Hines Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hines Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	10% 6% 2 6% 5% 5% 5% 4 6% 9% 42%	131093
	Night	10% 6% 2 6% 4 4 4 4 6% 8% 44%	129760
Transparency	Day	36% 7% 13% 19% 17% 8%	134867
	Night	36% 7% 12% 18% 19% 9%	134056
Seeing	Day	12% 7% 20% 24% 22% 14%	16758
	Night	12% 6% 22% 32% 23% 4	26841

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	10% 7% 2 7% 4 4 4 2 5% 6% 49%	5258
	11th to 20th	Day	14% 6% 2 4 3 2 3 3 6% 9% 48%	5280
	21st to end	Day	10% 8% 3 8% 4 4 5% 4 5% 7% 43%	6117
February	1st to 10th	Day	14% 7% 2 7% 5% 4 4 4 5% 6% 42%	2100
	11th to 20th	Day	15% 7% 2 6% 4 3 4 3 5% 6% 45%	1998
	21st to end	Day	14% 7% 2 6% 3 3 5% 3 4 6% 46%	1650
March	1st to 10th	Day	18% 8% 3 6% 5% 4 4 4 6% 7% 37%	4790
	11th to 20th	Day	7% 4 2 5% 3 4 4 4 6% 8% 55%	4980
	21st to end	Day	13% 6% 3 6% 4 4 4 3 5% 6% 47%	5465
April	1st to 10th	Day	9% 6% 2 5% 6% 4 5% 5% 6% 7% 46%	2200
	11th to 20th	Day	11% 5% 2 4 4 4 4 3 5% 8% 51%	2284
	21st to end	Day	6% 4 2 5% 4 4 5% 4 5% 7% 54%	2559
May	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 5% 9% 42%	5862
	11th to 20th	Day	13% 6% 3 6% 4 4 5% 4 6% 10% 40%	5861
	21st to end	Day	4 3 1 5% 5% 5% 6% 5% 7% 11% 47%	6433
June	1st to 10th	Day	9% 7% 2 8% 6% 6% 6% 5% 8% 10% 32%	2600
	11th to 20th	Day	4 4 2 6% 5% 6% 7% 6% 9% 12% 39%	2600
	21st to end	Day	6% 5% 3 7% 6% 6% 8% 7% 9% 11% 33%	2600
July	1st to 10th	Day	7% 7% 3 9% 8% 9% 9% 8% 10% 12% 18%	5900
	11th to 20th	Day	4 3 2 5% 4 5% 6% 6% 8% 14% 45%	5900
	21st to end	Day	5% 5% 3 8% 6% 7% 8% 7% 10% 13% 28%	6424
August	1st to 10th	Day	12% 6% 2 7% 6% 6% 7% 5% 9% 12% 27%	4234
	11th to 20th	Day	6% 5% 2 5% 5% 5% 6% 6% 8% 11% 43%	3881
	21st to end	Day	7% 6% 3 8% 6% 6% 6% 6% 9% 11% 32%	4185
September	1st to 10th	Day	10% 7% 3 8% 6% 6% 6% 4 7% 10% 33%	2200
	11th to 20th	Day	11% 7% 3 7% 5% 5% 5% 4 7% 9% 38%	2200
	21st to end	Day	11% 5% 3 6% 5% 4 5% 5% 6% 8% 45%	2200
October	1st to 10th	Day	12% 6% 2 6% 4 3 4 3 5% 5% 49%	2200
	11th to 20th	Day	9% 4 1 4 2 4 4 3 4 7% 58%	2073
	21st to end	Day	8% 4 2 5% 3 3 3 3 5% 6% 57%	2200
November	1st to 10th	Day	9% 5% 3 5% 4 3 3 3 5% 6% 54%	2031
	11th to 20th	Day	10% 6% 2 5% 3 3 3 2 4 4 57%	1890
	21st to end	Day	11% 6% 2 5% 4 4 3 3 6% 8% 47%	1890
December	1st to 10th	Day	12% 6% 2 6% 4 4 3 3 4 5% 51%	3020
	11th to 20th	Day	13% 6% 2 6% 4 3 3 3 6% 7% 47%	3463
	21st to end	Day	16% 11% 3 7% 3 3 3 3 4 6% 41%	4565
January	1st to 10th	Night	8% 5% 2 6% 4 4 5% 4 6% 7% 50%	7635
	11th to 20th	Night	12% 6% 3 7% 5% 4 4 3 5% 6% 46%	7680
	21st to end	Night	15% 8% 3 7% 4 4 3 3 5% 8% 41%	8061
February	1st to 10th	Night	12% 7% 2 6% 4 3 4 4 5% 6% 46%	2520
	11th to 20th	Night	15% 8% 2 6% 4 4 4 3 4 6% 45%	2399
	21st to end	Night	12% 6% 2 4 4 4 4 4 5% 7% 48%	1980
March	1st to 10th	Night	19% 8% 2 6% 4 3 4 2 4 6% 41%	5061
	11th to 20th	Night	8% 5% 1 4 4 4 4 4 7% 8% 51%	4980
	21st to end	Night	12% 5% 2 5% 4 4 4 4 5% 7% 49%	5467
April	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 5% 7% 49%	2200
	11th to 20th	Night	8% 4 2 5% 4 3 4 3 6% 7% 52%	2080
	21st to end	Night	6% 3 1 4 4 4 4 4 6% 7% 57%	1840
May	1st to 10th	Night	8% 6% 2 7% 5% 5% 5% 4 7% 9% 42%	4033
	11th to 20th	Night	14% 5% 2 5% 5% 4 4 4 6% 9% 41%	4032
	21st to end	Night	5% 4 2 6% 4 4 5% 4 8% 11% 47%	4421
June	1st to 10th	Night	7% 6% 3 7% 6% 6% 6% 6% 8% 11% 34%	1800
	11th to 20th	Night	4 3 2 6% 5% 5% 6% 7% 8% 12% 40%	1800
	21st to end	Night	5% 6% 2 6% 7% 6% 8% 6% 8% 10% 36%	1800
July	1st to 10th	Night	9% 10% 4 11% 7% 7% 7% 6% 9% 11% 19%	4058
	11th to 20th	Night	2 4 2 6% 5% 5% 7% 7% 12% 16% 35%	4057
	21st to end	Night	5% 7% 3 9% 7% 7% 8% 6% 10% 12% 27%	4427
August	1st to 10th	Night	12% 5% 2 8% 6% 6% 6% 7% 8% 12% 28%	2915
	11th to 20th	Night	5% 5% 2 6% 5% 5% 6% 4 8% 13% 41%	3162
	21st to end	Night	6% 5% 2 7% 5% 5% 6% 5% 8% 11% 39%	3680
September	1st to 10th	Night	9% 7% 3 8% 5% 5% 6% 5% 7% 11% 33%	2200
	11th to 20th	Night	10% 7% 2 7% 5% 5% 5% 5% 6% 8% 39%	2200
	21st to end	Night	9% 6% 2 8% 4 4 5% 4 5% 8% 44%	2200
October	1st to 10th	Night	10% 6% 2 6% 3 4 4 4 5% 7% 50%	2200
	11th to 20th	Night	9% 5% 2 4 3 2 3 3 4 5% 60%	2325
	21st to end	Night	8% 5% 2 5% 4 3 4 3 4 6% 56%	2640
November	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 5% 56%	2482
	11th to 20th	Night	10% 6% 2 5% 4 3 3 3 5% 6% 54%	2730
	21st to end	Night	11% 6% 2 5% 4 3 4 4 6% 8% 49%	2730
December	1st to 10th	Night	11% 5% 3 6% 3 3 3 3 4 6% 54%	4380
	11th to 20th	Night	13% 7% 3 6% 4 3 3 3 4 5% 51%	4952
	21st to end	Night	15% 9% 3 6% 4 3 3 3 4 5% 44%	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	36% 3 8% 17% 28% 8%	5259
	11th to 20th	Day	33% 3 6% 15% 25% 18%	5280
	21st to end	Day	42% 3 7% 18% 18% 12%	6117
February	1st to 10th	Day	41% 5% 7% 15% 22% 9%	2100
	11th to 20th	Day	40% 4 6% 16% 25% 10%	2008
	21st to end	Day	38% 5% 7% 18% 19% 12%	1650
March	1st to 10th	Day	46% 5% 8% 15% 17% 10%	4790
	11th to 20th	Day	27% 5% 10% 13% 22% 22%	4980
	21st to end	Day	38% 4 8% 21% 17% 11%	5465
April	1st to 10th	Day	36% 7% 9% 21% 16% 12%	2200
	11th to 20th	Day	33% 5% 7% 19% 25% 11%	2284
	21st to end	Day	29% 5% 10% 25% 23% 7%	2559
May	1st to 10th	Day	38% 7% 15% 23% 15% 3	5897
	11th to 20th	Day	39% 6% 15% 27% 12% 1	5865
	21st to end	Day	28% 9% 20% 23% 19% 1	6447
June	1st to 10th	Day	43% 11% 15% 19% 11% 1	2600
	11th to 20th	Day	33% 11% 18% 22% 14% 1	2600
	21st to end	Day	38% 14% 21% 17% 10% 2	2600
July	1st to 10th	Day	49% 18% 21% 9% 2	5835
	11th to 20th	Day	27% 14% 35% 22% 2	5831
	21st to end	Day	40% 17% 22% 18% 2	6240
August	1st to 10th	Day	45% 12% 20% 19% 4	4221
	11th to 20th	Day	31% 11% 18% 29% 11%	3918
	21st to end	Day	41% 11% 19% 24% 5% 1	4185
September	1st to 10th	Day	44% 10% 14% 18% 11% 3	2200
	11th to 20th	Day	42% 9% 11% 18% 16% 4	2200
	21st to end	Day	35% 7% 12% 20% 19% 6%	2200
October	1st to 10th	Day	40% 3 7% 11% 20% 18%	3590
	11th to 20th	Day	17% 3 5% 16% 26% 33%	3352
	21st to end	Day	20% 3 4 19% 38% 16%	3538
November	1st to 10th	Day	32% 4 8% 17% 24% 15%	2031
	11th to 20th	Day	31% 3 7% 17% 28% 15%	1890
	21st to end	Day	35% 5% 8% 19% 23% 11%	1890
December	1st to 10th	Day	36% 4 8% 14% 27% 11%	3020
	11th to 20th	Day	37% 3 6% 23% 23% 7%	3460
	21st to end	Day	45% 3 5% 18% 25% 4	4565
January	1st to 10th	Night	32% 4 10% 16% 28% 10%	7636
	11th to 20th	Night	39% 3 4 13% 27% 13%	7680
	21st to end	Night	43% 3 6% 18% 20% 9%	8061
February	1st to 10th	Night	38% 4 8% 16% 22% 12%	2520
	11th to 20th	Night	42% 3 7% 14% 21% 14%	2400
	21st to end	Night	36% 5% 7% 19% 18% 16%	1980
March	1st to 10th	Night	44% 5% 7% 15% 17% 12%	5061
	11th to 20th	Night	29% 5% 8% 17% 22% 19%	4980
	21st to end	Night	35% 4 9% 15% 23% 13%	5467
April	1st to 10th	Night	34% 6% 10% 18% 19% 13%	2200
	11th to 20th	Night	31% 5% 9% 18% 24% 12%	2080
	21st to end	Night	26% 5% 11% 25% 23% 9%	1840
May	1st to 10th	Night	35% 9% 16% 24% 12% 4	4060
	11th to 20th	Night	38% 8% 19% 23% 11% 2	4033
	21st to end	Night	29% 7% 17% 27% 17% 2	4429
June	1st to 10th	Night	39% 11% 19% 18% 12% 2	1800
	11th to 20th	Night	32% 13% 23% 21% 11% 1	1800
	21st to end	Night	37% 14% 19% 18% 9% 2	1800
July	1st to 10th	Night	54% 15% 20% 8% 2 1	4015
	11th to 20th	Night	30% 16% 32% 19% 2	4006
	21st to end	Night	44% 15% 23% 16% 2	4297
August	1st to 10th	Night	44% 13% 25% 16% 3	2904
	11th to 20th	Night	32% 13% 20% 27% 8% 1	3191
	21st to end	Night	35% 11% 24% 24% 4 1	3680
September	1st to 10th	Night	41% 12% 17% 17% 11% 2	2200
	11th to 20th	Night	41% 10% 13% 16% 16% 3	2200
	21st to end	Night	36% 7% 13% 19% 18% 7%	2200
October	1st to 10th	Night	38% 4 10% 13% 19% 16%	3590
	11th to 20th	Night	17% 3 4 14% 29% 34%	3785
	21st to end	Night	20% 3 5% 17% 36% 20%	4252
November	1st to 10th	Night	30% 4 8% 16% 24% 17%	2482
	11th to 20th	Night	31% 4 7% 15% 27% 16%	2730
	21st to end	Night	33% 5% 10% 19% 20% 13%	2730
December	1st to 10th	Night	32% 4 8% 17% 28% 11%	4380
	11th to 20th	Night	37% 3 7% 20% 23% 10%	4954
	21st to end	Night	43% 4 6% 17% 27% 3	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	14% 21% 38% 22% 4 1	330
	11th to 20th	Day	14% 19% 32% 25% 7% 3	330
	21st to end	Day	21% 16% 43% 15% 4 1	363
February	1st to 10th	Day	16% 18% 36% 21% 9%	330
	11th to 20th	Day	24% 15% 38% 18% 5%	315
	21st to end	Day	16% 17% 37% 26% 4 1	249
March	1st to 10th	Day	16% 12% 32% 29% 10% 1	428
	11th to 20th	Day	14% 10% 35% 29% 11% 1	500
	21st to end	Day	16% 12% 31% 29% 11% 1	550
April	1st to 10th	Day	16% 13% 27% 26% 16% 2	500
	11th to 20th	Day	12% 9% 30% 25% 19% 4	492
	21st to end	Day	9% 6% 20% 33% 25% 7%	500
May	1st to 10th	Day	7% 9% 25% 26% 24% 9%	503
	11th to 20th	Day	9% 4 20% 32% 21% 14%	527
	21st to end	Day	9% 3 13% 26% 29% 20%	573
June	1st to 10th	Day	9% 2 15% 25% 31% 18%	530
	11th to 20th	Day	6% 2 13% 21% 31% 27%	530
	21st to end	Day	7% 2 7% 20% 34% 30%	530
July	1st to 10th	Day	6% 2 6% 15% 33% 38%	525
	11th to 20th	Day	8% 2 13% 32% 44%	524
	21st to end	Day	7% 1 5% 13% 38% 35%	576
August	1st to 10th	Day	13% 2 6% 18% 30% 31%	526
	11th to 20th	Day	8% 1 4 13% 35% 40%	519
	21st to end	Day	12% 2 8% 22% 31% 26%	583
September	1st to 10th	Day	14% 3 9% 22% 27% 25%	530
	11th to 20th	Day	13% 2 8% 31% 32% 13%	530
	21st to end	Day	13% 1 11% 24% 31% 19%	530
October	1st to 10th	Day	13% 4 21% 33% 21% 7%	530
	11th to 20th	Day	9% 4 20% 29% 29% 9%	530
	21st to end	Day	11% 8% 22% 32% 24% 3	583
November	1st to 10th	Day	16% 5% 23% 30% 20% 6%	509
	11th to 20th	Day	12% 9% 35% 28% 11% 4	330
	21st to end	Day	9% 15% 31% 27% 14% 3	330
December	1st to 10th	Day	19% 12% 32% 25% 10% 3	330
	11th to 20th	Day	20% 12% 32% 25% 8% 2	330
	21st to end	Day	16% 13% 29% 28% 12% 1	363
January	1st to 10th	Night	15% 12% 36% 32% 5% 1	840
	11th to 20th	Night	14% 11% 32% 32% 10% 1	840
	21st to end	Night	19% 11% 36% 30% 4	924
February	1st to 10th	Night	16% 10% 32% 33% 8%	840
	11th to 20th	Night	18% 10% 35% 30% 6%	804
	21st to end	Night	15% 10% 36% 35% 4	660
March	1st to 10th	Night	15% 10% 31% 37% 8%	800
	11th to 20th	Night	13% 10% 32% 32% 12% 1	800
	21st to end	Night	11% 12% 33% 34% 10%	880
April	1st to 10th	Night	12% 9% 30% 35% 13% 1	800
	11th to 20th	Night	10% 7% 30% 38% 14% 1	698
	21st to end	Night	7% 5% 25% 39% 23% 1	600
May	1st to 10th	Night	7% 5% 24% 40% 22% 2	600
	11th to 20th	Night	8% 3 22% 38% 24% 4	591
	21st to end	Night	9% 2 14% 35% 36% 5%	650
June	1st to 10th	Night	9% 2 11% 38% 34% 7%	600
	11th to 20th	Night	6% 1 9% 32% 40% 11%	600
	21st to end	Night	7% 2 4 34% 44% 10%	600
July	1st to 10th	Night	9% 2 6% 24% 45% 14%	585
	11th to 20th	Night	8% 4 17% 53% 19%	582
	21st to end	Night	5% 1 4 23% 55% 12%	657
August	1st to 10th	Night	9% 1 6% 24% 49% 12%	597
	11th to 20th	Night	9% 1 5% 20% 52% 14%	594
	21st to end	Night	10% 2 7% 28% 39% 13%	715
September	1st to 10th	Night	11% 1 10% 29% 39% 11%	800
	11th to 20th	Night	12% 2 10% 36% 36% 5%	800
	21st to end	Night	12% 2 14% 30% 39% 4	800
October	1st to 10th	Night	13% 3 20% 36% 26% 3	800
	11th to 20th	Night	10% 4 20% 36% 27% 4	800
	21st to end	Night	11% 6% 23% 35% 25% 1	880
November	1st to 10th	Night	13% 6% 19% 39% 21% 2	820
	11th to 20th	Night	13% 9% 27% 34% 17% 1	840
	21st to end	Night	12% 8% 31% 34% 15% 1	840
December	1st to 10th	Night	15% 8% 32% 31% 13% 1	840
	11th to 20th	Night	17% 7% 30% 32% 13% 1	840
	21st to end	Night	21% 10% 25% 33% 12%	924

Page generated at 2026-02-13 01:42:12. Version ac07

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