30% 12% 1	734
June	1st to 10th	Day	9% 3 21% 44% 20% 3	680
	11th to 20th	Day	9% 3 22% 48% 17% 2	680
	21st to end	Day	10% 4 15% 40% 26% 5%	680
July	1st to 10th	Day	11% 3 12% 39% 28% 8%	671
	11th to 20th	Day	6% 1 8% 41% 38% 5%	668
	21st to end	Day	6% 2 7% 41% 36% 7%	739
August	1st to 10th	Day	3 1 5% 45% 38% 9%	597
	11th to 20th	Day	7% 1 6% 43% 36% 8%	477
	21st to end	Day	5% 2 6% 35% 38% 13%	469
September	1st to 10th	Day	12% 2 10% 32% 35% 10%	300
	11th to 20th	Day	7% 2 6% 35% 42% 9%	300
	21st to end	Day	7% 2 10% 39% 34% 9%	300
October	1st to 10th	Day	7% 3 17% 40% 27% 6%	300
	11th to 20th	Day	10% 5% 26% 40% 17% 2	300
	21st to end	Day	21% 11% 28% 24% 14% 3	330
November	1st to 10th	Day	20% 8% 31% 28% 11% 2	300
	11th to 20th	Day	22% 10% 32% 23% 10% 3	300
	21st to end	Day	26% 16% 33% 16% 8% 2	300
December	1st to 10th	Day	20% 11% 34% 26% 7% 2	300
	11th to 20th	Day	23% 17% 33% 20% 7% 1	300
	21st to end	Day	22% 11% 36% 25% 5% 1	330
January	1st to 10th	Night	19% 12% 30% 32% 8%	1000
	11th to 20th	Night	22% 14% 30% 29% 6%	1000
	21st to end	Night	23% 12% 31% 28% 6%	920
February	1st to 10th	Night	23% 12% 32% 26% 6%	800
	11th to 20th	Night	27% 13% 32% 24% 3	784
	21st to end	Night	26% 14% 39% 20% 1	628
March	1st to 10th	Night	23% 11% 37% 26% 2	631
	11th to 20th	Night	25% 9% 38% 25% 4	570
	21st to end	Night	19% 12% 43% 22% 3	627
April	1st to 10th	Night	15% 14% 41% 27% 3	570
	11th to 20th	Night	23% 13% 39% 23% 2	563
	21st to end	Night	22% 9% 43% 20% 5% 1	570
May	1st to 10th	Night	20% 10% 40% 26% 3	570
	11th to 20th	Night	17% 8% 37% 31% 7% 1	561
	21st to end	Night	16% 5% 36% 33% 9%	617
June	1st to 10th	Night	8% 1 28% 43% 19% 1	570
	11th to 20th	Night	7% 2 34% 46% 11%	570
	21st to end	Night	8% 2 22% 46% 19% 2	570
July	1st to 10th	Night	10% 3 14% 48% 23% 3	555
	11th to 20th	Night	4 1 12% 58% 24% 2	552
	21st to end	Night	7% 1 11% 55% 23% 4	624
August	1st to 10th	Night	2 1 10% 55% 29% 3	567
	11th to 20th	Night	6% 1 9% 54% 27% 4	564
	21st to end	Night	8% 2 6% 47% 33% 5%	627
September	1st to 10th	Night	11% 2 9% 42% 31% 5%	570
	11th to 20th	Night	13% 2 9% 41% 33% 3	570
	21st to end	Night	12% 1 12% 40% 31% 4	570
October	1st to 10th	Night	8% 1 13% 48% 26% 4	570
	11th to 20th	Night	14% 5% 22% 40% 18% 2	636
	21st to end	Night	17% 8% 27% 28% 20% 2	836
November	1st to 10th	Night	17% 6% 21% 39% 16% 2	780
	11th to 20th	Night	15% 7% 26% 36% 14% 1	969
	21st to end	Night	22% 12% 26% 29% 12% 1	1000
December	1st to 10th	Night	21% 9% 27% 34% 9% 1	1000
	11th to 20th	Night	25% 10% 29% 29% 7%	1000
	21st to end	Night	19% 11% 29% 33% 7%	1100

Page generated at 2025-02-18 02:26:30. Version ac07

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