Rattlesnake Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rattlesnake Lake clear sky chart from 2005-11-06 to 2026-08-30.

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	24% 13% 3 6% 4 3 3 3 4 5% 31%	149174
	Night	27% 15% 3 6% 3 3 3 3 4 4 29%	141735
Transparency	Day	55% 5% 11% 16% 10% 2	148894
	Night	60% 5% 9% 15% 10% 2	141597
Seeing	Day	32% 13% 24% 21% 8% 2	17225
	Night	37% 9% 19% 23% 12% 1	25512

By Month

Percentage Cloud Cover NW (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	39% 24% 3 7% 4 3 3 2 3 3 9%	4723
	11th to 20th	Day	20% 12% 4 7% 4 4 3 2 3 5% 36%	4716
	21st to end	Day	32% 11% 3 6% 3 2 3 2 3 5% 29%	5283
February	1st to 10th	Day	32% 19% 4 8% 4 4 3 2 4 4 14%	1890
	11th to 20th	Day	33% 22% 4 6% 3 3 2 2 3 3 18%	1845
	21st to end	Day	31% 27% 3 6% 2 2 3 2 2 4 17%	1655
March	1st to 10th	Day	20% 22% 5% 9% 5% 5% 5% 3 4 5% 17%	4663
	11th to 20th	Day	21% 26% 4 8% 4 4 2 2 2 2 24%	5110
	21st to end	Day	31% 21% 3 8% 5% 5% 4 3 4 4 11%	5831
April	1st to 10th	Day	30% 22% 3 7% 5% 4 4 3 5% 4 14%	2520
	11th to 20th	Day	27% 19% 4 7% 4 4 3 2 4 4 21%	2492
	21st to end	Day	32% 18% 3 8% 5% 4 3 2 3 4 18%	2787
May	1st to 10th	Day	17% 13% 3 7% 4 4 4 3 5% 6% 34%	8195
	11th to 20th	Day	31% 16% 3 7% 4 4 4 3 5% 5% 19%	8160
	21st to end	Day	28% 12% 3 6% 3 4 4 4 5% 7% 25%	8993
June	1st to 10th	Day	33% 10% 2 6% 4 4 4 3 5% 5% 23%	2940
	11th to 20th	Day	33% 13% 4 7% 4 5% 4 3 5% 5% 18%	3098
	21st to end	Day	23% 9% 3 6% 4 4 4 4 5% 6% 33%	3150
July	1st to 10th	Day	11% 7% 2 5% 3 3 3 3 4 6% 55%	8763
	11th to 20th	Day	7% 5% 1 4 3 3 3 2 4 5% 63%	8220
	21st to end	Day	10% 6% 2 5% 3 3 4 3 5% 7% 53%	8916
August	1st to 10th	Day	10% 4 2 4 3 3 3 3 4 6% 59%	4636
	11th to 20th	Day	15% 11% 3 8% 4 4 5% 3 4 7% 35%	4286
	21st to end	Day	23% 8% 2 6% 3 3 3 3 4 6% 39%	4536
September	1st to 10th	Day	16% 6% 2 5% 3 4 4 3 5% 6% 47%	2400
	11th to 20th	Day	22% 8% 2 6% 5% 4 4 3 4 5% 37%	2400
	21st to end	Day	25% 8% 3 6% 3 3 3 2 4 5% 38%	2126
October	1st to 10th	Day	22% 8% 2 4 3 4 3 4 6% 9% 36%	3280
	11th to 20th	Day	34% 11% 2 6% 3 3 3 3 4 5% 26%	3261
	21st to end	Day	35% 15% 3 7% 4 3 3 2 4 4 19%	3315
November	1st to 10th	Day	41% 12% 3 7% 3 3 4 3 3 4 16%	1727
	11th to 20th	Day	41% 17% 3 6% 3 2 3 2 3 3 18%	1680
	21st to end	Day	36% 17% 4 5% 3 3 3 2 3 4 19%	1680
December	1st to 10th	Day	27% 13% 2 5% 3 3 3 3 4 4 32%	2700
	11th to 20th	Day	36% 19% 4 7% 4 4 4 3 3 3 13%	3105
	21st to end	Day	33% 21% 4 8% 4 4 4 3 3 4 12%	4092
January	1st to 10th	Night	36% 26% 4 7% 4 3 2 2 3 3 11%	8172
	11th to 20th	Night	21% 13% 3 5% 3 3 3 3 4 6% 37%	8182
	21st to end	Night	28% 11% 3 7% 4 4 3 2 4 4 30%	8897
February	1st to 10th	Night	32% 20% 4 7% 4 4 3 3 4 5% 15%	2730
	11th to 20th	Night	29% 22% 3 5% 4 3 3 2 4 4 21%	2662
	21st to end	Night	30% 26% 3 6% 4 3 3 3 3 3 17%	2150
March	1st to 10th	Night	19% 22% 5% 8% 4 4 5% 3 5% 5% 20%	5438
	11th to 20th	Night	26% 24% 3 5% 3 2 2 2 3 3 26%	5109
	21st to end	Night	35% 22% 3 6% 4 4 3 3 4 4 11%	5386
April	1st to 10th	Night	28% 23% 3 6% 4 4 3 3 5% 5% 16%	2100
	11th to 20th	Night	29% 18% 3 6% 4 3 3 3 4 5% 24%	2079
	21st to end	Night	34% 16% 2 6% 4 3 3 2 4 4 20%	1829
May	1st to 10th	Night	19% 13% 3 5% 3 3 3 3 4 3 40%	4724
	11th to 20th	Night	29% 21% 3 6% 3 3 3 3 4 5% 20%	4700
	21st to end	Night	28% 15% 3 6% 4 3 3 3 5% 6% 24%	5178
June	1st to 10th	Night	31% 11% 3 6% 5% 4 4 2 4 5% 26%	1680
	11th to 20th	Night	32% 13% 2 6% 5% 3 3 4 3 5% 23%	1515
	21st to end	Night	25% 7% 2 6% 4 3 2 3 4 6% 38%	1470
July	1st to 10th	Night	18% 7% 2 4 2 2 2 2 3 4 53%	4193
	11th to 20th	Night	8% 4 1 3 2 3 2 3 4 4 66%	4740
	21st to end	Night	13% 5% 2 4 3 2 3 3 5% 6% 55%	5152
August	1st to 10th	Night	9% 3 1 3 2 2 2 3 4 7% 65%	2720
	11th to 20th	Night	21% 9% 2 5% 3 3 3 3 4 5% 42%	2992
	21st to end	Night	25% 8% 2 4 2 2 2 2 3 5% 45%	3580
September	1st to 10th	Night	17% 6% 2 4 2 3 4 2 4 5% 51%	2000
	11th to 20th	Night	24% 9% 2 7% 3 3 3 3 4 4 38%	2000
	21st to end	Night	23% 8% 3 7% 3 3 4 4 4 5% 37%	2270
October	1st to 10th	Night	23% 9% 2 5% 3 3 3 3 5% 8% 37%	3920
	11th to 20th	Night	34% 10% 2 5% 3 3 3 3 4 4 29%	3897
	21st to end	Night	36% 16% 3 5% 3 3 3 3 4 4 20%	4474
November	1st to 10th	Night	41% 14% 3 6% 3 3 3 3 4 4 16%	2790
	11th to 20th	Night	41% 14% 3 7% 4 3 3 2 2 3 19%	2940
	21st to end	Night	37% 16% 3 7% 3 3 3 2 3 5% 18%	2940
December	1st to 10th	Night	25% 13% 3 6% 4 4 3 3 4 5% 31%	4700
	11th to 20th	Night	35% 19% 4 8% 4 3 3 3 3 4 13%	5320
	21st to end	Night	33% 24% 4 7% 4 4 4 3 3 3 11%	7106

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	82% 3 4 7% 3	4723
	11th to 20th	Day	52% 4 8% 14% 18% 4	4740
	21st to end	Day	61% 3 5% 13% 17% 2	5284
February	1st to 10th	Day	74% 6% 7% 8% 5% 1	1890
	11th to 20th	Day	73% 5% 4 9% 7% 2	1854
	21st to end	Day	74% 3 7% 11% 5%	1655
March	1st to 10th	Day	67% 5% 11% 9% 7% 1	4663
	11th to 20th	Day	68% 4 6% 20% 2	5110
	21st to end	Day	76% 7% 8% 6% 2	5831
April	1st to 10th	Day	73% 6% 12% 8% 1	2520
	11th to 20th	Day	68% 6% 8% 12% 5% 1	2492
	21st to end	Day	72% 4 8% 10% 5% 1	2787
May	1st to 10th	Day	49% 5% 15% 21% 9%	8195
	11th to 20th	Day	66% 9% 13% 11% 2	8163
	21st to end	Day	58% 9% 20% 11% 2	8994
June	1st to 10th	Day	62% 7% 13% 12% 6%	2940
	11th to 20th	Day	67% 7% 10% 11% 4	3098
	21st to end	Day	51% 7% 14% 18% 9% 1	3150
July	1st to 10th	Day	31% 5% 16% 19% 19% 10%	8689
	11th to 20th	Day	23% 4 10% 24% 33% 6%	8145
	21st to end	Day	28% 5% 17% 33% 15% 2	8723
August	1st to 10th	Day	25% 4 9% 31% 27% 4	4619
	11th to 20th	Day	47% 7% 12% 22% 10% 2	4325
	21st to end	Day	46% 5% 10% 21% 16% 2	4535
September	1st to 10th	Day	37% 6% 13% 23% 15% 6%	2400
	11th to 20th	Day	49% 6% 9% 17% 16% 3	2400
	21st to end	Day	49% 6% 14% 17% 11% 3	2126
October	1st to 10th	Day	45% 4 9% 30% 11% 1	3280
	11th to 20th	Day	61% 5% 9% 14% 11%	3261
	21st to end	Day	70% 4 7% 9% 7% 3	3315
November	1st to 10th	Day	72% 6% 8% 10% 4	1727
	11th to 20th	Day	74% 4 6% 9% 6% 1	1680
	21st to end	Day	71% 4 4 10% 8% 3	1680
December	1st to 10th	Day	57% 4 6% 15% 15% 3	2700
	11th to 20th	Day	78% 5% 6% 10% 2	3108
	21st to end	Day	77% 5% 7% 7% 3	4092
January	1st to 10th	Night	81% 3 3 8% 4	8172
	11th to 20th	Night	49% 4 8% 16% 19% 5%	8220
	21st to end	Night	60% 4 6% 13% 15% 3	8897
February	1st to 10th	Night	73% 5% 7% 9% 4 1	2730
	11th to 20th	Night	68% 6% 7% 9% 8% 2	2664
	21st to end	Night	74% 3 7% 10% 5%	2150
March	1st to 10th	Night	63% 6% 11% 11% 7% 1	5438
	11th to 20th	Night	64% 4 10% 19% 2	5110
	21st to end	Night	76% 7% 8% 8% 1	5386
April	1st to 10th	Night	73% 6% 10% 9% 2	2100
	11th to 20th	Night	64% 7% 9% 12% 8% 1	2079
	21st to end	Night	69% 6% 8% 10% 6%	1829
May	1st to 10th	Night	49% 6% 11% 24% 10%	4721
	11th to 20th	Night	67% 7% 12% 12% 3	4700
	21st to end	Night	62% 9% 14% 13% 2	5182
June	1st to 10th	Night	63% 8% 11% 12% 6%	1680
	11th to 20th	Night	64% 8% 11% 14% 3	1515
	21st to end	Night	50% 9% 13% 20% 7% 2	1470
July	1st to 10th	Night	38% 5% 14% 18% 14% 12%	4158
	11th to 20th	Night	24% 4 11% 20% 36% 5%	4692
	21st to end	Night	32% 5% 15% 30% 15% 3	5040
August	1st to 10th	Night	22% 4 10% 31% 29% 4	2712
	11th to 20th	Night	48% 6% 14% 19% 10% 2	3019
	21st to end	Night	45% 4 11% 21% 17% 2	3580
September	1st to 10th	Night	36% 8% 12% 21% 18% 6%	2000
	11th to 20th	Night	54% 5% 6% 16% 13% 6%	2000
	21st to end	Night	50% 8% 10% 16% 12% 4	2270
October	1st to 10th	Night	47% 5% 9% 30% 9%	3920
	11th to 20th	Night	60% 4 7% 16% 11% 1	3897
	21st to end	Night	68% 5% 7% 11% 8% 1	4474
November	1st to 10th	Night	73% 6% 7% 10% 4	2790
	11th to 20th	Night	73% 5% 6% 9% 5% 2	2940
	21st to end	Night	71% 5% 5% 9% 8% 1	2940
December	1st to 10th	Night	58% 5% 7% 11% 17% 2	4700
	11th to 20th	Night	75% 5% 7% 10% 3	5316
	21st to end	Night	79% 6% 7% 7% 2	7106

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	46% 19% 20% 10% 5%	220
	11th to 20th	Day	38% 12% 26% 17% 6%	220
	21st to end	Day	45% 14% 18% 17% 6%	274
February	1st to 10th	Day	44% 17% 22% 12% 5%	430
	11th to 20th	Day	47% 14% 23% 13% 3	425
	21st to end	Day	41% 17% 22% 17% 2	408
March	1st to 10th	Day	39% 19% 20% 18% 4	540
	11th to 20th	Day	41% 16% 21% 14% 6% 1	540
	21st to end	Day	43% 18% 22% 12% 4	594
April	1st to 10th	Day	40% 16% 21% 16% 6% 1	540
	11th to 20th	Day	38% 17% 24% 14% 5% 1	532
	21st to end	Day	39% 17% 23% 17% 5% 1	540
May	1st to 10th	Day	28% 14% 31% 21% 5% 1	543
	11th to 20th	Day	39% 13% 23% 15% 8% 1	568
	21st to end	Day	38% 12% 22% 20% 6% 2	617
June	1st to 10th	Day	32% 13% 24% 22% 8% 1	570
	11th to 20th	Day	34% 18% 26% 16% 5% 1	728
	21st to end	Day	25% 9% 27% 28% 9% 2	780
July	1st to 10th	Day	19% 9% 32% 31% 8% 2	753
	11th to 20th	Day	15% 13% 30% 29% 10% 3	564
	21st to end	Day	12% 8% 32% 30% 14% 5%	619
August	1st to 10th	Day	16% 9% 23% 34% 13% 5%	567
	11th to 20th	Day	12% 8% 25% 37% 13% 5%	557
	21st to end	Day	19% 13% 25% 29% 10% 4	620
September	1st to 10th	Day	18% 10% 28% 32% 9% 2	530
	11th to 20th	Day	25% 11% 26% 24% 13% 1	530
	21st to end	Day	25% 10% 25% 28% 12%	530
October	1st to 10th	Day	28% 11% 25% 21% 14% 1	530
	11th to 20th	Day	37% 11% 22% 20% 10% 1	530
	21st to end	Day	42% 15% 20% 15% 6% 2	441
November	1st to 10th	Day	48% 16% 14% 19% 4	263
	11th to 20th	Day	47% 16% 14% 16% 5% 2	220
	21st to end	Day	41% 13% 23% 11% 12%	220
December	1st to 10th	Day	50% 9% 18% 12% 11% 1	220
	11th to 20th	Day	52% 12% 22% 12% 2	220
	21st to end	Day	45% 12% 23% 15% 5%	242
January	1st to 10th	Night	52% 12% 20% 14% 2	840
	11th to 20th	Night	40% 13% 24% 17% 5%	840
	21st to end	Night	40% 12% 24% 21% 3	924
February	1st to 10th	Night	41% 13% 27% 16% 3	840
	11th to 20th	Night	41% 10% 21% 22% 5%	824
	21st to end	Night	45% 10% 24% 16% 4 1	692
March	1st to 10th	Night	37% 12% 22% 20% 9%	840
	11th to 20th	Night	44% 9% 21% 20% 7% 1	840
	21st to end	Night	42% 13% 20% 19% 5% 1	924
April	1st to 10th	Night	40% 11% 16% 24% 8% 1	840
	11th to 20th	Night	35% 10% 22% 25% 7% 1	831
	21st to end	Night	41% 8% 18% 24% 9%	569
May	1st to 10th	Night	32% 8% 16% 30% 13% 1	420
	11th to 20th	Night	33% 6% 14% 28% 17% 2	414
	21st to end	Night	36% 7% 14% 23% 19% 2	455
June	1st to 10th	Night	35% 6% 13% 29% 17% 1	420
	11th to 20th	Night	37% 8% 13% 27% 15%	420
	21st to end	Night	25% 4 14% 30% 24% 2	420
July	1st to 10th	Night	19% 8% 12% 30% 30% 1	410
	11th to 20th	Night	18% 5% 11% 33% 29% 4	408
	21st to end	Night	13% 5% 9% 32% 37% 4	460
August	1st to 10th	Night	14% 2 9% 30% 39% 5%	436
	11th to 20th	Night	17% 3 8% 28% 35% 9%	624
	21st to end	Night	20% 4 11% 33% 28% 3	837
September	1st to 10th	Night	20% 2 12% 39% 23% 3	800
	11th to 20th	Night	27% 5% 15% 24% 26% 4	800
	21st to end	Night	27% 6% 15% 33% 19% 1	800
October	1st to 10th	Night	29% 5% 18% 32% 16% 1	800
	11th to 20th	Night	37% 6% 20% 22% 14% 1	800
	21st to end	Night	40% 9% 23% 20% 7% 1	880
November	1st to 10th	Night	48% 8% 25% 17% 2	820
	11th to 20th	Night	49% 12% 20% 16% 2 1	840
	21st to end	Night	44% 10% 23% 21% 3	840
December	1st to 10th	Night	38% 11% 26% 20% 5%	840
	11th to 20th	Night	50% 12% 24% 11% 3	840
	21st to end	Night	53% 12% 20% 12% 2	924

Page generated at 2026-08-27 01:07:11. Version ac07

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