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 2024-02-25.

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	17% 11% 5% 8% 6% 3 5% 5% 5% 7% 29%	78490
	Night	16% 12% 6% 8% 6% 3 4 4 5% 6% 30%	80807
Transparency	Day	54% 7% 10% 18% 10% 1	78208
	Night	55% 6% 10% 18% 10% 2	80608
Seeing	Day	26% 18% 24% 18% 10% 3	13779
	Night	25% 16% 23% 24% 11% 1	23604

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	17% 20% 7% 14% 6% 3 3 3 4 4 18%	2800
	11th to 20th	Day	10% 19% 10% 20% 11% 4 3 2 3 2 16%	2840
	21st to end	Day	21% 17% 12% 11% 9% 2 4 3 3 4 14%	3124
February	1st to 10th	Day	17% 15% 7% 10% 9% 4 5% 3 3 5% 22%	1710
	11th to 20th	Day	16% 15% 7% 12% 7% 3 5% 4 4 5% 22%	1663
	21st to end	Day	19% 17% 6% 12% 4 2 3 4 3 5% 24%	1529
March	1st to 10th	Day	14% 12% 7% 8% 6% 3 4 4 4 6% 31%	1980
	11th to 20th	Day	17% 15% 6% 8% 6% 3 4 4 4 5% 27%	1980
	21st to end	Day	21% 14% 5% 7% 6% 3 4 4 4 6% 27%	2178
April	1st to 10th	Day	15% 12% 7% 6% 6% 3 5% 6% 5% 7% 30%	2099
	11th to 20th	Day	16% 13% 4 6% 6% 3 5% 4 5% 6% 33%	2134
	21st to end	Day	23% 10% 4 6% 5% 2 4 5% 4 7% 31%	2308
May	1st to 10th	Day	19% 10% 4 6% 7% 4 6% 5% 5% 6% 28%	2340
	11th to 20th	Day	24% 8% 3 6% 6% 4 5% 5% 5% 7% 29%	2301
	21st to end	Day	14% 8% 3 6% 6% 4 5% 6% 7% 9% 33%	2699
June	1st to 10th	Day	16% 6% 3 6% 6% 4 7% 5% 5% 8% 34%	2520
	11th to 20th	Day	16% 6% 3 6% 7% 3 5% 5% 6% 10% 32%	2520
	21st to end	Day	17% 5% 2 5% 6% 3 6% 5% 5% 10% 34%	2520
July	1st to 10th	Day	15% 6% 2 6% 7% 4 6% 6% 6% 8% 35%	2471
	11th to 20th	Day	13% 6% 2 6% 6% 4 6% 6% 7% 9% 35%	2340
	21st to end	Day	15% 7% 4 6% 6% 4 6% 6% 6% 9% 31%	2547
August	1st to 10th	Day	13% 6% 3 6% 7% 5% 7% 7% 7% 10% 30%	2327
	11th to 20th	Day	14% 7% 3 6% 7% 3 5% 7% 7% 9% 32%	2303
	21st to end	Day	14% 6% 3 6% 5% 4 5% 6% 6% 9% 35%	2413
September	1st to 10th	Day	17% 6% 2 5% 5% 3 4 4 6% 8% 40%	2160
	11th to 20th	Day	13% 6% 3 5% 6% 3 4 5% 5% 8% 42%	2030
	21st to end	Day	18% 9% 3 7% 6% 3 4 5% 4 6% 35%	1953
October	1st to 10th	Day	18% 7% 3 5% 5% 3 4 5% 6% 8% 37%	1800
	11th to 20th	Day	21% 11% 2 5% 5% 3 4 4 4 6% 36%	1800
	21st to end	Day	23% 15% 3 5% 4 3 3 3 3 4 34%	1980
November	1st to 10th	Day	13% 12% 3 6% 4 2 4 4 4 6% 41%	1843
	11th to 20th	Day	17% 14% 6% 7% 4 3 4 5% 4 7% 30%	1556
	21st to end	Day	18% 17% 8% 8% 4 3 3 4 4 5% 25%	1520
December	1st to 10th	Day	19% 16% 11% 13% 5% 3 4 4 3 4 17%	1520
	11th to 20th	Day	17% 17% 8% 13% 5% 3 4 2 4 5% 22%	1910
	21st to end	Day	19% 21% 9% 10% 6% 3 4 3 5% 5% 16%	2772
January	1st to 10th	Night	13% 21% 10% 14% 7% 3 4 3 3 4 17%	4141
	11th to 20th	Night	13% 17% 10% 17% 7% 5% 5% 4 3 4 16%	4120
	21st to end	Night	23% 17% 9% 11% 10% 3 3 2 2 3 17%	4532
February	1st to 10th	Night	16% 14% 7% 11% 7% 3 4 4 4 4 25%	2470
	11th to 20th	Night	17% 15% 7% 11% 7% 4 5% 4 4 5% 21%	2403
	21st to end	Night	18% 15% 6% 12% 6% 3 4 3 5% 5% 23%	1820
March	1st to 10th	Night	15% 11% 6% 9% 7% 3 6% 3 4 6% 29%	1980
	11th to 20th	Night	18% 11% 5% 8% 6% 3 5% 4 4 7% 31%	1980
	21st to end	Night	18% 13% 6% 8% 4 3 4 3 4 6% 32%	2178
April	1st to 10th	Night	16% 14% 5% 6% 5% 3 4 5% 4 5% 33%	1859
	11th to 20th	Night	16% 9% 3 5% 6% 3 4 4 5% 7% 35%	1775
	21st to end	Night	17% 10% 3 5% 6% 2 4 4 6% 7% 35%	1666
May	1st to 10th	Night	20% 9% 3 6% 4 3 4 5% 5% 7% 33%	1620
	11th to 20th	Night	19% 9% 3 6% 5% 3 5% 5% 6% 6% 32%	1592
	21st to end	Night	10% 6% 3 5% 6% 3 4 5% 6% 10% 42%	1575
June	1st to 10th	Night	13% 5% 3 5% 4 3 5% 5% 6% 8% 42%	1440
	11th to 20th	Night	13% 7% 3 6% 6% 3 5% 5% 6% 9% 37%	1440
	21st to end	Night	14% 5% 2 5% 5% 3 4 5% 6% 10% 40%	1440
July	1st to 10th	Night	13% 6% 3 6% 5% 2 5% 4 6% 8% 41%	1484
	11th to 20th	Night	10% 5% 3 4 5% 3 5% 4 6% 9% 46%	1620
	21st to end	Night	12% 6% 3 5% 5% 3 6% 6% 8% 8% 38%	1771
August	1st to 10th	Night	10% 6% 3 6% 6% 4 4 6% 8% 9% 39%	1611
	11th to 20th	Night	11% 6% 3 6% 5% 4 4 5% 5% 8% 43%	1598
	21st to end	Night	12% 6% 2 5% 5% 2 4 5% 4 9% 45%	1923
September	1st to 10th	Night	14% 5% 2 6% 5% 3 4 4 6% 7% 44%	1800
	11th to 20th	Night	12% 5% 2 4 3 2 4 4 6% 8% 50%	1926
	21st to end	Night	17% 7% 3 6% 5% 3 3 5% 4 6% 41%	2006
October	1st to 10th	Night	17% 7% 2 5% 4 3 4 3 5% 8% 42%	2160
	11th to 20th	Night	17% 8% 3 5% 5% 3 4 5% 6% 8% 36%	2160
	21st to end	Night	25% 11% 3 5% 4 2 3 4 5% 5% 34%	2376
November	1st to 10th	Night	12% 12% 3 6% 4 3 4 3 4 6% 43%	2230
	11th to 20th	Night	17% 15% 5% 7% 7% 2 3 4 5% 5% 31%	2622
	21st to end	Night	16% 15% 7% 9% 5% 3 4 4 4 5% 28%	2660
December	1st to 10th	Night	20% 18% 10% 13% 5% 2 3 3 4 4 18%	2660
	11th to 20th	Night	18% 15% 8% 13% 7% 3 4 3 4 4 21%	3285
	21st to end	Night	18% 21% 10% 9% 7% 3 4 4 4 5% 15%	4884

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	69% 4 9% 12% 5% 1	2801
	11th to 20th	Day	76% 3 7% 8% 5% 1	2840
	21st to end	Day	74% 4 7% 11% 4	3124
February	1st to 10th	Day	65% 4 10% 12% 8%	1710
	11th to 20th	Day	65% 5% 8% 13% 9%	1672
	21st to end	Day	63% 5% 8% 13% 9% 2	1529
March	1st to 10th	Day	53% 6% 12% 19% 9% 1	1980
	11th to 20th	Day	59% 5% 9% 16% 9% 2	1980
	21st to end	Day	58% 6% 7% 17% 10% 2	2178
April	1st to 10th	Day	53% 7% 10% 17% 11% 2	2099
	11th to 20th	Day	51% 7% 10% 14% 15% 3	2134
	21st to end	Day	53% 7% 9% 21% 9% 1	2308
May	1st to 10th	Day	55% 7% 9% 16% 10% 1	2340
	11th to 20th	Day	55% 8% 11% 16% 10% 2	2305
	21st to end	Day	45% 10% 15% 20% 8% 1	2699
June	1st to 10th	Day	47% 8% 12% 22% 10% 1	2520
	11th to 20th	Day	48% 9% 12% 20% 10% 1	2520
	21st to end	Day	44% 9% 13% 24% 9%	2520
July	1st to 10th	Day	46% 10% 11% 19% 12% 3	2403
	11th to 20th	Day	43% 8% 13% 23% 13% 1	2271
	21st to end	Day	48% 8% 12% 18% 13% 1	2405
August	1st to 10th	Day	46% 12% 13% 21% 8%	2314
	11th to 20th	Day	45% 10% 12% 23% 10%	2303
	21st to end	Day	44% 9% 13% 21% 11% 2	2413
September	1st to 10th	Day	42% 8% 12% 19% 16% 3	2160
	11th to 20th	Day	38% 7% 11% 28% 13% 2	2030
	21st to end	Day	49% 7% 11% 19% 14% 1	1953
October	1st to 10th	Day	45% 8% 11% 20% 13% 2	1800
	11th to 20th	Day	51% 6% 11% 23% 10%	1800
	21st to end	Day	55% 4 8% 18% 12% 2	1980
November	1st to 10th	Day	44% 6% 10% 20% 15% 5%	1843
	11th to 20th	Day	54% 6% 9% 16% 12% 3	1556
	21st to end	Day	60% 7% 10% 14% 9%	1520
December	1st to 10th	Day	70% 6% 8% 11% 4 1	1520
	11th to 20th	Day	66% 3 8% 18% 4 1	1906
	21st to end	Day	69% 4 12% 10% 5%	2772
January	1st to 10th	Night	72% 4 6% 12% 6% 1	4141
	11th to 20th	Night	73% 4 10% 10% 3	4120
	21st to end	Night	75% 4 5% 12% 5% 1	4532
February	1st to 10th	Night	62% 5% 9% 16% 8%	2470
	11th to 20th	Night	66% 5% 7% 13% 8%	2405
	21st to end	Night	62% 5% 8% 14% 8% 2	1820
March	1st to 10th	Night	55% 6% 9% 19% 10% 1	1980
	11th to 20th	Night	53% 7% 10% 19% 9% 1	1980
	21st to end	Night	54% 5% 8% 19% 11% 3	2178
April	1st to 10th	Night	54% 6% 11% 16% 12% 2	1859
	11th to 20th	Night	48% 7% 12% 16% 14% 3	1775
	21st to end	Night	48% 8% 9% 24% 8% 3	1666
May	1st to 10th	Night	49% 7% 11% 20% 10% 3	1620
	11th to 20th	Night	50% 10% 10% 15% 13% 2	1593
	21st to end	Night	37% 9% 18% 26% 10% 1	1576
June	1st to 10th	Night	39% 9% 12% 26% 13% 1	1440
	11th to 20th	Night	42% 10% 12% 25% 10% 1	1440
	21st to end	Night	39% 8% 16% 23% 13% 1	1440
July	1st to 10th	Night	39% 9% 14% 23% 12% 3	1442
	11th to 20th	Night	34% 10% 13% 25% 16% 1	1566
	21st to end	Night	39% 9% 15% 22% 12% 2	1672
August	1st to 10th	Night	38% 11% 16% 23% 13%	1600
	11th to 20th	Night	40% 8% 14% 24% 13% 1	1598
	21st to end	Night	37% 8% 13% 26% 13% 3	1923
September	1st to 10th	Night	39% 8% 12% 23% 15% 4	1800
	11th to 20th	Night	31% 8% 13% 27% 20% 1	1926
	21st to end	Night	44% 6% 10% 22% 17% 1	2006
October	1st to 10th	Night	42% 7% 12% 23% 14% 2	2160
	11th to 20th	Night	43% 7% 12% 24% 13% 1	2160
	21st to end	Night	52% 4 10% 20% 12% 2	2376
November	1st to 10th	Night	43% 5% 11% 19% 17% 5%	2230
	11th to 20th	Night	56% 5% 8% 16% 13% 3	2622
	21st to end	Night	60% 5% 9% 17% 8% 2	2660
December	1st to 10th	Night	70% 5% 8% 12% 4 1	2660
	11th to 20th	Night	67% 4 8% 15% 6%	3288
	21st to end	Night	71% 5% 9% 10% 5%	4884

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	34% 33% 20% 10% 3 1	259
	11th to 20th	Day	41% 30% 24% 3 1	270
	21st to end	Day	36% 32% 25% 4 2	297
February	1st to 10th	Day	37% 29% 27% 6% 1	270
	11th to 20th	Day	44% 29% 22% 4	270
	21st to end	Day	42% 26% 26% 6%	277
March	1st to 10th	Day	33% 29% 24% 12% 2	420
	11th to 20th	Day	35% 26% 24% 10% 4 1	420
	21st to end	Day	34% 21% 27% 13% 4	462
April	1st to 10th	Day	30% 25% 24% 15% 6% 1	420
	11th to 20th	Day	26% 25% 26% 14% 7% 3	412
	21st to end	Day	34% 21% 24% 15% 5% 1	420
May	1st to 10th	Day	32% 20% 25% 13% 8% 2	423
	11th to 20th	Day	27% 18% 28% 16% 9% 2	447
	21st to end	Day	18% 14% 24% 25% 12% 7%	485
June	1st to 10th	Day	20% 12% 28% 25% 11% 4	450
	11th to 20th	Day	24% 14% 21% 23% 15% 3	450
	21st to end	Day	19% 14% 27% 25% 12% 3	450
July	1st to 10th	Day	19% 6% 23% 24% 17% 11%	445
	11th to 20th	Day	13% 7% 29% 28% 16% 7%	444
	21st to end	Day	17% 10% 25% 25% 18% 5%	488
August	1st to 10th	Day	15% 8% 23% 29% 19% 6%	446
	11th to 20th	Day	18% 7% 21% 28% 17% 9%	439
	21st to end	Day	17% 6% 26% 27% 16% 8%	495
September	1st to 10th	Day	22% 10% 17% 26% 16% 10%	450
	11th to 20th	Day	15% 3 25% 28% 20% 8%	450
	21st to end	Day	23% 16% 24% 19% 14% 3	450
October	1st to 10th	Day	21% 13% 24% 25% 15% 2	450
	11th to 20th	Day	29% 15% 28% 19% 8% 1	450
	21st to end	Day	35% 20% 24% 15% 6% 1	495
November	1st to 10th	Day	22% 19% 23% 24% 9% 2	441
	11th to 20th	Day	34% 23% 25% 12% 5% 1	196
	21st to end	Day	36% 23% 31% 8% 2	180
December	1st to 10th	Day	34% 34% 20% 7% 4 1	180
	11th to 20th	Day	40% 25% 24% 11% 1	180
	21st to end	Day	35% 32% 20% 10% 2 1	198
January	1st to 10th	Night	32% 29% 24% 11% 4	760
	11th to 20th	Night	35% 25% 26% 14% 1	760
	21st to end	Night	30% 30% 26% 13% 1	836
February	1st to 10th	Night	31% 23% 27% 17% 1	760
	11th to 20th	Night	34% 31% 24% 10% 1	743
	21st to end	Night	33% 27% 25% 13% 1	611
March	1st to 10th	Night	29% 23% 28% 18% 3	720
	11th to 20th	Night	30% 25% 20% 18% 6% 1	720
	21st to end	Night	28% 20% 28% 19% 4 1	792
April	1st to 10th	Night	28% 21% 27% 18% 5%	720
	11th to 20th	Night	24% 16% 27% 24% 9% 1	711
	21st to end	Night	24% 14% 31% 24% 7% 1	586
May	1st to 10th	Night	27% 12% 23% 27% 10% 1	540
	11th to 20th	Night	25% 11% 23% 29% 11% 1	531
	21st to end	Night	13% 6% 23% 33% 22% 3	415
June	1st to 10th	Night	14% 5% 24% 38% 20%	360
	11th to 20th	Night	15% 6% 23% 36% 18% 1	360
	21st to end	Night	16% 4 24% 38% 19%	360
July	1st to 10th	Night	14% 4 18% 37% 25% 3	392
	11th to 20th	Night	13% 4 16% 34% 28% 4	522
	21st to end	Night	16% 4 14% 39% 25% 2	591
August	1st to 10th	Night	12% 2 12% 44% 28% 1	537
	11th to 20th	Night	14% 6% 12% 36% 29% 4	534
	21st to end	Night	15% 5% 16% 34% 27% 4	735
September	1st to 10th	Night	16% 7% 18% 31% 25% 4	720
	11th to 20th	Night	14% 6% 19% 33% 26% 3	720
	21st to end	Night	22% 9% 20% 28% 19% 2	720
October	1st to 10th	Night	22% 10% 20% 28% 18% 3	720
	11th to 20th	Night	24% 14% 23% 28% 9% 1	720
	21st to end	Night	35% 13% 22% 23% 6%	792
November	1st to 10th	Night	21% 13% 23% 32% 10% 1	740
	11th to 20th	Night	29% 21% 23% 19% 7% 1	760
	21st to end	Night	30% 20% 27% 18% 4 1	760
December	1st to 10th	Night	36% 24% 22% 14% 4	760
	11th to 20th	Night	33% 24% 24% 17% 2	760
	21st to end	Night	32% 24% 26% 16% 1	836

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