21200 W Road And I-84
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the 21200 W Road And I-84 clear sky chart from 2005-11-06 to 2025-10-04.

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	6% 7% 2 6% 4 4 4 3 5% 6% 53%	135137
	Night	6% 8% 3 7% 4 4 4 3 5% 6% 50%	131642
Transparency	Day	31% 5% 10% 20% 20% 14%	129847
	Night	34% 5% 11% 20% 18% 11%	127934
Seeing	Day	9% 7% 20% 32% 24% 9%	16247
	Night	11% 4 17% 40% 25% 4	25998

By Month

Percentage Cloud Cover SW (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	8% 18% 6% 13% 6% 6% 4 3 5% 4 26%	3639
	11th to 20th	Day	6% 15% 4 12% 9% 7% 4 4 5% 5% 29%	4006
	21st to end	Day	7% 15% 5% 10% 7% 6% 5% 3 4 5% 33%	4510
February	1st to 10th	Day	12% 15% 5% 10% 5% 4 3 4 6% 7% 29%	1800
	11th to 20th	Day	11% 15% 4 9% 5% 4 4 4 5% 7% 33%	1839
	21st to end	Day	6% 13% 5% 8% 5% 6% 5% 4 7% 6% 34%	1650
March	1st to 10th	Day	6% 13% 5% 9% 5% 4 4 3 4 6% 42%	4729
	11th to 20th	Day	6% 9% 3 6% 4 4 4 4 6% 7% 48%	5000
	21st to end	Day	8% 13% 4 10% 6% 5% 6% 4 7% 8% 31%	5486
April	1st to 10th	Day	9% 10% 4 7% 4 4 4 3 5% 7% 43%	2235
	11th to 20th	Day	7% 9% 3 6% 5% 5% 4 4 6% 7% 44%	2567
	21st to end	Day	7% 8% 2 7% 5% 4 5% 4 6% 8% 44%	2600
May	1st to 10th	Day	5% 8% 2 6% 4 3 5% 3 5% 7% 53%	5880
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 4 6% 8% 48%	5841
	21st to end	Day	4 5% 2 5% 4 4 4 4 5% 8% 56%	6423
June	1st to 10th	Day	5% 4 2 4 4 4 5% 4 6% 8% 55%	2770
	11th to 20th	Day	6% 5% 1 4 3 3 4 3 5% 8% 58%	3000
	21st to end	Day	2 3 1 3 2 2 2 3 4 6% 71%	2893
July	1st to 10th	Day	2 1 1 2 2 2 2 3 4 5% 77%	6319
	11th to 20th	Day	2 3 1 3 2 3 4 3 6% 6% 67%	6105
	21st to end	Day	3 2 1 2 2 2 3 3 5% 7% 71%	6401
August	1st to 10th	Day	3 2 1 3 2 2 3 3 5% 7% 70%	5779
	11th to 20th	Day	3 1 1 3 2 3 3 3 4 6% 71%	5798
	21st to end	Day	5% 3 1 4 3 3 4 3 5% 5% 64%	6468
September	1st to 10th	Day	4 3 1 3 3 3 3 3 5% 5% 68%	2255
	11th to 20th	Day	4 4 1 4 3 3 3 4 4 6% 65%	2200
	21st to end	Day	7% 3 1 3 2 2 2 3 3 5% 68%	2200
October	1st to 10th	Day	5% 3 1 4 2 3 3 2 4 5% 68%	3404
	11th to 20th	Day	4 5% 2 4 3 3 3 3 5% 7% 61%	3150
	21st to end	Day	5% 6% 2 5% 3 3 4 3 5% 7% 58%	3296
November	1st to 10th	Day	10% 10% 3 6% 4 4 4 3 6% 6% 44%	1764
	11th to 20th	Day	11% 8% 2 5% 3 3 4 4 6% 8% 47%	1800
	21st to end	Day	8% 12% 4 8% 5% 5% 3 3 5% 6% 42%	1639
December	1st to 10th	Day	7% 8% 3 6% 5% 4 3 3 3 5% 54%	2620
	11th to 20th	Day	11% 12% 4 10% 7% 6% 6% 4 6% 7% 27%	3026
	21st to end	Day	11% 14% 6% 11% 5% 4 5% 4 5% 6% 28%	4045
January	1st to 10th	Night	7% 14% 4 9% 5% 4 4 4 6% 7% 37%	6306
	11th to 20th	Night	8% 13% 4 11% 5% 5% 5% 4 5% 7% 32%	5950
	21st to end	Night	7% 13% 3 9% 5% 5% 5% 3 5% 6% 40%	6468
February	1st to 10th	Night	11% 16% 5% 9% 5% 4 4 4 5% 5% 33%	2600
	11th to 20th	Night	10% 12% 3 8% 5% 4 4 3 5% 6% 41%	2443
	21st to end	Night	7% 13% 4 8% 6% 4 4 4 6% 6% 39%	1980
March	1st to 10th	Night	6% 14% 4 8% 4 4 4 4 6% 6% 40%	5164
	11th to 20th	Night	5% 10% 3 7% 5% 4 5% 5% 7% 7% 43%	5000
	21st to end	Night	7% 9% 3 8% 5% 5% 5% 4 6% 7% 39%	5489
April	1st to 10th	Night	8% 10% 3 8% 4 4 4 4 4 6% 45%	2160
	11th to 20th	Night	9% 7% 3 7% 4 5% 4 3 4 6% 47%	1781
	21st to end	Night	10% 7% 2 5% 4 3 5% 4 5% 6% 49%	1800
May	1st to 10th	Night	6% 6% 2 5% 4 3 4 3 5% 6% 55%	4100
	11th to 20th	Night	8% 6% 2 5% 3 3 4 3 6% 7% 53%	4074
	21st to end	Night	6% 4 2 5% 3 4 4 3 6% 6% 58%	4474
June	1st to 10th	Night	6% 4 1 4 3 4 5% 4 6% 6% 56%	1615
	11th to 20th	Night	6% 5% 1 4 4 2 3 3 6% 7% 59%	1400
	21st to end	Night	3 3 1 3 2 3 3 3 4 5% 71%	1497
July	1st to 10th	Night	3 2 1 4 3 3 3 3 4 5% 70%	3640
	11th to 20th	Night	2 3 1 5% 4 4 4 3 5% 6% 63%	3860
	21st to end	Night	3 3 1 4 3 3 4 3 5% 7% 65%	4472
August	1st to 10th	Night	3 3 1 3 3 3 4 4 5% 7% 63%	4033
	11th to 20th	Night	2 3 1 6% 3 3 4 3 4 5% 65%	4051
	21st to end	Night	5% 3 1 4 3 4 4 3 5% 6% 62%	4509
September	1st to 10th	Night	3 2 1 4 3 3 3 3 4 5% 69%	2139
	11th to 20th	Night	6% 3 1 3 2 2 3 3 5% 5% 67%	2200
	21st to end	Night	6% 4 1 4 3 2 2 2 3 4 69%	2200
October	1st to 10th	Night	4 3 1 3 2 2 2 2 3 5% 74%	3638
	11th to 20th	Night	8% 6% 2 5% 4 3 3 2 4 5% 58%	3778
	21st to end	Night	5% 6% 2 6% 4 3 5% 3 5% 7% 55%	4261
November	1st to 10th	Night	10% 8% 2 5% 5% 3 3 3 5% 7% 48%	2535
	11th to 20th	Night	8% 8% 3 6% 5% 3 4 3 5% 7% 48%	2600
	21st to end	Night	7% 10% 2 6% 5% 4 4 4 5% 7% 45%	2737
December	1st to 10th	Night	6% 7% 3 6% 3 3 3 2 3 6% 58%	4570
	11th to 20th	Night	10% 12% 4 9% 5% 5% 4 4 5% 7% 36%	5181
	21st to end	Night	11% 12% 5% 11% 7% 6% 5% 4 5% 6% 28%	6937

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	61% 7% 12% 13% 7% 1	3639
	11th to 20th	Day	57% 8% 10% 16% 6% 3	4006
	21st to end	Day	54% 7% 9% 17% 11% 1	4510
February	1st to 10th	Day	54% 8% 14% 16% 6% 2	1800
	11th to 20th	Day	49% 8% 14% 20% 7% 2	1848
	21st to end	Day	48% 7% 14% 19% 10% 2	1650
March	1st to 10th	Day	42% 6% 9% 29% 9% 4	4729
	11th to 20th	Day	34% 5% 11% 33% 16%	5000
	21st to end	Day	47% 8% 19% 21% 4	5486
April	1st to 10th	Day	40% 6% 14% 24% 13% 3	2235
	11th to 20th	Day	36% 6% 11% 26% 18% 2	2570
	21st to end	Day	35% 6% 12% 28% 18% 1	2600
May	1st to 10th	Day	28% 4 11% 26% 26% 4	5880
	11th to 20th	Day	30% 4 10% 27% 23% 6%	5844
	21st to end	Day	24% 4 9% 23% 35% 5%	6424
June	1st to 10th	Day	24% 6% 11% 25% 29% 5%	2770
	11th to 20th	Day	23% 4 10% 17% 28% 17%	3000
	21st to end	Day	14% 2 6% 13% 34% 30%	2893
July	1st to 10th	Day	11% 2 4 10% 16% 58%	6251
	11th to 20th	Day	16% 2 5% 7% 25% 45%	6030
	21st to end	Day	13% 2 4 7% 30% 43%	6217
August	1st to 10th	Day	17% 2 6% 13% 31% 31%	4208
	11th to 20th	Day	16% 2 5% 14% 35% 27%	4197
	21st to end	Day	15% 2 5% 14% 34% 31%	4663
September	1st to 10th	Day	18% 4 6% 16% 30% 27%	2255
	11th to 20th	Day	20% 4 7% 20% 30% 20%	2200
	21st to end	Day	19% 3 8% 22% 37% 12%	2200
October	1st to 10th	Day	20% 3 7% 18% 32% 21%	3404
	11th to 20th	Day	23% 4 8% 32% 26% 7%	3150
	21st to end	Day	26% 5% 15% 36% 18% 1	3296
November	1st to 10th	Day	40% 5% 15% 23% 16% 1	1764
	11th to 20th	Day	34% 7% 22% 27% 10% 1	1800
	21st to end	Day	43% 5% 17% 24% 11% 1	1639
December	1st to 10th	Day	35% 6% 16% 33% 8% 1	2620
	11th to 20th	Day	54% 9% 17% 17% 2	3024
	21st to end	Day	56% 7% 14% 16% 7% 1	4045
January	1st to 10th	Night	45% 9% 14% 24% 8% 1	6306
	11th to 20th	Night	48% 9% 13% 22% 7% 1	5950
	21st to end	Night	47% 7% 13% 18% 15% 1	6468
February	1st to 10th	Night	51% 8% 15% 20% 7% 1	2600
	11th to 20th	Night	44% 6% 14% 21% 13% 2	2444
	21st to end	Night	44% 5% 13% 24% 12% 3	1980
March	1st to 10th	Night	42% 6% 10% 24% 12% 6%	5164
	11th to 20th	Night	38% 5% 11% 26% 18% 1	5000
	21st to end	Night	40% 7% 18% 23% 10% 1	5489
April	1st to 10th	Night	39% 5% 12% 23% 15% 6%	2160
	11th to 20th	Night	37% 5% 11% 23% 22% 3	1781
	21st to end	Night	35% 4 11% 25% 22% 3	1800
May	1st to 10th	Night	27% 4 12% 22% 29% 7%	4097
	11th to 20th	Night	29% 4 7% 25% 28% 7%	4075
	21st to end	Night	26% 4 7% 22% 34% 6%	4474
June	1st to 10th	Night	25% 6% 11% 23% 27% 7%	1615
	11th to 20th	Night	25% 4 8% 16% 30% 17%	1400
	21st to end	Night	16% 3 4 13% 30% 34%	1497
July	1st to 10th	Night	18% 2 4 8% 16% 51%	3600
	11th to 20th	Night	22% 2 5% 5% 22% 45%	3812
	21st to end	Night	19% 2 4 6% 25% 43%	4344
August	1st to 10th	Night	20% 3 6% 11% 26% 34%	2928
	11th to 20th	Night	23% 3 6% 12% 27% 30%	2928
	21st to end	Night	17% 2 5% 10% 30% 36%	3245
September	1st to 10th	Night	18% 3 6% 14% 24% 35%	2139
	11th to 20th	Night	18% 4 7% 16% 31% 24%	2200
	21st to end	Night	20% 3 5% 18% 34% 19%	2200
October	1st to 10th	Night	16% 2 7% 14% 37% 24%	3638
	11th to 20th	Night	30% 2 5% 26% 27% 10%	3778
	21st to end	Night	29% 4 9% 34% 19% 4	4261
November	1st to 10th	Night	35% 5% 13% 24% 19% 3	2535
	11th to 20th	Night	35% 7% 15% 29% 14% 1	2600
	21st to end	Night	37% 6% 15% 30% 11% 2	2737
December	1st to 10th	Night	30% 4 13% 41% 10% 2	4570
	11th to 20th	Night	48% 8% 21% 19% 3	5182
	21st to end	Night	55% 6% 16% 14% 8%	6937

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	15% 7% 21% 30% 18% 11%	200
	11th to 20th	Day	14% 5% 21% 31% 20% 9%	280
	21st to end	Day	14% 5% 18% 34% 22% 7%	330
February	1st to 10th	Day	24% 9% 16% 33% 11% 6%	300
	11th to 20th	Day	16% 7% 23% 35% 13% 5%	384
	21st to end	Day	10% 6% 24% 38% 18% 4	414
March	1st to 10th	Day	14% 6% 15% 38% 21% 5%	500
	11th to 20th	Day	10% 6% 17% 36% 24% 7%	500
	21st to end	Day	14% 8% 24% 34% 17% 2	550
April	1st to 10th	Day	14% 8% 22% 30% 21% 4	500
	11th to 20th	Day	9% 15% 27% 30% 14% 5%	492
	21st to end	Day	10% 14% 26% 27% 17% 6%	500
May	1st to 10th	Day	6% 13% 26% 27% 23% 4	503
	11th to 20th	Day	12% 8% 24% 27% 23% 6%	528
	21st to end	Day	8% 9% 19% 29% 25% 10%	573
June	1st to 10th	Day	6% 6% 19% 30% 29% 11%	670
	11th to 20th	Day	8% 6% 24% 32% 25% 5%	730
	21st to end	Day	4 4 25% 38% 22% 8%	623
July	1st to 10th	Day	3 5% 20% 35% 28% 10%	526
	11th to 20th	Day	5% 2 21% 35% 30% 8%	524
	21st to end	Day	4 2 13% 33% 36% 13%	575
August	1st to 10th	Day	6% 3 17% 32% 31% 11%	527
	11th to 20th	Day	4 4 15% 32% 32% 14%	517
	21st to end	Day	5% 5% 13% 28% 31% 17%	583
September	1st to 10th	Day	3 4 14% 26% 34% 19%	530
	11th to 20th	Day	5% 4 17% 35% 27% 13%	530
	21st to end	Day	8% 4 18% 23% 35% 12%	530
October	1st to 10th	Day	9% 4 18% 29% 26% 14%	435
	11th to 20th	Day	7% 7% 21% 25% 24% 15%	300
	21st to end	Day	7% 11% 23% 35% 21% 3	330
November	1st to 10th	Day	14% 9% 21% 33% 16% 7%	300
	11th to 20th	Day	10% 8% 22% 39% 13% 8%	300
	21st to end	Day	14% 5% 20% 31% 23% 6%	300
December	1st to 10th	Day	14% 5% 20% 32% 19% 10%	300
	11th to 20th	Day	14% 6% 16% 40% 22% 3	300
	21st to end	Day	17% 10% 17% 34% 16% 6%	263
January	1st to 10th	Night	17% 3 16% 37% 22% 5%	800
	11th to 20th	Night	16% 4 17% 36% 23% 5%	800
	21st to end	Night	17% 4 16% 36% 23% 4	880
February	1st to 10th	Night	18% 3 23% 37% 16% 2	800
	11th to 20th	Night	16% 3 19% 42% 18% 2	784
	21st to end	Night	11% 4 22% 41% 20% 2	660
March	1st to 10th	Night	15% 6% 16% 43% 17% 3	800
	11th to 20th	Night	10% 3 19% 42% 23% 2	800
	21st to end	Night	13% 6% 19% 42% 18% 2	880
April	1st to 10th	Night	12% 8% 19% 40% 19% 3	800
	11th to 20th	Night	11% 6% 22% 42% 15% 3	593
	21st to end	Night	11% 7% 23% 34% 22% 3	600
May	1st to 10th	Night	9% 6% 24% 39% 20% 3	600
	11th to 20th	Night	11% 5% 17% 36% 27% 4	591
	21st to end	Night	11% 6% 16% 33% 30% 4	650
June	1st to 10th	Night	6% 7% 16% 38% 32% 2	565
	11th to 20th	Night	8% 6% 22% 40% 23% 1	400
	21st to end	Night	4 3 16% 44% 29% 4	400
July	1st to 10th	Night	2 3 15% 39% 37% 4	390
	11th to 20th	Night	4 2 16% 48% 30% 1	478
	21st to end	Night	3 1 10% 44% 37% 4	657
August	1st to 10th	Night	5% 2 11% 48% 32% 3	597
	11th to 20th	Night	4 2 12% 44% 36% 2	594
	21st to end	Night	7% 3 13% 38% 35% 5%	660
September	1st to 10th	Night	4 2 11% 38% 36% 8%	770
	11th to 20th	Night	6% 3 13% 37% 34% 8%	800
	21st to end	Night	7% 3 14% 36% 31% 9%	800
October	1st to 10th	Night	9% 3 14% 39% 31% 5%	772
	11th to 20th	Night	10% 5% 12% 37% 31% 5%	760
	21st to end	Night	8% 4 19% 39% 26% 4	836
November	1st to 10th	Night	12% 5% 13% 39% 26% 4	780
	11th to 20th	Night	13% 3 18% 44% 20% 3	800
	21st to end	Night	10% 5% 18% 41% 23% 4	937
December	1st to 10th	Night	14% 5% 16% 42% 22% 2	1000
	11th to 20th	Night	18% 4 19% 40% 17% 2	1000
	21st to end	Night	20% 5% 20% 38% 14% 2	964

Page generated at 2025-09-30 23:39:07. Version ac07

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