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 2026-06-16.

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%	137794
	Night	14% 6% 2 5% 3 3 3 3 5% 6% 50%	134600
Transparency	Day	37% 4 10% 20% 19% 10%	141599
	Night	36% 4 10% 19% 19% 13%	138862
Seeing	Day	15% 9% 25% 30% 17% 3	16908
	Night	17% 8% 27% 35% 13% 1	26872

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	11% 8% 3 5% 3 3 3 2 4 4 54%	5289
	11th to 20th	Day	13% 6% 2 4 3 3 3 2 5% 4 56%	5310
	21st to end	Day	21% 8% 2 5% 3 2 3 2 4 6% 43%	6404
February	1st to 10th	Day	17% 7% 3 6% 3 3 4 2 5% 5% 47%	2100
	11th to 20th	Day	16% 6% 2 6% 4 3 4 2 5% 5% 47%	2049
	21st to end	Day	16% 6% 1 5% 3 2 2 2 4 5% 53%	1730
March	1st to 10th	Day	13% 6% 2 4 3 3 3 2 4 6% 53%	4910
	11th to 20th	Day	15% 7% 2 5% 3 3 3 2 3 4 52%	5090
	21st to end	Day	15% 7% 2 5% 3 3 3 3 4 6% 47%	5583
April	1st to 10th	Day	14% 8% 3 7% 4 4 3 4 4 5% 45%	2310
	11th to 20th	Day	20% 6% 2 6% 4 4 3 4 4 5% 41%	2345
	21st to end	Day	15% 7% 2 6% 5% 4 4 3 5% 6% 43%	2630
May	1st to 10th	Day	21% 7% 3 6% 4 4 4 3 5% 7% 38%	7617
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 5% 8% 45%	7627
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 4 7% 8% 35%	8392
June	1st to 10th	Day	7% 5% 2 5% 5% 5% 6% 5% 7% 8% 47%	2730
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 5% 7% 8% 45%	2666
	21st to end	Day	8% 5% 2 6% 6% 6% 7% 5% 7% 9% 39%	2600
July	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 5% 6% 8% 37%	5900
	11th to 20th	Day	5% 5% 2 5% 5% 5% 6% 4 7% 10% 46%	5900
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 10% 43%	6422
August	1st to 10th	Day	1 1 1 2 2 3 4 3 5% 9% 68%	4086
	11th to 20th	Day	4 2 1 4 3 3 3 3 7% 11% 59%	3846
	21st to end	Day	5% 3 1 6% 5% 5% 5% 5% 7% 11% 47%	4186
September	1st to 10th	Day	12% 7% 2 6% 5% 5% 5% 4 5% 6% 43%	2200
	11th to 20th	Day	12% 6% 2 5% 5% 4 6% 4 6% 9% 41%	2200
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 6% 8% 48%	2200
October	1st to 10th	Day	7% 5% 2 4 4 4 5% 3 5% 6% 57%	2200
	11th to 20th	Day	13% 5% 2 4 3 3 4 3 4 6% 54%	2108
	21st to end	Day	12% 4 2 5% 4 2 3 3 3 6% 58%	2200
November	1st to 10th	Day	14% 7% 2 6% 3 4 3 3 4 6% 47%	2058
	11th to 20th	Day	12% 5% 2 6% 3 3 4 3 5% 5% 51%	1918
	21st to end	Day	17% 8% 2 6% 3 3 3 3 4 6% 46%	1890
December	1st to 10th	Day	26% 8% 2 7% 3 3 2 2 3 5% 41%	3030
	11th to 20th	Day	16% 8% 2 6% 4 3 4 3 4 5% 45%	3481
	21st to end	Day	13% 5% 2 4 2 2 3 2 3 3 60%	4587
January	1st to 10th	Night	14% 5% 2 5% 2 3 2 3 4 5% 55%	7634
	11th to 20th	Night	9% 4 1 3 2 2 2 3 4 4 64%	7680
	21st to end	Night	26% 6% 2 5% 3 3 3 2 4 4 42%	7866
February	1st to 10th	Night	16% 7% 2 5% 3 2 3 3 4 5% 50%	2520
	11th to 20th	Night	17% 7% 2 5% 3 3 3 2 4 5% 50%	2458
	21st to end	Night	18% 7% 2 5% 3 3 3 3 4 5% 49%	2076
March	1st to 10th	Night	15% 8% 2 5% 3 3 4 3 5% 6% 46%	5169
	11th to 20th	Night	15% 9% 2 4 3 3 3 2 3 5% 53%	5090
	21st to end	Night	18% 9% 2 5% 3 3 3 3 4 6% 45%	5588
April	1st to 10th	Night	13% 6% 2 6% 4 3 3 3 5% 7% 46%	2310
	11th to 20th	Night	20% 7% 2 6% 4 2 3 2 4 5% 45%	2219
	21st to end	Night	20% 7% 3 5% 4 3 3 2 4 5% 45%	1987
May	1st to 10th	Night	24% 10% 3 6% 3 4 3 3 4 6% 36%	5266
	11th to 20th	Night	11% 6% 2 6% 4 4 5% 4 6% 7% 46%	5265
	21st to end	Night	15% 7% 2 6% 4 4 4 4 6% 10% 37%	5793
June	1st to 10th	Night	6% 4 1 4 3 3 3 3 5% 10% 57%	1890
	11th to 20th	Night	5% 4 1 5% 4 4 4 3 5% 8% 57%	1845
	21st to end	Night	8% 4 1 5% 4 4 4 4 6% 10% 51%	1800
July	1st to 10th	Night	12% 6% 2 5% 4 4 5% 4 6% 10% 42%	4078
	11th to 20th	Night	3 4 1 5% 4 4 5% 4 7% 10% 51%	4077
	21st to end	Night	9% 6% 2 5% 4 4 4 4 7% 9% 46%	4449
August	1st to 10th	Night	1 2 2 2 2 3 2 5% 8% 73%	3055
	11th to 20th	Night	3 3 1 3 3 3 4 3 5% 12% 61%	3213
	21st to end	Night	5% 4 2 4 3 4 4 4 6% 10% 54%	3715
September	1st to 10th	Night	8% 6% 2 6% 4 4 5% 3 6% 7% 48%	2200
	11th to 20th	Night	9% 6% 3 5% 4 3 4 4 7% 8% 46%	2200
	21st to end	Night	9% 5% 1 5% 3 4 5% 3 6% 8% 52%	2200
October	1st to 10th	Night	6% 4 1 5% 3 3 3 3 4 6% 62%	2200
	11th to 20th	Night	12% 5% 1 4 3 3 3 3 5% 6% 56%	2270
	21st to end	Night	14% 4 1 3 2 3 2 2 3 4 61%	2640
November	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 3 5% 54%	2460
	11th to 20th	Night	10% 7% 2 4 3 3 3 3 5% 5% 56%	2698
	21st to end	Night	18% 6% 2 4 3 3 3 3 4 7% 48%	2730
December	1st to 10th	Night	23% 8% 3 6% 3 3 3 2 4 5% 40%	4380
	11th to 20th	Night	19% 9% 2 6% 3 3 3 2 2 4 48%	4946
	21st to end	Night	16% 6% 2 4 2 2 2 2 3 4 58%	6633

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	37% 2 6% 20% 25% 12%	5290
	11th to 20th	Day	34% 1 4 14% 26% 21%	5310
	21st to end	Day	45% 2 5% 14% 24% 11%	6404
February	1st to 10th	Day	42% 3 5% 12% 21% 17%	2100
	11th to 20th	Day	42% 3 7% 18% 18% 13%	2059
	21st to end	Day	36% 3 6% 16% 22% 16%	1730
March	1st to 10th	Day	34% 3 5% 17% 17% 23%	4910
	11th to 20th	Day	38% 3 4 9% 15% 31%	5090
	21st to end	Day	40% 3 8% 18% 20% 11%	5585
April	1st to 10th	Day	43% 4 8% 16% 17% 12%	2310
	11th to 20th	Day	45% 4 7% 15% 17% 11%	2345
	21st to end	Day	45% 4 7% 18% 16% 10%	2630
May	1st to 10th	Day	48% 4 10% 19% 15% 4	7655
	11th to 20th	Day	39% 4 11% 18% 21% 7%	7631
	21st to end	Day	44% 6% 11% 26% 12% 1	8405
June	1st to 10th	Day	33% 8% 12% 26% 18% 3	2730
	11th to 20th	Day	35% 7% 11% 24% 20% 4	2666
	21st to end	Day	39% 8% 12% 24% 14% 3	2600
July	1st to 10th	Day	45% 5% 17% 28% 5% 1	5836
	11th to 20th	Day	33% 5% 19% 30% 12% 1	5831
	21st to end	Day	35% 6% 17% 26% 15% 2	6240
August	1st to 10th	Day	15% 3 11% 44% 26%	4072
	11th to 20th	Day	21% 4 24% 38% 11% 2	3882
	21st to end	Day	31% 7% 18% 24% 18% 2	4185
September	1st to 10th	Day	41% 7% 9% 18% 17% 8%	2200
	11th to 20th	Day	37% 9% 12% 19% 18% 5%	2200
	21st to end	Day	32% 7% 9% 22% 17% 13%	2200
October	1st to 10th	Day	18% 3 6% 12% 26% 35%	3590
	11th to 20th	Day	25% 3 8% 19% 23% 21%	3413
	21st to end	Day	27% 4 5% 17% 33% 14%	3538
November	1st to 10th	Day	39% 4 7% 18% 19% 12%	2058
	11th to 20th	Day	36% 4 7% 16% 21% 16%	1918
	21st to end	Day	42% 4 6% 15% 20% 13%	1890
December	1st to 10th	Day	50% 2 5% 18% 14% 10%	3030
	11th to 20th	Day	42% 3 8% 12% 20% 15%	3479
	21st to end	Day	32% 2 4 15% 29% 18%	4587
January	1st to 10th	Night	35% 2 6% 18% 24% 15%	7635
	11th to 20th	Night	25% 1 4 16% 29% 24%	7680
	21st to end	Night	48% 2 3 11% 22% 14%	7866
February	1st to 10th	Night	38% 3 6% 12% 22% 19%	2520
	11th to 20th	Night	40% 2 6% 17% 19% 16%	2458
	21st to end	Night	40% 3 6% 14% 20% 17%	2076
March	1st to 10th	Night	40% 3 6% 17% 17% 17%	5169
	11th to 20th	Night	38% 3 6% 11% 15% 27%	5090
	21st to end	Night	42% 3 8% 18% 18% 11%	5588
April	1st to 10th	Night	39% 4 10% 17% 15% 15%	2310
	11th to 20th	Night	43% 4 8% 14% 16% 15%	2219
	21st to end	Night	45% 4 9% 18% 16% 9%	1987
May	1st to 10th	Night	53% 3 9% 16% 13% 6%	5293
	11th to 20th	Night	38% 5% 10% 18% 20% 9%	5266
	21st to end	Night	44% 4 17% 25% 8% 2	5805
June	1st to 10th	Night	25% 6% 17% 31% 19% 3	1890
	11th to 20th	Night	28% 4 14% 29% 20% 5%	1845
	21st to end	Night	30% 7% 18% 26% 16% 3	1800
July	1st to 10th	Night	39% 7% 24% 26% 4 1	4035
	11th to 20th	Night	29% 6% 24% 29% 11% 1	4026
	21st to end	Night	34% 6% 21% 24% 12% 3	4320
August	1st to 10th	Night	12% 4 16% 46% 22%	3045
	11th to 20th	Night	20% 6% 31% 34% 8% 2	3243
	21st to end	Night	26% 10% 21% 25% 16% 2	3715
September	1st to 10th	Night	34% 6% 15% 20% 17% 8%	2200
	11th to 20th	Night	35% 8% 15% 22% 14% 5%	2200
	21st to end	Night	30% 7% 11% 22% 18% 12%	2200
October	1st to 10th	Night	17% 4 8% 14% 22% 36%	3590
	11th to 20th	Night	23% 3 9% 16% 30% 19%	3691
	21st to end	Night	26% 2 6% 18% 32% 15%	4252
November	1st to 10th	Night	35% 3 7% 18% 22% 14%	2460
	11th to 20th	Night	32% 4 8% 18% 20% 17%	2698
	21st to end	Night	38% 4 8% 16% 20% 14%	2730
December	1st to 10th	Night	49% 3 6% 15% 15% 12%	4380
	11th to 20th	Night	44% 2 5% 10% 21% 18%	4947
	21st to end	Night	34% 2 4 14% 28% 18%	6633

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% 19% 5%	330
	11th to 20th	Day	24% 17% 34% 19% 6%	330
	21st to end	Day	24% 18% 36% 17% 4	363
February	1st to 10th	Day	25% 18% 28% 25% 4	330
	11th to 20th	Day	25% 22% 38% 12% 3	330
	21st to end	Day	25% 16% 45% 11% 3	273
March	1st to 10th	Day	25% 18% 41% 16% 1	330
	11th to 20th	Day	19% 23% 38% 17% 3	330
	21st to end	Day	20% 16% 40% 19% 3 2	363
April	1st to 10th	Day	22% 19% 41% 18% 1	330
	11th to 20th	Day	24% 19% 34% 20% 3	383
	21st to end	Day	22% 16% 40% 19% 3	650
May	1st to 10th	Day	22% 12% 40% 21% 5%	753
	11th to 20th	Day	18% 11% 33% 31% 5% 2	775
	21st to end	Day	18% 7% 33% 30% 11% 1	844
June	1st to 10th	Day	9% 4 24% 42% 19% 3	780
	11th to 20th	Day	9% 3 24% 46% 16% 2	756
	21st to end	Day	9% 4 17% 38% 26% 5%	730
July	1st to 10th	Day	12% 3 13% 38% 27% 7%	721
	11th to 20th	Day	7% 2 9% 41% 36% 5%	718
	21st to end	Day	5% 3 8% 41% 35% 7%	794
August	1st to 10th	Day	3 1 5% 44% 38% 9%	643
	11th to 20th	Day	6% 1 6% 43% 36% 8%	517
	21st to end	Day	5% 2 6% 35% 39% 13%	509
September	1st to 10th	Day	12% 2 9% 34% 34% 9%	330
	11th to 20th	Day	6% 2 7% 36% 42% 8%	330
	21st to end	Day	7% 2 10% 38% 34% 8%	330
October	1st to 10th	Day	6% 2 16% 39% 30% 7%	330
	11th to 20th	Day	9% 5% 26% 39% 18% 3	330
	21st to end	Day	19% 11% 28% 25% 14% 3	363
November	1st to 10th	Day	18% 8% 30% 29% 13% 2	330
	11th to 20th	Day	21% 10% 32% 25% 9% 3	330
	21st to end	Day	25% 16% 33% 16% 8% 2	330
December	1st to 10th	Day	19% 12% 35% 25% 7% 2	330
	11th to 20th	Day	22% 16% 33% 20% 8% 1	330
	21st to end	Day	22% 10% 36% 25% 5% 1	363
January	1st to 10th	Night	19% 12% 30% 32% 8%	1050
	11th to 20th	Night	21% 14% 31% 29% 5%	1050
	21st to end	Night	24% 12% 30% 28% 6%	966
February	1st to 10th	Night	23% 12% 32% 27% 6%	840
	11th to 20th	Night	26% 14% 32% 24% 3	824
	21st to end	Night	25% 13% 39% 21% 1	692
March	1st to 10th	Night	26% 12% 36% 25% 2	697
	11th to 20th	Night	24% 10% 38% 24% 4	630
	21st to end	Night	19% 11% 43% 23% 3	693
April	1st to 10th	Night	16% 14% 40% 26% 3	630
	11th to 20th	Night	24% 15% 37% 22% 2	623
	21st to end	Night	23% 10% 42% 19% 5%	630
May	1st to 10th	Night	21% 11% 40% 26% 3	630
	11th to 20th	Night	17% 11% 35% 30% 7% 1	621
	21st to end	Night	16% 6% 36% 33% 9%	683
June	1st to 10th	Night	8% 2 30% 41% 18% 1	630
	11th to 20th	Night	7% 2 36% 45% 10%	615
	21st to end	Night	8% 3 23% 46% 19% 2	600
July	1st to 10th	Night	12% 3 13% 47% 23% 2	585
	11th to 20th	Night	4 1 14% 56% 22% 2	582
	21st to end	Night	7% 1 13% 54% 23% 3	657
August	1st to 10th	Night	2 1 11% 55% 28% 3	597
	11th to 20th	Night	6% 1 10% 54% 26% 4	594
	21st to end	Night	7% 2 6% 47% 33% 5%	660
September	1st to 10th	Night	10% 2 10% 42% 32% 5%	600
	11th to 20th	Night	12% 2 10% 41% 33% 3	600
	21st to end	Night	12% 1 12% 40% 31% 4	600
October	1st to 10th	Night	8% 1 13% 49% 27% 3	600
	11th to 20th	Night	13% 4 22% 40% 19% 1	670
	21st to end	Night	16% 8% 27% 28% 19% 2	880
November	1st to 10th	Night	16% 6% 21% 39% 15% 2	820
	11th to 20th	Night	15% 7% 26% 37% 13% 1	1018
	21st to end	Night	22% 12% 25% 29% 11%	1050
December	1st to 10th	Night	21% 9% 28% 33% 8% 1	1050
	11th to 20th	Night	24% 9% 29% 29% 8%	1050
	21st to end	Night	19% 11% 29% 34% 7%	1155

Page generated at 2026-06-13 01:31:38. Version ac07

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