Spring Hill School Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spring Hill School 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	14% 10% 4 9% 6% 5% 5% 4 6% 7% 31%	131675
	Night	14% 10% 4 9% 6% 5% 5% 4 5% 7% 32%	129146
Transparency	Day	50% 7% 12% 19% 11% 1	135270
	Night	49% 7% 12% 19% 12% 1	133640
Seeing	Day	18% 13% 28% 23% 13% 5%	16414
	Night	18% 11% 30% 28% 11% 1	25624

By Month

Percentage Cloud Cover NE (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	14% 15% 6% 18% 7% 6% 5% 3 4 5% 17%	4838
	11th to 20th	Day	9% 21% 8% 21% 9% 7% 4 3 3 4 11%	5246
	21st to end	Day	15% 15% 6% 16% 8% 7% 5% 3 4 5% 14%	5780
February	1st to 10th	Day	14% 18% 5% 12% 7% 6% 5% 4 5% 5% 19%	1890
	11th to 20th	Day	13% 14% 8% 13% 7% 5% 5% 3 5% 5% 22%	1798
	21st to end	Day	16% 15% 4 12% 6% 4 6% 3 5% 6% 23%	1600
March	1st to 10th	Day	18% 12% 5% 11% 7% 4 4 3 4 5% 27%	4882
	11th to 20th	Day	11% 11% 4 10% 6% 4 5% 4 6% 7% 31%	4980
	21st to end	Day	17% 12% 4 11% 6% 5% 4 3 5% 5% 29%	5465
April	1st to 10th	Day	16% 13% 5% 8% 6% 5% 4 3 5% 7% 28%	2309
	11th to 20th	Day	17% 11% 3 7% 5% 5% 4 4 4 7% 34%	2510
	21st to end	Day	19% 11% 3 8% 5% 4 4 3 4 5% 34%	2600
May	1st to 10th	Day	19% 9% 3 7% 5% 4 5% 3 5% 7% 32%	5888
	11th to 20th	Day	17% 10% 3 8% 5% 4 4 4 6% 8% 32%	6036
	21st to end	Day	16% 7% 2 6% 5% 5% 4 4 5% 7% 39%	6949
June	1st to 10th	Day	13% 7% 2 6% 5% 5% 5% 5% 6% 8% 38%	2800
	11th to 20th	Day	11% 7% 2 7% 5% 5% 5% 5% 8% 10% 37%	2884
	21st to end	Day	12% 5% 2 7% 5% 4 5% 4 7% 9% 40%	3000
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 10% 41%	6727
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 4 7% 10% 43%	6112
	21st to end	Day	8% 5% 2 6% 4 5% 5% 4 6% 8% 47%	6424
August	1st to 10th	Day	15% 6% 2 5% 5% 5% 5% 5% 6% 8% 36%	4234
	11th to 20th	Day	11% 6% 3 7% 5% 5% 6% 5% 7% 9% 37%	4174
	21st to end	Day	8% 5% 2 7% 5% 4 6% 5% 7% 9% 41%	4558
September	1st to 10th	Day	11% 6% 3 7% 5% 5% 4 5% 6% 9% 40%	2399
	11th to 20th	Day	9% 6% 2 5% 4 4 4 4 6% 8% 46%	2200
	21st to end	Day	15% 6% 3 7% 5% 5% 5% 5% 6% 8% 35%	2200
October	1st to 10th	Day	16% 8% 3 7% 5% 4 4 3 5% 7% 38%	2010
	11th to 20th	Day	19% 9% 3 8% 5% 5% 4 4 5% 8% 31%	2000
	21st to end	Day	22% 11% 3 9% 5% 5% 5% 4 6% 7% 23%	2200
November	1st to 10th	Day	13% 10% 4 10% 6% 4 4 4 6% 7% 32%	1828
	11th to 20th	Day	14% 14% 4 10% 6% 5% 5% 3 7% 8% 25%	1680
	21st to end	Day	17% 16% 6% 9% 6% 5% 5% 4 7% 6% 19%	1680
December	1st to 10th	Day	18% 17% 5% 12% 7% 6% 6% 5% 6% 5% 13%	2680
	11th to 20th	Day	21% 19% 5% 11% 5% 4 4 4 5% 5% 18%	3066
	21st to end	Day	25% 19% 6% 12% 6% 5% 4 3 4 4 11%	4048
January	1st to 10th	Night	14% 13% 5% 16% 6% 5% 6% 4 5% 6% 18%	8054
	11th to 20th	Night	10% 16% 7% 16% 9% 6% 6% 4 6% 5% 14%	7639
	21st to end	Night	14% 13% 6% 14% 7% 6% 5% 4 5% 6% 21%	8401
February	1st to 10th	Night	15% 15% 4 11% 7% 6% 5% 4 5% 6% 22%	2730
	11th to 20th	Night	13% 13% 5% 10% 6% 5% 5% 4 6% 7% 25%	2599
	21st to end	Night	15% 13% 4 12% 7% 5% 5% 4 6% 6% 23%	2026
March	1st to 10th	Night	14% 9% 3 9% 7% 5% 5% 4 6% 6% 32%	4957
	11th to 20th	Night	10% 8% 3 8% 5% 4 5% 4 7% 9% 37%	4979
	21st to end	Night	19% 10% 3 7% 4 4 3 3 5% 5% 37%	5467
April	1st to 10th	Night	19% 10% 4 7% 5% 4 4 4 5% 6% 32%	2090
	11th to 20th	Night	17% 9% 3 7% 5% 4 5% 3 5% 6% 36%	1856
	21st to end	Night	13% 9% 2 7% 6% 4 5% 4 6% 8% 36%	1800
May	1st to 10th	Night	20% 8% 2 7% 4 4 4 4 5% 6% 37%	4053
	11th to 20th	Night	13% 8% 3 8% 5% 5% 5% 4 5% 7% 36%	3854
	21st to end	Night	18% 5% 2 4 3 3 4 3 5% 8% 45%	3926
June	1st to 10th	Night	11% 6% 2 5% 5% 4 5% 4 7% 8% 45%	1600
	11th to 20th	Night	10% 5% 2 5% 5% 4 5% 5% 7% 9% 44%	1508
	21st to end	Night	10% 6% 2 5% 4 4 4 5% 7% 7% 47%	1400
July	1st to 10th	Night	6% 5% 2 6% 5% 4 4 4 7% 10% 49%	3243
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 5% 8% 50%	3830
	21st to end	Night	7% 4 2 6% 4 4 4 4 6% 7% 51%	4428
August	1st to 10th	Night	10% 5% 3 6% 4 4 4 4 6% 7% 46%	2915
	11th to 20th	Night	11% 5% 2 5% 3 4 4 4 6% 8% 47%	2875
	21st to end	Night	8% 4 2 5% 4 4 4 4 6% 9% 49%	3307
September	1st to 10th	Night	10% 5% 2 6% 4 4 5% 4 5% 6% 49%	2000
	11th to 20th	Night	9% 4 2 4 5% 3 3 3 6% 7% 55%	2198
	21st to end	Night	16% 6% 2 7% 5% 4 4 4 5% 7% 40%	2200
October	1st to 10th	Night	15% 7% 2 6% 4 3 4 3 4 7% 44%	2390
	11th to 20th	Night	17% 9% 3 8% 4 5% 4 3 5% 7% 36%	2400
	21st to end	Night	21% 8% 3 8% 5% 5% 4 4 5% 7% 30%	2640
November	1st to 10th	Night	14% 9% 2 7% 5% 4 5% 4 7% 7% 38%	2683
	11th to 20th	Night	15% 12% 3 9% 7% 5% 6% 4 6% 7% 27%	2940
	21st to end	Night	19% 12% 4 10% 6% 5% 5% 4 6% 6% 23%	2940
December	1st to 10th	Night	14% 15% 5% 11% 7% 6% 6% 5% 7% 6% 16%	4720
	11th to 20th	Night	15% 20% 5% 10% 6% 5% 4 3 5% 6% 23%	5348
	21st to end	Night	24% 18% 6% 11% 6% 4 4 3 4 4 16%	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	66% 7% 9% 10% 7%	4839
	11th to 20th	Day	78% 4 7% 7% 4	5280
	21st to end	Day	71% 6% 10% 8% 6%	5780
February	1st to 10th	Day	65% 7% 10% 14% 5%	1890
	11th to 20th	Day	63% 6% 10% 14% 6%	1807
	21st to end	Day	63% 5% 10% 14% 6% 1	1600
March	1st to 10th	Day	58% 5% 13% 16% 6% 1	4885
	11th to 20th	Day	50% 6% 17% 18% 6% 2	4980
	21st to end	Day	56% 5% 10% 19% 10% 1	5465
April	1st to 10th	Day	55% 6% 11% 16% 11% 1	2309
	11th to 20th	Day	50% 5% 12% 20% 12% 1	2510
	21st to end	Day	53% 6% 10% 18% 11% 2	2600
May	1st to 10th	Day	51% 6% 11% 17% 10% 5%	5897
	11th to 20th	Day	49% 8% 11% 19% 11% 2	6040
	21st to end	Day	44% 7% 11% 19% 16% 3	6949
June	1st to 10th	Day	41% 8% 12% 23% 15% 2	2800
	11th to 20th	Day	40% 8% 14% 22% 15% 1	2884
	21st to end	Day	39% 8% 14% 24% 15%	3000
July	1st to 10th	Day	35% 8% 14% 25% 17% 2	6656
	11th to 20th	Day	33% 7% 14% 30% 15% 1	6038
	21st to end	Day	32% 8% 14% 26% 20% 1	6240
August	1st to 10th	Day	42% 9% 14% 21% 13% 1	4221
	11th to 20th	Day	40% 9% 17% 25% 9% 1	4213
	21st to end	Day	35% 8% 14% 31% 11% 1	4558
September	1st to 10th	Day	39% 8% 14% 26% 12% 1	2399
	11th to 20th	Day	33% 7% 14% 31% 15%	2200
	21st to end	Day	44% 8% 13% 25% 10% 1	2200
October	1st to 10th	Day	37% 7% 14% 24% 17% 1	3270
	11th to 20th	Day	39% 6% 15% 22% 18%	3240
	21st to end	Day	49% 7% 15% 21% 8%	3538
November	1st to 10th	Day	49% 7% 13% 23% 7% 1	1828
	11th to 20th	Day	54% 8% 14% 14% 9%	1680
	21st to end	Day	62% 8% 13% 13% 4	1680
December	1st to 10th	Day	69% 10% 10% 8% 3	2680
	11th to 20th	Day	66% 7% 9% 11% 7%	3066
	21st to end	Day	75% 6% 7% 8% 4	4048
January	1st to 10th	Night	60% 10% 11% 14% 6%	8054
	11th to 20th	Night	67% 8% 9% 9% 6%	7680
	21st to end	Night	62% 6% 9% 13% 9%	8401
February	1st to 10th	Night	62% 6% 9% 17% 5% 1	2730
	11th to 20th	Night	57% 8% 12% 16% 7%	2601
	21st to end	Night	58% 8% 10% 16% 7%	2026
March	1st to 10th	Night	49% 6% 14% 19% 9% 2	4967
	11th to 20th	Night	40% 7% 19% 21% 12% 1	4980
	21st to end	Night	48% 5% 11% 19% 17% 2	5467
April	1st to 10th	Night	52% 5% 11% 18% 13% 1	2090
	11th to 20th	Night	47% 6% 10% 19% 16% 2	1856
	21st to end	Night	45% 7% 11% 20% 16% 3	1800
May	1st to 10th	Night	47% 6% 11% 15% 12% 8%	4060
	11th to 20th	Night	47% 7% 13% 21% 10% 2	3855
	21st to end	Night	37% 6% 13% 24% 18% 2	3927
June	1st to 10th	Night	37% 7% 11% 26% 17% 3	1600
	11th to 20th	Night	34% 8% 13% 25% 18% 2	1508
	21st to end	Night	35% 7% 14% 23% 19% 1	1400
July	1st to 10th	Night	30% 7% 14% 25% 20% 4	3207
	11th to 20th	Night	32% 7% 12% 28% 19% 3	3781
	21st to end	Night	31% 7% 12% 24% 23% 3	4297
August	1st to 10th	Night	35% 8% 13% 25% 17% 2	2904
	11th to 20th	Night	33% 8% 17% 28% 13% 1	2902
	21st to end	Night	29% 8% 13% 32% 16% 1	3307
September	1st to 10th	Night	36% 6% 13% 28% 16% 2	2000
	11th to 20th	Night	28% 7% 12% 33% 19% 1	2198
	21st to end	Night	43% 6% 12% 25% 13% 1	2200
October	1st to 10th	Night	31% 5% 9% 30% 24% 1	3910
	11th to 20th	Night	41% 5% 13% 24% 16%	3899
	21st to end	Night	40% 6% 15% 26% 13% 1	4252
November	1st to 10th	Night	43% 6% 14% 23% 13% 1	2683
	11th to 20th	Night	52% 9% 13% 17% 8% 1	2940
	21st to end	Night	58% 8% 15% 13% 5%	2940
December	1st to 10th	Night	62% 10% 14% 11% 3	4720
	11th to 20th	Night	61% 6% 10% 14% 9% 1	5348
	21st to end	Night	70% 7% 10% 8% 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	23% 25% 30% 16% 6%	252
	11th to 20th	Day	26% 24% 35% 11% 3 1	330
	21st to end	Day	25% 22% 33% 16% 4	363
February	1st to 10th	Day	22% 22% 38% 13% 5%	330
	11th to 20th	Day	26% 23% 35% 15% 1	315
	21st to end	Day	34% 22% 31% 12%	249
March	1st to 10th	Day	23% 16% 36% 19% 5% 1	474
	11th to 20th	Day	24% 17% 35% 18% 6% 1	500
	21st to end	Day	29% 17% 28% 19% 7%	550
April	1st to 10th	Day	23% 20% 29% 20% 6% 1	500
	11th to 20th	Day	27% 15% 25% 19% 11% 3	492
	21st to end	Day	25% 12% 33% 19% 8% 3	500
May	1st to 10th	Day	24% 13% 26% 22% 10% 4	503
	11th to 20th	Day	17% 10% 30% 22% 17% 4	527
	21st to end	Day	14% 8% 28% 27% 16% 7%	573
June	1st to 10th	Day	14% 12% 31% 29% 11% 3	530
	11th to 20th	Day	13% 9% 30% 27% 15% 5%	614
	21st to end	Day	13% 10% 29% 32% 13% 3	730
July	1st to 10th	Day	8% 7% 22% 29% 25% 8%	683
	11th to 20th	Day	13% 7% 22% 29% 19% 11%	524
	21st to end	Day	8% 4 24% 33% 23% 8%	576
August	1st to 10th	Day	10% 5% 20% 28% 24% 13%	526
	11th to 20th	Day	12% 5% 22% 24% 24% 14%	519
	21st to end	Day	11% 5% 23% 28% 22% 11%	583
September	1st to 10th	Day	12% 7% 23% 23% 19% 15%	530
	11th to 20th	Day	8% 4 17% 24% 28% 17%	530
	21st to end	Day	16% 10% 25% 26% 15% 8%	530
October	1st to 10th	Day	18% 10% 25% 26% 14% 7%	530
	11th to 20th	Day	19% 16% 35% 21% 7% 3	530
	21st to end	Day	25% 16% 31% 21% 7% 1	583
November	1st to 10th	Day	21% 23% 32% 16% 7% 2	316
	11th to 20th	Day	25% 22% 29% 15% 7% 1	220
	21st to end	Day	27% 22% 32% 18% 1	220
December	1st to 10th	Day	28% 21% 33% 13% 5%	220
	11th to 20th	Day	27% 26% 31% 12% 3	220
	21st to end	Day	30% 21% 35% 12% 3	242
January	1st to 10th	Night	20% 19% 37% 17% 6% 1	840
	11th to 20th	Night	27% 17% 33% 19% 3	840
	21st to end	Night	19% 15% 37% 24% 5%	924
February	1st to 10th	Night	23% 16% 36% 23% 2	840
	11th to 20th	Night	23% 15% 34% 24% 4	804
	21st to end	Night	22% 17% 36% 22% 3	660
March	1st to 10th	Night	19% 14% 36% 25% 6%	800
	11th to 20th	Night	20% 15% 36% 23% 6% 1	800
	21st to end	Night	21% 12% 33% 25% 8%	880
April	1st to 10th	Night	24% 14% 31% 26% 5%	800
	11th to 20th	Night	21% 14% 30% 25% 9% 1	672
	21st to end	Night	17% 10% 33% 32% 8%	600
May	1st to 10th	Night	18% 7% 31% 30% 13% 1	600
	11th to 20th	Night	16% 5% 29% 36% 12% 1	517
	21st to end	Night	12% 5% 30% 35% 18% 1	433
June	1st to 10th	Night	13% 6% 31% 38% 12% 1	400
	11th to 20th	Night	12% 6% 28% 41% 14% 1	400
	21st to end	Night	12% 5% 28% 40% 15% 1	400
July	1st to 10th	Night	5% 2 23% 47% 21% 2	390
	11th to 20th	Night	9% 3 23% 40% 22% 3	487
	21st to end	Night	9% 3 18% 47% 21% 2	657
August	1st to 10th	Night	7% 3 17% 43% 27% 3	597
	11th to 20th	Night	9% 2 15% 40% 32% 3	594
	21st to end	Night	10% 2 21% 37% 27% 3	705
September	1st to 10th	Night	11% 5% 22% 34% 24% 4	800
	11th to 20th	Night	10% 5% 17% 35% 31% 4	800
	21st to end	Night	18% 7% 21% 30% 19% 5%	800
October	1st to 10th	Night	16% 7% 25% 29% 20% 3	800
	11th to 20th	Night	20% 11% 31% 27% 9% 2	800
	21st to end	Night	24% 11% 32% 28% 5%	880
November	1st to 10th	Night	18% 13% 30% 28% 10% 2	820
	11th to 20th	Night	20% 18% 32% 23% 6% 1	840
	21st to end	Night	26% 14% 33% 20% 7%	840
December	1st to 10th	Night	27% 17% 31% 19% 5%	840
	11th to 20th	Night	20% 20% 38% 17% 4	840
	21st to end	Night	31% 13% 34% 19% 3	924

Page generated at 2026-02-13 01:42:14. Version ac07

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