Cache Creek
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cache Creek clear sky chart from 2005-11-06 to 2024-09-14.

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% 9% 3 9% 6% 6% 6% 5% 7% 8% 35%	101101
	Night	7% 11% 4 10% 6% 6% 6% 4 6% 7% 32%	96845
Transparency	Day	44% 10% 15% 19% 10% 2	100852
	Night	48% 10% 15% 17% 9% 2	96740
Seeing	Day	11% 16% 31% 26% 13% 3	15500
	Night	11% 9% 30% 32% 17% 2	23682

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	8% 14% 7% 20% 10% 7% 7% 5% 6% 5% 11%	2219
	11th to 20th	Day	10% 12% 5% 13% 7% 7% 5% 4 6% 9% 22%	2199
	21st to end	Day	10% 22% 7% 14% 6% 6% 5% 4 5% 6% 15%	2775
February	1st to 10th	Day	7% 15% 5% 11% 7% 8% 6% 5% 7% 8% 22%	1605
	11th to 20th	Day	5% 12% 3 11% 6% 5% 6% 5% 8% 8% 31%	1665
	21st to end	Day	3 13% 5% 13% 9% 8% 7% 6% 7% 8% 24%	1470
March	1st to 10th	Day	2 10% 5% 11% 7% 7% 7% 5% 8% 9% 29%	3138
	11th to 20th	Day	2 7% 3 8% 5% 6% 6% 5% 9% 10% 39%	3449
	21st to end	Day	3 11% 4 12% 8% 7% 7% 6% 7% 9% 26%	4031
April	1st to 10th	Day	5% 12% 5% 11% 9% 6% 6% 6% 8% 8% 24%	2280
	11th to 20th	Day	7% 12% 4 10% 8% 7% 6% 5% 7% 8% 26%	2300
	21st to end	Day	6% 12% 5% 11% 7% 7% 5% 5% 7% 8% 27%	2631
May	1st to 10th	Day	6% 10% 4 9% 6% 5% 7% 4 6% 7% 35%	4420
	11th to 20th	Day	7% 9% 3 9% 7% 7% 6% 5% 8% 11% 28%	4377
	21st to end	Day	14% 15% 4 11% 7% 7% 7% 5% 6% 6% 18%	4924
June	1st to 10th	Day	12% 10% 4 9% 7% 6% 6% 5% 6% 6% 28%	2983
	11th to 20th	Day	14% 10% 4 9% 8% 7% 7% 5% 7% 6% 23%	3040
	21st to end	Day	7% 8% 3 9% 7% 6% 7% 5% 7% 7% 35%	3017
July	1st to 10th	Day	4 4 1 4 4 4 5% 4 6% 7% 57%	4740
	11th to 20th	Day	4 3 1 3 3 3 3 4 5% 7% 64%	4740
	21st to end	Day	5% 4 2 5% 5% 5% 5% 4 6% 9% 52%	4864
August	1st to 10th	Day	3 3 1 4 3 4 5% 4 8% 9% 54%	4401
	11th to 20th	Day	3 3 2 7% 4 6% 6% 5% 8% 10% 46%	4214
	21st to end	Day	7% 6% 2 6% 4 5% 5% 5% 6% 10% 44%	4366
September	1st to 10th	Day	6% 5% 2 7% 5% 4 4 4 6% 7% 50%	2280
	11th to 20th	Day	10% 7% 3 8% 5% 4 5% 4 5% 7% 41%	2198
	21st to end	Day	9% 7% 3 9% 5% 5% 5% 4 6% 7% 39%	1918
October	1st to 10th	Day	8% 9% 3 8% 6% 4 5% 4 5% 7% 40%	1800
	11th to 20th	Day	8% 8% 3 8% 5% 6% 6% 5% 8% 9% 34%	1795
	21st to end	Day	8% 11% 4 10% 6% 5% 5% 5% 6% 8% 31%	1777
November	1st to 10th	Day	9% 13% 4 12% 7% 5% 6% 5% 5% 8% 27%	1488
	11th to 20th	Day	5% 11% 4 9% 6% 5% 6% 5% 7% 8% 34%	1520
	21st to end	Day	6% 14% 5% 12% 9% 7% 5% 6% 9% 7% 22%	1474
December	1st to 10th	Day	4 17% 5% 13% 8% 9% 7% 4 5% 6% 21%	1297
	11th to 20th	Day	6% 21% 7% 17% 9% 5% 5% 4 5% 6% 17%	1442
	21st to end	Day	5% 12% 6% 16% 9% 8% 8% 5% 7% 7% 18%	2264
January	1st to 10th	Night	8% 19% 8% 16% 8% 6% 6% 5% 6% 6% 12%	4726
	11th to 20th	Night	9% 17% 6% 14% 7% 6% 4 3 9% 6% 20%	4699
	21st to end	Night	9% 19% 8% 17% 7% 7% 6% 4 4 5% 14%	4880
February	1st to 10th	Night	9% 12% 5% 14% 9% 7% 5% 5% 6% 6% 22%	2573
	11th to 20th	Night	6% 12% 4 9% 6% 7% 6% 4 6% 8% 32%	2402
	21st to end	Night	4 13% 5% 13% 7% 6% 6% 5% 8% 8% 25%	1984
March	1st to 10th	Night	3 12% 5% 10% 6% 5% 6% 5% 8% 9% 31%	3736
	11th to 20th	Night	1 6% 2 6% 4 4 5% 5% 8% 10% 49%	3528
	21st to end	Night	6% 12% 4 9% 7% 6% 5% 5% 6% 9% 32%	3622
April	1st to 10th	Night	4 10% 4 8% 7% 6% 7% 5% 7% 9% 34%	1900
	11th to 20th	Night	2 8% 3 10% 8% 5% 7% 5% 8% 9% 34%	1829
	21st to end	Night	6% 9% 4 9% 5% 6% 6% 4 7% 11% 33%	1549
May	1st to 10th	Night	6% 11% 4 10% 7% 5% 5% 3 5% 6% 39%	2540
	11th to 20th	Night	7% 9% 3 9% 6% 7% 6% 5% 8% 10% 31%	2516
	21st to end	Night	11% 10% 4 10% 7% 7% 6% 5% 7% 9% 24%	2648
June	1st to 10th	Night	12% 8% 3 8% 5% 6% 6% 6% 6% 8% 33%	1193
	11th to 20th	Night	10% 8% 2 8% 8% 6% 6% 5% 9% 10% 28%	1140
	21st to end	Night	5% 7% 3 7% 6% 5% 6% 4 7% 9% 41%	1157
July	1st to 10th	Night	6% 5% 2 5% 5% 4 4 4 5% 7% 52%	2220
	11th to 20th	Night	3 3 1 4 4 4 4 4 5% 7% 61%	2220
	21st to end	Night	5% 4 1 5% 5% 4 5% 5% 7% 8% 52%	2685
August	1st to 10th	Night	3 3 2 5% 5% 5% 5% 4 5% 8% 55%	2529
	11th to 20th	Night	4 5% 2 6% 5% 5% 6% 6% 7% 11% 44%	2615
	21st to end	Night	8% 5% 2 7% 5% 5% 5% 5% 6% 8% 44%	3306
September	1st to 10th	Night	5% 5% 2 7% 4 5% 4 5% 4 7% 53%	1900
	11th to 20th	Night	8% 6% 3 6% 4 6% 5% 3 7% 8% 45%	1840
	21st to end	Night	8% 8% 3 7% 4 4 5% 4 6% 8% 45%	2038
October	1st to 10th	Night	7% 8% 2 8% 6% 5% 6% 4 5% 8% 41%	2160
	11th to 20th	Night	9% 9% 3 7% 6% 5% 5% 5% 6% 7% 39%	2164
	21st to end	Night	7% 11% 4 10% 6% 4 5% 4 6% 7% 35%	2578
November	1st to 10th	Night	10% 14% 4 9% 7% 6% 6% 4 7% 7% 26%	2590
	11th to 20th	Night	5% 12% 5% 9% 7% 6% 6% 5% 7% 7% 30%	2660
	21st to end	Night	8% 17% 6% 11% 7% 6% 5% 5% 7% 7% 22%	2683
December	1st to 10th	Night	5% 16% 6% 12% 8% 7% 5% 4 6% 6% 24%	2879
	11th to 20th	Night	11% 20% 7% 16% 7% 5% 5% 4 5% 5% 14%	3750
	21st to end	Night	7% 15% 6% 14% 9% 9% 7% 5% 6% 6% 17%	5406

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% 13% 9% 9% 1	2219
	11th to 20th	Day	57% 9% 14% 12% 3 6%	2220
	21st to end	Day	68% 11% 9% 9% 2	2775
February	1st to 10th	Day	57% 11% 13% 11% 7% 1	1605
	11th to 20th	Day	48% 12% 17% 15% 8%	1674
	21st to end	Day	54% 13% 15% 14% 4 1	1470
March	1st to 10th	Day	44% 15% 15% 17% 8%	3138
	11th to 20th	Day	34% 13% 21% 27% 4	3449
	21st to end	Day	48% 15% 17% 13% 8%	4031
April	1st to 10th	Day	52% 14% 17% 14% 4	2280
	11th to 20th	Day	52% 12% 13% 13% 9% 1	2300
	21st to end	Day	51% 12% 14% 15% 7%	2631
May	1st to 10th	Day	40% 9% 16% 24% 9% 1	4420
	11th to 20th	Day	43% 13% 17% 20% 7%	4380
	21st to end	Day	58% 14% 12% 12% 3 1	4925
June	1st to 10th	Day	52% 11% 13% 13% 9% 3	2983
	11th to 20th	Day	57% 12% 14% 12% 6%	3040
	21st to end	Day	44% 11% 16% 17% 10% 2	3017
July	1st to 10th	Day	24% 6% 11% 17% 28% 14%	4666
	11th to 20th	Day	19% 6% 10% 25% 30% 9%	4659
	21st to end	Day	28% 5% 17% 26% 18% 6%	4710
August	1st to 10th	Day	24% 6% 12% 30% 26% 2	4384
	11th to 20th	Day	31% 7% 22% 30% 9% 1	4256
	21st to end	Day	35% 7% 26% 26% 6% 1	4365
September	1st to 10th	Day	32% 9% 17% 27% 12% 3	2280
	11th to 20th	Day	41% 9% 13% 22% 13% 2	2198
	21st to end	Day	43% 10% 19% 17% 9% 1	1918
October	1st to 10th	Day	42% 10% 16% 25% 7%	1800
	11th to 20th	Day	43% 13% 18% 20% 6%	1795
	21st to end	Day	50% 12% 14% 19% 4	1777
November	1st to 10th	Day	54% 12% 13% 15% 6%	1488
	11th to 20th	Day	47% 15% 14% 17% 7%	1520
	21st to end	Day	57% 12% 15% 12% 4	1474
December	1st to 10th	Day	64% 9% 13% 11% 2	1297
	11th to 20th	Day	69% 10% 10% 8% 3	1444
	21st to end	Day	58% 15% 12% 12% 3	2264
January	1st to 10th	Night	69% 11% 11% 7% 2	4726
	11th to 20th	Night	63% 7% 12% 10% 3 5%	4740
	21st to end	Night	70% 11% 9% 6% 3	4880
February	1st to 10th	Night	60% 10% 12% 12% 5% 1	2573
	11th to 20th	Night	51% 8% 15% 16% 9% 1	2404
	21st to end	Night	52% 11% 16% 15% 6%	1984
March	1st to 10th	Night	42% 14% 15% 19% 11%	3736
	11th to 20th	Night	27% 11% 20% 32% 9%	3529
	21st to end	Night	46% 13% 15% 16% 11%	3622
April	1st to 10th	Night	44% 13% 18% 19% 6%	1900
	11th to 20th	Night	43% 12% 16% 18% 10%	1829
	21st to end	Night	42% 11% 17% 19% 10%	1549
May	1st to 10th	Night	42% 9% 13% 25% 9% 2	2540
	11th to 20th	Night	41% 15% 16% 21% 6%	2516
	21st to end	Night	49% 15% 15% 16% 4	2648
June	1st to 10th	Night	47% 10% 13% 18% 9% 3	1193
	11th to 20th	Night	46% 12% 19% 14% 8% 1	1140
	21st to end	Night	39% 9% 19% 20% 10% 3	1157
July	1st to 10th	Night	32% 6% 10% 13% 27% 11%	2185
	11th to 20th	Night	21% 7% 11% 25% 30% 6%	2178
	21st to end	Night	29% 6% 14% 23% 18% 9%	2600
August	1st to 10th	Night	27% 6% 12% 26% 27% 2	2521
	11th to 20th	Night	33% 5% 23% 27% 10% 2	2639
	21st to end	Night	37% 6% 25% 22% 9% 1	3306
September	1st to 10th	Night	30% 9% 15% 30% 14% 3	1900
	11th to 20th	Night	36% 9% 15% 22% 15% 3	1840
	21st to end	Night	37% 10% 20% 20% 11% 2	2038
October	1st to 10th	Night	42% 11% 14% 25% 8%	2160
	11th to 20th	Night	43% 11% 17% 21% 7%	2164
	21st to end	Night	48% 9% 17% 20% 6%	2578
November	1st to 10th	Night	55% 11% 14% 13% 6% 1	2590
	11th to 20th	Night	50% 14% 14% 15% 6% 1	2660
	21st to end	Night	60% 11% 14% 11% 4	2683
December	1st to 10th	Night	60% 10% 13% 13% 4	2879
	11th to 20th	Night	72% 9% 9% 8% 2	3747
	21st to end	Night	60% 13% 11% 11% 4	5406

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	8% 17% 39% 22% 8% 6%	180
	11th to 20th	Day	9% 15% 31% 32% 11% 3	180
	21st to end	Day	14% 12% 37% 28% 8% 1	198
February	1st to 10th	Day	10% 20% 46% 20% 4 1	265
	11th to 20th	Day	10% 13% 40% 25% 10% 1	365
	21st to end	Day	8% 18% 41% 21% 10% 1	334
March	1st to 10th	Day	11% 22% 33% 22% 12% 1	460
	11th to 20th	Day	9% 22% 34% 19% 13% 3	460
	21st to end	Day	12% 25% 31% 20% 10% 1	506
April	1st to 10th	Day	15% 22% 31% 23% 8% 1	460
	11th to 20th	Day	17% 21% 29% 19% 11% 3	452
	21st to end	Day	12% 23% 30% 23% 9% 2	460
May	1st to 10th	Day	12% 21% 31% 24% 10% 2	463
	11th to 20th	Day	13% 12% 29% 30% 12% 4	488
	21st to end	Day	13% 16% 29% 28% 11% 3	588
June	1st to 10th	Day	15% 18% 33% 24% 9% 1	813
	11th to 20th	Day	16% 15% 28% 26% 13% 1	870
	21st to end	Day	11% 11% 28% 29% 19% 2	847
July	1st to 10th	Day	8% 11% 30% 31% 17% 2	671
	11th to 20th	Day	6% 12% 29% 31% 17% 4	668
	21st to end	Day	4 8% 24% 36% 24% 4	579
August	1st to 10th	Day	6% 7% 22% 32% 26% 6%	487
	11th to 20th	Day	6% 11% 22% 34% 22% 6%	477
	21st to end	Day	9% 12% 29% 30% 15% 5%	539
September	1st to 10th	Day	9% 15% 28% 31% 13% 5%	490
	11th to 20th	Day	15% 14% 25% 27% 16% 4	463
	21st to end	Day	15% 18% 32% 22% 11% 2	450
October	1st to 10th	Day	15% 16% 28% 24% 13% 4	450
	11th to 20th	Day	14% 23% 29% 20% 13% 2	445
	21st to end	Day	17% 14% 37% 20% 11%	294
November	1st to 10th	Day	14% 22% 33% 21% 10% 1	180
	11th to 20th	Day	11% 11% 42% 23% 10% 3	180
	21st to end	Day	11% 13% 37% 27% 7% 6%	180
December	1st to 10th	Day	8% 9% 36% 23% 19% 4	180
	11th to 20th	Day	9% 11% 39% 25% 11% 5%	180
	21st to end	Day	11% 10% 37% 30% 10% 1	198
January	1st to 10th	Night	16% 11% 38% 24% 9% 2	850
	11th to 20th	Night	13% 8% 35% 32% 10% 1	850
	21st to end	Night	10% 8% 37% 30% 13% 2	839
February	1st to 10th	Night	14% 9% 36% 30% 10% 1	760
	11th to 20th	Night	11% 7% 33% 36% 12% 1	744
	21st to end	Night	7% 10% 36% 29% 16% 2	628
March	1st to 10th	Night	10% 9% 33% 30% 16% 1	760
	11th to 20th	Night	5% 14% 34% 31% 15% 1	760
	21st to end	Night	8% 10% 32% 32% 16% 2	836
April	1st to 10th	Night	8% 11% 27% 34% 19% 2	760
	11th to 20th	Night	6% 9% 35% 33% 15% 1	701
	21st to end	Night	9% 9% 36% 30% 16% 1	409
May	1st to 10th	Night	10% 9% 31% 31% 18% 2	380
	11th to 20th	Night	9% 6% 21% 41% 20% 2	374
	21st to end	Night	17% 5% 21% 35% 20% 2	411
June	1st to 10th	Night	12% 8% 28% 33% 17% 2	380
	11th to 20th	Night	12% 6% 27% 37% 16% 2	380
	21st to end	Night	9% 4 25% 36% 24% 1	380
July	1st to 10th	Night	7% 5% 23% 40% 23% 2	370
	11th to 20th	Night	7% 5% 20% 40% 26% 2	368
	21st to end	Night	6% 3 14% 40% 34% 3	416
August	1st to 10th	Night	5% 3 13% 35% 39% 5%	378
	11th to 20th	Night	5% 4 15% 36% 35% 5%	442
	21st to end	Night	10% 5% 20% 35% 28% 2	722
September	1st to 10th	Night	8% 6% 19% 40% 25% 3	760
	11th to 20th	Night	11% 5% 18% 29% 30% 7%	736
	21st to end	Night	13% 12% 23% 30% 19% 3	720
October	1st to 10th	Night	12% 10% 22% 32% 22% 3	720
	11th to 20th	Night	14% 10% 27% 30% 17% 3	720
	21st to end	Night	16% 9% 32% 29% 11% 3	792
November	1st to 10th	Night	17% 11% 33% 30% 8% 1	740
	11th to 20th	Night	13% 10% 34% 30% 11% 2	760
	21st to end	Night	15% 11% 34% 27% 9% 3	783
December	1st to 10th	Night	12% 8% 33% 33% 13% 2	964
	11th to 20th	Night	16% 9% 36% 29% 8%	1040
	21st to end	Night	13% 11% 36% 30% 9% 1	1049

Page generated at 2024-09-11 07:00:03. Version ac07

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