The Ranch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Ranch Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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% 3 6% 4 4 5% 4 7% 9% 45%	105493
	Night	4 7% 3 7% 4 4 4 4 6% 9% 48%	102133
Transparency	Day	34% 7% 18% 24% 12% 3	105246
	Night	34% 8% 20% 23% 12% 3	102026
Seeing	Day	16% 18% 28% 22% 12% 3	15315
	Night	12% 19% 30% 26% 10% 2	24443

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	1 5% 3 11% 7% 5% 5% 4 9% 11% 39%	2219
	11th to 20th	Day	1 9% 3 9% 7% 10% 7% 6% 8% 7% 34%	2388
	21st to end	Day	2 6% 3 8% 4 5% 5% 3 6% 13% 45%	2794
February	1st to 10th	Day	2 9% 4 11% 6% 6% 7% 5% 7% 10% 33%	1573
	11th to 20th	Day	2 11% 5% 10% 5% 5% 6% 4 8% 9% 36%	1665
	21st to end	Day	2 8% 4 11% 7% 5% 5% 4 7% 9% 38%	1556
March	1st to 10th	Day	1 5% 3 7% 5% 4 5% 6% 9% 11% 44%	3138
	11th to 20th	Day	2 10% 3 6% 4 3 4 4 6% 11% 49%	3479
	21st to end	Day	3 14% 7% 13% 5% 4 5% 4 6% 8% 30%	3844
April	1st to 10th	Day	4 13% 4 8% 5% 6% 5% 5% 8% 8% 34%	2317
	11th to 20th	Day	6% 13% 4 9% 6% 5% 4 4 6% 9% 34%	2440
	21st to end	Day	8% 14% 4 7% 4 4 5% 5% 7% 8% 35%	2470
May	1st to 10th	Day	12% 18% 4 7% 5% 4 4 5% 5% 6% 28%	4110
	11th to 20th	Day	9% 9% 2 7% 4 6% 6% 5% 8% 9% 34%	4648
	21st to end	Day	14% 9% 2 6% 4 4 5% 5% 8% 9% 34%	5161
June	1st to 10th	Day	12% 8% 2 7% 6% 5% 5% 5% 7% 8% 35%	2850
	11th to 20th	Day	14% 7% 3 7% 5% 6% 6% 5% 7% 8% 32%	2850
	21st to end	Day	7% 3 2 4 4 4 6% 5% 8% 8% 48%	2850
July	1st to 10th	Day	4 4 2 5% 4 4 5% 4 7% 8% 54%	4740
	11th to 20th	Day	3 2 1 3 2 3 5% 4 6% 7% 63%	4740
	21st to end	Day	2 2 1 2 1 3 3 3 6% 8% 70%	4860
August	1st to 10th	Day	10% 6% 2 5% 4 4 5% 4 7% 8% 45%	4340
	11th to 20th	Day	5% 3 1 4 3 4 5% 5% 6% 8% 56%	4028
	21st to end	Day	7% 3 1 3 3 3 3 3 6% 7% 61%	4521
September	1st to 10th	Day	11% 5% 2 4 3 3 4 3 5% 7% 54%	2313
	11th to 20th	Day	9% 5% 2 5% 3 3 4 4 7% 8% 50%	2090
	21st to end	Day	9% 7% 2 3 2 3 3 3 5% 8% 54%	2090
October	1st to 10th	Day	6% 6% 2 5% 4 3 4 3 6% 8% 54%	3366
	11th to 20th	Day	3 5% 2 5% 4 4 5% 4 7% 11% 52%	2976
	21st to end	Day	4 7% 3 6% 4 4 5% 3 7% 9% 49%	3101
November	1st to 10th	Day	5% 8% 3 7% 5% 4 4 4 7% 9% 43%	1648
	11th to 20th	Day	1 9% 3 6% 4 3 3 5% 7% 12% 48%	1520
	21st to end	Day	2 10% 3 9% 5% 5% 5% 3 5% 9% 44%	1354
December	1st to 10th	Day	1 8% 4 7% 5% 5% 6% 5% 8% 8% 43%	1330
	11th to 20th	Day	2 9% 2 9% 6% 4 5% 5% 7% 10% 40%	1682
	21st to end	Day	1 7% 3 7% 5% 5% 5% 4 6% 9% 47%	2442
January	1st to 10th	Night	1 6% 3 10% 6% 5% 6% 5% 8% 10% 41%	4726
	11th to 20th	Night	2 6% 3 9% 7% 7% 7% 6% 6% 9% 36%	4538
	21st to end	Night	2 8% 2 8% 4 5% 5% 4 8% 10% 46%	4862
February	1st to 10th	Night	2 10% 4 12% 6% 5% 5% 4 5% 9% 37%	2606
	11th to 20th	Night	1 11% 4 9% 5% 5% 6% 4 6% 10% 39%	2402
	21st to end	Night	1 8% 3 11% 6% 7% 6% 5% 7% 9% 38%	1894
March	1st to 10th	Night	4 3 10% 7% 6% 5% 4 7% 10% 43%	3736
	11th to 20th	Night	2 5% 2 6% 4 3 4 4 6% 10% 54%	3500
	21st to end	Night	3 14% 7% 13% 6% 4 4 4 5% 9% 31%	3808
April	1st to 10th	Night	5% 11% 3 6% 5% 4 4 4 6% 9% 43%	1862
	11th to 20th	Night	5% 12% 4 8% 4 3 4 4 6% 8% 41%	1691
	21st to end	Night	7% 11% 3 7% 6% 5% 4 4 6% 7% 41%	1710
May	1st to 10th	Night	12% 16% 4 8% 6% 5% 5% 4 5% 5% 33%	2845
	11th to 20th	Night	7% 6% 2 5% 4 3 6% 5% 7% 9% 46%	2243
	21st to end	Night	15% 9% 2 6% 5% 5% 5% 4 7% 7% 34%	2414
June	1st to 10th	Night	12% 6% 2 7% 5% 3 4 3 6% 8% 44%	1330
	11th to 20th	Night	15% 6% 1 5% 3 5% 5% 4 6% 8% 41%	1330
	21st to end	Night	5% 3 2 4 4 3 4 3 7% 8% 58%	1330
July	1st to 10th	Night	4 3 1 2 3 3 4 3 5% 7% 64%	2220
	11th to 20th	Night	3 2 2 2 3 3 3 4 6% 71%	2220
	21st to end	Night	3 2 1 4 2 1 3 3 3 5% 74%	2689
August	1st to 10th	Night	10% 4 2 3 3 3 4 3 5% 7% 56%	2592
	11th to 20th	Night	5% 2 1 5% 3 4 5% 5% 7% 10% 53%	2811
	21st to end	Night	5% 3 1 3 2 3 3 3 5% 8% 64%	3146
September	1st to 10th	Night	11% 4 2 3 3 3 4 3 6% 7% 56%	1864
	11th to 20th	Night	9% 6% 2 4 3 3 4 3 5% 8% 53%	2090
	21st to end	Night	8% 5% 2 4 3 3 4 3 5% 8% 55%	2090
October	1st to 10th	Night	5% 6% 1 3 2 2 3 3 5% 8% 60%	3609
	11th to 20th	Night	2 4 1 4 2 3 4 4 7% 10% 58%	3948
	21st to end	Night	3 5% 3 7% 4 3 3 3 6% 9% 55%	4457
November	1st to 10th	Night	4 10% 3 8% 5% 4 5% 4 6% 8% 43%	2511
	11th to 20th	Night	1 7% 2 6% 4 4 4 4 6% 10% 51%	2660
	21st to end	Night	1 9% 3 8% 6% 4 4 3 6% 11% 44%	2825
December	1st to 10th	Night	1 7% 4 8% 6% 4 5% 5% 7% 10% 44%	2850
	11th to 20th	Night	2 6% 4 10% 6% 5% 6% 5% 8% 9% 38%	3510
	21st to end	Night	1 7% 4 8% 6% 5% 4 3 6% 9% 47%	5214

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	39% 9% 28% 17% 6% 1	2219
	11th to 20th	Day	44% 11% 17% 11% 10% 6%	2412
	21st to end	Day	33% 16% 27% 19% 4 1	2794
February	1st to 10th	Day	45% 11% 23% 13% 6% 1	1573
	11th to 20th	Day	42% 10% 21% 19% 7% 1	1674
	21st to end	Day	42% 10% 21% 17% 8% 2	1556
March	1st to 10th	Day	29% 8% 24% 27% 11% 1	3138
	11th to 20th	Day	30% 7% 31% 27% 4 1	3480
	21st to end	Day	52% 8% 22% 14% 5%	3844
April	1st to 10th	Day	43% 9% 22% 22% 4	2317
	11th to 20th	Day	46% 8% 17% 21% 7% 1	2440
	21st to end	Day	45% 8% 20% 20% 7% 1	2470
May	1st to 10th	Day	54% 6% 14% 21% 5%	4110
	11th to 20th	Day	41% 8% 18% 26% 7%	4652
	21st to end	Day	44% 10% 21% 21% 5%	5162
June	1st to 10th	Day	44% 9% 16% 22% 8%	2850
	11th to 20th	Day	46% 8% 16% 19% 10% 1	2850
	21st to end	Day	29% 6% 22% 28% 13% 2	2850
July	1st to 10th	Day	25% 5% 10% 31% 21% 7%	4666
	11th to 20th	Day	18% 5% 9% 27% 29% 12%	4659
	21st to end	Day	14% 4 7% 27% 29% 21%	4706
August	1st to 10th	Day	34% 4 14% 22% 20% 6%	4323
	11th to 20th	Day	26% 3 17% 34% 18% 2	4067
	21st to end	Day	23% 3 10% 42% 19% 3	4520
September	1st to 10th	Day	30% 4 13% 27% 18% 8%	2313
	11th to 20th	Day	30% 6% 17% 29% 15% 4	2090
	21st to end	Day	30% 5% 20% 30% 12% 3	2090
October	1st to 10th	Day	28% 3 16% 36% 16% 1	3366
	11th to 20th	Day	27% 4 23% 27% 17% 1	2976
	21st to end	Day	33% 6% 18% 29% 12% 2	3101
November	1st to 10th	Day	38% 11% 22% 20% 7% 1	1648
	11th to 20th	Day	30% 13% 27% 22% 7% 1	1520
	21st to end	Day	38% 10% 24% 17% 9% 2	1354
December	1st to 10th	Day	38% 13% 22% 17% 6% 3	1330
	11th to 20th	Day	37% 12% 29% 16% 5% 1	1684
	21st to end	Day	33% 11% 20% 18% 14% 4	2442
January	1st to 10th	Night	39% 11% 29% 14% 6% 2	4726
	11th to 20th	Night	43% 9% 18% 16% 8% 7%	4576
	21st to end	Night	33% 19% 23% 19% 6%	4862
February	1st to 10th	Night	44% 9% 19% 18% 7% 2	2606
	11th to 20th	Night	41% 10% 23% 17% 8% 1	2403
	21st to end	Night	43% 9% 20% 17% 8% 2	1894
March	1st to 10th	Night	36% 10% 20% 25% 9%	3736
	11th to 20th	Night	27% 9% 32% 25% 6% 1	3500
	21st to end	Night	52% 9% 21% 13% 5%	3808
April	1st to 10th	Night	37% 9% 22% 25% 7%	1862
	11th to 20th	Night	40% 9% 18% 25% 7%	1691
	21st to end	Night	41% 8% 18% 24% 8% 1	1710
May	1st to 10th	Night	51% 8% 14% 20% 7%	2845
	11th to 20th	Night	31% 8% 19% 30% 13%	2243
	21st to end	Night	48% 8% 22% 17% 4	2414
June	1st to 10th	Night	38% 7% 21% 25% 9%	1330
	11th to 20th	Night	41% 6% 18% 21% 13% 1	1330
	21st to end	Night	23% 7% 22% 29% 16% 3	1330
July	1st to 10th	Night	19% 3 15% 31% 22% 10%	2185
	11th to 20th	Night	15% 4 10% 29% 26% 17%	2178
	21st to end	Night	15% 3 8% 26% 28% 20%	2604
August	1st to 10th	Night	27% 3 17% 24% 20% 9%	2584
	11th to 20th	Night	26% 4 18% 34% 16% 3	2838
	21st to end	Night	20% 3 10% 39% 24% 5%	3146
September	1st to 10th	Night	27% 4 15% 28% 17% 8%	1864
	11th to 20th	Night	30% 5% 17% 29% 14% 4	2090
	21st to end	Night	27% 5% 22% 28% 15% 3	2090
October	1st to 10th	Night	23% 4 14% 34% 22% 3	3609
	11th to 20th	Night	22% 5% 23% 30% 18% 2	3948
	21st to end	Night	28% 7% 16% 34% 14% 1	4457
November	1st to 10th	Night	40% 10% 22% 19% 8% 2	2511
	11th to 20th	Night	30% 12% 25% 24% 8% 1	2660
	21st to end	Night	35% 12% 23% 19% 8% 3	2825
December	1st to 10th	Night	35% 13% 23% 19% 7% 3	2850
	11th to 20th	Night	40% 14% 24% 17% 4 1	3507
	21st to end	Night	34% 9% 22% 17% 12% 6%	5214

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	14% 24% 31% 18% 8% 5%	180
	11th to 20th	Day	12% 22% 32% 25% 7% 2	180
	21st to end	Day	16% 31% 31% 13% 4 4	198
February	1st to 10th	Day	20% 17% 35% 21% 6% 1	205
	11th to 20th	Day	13% 26% 33% 19% 9%	270
	21st to end	Day	17% 27% 33% 17% 6% 1	368
March	1st to 10th	Day	12% 28% 34% 17% 6% 2	460
	11th to 20th	Day	13% 23% 31% 20% 10% 3	460
	21st to end	Day	14% 25% 34% 19% 7% 2	506
April	1st to 10th	Day	18% 25% 35% 15% 6%	460
	11th to 20th	Day	17% 27% 29% 17% 8% 2	452
	21st to end	Day	22% 18% 27% 21% 8% 3	460
May	1st to 10th	Day	21% 17% 24% 21% 14% 3	463
	11th to 20th	Day	23% 12% 24% 19% 17% 5%	642
	21st to end	Day	26% 10% 19% 22% 17% 6%	734
June	1st to 10th	Day	19% 18% 24% 23% 13% 3	680
	11th to 20th	Day	21% 22% 23% 20% 12% 3	680
	21st to end	Day	10% 12% 26% 28% 18% 5%	680
July	1st to 10th	Day	9% 10% 25% 30% 20% 6%	671
	11th to 20th	Day	8% 9% 29% 33% 17% 4	668
	21st to end	Day	6% 10% 29% 33% 17% 5%	575
August	1st to 10th	Day	12% 9% 25% 29% 18% 6%	487
	11th to 20th	Day	7% 7% 30% 32% 19% 5%	477
	21st to end	Day	12% 14% 29% 28% 13% 4	539
September	1st to 10th	Day	14% 18% 26% 21% 15% 7%	490
	11th to 20th	Day	12% 16% 32% 23% 15% 3	490
	21st to end	Day	21% 20% 27% 21% 9% 2	490
October	1st to 10th	Day	17% 17% 29% 21% 13% 3	418
	11th to 20th	Day	14% 22% 29% 21% 11% 4	300
	21st to end	Day	19% 26% 31% 14% 8% 2	330
November	1st to 10th	Day	21% 26% 30% 17% 5%	282
	11th to 20th	Day	19% 34% 29% 15% 3	270
	21st to end	Day	17% 38% 19% 13% 9% 4	192
December	1st to 10th	Day	13% 32% 31% 17% 6% 3	180
	11th to 20th	Day	22% 23% 27% 22% 4 1	180
	21st to end	Day	11% 26% 28% 21% 11% 4	198
January	1st to 10th	Night	12% 30% 34% 18% 5% 1	950
	11th to 20th	Night	11% 27% 35% 20% 6% 1	851
	21st to end	Night	11% 29% 36% 16% 5% 2	836
February	1st to 10th	Night	11% 22% 34% 23% 8% 2	760
	11th to 20th	Night	11% 25% 34% 23% 7%	744
	21st to end	Night	10% 23% 32% 25% 9% 1	628
March	1st to 10th	Night	7% 21% 36% 26% 8% 2	760
	11th to 20th	Night	8% 25% 31% 26% 8% 2	760
	21st to end	Night	11% 19% 29% 28% 11% 2	836
April	1st to 10th	Night	11% 19% 33% 25% 10% 1	722
	11th to 20th	Night	13% 17% 34% 24% 10% 1	563
	21st to end	Night	13% 16% 32% 28% 10% 1	570
May	1st to 10th	Night	17% 14% 25% 29% 15% 1	568
	11th to 20th	Night	12% 7% 24% 36% 18% 2	374
	21st to end	Night	25% 9% 19% 27% 18% 2	411
June	1st to 10th	Night	17% 13% 24% 31% 15% 1	380
	11th to 20th	Night	18% 16% 27% 28% 9% 1	380
	21st to end	Night	7% 7% 30% 36% 19% 1	380
July	1st to 10th	Night	8% 8% 17% 40% 25% 2	370
	11th to 20th	Night	7% 5% 29% 42% 16% 1	368
	21st to end	Night	7% 4 22% 42% 22% 2	416
August	1st to 10th	Night	8% 1 20% 43% 25% 2	415
	11th to 20th	Night	6% 4 21% 41% 24% 3	564
	21st to end	Night	10% 8% 25% 41% 14% 2	627
September	1st to 10th	Night	14% 7% 23% 32% 19% 5%	687
	11th to 20th	Night	13% 7% 24% 34% 17% 5%	760
	21st to end	Night	14% 13% 30% 28% 12% 3	760
October	1st to 10th	Night	16% 15% 23% 31% 13% 2	760
	11th to 20th	Night	10% 20% 28% 29% 10% 2	760
	21st to end	Night	14% 24% 33% 22% 5% 2	836
November	1st to 10th	Night	16% 24% 34% 21% 4	802
	11th to 20th	Night	14% 32% 35% 16% 2 1	950
	21st to end	Night	12% 30% 33% 19% 5% 1	950
December	1st to 10th	Night	10% 24% 37% 20% 7% 1	950
	11th to 20th	Night	9% 28% 37% 20% 5%	950
	21st to end	Night	11% 26% 35% 22% 6% 1	1045

Page generated at 2024-11-03 00:11:30. Version ac07

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