Thornton Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thornton Observing Site clear sky chart from 2005-11-06 to 2026-07-31.

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	15% 11% 4 10% 6% 5% 5% 4 5% 7% 29%	138115
	Night	15% 13% 5% 10% 6% 4 4 3 5% 6% 29%	134885
Transparency	Day	52% 7% 10% 19% 10% 1	141718
	Night	54% 6% 10% 18% 10% 2	139352
Seeing	Day	28% 18% 25% 18% 9% 3	17011
	Night	26% 16% 23% 24% 10% 1	26663

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	15% 19% 7% 21% 8% 4 3 3 4 4 14%	5195
	11th to 20th	Day	9% 17% 9% 24% 12% 6% 3 2 3 2 14%	5280
	21st to end	Day	15% 14% 9% 10% 11% 6% 4 3 4 5% 20%	5780
February	1st to 10th	Day	15% 17% 6% 12% 8% 5% 4 3 3 5% 22%	1890
	11th to 20th	Day	16% 16% 7% 14% 5% 4 5% 3 4 5% 22%	1843
	21st to end	Day	17% 18% 6% 13% 4 3 3 3 4 5% 25%	1745
March	1st to 10th	Day	14% 13% 9% 8% 5% 4 3 3 4 6% 30%	5045
	11th to 20th	Day	12% 12% 4 9% 5% 4 4 3 5% 6% 35%	5090
	21st to end	Day	15% 12% 4 9% 6% 5% 4 3 4 6% 31%	5586
April	1st to 10th	Day	16% 13% 5% 7% 5% 4 4 4 5% 6% 28%	2450
	11th to 20th	Day	14% 14% 3 7% 5% 4 5% 3 5% 6% 33%	2494
	21st to end	Day	21% 10% 3 7% 4 3 4 3 5% 7% 32%	2694
May	1st to 10th	Day	18% 10% 3 9% 6% 6% 6% 4 6% 7% 26%	7616
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 5% 6% 8% 29%	7626
	21st to end	Day	23% 9% 3 7% 5% 4 5% 4 5% 7% 29%	8964
June	1st to 10th	Day	15% 7% 3 7% 5% 5% 5% 4 6% 8% 34%	2940
	11th to 20th	Day	16% 7% 3 7% 6% 4 5% 5% 7% 9% 32%	2940
	21st to end	Day	16% 6% 2 6% 6% 4 5% 4 6% 10% 34%	2940
July	1st to 10th	Day	10% 6% 2 7% 6% 6% 6% 4 6% 8% 39%	6364
	11th to 20th	Day	12% 8% 2 8% 6% 5% 6% 5% 7% 10% 30%	6030
	21st to end	Day	13% 8% 3 7% 5% 5% 5% 4 6% 9% 35%	6554
August	1st to 10th	Day	14% 8% 3 8% 5% 5% 6% 5% 7% 9% 29%	4234
	11th to 20th	Day	12% 7% 3 8% 5% 5% 6% 5% 6% 8% 34%	4174
	21st to end	Day	8% 4 2 6% 4 4 6% 5% 7% 10% 42%	4379
September	1st to 10th	Day	16% 6% 2 5% 4 4 4 3 6% 7% 43%	2400
	11th to 20th	Day	13% 7% 2 5% 4 4 4 4 5% 8% 44%	2255
	21st to end	Day	19% 10% 3 8% 5% 4 4 4 4 6% 32%	2169
October	1st to 10th	Day	17% 7% 2 5% 4 4 4 4 5% 8% 40%	2000
	11th to 20th	Day	19% 11% 2 7% 4 4 4 3 4 5% 37%	2000
	21st to end	Day	21% 15% 3 6% 3 3 3 3 4 5% 35%	2200
November	1st to 10th	Day	12% 12% 3 6% 4 3 3 4 4 6% 42%	2041
	11th to 20th	Day	16% 16% 5% 8% 4 3 5% 4 4 6% 29%	1719
	21st to end	Day	17% 19% 8% 10% 4 3 3 3 4 5% 24%	1680
December	1st to 10th	Day	15% 12% 7% 25% 6% 4 4 3 4 3 16%	2680
	11th to 20th	Day	20% 17% 6% 13% 4 4 3 2 4 5% 22%	3070
	21st to end	Day	16% 22% 7% 12% 6% 4 4 3 5% 6% 16%	4048
January	1st to 10th	Night	12% 19% 8% 21% 10% 5% 4 3 3 3 13%	7694
	11th to 20th	Night	10% 15% 8% 21% 10% 6% 4 3 3 3 16%	7680
	21st to end	Night	15% 14% 6% 10% 9% 7% 5% 4 4 4 21%	8401
February	1st to 10th	Night	15% 15% 6% 12% 6% 5% 4 3 4 4 25%	2730
	11th to 20th	Night	17% 16% 6% 14% 6% 5% 5% 3 4 5% 20%	2663
	21st to end	Night	16% 15% 5% 13% 6% 4 3 3 5% 5% 24%	2058
March	1st to 10th	Night	16% 14% 6% 9% 6% 4 4 3 4 5% 30%	5028
	11th to 20th	Night	10% 11% 7% 8% 4 4 4 3 4 7% 38%	5090
	21st to end	Night	13% 12% 5% 9% 4 4 4 4 5% 6% 33%	5588
April	1st to 10th	Night	16% 16% 4 7% 5% 4 4 4 4 5% 32%	2168
	11th to 20th	Night	16% 11% 3 7% 5% 5% 4 3 6% 7% 34%	2075
	21st to end	Night	15% 11% 3 6% 5% 4 4 3 6% 7% 35%	1943
May	1st to 10th	Night	20% 13% 4 8% 5% 4 5% 4 5% 6% 26%	5237
	11th to 20th	Night	15% 12% 3 8% 5% 5% 5% 4 6% 8% 29%	5234
	21st to end	Night	24% 12% 3 7% 5% 4 3 3 4 6% 29%	5175
June	1st to 10th	Night	13% 6% 3 6% 4 4 4 4 6% 8% 43%	1680
	11th to 20th	Night	13% 7% 3 7% 5% 4 4 4 6% 9% 37%	1680
	21st to end	Night	14% 7% 2 6% 4 5% 3 3 6% 10% 39%	1680
July	1st to 10th	Night	12% 10% 3 8% 5% 4 4 4 6% 8% 36%	3808
	11th to 20th	Night	14% 8% 3 7% 5% 5% 5% 4 7% 8% 33%	4147
	21st to end	Night	13% 8% 3 6% 5% 5% 4 4 6% 8% 38%	4517
August	1st to 10th	Night	14% 9% 3 7% 5% 5% 4 4 7% 10% 32%	2915
	11th to 20th	Night	16% 8% 3 6% 4 4 4 3 5% 7% 40%	2875
	21st to end	Night	8% 5% 2 6% 4 5% 5% 5% 6% 9% 45%	3484
September	1st to 10th	Night	13% 5% 2 6% 5% 4 4 3 5% 7% 46%	2000
	11th to 20th	Night	11% 5% 2 4 3 3 4 4 6% 8% 50%	2140
	21st to end	Night	18% 8% 3 7% 5% 3 4 4 4 6% 38%	2230
October	1st to 10th	Night	17% 7% 1 6% 3 4 4 3 5% 7% 44%	2400
	11th to 20th	Night	16% 8% 2 5% 4 4 4 4 6% 8% 38%	2400
	21st to end	Night	22% 12% 2 5% 3 3 4 3 4 5% 36%	2640
November	1st to 10th	Night	12% 12% 3 6% 4 4 3 3 4 6% 44%	2472
	11th to 20th	Night	17% 16% 5% 9% 5% 3 3 3 5% 5% 29%	2898
	21st to end	Night	16% 17% 7% 11% 4 4 4 3 4 4 27%	2940
December	1st to 10th	Night	16% 16% 7% 22% 5% 3 2 2 3 3 20%	4720
	11th to 20th	Night	19% 15% 8% 14% 5% 3 3 3 4 5% 22%	5345
	21st to end	Night	15% 20% 9% 12% 6% 4 4 3 4 5% 17%	7150

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	74% 4 8% 10% 4 1	5196
	11th to 20th	Day	76% 4 6% 7% 5% 2	5280
	21st to end	Day	66% 4 8% 14% 7% 1	5780
February	1st to 10th	Day	65% 4 10% 13% 9%	1890
	11th to 20th	Day	66% 5% 8% 13% 8%	1852
	21st to end	Day	62% 5% 9% 14% 8% 2	1745
March	1st to 10th	Day	54% 5% 9% 23% 9% 2	5045
	11th to 20th	Day	48% 5% 12% 25% 9% 2	5090
	21st to end	Day	53% 6% 9% 18% 12% 1	5586
April	1st to 10th	Day	54% 7% 10% 16% 11% 2	2450
	11th to 20th	Day	50% 7% 11% 15% 14% 2	2494
	21st to end	Day	52% 7% 9% 23% 9% 1	2694
May	1st to 10th	Day	54% 8% 10% 20% 6% 1	7651
	11th to 20th	Day	50% 9% 11% 23% 8% 1	7630
	21st to end	Day	52% 8% 10% 14% 13% 3	8979
June	1st to 10th	Day	46% 8% 11% 22% 12% 1	2940
	11th to 20th	Day	47% 9% 12% 21% 10% 1	2940
	21st to end	Day	44% 9% 13% 24% 10%	2940
July	1st to 10th	Day	40% 8% 11% 25% 13% 2	6296
	11th to 20th	Day	44% 10% 14% 22% 9%	5961
	21st to end	Day	43% 7% 10% 22% 17% 1	6370
August	1st to 10th	Day	48% 10% 13% 21% 8%	4221
	11th to 20th	Day	43% 9% 10% 29% 9%	4213
	21st to end	Day	34% 10% 15% 27% 13% 1	4379
September	1st to 10th	Day	39% 8% 11% 21% 18% 3	2400
	11th to 20th	Day	37% 7% 11% 28% 15% 2	2255
	21st to end	Day	51% 7% 11% 17% 13% 1	2169
October	1st to 10th	Day	35% 6% 8% 27% 19% 4	3260
	11th to 20th	Day	43% 6% 11% 20% 11% 8%	3240
	21st to end	Day	36% 4 9% 28% 21% 1	3538
November	1st to 10th	Day	43% 6% 10% 20% 16% 5%	2041
	11th to 20th	Day	55% 6% 9% 16% 12% 3	1719
	21st to end	Day	61% 6% 10% 13% 9%	1680
December	1st to 10th	Day	70% 6% 13% 8% 2	2680
	11th to 20th	Day	65% 4 10% 16% 5% 1	3066
	21st to end	Day	69% 4 10% 11% 5%	4048
January	1st to 10th	Night	75% 4 6% 10% 4	7695
	11th to 20th	Night	70% 5% 8% 10% 5% 2	7680
	21st to end	Night	64% 5% 9% 14% 7% 1	8401
February	1st to 10th	Night	62% 5% 8% 15% 9%	2730
	11th to 20th	Night	66% 5% 8% 13% 8%	2665
	21st to end	Night	61% 5% 8% 15% 9% 2	2058
March	1st to 10th	Night	56% 6% 8% 21% 8% 2	5028
	11th to 20th	Night	44% 7% 13% 26% 7% 4	5090
	21st to end	Night	49% 6% 11% 19% 13% 2	5588
April	1st to 10th	Night	55% 5% 10% 16% 11% 2	2168
	11th to 20th	Night	48% 7% 12% 16% 13% 3	2075
	21st to end	Night	47% 8% 10% 25% 8% 3	1943
May	1st to 10th	Night	56% 8% 12% 14% 8% 2	5264
	11th to 20th	Night	49% 10% 11% 23% 7% 1	5235
	21st to end	Night	56% 6% 10% 17% 9% 2	5182
June	1st to 10th	Night	39% 8% 11% 26% 14% 1	1680
	11th to 20th	Night	42% 9% 12% 24% 11% 1	1680
	21st to end	Night	40% 8% 15% 23% 14% 1	1680
July	1st to 10th	Night	44% 8% 11% 23% 12% 2	3768
	11th to 20th	Night	45% 10% 16% 19% 10% 1	4096
	21st to end	Night	41% 8% 11% 22% 17% 1	4387
August	1st to 10th	Night	44% 10% 16% 21% 8%	2904
	11th to 20th	Night	43% 7% 10% 26% 14% 1	2902
	21st to end	Night	33% 9% 13% 31% 12% 2	3484
September	1st to 10th	Night	38% 8% 11% 22% 17% 4	2000
	11th to 20th	Night	31% 8% 12% 28% 20% 1	2140
	21st to end	Night	47% 6% 10% 20% 16% 1	2230
October	1st to 10th	Night	35% 5% 10% 26% 22% 2	3920
	11th to 20th	Night	35% 7% 11% 22% 16% 9%	3899
	21st to end	Night	37% 4 9% 27% 21% 2	4252
November	1st to 10th	Night	43% 5% 10% 20% 18% 5%	2472
	11th to 20th	Night	57% 5% 7% 16% 12% 3	2898
	21st to end	Night	61% 5% 9% 16% 8% 1	2940
December	1st to 10th	Night	68% 6% 11% 12% 3 1	4720
	11th to 20th	Night	66% 4 11% 14% 5%	5348
	21st to end	Night	70% 5% 9% 10% 5%	7150

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	38% 31% 20% 9% 3 1	317
	11th to 20th	Day	47% 28% 22% 3 1	330
	21st to end	Day	40% 32% 23% 4 2	363
February	1st to 10th	Day	40% 28% 25% 6% 1	330
	11th to 20th	Day	48% 29% 20% 4	330
	21st to end	Day	42% 27% 25% 6%	352
March	1st to 10th	Day	35% 28% 24% 11% 2	540
	11th to 20th	Day	38% 24% 24% 10% 4 1	540
	21st to end	Day	35% 22% 28% 12% 3	594
April	1st to 10th	Day	35% 23% 24% 13% 4 1	540
	11th to 20th	Day	29% 25% 27% 12% 5% 2	532
	21st to end	Day	33% 21% 26% 15% 4 1	540
May	1st to 10th	Day	33% 20% 26% 13% 7% 2	543
	11th to 20th	Day	28% 18% 28% 17% 8% 2	567
	21st to end	Day	22% 15% 24% 23% 10% 6%	617
June	1st to 10th	Day	20% 14% 28% 24% 10% 5%	570
	11th to 20th	Day	25% 15% 22% 21% 13% 3	570
	21st to end	Day	19% 13% 27% 25% 13% 2	570
July	1st to 10th	Day	18% 7% 25% 25% 17% 9%	565
	11th to 20th	Day	15% 8% 28% 28% 16% 6%	564
	21st to end	Day	19% 10% 25% 23% 18% 4	616
August	1st to 10th	Day	15% 8% 26% 28% 17% 6%	526
	11th to 20th	Day	18% 8% 22% 28% 17% 8%	519
	21st to end	Day	15% 7% 25% 28% 17% 8%	583
September	1st to 10th	Day	21% 10% 18% 25% 16% 9%	530
	11th to 20th	Day	14% 4 25% 29% 21% 8%	530
	21st to end	Day	26% 16% 25% 17% 13% 3	530
October	1st to 10th	Day	20% 12% 23% 26% 15% 3	530
	11th to 20th	Day	28% 15% 28% 19% 8% 2	530
	21st to end	Day	33% 20% 25% 15% 6% 1	583
November	1st to 10th	Day	24% 20% 24% 22% 8% 2	519
	11th to 20th	Day	38% 21% 25% 11% 4 1	239
	21st to end	Day	42% 21% 28% 8% 1	220
December	1st to 10th	Day	36% 35% 19% 5% 3 1	220
	11th to 20th	Day	41% 25% 25% 9%	220
	21st to end	Day	39% 30% 20% 9% 2	242
January	1st to 10th	Night	33% 29% 24% 10% 3	840
	11th to 20th	Night	37% 25% 25% 13% 1	840
	21st to end	Night	32% 30% 25% 12% 1	924
February	1st to 10th	Night	32% 23% 27% 16% 1	840
	11th to 20th	Night	36% 29% 25% 10%	823
	21st to end	Night	33% 27% 26% 13% 1	692
March	1st to 10th	Night	30% 22% 28% 17% 3	840
	11th to 20th	Night	32% 24% 20% 18% 5% 1	840
	21st to end	Night	29% 22% 28% 17% 4 1	924
April	1st to 10th	Night	30% 21% 28% 18% 4	840
	11th to 20th	Night	26% 16% 27% 23% 8% 1	831
	21st to end	Night	25% 13% 29% 26% 7% 1	683
May	1st to 10th	Night	27% 12% 25% 27% 9% 1	630
	11th to 20th	Night	25% 11% 24% 30% 10% 1	621
	21st to end	Night	17% 8% 23% 30% 20% 2	484
June	1st to 10th	Night	15% 6% 24% 37% 18%	420
	11th to 20th	Night	16% 6% 23% 37% 16% 1	420
	21st to end	Night	16% 4 24% 36% 19% 1	420
July	1st to 10th	Night	14% 5% 18% 36% 25% 3	460
	11th to 20th	Night	14% 4 16% 35% 27% 4	612
	21st to end	Night	17% 4 14% 40% 23% 2	687
August	1st to 10th	Night	12% 3 13% 45% 26% 1	597
	11th to 20th	Night	14% 5% 12% 37% 28% 3	594
	21st to end	Night	14% 5% 16% 34% 27% 3	817
September	1st to 10th	Night	16% 6% 19% 31% 23% 4	800
	11th to 20th	Night	13% 5% 19% 34% 26% 3	800
	21st to end	Night	23% 9% 20% 28% 18% 2	800
October	1st to 10th	Night	21% 10% 19% 30% 18% 3	800
	11th to 20th	Night	23% 14% 24% 29% 10% 1	800
	21st to end	Night	33% 13% 22% 24% 6% 1	880
November	1st to 10th	Night	22% 13% 24% 31% 9% 1	820
	11th to 20th	Night	31% 21% 22% 18% 7% 1	840
	21st to end	Night	32% 19% 27% 17% 4 1	840
December	1st to 10th	Night	36% 24% 23% 13% 4	840
	11th to 20th	Night	35% 24% 24% 16% 2	840
	21st to end	Night	33% 23% 27% 16% 1	924

Page generated at 2026-07-28 03:25:20. Version ac07

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