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 2026-06-01.

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%	135275
	Night	6% 10% 4 10% 6% 6% 6% 5% 7% 8% 33%	136571
Transparency	Day	44% 8% 16% 20% 10% 2	134985
	Night	48% 8% 16% 19% 8% 1	136439
Seeing	Day	12% 15% 31% 26% 13% 3	17630
	Night	11% 8% 30% 32% 16% 2	25988

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	6% 14% 6% 17% 8% 7% 7% 5% 6% 7% 16%	4125
	11th to 20th	Day	5% 8% 4 9% 6% 5% 5% 6% 8% 9% 34%	4119
	21st to end	Day	6% 16% 5% 12% 6% 6% 6% 5% 7% 7% 25%	5167
February	1st to 10th	Day	6% 14% 5% 11% 8% 8% 7% 5% 7% 8% 22%	1775
	11th to 20th	Day	5% 11% 3 11% 6% 6% 6% 6% 8% 8% 30%	1845
	21st to end	Day	3 12% 5% 12% 9% 8% 7% 6% 7% 8% 24%	1620
March	1st to 10th	Day	2 10% 5% 10% 7% 7% 7% 5% 8% 8% 30%	4618
	11th to 20th	Day	1 6% 3 9% 6% 7% 7% 6% 10% 10% 34%	5054
	21st to end	Day	4 11% 4 12% 8% 7% 7% 7% 7% 9% 24%	5910
April	1st to 10th	Day	5% 11% 5% 10% 8% 6% 7% 6% 8% 9% 25%	2520
	11th to 20th	Day	6% 11% 4 10% 8% 7% 6% 5% 7% 8% 28%	2546
	21st to end	Day	6% 11% 5% 11% 6% 6% 5% 5% 7% 8% 31%	2908
May	1st to 10th	Day	5% 9% 3 8% 6% 5% 6% 4 6% 7% 40%	6460
	11th to 20th	Day	6% 10% 4 12% 7% 8% 8% 6% 8% 9% 23%	6417
	21st to end	Day	11% 11% 3 10% 6% 6% 6% 5% 7% 7% 27%	7213
June	1st to 10th	Day	12% 10% 4 9% 7% 6% 6% 5% 6% 7% 29%	3141
	11th to 20th	Day	14% 10% 4 9% 8% 7% 7% 5% 7% 7% 23%	3200
	21st to end	Day	6% 8% 3 9% 7% 6% 7% 5% 7% 8% 34%	3176
July	1st to 10th	Day	3 4 2 6% 5% 5% 5% 5% 6% 8% 51%	6780
	11th to 20th	Day	4 3 1 4 4 4 4 4 6% 8% 58%	6780
	21st to end	Day	4 4 2 5% 4 4 5% 4 6% 9% 54%	6976
August	1st to 10th	Day	3 3 1 4 3 4 5% 5% 8% 9% 54%	4541
	11th to 20th	Day	3 3 2 7% 4 6% 6% 5% 8% 10% 46%	4350
	21st to end	Day	7% 5% 2 6% 4 5% 5% 5% 6% 10% 45%	4504
September	1st to 10th	Day	6% 5% 2 7% 4 4 4 4 6% 7% 50%	2400
	11th to 20th	Day	9% 7% 3 8% 5% 5% 5% 4 5% 8% 43%	2400
	21st to end	Day	9% 7% 3 9% 5% 6% 6% 4 6% 8% 38%	2130
October	1st to 10th	Day	5% 8% 3 9% 6% 5% 6% 4 5% 8% 41%	3280
	11th to 20th	Day	8% 9% 3 8% 5% 5% 5% 5% 7% 9% 35%	3242
	21st to end	Day	6% 9% 3 9% 6% 6% 6% 6% 7% 9% 33%	3181
November	1st to 10th	Day	9% 12% 4 12% 7% 5% 5% 5% 5% 8% 27%	1648
	11th to 20th	Day	6% 12% 4 9% 6% 5% 6% 5% 7% 8% 32%	1680
	21st to end	Day	5% 14% 6% 12% 9% 7% 5% 6% 8% 7% 22%	1628
December	1st to 10th	Day	5% 14% 4 11% 9% 9% 9% 6% 6% 8% 19%	2297
	11th to 20th	Day	6% 19% 6% 18% 9% 6% 5% 4 5% 5% 16%	2322
	21st to end	Day	4 14% 6% 16% 9% 8% 7% 5% 7% 7% 18%	3322
January	1st to 10th	Night	6% 19% 7% 16% 8% 7% 7% 5% 6% 6% 13%	8770
	11th to 20th	Night	5% 10% 4 10% 5% 5% 5% 4 8% 8% 34%	8779
	21st to end	Night	6% 14% 6% 13% 6% 6% 6% 4 5% 6% 28%	9012
February	1st to 10th	Night	9% 12% 5% 14% 9% 7% 6% 5% 6% 7% 22%	2843
	11th to 20th	Night	6% 12% 4 9% 6% 7% 6% 4 6% 8% 32%	2662
	21st to end	Night	4 13% 5% 13% 7% 6% 6% 5% 8% 9% 25%	2186
March	1st to 10th	Night	2 11% 5% 10% 6% 5% 6% 5% 8% 10% 34%	5496
	11th to 20th	Night	1 6% 2 7% 5% 5% 6% 5% 9% 10% 43%	5162
	21st to end	Night	6% 11% 4 10% 7% 6% 6% 5% 7% 9% 28%	5307
April	1st to 10th	Night	4 10% 4 8% 6% 6% 7% 5% 7% 9% 33%	2100
	11th to 20th	Night	3 9% 3 10% 8% 5% 6% 5% 8% 9% 34%	2023
	21st to end	Night	5% 8% 4 9% 5% 6% 6% 4 7% 11% 35%	1712
May	1st to 10th	Night	5% 9% 4 9% 6% 5% 5% 3 5% 6% 42%	3720
	11th to 20th	Night	6% 10% 4 9% 6% 7% 5% 5% 8% 9% 31%	3696
	21st to end	Night	8% 8% 3 8% 6% 6% 6% 5% 7% 9% 34%	3901
June	1st to 10th	Night	11% 8% 3 8% 5% 6% 6% 6% 7% 8% 34%	1256
	11th to 20th	Night	10% 8% 2 8% 8% 6% 6% 5% 9% 10% 29%	1200
	21st to end	Night	6% 7% 3 7% 6% 5% 5% 4 7% 9% 41%	1218
July	1st to 10th	Night	5% 6% 3 7% 5% 5% 5% 5% 6% 7% 46%	3180
	11th to 20th	Night	3 3 1 4 4 3 3 4 5% 7% 62%	3180
	21st to end	Night	5% 4 2 5% 5% 4 5% 4 6% 8% 53%	3864
August	1st to 10th	Night	3 3 2 5% 5% 5% 5% 4 5% 8% 54%	2609
	11th to 20th	Night	4 5% 2 6% 5% 5% 6% 6% 7% 11% 44%	2699
	21st to end	Night	8% 5% 2 7% 5% 5% 4 5% 6% 8% 45%	3410
September	1st to 10th	Night	5% 5% 2 7% 4 5% 4 5% 4 7% 54%	2000
	11th to 20th	Night	8% 6% 2 6% 4 5% 5% 3 6% 8% 46%	2000
	21st to end	Night	8% 8% 3 7% 4 4 5% 4 5% 7% 44%	2266
October	1st to 10th	Night	6% 6% 2 7% 6% 5% 6% 4 6% 9% 43%	3920
	11th to 20th	Night	6% 7% 3 8% 6% 4 6% 5% 8% 10% 37%	3914
	21st to end	Night	6% 9% 3 9% 6% 5% 7% 6% 6% 7% 35%	4616
November	1st to 10th	Night	9% 14% 4 9% 8% 6% 6% 4 7% 8% 26%	2870
	11th to 20th	Night	6% 12% 4 9% 8% 6% 7% 6% 7% 7% 29%	2940
	21st to end	Night	7% 17% 6% 11% 7% 6% 5% 5% 7% 7% 21%	2966
December	1st to 10th	Night	5% 11% 4 11% 10% 8% 6% 5% 6% 7% 26%	5099
	11th to 20th	Night	9% 17% 6% 16% 9% 7% 6% 5% 5% 5% 15%	6090
	21st to end	Night	6% 15% 5% 13% 9% 8% 7% 5% 7% 7% 18%	7905

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	64% 8% 14% 12% 2	4125
	11th to 20th	Day	44% 6% 21% 21% 4 3	4140
	21st to end	Day	57% 9% 15% 18% 2	5168
February	1st to 10th	Day	57% 11% 13% 10% 9% 1	1775
	11th to 20th	Day	48% 11% 16% 16% 8%	1854
	21st to end	Day	54% 12% 15% 13% 5% 1	1620
March	1st to 10th	Day	44% 10% 18% 18% 9%	4618
	11th to 20th	Day	40% 10% 16% 26% 8%	5054
	21st to end	Day	50% 12% 14% 17% 7%	5910
April	1st to 10th	Day	50% 12% 18% 15% 4	2520
	11th to 20th	Day	50% 11% 14% 14% 9% 1	2546
	21st to end	Day	48% 11% 14% 17% 9% 1	2908
May	1st to 10th	Day	37% 7% 13% 29% 13% 1	6460
	11th to 20th	Day	51% 10% 14% 19% 6%	6420
	21st to end	Day	50% 10% 15% 21% 4 1	7214
June	1st to 10th	Day	51% 10% 13% 14% 10% 3	3141
	11th to 20th	Day	57% 11% 13% 12% 6%	3200
	21st to end	Day	44% 10% 16% 17% 10% 2	3176
July	1st to 10th	Day	29% 5% 16% 19% 22% 10%	6706
	11th to 20th	Day	24% 5% 16% 26% 23% 6%	6699
	21st to end	Day	26% 4 20% 28% 18% 4	6780
August	1st to 10th	Day	24% 6% 13% 30% 25% 2	4524
	11th to 20th	Day	31% 7% 22% 29% 10% 1	4392
	21st to end	Day	34% 7% 25% 26% 7% 2	4503
September	1st to 10th	Day	31% 8% 17% 27% 13% 3	2400
	11th to 20th	Day	40% 8% 14% 24% 12% 2	2400
	21st to end	Day	44% 9% 20% 16% 10% 1	2130
October	1st to 10th	Day	42% 6% 20% 23% 9%	3280
	11th to 20th	Day	45% 7% 18% 23% 7%	3242
	21st to end	Day	47% 7% 22% 21% 3	3181
November	1st to 10th	Day	54% 11% 13% 16% 6%	1648
	11th to 20th	Day	50% 14% 14% 16% 6%	1680
	21st to end	Day	57% 11% 16% 12% 4	1628
December	1st to 10th	Day	65% 7% 17% 9% 1	2297
	11th to 20th	Day	70% 7% 9% 10% 4	2324
	21st to end	Day	61% 11% 12% 13% 2	3322
January	1st to 10th	Night	69% 7% 11% 9% 3	8770
	11th to 20th	Night	46% 7% 20% 19% 5% 3	8820
	21st to end	Night	56% 8% 15% 19% 2	9012
February	1st to 10th	Night	59% 10% 12% 13% 6% 1	2843
	11th to 20th	Night	51% 8% 15% 16% 9% 1	2664
	21st to end	Night	52% 10% 17% 14% 6%	2186
March	1st to 10th	Night	40% 10% 20% 21% 9%	5496
	11th to 20th	Night	34% 8% 17% 27% 13%	5163
	21st to end	Night	50% 10% 13% 16% 10%	5307
April	1st to 10th	Night	45% 12% 19% 19% 6%	2100
	11th to 20th	Night	43% 11% 17% 18% 10% 1	2023
	21st to end	Night	41% 10% 16% 20% 11% 1	1712
May	1st to 10th	Night	41% 6% 12% 27% 12% 2	3717
	11th to 20th	Night	44% 10% 15% 25% 6%	3696
	21st to end	Night	42% 10% 18% 24% 5% 1	3901
June	1st to 10th	Night	46% 10% 13% 18% 10% 4	1256
	11th to 20th	Night	46% 11% 18% 15% 10% 1	1200
	21st to end	Night	38% 9% 21% 20% 9% 3	1218
July	1st to 10th	Night	37% 4 15% 16% 20% 8%	3145
	11th to 20th	Night	22% 5% 20% 27% 22% 4	3138
	21st to end	Night	29% 4 18% 27% 16% 7%	3755
August	1st to 10th	Night	28% 6% 12% 26% 26% 2	2601
	11th to 20th	Night	33% 5% 23% 28% 9% 2	2723
	21st to end	Night	36% 6% 25% 22% 10% 1	3410
September	1st to 10th	Night	29% 9% 15% 30% 15% 3	2000
	11th to 20th	Night	36% 8% 16% 23% 14% 3	2000
	21st to end	Night	38% 9% 20% 20% 11% 2	2266
October	1st to 10th	Night	40% 6% 20% 24% 9%	3920
	11th to 20th	Night	41% 7% 23% 22% 7%	3914
	21st to end	Night	49% 6% 19% 22% 5%	4616
November	1st to 10th	Night	55% 10% 15% 13% 6% 1	2870
	11th to 20th	Night	51% 13% 14% 15% 6% 1	2940
	21st to end	Night	61% 10% 14% 11% 4	2966
December	1st to 10th	Night	58% 10% 16% 13% 3	5099
	11th to 20th	Night	71% 6% 11% 8% 3	6087
	21st to end	Night	61% 10% 13% 14% 3	7905

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	10% 14% 37% 27% 8% 5%	220
	11th to 20th	Day	9% 13% 30% 33% 13% 3	220
	21st to end	Day	14% 10% 35% 29% 10% 1	242
February	1st to 10th	Day	9% 17% 44% 22% 6% 2	315
	11th to 20th	Day	11% 12% 40% 25% 10% 1	425
	21st to end	Day	11% 17% 41% 21% 10% 1	388
March	1st to 10th	Day	11% 20% 34% 22% 11% 1	540
	11th to 20th	Day	11% 21% 34% 20% 11% 2	540
	21st to end	Day	13% 24% 31% 20% 9% 1	594
April	1st to 10th	Day	14% 22% 32% 24% 8% 1	540
	11th to 20th	Day	18% 20% 29% 20% 10% 3	532
	21st to end	Day	12% 22% 29% 22% 11% 3	540
May	1st to 10th	Day	12% 20% 32% 24% 10% 2	543
	11th to 20th	Day	15% 12% 29% 29% 11% 4	568
	21st to end	Day	15% 16% 30% 26% 11% 2	682
June	1st to 10th	Day	14% 18% 34% 24% 9% 1	871
	11th to 20th	Day	16% 14% 28% 26% 14% 2	930
	21st to end	Day	11% 11% 28% 29% 18% 2	906
July	1st to 10th	Day	8% 12% 31% 30% 16% 2	721
	11th to 20th	Day	7% 12% 29% 31% 17% 4	718
	21st to end	Day	4 8% 24% 36% 23% 5%	625
August	1st to 10th	Day	6% 7% 23% 33% 26% 6%	527
	11th to 20th	Day	7% 11% 23% 32% 21% 6%	517
	21st to end	Day	8% 11% 28% 30% 16% 7%	583
September	1st to 10th	Day	8% 14% 26% 30% 16% 6%	530
	11th to 20th	Day	14% 13% 25% 28% 15% 5%	530
	21st to end	Day	16% 18% 34% 20% 10% 2	530
October	1st to 10th	Day	15% 16% 30% 24% 11% 4	530
	11th to 20th	Day	15% 22% 29% 20% 13% 2	523
	21st to end	Day	18% 13% 36% 23% 10% 1	358
November	1st to 10th	Day	15% 19% 36% 21% 8%	220
	11th to 20th	Day	13% 11% 38% 26% 9% 3	220
	21st to end	Day	10% 11% 38% 29% 6% 5%	220
December	1st to 10th	Day	10% 8% 33% 25% 20% 4	220
	11th to 20th	Day	13% 11% 36% 25% 10% 4	220
	21st to end	Day	13% 9% 36% 31% 9% 2	242
January	1st to 10th	Night	16% 10% 37% 27% 9% 1	950
	11th to 20th	Night	13% 7% 35% 33% 10% 1	950
	21st to end	Night	10% 7% 36% 32% 13% 2	927
February	1st to 10th	Night	13% 9% 35% 30% 11% 1	840
	11th to 20th	Night	12% 6% 31% 38% 12% 1	824
	21st to end	Night	7% 10% 38% 28% 15% 2	692
March	1st to 10th	Night	10% 9% 33% 32% 15% 1	840
	11th to 20th	Night	5% 13% 36% 30% 15% 1	840
	21st to end	Night	9% 10% 33% 31% 15% 2	924
April	1st to 10th	Night	8% 10% 27% 34% 19% 2	840
	11th to 20th	Night	8% 9% 34% 32% 15% 1	775
	21st to end	Night	9% 8% 35% 30% 16% 1	452
May	1st to 10th	Night	9% 8% 31% 32% 18% 1	420
	11th to 20th	Night	9% 6% 22% 41% 20% 2	414
	21st to end	Night	17% 5% 22% 35% 20% 2	455
June	1st to 10th	Night	12% 8% 29% 33% 17% 2	400
	11th to 20th	Night	12% 6% 26% 38% 16% 2	400
	21st to end	Night	10% 4 24% 36% 25% 1	400
July	1st to 10th	Night	7% 5% 23% 40% 23% 2	390
	11th to 20th	Night	7% 5% 21% 40% 26% 2	388
	21st to end	Night	6% 3 14% 40% 34% 3	438
August	1st to 10th	Night	6% 3 13% 36% 39% 5%	398
	11th to 20th	Night	5% 4 16% 35% 34% 5%	466
	21st to end	Night	9% 5% 20% 34% 29% 2	760
September	1st to 10th	Night	8% 6% 18% 41% 26% 3	800
	11th to 20th	Night	11% 5% 17% 30% 30% 7%	800
	21st to end	Night	14% 11% 25% 30% 18% 3	800
October	1st to 10th	Night	13% 9% 24% 32% 20% 2	800
	11th to 20th	Night	14% 9% 28% 31% 16% 3	800
	21st to end	Night	16% 9% 32% 30% 11% 3	880
November	1st to 10th	Night	17% 11% 33% 30% 8% 1	820
	11th to 20th	Night	14% 9% 34% 31% 11% 2	840
	21st to end	Night	15% 11% 33% 29% 9% 3	866
December	1st to 10th	Night	12% 7% 32% 34% 13% 2	1068
	11th to 20th	Night	16% 9% 36% 29% 9% 1	1160
	21st to end	Night	14% 10% 36% 30% 9% 1	1171

Page generated at 2026-05-28 22:53:30. Version ac07

Copyright © 2026 Attilla Danko.