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 2024-12-24.

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%	102199
	Night	9% 6% 2 5% 3 3 4 3 5% 7% 53%	98023
Transparency	Day	34% 5% 13% 26% 19% 3	105880
	Night	31% 5% 13% 26% 21% 4	102461
Seeing	Day	12% 9% 31% 31% 14% 2	15869
	Night	11% 9% 34% 35% 10% 1	25835

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	7% 13% 4 10% 5% 4 5% 4 6% 5% 36%	2847
	11th to 20th	Day	7% 12% 5% 10% 5% 5% 4 3 5% 6% 40%	2850
	21st to end	Day	7% 12% 5% 13% 4 3 3 3 4 5% 43%	3135
February	1st to 10th	Day	9% 10% 4 7% 5% 4 5% 3 6% 7% 41%	1710
	11th to 20th	Day	6% 7% 2 7% 4 5% 4 4 6% 8% 47%	1844
	21st to end	Day	7% 7% 2 6% 4 4 4 3 5% 6% 52%	1570
March	1st to 10th	Day	10% 5% 2 4 3 3 3 3 4 5% 57%	3248
	11th to 20th	Day	11% 6% 2 4 3 3 3 2 4 6% 56%	3480
	21st to end	Day	15% 9% 3 7% 4 3 3 3 5% 7% 43%	3814
April	1st to 10th	Day	12% 6% 2 5% 4 3 4 3 4 6% 51%	2137
	11th to 20th	Day	13% 6% 2 7% 4 4 4 4 5% 7% 43%	2385
	21st to end	Day	18% 6% 2 5% 4 3 4 3 6% 6% 43%	2470
May	1st to 10th	Day	14% 6% 2 5% 4 3 4 3 6% 8% 45%	4082
	11th to 20th	Day	14% 5% 2 7% 4 4 4 3 4 6% 46%	4082
	21st to end	Day	13% 5% 2 6% 4 4 5% 4 6% 7% 43%	4477
June	1st to 10th	Day	5% 4 2 4 4 4 4 4 6% 8% 57%	2470
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 6% 9% 51%	2545
	21st to end	Day	6% 4 2 5% 4 4 4 3 6% 7% 56%	2660
July	1st to 10th	Day	10% 4 2 5% 4 4 4 4 6% 8% 48%	4404
	11th to 20th	Day	6% 4 1 5% 3 4 5% 4 7% 9% 52%	4120
	21st to end	Day	5% 3 1 4 3 4 5% 4 7% 8% 56%	4465
August	1st to 10th	Day	8% 5% 2 5% 4 4 4 3 6% 8% 50%	4104
	11th to 20th	Day	13% 5% 2 6% 4 3 5% 3 5% 7% 47%	4041
	21st to end	Day	5% 3 1 4 3 4 4 4 5% 7% 59%	4417
September	1st to 10th	Day	13% 4 2 4 3 3 3 3 4 6% 55%	2128
	11th to 20th	Day	9% 5% 2 4 3 3 3 3 5% 8% 56%	2090
	21st to end	Day	9% 3 1 4 3 3 4 2 5% 6% 60%	2090
October	1st to 10th	Day	15% 5% 2 4 3 2 2 2 2 4 57%	2047
	11th to 20th	Day	10% 4 1 4 3 3 3 2 3 5% 62%	1900
	21st to end	Day	13% 6% 2 4 3 3 3 2 4 4 57%	2011
November	1st to 10th	Day	9% 4 2 4 3 3 3 3 5% 6% 58%	1763
	11th to 20th	Day	8% 6% 2 4 5% 4 3 3 4 5% 58%	1800
	21st to end	Day	11% 8% 3 4 3 2 3 3 3 8% 52%	1800
December	1st to 10th	Day	7% 7% 2 5% 5% 3 3 3 5% 8% 51%	1800
	11th to 20th	Day	8% 7% 4 7% 4 3 4 3 5% 6% 50%	2251
	21st to end	Day	12% 10% 3 12% 5% 4 5% 3 4 5% 36%	3162
January	1st to 10th	Night	6% 10% 5% 13% 4 3 4 3 5% 6% 41%	4108
	11th to 20th	Night	5% 9% 3 6% 4 4 5% 3 4 5% 51%	4120
	21st to end	Night	11% 9% 5% 10% 4 3 3 3 4 5% 43%	4532
February	1st to 10th	Night	9% 11% 3 6% 4 4 4 4 5% 6% 44%	2470
	11th to 20th	Night	7% 7% 2 6% 4 4 5% 4 6% 8% 47%	2237
	21st to end	Night	7% 6% 2 6% 4 3 4 3 4 6% 55%	1884
March	1st to 10th	Night	10% 5% 2 5% 3 3 3 2 4 5% 59%	3611
	11th to 20th	Night	9% 5% 2 5% 3 3 2 3 4 6% 58%	3480
	21st to end	Night	16% 7% 2 5% 3 3 4 4 5% 7% 44%	3817
April	1st to 10th	Night	9% 8% 2 5% 4 4 3 3 5% 6% 51%	2038
	11th to 20th	Night	11% 7% 3 4 3 4 5% 3 5% 7% 48%	1745
	21st to end	Night	13% 6% 2 6% 3 3 3 3 4 7% 50%	1710
May	1st to 10th	Night	22% 5% 1 4 4 3 3 3 5% 6% 43%	2823
	11th to 20th	Night	13% 6% 2 5% 4 4 4 4 5% 6% 49%	2823
	21st to end	Night	13% 5% 2 5% 4 4 4 3 6% 9% 45%	3090
June	1st to 10th	Night	5% 2 1 4 3 4 3 4 6% 9% 58%	1710
	11th to 20th	Night	8% 3 2 4 4 3 4 4 5% 9% 55%	1627
	21st to end	Night	4 3 1 3 3 3 4 4 8% 10% 56%	1520
July	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 5% 8% 50%	2561
	11th to 20th	Night	4 3 1 4 3 4 4 4 7% 10% 56%	2850
	21st to end	Night	3 3 1 4 2 3 3 3 6% 8% 62%	3098
August	1st to 10th	Night	4 3 1 5% 3 4 4 4 6% 11% 56%	2836
	11th to 20th	Night	11% 4 2 4 4 3 3 4 6% 8% 51%	2798
	21st to end	Night	5% 3 2 4 3 3 4 4 5% 9% 58%	3248
September	1st to 10th	Night	8% 3 1 4 2 3 3 2 4 6% 64%	2051
	11th to 20th	Night	8% 4 1 3 2 3 4 4 5% 7% 58%	2090
	21st to end	Night	7% 3 1 4 3 3 3 3 5% 5% 62%	2090
October	1st to 10th	Night	12% 4 2 4 3 3 4 3 4 5% 56%	2113
	11th to 20th	Night	10% 5% 1 4 3 2 3 2 3 5% 61%	2280
	21st to end	Night	12% 4 1 4 3 2 3 3 4 5% 60%	2584
November	1st to 10th	Night	7% 4 2 4 3 3 3 2 4 7% 63%	2535
	11th to 20th	Night	8% 5% 2 4 3 3 3 3 4 6% 60%	2600
	21st to end	Night	9% 5% 2 4 3 2 3 3 5% 6% 58%	2600
December	1st to 10th	Night	5% 6% 2 5% 3 3 4 4 4 7% 56%	2600
	11th to 20th	Night	8% 6% 2 5% 3 3 4 3 5% 7% 55%	3166
	21st to end	Night	15% 10% 3 7% 3 3 4 3 4 7% 41%	4578

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% 6% 15% 20% 10% 1	2848
	11th to 20th	Day	44% 6% 15% 20% 12% 2	2850
	21st to end	Day	45% 4 14% 24% 12% 1	3135
February	1st to 10th	Day	42% 5% 17% 22% 11% 3	1710
	11th to 20th	Day	35% 6% 14% 27% 15% 3	1854
	21st to end	Day	33% 5% 12% 25% 19% 5%	1570
March	1st to 10th	Day	30% 4 10% 25% 26% 5%	3248
	11th to 20th	Day	31% 5% 13% 28% 21% 2	3480
	21st to end	Day	43% 7% 22% 23% 5% 1	3814
April	1st to 10th	Day	35% 5% 14% 25% 19% 2	2137
	11th to 20th	Day	39% 5% 14% 24% 15% 3	2385
	21st to end	Day	42% 6% 11% 26% 14% 1	2470
May	1st to 10th	Day	36% 7% 13% 25% 14% 6%	4120
	11th to 20th	Day	38% 7% 11% 27% 16% 1	4086
	21st to end	Day	37% 9% 12% 28% 13% 1	4489
June	1st to 10th	Day	24% 8% 18% 34% 14% 2	2470
	11th to 20th	Day	29% 7% 14% 30% 17% 3	2545
	21st to end	Day	26% 7% 13% 31% 19% 4	2660
July	1st to 10th	Day	34% 5% 11% 26% 21% 3	4335
	11th to 20th	Day	29% 5% 14% 28% 22% 3	4051
	21st to end	Day	25% 5% 14% 23% 26% 7%	4325
August	1st to 10th	Day	33% 6% 14% 25% 17% 5%	4090
	11th to 20th	Day	38% 4 16% 22% 18% 1	4080
	21st to end	Day	25% 4 15% 24% 23% 8%	4416
September	1st to 10th	Day	32% 5% 11% 24% 22% 7%	2128
	11th to 20th	Day	27% 6% 11% 27% 26% 3	2090
	21st to end	Day	26% 5% 10% 30% 27% 2	2090
October	1st to 10th	Day	23% 3 4 20% 45% 6%	3398
	11th to 20th	Day	21% 2 11% 34% 23% 9%	3140
	21st to end	Day	29% 2 9% 29% 28% 3	3292
November	1st to 10th	Day	27% 4 12% 30% 23% 4	1763
	11th to 20th	Day	29% 5% 9% 25% 28% 3	1800
	21st to end	Day	34% 4 11% 27% 20% 4	1800
December	1st to 10th	Day	32% 5% 15% 30% 16% 2	1800
	11th to 20th	Day	35% 5% 14% 32% 13% 1	2249
	21st to end	Day	50% 5% 11% 20% 12% 1	3162
January	1st to 10th	Night	44% 4 14% 24% 13% 1	4108
	11th to 20th	Night	35% 5% 14% 26% 19% 1	4120
	21st to end	Night	45% 4 13% 21% 15% 2	4532
February	1st to 10th	Night	41% 6% 12% 21% 15% 4	2470
	11th to 20th	Night	34% 6% 13% 29% 15% 3	2237
	21st to end	Night	31% 4 12% 22% 22% 9%	1884
March	1st to 10th	Night	28% 4 9% 25% 27% 7%	3611
	11th to 20th	Night	28% 4 9% 26% 29% 4	3480
	21st to end	Night	39% 7% 23% 23% 7% 1	3817
April	1st to 10th	Night	35% 5% 12% 23% 22% 3	2038
	11th to 20th	Night	35% 5% 14% 26% 17% 2	1745
	21st to end	Night	35% 6% 12% 27% 18% 2	1710
May	1st to 10th	Night	41% 6% 10% 24% 15% 5%	2850
	11th to 20th	Night	34% 8% 14% 24% 20% 1	2824
	21st to end	Night	34% 9% 19% 24% 14% 1	3099
June	1st to 10th	Night	22% 8% 17% 31% 19% 2	1710
	11th to 20th	Night	27% 5% 15% 29% 18% 5%	1627
	21st to end	Night	21% 6% 16% 30% 22% 5%	1520
July	1st to 10th	Night	32% 5% 14% 24% 22% 4	2521
	11th to 20th	Night	23% 4 14% 33% 20% 5%	2796
	21st to end	Night	18% 5% 16% 26% 28% 7%	2999
August	1st to 10th	Night	23% 5% 18% 28% 20% 6%	2826
	11th to 20th	Night	32% 5% 19% 25% 17% 2	2825
	21st to end	Night	24% 4 17% 27% 19% 9%	3248
September	1st to 10th	Night	23% 4 11% 27% 25% 10%	2051
	11th to 20th	Night	25% 5% 11% 28% 25% 6%	2090
	21st to end	Night	24% 5% 9% 26% 30% 5%	2090
October	1st to 10th	Night	20% 4 5% 18% 44% 9%	3529
	11th to 20th	Night	21% 2 9% 30% 27% 11%	3779
	21st to end	Night	25% 3 9% 29% 30% 4	4245
November	1st to 10th	Night	24% 3 11% 29% 26% 7%	2535
	11th to 20th	Night	26% 5% 9% 29% 26% 5%	2600
	21st to end	Night	29% 4 12% 28% 23% 4	2600
December	1st to 10th	Night	28% 5% 15% 31% 19% 2	2600
	11th to 20th	Night	30% 4 11% 36% 17% 1	3167
	21st to end	Night	43% 6% 12% 26% 11% 2	4578

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	13% 10% 38% 29% 8% 2	270
	11th to 20th	Day	18% 14% 32% 27% 8% 1	270
	21st to end	Day	12% 9% 44% 29% 5% 1	297
February	1st to 10th	Day	14% 13% 46% 24% 4	270
	11th to 20th	Day	9% 11% 44% 28% 6% 1	270
	21st to end	Day	9% 13% 56% 18% 4	225
March	1st to 10th	Day	15% 14% 43% 21% 6%	270
	11th to 20th	Day	26% 10% 33% 22% 9%	270
	21st to end	Day	22% 20% 36% 18% 4	297
April	1st to 10th	Day	15% 18% 42% 20% 5%	317
	11th to 20th	Day	17% 15% 39% 22% 7%	583
	21st to end	Day	20% 18% 32% 19% 9% 2	650
May	1st to 10th	Day	14% 12% 36% 28% 10%	653
	11th to 20th	Day	16% 11% 32% 26% 13% 3	675
	21st to end	Day	15% 9% 33% 28% 12% 2	734
June	1st to 10th	Day	4 6% 26% 34% 23% 6%	680
	11th to 20th	Day	10% 5% 35% 35% 14% 1	680
	21st to end	Day	7% 8% 34% 35% 16% 1	680
July	1st to 10th	Day	8% 3 21% 37% 26% 5%	671
	11th to 20th	Day	4 2 19% 46% 23% 5%	668
	21st to end	Day	6% 2 17% 41% 28% 6%	739
August	1st to 10th	Day	8% 3 18% 45% 23% 4	676
	11th to 20th	Day	7% 4 18% 44% 23% 3	665
	21st to end	Day	10% 2 20% 38% 25% 5%	685
September	1st to 10th	Day	14% 5% 21% 41% 17% 2	338
	11th to 20th	Day	6% 3 28% 41% 18% 4	300
	21st to end	Day	13% 8% 29% 27% 18% 5%	300
October	1st to 10th	Day	19% 8% 21% 32% 16% 4	300
	11th to 20th	Day	6% 15% 41% 25% 12% 2	300
	21st to end	Day	20% 13% 36% 23% 6% 2	330
November	1st to 10th	Day	14% 14% 38% 24% 8% 2	300
	11th to 20th	Day	11% 18% 42% 21% 6% 2	300
	21st to end	Day	21% 13% 31% 25% 9% 1	300
December	1st to 10th	Day	7% 10% 45% 25% 12% 1	300
	11th to 20th	Day	16% 9% 43% 25% 7%	300
	21st to end	Day	17% 15% 39% 26% 3	306
January	1st to 10th	Night	12% 11% 35% 33% 8% 1	950
	11th to 20th	Night	12% 11% 37% 33% 7% 1	950
	21st to end	Night	10% 11% 39% 32% 8%	1045
February	1st to 10th	Night	16% 9% 42% 29% 5%	950
	11th to 20th	Night	11% 11% 42% 31% 4 1	763
	21st to end	Night	10% 14% 43% 28% 4	628
March	1st to 10th	Night	12% 14% 40% 29% 5%	689
	11th to 20th	Night	16% 11% 38% 28% 6% 1	570
	21st to end	Night	17% 15% 40% 24% 4	627
April	1st to 10th	Night	14% 14% 40% 26% 4	570
	11th to 20th	Night	16% 14% 39% 27% 4	563
	21st to end	Night	15% 11% 38% 29% 7% 1	570
May	1st to 10th	Night	22% 8% 35% 29% 7%	570
	11th to 20th	Night	17% 8% 34% 33% 8%	561
	21st to end	Night	15% 9% 31% 34% 12% 1	617
June	1st to 10th	Night	5% 5% 29% 43% 16% 1	570
	11th to 20th	Night	8% 5% 35% 40% 12%	570
	21st to end	Night	4 5% 36% 45% 11%	570
July	1st to 10th	Night	7% 3 22% 42% 26%	555
	11th to 20th	Night	3 2 22% 52% 20% 2	552
	21st to end	Night	4 1 18% 51% 26% 1	624
August	1st to 10th	Night	5% 2 18% 54% 19% 1	567
	11th to 20th	Night	9% 2 22% 47% 19% 1	564
	21st to end	Night	7% 2 24% 41% 24% 2	627
September	1st to 10th	Night	9% 2 26% 45% 17% 1	570
	11th to 20th	Night	9% 3 34% 38% 16% 1	570
	21st to end	Night	8% 6% 34% 37% 14% 1	570
October	1st to 10th	Night	14% 7% 31% 34% 13%	593
	11th to 20th	Night	13% 9% 39% 31% 7%	760
	21st to end	Night	14% 10% 34% 32% 10% 1	912
November	1st to 10th	Night	10% 10% 33% 38% 8%	975
	11th to 20th	Night	10% 12% 40% 32% 7%	1000
	21st to end	Night	11% 12% 32% 34% 9% 1	1000
December	1st to 10th	Night	8% 11% 41% 35% 5%	1000
	11th to 20th	Night	14% 11% 39% 30% 7%	1000
	21st to end	Night	16% 10% 35% 34% 5%	1063

Page generated at 2024-12-21 03:13:53. Version ac07

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