Ritter Butte
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ritter Butte 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	8% 10% 3 6% 4 4 4 3 5% 6% 48%	135413
	Night	10% 12% 3 6% 4 4 4 3 5% 6% 44%	131274
Transparency	Day	38% 5% 10% 20% 18% 9%	130178
	Night	42% 5% 10% 21% 16% 6%	127520
Seeing	Day	14% 12% 26% 27% 16% 6%	15648
	Night	17% 7% 17% 35% 21% 3	24722

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	17% 22% 7% 10% 5% 4 5% 4 5% 5% 18%	3639
	11th to 20th	Day	12% 17% 5% 9% 4 3 4 4 5% 5% 33%	3640
	21st to end	Day	12% 13% 3 7% 4 4 3 3 4 6% 40%	4004
February	1st to 10th	Day	15% 18% 6% 10% 5% 5% 4 3 4 6% 25%	1781
	11th to 20th	Day	13% 19% 5% 9% 5% 4 4 5% 5% 6% 25%	1859
	21st to end	Day	11% 18% 8% 10% 5% 4 4 3 5% 6% 25%	1650
March	1st to 10th	Day	8% 13% 5% 11% 6% 5% 6% 4 7% 7% 29%	4518
	11th to 20th	Day	6% 16% 6% 8% 5% 5% 5% 4 5% 5% 34%	4731
	21st to end	Day	9% 18% 5% 13% 7% 7% 6% 5% 5% 6% 21%	5942
April	1st to 10th	Day	12% 18% 6% 9% 5% 4 5% 3 5% 7% 27%	2400
	11th to 20th	Day	10% 13% 5% 9% 5% 5% 4 3 5% 7% 34%	2369
	21st to end	Day	11% 14% 4 8% 5% 4 4 4 5% 6% 35%	2530
May	1st to 10th	Day	9% 11% 3 6% 4 3 3 3 5% 6% 46%	6185
	11th to 20th	Day	10% 9% 2 7% 5% 4 4 3 5% 6% 43%	6277
	21st to end	Day	7% 9% 2 4 3 3 3 3 5% 8% 50%	6902
June	1st to 10th	Day	15% 8% 3 6% 5% 4 5% 4 5% 6% 42%	2800
	11th to 20th	Day	11% 9% 3 5% 4 3 4 4 5% 7% 46%	2800
	21st to end	Day	5% 4 1 4 3 2 3 3 5% 6% 63%	2800
July	1st to 10th	Day	2 2 1 2 2 2 2 2 4 5% 77%	6319
	11th to 20th	Day	1 2 1 1 2 1 2 2 3 3 82%	6320
	21st to end	Day	1 2 1 2 2 3 2 2 4 6% 74%	6880
August	1st to 10th	Day	4 4 1 4 3 3 3 3 5% 7% 63%	6210
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 5% 77%	5641
	21st to end	Day	3 5% 2 4 3 3 3 3 5% 6% 64%	5984
September	1st to 10th	Day	3 2 1 3 2 3 3 3 4 6% 69%	2400
	11th to 20th	Day	7% 5% 2 4 3 3 3 3 5% 6% 58%	2345
	21st to end	Day	8% 7% 2 6% 3 3 3 3 5% 5% 55%	2200
October	1st to 10th	Day	9% 6% 2 4 3 2 3 2 3 5% 61%	3281
	11th to 20th	Day	9% 7% 2 6% 4 4 4 3 6% 9% 47%	3161
	21st to end	Day	13% 12% 4 9% 5% 5% 5% 4 5% 6% 32%	3102
November	1st to 10th	Day	18% 14% 4 7% 5% 4 4 3 6% 8% 28%	1764
	11th to 20th	Day	16% 16% 4 9% 6% 5% 5% 4 5% 7% 23%	1727
	21st to end	Day	14% 17% 4 8% 5% 4 4 3 5% 6% 32%	1600
December	1st to 10th	Day	9% 13% 2 6% 4 3 3 3 3 6% 48%	2620
	11th to 20th	Day	13% 18% 4 11% 5% 5% 5% 4 6% 7% 22%	3028
	21st to end	Day	16% 18% 5% 10% 5% 5% 4 4 5% 6% 21%	4004
January	1st to 10th	Night	18% 23% 6% 9% 5% 5% 4 3 5% 5% 17%	6306
	11th to 20th	Night	15% 18% 5% 7% 4 4 4 2 3 5% 34%	6320
	21st to end	Night	12% 12% 3 8% 4 4 3 3 4 6% 40%	6952
February	1st to 10th	Night	16% 18% 5% 10% 6% 4 3 3 6% 5% 25%	2615
	11th to 20th	Night	13% 21% 5% 9% 4 4 4 3 4 5% 28%	2426
	21st to end	Night	11% 21% 6% 9% 5% 4 4 3 5% 7% 25%	1980
March	1st to 10th	Night	8% 11% 4 8% 5% 5% 5% 5% 8% 9% 32%	5376
	11th to 20th	Night	9% 15% 5% 10% 5% 4 4 3 4 5% 37%	5264
	21st to end	Night	11% 19% 5% 9% 6% 5% 6% 4 7% 7% 22%	5033
April	1st to 10th	Night	12% 17% 4 7% 6% 5% 4 3 5% 6% 32%	2000
	11th to 20th	Night	12% 13% 3 7% 5% 3 4 3 5% 7% 39%	1979
	21st to end	Night	11% 13% 3 6% 5% 5% 4 3 5% 7% 37%	1869
May	1st to 10th	Night	8% 13% 2 5% 3 3 4 3 4 5% 49%	3778
	11th to 20th	Night	13% 10% 2 5% 3 3 3 3 4 6% 48%	3616
	21st to end	Night	11% 9% 2 4 3 3 3 3 3 6% 53%	3972
June	1st to 10th	Night	12% 7% 2 4 5% 4 4 4 5% 7% 48%	1600
	11th to 20th	Night	13% 7% 2 5% 3 3 3 3 5% 7% 48%	1600
	21st to end	Night	5% 3 1 3 4 4 3 3 5% 5% 64%	1600
July	1st to 10th	Night	1 2 1 2 2 2 2 2 4 5% 77%	3640
	11th to 20th	Night	1 1 2 1 1 1 2 3 4 84%	3640
	21st to end	Night	3 2 1 2 1 2 3 3 4 6% 73%	3970
August	1st to 10th	Night	2 3 1 3 3 4 4 4 6% 8% 61%	3582
	11th to 20th	Night	2 2 1 3 2 3 2 2 4 5% 75%	4200
	21st to end	Night	3 4 1 4 3 2 3 3 4 5% 66%	5015
September	1st to 10th	Night	4 2 1 3 2 2 2 2 4 7% 71%	2000
	11th to 20th	Night	6% 5% 1 4 3 3 4 3 5% 6% 60%	2055
	21st to end	Night	8% 6% 2 5% 3 3 3 2 4 6% 57%	2200
October	1st to 10th	Night	7% 6% 2 4 2 2 2 2 3 4 65%	3762
	11th to 20th	Night	10% 7% 2 6% 4 3 4 3 5% 7% 51%	3777
	21st to end	Night	11% 12% 3 8% 4 4 4 3 5% 7% 39%	4454
November	1st to 10th	Night	18% 13% 3 7% 5% 5% 5% 3 4 6% 32%	2535
	11th to 20th	Night	20% 14% 4 7% 5% 5% 4 4 5% 5% 29%	2670
	21st to end	Night	13% 17% 4 6% 4 3 5% 4 6% 6% 33%	2800
December	1st to 10th	Night	10% 10% 4 6% 3 3 4 3 5% 6% 48%	4560
	11th to 20th	Night	15% 20% 5% 10% 6% 6% 5% 4 6% 6% 18%	5176
	21st to end	Night	17% 19% 6% 9% 5% 5% 4 3 4 5% 23%	6952

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	67% 8% 8% 14% 3	3639
	11th to 20th	Day	53% 5% 11% 20% 10% 1	3640
	21st to end	Day	45% 7% 8% 16% 19% 5%	4004
February	1st to 10th	Day	62% 7% 11% 14% 6%	1781
	11th to 20th	Day	61% 7% 11% 12% 8% 1	1868
	21st to end	Day	62% 7% 9% 13% 8% 2	1650
March	1st to 10th	Day	52% 4 11% 18% 10% 4	4518
	11th to 20th	Day	51% 5% 10% 24% 10%	4731
	21st to end	Day	62% 7% 12% 16% 4	5942
April	1st to 10th	Day	58% 6% 13% 16% 6% 1	2400
	11th to 20th	Day	52% 6% 12% 17% 12% 2	2372
	21st to end	Day	51% 6% 10% 20% 12% 1	2530
May	1st to 10th	Day	38% 5% 9% 26% 22% 1	6185
	11th to 20th	Day	41% 4 11% 23% 18% 2	6280
	21st to end	Day	33% 5% 16% 32% 13% 2	6904
June	1st to 10th	Day	45% 6% 10% 26% 13% 1	2800
	11th to 20th	Day	38% 6% 11% 26% 16% 3	2800
	21st to end	Day	23% 4 11% 29% 26% 8%	2800
July	1st to 10th	Day	12% 2 5% 17% 28% 35%	6251
	11th to 20th	Day	9% 2 4 9% 28% 47%	6245
	21st to end	Day	12% 2 5% 24% 35% 21%	6681
August	1st to 10th	Day	19% 3 5% 14% 34% 25%	4524
	11th to 20th	Day	14% 2 5% 25% 38% 16%	4090
	21st to end	Day	17% 3 7% 28% 24% 21%	4311
September	1st to 10th	Day	18% 4 6% 24% 30% 19%	2400
	11th to 20th	Day	27% 5% 10% 22% 25% 11%	2345
	21st to end	Day	32% 5% 9% 23% 23% 9%	2200
October	1st to 10th	Day	27% 4 10% 18% 29% 12%	3281
	11th to 20th	Day	35% 4 14% 26% 18% 2	3161
	21st to end	Day	52% 4 9% 22% 13% 1	3102
November	1st to 10th	Day	55% 6% 13% 17% 9% 1	1764
	11th to 20th	Day	60% 7% 11% 16% 5%	1727
	21st to end	Day	55% 6% 9% 16% 12% 2	1600
December	1st to 10th	Day	40% 5% 11% 25% 18% 1	2620
	11th to 20th	Day	59% 7% 19% 11% 4	3028
	21st to end	Day	63% 6% 13% 15% 4	4004
January	1st to 10th	Night	69% 6% 7% 15% 3	6306
	11th to 20th	Night	54% 4 9% 20% 11% 1	6320
	21st to end	Night	46% 7% 11% 16% 16% 5%	6952
February	1st to 10th	Night	62% 8% 11% 12% 7%	2615
	11th to 20th	Night	60% 6% 11% 14% 8% 3	2428
	21st to end	Night	61% 6% 9% 15% 8% 1	1980
March	1st to 10th	Night	44% 5% 11% 22% 13% 4	5376
	11th to 20th	Night	49% 5% 11% 27% 8%	5264
	21st to end	Night	59% 9% 11% 17% 3	5033
April	1st to 10th	Night	54% 7% 13% 21% 5% 1	2000
	11th to 20th	Night	47% 7% 12% 22% 10% 2	1979
	21st to end	Night	46% 8% 13% 17% 15% 1	1869
May	1st to 10th	Night	38% 4 12% 22% 23% 1	3775
	11th to 20th	Night	40% 4 12% 30% 14%	3616
	21st to end	Night	34% 6% 17% 32% 11%	3972
June	1st to 10th	Night	38% 8% 12% 27% 14% 1	1600
	11th to 20th	Night	37% 6% 12% 27% 13% 4	1600
	21st to end	Night	22% 5% 13% 30% 23% 7%	1600
July	1st to 10th	Night	12% 2 6% 19% 26% 34%	3600
	11th to 20th	Night	7% 2 6% 11% 30% 44%	3592
	21st to end	Night	14% 2 5% 29% 31% 19%	3857
August	1st to 10th	Night	17% 3 7% 17% 36% 20%	2601
	11th to 20th	Night	13% 3 5% 34% 33% 12%	3036
	21st to end	Night	16% 3 8% 29% 25% 19%	3608
September	1st to 10th	Night	15% 3 9% 25% 32% 16%	2000
	11th to 20th	Night	25% 5% 8% 25% 24% 13%	2055
	21st to end	Night	29% 4 9% 24% 24% 10%	2200
October	1st to 10th	Night	26% 3 6% 23% 32% 10%	3762
	11th to 20th	Night	34% 4 17% 19% 23% 3	3777
	21st to end	Night	45% 5% 9% 24% 15% 2	4454
November	1st to 10th	Night	53% 7% 9% 18% 11% 2	2535
	11th to 20th	Night	57% 7% 9% 16% 10%	2670
	21st to end	Night	51% 7% 10% 14% 16% 2	2800
December	1st to 10th	Night	38% 6% 9% 26% 19% 2	4560
	11th to 20th	Night	65% 6% 12% 12% 5%	5176
	21st to end	Night	62% 6% 12% 13% 5% 1	6952

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	25% 22% 23% 17% 12% 3	200
	11th to 20th	Day	18% 16% 27% 23% 12% 5%	200
	21st to end	Day	17% 15% 27% 23% 16% 1	220
February	1st to 10th	Day	24% 21% 26% 20% 7% 2	381
	11th to 20th	Day	24% 19% 26% 20% 10% 1	448
	21st to end	Day	20% 20% 26% 25% 8% 1	414
March	1st to 10th	Day	19% 17% 32% 24% 7% 1	500
	11th to 20th	Day	17% 19% 28% 24% 10% 3	500
	21st to end	Day	22% 16% 33% 21% 7% 1	550
April	1st to 10th	Day	21% 18% 24% 24% 11% 4	500
	11th to 20th	Day	16% 19% 29% 20% 12% 4	492
	21st to end	Day	16% 17% 25% 27% 13% 3	500
May	1st to 10th	Day	13% 17% 27% 25% 15% 4	503
	11th to 20th	Day	22% 13% 24% 23% 14% 5%	528
	21st to end	Day	16% 9% 25% 23% 20% 8%	573
June	1st to 10th	Day	16% 9% 22% 26% 18% 8%	530
	11th to 20th	Day	12% 16% 31% 24% 15% 3	530
	21st to end	Day	4 7% 28% 33% 20% 8%	530
July	1st to 10th	Day	3 5% 28% 38% 21% 6%	526
	11th to 20th	Day	2 6% 30% 33% 22% 7%	524
	21st to end	Day	2 5% 30% 33% 22% 9%	575
August	1st to 10th	Day	4 6% 23% 37% 21% 9%	527
	11th to 20th	Day	2 4 22% 32% 26% 14%	517
	21st to end	Day	5% 7% 22% 33% 22% 11%	583
September	1st to 10th	Day	4 4 23% 31% 26% 12%	530
	11th to 20th	Day	8% 6% 22% 33% 24% 7%	530
	21st to end	Day	12% 8% 23% 27% 22% 8%	530
October	1st to 10th	Day	14% 10% 22% 27% 19% 8%	503
	11th to 20th	Day	14% 13% 23% 26% 18% 8%	490
	21st to end	Day	21% 12% 24% 23% 15% 5%	331
November	1st to 10th	Day	20% 17% 25% 24% 12% 2	300
	11th to 20th	Day	20% 16% 24% 24% 13% 3	263
	21st to end	Day	22% 9% 27% 30% 11% 3	200
December	1st to 10th	Day	19% 18% 18% 23% 12% 12%	200
	11th to 20th	Day	27% 16% 35% 15% 9%	200
	21st to end	Day	22% 17% 29% 21% 10% 1	220
January	1st to 10th	Night	24% 12% 24% 25% 15% 2	800
	11th to 20th	Night	26% 10% 19% 26% 17% 2	800
	21st to end	Night	22% 7% 18% 35% 16% 2	880
February	1st to 10th	Night	24% 9% 23% 32% 10% 1	800
	11th to 20th	Night	21% 10% 20% 32% 16% 2	784
	21st to end	Night	20% 14% 24% 31% 9% 2	660
March	1st to 10th	Night	19% 11% 24% 32% 12% 2	800
	11th to 20th	Night	16% 10% 24% 32% 16% 2	800
	21st to end	Night	20% 10% 23% 33% 13% 1	880
April	1st to 10th	Night	19% 11% 18% 34% 16% 2	800
	11th to 20th	Night	17% 8% 25% 30% 17% 3	791
	21st to end	Night	17% 8% 19% 35% 20% 1	669
May	1st to 10th	Night	13% 8% 18% 38% 21% 2	460
	11th to 20th	Night	15% 4 14% 34% 29% 4	394
	21st to end	Night	18% 7% 12% 31% 29% 3	433
June	1st to 10th	Night	14% 5% 14% 41% 23% 3	400
	11th to 20th	Night	16% 5% 15% 43% 20% 2	400
	21st to end	Night	6% 2 12% 47% 33% 2	400
July	1st to 10th	Night	3 1 9% 54% 31% 2	390
	11th to 20th	Night	2 9% 60% 27% 1	388
	21st to end	Night	2 8% 52% 35% 3	438
August	1st to 10th	Night	3 1 6% 48% 40% 2	398
	11th to 20th	Night	2 1 8% 39% 42% 8%	649
	21st to end	Night	5% 3 11% 45% 33% 4	880
September	1st to 10th	Night	5% 2 9% 42% 37% 4	800
	11th to 20th	Night	7% 2 9% 39% 34% 8%	800
	21st to end	Night	11% 4 13% 38% 29% 6%	800
October	1st to 10th	Night	15% 3 11% 42% 26% 4	772
	11th to 20th	Night	15% 5% 17% 36% 21% 5%	760
	21st to end	Night	18% 9% 17% 35% 16% 5%	836
November	1st to 10th	Night	23% 8% 19% 35% 15% 1	780
	11th to 20th	Night	26% 9% 19% 30% 13% 2	800
	21st to end	Night	20% 9% 19% 31% 16% 5%	800
December	1st to 10th	Night	20% 8% 20% 32% 17% 3	800
	11th to 20th	Night	25% 9% 23% 29% 13% 1	800
	21st to end	Night	28% 11% 27% 24% 9% 1	880

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