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 2025-03-27.

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% 12% 4 9% 6% 4 4 4 5% 7% 30%	109028
	Night	15% 13% 5% 11% 6% 4 4 3 5% 6% 29%	110311
Transparency	Day	52% 7% 11% 19% 10% 1	112675
	Night	54% 6% 10% 19% 10% 2	114803
Seeing	Day	27% 18% 25% 18% 9% 3	15204
	Night	25% 16% 23% 24% 11% 1	25018

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	13% 19% 7% 22% 8% 4 3 2 3 3 15%	4006
	11th to 20th	Day	10% 18% 9% 24% 11% 6% 3 2 2 2 13%	4060
	21st to end	Day	17% 16% 11% 11% 8% 3 4 2 4 5% 20%	4466
February	1st to 10th	Day	16% 17% 6% 12% 8% 4 4 3 3 5% 22%	1800
	11th to 20th	Day	16% 17% 7% 13% 5% 4 5% 3 4 5% 22%	1753
	21st to end	Day	18% 18% 5% 13% 4 3 3 3 4 5% 24%	1664
March	1st to 10th	Day	17% 13% 7% 8% 5% 4 3 3 5% 6% 28%	3545
	11th to 20th	Day	13% 13% 5% 11% 6% 4 4 3 5% 6% 31%	3590
	21st to end	Day	18% 12% 4 7% 5% 5% 4 3 4 6% 31%	3881
April	1st to 10th	Day	15% 14% 5% 7% 5% 4 5% 4 5% 6% 29%	2216
	11th to 20th	Day	15% 14% 4 6% 5% 4 5% 3 5% 6% 34%	2254
	21st to end	Day	22% 10% 3 7% 4 4 4 4 4 7% 31%	2437
May	1st to 10th	Day	23% 10% 4 8% 5% 5% 5% 4 5% 5% 26%	4082
	11th to 20th	Day	22% 9% 3 8% 5% 5% 5% 4 6% 7% 28%	4081
	21st to end	Day	13% 8% 2 7% 5% 5% 6% 5% 7% 9% 33%	4783
June	1st to 10th	Day	15% 7% 3 7% 5% 6% 6% 4 6% 8% 33%	2660
	11th to 20th	Day	15% 7% 3 7% 6% 4 5% 5% 7% 10% 32%	2660
	21st to end	Day	16% 6% 2 6% 6% 5% 5% 4 6% 10% 34%	2660
July	1st to 10th	Day	11% 6% 2 6% 6% 6% 6% 4 7% 8% 39%	4349
	11th to 20th	Day	12% 7% 2 6% 5% 4 5% 5% 7% 10% 37%	4120
	21st to end	Day	14% 9% 3 8% 5% 5% 5% 4 6% 8% 32%	4466
August	1st to 10th	Day	14% 8% 3 8% 5% 5% 6% 5% 7% 9% 29%	4104
	11th to 20th	Day	12% 7% 3 8% 5% 5% 6% 5% 6% 8% 34%	4044
	21st to end	Day	9% 4 2 6% 4 4 6% 5% 7% 10% 42%	4245
September	1st to 10th	Day	17% 6% 2 5% 4 4 4 3 6% 7% 42%	2280
	11th to 20th	Day	13% 7% 2 5% 5% 3 4 4 5% 8% 43%	2142
	21st to end	Day	19% 10% 3 8% 5% 4 4 4 4 6% 33%	2061
October	1st to 10th	Day	17% 7% 2 5% 4 4 4 5% 6% 8% 38%	1900
	11th to 20th	Day	20% 11% 2 6% 4 4 4 3 4 5% 37%	1900
	21st to end	Day	21% 15% 3 5% 3 3 3 3 3 5% 36%	2090
November	1st to 10th	Day	13% 13% 3 6% 4 3 3 4 4 6% 42%	1942
	11th to 20th	Day	15% 16% 5% 8% 4 3 5% 4 5% 6% 30%	1637
	21st to end	Day	17% 19% 7% 10% 4 3 3 3 4 5% 24%	1600
December	1st to 10th	Day	16% 12% 7% 25% 6% 4 4 3 4 3 17%	2600
	11th to 20th	Day	20% 17% 7% 13% 3 4 3 2 4 5% 22%	2990
	21st to end	Day	16% 22% 7% 12% 6% 4 4 3 5% 6% 16%	3960
January	1st to 10th	Night	11% 19% 8% 23% 11% 4 3 2 2 3 13%	5935
	11th to 20th	Night	11% 15% 8% 21% 9% 7% 4 3 3 3 15%	5900
	21st to end	Night	18% 15% 7% 12% 9% 4 4 3 4 4 22%	6490
February	1st to 10th	Night	16% 16% 6% 12% 6% 4 4 3 4 4 25%	2600
	11th to 20th	Night	17% 16% 6% 13% 6% 5% 5% 3 4 5% 21%	2533
	21st to end	Night	16% 15% 5% 14% 6% 4 4 3 5% 5% 23%	1963
March	1st to 10th	Night	19% 14% 5% 8% 5% 3 4 3 4 6% 29%	3528
	11th to 20th	Night	13% 12% 9% 10% 5% 4 4 3 4 7% 31%	3590
	21st to end	Night	15% 13% 5% 8% 4 4 4 3 4 6% 36%	3890
April	1st to 10th	Night	16% 16% 4 7% 5% 4 4 4 4 5% 32%	1962
	11th to 20th	Night	16% 11% 3 7% 5% 5% 4 3 5% 7% 35%	1875
	21st to end	Night	16% 11% 3 6% 5% 4 4 3 6% 7% 36%	1758
May	1st to 10th	Night	27% 12% 3 8% 4 4 4 4 4 5% 26%	2813
	11th to 20th	Night	19% 13% 3 8% 5% 4 4 3 5% 6% 31%	2812
	21st to end	Night	13% 10% 3 7% 5% 4 4 4 5% 8% 36%	2771
June	1st to 10th	Night	13% 6% 2 6% 4 5% 4 4 6% 9% 42%	1520
	11th to 20th	Night	12% 7% 3 7% 5% 4 4 5% 7% 9% 38%	1520
	21st to end	Night	14% 6% 2 6% 4 5% 3 4 6% 10% 40%	1520
July	1st to 10th	Night	11% 9% 3 8% 5% 4 5% 4 6% 9% 36%	2606
	11th to 20th	Night	11% 7% 3 7% 5% 5% 5% 3 7% 8% 40%	2840
	21st to end	Night	14% 9% 3 7% 5% 5% 5% 4 6% 6% 37%	3087
August	1st to 10th	Night	15% 9% 3 7% 5% 5% 4 4 7% 10% 31%	2825
	11th to 20th	Night	16% 8% 3 6% 4 4 4 3 5% 7% 40%	2785
	21st to end	Night	8% 5% 2 6% 4 5% 5% 5% 6% 9% 45%	3377
September	1st to 10th	Night	13% 5% 2 6% 5% 4 4 3 6% 7% 44%	1900
	11th to 20th	Night	12% 5% 2 4 3 3 4 4 6% 8% 50%	2033
	21st to end	Night	18% 8% 3 7% 5% 3 4 4 4 6% 40%	2118
October	1st to 10th	Night	17% 7% 1 6% 3 4 4 3 5% 7% 43%	2280
	11th to 20th	Night	16% 9% 3 5% 4 4 4 4 5% 8% 37%	2280
	21st to end	Night	23% 12% 2 5% 3 3 4 3 4 5% 36%	2508
November	1st to 10th	Night	12% 12% 3 6% 4 4 3 2 4 6% 44%	2351
	11th to 20th	Night	17% 16% 5% 8% 5% 3 3 3 5% 5% 30%	2760
	21st to end	Night	16% 17% 7% 10% 4 4 4 3 4 4 27%	2800
December	1st to 10th	Night	17% 16% 6% 22% 5% 2 2 2 3 3 20%	4580
	11th to 20th	Night	20% 15% 8% 14% 5% 3 3 3 4 5% 22%	5205
	21st to end	Night	15% 21% 9% 12% 6% 4 4 3 4 5% 17%	6996

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% 9% 4 1	4007
	11th to 20th	Day	80% 3 6% 6% 5% 1	4060
	21st to end	Day	67% 4 8% 13% 8% 1	4466
February	1st to 10th	Day	66% 4 10% 12% 8%	1800
	11th to 20th	Day	66% 5% 8% 13% 9%	1762
	21st to end	Day	63% 5% 9% 13% 8% 2	1664
March	1st to 10th	Day	55% 5% 9% 23% 7% 1	3545
	11th to 20th	Day	52% 4 11% 23% 8% 1	3590
	21st to end	Day	53% 6% 10% 18% 11% 1	3881
April	1st to 10th	Day	54% 7% 10% 16% 11% 2	2216
	11th to 20th	Day	50% 7% 10% 15% 15% 2	2254
	21st to end	Day	53% 7% 9% 21% 9% 1	2437
May	1st to 10th	Day	58% 7% 7% 20% 7% 1	4120
	11th to 20th	Day	53% 9% 10% 21% 6% 1	4085
	21st to end	Day	43% 10% 15% 20% 11% 2	4797
June	1st to 10th	Day	47% 8% 12% 23% 10% 1	2660
	11th to 20th	Day	46% 9% 12% 21% 11% 1	2660
	21st to end	Day	44% 9% 13% 24% 10%	2660
July	1st to 10th	Day	40% 9% 13% 24% 12% 2	4281
	11th to 20th	Day	39% 8% 14% 26% 12% 1	4051
	21st to end	Day	48% 7% 10% 20% 15% 1	4324
August	1st to 10th	Day	48% 11% 13% 21% 7%	4091
	11th to 20th	Day	43% 9% 10% 29% 9%	4083
	21st to end	Day	35% 10% 14% 27% 13% 1	4245
September	1st to 10th	Day	40% 8% 11% 20% 18% 4	2280
	11th to 20th	Day	38% 7% 11% 28% 14% 2	2142
	21st to end	Day	51% 7% 10% 18% 13% 1	2061
October	1st to 10th	Day	35% 7% 9% 27% 18% 4	3160
	11th to 20th	Day	43% 7% 11% 20% 11% 9%	3140
	21st to end	Day	36% 4 9% 28% 22% 1	3428
November	1st to 10th	Day	44% 6% 10% 20% 16% 5%	1942
	11th to 20th	Day	54% 6% 9% 16% 13% 3	1637
	21st to end	Day	61% 7% 10% 13% 9%	1600
December	1st to 10th	Day	69% 6% 13% 8% 3 1	2600
	11th to 20th	Day	65% 4 10% 15% 5% 1	2986
	21st to end	Day	69% 4 11% 11% 5%	3960
January	1st to 10th	Night	77% 4 4 9% 5% 1	5935
	11th to 20th	Night	72% 4 8% 10% 4 1	5900
	21st to end	Night	65% 5% 8% 13% 7% 1	6490
February	1st to 10th	Night	62% 5% 9% 16% 8%	2600
	11th to 20th	Night	66% 5% 8% 13% 8%	2535
	21st to end	Night	62% 5% 8% 14% 9% 2	1963
March	1st to 10th	Night	56% 6% 7% 23% 7% 1	3528
	11th to 20th	Night	51% 7% 11% 24% 6% 1	3590
	21st to end	Night	48% 5% 9% 23% 13% 2	3890
April	1st to 10th	Night	54% 6% 10% 16% 12% 2	1962
	11th to 20th	Night	48% 7% 12% 17% 14% 3	1875
	21st to end	Night	47% 8% 10% 25% 7% 3	1758
May	1st to 10th	Night	59% 6% 8% 17% 7% 2	2840
	11th to 20th	Night	52% 11% 8% 20% 8% 1	2813
	21st to end	Night	44% 8% 14% 21% 8% 4	2778
June	1st to 10th	Night	39% 9% 12% 27% 13% 1	1520
	11th to 20th	Night	40% 9% 13% 26% 10% 1	1520
	21st to end	Night	40% 7% 15% 23% 13% 1	1520
July	1st to 10th	Night	43% 8% 12% 23% 11% 3	2564
	11th to 20th	Night	39% 9% 15% 24% 13% 1	2786
	21st to end	Night	44% 8% 12% 20% 16% 1	2988
August	1st to 10th	Night	45% 10% 17% 20% 8%	2814
	11th to 20th	Night	44% 7% 10% 25% 14% 1	2812
	21st to end	Night	32% 9% 13% 31% 12% 2	3377
September	1st to 10th	Night	38% 8% 11% 22% 17% 4	1900
	11th to 20th	Night	31% 8% 12% 28% 20% 1	2033
	21st to end	Night	46% 6% 10% 21% 16% 1	2118
October	1st to 10th	Night	36% 5% 10% 26% 21% 2	3800
	11th to 20th	Night	36% 7% 11% 22% 16% 9%	3779
	21st to end	Night	37% 4 9% 27% 21% 2	4120
November	1st to 10th	Night	42% 5% 11% 20% 18% 5%	2351
	11th to 20th	Night	56% 5% 8% 16% 12% 3	2760
	21st to end	Night	60% 5% 9% 16% 8% 1	2800
December	1st to 10th	Night	67% 6% 11% 13% 3 1	4580
	11th to 20th	Night	66% 4 11% 14% 5%	5208
	21st to end	Night	70% 5% 9% 10% 5%	6996

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	36% 33% 18% 9% 3 1	288
	11th to 20th	Day	44% 28% 24% 3 1	300
	21st to end	Day	37% 33% 23% 4 2	330
February	1st to 10th	Day	40% 29% 25% 5% 1	300
	11th to 20th	Day	48% 29% 20% 4	300
	21st to end	Day	43% 26% 25% 6%	324
March	1st to 10th	Day	34% 28% 24% 12% 2	500
	11th to 20th	Day	36% 26% 24% 11% 4 1	500
	21st to end	Day	34% 22% 28% 12% 3	530
April	1st to 10th	Day	32% 24% 25% 13% 5% 1	460
	11th to 20th	Day	27% 25% 26% 13% 6% 3	452
	21st to end	Day	33% 21% 25% 15% 4 1	460
May	1st to 10th	Day	32% 19% 25% 14% 7% 2	463
	11th to 20th	Day	27% 17% 28% 16% 9% 2	487
	21st to end	Day	17% 14% 25% 26% 12% 6%	529
June	1st to 10th	Day	20% 13% 28% 24% 10% 5%	490
	11th to 20th	Day	23% 13% 22% 23% 15% 4	490
	21st to end	Day	19% 14% 29% 24% 12% 3	490
July	1st to 10th	Day	18% 7% 25% 24% 16% 10%	485
	11th to 20th	Day	13% 7% 29% 28% 16% 6%	484
	21st to end	Day	18% 10% 25% 24% 18% 5%	532
August	1st to 10th	Day	15% 8% 25% 28% 18% 6%	486
	11th to 20th	Day	18% 8% 22% 27% 16% 8%	479
	21st to end	Day	16% 6% 25% 28% 17% 8%	539
September	1st to 10th	Day	21% 9% 19% 26% 15% 9%	490
	11th to 20th	Day	15% 3 25% 29% 20% 8%	490
	21st to end	Day	25% 16% 24% 18% 13% 3	490
October	1st to 10th	Day	20% 13% 24% 25% 15% 3	490
	11th to 20th	Day	28% 15% 27% 19% 9% 2	490
	21st to end	Day	32% 20% 25% 16% 7% 1	539
November	1st to 10th	Day	23% 19% 24% 23% 8% 2	480
	11th to 20th	Day	34% 23% 27% 12% 4 1	217
	21st to end	Day	39% 23% 30% 8% 2	200
December	1st to 10th	Day	35% 35% 20% 6% 4 1	200
	11th to 20th	Day	41% 25% 25% 10% 1	200
	21st to end	Day	36% 31% 20% 10% 2	220
January	1st to 10th	Night	33% 30% 23% 10% 4	800
	11th to 20th	Night	35% 25% 25% 14% 1	800
	21st to end	Night	31% 30% 26% 13% 1	880
February	1st to 10th	Night	33% 24% 26% 16% 1	800
	11th to 20th	Night	35% 31% 24% 10% 1	783
	21st to end	Night	32% 27% 26% 13% 2	660
March	1st to 10th	Night	30% 22% 28% 17% 3	800
	11th to 20th	Night	31% 24% 20% 18% 6% 1	800
	21st to end	Night	29% 22% 27% 18% 4 1	863
April	1st to 10th	Night	28% 21% 28% 18% 5%	760
	11th to 20th	Night	25% 16% 27% 23% 9% 1	751
	21st to end	Night	23% 13% 30% 25% 7% 1	618
May	1st to 10th	Night	27% 12% 24% 27% 10% 1	570
	11th to 20th	Night	25% 11% 23% 29% 11% 1	561
	21st to end	Night	13% 7% 23% 33% 22% 3	438
June	1st to 10th	Night	13% 6% 25% 37% 19%	380
	11th to 20th	Night	14% 6% 23% 38% 18% 1	380
	21st to end	Night	16% 4 25% 37% 18% 1	380
July	1st to 10th	Night	14% 4 18% 36% 24% 3	415
	11th to 20th	Night	13% 4 16% 36% 27% 4	552
	21st to end	Night	16% 4 14% 40% 24% 2	624
August	1st to 10th	Night	12% 3 13% 44% 27% 1	567
	11th to 20th	Night	15% 6% 12% 36% 28% 4	564
	21st to end	Night	14% 5% 15% 35% 27% 3	776
September	1st to 10th	Night	16% 7% 19% 31% 24% 4	760
	11th to 20th	Night	13% 5% 19% 33% 26% 3	760
	21st to end	Night	23% 9% 20% 27% 19% 2	760
October	1st to 10th	Night	21% 10% 20% 28% 18% 2	760
	11th to 20th	Night	23% 14% 23% 28% 10% 1	760
	21st to end	Night	33% 13% 22% 24% 7% 1	836
November	1st to 10th	Night	21% 13% 24% 32% 9% 1	780
	11th to 20th	Night	30% 21% 23% 19% 7% 1	800
	21st to end	Night	31% 20% 27% 17% 4 1	800
December	1st to 10th	Night	36% 24% 22% 14% 4	800
	11th to 20th	Night	34% 23% 24% 16% 2	800
	21st to end	Night	32% 24% 27% 16% 1	880

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