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 2025-04-11.

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	8% 5% 2 5% 3 3 4 3 5% 7% 55%	117461
	Night	9% 6% 2 5% 4 4 4 3 5% 7% 51%	119718
Transparency	Day	27% 6% 10% 20% 22% 14%	117221
	Night	32% 7% 10% 18% 20% 13%	119548
Seeing	Day	9% 5% 18% 32% 28% 8%	15991
	Night	11% 3 11% 32% 33% 10%	25500

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	12% 5% 1 5% 2 3 2 3 5% 8% 53%	3961
	11th to 20th	Day	12% 5% 2 6% 3 4 3 3 4 5% 54%	4100
	21st to end	Day	15% 7% 3 6% 4 4 4 3 5% 6% 43%	4510
February	1st to 10th	Day	20% 5% 1 5% 4 3 4 3 5% 6% 46%	1800
	11th to 20th	Day	13% 5% 1 6% 4 4 4 5% 5% 8% 45%	1868
	21st to end	Day	13% 5% 2 6% 5% 3 4 3 5% 6% 47%	1650
March	1st to 10th	Day	12% 7% 3 7% 5% 5% 6% 5% 7% 9% 34%	4868
	11th to 20th	Day	13% 4 1 4 3 4 4 3 5% 8% 51%	5000
	21st to end	Day	14% 8% 2 5% 4 4 5% 4 7% 10% 36%	5569
April	1st to 10th	Day	11% 5% 2 4 4 4 3 4 5% 8% 50%	2400
	11th to 20th	Day	8% 5% 2 5% 3 3 3 4 5% 8% 53%	2251
	21st to end	Day	5% 5% 1 6% 5% 3 4 3 6% 9% 54%	2280
May	1st to 10th	Day	6% 3 1 2 2 2 2 2 4 6% 70%	3855
	11th to 20th	Day	3 9% 2 6% 3 3 3 3 4 9% 56%	4117
	21st to end	Day	3 7% 2 4 3 3 3 3 4 7% 61%	4813
June	1st to 10th	Day	4 6% 2 5% 3 4 4 3 6% 7% 55%	2660
	11th to 20th	Day	2 5% 1 4 3 3 3 3 5% 9% 62%	2660
	21st to end	Day	2 4 2 5% 3 3 3 3 5% 8% 62%	2660
July	1st to 10th	Day	1 3 1 4 3 3 3 3 3 6% 68%	4369
	11th to 20th	Day	2 4 2 6% 4 4 3 3 5% 7% 61%	4110
	21st to end	Day	5% 2 5% 4 4 3 3 4 8% 62%	4455
August	1st to 10th	Day	1 4 3 6% 5% 5% 5% 4 5% 8% 55%	4091
	11th to 20th	Day	3 1 4 4 4 4 3 4 7% 65%	3788
	21st to end	Day	1 5% 3 5% 3 4 4 3 4 8% 62%	4180
September	1st to 10th	Day	2 3 1 4 2 3 3 2 3 6% 70%	2280
	11th to 20th	Day	3 5% 2 5% 3 3 3 4 5% 8% 59%	2280
	21st to end	Day	3 2 1 3 2 3 3 2 4 6% 71%	2020
October	1st to 10th	Day	2 2 1 3 2 3 4 3 4 7% 69%	3180
	11th to 20th	Day	3 3 2 4 3 3 4 2 4 6% 66%	3161
	21st to end	Day	4 4 1 3 2 3 4 3 5% 8% 63%	3448
November	1st to 10th	Day	8% 4 1 4 3 4 5% 3 5% 7% 56%	1941
	11th to 20th	Day	9% 5% 2 4 3 3 4 4 5% 6% 54%	1800
	21st to end	Day	12% 4 1 4 2 2 3 3 4 8% 57%	1684
December	1st to 10th	Day	9% 3 1 4 3 4 3 4 5% 9% 54%	2620
	11th to 20th	Day	22% 7% 2 4 3 3 4 3 5% 7% 39%	3028
	21st to end	Day	26% 8% 2 5% 3 3 3 3 5% 7% 35%	4004
January	1st to 10th	Night	9% 5% 2 5% 3 2 3 3 5% 7% 55%	5982
	11th to 20th	Night	14% 5% 2 4 3 4 4 4 4 6% 50%	5880
	21st to end	Night	15% 7% 2 6% 3 3 4 3 5% 6% 46%	6468
February	1st to 10th	Night	19% 5% 2 4 3 3 3 3 4 6% 48%	2600
	11th to 20th	Night	13% 6% 2 5% 3 3 4 4 6% 7% 46%	2418
	21st to end	Night	12% 6% 2 4 3 3 3 3 6% 6% 53%	1980
March	1st to 10th	Night	15% 9% 2 7% 5% 4 5% 4 6% 7% 36%	5013
	11th to 20th	Night	14% 5% 2 5% 5% 3 3 2 4 7% 49%	5000
	21st to end	Night	16% 9% 2 6% 4 3 4 3 5% 7% 42%	5404
April	1st to 10th	Night	11% 6% 2 4 4 3 5% 4 5% 8% 49%	2000
	11th to 20th	Night	10% 6% 2 4 4 3 3 3 5% 6% 53%	1883
	21st to end	Night	5% 6% 2 4 3 3 4 3 5% 9% 57%	1900
May	1st to 10th	Night	3 4 1 3 2 2 2 2 4 5% 71%	3119
	11th to 20th	Night	2 17% 5% 10% 5% 6% 3 3 4 5% 40%	2780
	21st to end	Night	2 8% 2 6% 4 4 3 2 4 7% 57%	2762
June	1st to 10th	Night	4 5% 3 5% 4 3 2 3 5% 8% 58%	1520
	11th to 20th	Night	1 7% 2 5% 4 3 4 4 5% 7% 59%	1520
	21st to end	Night	2 5% 2 6% 3 3 5% 4 5% 8% 57%	1520
July	1st to 10th	Night	1 5% 2 7% 3 3 4 2 5% 7% 63%	2588
	11th to 20th	Night	1 7% 5% 8% 5% 4 6% 3 5% 8% 48%	2860
	21st to end	Night	1 13% 5% 8% 6% 4 4 4 6% 7% 44%	3109
August	1st to 10th	Night	2 7% 4 11% 7% 8% 7% 3 6% 7% 38%	2848
	11th to 20th	Night	1 4 2 7% 7% 8% 6% 3 6% 7% 50%	3057
	21st to end	Night	2 7% 5% 7% 4 4 5% 2 3 6% 55%	3498
September	1st to 10th	Night	1 4 2 4 4 4 5% 3 4 6% 63%	1900
	11th to 20th	Night	4 6% 2 5% 3 3 3 2 5% 7% 59%	1900
	21st to end	Night	2 6% 2 3 3 2 3 2 4 6% 67%	2154
October	1st to 10th	Night	2 3 1 5% 3 3 3 2 5% 7% 66%	3800
	11th to 20th	Night	4 6% 2 7% 6% 4 3 3 4 5% 57%	3777
	21st to end	Night	5% 3 2 5% 3 3 3 3 4 6% 63%	4121
November	1st to 10th	Night	8% 4 2 3 4 3 4 2 5% 7% 59%	2355
	11th to 20th	Night	10% 4 1 4 4 3 3 3 6% 9% 54%	2600
	21st to end	Night	9% 4 1 4 3 4 4 3 5% 5% 57%	2714
December	1st to 10th	Night	9% 4 1 3 2 3 4 3 5% 6% 59%	4560
	11th to 20th	Night	22% 7% 2 5% 4 3 4 4 8% 9% 35%	5176
	21st to end	Night	27% 8% 2 5% 4 4 4 3 4 5% 35%	6952

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	29% 5% 20% 24% 17% 4	3961
	11th to 20th	Day	33% 6% 9% 15% 18% 19%	4100
	21st to end	Day	41% 6% 8% 13% 18% 14%	4510
February	1st to 10th	Day	38% 5% 8% 19% 19% 10%	1800
	11th to 20th	Day	36% 7% 12% 19% 18% 8%	1877
	21st to end	Day	36% 7% 9% 24% 17% 7%	1650
March	1st to 10th	Day	40% 11% 9% 18% 19% 2	4868
	11th to 20th	Day	31% 4 17% 29% 11% 8%	5000
	21st to end	Day	39% 12% 18% 20% 10% 2	5569
April	1st to 10th	Day	32% 8% 14% 23% 19% 4	2400
	11th to 20th	Day	28% 7% 15% 26% 18% 6%	2254
	21st to end	Day	27% 7% 15% 27% 19% 4	2280
May	1st to 10th	Day	17% 4 13% 27% 26% 12%	3855
	11th to 20th	Day	21% 8% 14% 29% 21% 7%	4120
	21st to end	Day	23% 7% 14% 26% 26% 3	4814
June	1st to 10th	Day	27% 8% 13% 24% 22% 5%	2660
	11th to 20th	Day	21% 4 12% 24% 28% 11%	2660
	21st to end	Day	22% 3 9% 17% 28% 21%	2660
July	1st to 10th	Day	16% 5% 3 10% 15% 50%	4301
	11th to 20th	Day	21% 7% 5% 10% 14% 43%	4041
	21st to end	Day	20% 5% 4 12% 32% 27%	4315
August	1st to 10th	Day	24% 7% 5% 14% 26% 24%	4076
	11th to 20th	Day	15% 9% 5% 13% 41% 17%	3825
	21st to end	Day	20% 6% 6% 14% 33% 21%	4179
September	1st to 10th	Day	18% 3 4 15% 27% 34%	2280
	11th to 20th	Day	23% 5% 11% 21% 28% 12%	2280
	21st to end	Day	16% 4 7% 22% 34% 17%	2020
October	1st to 10th	Day	15% 5% 6% 12% 27% 35%	3180
	11th to 20th	Day	19% 6% 8% 24% 26% 17%	3161
	21st to end	Day	21% 3 7% 24% 30% 15%	3448
November	1st to 10th	Day	27% 6% 9% 25% 23% 10%	1941
	11th to 20th	Day	29% 5% 12% 22% 23% 8%	1800
	21st to end	Day	28% 5% 11% 19% 27% 11%	1684
December	1st to 10th	Day	27% 4 7% 22% 33% 7%	2620
	11th to 20th	Day	44% 5% 12% 17% 15% 7%	3028
	21st to end	Day	49% 5% 13% 20% 11% 3	4004
January	1st to 10th	Night	27% 5% 21% 26% 16% 5%	5982
	11th to 20th	Night	34% 7% 11% 17% 20% 12%	5880
	21st to end	Night	39% 6% 9% 13% 19% 14%	6468
February	1st to 10th	Night	39% 4 9% 17% 19% 13%	2600
	11th to 20th	Night	35% 6% 12% 19% 20% 7%	2420
	21st to end	Night	31% 6% 12% 29% 16% 7%	1980
March	1st to 10th	Night	44% 8% 11% 17% 17% 3	5013
	11th to 20th	Night	34% 6% 13% 23% 16% 8%	5000
	21st to end	Night	41% 9% 14% 22% 12% 2	5404
April	1st to 10th	Night	32% 8% 15% 23% 17% 5%	2000
	11th to 20th	Night	31% 6% 13% 28% 15% 7%	1883
	21st to end	Night	23% 8% 14% 29% 20% 6%	1900
May	1st to 10th	Night	17% 4 16% 24% 25% 14%	3119
	11th to 20th	Night	38% 13% 14% 13% 14% 9%	2781
	21st to end	Night	24% 9% 16% 21% 27% 3	2762
June	1st to 10th	Night	26% 8% 13% 23% 23% 8%	1520
	11th to 20th	Night	26% 6% 9% 16% 28% 16%	1520
	21st to end	Night	25% 7% 8% 12% 25% 23%	1520
July	1st to 10th	Night	21% 6% 2 7% 16% 48%	2547
	11th to 20th	Night	28% 12% 4 9% 14% 33%	2806
	21st to end	Night	36% 6% 4 10% 20% 25%	3011
August	1st to 10th	Night	37% 13% 4 11% 20% 15%	2839
	11th to 20th	Night	23% 14% 3 11% 31% 19%	3086
	21st to end	Night	27% 9% 6% 11% 33% 15%	3498
September	1st to 10th	Night	21% 4 4 11% 28% 31%	1900
	11th to 20th	Night	26% 5% 7% 21% 28% 13%	1900
	21st to end	Night	20% 4 5% 18% 33% 20%	2154
October	1st to 10th	Night	18% 5% 5% 11% 23% 37%	3800
	11th to 20th	Night	25% 10% 6% 22% 20% 18%	3777
	21st to end	Night	23% 4 6% 22% 29% 16%	4121
November	1st to 10th	Night	26% 5% 9% 24% 23% 13%	2355
	11th to 20th	Night	28% 5% 12% 23% 23% 9%	2600
	21st to end	Night	29% 5% 8% 20% 23% 15%	2714
December	1st to 10th	Night	27% 4 8% 24% 27% 11%	4560
	11th to 20th	Night	45% 5% 12% 16% 18% 5%	5176
	21st to end	Night	52% 6% 13% 18% 10% 3	6952

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% 17% 27% 18% 6%	340
	11th to 20th	Day	15% 4 20% 28% 28% 5%	400
	21st to end	Day	13% 5% 20% 31% 24% 7%	440
February	1st to 10th	Day	22% 7% 21% 22% 21% 9%	400
	11th to 20th	Day	14% 12% 23% 29% 18% 4	452
	21st to end	Day	11% 9% 23% 35% 19% 4	414
March	1st to 10th	Day	20% 11% 22% 29% 15% 2	500
	11th to 20th	Day	20% 7% 22% 26% 19% 6%	500
	21st to end	Day	13% 9% 28% 30% 16% 4	550
April	1st to 10th	Day	12% 7% 27% 31% 18% 5%	500
	11th to 20th	Day	9% 11% 25% 31% 18% 6%	453
	21st to end	Day	5% 8% 30% 30% 21% 6%	460
May	1st to 10th	Day	8% 7% 28% 31% 21% 5%	463
	11th to 20th	Day	10% 5% 23% 31% 26% 6%	488
	21st to end	Day	9% 8% 21% 30% 26% 7%	529
June	1st to 10th	Day	6% 4 18% 41% 24% 7%	490
	11th to 20th	Day	4 4 23% 36% 28% 4	490
	21st to end	Day	2 2 13% 42% 33% 7%	490
July	1st to 10th	Day	6% 11% 39% 36% 9%	486
	11th to 20th	Day	4 1 6% 39% 39% 11%	484
	21st to end	Day	2 1 7% 37% 41% 12%	531
August	1st to 10th	Day	5% 11% 41% 36% 7%	487
	11th to 20th	Day	3 9% 42% 38% 8%	477
	21st to end	Day	4 1 8% 35% 39% 12%	539
September	1st to 10th	Day	2 2 10% 32% 40% 13%	490
	11th to 20th	Day	5% 3 16% 36% 30% 10%	490
	21st to end	Day	3 3 11% 30% 40% 12%	490
October	1st to 10th	Day	2 3 19% 28% 34% 14%	490
	11th to 20th	Day	6% 3 16% 28% 32% 15%	490
	21st to end	Day	9% 4 17% 28% 29% 14%	539
November	1st to 10th	Day	8% 6% 16% 25% 31% 15%	477
	11th to 20th	Day	10% 5% 16% 31% 27% 11%	300
	21st to end	Day	11% 2 15% 33% 30% 8%	242
December	1st to 10th	Day	16% 7% 15% 24% 29% 10%	200
	11th to 20th	Day	17% 6% 18% 24% 26% 10%	200
	21st to end	Day	17% 11% 20% 27% 17% 7%	220
January	1st to 10th	Night	18% 5% 13% 33% 25% 6%	800
	11th to 20th	Night	15% 3 13% 33% 27% 8%	800
	21st to end	Night	16% 5% 10% 35% 30% 5%	880
February	1st to 10th	Night	21% 6% 13% 29% 25% 7%	800
	11th to 20th	Night	16% 6% 16% 34% 23% 5%	784
	21st to end	Night	13% 7% 16% 37% 24% 3	660
March	1st to 10th	Night	19% 8% 14% 32% 23% 4	800
	11th to 20th	Night	18% 5% 14% 32% 27% 6%	800
	21st to end	Night	16% 5% 18% 35% 24% 4	880
April	1st to 10th	Night	13% 5% 16% 37% 26% 3	800
	11th to 20th	Night	11% 8% 16% 34% 24% 6%	753
	21st to end	Night	8% 4 20% 33% 26% 9%	760
May	1st to 10th	Night	8% 3 17% 34% 31% 7%	725
	11th to 20th	Night	7% 2 10% 36% 35% 11%	534
	21st to end	Night	8% 4 15% 34% 31% 8%	411
June	1st to 10th	Night	6% 2 10% 39% 35% 8%	380
	11th to 20th	Night	5% 2 10% 39% 40% 5%	380
	21st to end	Night	4 6% 32% 48% 10%	380
July	1st to 10th	Night	6% 4 26% 53% 12%	391
	11th to 20th	Night	7% 1 1 20% 48% 24%	552
	21st to end	Night	6% 1 2 25% 48% 18%	624
August	1st to 10th	Night	8% 1 6% 26% 43% 14%	567
	11th to 20th	Night	5% 1 4 28% 48% 13%	707
	21st to end	Night	6% 1 4 24% 46% 18%	836
September	1st to 10th	Night	3 5% 25% 48% 18%	760
	11th to 20th	Night	6% 2 6% 29% 43% 14%	760
	21st to end	Night	5% 2 7% 24% 45% 18%	760
October	1st to 10th	Night	6% 2 8% 31% 38% 16%	760
	11th to 20th	Night	8% 2 7% 28% 39% 15%	760
	21st to end	Night	8% 1 9% 29% 34% 18%	836
November	1st to 10th	Night	9% 2 12% 29% 36% 12%	780
	11th to 20th	Night	12% 2 9% 35% 35% 8%	800
	21st to end	Night	10% 2 13% 38% 30% 8%	800
December	1st to 10th	Night	16% 4 12% 33% 28% 8%	800
	11th to 20th	Night	21% 4 13% 34% 23% 4	800
	21st to end	Night	20% 6% 17% 34% 19% 4	880

Page generated at 2025-04-08 06:17:24. Version ac07

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