Smith Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Smith Observing Site clear sky chart from 2005-11-06 to 2026-03-18.

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	13% 9% 3 8% 5% 4 4 4 6% 7% 37%	131072
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 38%	130716
Transparency	Day	44% 6% 11% 24% 13% 2	134583
	Night	46% 5% 10% 23% 14% 2	135280
Seeing	Day	16% 13% 31% 25% 12% 4	17628
	Night	18% 11% 31% 28% 12% 1	27642

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	12% 12% 7% 12% 5% 5% 5% 4 6% 5% 26%	4691
	11th to 20th	Day	7% 12% 6% 12% 5% 4 5% 6% 6% 5% 32%	4710
	21st to end	Day	12% 15% 6% 9% 4 4 4 2 4 6% 34%	5253
February	1st to 10th	Day	11% 16% 5% 9% 6% 4 5% 4 6% 7% 27%	1920
	11th to 20th	Day	13% 11% 4 9% 5% 4 4 3 5% 4 37%	2048
	21st to end	Day	12% 15% 4 11% 5% 4 4 3 5% 7% 31%	1730
March	1st to 10th	Day	15% 10% 3 7% 4 4 3 3 5% 5% 42%	4580
	11th to 20th	Day	10% 7% 3 7% 5% 5% 4 4 6% 8% 41%	4754
	21st to end	Day	19% 14% 6% 12% 5% 5% 4 3 4 5% 23%	5967
April	1st to 10th	Day	18% 14% 3 8% 6% 5% 4 3 4 6% 31%	2400
	11th to 20th	Day	16% 9% 3 7% 6% 5% 4 3 6% 7% 34%	2374
	21st to end	Day	19% 8% 3 7% 5% 5% 4 3 5% 7% 33%	2521
May	1st to 10th	Day	16% 9% 3 7% 4 5% 4 3 6% 8% 36%	5862
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 4 6% 8% 40%	6223
	21st to end	Day	15% 7% 2 6% 4 4 4 4 6% 9% 39%	6935
June	1st to 10th	Day	9% 6% 2 6% 5% 5% 4 4 6% 8% 45%	2800
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 4 7% 9% 44%	2800
	21st to end	Day	10% 7% 2 7% 5% 4 5% 4 7% 9% 39%	2800
July	1st to 10th	Day	10% 7% 3 7% 5% 5% 6% 5% 7% 9% 36%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	6360
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 4 7% 9% 42%	6924
August	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 7% 9% 43%	4498
	11th to 20th	Day	10% 6% 2 5% 5% 4 4 4 6% 8% 45%	3893
	21st to end	Day	7% 6% 2 6% 4 4 5% 4 6% 9% 47%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 5% 8% 47%	2400
	11th to 20th	Day	12% 4 1 4 3 3 5% 4 6% 7% 52%	2273
	21st to end	Day	11% 7% 2 6% 4 4 4 4 6% 8% 43%	2200
October	1st to 10th	Day	12% 8% 2 7% 4 3 4 3 4 6% 47%	2142
	11th to 20th	Day	18% 8% 3 8% 5% 5% 3 4 5% 7% 35%	1977
	21st to end	Day	19% 10% 3 6% 4 5% 4 3 4 6% 37%	1980
November	1st to 10th	Day	12% 9% 3 7% 4 4 4 3 5% 7% 43%	1852
	11th to 20th	Day	15% 12% 3 10% 6% 4 4 3 4 6% 33%	1890
	21st to end	Day	17% 14% 3 8% 5% 4 3 2 4 5% 35%	1795
December	1st to 10th	Day	9% 9% 4 9% 5% 5% 5% 3 6% 7% 39%	2690
	11th to 20th	Day	11% 15% 5% 11% 5% 5% 6% 4 6% 6% 26%	3088
	21st to end	Day	21% 23% 8% 11% 5% 4 4 3 4 4 14%	4070
January	1st to 10th	Night	12% 12% 7% 12% 6% 4 3 3 4 5% 32%	8232
	11th to 20th	Night	10% 14% 6% 14% 7% 6% 6% 5% 5% 5% 23%	8280
	21st to end	Night	13% 14% 6% 8% 5% 4 4 3 5% 5% 32%	8956
February	1st to 10th	Night	13% 15% 5% 9% 6% 4 4 3 4 5% 33%	2698
	11th to 20th	Night	12% 11% 4 8% 5% 4 4 3 5% 6% 38%	2458
	21st to end	Night	17% 11% 4 7% 5% 4 4 3 4 5% 37%	2076
March	1st to 10th	Night	16% 10% 3 7% 4 4 3 2 4 5% 41%	5493
	11th to 20th	Night	10% 7% 2 6% 4 4 4 3 5% 8% 47%	5403
	21st to end	Night	20% 12% 5% 10% 5% 5% 4 3 4 4 28%	4962
April	1st to 10th	Night	17% 9% 3 7% 4 5% 4 4 5% 6% 38%	2000
	11th to 20th	Night	18% 8% 4 6% 4 5% 4 3 5% 5% 38%	1975
	21st to end	Night	17% 8% 3 8% 5% 3 3 4 5% 5% 39%	1877
May	1st to 10th	Night	18% 7% 3 7% 4 5% 4 4 5% 7% 36%	4033
	11th to 20th	Night	12% 7% 3 7% 4 4 4 4 6% 9% 41%	3684
	21st to end	Night	17% 8% 2 7% 5% 4 4 3 4 6% 40%	3942
June	1st to 10th	Night	9% 4 2 5% 4 5% 5% 4 6% 8% 47%	1600
	11th to 20th	Night	11% 6% 2 4 5% 4 4 4 6% 8% 45%	1600
	21st to end	Night	8% 6% 3 7% 5% 4 5% 4 7% 9% 43%	1600
July	1st to 10th	Night	10% 5% 3 7% 5% 5% 6% 4 6% 9% 41%	3618
	11th to 20th	Night	6% 5% 1 6% 4 4 5% 4 6% 7% 50%	3617
	21st to end	Night	8% 5% 2 6% 4 5% 5% 5% 7% 7% 47%	3947
August	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 7% 8% 50%	2657
	11th to 20th	Night	9% 5% 2 6% 4 4 3 3 6% 8% 50%	3153
	21st to end	Night	7% 4 2 5% 4 4 5% 3 6% 8% 52%	3586
September	1st to 10th	Night	9% 6% 2 5% 4 3 3 3 5% 6% 53%	2000
	11th to 20th	Night	9% 5% 2 5% 3 3 3 3 4 8% 55%	2105
	21st to end	Night	12% 6% 2 5% 4 5% 4 3 5% 7% 48%	2200
October	1st to 10th	Night	11% 5% 2 5% 4 3 3 3 5% 5% 52%	2255
	11th to 20th	Night	17% 7% 2 6% 4 3 4 3 4 5% 44%	2422
	21st to end	Night	19% 9% 2 7% 3 4 4 3 5% 6% 38%	2860
November	1st to 10th	Night	12% 9% 3 7% 4 4 3 3 5% 6% 46%	2666
	11th to 20th	Night	14% 11% 3 9% 5% 4 4 3 4 7% 36%	2730
	21st to end	Night	16% 10% 4 9% 4 4 3 4 4 6% 36%	2822
December	1st to 10th	Night	11% 9% 4 8% 5% 5% 5% 4 5% 6% 40%	4720
	11th to 20th	Night	15% 16% 4 9% 5% 5% 4 3 5% 5% 28%	5339
	21st to end	Night	21% 20% 6% 10% 5% 4 4 3 4 5% 18%	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	60% 3 14% 16% 7%	4692
	11th to 20th	Day	52% 4 9% 17% 15% 3	4710
	21st to end	Day	54% 2 10% 23% 11%	5253
February	1st to 10th	Day	55% 6% 12% 17% 10% 1	1920
	11th to 20th	Day	50% 4 9% 24% 11% 3	2058
	21st to end	Day	54% 6% 9% 20% 11% 1	1730
March	1st to 10th	Day	45% 5% 9% 24% 12% 5%	4580
	11th to 20th	Day	40% 6% 12% 29% 11% 1	4754
	21st to end	Day	63% 6% 10% 12% 8% 1	5969
April	1st to 10th	Day	54% 6% 10% 18% 11%	2400
	11th to 20th	Day	48% 6% 10% 22% 12% 3	2374
	21st to end	Day	50% 7% 12% 17% 12% 1	2521
May	1st to 10th	Day	45% 7% 10% 23% 13% 2	5897
	11th to 20th	Day	39% 9% 12% 25% 14% 1	6227
	21st to end	Day	40% 6% 12% 25% 13% 3	6949
June	1st to 10th	Day	36% 7% 12% 26% 17% 2	2800
	11th to 20th	Day	35% 7% 14% 23% 19% 3	2800
	21st to end	Day	39% 9% 13% 22% 15% 2	2800
July	1st to 10th	Day	41% 7% 12% 26% 13% 2	6291
	11th to 20th	Day	35% 7% 15% 28% 15% 1	6285
	21st to end	Day	37% 7% 16% 22% 17% 1	6728
August	1st to 10th	Day	36% 7% 15% 29% 12% 1	4483
	11th to 20th	Day	35% 7% 12% 33% 13% 1	3930
	21st to end	Day	33% 7% 12% 25% 21% 2	4311
September	1st to 10th	Day	35% 8% 11% 26% 17% 2	2400
	11th to 20th	Day	29% 6% 12% 31% 19% 2	2273
	21st to end	Day	37% 6% 11% 29% 17% 1	2200
October	1st to 10th	Day	26% 4 7% 39% 24% 1	3493
	11th to 20th	Day	36% 5% 10% 27% 21% 1	3193
	21st to end	Day	44% 4 7% 28% 13% 4	3180
November	1st to 10th	Day	41% 5% 13% 23% 16% 1	1852
	11th to 20th	Day	52% 5% 10% 22% 10% 1	1890
	21st to end	Day	53% 4 11% 24% 8%	1795
December	1st to 10th	Day	44% 4 19% 26% 7% 1	2690
	11th to 20th	Day	57% 5% 12% 18% 7% 1	3085
	21st to end	Day	75% 3 6% 10% 5%	4070
January	1st to 10th	Night	56% 3 9% 21% 10% 1	8233
	11th to 20th	Night	63% 3 8% 11% 14% 1	8280
	21st to end	Night	54% 4 12% 21% 10%	8956
February	1st to 10th	Night	53% 5% 10% 18% 12% 2	2698
	11th to 20th	Night	47% 6% 10% 22% 14% 2	2459
	21st to end	Night	50% 5% 9% 22% 10% 4	2076
March	1st to 10th	Night	46% 4 12% 24% 12% 3	5493
	11th to 20th	Night	36% 4 11% 31% 14% 3	5403
	21st to end	Night	59% 5% 9% 17% 10% 1	4962
April	1st to 10th	Night	47% 6% 10% 22% 14% 1	2000
	11th to 20th	Night	48% 4 8% 18% 17% 4	1975
	21st to end	Night	46% 6% 10% 22% 15% 2	1877
May	1st to 10th	Night	46% 6% 12% 17% 15% 3	4060
	11th to 20th	Night	39% 8% 13% 23% 14% 4	3685
	21st to end	Night	46% 5% 8% 22% 15% 4	3950
June	1st to 10th	Night	33% 7% 11% 27% 18% 3	1600
	11th to 20th	Night	35% 6% 10% 26% 21% 2	1600
	21st to end	Night	36% 8% 15% 22% 17% 2	1600
July	1st to 10th	Night	39% 7% 13% 28% 11% 3	3580
	11th to 20th	Night	30% 6% 14% 31% 17% 2	3572
	21st to end	Night	33% 7% 13% 24% 20% 2	3832
August	1st to 10th	Night	30% 7% 15% 28% 19% 1	2648
	11th to 20th	Night	31% 6% 13% 33% 17% 1	3182
	21st to end	Night	29% 6% 12% 31% 18% 4	3586
September	1st to 10th	Night	31% 7% 9% 28% 22% 2	2000
	11th to 20th	Night	29% 5% 8% 33% 23% 2	2105
	21st to end	Night	37% 6% 8% 31% 16% 1	2200
October	1st to 10th	Night	24% 4 7% 38% 26% 2	3684
	11th to 20th	Night	37% 4 8% 24% 26% 2	3945
	21st to end	Night	41% 5% 9% 25% 18% 3	4610
November	1st to 10th	Night	40% 4 9% 26% 18% 3	2666
	11th to 20th	Night	49% 5% 11% 23% 11% 2	2730
	21st to end	Night	50% 5% 11% 22% 11% 1	2822
December	1st to 10th	Night	45% 4 14% 26% 9% 2	4720
	11th to 20th	Night	58% 4 11% 16% 11% 1	5341
	21st to end	Night	69% 4 8% 13% 6%	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	21% 16% 36% 22% 5% 1	330
	11th to 20th	Day	21% 23% 35% 18% 2 1	330
	21st to end	Day	22% 19% 39% 15% 4 1	363
February	1st to 10th	Day	16% 24% 39% 17% 3	330
	11th to 20th	Day	25% 22% 35% 16% 3	330
	21st to end	Day	24% 25% 40% 10% 2 1	273
March	1st to 10th	Day	22% 24% 41% 12% 2	330
	11th to 20th	Day	26% 18% 37% 14% 5% 1	347
	21st to end	Day	27% 15% 31% 20% 6% 1	550
April	1st to 10th	Day	25% 17% 32% 22% 4	500
	11th to 20th	Day	22% 23% 26% 19% 8% 2	492
	21st to end	Day	22% 17% 31% 18% 10% 2	500
May	1st to 10th	Day	20% 14% 34% 21% 9% 2	503
	11th to 20th	Day	20% 12% 31% 24% 9% 3	681
	21st to end	Day	17% 9% 32% 23% 14% 4	789
June	1st to 10th	Day	10% 10% 33% 29% 15% 3	730
	11th to 20th	Day	13% 8% 29% 34% 12% 4	730
	21st to end	Day	12% 8% 31% 32% 14% 3	730
July	1st to 10th	Day	7% 4 26% 31% 22% 11%	721
	11th to 20th	Day	9% 6% 30% 31% 16% 8%	718
	21st to end	Day	8% 4 27% 39% 17% 6%	794
August	1st to 10th	Day	8% 5% 23% 34% 21% 9%	687
	11th to 20th	Day	12% 5% 20% 29% 23% 11%	519
	21st to end	Day	7% 4 24% 31% 26% 8%	583
September	1st to 10th	Day	14% 6% 25% 27% 21% 7%	530
	11th to 20th	Day	9% 4 20% 32% 22% 12%	530
	21st to end	Day	15% 12% 24% 26% 15% 8%	530
October	1st to 10th	Day	15% 12% 26% 27% 15% 4	472
	11th to 20th	Day	22% 18% 32% 21% 6%	330
	21st to end	Day	29% 16% 33% 17% 5% 1	363
November	1st to 10th	Day	17% 18% 33% 23% 8% 2	330
	11th to 20th	Day	23% 20% 35% 16% 5%	330
	21st to end	Day	23% 12% 44% 18% 2	330
December	1st to 10th	Day	15% 23% 42% 14% 5% 1	330
	11th to 20th	Day	20% 20% 38% 17% 5%	330
	21st to end	Day	27% 23% 35% 13% 2 1	363
January	1st to 10th	Night	18% 17% 38% 24% 3	1050
	11th to 20th	Night	21% 18% 37% 18% 5% 1	1050
	21st to end	Night	21% 15% 38% 23% 4	1155
February	1st to 10th	Night	21% 14% 38% 22% 5%	1018
	11th to 20th	Night	21% 15% 36% 25% 3	823
	21st to end	Night	24% 15% 38% 20% 3	692
March	1st to 10th	Night	20% 14% 36% 22% 8%	840
	11th to 20th	Night	16% 14% 37% 25% 7%	813
	21st to end	Night	23% 11% 31% 26% 8% 1	660
April	1st to 10th	Night	20% 12% 36% 25% 6% 1	600
	11th to 20th	Night	23% 13% 31% 23% 8% 1	593
	21st to end	Night	23% 10% 27% 31% 9% 1	600
May	1st to 10th	Night	17% 8% 34% 32% 9% 1	600
	11th to 20th	Night	19% 6% 27% 30% 16% 1	591
	21st to end	Night	14% 6% 25% 34% 19% 2	650
June	1st to 10th	Night	10% 4 26% 41% 19% 1	600
	11th to 20th	Night	14% 5% 28% 38% 15% 1	600
	21st to end	Night	10% 4 27% 39% 19% 2	600
July	1st to 10th	Night	8% 3 16% 40% 31% 3	585
	11th to 20th	Night	9% 3 21% 37% 25% 4	582
	21st to end	Night	7% 2 17% 47% 25% 2	657
August	1st to 10th	Night	6% 2 16% 44% 29% 4	597
	11th to 20th	Night	9% 3 15% 36% 31% 5%	594
	21st to end	Night	8% 3 18% 38% 30% 3	660
September	1st to 10th	Night	10% 4 20% 34% 27% 5%	600
	11th to 20th	Night	11% 4 19% 32% 29% 6%	705
	21st to end	Night	13% 9% 24% 29% 22% 4	800
October	1st to 10th	Night	13% 7% 25% 32% 20% 2	800
	11th to 20th	Night	21% 12% 31% 27% 9%	800
	21st to end	Night	24% 12% 32% 24% 8% 1	880
November	1st to 10th	Night	17% 15% 35% 24% 9% 1	820
	11th to 20th	Night	20% 19% 33% 22% 5% 1	840
	21st to end	Night	22% 15% 36% 23% 6%	932
December	1st to 10th	Night	21% 17% 38% 20% 5%	1050
	11th to 20th	Night	22% 15% 37% 22% 4	1050
	21st to end	Night	30% 15% 35% 18% 2	1155

Page generated at 2026-03-15 01:23:53. Version ac07

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