Hansen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hansen Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	10% 6% 2 6% 4 4 4 3 5% 7% 50%	132536
	Night	9% 6% 2 6% 3 3 4 3 5% 7% 52%	129947
Transparency	Day	34% 4 12% 26% 19% 4	136178
	Night	32% 4 12% 26% 21% 5%	134361
Seeing	Day	12% 9% 32% 30% 14% 2	17602
	Night	11% 9% 35% 35% 10% 1	27647

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	9% 10% 4 9% 5% 4 4 4 5% 5% 41%	5289
	11th to 20th	Day	4 8% 3 10% 5% 5% 5% 3 5% 6% 45%	5310
	21st to end	Day	5% 11% 4 11% 4 3 3 3 4 5% 47%	5812
February	1st to 10th	Day	9% 9% 4 7% 5% 4 4 3 6% 7% 41%	1890
	11th to 20th	Day	6% 8% 3 7% 4 5% 4 4 6% 8% 45%	2044
	21st to end	Day	6% 7% 2 6% 4 4 3 3 5% 6% 54%	1730
March	1st to 10th	Day	10% 7% 2 5% 3 2 3 2 4 5% 57%	4774
	11th to 20th	Day	9% 6% 2 4 3 3 3 3 4 7% 56%	5090
	21st to end	Day	12% 8% 3 6% 4 3 4 3 5% 7% 45%	5583
April	1st to 10th	Day	12% 7% 2 6% 4 3 4 3 5% 6% 49%	2362
	11th to 20th	Day	12% 6% 2 7% 4 4 4 4 5% 7% 44%	2588
	21st to end	Day	19% 7% 2 5% 4 3 4 3 6% 6% 42%	2600
May	1st to 10th	Day	12% 5% 2 5% 4 3 4 3 6% 7% 50%	5862
	11th to 20th	Day	11% 4 2 6% 4 4 3 3 4 6% 54%	5862
	21st to end	Day	18% 6% 2 6% 4 4 4 4 6% 6% 40%	6434
June	1st to 10th	Day	6% 4 2 4 4 4 4 4 6% 8% 56%	2600
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 6% 9% 52%	2679
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 7% 56%	2800
July	1st to 10th	Day	8% 4 2 5% 4 4 4 4 6% 8% 52%	6301
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 7% 9% 50%	5900
	21st to end	Day	7% 5% 2 4 4 4 5% 4 6% 8% 52%	6422
August	1st to 10th	Day	8% 5% 2 5% 4 4 4 4 6% 8% 50%	4234
	11th to 20th	Day	12% 5% 2 6% 4 3 5% 3 5% 7% 48%	4171
	21st to end	Day	5% 4 2 4 4 4 4 4 5% 7% 58%	4556
September	1st to 10th	Day	13% 5% 2 4 3 3 4 3 4 6% 54%	2240
	11th to 20th	Day	9% 5% 2 4 3 3 3 3 6% 8% 55%	2200
	21st to end	Day	10% 3 2 3 3 3 3 2 5% 6% 60%	2200
October	1st to 10th	Day	15% 5% 2 4 3 2 2 2 3 4 57%	2155
	11th to 20th	Day	10% 4 1 5% 3 3 3 2 3 5% 62%	2000
	21st to end	Day	14% 6% 2 4 3 3 3 2 4 4 56%	2117
November	1st to 10th	Day	8% 4 2 3 3 3 3 3 5% 6% 59%	1853
	11th to 20th	Day	7% 6% 2 4 4 3 3 3 4 5% 58%	1890
	21st to end	Day	11% 8% 2 5% 3 2 3 3 3 8% 51%	1890
December	1st to 10th	Day	5% 5% 2 5% 4 3 3 4 4 7% 58%	3030
	11th to 20th	Day	6% 6% 3 7% 4 3 4 2 5% 6% 54%	3481
	21st to end	Day	13% 10% 3 11% 5% 4 5% 3 4 5% 38%	4587
January	1st to 10th	Night	7% 9% 5% 12% 4 3 4 3 4 6% 42%	7634
	11th to 20th	Night	3 7% 2 7% 4 5% 5% 3 5% 6% 54%	7680
	21st to end	Night	8% 10% 5% 9% 4 4 4 3 4 5% 46%	8402
February	1st to 10th	Night	9% 10% 3 6% 4 4 4 4 5% 6% 45%	2730
	11th to 20th	Night	7% 8% 2 7% 4 4 5% 4 6% 7% 46%	2479
	21st to end	Night	6% 6% 2 6% 4 3 4 3 4 6% 56%	2076
March	1st to 10th	Night	8% 5% 2 4 3 3 3 2 4 6% 60%	5311
	11th to 20th	Night	8% 4 1 4 3 3 3 3 4 7% 59%	5090
	21st to end	Night	11% 7% 2 5% 4 4 5% 5% 7% 7% 43%	5588
April	1st to 10th	Night	9% 8% 2 6% 4 4 3 3 4 6% 50%	2252
	11th to 20th	Night	11% 7% 3 4 3 4 5% 4 5% 6% 48%	1895
	21st to end	Night	14% 6% 2 6% 3 3 3 3 4 6% 50%	1800
May	1st to 10th	Night	19% 5% 1 4 3 3 3 3 5% 7% 47%	4053
	11th to 20th	Night	12% 5% 1 4 3 3 3 3 4 6% 56%	4053
	21st to end	Night	17% 6% 2 5% 4 3 3 3 5% 8% 45%	4443
June	1st to 10th	Night	5% 2 1 4 3 4 3 4 7% 9% 58%	1800
	11th to 20th	Night	7% 3 2 4 4 2 4 4 5% 9% 55%	1712
	21st to end	Night	5% 3 1 4 3 3 4 4 8% 10% 56%	1600
July	1st to 10th	Night	8% 4 2 5% 4 4 3 3 5% 9% 52%	3672
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 7% 10% 48%	4077
	21st to end	Night	6% 5% 1 4 3 4 4 3 5% 8% 57%	4449
August	1st to 10th	Night	4 3 1 4 3 4 4 4 6% 11% 56%	2926
	11th to 20th	Night	11% 4 2 4 4 3 3 4 6% 8% 51%	2888
	21st to end	Night	6% 3 2 4 3 3 4 4 5% 9% 58%	3350
September	1st to 10th	Night	8% 4 1 4 2 3 3 3 4 6% 63%	2159
	11th to 20th	Night	8% 4 1 3 3 3 4 4 5% 7% 58%	2200
	21st to end	Night	7% 3 2 4 3 3 3 3 5% 5% 62%	2200
October	1st to 10th	Night	13% 4 2 4 3 3 3 3 4 5% 56%	2224
	11th to 20th	Night	10% 5% 1 5% 3 2 3 2 3 5% 60%	2400
	21st to end	Night	13% 4 1 4 3 2 3 3 4 5% 59%	2720
November	1st to 10th	Night	7% 4 2 4 3 3 3 2 4 6% 63%	2665
	11th to 20th	Night	8% 5% 2 4 3 3 3 3 4 6% 60%	2730
	21st to end	Night	9% 5% 2 5% 3 3 3 3 5% 6% 57%	2730
December	1st to 10th	Night	3 4 1 4 3 3 4 4 4 6% 64%	4380
	11th to 20th	Night	8% 5% 2 4 3 3 4 3 5% 8% 56%	4946
	21st to end	Night	13% 10% 3 8% 4 4 4 3 4 7% 40%	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	46% 4 11% 24% 14% 1	5290
	11th to 20th	Day	40% 4 14% 22% 19% 2	5310
	21st to end	Day	43% 2 10% 26% 16% 3	5812
February	1st to 10th	Day	41% 5% 17% 22% 12% 3	1890
	11th to 20th	Day	37% 5% 14% 26% 14% 3	2054
	21st to end	Day	31% 5% 12% 27% 21% 5%	1730
March	1st to 10th	Day	32% 3 7% 23% 25% 9%	4774
	11th to 20th	Day	30% 3 9% 24% 25% 8%	5090
	21st to end	Day	40% 5% 21% 26% 8% 1	5585
April	1st to 10th	Day	36% 5% 13% 25% 19% 2	2362
	11th to 20th	Day	39% 5% 13% 25% 16% 3	2588
	21st to end	Day	43% 6% 11% 25% 14% 1	2600
May	1st to 10th	Day	32% 6% 12% 28% 18% 4	5900
	11th to 20th	Day	32% 5% 9% 24% 22% 7%	5866
	21st to end	Day	43% 7% 9% 26% 14% 1	6447
June	1st to 10th	Day	25% 7% 17% 33% 15% 2	2600
	11th to 20th	Day	29% 7% 14% 30% 17% 3	2679
	21st to end	Day	27% 7% 12% 30% 19% 5%	2800
July	1st to 10th	Day	31% 4 12% 29% 22% 2	6232
	11th to 20th	Day	32% 4 17% 27% 18% 2	5831
	21st to end	Day	30% 4 13% 25% 22% 5%	6240
August	1st to 10th	Day	33% 6% 14% 26% 17% 5%	4220
	11th to 20th	Day	37% 4 16% 23% 18% 1	4210
	21st to end	Day	26% 4 15% 24% 23% 8%	4555
September	1st to 10th	Day	32% 4 12% 24% 21% 7%	2240
	11th to 20th	Day	28% 5% 12% 28% 25% 3	2200
	21st to end	Day	26% 4 10% 30% 27% 2	2200
October	1st to 10th	Day	23% 3 4 20% 44% 6%	3506
	11th to 20th	Day	21% 2 11% 34% 23% 9%	3240
	21st to end	Day	30% 2 9% 29% 28% 3	3398
November	1st to 10th	Day	26% 4 12% 31% 24% 3	1853
	11th to 20th	Day	29% 5% 9% 26% 28% 3	1890
	21st to end	Day	35% 4 12% 27% 19% 3	1890
December	1st to 10th	Day	28% 3 10% 35% 20% 3	3030
	11th to 20th	Day	33% 3 17% 33% 13% 1	3479
	21st to end	Day	49% 4 10% 26% 11% 1	4587
January	1st to 10th	Night	45% 3 12% 21% 17% 3	7635
	11th to 20th	Night	32% 3 12% 28% 22% 2	7680
	21st to end	Night	43% 2 10% 21% 17% 6%	8402
February	1st to 10th	Night	39% 6% 12% 22% 16% 4	2730
	11th to 20th	Night	36% 5% 13% 28% 15% 3	2479
	21st to end	Night	30% 4 12% 22% 24% 9%	2076
March	1st to 10th	Night	28% 3 7% 25% 30% 7%	5311
	11th to 20th	Night	27% 3 7% 20% 31% 12%	5090
	21st to end	Night	39% 5% 22% 24% 8% 1	5588
April	1st to 10th	Night	36% 5% 12% 23% 22% 3	2252
	11th to 20th	Night	35% 5% 14% 26% 17% 3	1895
	21st to end	Night	36% 6% 12% 27% 17% 2	1800
May	1st to 10th	Night	37% 4 10% 26% 18% 3	4080
	11th to 20th	Night	30% 6% 11% 23% 22% 8%	4054
	21st to end	Night	38% 6% 15% 24% 16% 1	4452
June	1st to 10th	Night	22% 8% 17% 31% 20% 2	1800
	11th to 20th	Night	27% 5% 14% 29% 19% 5%	1712
	21st to end	Night	22% 6% 16% 30% 22% 5%	1600
July	1st to 10th	Night	30% 4 14% 29% 21% 3	3634
	11th to 20th	Night	32% 4 15% 28% 18% 4	4026
	21st to end	Night	26% 4 15% 25% 25% 5%	4320
August	1st to 10th	Night	23% 5% 18% 29% 19% 5%	2916
	11th to 20th	Night	32% 5% 19% 26% 17% 2	2915
	21st to end	Night	25% 4 17% 27% 19% 9%	3350
September	1st to 10th	Night	24% 4 12% 27% 24% 10%	2159
	11th to 20th	Night	25% 5% 12% 28% 24% 5%	2200
	21st to end	Night	25% 5% 9% 26% 30% 5%	2200
October	1st to 10th	Night	20% 4 5% 19% 43% 8%	3640
	11th to 20th	Night	22% 2 9% 30% 27% 11%	3899
	21st to end	Night	25% 3 9% 28% 30% 4	4381
November	1st to 10th	Night	24% 3 11% 30% 27% 7%	2665
	11th to 20th	Night	26% 4 10% 29% 25% 5%	2730
	21st to end	Night	30% 4 12% 28% 23% 4	2730
December	1st to 10th	Night	21% 3 15% 37% 21% 2	4380
	11th to 20th	Night	28% 3 14% 35% 17% 3	4947
	21st to end	Night	45% 4 10% 27% 12% 1	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	15% 11% 37% 28% 8% 2	330
	11th to 20th	Day	16% 16% 35% 25% 7% 1	330
	21st to end	Day	11% 9% 45% 29% 5% 1	363
February	1st to 10th	Day	14% 13% 47% 22% 4	330
	11th to 20th	Day	12% 12% 42% 26% 7% 1	330
	21st to end	Day	7% 14% 53% 21% 4	273
March	1st to 10th	Day	17% 13% 45% 20% 6%	330
	11th to 20th	Day	24% 11% 35% 20% 9% 1	330
	21st to end	Day	20% 20% 38% 18% 4	363
April	1st to 10th	Day	16% 19% 41% 19% 4 1	382
	11th to 20th	Day	16% 16% 40% 22% 6%	658
	21st to end	Day	22% 17% 32% 19% 8% 2	700
May	1st to 10th	Day	13% 11% 34% 29% 12% 1	703
	11th to 20th	Day	16% 11% 31% 26% 13% 3	725
	21st to end	Day	16% 9% 33% 28% 12% 2	789
June	1st to 10th	Day	5% 6% 27% 34% 22% 6%	730
	11th to 20th	Day	9% 5% 35% 34% 15% 2	730
	21st to end	Day	7% 8% 34% 35% 15% 1	730
July	1st to 10th	Day	8% 3 21% 37% 26% 5%	721
	11th to 20th	Day	5% 2 20% 45% 23% 5%	718
	21st to end	Day	7% 2 17% 41% 28% 6%	794
August	1st to 10th	Day	8% 3 20% 44% 22% 4	726
	11th to 20th	Day	7% 4 18% 44% 23% 4	715
	21st to end	Day	10% 2 21% 37% 25% 5%	736
September	1st to 10th	Day	14% 5% 22% 40% 17% 2	370
	11th to 20th	Day	6% 5% 27% 39% 18% 5%	330
	21st to end	Day	13% 8% 29% 27% 18% 5%	330
October	1st to 10th	Day	18% 8% 23% 32% 15% 3	330
	11th to 20th	Day	7% 15% 42% 24% 11% 2	330
	21st to end	Day	22% 13% 35% 22% 6% 2	363
November	1st to 10th	Day	14% 14% 40% 23% 7% 2	330
	11th to 20th	Day	11% 16% 43% 22% 7% 2	330
	21st to end	Day	21% 13% 31% 25% 9% 1	330
December	1st to 10th	Day	7% 11% 45% 26% 11% 1	330
	11th to 20th	Day	15% 10% 43% 25% 7%	330
	21st to end	Day	16% 14% 40% 26% 4	363
January	1st to 10th	Night	12% 10% 36% 34% 8%	1050
	11th to 20th	Night	11% 12% 38% 32% 7% 1	1050
	21st to end	Night	10% 11% 40% 32% 7%	1155
February	1st to 10th	Night	15% 9% 42% 29% 4	1050
	11th to 20th	Night	11% 12% 42% 30% 4 1	845
	21st to end	Night	9% 14% 44% 29% 4	692
March	1st to 10th	Night	12% 15% 40% 29% 5%	761
	11th to 20th	Night	16% 12% 39% 27% 6% 1	630
	21st to end	Night	16% 15% 41% 25% 4	693
April	1st to 10th	Night	15% 14% 40% 26% 4	630
	11th to 20th	Night	16% 15% 40% 25% 4	611
	21st to end	Night	15% 11% 38% 29% 7% 1	600
May	1st to 10th	Night	21% 7% 33% 30% 8%	600
	11th to 20th	Night	16% 9% 34% 32% 9%	591
	21st to end	Night	15% 9% 31% 33% 12% 1	650
June	1st to 10th	Night	6% 5% 29% 43% 16% 1	600
	11th to 20th	Night	8% 5% 35% 40% 13%	600
	21st to end	Night	5% 5% 36% 44% 11%	600
July	1st to 10th	Night	7% 3 22% 42% 25%	585
	11th to 20th	Night	3 2 23% 50% 20% 2	582
	21st to end	Night	4 1 18% 51% 26% 1	657
August	1st to 10th	Night	5% 2 19% 55% 18% 1	597
	11th to 20th	Night	8% 2 22% 47% 20% 1	594
	21st to end	Night	8% 2 23% 41% 24% 2	660
September	1st to 10th	Night	10% 2 27% 45% 16% 1	600
	11th to 20th	Night	9% 3 34% 39% 15% 1	600
	21st to end	Night	9% 6% 33% 38% 15% 1	600
October	1st to 10th	Night	14% 8% 31% 34% 13%	624
	11th to 20th	Night	13% 10% 40% 30% 7%	800
	21st to end	Night	15% 10% 33% 31% 10% 1	960
November	1st to 10th	Night	10% 10% 35% 37% 8%	1025
	11th to 20th	Night	10% 11% 40% 32% 6%	1050
	21st to end	Night	11% 12% 32% 34% 9% 1	1050
December	1st to 10th	Night	8% 11% 41% 35% 5%	1050
	11th to 20th	Night	13% 12% 40% 29% 7%	1050
	21st to end	Night	16% 10% 36% 34% 5%	1155

Page generated at 2026-04-14 01:44:26. Version ac07

Copyright © 2026 Attilla Danko.