SIX A RANCH OBSERVATORY
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SIX A RANCH OBSERVATORY clear sky chart from 2005-11-06 to 2024-12-24.

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	13% 8% 3 7% 4 4 4 4 6% 7% 39%	101466
	Night	14% 9% 3 7% 4 4 4 3 5% 7% 40%	98679
Transparency	Day	44% 3 10% 25% 17% 2	105136
	Night	44% 3 9% 24% 17% 3	103150
Seeing	Day	18% 9% 25% 25% 16% 7%	15596
	Night	18% 7% 26% 30% 16% 2	25429

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	13% 17% 7% 9% 4 4 4 3 4 6% 29%	2527
	11th to 20th	Day	18% 10% 4 11% 7% 5% 4 3 4 5% 29%	2530
	21st to end	Day	26% 15% 7% 8% 4 3 3 2 4 5% 22%	3095
February	1st to 10th	Day	17% 11% 6% 9% 4 3 3 3 5% 6% 32%	1900
	11th to 20th	Day	15% 10% 4 8% 5% 4 4 4 6% 6% 35%	1848
	21st to end	Day	16% 13% 5% 9% 4 4 4 3 6% 6% 30%	1570
March	1st to 10th	Day	23% 15% 3 7% 4 3 3 2 4 4 32%	3120
	11th to 20th	Day	14% 7% 3 6% 4 3 4 3 4 7% 44%	3371
	21st to end	Day	14% 9% 3 7% 5% 5% 5% 4 6% 7% 34%	3968
April	1st to 10th	Day	14% 9% 3 6% 5% 4 5% 4 5% 7% 39%	2280
	11th to 20th	Day	16% 10% 3 7% 4 5% 4 3 5% 6% 38%	2254
	21st to end	Day	18% 7% 2 7% 5% 4 4 4 5% 6% 38%	2418
May	1st to 10th	Day	19% 9% 3 6% 5% 5% 5% 5% 7% 8% 30%	4082
	11th to 20th	Day	19% 8% 2 6% 5% 4 4 3 4 6% 38%	4081
	21st to end	Day	11% 8% 2 8% 6% 5% 5% 4 7% 8% 36%	4476
June	1st to 10th	Day	10% 7% 2 6% 5% 5% 4 4 6% 7% 43%	2626
	11th to 20th	Day	7% 6% 2 7% 4 4 5% 5% 7% 8% 45%	2660
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 9% 43%	2660
July	1st to 10th	Day	14% 6% 2 6% 4 4 5% 4 6% 10% 39%	4440
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 7% 10% 49%	4440
	21st to end	Day	10% 8% 3 9% 7% 7% 6% 6% 8% 9% 28%	4560
August	1st to 10th	Day	7% 5% 2 5% 4 3 4 4 6% 11% 48%	4104
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 10% 40%	3797
	21st to end	Day	5% 4 2 5% 4 3 4 3 7% 8% 55%	4180
September	1st to 10th	Day	11% 5% 2 7% 4 5% 5% 4 5% 6% 46%	2239
	11th to 20th	Day	9% 6% 1 6% 4 4 4 3 7% 8% 48%	2090
	21st to end	Day	12% 6% 2 6% 4 4 3 3 4 7% 49%	2090
October	1st to 10th	Day	10% 5% 2 5% 4 3 4 3 5% 6% 53%	2090
	11th to 20th	Day	11% 6% 2 7% 4 3 5% 3 6% 6% 47%	1982
	21st to end	Day	18% 9% 2 5% 4 4 4 3 5% 6% 40%	1881
November	1st to 10th	Day	11% 5% 2 4 4 4 4 3 5% 6% 53%	1762
	11th to 20th	Day	17% 10% 3 5% 4 3 4 3 5% 7% 39%	1800
	21st to end	Day	19% 10% 2 6% 4 4 3 3 5% 6% 39%	1800
December	1st to 10th	Day	20% 14% 4 7% 4 3 3 3 4 5% 34%	1800
	11th to 20th	Day	15% 16% 5% 9% 5% 4 4 3 5% 6% 30%	2138
	21st to end	Day	19% 20% 6% 7% 5% 4 4 4 4 6% 22%	2807
January	1st to 10th	Night	16% 16% 5% 9% 5% 4 4 3 5% 5% 26%	4428
	11th to 20th	Night	15% 15% 6% 12% 8% 5% 3 3 4 4 25%	4440
	21st to end	Night	24% 17% 6% 7% 4 3 3 2 4 4 26%	4559
February	1st to 10th	Night	20% 11% 5% 8% 5% 5% 4 4 5% 5% 29%	2280
	11th to 20th	Night	16% 10% 4 8% 5% 4 5% 3 5% 6% 34%	2218
	21st to end	Night	20% 11% 4 10% 4 3 4 4 4 6% 31%	1884
March	1st to 10th	Night	22% 16% 2 6% 4 3 3 3 4 4 33%	3733
	11th to 20th	Night	12% 8% 2 6% 4 3 3 3 5% 6% 48%	3587
	21st to end	Night	15% 7% 3 7% 5% 5% 4 3 5% 5% 40%	3692
April	1st to 10th	Night	12% 8% 3 6% 4 4 5% 4 6% 8% 41%	1900
	11th to 20th	Night	15% 8% 3 5% 4 4 4 3 5% 6% 42%	1875
	21st to end	Night	17% 7% 3 6% 4 4 3 4 5% 8% 38%	1762
May	1st to 10th	Night	17% 11% 4 9% 5% 5% 4 4 6% 6% 29%	2813
	11th to 20th	Night	17% 6% 2 6% 4 4 4 3 6% 7% 41%	2812
	21st to end	Night	8% 6% 3 7% 4 5% 6% 4 6% 9% 41%	3079
June	1st to 10th	Night	9% 6% 2 5% 5% 4 5% 3 6% 10% 47%	1550
	11th to 20th	Night	6% 5% 1 6% 3 4 5% 4 6% 9% 50%	1520
	21st to end	Night	5% 3 1 4 4 3 5% 4 9% 11% 51%	1520
July	1st to 10th	Night	9% 5% 3 7% 4 3 4 4 5% 9% 47%	2530
	11th to 20th	Night	5% 4 2 4 3 4 4 4 6% 9% 55%	2530
	21st to end	Night	10% 6% 3 7% 6% 6% 7% 5% 8% 10% 34%	2992
August	1st to 10th	Night	6% 4 2 6% 3 4 4 3 6% 9% 53%	2825
	11th to 20th	Night	6% 4 1 6% 4 4 5% 4 7% 10% 47%	3029
	21st to end	Night	4 3 1 3 3 3 4 3 5% 10% 62%	3476
September	1st to 10th	Night	11% 5% 2 5% 4 3 4 3 5% 7% 51%	1920
	11th to 20th	Night	7% 5% 2 5% 3 3 4 4 6% 9% 52%	2090
	21st to end	Night	13% 5% 2 6% 4 3 3 2 4 6% 52%	2090
October	1st to 10th	Night	7% 5% 2 5% 3 3 3 2 3 7% 58%	2090
	11th to 20th	Night	14% 6% 2 5% 4 4 5% 2 5% 5% 48%	2195
	21st to end	Night	17% 8% 3 6% 4 3 3 3 4 5% 44%	2717
November	1st to 10th	Night	13% 7% 2 5% 4 3 3 3 4 6% 49%	2536
	11th to 20th	Night	15% 9% 3 6% 4 3 3 4 5% 6% 43%	2600
	21st to end	Night	17% 11% 4 6% 4 4 3 3 4 6% 37%	2600
December	1st to 10th	Night	18% 12% 5% 7% 4 4 4 3 5% 7% 33%	2600
	11th to 20th	Night	18% 12% 4 9% 4 4 4 3 5% 5% 32%	3274
	21st to end	Night	19% 18% 6% 11% 5% 5% 4 3 4 5% 20%	4933

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	59% 1 9% 21% 10% 1	2528
	11th to 20th	Day	60% 2 8% 20% 10% 1	2530
	21st to end	Day	66% 2 7% 20% 4 1	3095
February	1st to 10th	Day	54% 2 8% 22% 12% 2	1900
	11th to 20th	Day	49% 3 9% 24% 14% 2	1858
	21st to end	Day	55% 2 8% 21% 10% 3	1570
March	1st to 10th	Day	58% 1 5% 19% 12% 4	3120
	11th to 20th	Day	42% 2 9% 28% 17% 2	3371
	21st to end	Day	49% 2 12% 24% 13% 1	3968
April	1st to 10th	Day	45% 3 11% 24% 16% 1	2280
	11th to 20th	Day	47% 3 8% 24% 15% 3	2254
	21st to end	Day	47% 4 9% 25% 13% 3	2418
May	1st to 10th	Day	52% 2 14% 22% 9%	4120
	11th to 20th	Day	49% 2 10% 24% 14% 1	4085
	21st to end	Day	45% 3 11% 18% 21% 1	4489
June	1st to 10th	Day	39% 3 11% 26% 18% 3	2626
	11th to 20th	Day	35% 5% 13% 25% 20% 2	2660
	21st to end	Day	36% 6% 14% 22% 18% 4	2660
July	1st to 10th	Day	42% 3 11% 22% 19% 2	4371
	11th to 20th	Day	30% 4 11% 35% 18% 2	4365
	21st to end	Day	51% 4 13% 19% 12% 1	4416
August	1st to 10th	Day	31% 4 14% 35% 15% 1	4091
	11th to 20th	Day	37% 4 11% 30% 17% 1	3835
	21st to end	Day	27% 4 13% 36% 17% 4	4179
September	1st to 10th	Day	38% 3 9% 24% 20% 5%	2239
	11th to 20th	Day	34% 3 9% 30% 21% 4	2090
	21st to end	Day	37% 3 8% 26% 24% 3	2090
October	1st to 10th	Day	22% 2 5% 29% 40% 2	3480
	11th to 20th	Day	29% 2 7% 29% 30% 3	3263
	21st to end	Day	36% 2 8% 26% 25% 4	3081
November	1st to 10th	Day	33% 3 9% 31% 20% 5%	1762
	11th to 20th	Day	45% 3 7% 24% 17% 4	1800
	21st to end	Day	48% 2 7% 24% 17% 2	1800
December	1st to 10th	Day	54% 2 10% 22% 12% 1	1800
	11th to 20th	Day	57% 2 7% 22% 12% 1	2135
	21st to end	Day	66% 1 9% 15% 9% 1	2807
January	1st to 10th	Night	62% 2 9% 17% 10% 1	4428
	11th to 20th	Night	64% 1 8% 17% 9% 1	4440
	21st to end	Night	64% 2 5% 18% 10% 1	4559
February	1st to 10th	Night	58% 2 9% 21% 8% 1	2280
	11th to 20th	Night	52% 2 8% 23% 13% 2	2219
	21st to end	Night	56% 2 10% 18% 11% 3	1884
March	1st to 10th	Night	56% 2 6% 22% 13% 1	3733
	11th to 20th	Night	39% 2 11% 26% 21% 2	3587
	21st to end	Night	47% 2 9% 29% 12% 1	3692
April	1st to 10th	Night	42% 3 10% 27% 15% 3	1900
	11th to 20th	Night	43% 2 9% 21% 21% 5%	1875
	21st to end	Night	46% 2 9% 26% 14% 3	1762
May	1st to 10th	Night	57% 2 11% 22% 7% 2	2840
	11th to 20th	Night	43% 4 13% 29% 10% 1	2813
	21st to end	Night	39% 3 13% 25% 16% 4	3087
June	1st to 10th	Night	34% 4 10% 27% 20% 5%	1550
	11th to 20th	Night	30% 4 15% 24% 21% 6%	1520
	21st to end	Night	25% 6% 18% 24% 21% 7%	1520
July	1st to 10th	Night	35% 5% 11% 26% 19% 4	2490
	11th to 20th	Night	26% 4 11% 35% 21% 3	2482
	21st to end	Night	43% 5% 15% 23% 12% 2	2897
August	1st to 10th	Night	30% 4 14% 38% 14% 2	2814
	11th to 20th	Night	32% 4 13% 32% 18% 1	3057
	21st to end	Night	20% 3 15% 36% 23% 3	3476
September	1st to 10th	Night	33% 3 9% 27% 20% 7%	1920
	11th to 20th	Night	29% 3 8% 29% 26% 5%	2090
	21st to end	Night	36% 2 8% 25% 25% 4	2090
October	1st to 10th	Night	20% 2 5% 20% 49% 4	3480
	11th to 20th	Night	26% 2 6% 25% 37% 3	3653
	21st to end	Night	33% 1 7% 23% 29% 6%	4467
November	1st to 10th	Night	37% 3 8% 25% 23% 4	2536
	11th to 20th	Night	42% 3 7% 23% 21% 5%	2600
	21st to end	Night	49% 2 8% 23% 16% 1	2600
December	1st to 10th	Night	52% 3 10% 23% 12% 1	2600
	11th to 20th	Night	56% 2 8% 20% 13% 1	3276
	21st to end	Night	68% 2 8% 14% 7% 1	4933

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% 18% 33% 17% 6% 1	270
	11th to 20th	Day	31% 18% 34% 15% 2	270
	21st to end	Day	27% 17% 38% 14% 3	297
February	1st to 10th	Day	26% 21% 35% 15% 3	270
	11th to 20th	Day	27% 23% 35% 11% 3	270
	21st to end	Day	32% 16% 36% 12% 3	225
March	1st to 10th	Day	27% 17% 40% 12% 4	270
	11th to 20th	Day	25% 16% 35% 18% 5% 1	390
	21st to end	Day	30% 15% 32% 17% 6% 1	506
April	1st to 10th	Day	21% 16% 30% 23% 9% 1	460
	11th to 20th	Day	23% 14% 27% 21% 12% 4	452
	21st to end	Day	23% 13% 34% 19% 7% 4	460
May	1st to 10th	Day	24% 10% 29% 24% 10% 3	463
	11th to 20th	Day	22% 7% 21% 30% 15% 5%	487
	21st to end	Day	13% 4 24% 29% 18% 12%	529
June	1st to 10th	Day	14% 4 22% 32% 20% 9%	646
	11th to 20th	Day	10% 5% 20% 34% 23% 9%	680
	21st to end	Day	11% 4 26% 29% 20% 9%	680
July	1st to 10th	Day	15% 3 13% 25% 26% 18%	671
	11th to 20th	Day	9% 3 19% 29% 29% 12%	668
	21st to end	Day	12% 2 15% 33% 25% 13%	587
August	1st to 10th	Day	10% 3 14% 30% 27% 18%	486
	11th to 20th	Day	16% 3 15% 30% 25% 11%	479
	21st to end	Day	9% 1 12% 27% 33% 18%	539
September	1st to 10th	Day	11% 2 14% 32% 29% 11%	490
	11th to 20th	Day	12% 1 11% 29% 31% 15%	490
	21st to end	Day	16% 8% 18% 23% 23% 11%	490
October	1st to 10th	Day	10% 5% 23% 33% 23% 6%	490
	11th to 20th	Day	15% 13% 31% 27% 10% 5%	445
	21st to end	Day	28% 8% 35% 24% 5%	330
November	1st to 10th	Day	14% 13% 35% 25% 11% 3	300
	11th to 20th	Day	23% 16% 33% 19% 8% 2	300
	21st to end	Day	28% 10% 33% 22% 7%	300
December	1st to 10th	Day	23% 18% 35% 16% 7% 1	300
	11th to 20th	Day	25% 15% 36% 17% 5% 1	300
	21st to end	Day	32% 17% 35% 13% 3 1	306
January	1st to 10th	Night	22% 14% 33% 26% 4	950
	11th to 20th	Night	29% 12% 35% 20% 4	950
	21st to end	Night	22% 12% 36% 26% 3	892
February	1st to 10th	Night	27% 11% 34% 25% 3	760
	11th to 20th	Night	23% 13% 38% 25% 2	743
	21st to end	Night	28% 11% 39% 20% 2	628
March	1st to 10th	Night	24% 11% 33% 27% 5%	760
	11th to 20th	Night	23% 9% 37% 24% 6% 1	760
	21st to end	Night	25% 7% 30% 30% 7% 1	762
April	1st to 10th	Night	18% 9% 33% 32% 8%	570
	11th to 20th	Night	20% 8% 29% 30% 13%	563
	21st to end	Night	19% 6% 34% 31% 9% 2	570
May	1st to 10th	Night	19% 6% 29% 32% 14% 1	570
	11th to 20th	Night	15% 5% 19% 37% 20% 3	561
	21st to end	Night	9% 3 18% 37% 29% 5%	617
June	1st to 10th	Night	12% 2 17% 41% 25% 3	570
	11th to 20th	Night	8% 2 15% 44% 28% 2	570
	21st to end	Night	6% 2 19% 40% 31% 2	570
July	1st to 10th	Night	12% 2 8% 32% 37% 10%	555
	11th to 20th	Night	7% 1 10% 39% 37% 6%	552
	21st to end	Night	9% 2 10% 41% 32% 6%	624
August	1st to 10th	Night	10% 2 8% 36% 40% 5%	567
	11th to 20th	Night	7% 1 10% 40% 35% 7%	564
	21st to end	Night	8% 1 8% 32% 43% 8%	627
September	1st to 10th	Night	14% 2 10% 33% 36% 6%	590
	11th to 20th	Night	9% 2 12% 34% 39% 5%	760
	21st to end	Night	14% 5% 18% 31% 28% 4	760
October	1st to 10th	Night	10% 4 20% 35% 25% 6%	760
	11th to 20th	Night	16% 6% 26% 32% 18% 2	760
	21st to end	Night	21% 7% 31% 28% 11% 1	836
November	1st to 10th	Night	18% 6% 31% 30% 14% 2	780
	11th to 20th	Night	20% 12% 29% 30% 9% 1	800
	21st to end	Night	25% 10% 27% 29% 8% 1	800
December	1st to 10th	Night	24% 9% 34% 26% 6%	800
	11th to 20th	Night	27% 9% 32% 25% 7%	865
	21st to end	Night	28% 11% 33% 24% 3	1063

Page generated at 2024-12-21 04:13:47. Version ac07

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