Mountain City
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain City clear sky chart from 2005-11-06 to 2024-11-30.

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	14% 10% 4 8% 5% 5% 5% 4 6% 7% 33%	101433
	Night	13% 11% 4 8% 5% 4 4 4 6% 7% 35%	98119
Transparency	Day	47% 7% 11% 19% 12% 4	105149
	Night	46% 7% 12% 19% 13% 4	102542
Seeing	Day	23% 16% 24% 19% 12% 6%	15193
	Night	21% 15% 20% 25% 17% 3	25272

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% 15% 5% 14% 6% 3 3 2 4 6% 27%	2836
	11th to 20th	Day	13% 16% 9% 13% 5% 7% 3 3 4 4 24%	2840
	21st to end	Day	15% 18% 7% 8% 4 4 4 3 4 4 29%	3124
February	1st to 10th	Day	16% 14% 4 9% 6% 5% 4 4 4 6% 30%	1710
	11th to 20th	Day	20% 13% 5% 10% 4 4 3 2 4 5% 30%	1848
	21st to end	Day	21% 13% 4 9% 3 3 3 2 4 6% 32%	1570
March	1st to 10th	Day	17% 14% 5% 7% 4 4 4 3 5% 5% 32%	3419
	11th to 20th	Day	13% 9% 3 7% 4 4 4 2 4 6% 44%	3480
	21st to end	Day	18% 10% 3 6% 4 3 4 3 6% 6% 36%	3815
April	1st to 10th	Day	13% 11% 4 5% 5% 4 4 4 6% 7% 37%	2090
	11th to 20th	Day	16% 10% 3 5% 4 4 4 4 5% 6% 38%	2283
	21st to end	Day	16% 10% 3 7% 5% 4 5% 4 5% 6% 36%	2470
May	1st to 10th	Day	19% 10% 4 9% 6% 6% 5% 4 5% 6% 26%	4082
	11th to 20th	Day	18% 10% 4 9% 6% 6% 5% 4 6% 6% 25%	4081
	21st to end	Day	10% 7% 2 7% 6% 5% 6% 5% 9% 11% 32%	4476
June	1st to 10th	Day	12% 8% 2 7% 6% 6% 6% 5% 7% 8% 32%	2470
	11th to 20th	Day	9% 7% 3 8% 7% 7% 6% 5% 8% 8% 32%	2470
	21st to end	Day	10% 7% 3 7% 7% 7% 7% 5% 7% 8% 31%	2470
July	1st to 10th	Day	8% 6% 3 7% 6% 6% 7% 6% 8% 10% 32%	4120
	11th to 20th	Day	11% 7% 3 9% 5% 6% 6% 5% 7% 9% 33%	4120
	21st to end	Day	18% 11% 4 11% 8% 7% 7% 5% 6% 6% 17%	4466
August	1st to 10th	Day	12% 8% 3 10% 6% 7% 7% 6% 9% 9% 23%	4104
	11th to 20th	Day	11% 7% 3 8% 6% 7% 7% 6% 8% 10% 28%	4044
	21st to end	Day	6% 5% 2 6% 5% 5% 5% 5% 7% 9% 46%	4300
September	1st to 10th	Day	12% 6% 3 7% 5% 6% 6% 5% 7% 8% 36%	2090
	11th to 20th	Day	14% 8% 4 8% 6% 5% 6% 5% 5% 7% 34%	2090
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 4 7% 7% 34%	2090
October	1st to 10th	Day	14% 7% 2 6% 5% 3 4 3 4 8% 44%	2009
	11th to 20th	Day	15% 7% 2 5% 3 3 4 3 4 5% 48%	1900
	21st to end	Day	14% 11% 2 6% 3 3 3 2 5% 6% 45%	2090
November	1st to 10th	Day	11% 9% 2 5% 3 3 4 4 5% 7% 48%	1892
	11th to 20th	Day	11% 11% 2 6% 4 4 4 4 6% 7% 41%	1800
	21st to end	Day	12% 11% 3 7% 5% 3 4 3 6% 5% 41%	1797
December	1st to 10th	Day	17% 14% 5% 9% 5% 4 4 3 5% 5% 28%	1710
	11th to 20th	Day	15% 13% 6% 7% 3 3 4 4 5% 7% 33%	2153
	21st to end	Day	12% 14% 7% 10% 5% 5% 6% 4 5% 5% 27%	3124
January	1st to 10th	Night	13% 20% 6% 15% 5% 4 3 3 3 4 24%	4109
	11th to 20th	Night	14% 14% 4 10% 6% 5% 4 3 5% 4 31%	4120
	21st to end	Night	16% 17% 5% 9% 4 4 5% 3 5% 5% 28%	4532
February	1st to 10th	Night	14% 12% 4 9% 5% 4 4 3 6% 6% 33%	2466
	11th to 20th	Night	18% 14% 4 11% 5% 5% 4 3 4 4 28%	2218
	21st to end	Night	18% 14% 5% 10% 4 3 4 3 5% 5% 30%	1884
March	1st to 10th	Night	21% 15% 4 7% 4 4 4 3 3 5% 30%	3432
	11th to 20th	Night	14% 9% 3 9% 3 3 4 3 5% 6% 42%	3480
	21st to end	Night	15% 8% 3 6% 4 4 4 3 6% 7% 40%	3817
April	1st to 10th	Night	13% 13% 3 6% 4 3 4 4 6% 7% 37%	2090
	11th to 20th	Night	13% 10% 3 6% 3 3 5% 4 6% 8% 39%	1859
	21st to end	Night	13% 9% 2 6% 6% 4 4 3 6% 6% 41%	1710
May	1st to 10th	Night	18% 12% 3 8% 5% 5% 4 4 6% 7% 28%	2813
	11th to 20th	Night	19% 10% 3 8% 5% 5% 4 4 7% 7% 28%	2812
	21st to end	Night	11% 8% 3 7% 5% 5% 5% 5% 7% 10% 34%	3079
June	1st to 10th	Night	9% 6% 2 6% 5% 6% 5% 5% 7% 11% 36%	1710
	11th to 20th	Night	6% 6% 2 7% 5% 5% 6% 6% 7% 11% 40%	1710
	21st to end	Night	9% 7% 3 6% 5% 5% 5% 4 8% 10% 37%	1710
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 6% 6% 9% 11% 33%	2840
	11th to 20th	Night	10% 7% 2 6% 4 4 5% 5% 7% 10% 40%	2840
	21st to end	Night	15% 10% 4 11% 7% 6% 6% 5% 7% 7% 21%	3087
August	1st to 10th	Night	12% 8% 3 8% 5% 6% 5% 6% 7% 10% 29%	2825
	11th to 20th	Night	12% 7% 2 8% 5% 6% 5% 5% 8% 8% 33%	2785
	21st to end	Night	5% 4 1 4 3 4 5% 4 7% 10% 52%	3318
September	1st to 10th	Night	11% 8% 2 6% 5% 5% 3 4 6% 8% 43%	2090
	11th to 20th	Night	14% 7% 3 6% 4 4 4 4 6% 7% 41%	2090
	21st to end	Night	13% 7% 2 6% 4 4 4 5% 6% 6% 41%	2090
October	1st to 10th	Night	15% 6% 2 6% 3 3 4 3 5% 7% 47%	2171
	11th to 20th	Night	14% 9% 2 4 4 3 4 3 5% 5% 46%	2280
	21st to end	Night	12% 11% 3 5% 3 4 3 3 4 7% 47%	2508
November	1st to 10th	Night	12% 9% 2 6% 3 4 4 3 5% 5% 47%	2401
	11th to 20th	Night	13% 11% 3 7% 4 3 4 3 5% 7% 41%	2600
	21st to end	Night	12% 11% 3 6% 3 4 4 3 5% 5% 42%	2599
December	1st to 10th	Night	17% 13% 4 11% 6% 4 4 3 5% 6% 28%	2470
	11th to 20th	Night	15% 14% 5% 12% 4 4 4 2 4 5% 31%	3042
	21st to end	Night	12% 15% 9% 12% 6% 5% 5% 3 6% 7% 21%	4532

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	56% 5% 6% 13% 16% 4	2837
	11th to 20th	Day	62% 4 6% 12% 14% 2	2840
	21st to end	Day	58% 3 7% 14% 15% 2	3124
February	1st to 10th	Day	55% 5% 8% 15% 15% 2	1710
	11th to 20th	Day	57% 3 7% 14% 15% 3	1858
	21st to end	Day	52% 5% 9% 14% 14% 5%	1570
March	1st to 10th	Day	52% 4 9% 18% 14% 4	3419
	11th to 20th	Day	41% 5% 9% 23% 16% 6%	3480
	21st to end	Day	47% 5% 10% 22% 12% 4	3815
April	1st to 10th	Day	44% 7% 11% 21% 14% 3	2090
	11th to 20th	Day	44% 7% 9% 19% 17% 4	2283
	21st to end	Day	47% 8% 12% 21% 11% 2	2470
May	1st to 10th	Day	56% 8% 10% 18% 7%	4120
	11th to 20th	Day	54% 10% 12% 19% 4 1	4085
	21st to end	Day	39% 12% 17% 19% 11% 2	4489
June	1st to 10th	Day	47% 10% 15% 18% 10% 1	2470
	11th to 20th	Day	47% 11% 14% 16% 11% 1	2470
	21st to end	Day	45% 11% 14% 17% 12% 1	2470
July	1st to 10th	Day	43% 11% 19% 17% 9% 1	4055
	11th to 20th	Day	46% 8% 18% 21% 5% 1	4051
	21st to end	Day	64% 9% 9% 13% 5%	4324
August	1st to 10th	Day	53% 8% 18% 16% 4	4091
	11th to 20th	Day	47% 9% 16% 19% 7% 1	4083
	21st to end	Day	34% 8% 18% 29% 10% 1	4300
September	1st to 10th	Day	42% 11% 12% 17% 14% 4	2090
	11th to 20th	Day	48% 8% 9% 19% 15% 2	2090
	21st to end	Day	47% 8% 12% 17% 13% 4	2090
October	1st to 10th	Day	32% 5% 7% 19% 27% 10%	3334
	11th to 20th	Day	30% 4 7% 25% 14% 21%	3140
	21st to end	Day	30% 3 6% 30% 16% 15%	3428
November	1st to 10th	Day	36% 5% 9% 19% 25% 6%	1892
	11th to 20th	Day	41% 4 8% 17% 24% 6%	1800
	21st to end	Day	44% 4 7% 21% 19% 4	1797
December	1st to 10th	Day	57% 5% 8% 15% 12% 3	1710
	11th to 20th	Day	48% 4 10% 15% 20% 3	2150
	21st to end	Day	54% 5% 8% 21% 11% 1	3124
January	1st to 10th	Night	62% 5% 7% 10% 10% 6%	4109
	11th to 20th	Night	54% 5% 7% 13% 18% 4	4120
	21st to end	Night	57% 4 8% 13% 15% 3	4532
February	1st to 10th	Night	51% 5% 9% 18% 15% 2	2466
	11th to 20th	Night	58% 5% 7% 14% 12% 4	2219
	21st to end	Night	55% 5% 8% 17% 10% 4	1884
March	1st to 10th	Night	57% 6% 7% 14% 13% 2	3432
	11th to 20th	Night	41% 6% 10% 23% 12% 8%	3480
	21st to end	Night	42% 6% 11% 23% 14% 4	3817
April	1st to 10th	Night	44% 7% 12% 20% 13% 3	2090
	11th to 20th	Night	41% 8% 12% 19% 16% 4	1859
	21st to end	Night	42% 7% 13% 26% 11% 1	1710
May	1st to 10th	Night	53% 10% 12% 19% 5%	2840
	11th to 20th	Night	49% 12% 14% 20% 4	2813
	21st to end	Night	39% 13% 18% 20% 9% 1	3087
June	1st to 10th	Night	39% 13% 18% 19% 11%	1710
	11th to 20th	Night	35% 11% 21% 23% 8% 1	1710
	21st to end	Night	39% 11% 17% 19% 12% 2	1710
July	1st to 10th	Night	41% 10% 21% 19% 7% 1	2795
	11th to 20th	Night	37% 9% 26% 22% 5% 1	2786
	21st to end	Night	57% 10% 14% 15% 4	2988
August	1st to 10th	Night	48% 9% 21% 17% 5%	2814
	11th to 20th	Night	45% 9% 19% 20% 7%	2812
	21st to end	Night	27% 7% 20% 36% 10% 1	3318
September	1st to 10th	Night	39% 10% 15% 19% 13% 4	2090
	11th to 20th	Night	41% 8% 15% 20% 16% 1	2090
	21st to end	Night	42% 9% 11% 21% 15% 2	2090
October	1st to 10th	Night	36% 5% 7% 20% 20% 12%	3626
	11th to 20th	Night	32% 4 7% 20% 18% 19%	3779
	21st to end	Night	27% 3 6% 30% 18% 16%	4120
November	1st to 10th	Night	38% 6% 10% 19% 22% 5%	2401
	11th to 20th	Night	42% 5% 7% 18% 21% 7%	2600
	21st to end	Night	43% 4 7% 21% 20% 5%	2599
December	1st to 10th	Night	57% 5% 7% 15% 12% 3	2470
	11th to 20th	Night	56% 4 6% 13% 17% 4	3044
	21st to end	Night	59% 6% 8% 13% 11% 1	4532

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	35% 32% 24% 7% 1 1	270
	11th to 20th	Day	41% 27% 26% 6% 1	270
	21st to end	Day	37% 35% 20% 7% 1	297
February	1st to 10th	Day	35% 30% 26% 8% 2	270
	11th to 20th	Day	51% 25% 21% 3	270
	21st to end	Day	47% 27% 22% 4	225
March	1st to 10th	Day	28% 28% 25% 14% 4 1	444
	11th to 20th	Day	36% 28% 21% 11% 3 1	460
	21st to end	Day	33% 25% 25% 13% 4	506
April	1st to 10th	Day	30% 23% 26% 13% 6% 1	460
	11th to 20th	Day	27% 22% 25% 15% 9% 2	452
	21st to end	Day	28% 19% 25% 15% 9% 4	460
May	1st to 10th	Day	26% 21% 29% 12% 8% 4	463
	11th to 20th	Day	23% 14% 24% 22% 10% 6%	487
	21st to end	Day	16% 12% 24% 21% 16% 12%	529
June	1st to 10th	Day	15% 10% 29% 23% 17% 7%	490
	11th to 20th	Day	13% 10% 25% 25% 19% 8%	490
	21st to end	Day	15% 11% 27% 26% 14% 7%	490
July	1st to 10th	Day	14% 5% 19% 28% 20% 14%	485
	11th to 20th	Day	12% 5% 19% 31% 23% 10%	484
	21st to end	Day	14% 5% 24% 25% 20% 12%	532
August	1st to 10th	Day	16% 6% 20% 27% 17% 14%	486
	11th to 20th	Day	15% 5% 22% 23% 24% 12%	479
	21st to end	Day	12% 4 17% 29% 20% 18%	539
September	1st to 10th	Day	14% 4 18% 27% 26% 12%	490
	11th to 20th	Day	19% 4 18% 24% 25% 10%	490
	21st to end	Day	19% 11% 24% 23% 17% 6%	490
October	1st to 10th	Day	20% 10% 22% 24% 18% 6%	490
	11th to 20th	Day	20% 19% 20% 23% 16% 3	490
	21st to end	Day	23% 15% 27% 21% 10% 4	539
November	1st to 10th	Day	19% 17% 30% 21% 10% 3	430
	11th to 20th	Day	27% 25% 25% 17% 5% 1	300
	21st to end	Day	27% 24% 30% 13% 5% 1	299
December	1st to 10th	Day	36% 23% 29% 10% 2	270
	11th to 20th	Day	31% 24% 29% 13% 3	270
	21st to end	Day	31% 28% 28% 9% 4	297
January	1st to 10th	Night	28% 29% 21% 18% 4	760
	11th to 20th	Night	32% 22% 23% 20% 4	760
	21st to end	Night	25% 31% 24% 17% 3	836
February	1st to 10th	Night	25% 24% 25% 23% 3	760
	11th to 20th	Night	34% 30% 23% 13% 1	743
	21st to end	Night	32% 28% 26% 12% 2	628
March	1st to 10th	Night	24% 24% 28% 19% 4	760
	11th to 20th	Night	29% 21% 25% 19% 6% 1	760
	21st to end	Night	23% 20% 30% 21% 6%	836
April	1st to 10th	Night	25% 19% 26% 22% 7% 1	760
	11th to 20th	Night	21% 18% 26% 22% 11% 1	701
	21st to end	Night	18% 17% 21% 35% 9% 1	570
May	1st to 10th	Night	22% 11% 26% 26% 14% 1	570
	11th to 20th	Night	18% 9% 21% 31% 19% 3	561
	21st to end	Night	12% 6% 18% 30% 28% 7%	617
June	1st to 10th	Night	11% 4 18% 34% 29% 4	570
	11th to 20th	Night	9% 5% 15% 32% 32% 6%	570
	21st to end	Night	12% 4 14% 35% 31% 4	570
July	1st to 10th	Night	14% 3 11% 28% 40% 5%	555
	11th to 20th	Night	12% 3 7% 29% 43% 6%	552
	21st to end	Night	11% 2 9% 30% 39% 9%	624
August	1st to 10th	Night	14% 1 10% 31% 39% 5%	567
	11th to 20th	Night	15% 2 8% 32% 34% 8%	564
	21st to end	Night	11% 1 11% 30% 38% 9%	706
September	1st to 10th	Night	14% 4 12% 30% 33% 7%	760
	11th to 20th	Night	17% 6% 13% 28% 31% 5%	760
	21st to end	Night	18% 6% 17% 30% 26% 3	760
October	1st to 10th	Night	19% 10% 17% 24% 23% 6%	760
	11th to 20th	Night	20% 12% 23% 26% 16% 3	760
	21st to end	Night	20% 14% 23% 28% 13% 2	836
November	1st to 10th	Night	18% 12% 23% 28% 15% 3	780
	11th to 20th	Night	23% 18% 20% 29% 9% 1	800
	21st to end	Night	22% 19% 22% 26% 10% 1	800
December	1st to 10th	Night	30% 20% 24% 20% 6%	760
	11th to 20th	Night	26% 24% 19% 24% 6% 1	760
	21st to end	Night	31% 19% 22% 23% 4	836

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