Rattle Snake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rattle Snake Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	12% 8% 3 6% 4 4 4 3 5% 7% 44%	131178
	Night	12% 8% 3 7% 4 4 4 3 5% 6% 46%	130286
Transparency	Day	38% 6% 11% 25% 16% 3	134785
	Night	38% 5% 11% 25% 18% 3	134756
Seeing	Day	17% 13% 29% 25% 13% 3	17378
	Night	16% 12% 32% 28% 12% 1	27361

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	14% 11% 4 8% 4 4 4 3 4 5% 39%	5148
	11th to 20th	Day	6% 9% 4 9% 5% 6% 6% 3 5% 6% 42%	5310
	21st to end	Day	14% 12% 4 9% 4 3 3 3 3 4 41%	5812
February	1st to 10th	Day	15% 11% 4 8% 5% 4 4 3 4 5% 36%	1890
	11th to 20th	Day	14% 10% 3 7% 5% 4 3 3 4 6% 40%	1852
	21st to end	Day	15% 10% 4 5% 4 4 3 4 5% 6% 41%	1767
March	1st to 10th	Day	15% 10% 3 4 3 2 3 2 3 5% 50%	4958
	11th to 20th	Day	11% 7% 2 4 3 3 4 3 5% 7% 53%	4980
	21st to end	Day	14% 12% 3 8% 5% 5% 4 4 5% 7% 34%	5462
April	1st to 10th	Day	14% 7% 3 7% 4 3 4 4 5% 6% 43%	2390
	11th to 20th	Day	18% 9% 2 5% 4 3 4 4 5% 6% 41%	2374
	21st to end	Day	19% 8% 3 7% 6% 4 4 3 5% 6% 35%	2400
May	1st to 10th	Day	16% 8% 2 6% 5% 4 4 4 6% 8% 36%	5858
	11th to 20th	Day	13% 7% 2 7% 4 5% 5% 4 6% 7% 40%	5903
	21st to end	Day	18% 6% 2 5% 4 4 4 3 6% 7% 41%	6935
June	1st to 10th	Day	8% 4 2 5% 4 4 5% 4 7% 10% 46%	2800
	11th to 20th	Day	9% 4 2 5% 4 4 4 5% 7% 9% 47%	2800
	21st to end	Day	6% 4 1 4 4 5% 4 5% 7% 10% 50%	2800
July	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 5% 8% 46%	6317
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 7% 10% 44%	5900
	21st to end	Day	8% 5% 2 5% 5% 5% 6% 5% 8% 11% 42%	6422
August	1st to 10th	Day	5% 3 1 5% 3 4 5% 5% 7% 10% 51%	4234
	11th to 20th	Day	11% 5% 2 5% 4 3 4 3 6% 8% 47%	4105
	21st to end	Day	6% 3 2 5% 3 3 4 4 6% 9% 55%	4312
September	1st to 10th	Day	11% 5% 2 4 4 4 4 4 5% 7% 51%	2400
	11th to 20th	Day	10% 6% 2 5% 4 4 4 4 5% 7% 51%	2358
	21st to end	Day	9% 5% 2 6% 3 2 3 4 5% 6% 54%	2108
October	1st to 10th	Day	11% 5% 2 4 4 3 4 3 4 6% 56%	2000
	11th to 20th	Day	14% 6% 3 5% 2 2 3 3 3 6% 54%	2000
	21st to end	Day	18% 8% 2 6% 3 3 3 3 3 5% 46%	2200
November	1st to 10th	Day	11% 6% 2 4 3 3 2 3 5% 6% 54%	2042
	11th to 20th	Day	14% 7% 2 5% 4 3 3 3 4 7% 49%	1813
	21st to end	Day	16% 10% 3 6% 2 3 4 2 3 6% 44%	1680
December	1st to 10th	Day	8% 8% 3 7% 4 3 3 3 4 6% 51%	2690
	11th to 20th	Day	10% 9% 5% 8% 4 3 3 2 4 5% 47%	3088
	21st to end	Day	18% 20% 6% 11% 5% 4 4 2 3 4 23%	4070
January	1st to 10th	Night	13% 12% 5% 13% 5% 4 3 2 3 3 37%	7770
	11th to 20th	Night	9% 8% 3 11% 5% 5% 4 3 3 4 45%	7680
	21st to end	Night	13% 12% 4 8% 5% 4 3 3 4 5% 39%	8402
February	1st to 10th	Night	13% 11% 5% 7% 5% 4 5% 3 5% 6% 37%	2730
	11th to 20th	Night	14% 9% 3 7% 4 4 4 3 4 6% 42%	2653
	21st to end	Night	13% 9% 3 7% 4 3 4 4 4 5% 45%	2056
March	1st to 10th	Night	17% 11% 3 5% 3 3 3 3 4 5% 45%	4941
	11th to 20th	Night	11% 7% 1 4 3 3 3 3 5% 7% 53%	4980
	21st to end	Night	13% 10% 4 7% 5% 5% 4 4 5% 6% 37%	5467
April	1st to 10th	Night	11% 7% 2 6% 4 3 5% 3 4 7% 49%	2010
	11th to 20th	Night	14% 8% 2 5% 4 3 3 4 5% 7% 45%	1975
	21st to end	Night	17% 7% 3 6% 5% 4 4 3 5% 6% 40%	2000
May	1st to 10th	Night	16% 8% 3 6% 4 4 4 3 5% 8% 40%	4053
	11th to 20th	Night	13% 7% 2 6% 3 3 3 3 6% 9% 45%	4008
	21st to end	Night	16% 6% 2 5% 3 3 3 3 5% 7% 46%	3942
June	1st to 10th	Night	7% 3 1 4 4 4 4 4 6% 10% 54%	1600
	11th to 20th	Night	6% 4 1 4 4 3 4 3 7% 12% 51%	1600
	21st to end	Night	4 2 1 3 4 4 5% 3 7% 9% 59%	1600
July	1st to 10th	Night	9% 5% 2 6% 3 3 3 3 5% 10% 49%	3655
	11th to 20th	Night	6% 4 2 6% 4 4 4 4 7% 10% 50%	4077
	21st to end	Night	6% 4 2 4 3 3 5% 4 8% 12% 48%	4449
August	1st to 10th	Night	4 2 1 3 3 3 3 3 6% 10% 62%	2926
	11th to 20th	Night	10% 6% 2 5% 4 3 4 3 5% 7% 52%	2952
	21st to end	Night	5% 3 1 4 3 4 4 3 6% 8% 60%	3586
September	1st to 10th	Night	10% 5% 1 5% 3 3 3 3 4 7% 57%	2000
	11th to 20th	Night	9% 4 2 5% 3 4 4 4 6% 7% 52%	2038
	21st to end	Night	8% 3 2 5% 4 3 3 2 5% 7% 58%	2271
October	1st to 10th	Night	8% 4 1 4 3 3 4 3 5% 6% 60%	2400
	11th to 20th	Night	14% 4 2 5% 4 4 3 3 4 5% 54%	2400
	21st to end	Night	16% 7% 2 5% 4 2 4 2 4 4 50%	2640
November	1st to 10th	Night	11% 6% 2 4 2 3 3 4 4 5% 57%	2472
	11th to 20th	Night	10% 7% 2 6% 3 3 3 3 4 6% 51%	2804
	21st to end	Night	15% 10% 3 6% 3 3 3 2 4 5% 47%	2940
December	1st to 10th	Night	9% 8% 3 6% 4 3 3 3 5% 6% 50%	4720
	11th to 20th	Night	13% 10% 3 7% 4 3 3 3 4 5% 44%	5339
	21st to end	Night	20% 17% 6% 10% 4 3 3 3 3 4 27%	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	48% 4 10% 24% 12% 1	5149
	11th to 20th	Day	42% 4 10% 25% 14% 5%	5310
	21st to end	Day	48% 3 9% 19% 17% 4	5812
February	1st to 10th	Day	50% 5% 11% 22% 12% 1	1890
	11th to 20th	Day	46% 5% 11% 21% 13% 4	1861
	21st to end	Day	43% 4 9% 24% 16% 4	1767
March	1st to 10th	Day	39% 4 7% 24% 23% 5%	4958
	11th to 20th	Day	32% 5% 8% 23% 25% 7%	4980
	21st to end	Day	48% 7% 16% 24% 5%	5464
April	1st to 10th	Day	42% 6% 11% 24% 16% 2	2390
	11th to 20th	Day	43% 5% 12% 23% 15% 2	2374
	21st to end	Day	50% 6% 12% 21% 11% 1	2400
May	1st to 10th	Day	43% 9% 13% 22% 12% 1	5896
	11th to 20th	Day	41% 9% 11% 24% 13% 3	5907
	21st to end	Day	41% 8% 9% 27% 14% 1	6949
June	1st to 10th	Day	31% 9% 16% 28% 15% 1	2800
	11th to 20th	Day	31% 9% 14% 27% 16% 3	2800
	21st to end	Day	27% 8% 17% 30% 14% 5%	2800
July	1st to 10th	Day	36% 6% 11% 26% 19% 2	6248
	11th to 20th	Day	35% 7% 14% 30% 13% 1	5831
	21st to end	Day	32% 9% 18% 32% 9% 1	6240
August	1st to 10th	Day	25% 8% 18% 31% 17% 1	4220
	11th to 20th	Day	35% 6% 14% 29% 15% 2	4144
	21st to end	Day	26% 7% 14% 35% 14% 4	4311
September	1st to 10th	Day	32% 6% 11% 26% 18% 6%	2400
	11th to 20th	Day	32% 6% 9% 27% 24% 2	2358
	21st to end	Day	30% 6% 11% 26% 25% 3	2108
October	1st to 10th	Day	19% 3 6% 20% 38% 14%	3260
	11th to 20th	Day	22% 3 5% 23% 39% 9%	3240
	21st to end	Day	37% 3 7% 24% 26% 3	3538
November	1st to 10th	Day	32% 5% 12% 26% 21% 5%	2042
	11th to 20th	Day	36% 4 10% 24% 22% 4	1813
	21st to end	Day	44% 3 8% 29% 15% 1	1680
December	1st to 10th	Day	35% 4 11% 34% 16% 1	2690
	11th to 20th	Day	41% 4 12% 29% 14% 1	3085
	21st to end	Day	66% 4 11% 13% 6% 1	4070
January	1st to 10th	Night	53% 4 10% 23% 10%	7771
	11th to 20th	Night	44% 4 7% 24% 17% 4	7680
	21st to end	Night	49% 4 11% 18% 15% 3	8402
February	1st to 10th	Night	48% 6% 11% 20% 13% 2	2730
	11th to 20th	Night	45% 5% 11% 21% 16% 3	2654
	21st to end	Night	41% 4 9% 22% 18% 4	2056
March	1st to 10th	Night	43% 4 7% 21% 21% 4	4941
	11th to 20th	Night	32% 5% 7% 23% 27% 7%	4980
	21st to end	Night	47% 7% 12% 23% 9% 2	5467
April	1st to 10th	Night	35% 6% 10% 25% 20% 4	2010
	11th to 20th	Night	38% 7% 8% 26% 17% 5%	1975
	21st to end	Night	44% 5% 10% 24% 15% 1	2000
May	1st to 10th	Night	42% 8% 15% 20% 14% 1	4080
	11th to 20th	Night	36% 8% 12% 23% 16% 5%	4009
	21st to end	Night	38% 7% 11% 24% 20% 1	3950
June	1st to 10th	Night	24% 8% 16% 33% 16% 3	1600
	11th to 20th	Night	25% 9% 13% 27% 23% 4	1600
	21st to end	Night	19% 8% 16% 31% 21% 6%	1600
July	1st to 10th	Night	31% 6% 12% 27% 22% 2	3617
	11th to 20th	Night	28% 8% 12% 36% 15% 2	4026
	21st to end	Night	26% 7% 18% 38% 10% 1	4320
August	1st to 10th	Night	17% 7% 22% 30% 23% 2	2916
	11th to 20th	Night	32% 5% 12% 30% 19% 1	2979
	21st to end	Night	22% 5% 14% 37% 17% 4	3586
September	1st to 10th	Night	28% 5% 12% 25% 23% 7%	2000
	11th to 20th	Night	29% 7% 11% 24% 25% 4	2038
	21st to end	Night	27% 5% 9% 27% 28% 4	2271
October	1st to 10th	Night	16% 3 6% 22% 43% 11%	3920
	11th to 20th	Night	23% 3 7% 22% 29% 16%	3899
	21st to end	Night	35% 4 7% 28% 25% 3	4252
November	1st to 10th	Night	30% 5% 11% 29% 20% 5%	2472
	11th to 20th	Night	34% 4 9% 25% 23% 5%	2804
	21st to end	Night	42% 4 9% 27% 16% 2	2940
December	1st to 10th	Night	36% 4 10% 35% 15% 1	4720
	11th to 20th	Night	43% 4 11% 25% 16% 1	5341
	21st to end	Night	62% 5% 11% 16% 6% 1	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	24% 24% 35% 15% 2	330
	11th to 20th	Day	28% 24% 39% 8% 1	330
	21st to end	Day	19% 25% 42% 11% 2	363
February	1st to 10th	Day	25% 30% 32% 11% 2	330
	11th to 20th	Day	26% 26% 35% 10% 2	330
	21st to end	Day	32% 27% 31% 9% 2	273
March	1st to 10th	Day	21% 25% 42% 11% 1	306
	11th to 20th	Day	33% 18% 33% 14% 1	300
	21st to end	Day	36% 22% 30% 9% 3	330
April	1st to 10th	Day	24% 18% 37% 16% 5% 1	490
	11th to 20th	Day	24% 21% 31% 17% 5%	492
	21st to end	Day	28% 13% 34% 20% 5%	500
May	1st to 10th	Day	21% 16% 35% 22% 6%	699
	11th to 20th	Day	21% 14% 25% 27% 11% 2	725
	21st to end	Day	16% 10% 32% 25% 11% 4	789
June	1st to 10th	Day	9% 7% 23% 34% 22% 5%	730
	11th to 20th	Day	10% 4 27% 38% 18% 3	730
	21st to end	Day	7% 7% 34% 35% 15% 3	730
July	1st to 10th	Day	12% 4 16% 32% 28% 9%	721
	11th to 20th	Day	6% 3 26% 34% 25% 6%	718
	21st to end	Day	9% 3 18% 34% 30% 6%	794
August	1st to 10th	Day	8% 3 22% 39% 24% 4	726
	11th to 20th	Day	10% 3 21% 35% 25% 6%	675
	21st to end	Day	9% 2 18% 32% 31% 8%	583
September	1st to 10th	Day	13% 3 18% 36% 25% 5%	530
	11th to 20th	Day	8% 4 20% 39% 23% 6%	488
	21st to end	Day	13% 11% 23% 32% 16% 5%	330
October	1st to 10th	Day	15% 8% 32% 29% 12% 3	330
	11th to 20th	Day	18% 21% 32% 21% 7%	330
	21st to end	Day	31% 21% 30% 17% 1 1	363
November	1st to 10th	Day	17% 18% 38% 18% 6% 2	330
	11th to 20th	Day	20% 22% 33% 18% 5% 1	330
	21st to end	Day	30% 18% 28% 20% 3	330
December	1st to 10th	Day	20% 22% 35% 17% 5% 1	330
	11th to 20th	Day	23% 21% 37% 17% 2 1	330
	21st to end	Day	25% 28% 37% 10%	363
January	1st to 10th	Night	18% 17% 39% 20% 6%	1050
	11th to 20th	Night	20% 23% 36% 17% 4	1050
	21st to end	Night	18% 19% 40% 21% 2	1155
February	1st to 10th	Night	19% 17% 40% 20% 3	1050
	11th to 20th	Night	22% 17% 36% 22% 2	1019
	21st to end	Night	21% 19% 41% 16% 3	672
March	1st to 10th	Night	20% 16% 40% 21% 3	606
	11th to 20th	Night	22% 15% 33% 27% 4	600
	21st to end	Night	26% 17% 35% 19% 3	660
April	1st to 10th	Night	15% 19% 37% 25% 4 1	600
	11th to 20th	Night	21% 14% 32% 26% 8% 1	593
	21st to end	Night	20% 11% 38% 24% 8%	600
May	1st to 10th	Night	17% 7% 35% 32% 8%	600
	11th to 20th	Night	16% 8% 27% 34% 15% 1	591
	21st to end	Night	12% 8% 28% 32% 17% 2	650
June	1st to 10th	Night	8% 3 20% 44% 25% 1	600
	11th to 20th	Night	7% 3 24% 45% 20% 2	600
	21st to end	Night	3 3 31% 43% 18% 2	600
July	1st to 10th	Night	11% 2 13% 42% 28% 5%	585
	11th to 20th	Night	5% 1 19% 42% 31% 2	582
	21st to end	Night	8% 1 18% 40% 30% 3	657
August	1st to 10th	Night	6% 3 16% 40% 33% 2	597
	11th to 20th	Night	6% 2 15% 44% 29% 3	594
	21st to end	Night	6% 1 15% 39% 36% 3	660
September	1st to 10th	Night	11% 3 18% 39% 26% 4	600
	11th to 20th	Night	10% 3 19% 36% 28% 5%	600
	21st to end	Night	9% 6% 23% 38% 22% 3	671
October	1st to 10th	Night	9% 6% 29% 32% 21% 3	800
	11th to 20th	Night	16% 9% 37% 27% 10%	800
	21st to end	Night	20% 14% 34% 24% 7%	880
November	1st to 10th	Night	13% 13% 36% 30% 8%	820
	11th to 20th	Night	15% 17% 37% 25% 6%	914
	21st to end	Night	22% 15% 35% 22% 5%	1050
December	1st to 10th	Night	20% 14% 38% 22% 5%	1050
	11th to 20th	Night	22% 14% 38% 22% 4	1050
	21st to end	Night	22% 19% 38% 19% 2	1155

Page generated at 2026-02-28 00:40:21. Version ac07

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