Santa Cruz
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Santa Cruz clear sky chart from 2005-11-06 to 2026-08-15.

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	7% 5% 2 5% 3 4 4 3 5% 7% 56%	151960
	Night	9% 7% 2 6% 4 4 4 3 5% 7% 50%	147453
Transparency	Day	26% 7% 9% 20% 24% 15%	146906
	Night	32% 7% 9% 18% 21% 13%	143495
Seeing	Day	9% 5% 18% 32% 28% 9%	17818
	Night	11% 4 11% 31% 33% 10%	27167

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% 4 1 5% 3 3 3 3 5% 8% 49%	5142
	11th to 20th	Day	9% 4 1 5% 3 3 3 3 4 6% 59%	5340
	21st to end	Day	12% 7% 3 7% 4 4 4 3 6% 7% 42%	5843
February	1st to 10th	Day	19% 4 1 4 4 3 4 3 5% 6% 47%	1890
	11th to 20th	Day	14% 5% 1 6% 4 5% 4 5% 5% 8% 43%	1964
	21st to end	Day	13% 5% 2 6% 5% 3 4 3 6% 7% 46%	1730
March	1st to 10th	Day	12% 7% 3 7% 5% 5% 6% 5% 7% 9% 35%	4976
	11th to 20th	Day	12% 4 1 4 3 4 4 3 5% 8% 52%	5110
	21st to end	Day	14% 8% 2 5% 4 4 5% 4 7% 10% 36%	5692
April	1st to 10th	Day	11% 6% 2 4 4 4 3 4 5% 8% 50%	2520
	11th to 20th	Day	8% 5% 2 5% 3 3 4 5% 5% 8% 52%	2489
	21st to end	Day	6% 5% 2 6% 4 3 5% 3 5% 9% 52%	2520
May	1st to 10th	Day	4 4 1 4 3 3 3 3 5% 7% 63%	7164
	11th to 20th	Day	2 5% 1 4 2 2 3 3 4 8% 65%	7667
	21st to end	Day	2 5% 2 4 3 4 4 3 4 8% 62%	8992
June	1st to 10th	Day	4 6% 2 5% 3 3 4 3 6% 8% 55%	2940
	11th to 20th	Day	2 6% 1 4 3 3 2 3 5% 8% 62%	2940
	21st to end	Day	2 4 2 5% 3 3 3 3 5% 7% 63%	2940
July	1st to 10th	Day	1 6% 2 6% 4 4 3 2 3 6% 61%	8129
	11th to 20th	Day	3 7% 4 7% 4 4 3 3 4 6% 55%	7650
	21st to end	Day	1 4 1 5% 4 4 3 3 4 7% 63%	8304
August	1st to 10th	Day	1 3 2 6% 4 4 4 3 4 7% 62%	5907
	11th to 20th	Day	3 2 4 3 5% 4 3 4 6% 65%	5505
	21st to end	Day	1 4 3 5% 4 5% 4 3 4 7% 61%	5984
September	1st to 10th	Day	2 4 1 4 3 3 3 2 3 6% 69%	2400
	11th to 20th	Day	3 5% 2 5% 3 3 3 4 5% 8% 58%	2400
	21st to end	Day	4 3 1 3 2 3 3 2 4 6% 70%	2125
October	1st to 10th	Day	2 2 1 3 2 3 4 3 4 7% 69%	3280
	11th to 20th	Day	3 3 2 4 3 3 4 2 4 6% 67%	3261
	21st to end	Day	4 4 1 3 2 3 4 3 5% 8% 63%	3558
November	1st to 10th	Day	8% 4 1 4 3 4 4 3 5% 7% 57%	2040
	11th to 20th	Day	10% 5% 2 5% 4 3 4 4 5% 6% 52%	1890
	21st to end	Day	11% 4 1 4 2 2 3 3 4 8% 58%	1768
December	1st to 10th	Day	9% 3 1 4 3 3 3 4 5% 9% 55%	2700
	11th to 20th	Day	21% 6% 2 5% 3 4 4 3 5% 8% 39%	3108
	21st to end	Day	26% 8% 1 5% 3 3 3 3 5% 7% 35%	4092
January	1st to 10th	Night	17% 4 2 4 3 2 3 3 5% 6% 50%	7757
	11th to 20th	Night	11% 4 1 4 3 3 4 3 4 8% 55%	7650
	21st to end	Night	13% 8% 3 6% 4 4 4 3 5% 8% 43%	8371
February	1st to 10th	Night	18% 5% 2 4 3 3 3 3 4 6% 49%	2730
	11th to 20th	Night	14% 7% 2 5% 4 3 4 4 6% 7% 45%	2542
	21st to end	Night	12% 6% 2 4 4 3 3 3 6% 6% 51%	2076
March	1st to 10th	Night	15% 9% 2 7% 5% 4 5% 4 6% 7% 36%	5125
	11th to 20th	Night	14% 5% 2 5% 5% 3 3 2 4 7% 50%	5110
	21st to end	Night	16% 9% 2 6% 4 4 4 3 5% 7% 42%	5523
April	1st to 10th	Night	11% 6% 2 4 4 3 5% 4 5% 8% 49%	2100
	11th to 20th	Night	10% 7% 2 5% 4 3 3 3 5% 6% 51%	2079
	21st to end	Night	5% 6% 2 5% 4 3 3 3 5% 8% 56%	2100
May	1st to 10th	Night	3 7% 2 5% 5% 4 4 3 4 5% 59%	5804
	11th to 20th	Night	1 10% 3 6% 4 5% 3 3 4 6% 55%	5200
	21st to end	Night	2 8% 3 6% 4 6% 4 2 4 7% 55%	5181
June	1st to 10th	Night	4 6% 3 5% 4 3 3 3 6% 7% 57%	1680
	11th to 20th	Night	2 8% 3 5% 4 3 4 3 5% 6% 58%	1680
	21st to end	Night	2 6% 2 6% 3 4 4 4 5% 8% 57%	1680
July	1st to 10th	Night	1 13% 3 7% 5% 4 3 2 4 6% 52%	4816
	11th to 20th	Night	2 14% 6% 11% 5% 5% 4 2 4 6% 40%	5340
	21st to end	Night	1 9% 3 8% 6% 6% 4 3 5% 8% 46%	5806
August	1st to 10th	Night	1 7% 4 10% 7% 7% 6% 3 6% 6% 42%	4125
	11th to 20th	Night	1 5% 3 8% 6% 8% 5% 3 5% 6% 51%	4447
	21st to end	Night	1 7% 5% 7% 5% 6% 6% 2 3 6% 51%	5015
September	1st to 10th	Night	1 5% 2 5% 4 4 5% 3 4 7% 61%	2000
	11th to 20th	Night	4 6% 3 5% 4 3 3 3 5% 7% 58%	2000
	21st to end	Night	4 6% 2 3 3 2 3 2 4 6% 66%	2268
October	1st to 10th	Night	3 3 1 5% 3 3 3 2 5% 7% 65%	3920
	11th to 20th	Night	4 5% 2 7% 5% 4 3 3 4 5% 57%	3897
	21st to end	Night	5% 4 2 5% 3 3 3 3 4 6% 63%	4253
November	1st to 10th	Night	8% 4 2 3 4 3 4 2 5% 7% 58%	2476
	11th to 20th	Night	11% 4 1 4 4 3 3 3 6% 8% 52%	2730
	21st to end	Night	9% 4 1 4 3 4 4 3 5% 5% 58%	2850
December	1st to 10th	Night	9% 4 1 3 2 3 4 3 5% 6% 60%	4700
	11th to 20th	Night	21% 7% 2 5% 4 3 3 4 7% 9% 35%	5316
	21st to end	Night	27% 8% 2 5% 4 4 4 3 4 5% 35%	7106

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	34% 5% 18% 22% 17% 4	5143
	11th to 20th	Day	27% 5% 7% 13% 22% 25%	5340
	21st to end	Day	39% 6% 8% 16% 21% 10%	5843
February	1st to 10th	Day	38% 5% 8% 20% 19% 11%	1890
	11th to 20th	Day	37% 7% 12% 19% 18% 7%	1973
	21st to end	Day	36% 7% 10% 24% 16% 7%	1730
March	1st to 10th	Day	40% 11% 9% 19% 20% 2	4976
	11th to 20th	Day	31% 4 16% 29% 11% 8%	5110
	21st to end	Day	38% 12% 18% 20% 11% 2	5692
April	1st to 10th	Day	32% 8% 14% 22% 20% 4	2520
	11th to 20th	Day	29% 7% 15% 25% 18% 6%	2492
	21st to end	Day	28% 7% 15% 27% 19% 4	2520
May	1st to 10th	Day	19% 7% 9% 27% 29% 10%	7164
	11th to 20th	Day	14% 8% 10% 30% 30% 8%	7670
	21st to end	Day	19% 9% 9% 29% 31% 2	8994
June	1st to 10th	Day	26% 8% 13% 24% 24% 5%	2940
	11th to 20th	Day	21% 5% 11% 22% 29% 11%	2940
	21st to end	Day	21% 3 8% 16% 29% 21%	2940
July	1st to 10th	Day	21% 8% 2 9% 23% 37%	8062
	11th to 20th	Day	28% 7% 3 6% 16% 40%	7581
	21st to end	Day	17% 9% 3 16% 29% 25%	8119
August	1st to 10th	Day	24% 7% 4 14% 28% 23%	4336
	11th to 20th	Day	16% 9% 5% 13% 40% 17%	3998
	21st to end	Day	20% 6% 5% 14% 34% 20%	4311
September	1st to 10th	Day	18% 3 4 16% 27% 32%	2400
	11th to 20th	Day	23% 5% 11% 22% 28% 12%	2400
	21st to end	Day	17% 4 7% 22% 34% 16%	2125
October	1st to 10th	Day	15% 5% 6% 13% 27% 34%	3280
	11th to 20th	Day	19% 6% 8% 25% 26% 16%	3261
	21st to end	Day	21% 3 7% 24% 30% 15%	3558
November	1st to 10th	Day	26% 6% 10% 24% 24% 10%	2040
	11th to 20th	Day	31% 5% 12% 22% 22% 8%	1890
	21st to end	Day	26% 5% 11% 20% 27% 11%	1768
December	1st to 10th	Day	26% 4 7% 23% 32% 7%	2700
	11th to 20th	Day	44% 5% 12% 17% 15% 7%	3108
	21st to end	Day	49% 5% 13% 20% 11% 3	4092
January	1st to 10th	Night	34% 5% 19% 24% 15% 4	7757
	11th to 20th	Night	28% 6% 9% 16% 25% 18%	7650
	21st to end	Night	39% 6% 8% 17% 20% 11%	8371
February	1st to 10th	Night	38% 4 9% 17% 19% 14%	2730
	11th to 20th	Night	37% 6% 12% 19% 19% 7%	2544
	21st to end	Night	32% 6% 12% 29% 15% 7%	2076
March	1st to 10th	Night	44% 8% 11% 17% 18% 3	5125
	11th to 20th	Night	34% 6% 13% 23% 17% 8%	5110
	21st to end	Night	40% 9% 14% 22% 12% 2	5523
April	1st to 10th	Night	32% 8% 15% 23% 18% 5%	2100
	11th to 20th	Night	32% 6% 12% 28% 15% 7%	2079
	21st to end	Night	25% 8% 14% 30% 18% 5%	2100
May	1st to 10th	Night	24% 8% 13% 21% 26% 8%	5802
	11th to 20th	Night	23% 11% 10% 21% 25% 10%	5201
	21st to end	Night	24% 12% 12% 22% 28% 2	5182
June	1st to 10th	Night	26% 8% 12% 24% 23% 7%	1680
	11th to 20th	Night	27% 6% 8% 15% 28% 16%	1680
	21st to end	Night	25% 7% 7% 13% 26% 22%	1680
July	1st to 10th	Night	30% 8% 2 8% 19% 33%	4775
	11th to 20th	Night	39% 11% 2 5% 14% 28%	5286
	21st to end	Night	27% 12% 4 12% 22% 22%	5679
August	1st to 10th	Night	36% 13% 4 12% 21% 14%	3019
	11th to 20th	Night	24% 13% 3 11% 31% 18%	3222
	21st to end	Night	27% 9% 6% 11% 33% 15%	3608
September	1st to 10th	Night	22% 5% 4 12% 27% 30%	2000
	11th to 20th	Night	26% 6% 7% 22% 27% 13%	2000
	21st to end	Night	22% 4 5% 18% 32% 19%	2268
October	1st to 10th	Night	18% 6% 5% 12% 23% 36%	3920
	11th to 20th	Night	25% 9% 6% 22% 21% 17%	3897
	21st to end	Night	23% 4 6% 22% 29% 16%	4253
November	1st to 10th	Night	26% 6% 9% 24% 23% 12%	2476
	11th to 20th	Night	31% 5% 11% 22% 22% 9%	2730
	21st to end	Night	28% 5% 8% 22% 23% 14%	2850
December	1st to 10th	Night	26% 4 7% 26% 27% 11%	4700
	11th to 20th	Night	45% 5% 12% 16% 17% 5%	5316
	21st to end	Night	52% 5% 12% 17% 10% 3	7106

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% 7% 19% 26% 18% 6%	367
	11th to 20th	Day	14% 3 18% 27% 31% 7%	430
	21st to end	Day	13% 5% 19% 32% 24% 6%	473
February	1st to 10th	Day	20% 6% 20% 23% 22% 9%	430
	11th to 20th	Day	16% 12% 22% 29% 17% 3	488
	21st to end	Day	11% 9% 24% 35% 19% 3	446
March	1st to 10th	Day	19% 10% 22% 31% 15% 2	540
	11th to 20th	Day	19% 7% 20% 26% 21% 7%	540
	21st to end	Day	13% 9% 26% 30% 18% 4	594
April	1st to 10th	Day	12% 7% 26% 32% 19% 5%	540
	11th to 20th	Day	9% 10% 25% 31% 18% 6%	532
	21st to end	Day	6% 8% 28% 30% 21% 6%	540
May	1st to 10th	Day	7% 6% 28% 30% 22% 7%	543
	11th to 20th	Day	8% 6% 23% 30% 26% 6%	568
	21st to end	Day	9% 7% 20% 30% 26% 7%	617
June	1st to 10th	Day	6% 4 17% 41% 25% 8%	570
	11th to 20th	Day	5% 5% 21% 36% 27% 5%	570
	21st to end	Day	2 4 14% 39% 32% 9%	570
July	1st to 10th	Day	5% 1 12% 38% 35% 9%	566
	11th to 20th	Day	4 2 7% 38% 39% 11%	564
	21st to end	Day	2 1 7% 38% 40% 12%	619
August	1st to 10th	Day	4 1 11% 37% 37% 10%	567
	11th to 20th	Day	4 1 8% 40% 38% 8%	534
	21st to end	Day	4 2 8% 33% 40% 13%	583
September	1st to 10th	Day	2 2 10% 32% 40% 13%	530
	11th to 20th	Day	5% 3 15% 35% 31% 11%	530
	21st to end	Day	6% 3 11% 31% 38% 12%	530
October	1st to 10th	Day	3 3 19% 28% 32% 14%	530
	11th to 20th	Day	6% 3 16% 28% 32% 15%	530
	21st to end	Day	8% 4 16% 28% 30% 14%	583
November	1st to 10th	Day	8% 6% 15% 24% 32% 15%	516
	11th to 20th	Day	12% 5% 17% 31% 25% 11%	330
	21st to end	Day	10% 2 14% 33% 31% 9%	266
December	1st to 10th	Day	14% 6% 14% 26% 30% 11%	220
	11th to 20th	Day	16% 5% 18% 23% 26% 11%	220
	21st to end	Day	19% 12% 21% 26% 16% 6%	242
January	1st to 10th	Night	19% 5% 13% 33% 25% 6%	840
	11th to 20th	Night	15% 3 12% 33% 28% 9%	840
	21st to end	Night	15% 5% 10% 34% 31% 5%	924
February	1st to 10th	Night	20% 6% 12% 30% 25% 6%	840
	11th to 20th	Night	17% 6% 16% 33% 22% 5%	824
	21st to end	Night	13% 7% 16% 37% 23% 3	692
March	1st to 10th	Night	18% 8% 14% 33% 23% 4	840
	11th to 20th	Night	17% 4 14% 31% 28% 6%	840
	21st to end	Night	15% 4 17% 35% 25% 4	924
April	1st to 10th	Night	13% 5% 16% 36% 27% 4	840
	11th to 20th	Night	12% 8% 15% 34% 25% 6%	831
	21st to end	Night	8% 4 19% 33% 26% 9%	840
May	1st to 10th	Night	8% 3 16% 34% 31% 8%	801
	11th to 20th	Night	6% 2 10% 36% 35% 10%	591
	21st to end	Night	8% 4 14% 34% 32% 8%	455
June	1st to 10th	Night	6% 3 10% 37% 36% 9%	420
	11th to 20th	Night	6% 3 9% 36% 40% 6%	420
	21st to end	Night	5% 1 7% 30% 47% 11%	420
July	1st to 10th	Night	6% 2 3 24% 52% 12%	433
	11th to 20th	Night	8% 1 1 19% 46% 24%	612
	21st to end	Night	6% 2 2 23% 49% 17%	690
August	1st to 10th	Night	8% 2 6% 25% 43% 17%	627
	11th to 20th	Night	6% 1 4 28% 48% 13%	759
	21st to end	Night	6% 2 4 24% 46% 19%	880
September	1st to 10th	Night	3 1 5% 25% 48% 18%	800
	11th to 20th	Night	6% 2 6% 28% 43% 15%	800
	21st to end	Night	6% 2 6% 24% 44% 17%	800
October	1st to 10th	Night	6% 2 8% 31% 38% 16%	800
	11th to 20th	Night	8% 2 7% 29% 39% 16%	800
	21st to end	Night	9% 2 9% 29% 33% 18%	880
November	1st to 10th	Night	9% 2 11% 28% 37% 13%	820
	11th to 20th	Night	13% 2 9% 34% 34% 7%	840
	21st to end	Night	10% 2 12% 37% 32% 8%	840
December	1st to 10th	Night	15% 4 11% 33% 29% 8%	840
	11th to 20th	Night	20% 4 13% 34% 23% 5%	840
	21st to end	Night	22% 6% 17% 34% 18% 4	924

Page generated at 2026-08-12 00:37:13. Version ac07

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