Santa Maria
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Santa Maria clear sky chart from 2005-11-06 to 2025-07-21.

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	5% 4 1 4 3 3 3 3 5% 7% 63%	123635
	Night	6% 6% 3 6% 3 3 4 3 5% 7% 53%	124423
Transparency	Day	20% 5% 8% 19% 27% 21%	123393
	Night	27% 7% 8% 16% 22% 19%	124252
Seeing	Day	7% 8% 25% 34% 19% 6%	16529
	Night	8% 3 14% 35% 32% 8%	26143

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	5% 2 1 4 3 3 3 2 4 6% 67%	3897
	11th to 20th	Day	8% 4 1 4 2 3 3 2 4 5% 63%	4100
	21st to end	Day	10% 5% 2 5% 3 4 4 4 6% 9% 48%	4510
February	1st to 10th	Day	13% 5% 2 4 2 3 3 3 4 5% 56%	1905
	11th to 20th	Day	8% 6% 1 5% 4 4 3 3 5% 7% 54%	1950
	21st to end	Day	11% 5% 2 5% 3 4 4 4 6% 6% 52%	1650
March	1st to 10th	Day	12% 6% 2 6% 4 4 3 4 6% 8% 46%	4636
	11th to 20th	Day	9% 5% 2 5% 4 3 3 2 4 6% 58%	5000
	21st to end	Day	8% 7% 2 6% 3 4 4 4 6% 10% 45%	5744
April	1st to 10th	Day	8% 4 1 4 4 3 4 3 6% 6% 58%	2400
	11th to 20th	Day	5% 5% 2 5% 3 2 3 3 5% 7% 60%	2369
	21st to end	Day	7% 5% 1 4 2 2 3 3 5% 7% 62%	2400
May	1st to 10th	Day	3 3 1 3 2 2 1 1 3 5% 77%	5440
	11th to 20th	Day	1 5% 2 5% 3 2 3 3 4 7% 65%	5574
	21st to end	Day	2 3 1 5% 2 2 3 3 4 6% 67%	6830
June	1st to 10th	Day	4 6% 2 5% 4 3 3 3 5% 8% 60%	2800
	11th to 20th	Day	1 4 1 4 2 2 3 3 4 7% 69%	2800
	21st to end	Day	1 3 1 4 3 2 3 3 4 7% 68%	2800
July	1st to 10th	Day	1 2 1 3 2 2 2 3 4 6% 74%	4560
	11th to 20th	Day	1 3 2 4 3 2 3 3 4 7% 68%	4247
	21st to end	Day	1 2 1 5% 3 2 3 3 5% 7% 67%	4456
August	1st to 10th	Day	2 4 1 4 2 2 3 2 5% 7% 68%	3858
	11th to 20th	Day	1 3 1 2 1 2 2 3 4 5% 77%	3724
	21st to end	Day	1 2 1 4 2 2 2 2 4 6% 74%	4180
September	1st to 10th	Day	2 3 1 3 2 2 3 3 6% 6% 68%	2280
	11th to 20th	Day	2 4 2 4 2 2 3 3 4 8% 66%	2266
	21st to end	Day	1 3 1 3 2 1 2 3 5% 6% 73%	2090
October	1st to 10th	Day	2 2 1 2 2 2 2 2 4 5% 76%	3216
	11th to 20th	Day	2 2 1 3 2 3 3 2 4 4 74%	3161
	21st to end	Day	3 2 1 3 2 3 4 3 4 6% 68%	3448
November	1st to 10th	Day	6% 4 1 3 3 2 2 3 5% 6% 66%	1879
	11th to 20th	Day	5% 3 1 3 3 2 3 3 4 6% 68%	1800
	21st to end	Day	8% 4 1 3 3 3 3 4 5% 7% 59%	1800
December	1st to 10th	Day	5% 4 2 4 4 4 4 3 4 7% 59%	2833
	11th to 20th	Day	13% 6% 2 5% 3 3 4 3 5% 6% 50%	3028
	21st to end	Day	12% 5% 2 6% 4 3 4 4 6% 10% 43%	4004
January	1st to 10th	Night	6% 5% 2 4 3 3 3 3 4 6% 62%	6042
	11th to 20th	Night	8% 6% 2 4 3 3 3 3 5% 5% 59%	5880
	21st to end	Night	12% 7% 2 5% 3 4 5% 4 5% 7% 46%	6468
February	1st to 10th	Night	11% 7% 2 5% 3 3 3 2 4 5% 54%	2493
	11th to 20th	Night	9% 6% 1 4 4 3 3 3 5% 8% 54%	2338
	21st to end	Night	11% 6% 2 5% 3 3 3 2 3 6% 55%	1980
March	1st to 10th	Night	11% 10% 3 6% 4 4 4 3 6% 8% 41%	5254
	11th to 20th	Night	8% 7% 2 5% 3 3 4 3 4 6% 55%	5000
	21st to end	Night	11% 9% 3 7% 4 3 3 2 5% 7% 47%	5229
April	1st to 10th	Night	6% 6% 2 6% 4 4 4 3 5% 6% 54%	2000
	11th to 20th	Night	6% 7% 3 5% 3 3 4 3 4 6% 58%	1979
	21st to end	Night	6% 10% 2 4 3 2 3 3 5% 9% 54%	2000
May	1st to 10th	Night	2 7% 3 5% 3 2 2 2 5% 6% 62%	4560
	11th to 20th	Night	2 13% 7% 8% 3 3 3 3 6% 7% 45%	4349
	21st to end	Night	2 10% 8% 13% 3 2 3 2 4 4 50%	4029
June	1st to 10th	Night	2 10% 4 7% 3 4 4 3 6% 8% 49%	1600
	11th to 20th	Night	2 8% 3 6% 3 3 3 3 5% 8% 56%	1600
	21st to end	Night	1 5% 3 8% 3 2 5% 5% 6% 10% 52%	1600
July	1st to 10th	Night	3 2 5% 3 3 2 4 6% 9% 62%	2620
	11th to 20th	Night	1 3 4 11% 3 3 4 5% 8% 9% 48%	2938
	21st to end	Night	1 6% 5% 16% 6% 4 5% 4 6% 9% 38%	3109
August	1st to 10th	Night	1 5% 3 9% 4 4 4 3 7% 11% 49%	3089
	11th to 20th	Night	1 3 2 7% 5% 3 4 4 6% 9% 58%	3126
	21st to end	Night	6% 4 9% 5% 3 3 3 5% 8% 55%	3498
September	1st to 10th	Night	2 5% 2 5% 2 3 3 4 5% 7% 62%	1900
	11th to 20th	Night	3 6% 3 6% 3 3 4 4 5% 8% 55%	1914
	21st to end	Night	2 4 1 5% 3 1 3 3 4 6% 68%	2090
October	1st to 10th	Night	2 3 1 3 1 2 2 2 5% 7% 72%	3760
	11th to 20th	Night	3 6% 5% 6% 2 3 5% 4 7% 5% 55%	3777
	21st to end	Night	3 6% 2 3 2 2 2 2 3 6% 68%	4121
November	1st to 10th	Night	4 5% 1 4 3 3 3 3 4 8% 62%	2416
	11th to 20th	Night	4 3 2 4 3 3 3 3 5% 7% 65%	2600
	21st to end	Night	5% 6% 2 4 3 3 4 3 5% 6% 59%	2600
December	1st to 10th	Night	6% 3 2 4 3 3 3 4 5% 7% 60%	4336
	11th to 20th	Night	12% 6% 2 5% 3 3 4 5% 7% 7% 46%	5176
	21st to end	Night	10% 8% 3 8% 5% 6% 6% 5% 7% 8% 34%	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	19% 3 8% 32% 24% 13%	3897
	11th to 20th	Day	25% 4 8% 13% 21% 29%	4100
	21st to end	Day	31% 5% 8% 18% 22% 16%	4510
February	1st to 10th	Day	32% 3 7% 19% 21% 18%	1905
	11th to 20th	Day	29% 5% 8% 24% 23% 10%	1960
	21st to end	Day	31% 5% 10% 19% 26% 9%	1650
March	1st to 10th	Day	34% 6% 13% 18% 23% 6%	4636
	11th to 20th	Day	28% 5% 12% 30% 17% 9%	5000
	21st to end	Day	32% 6% 16% 27% 16% 3	5744
April	1st to 10th	Day	26% 6% 13% 27% 21% 8%	2400
	11th to 20th	Day	24% 5% 14% 26% 24% 8%	2372
	21st to end	Day	22% 5% 11% 27% 27% 8%	2400
May	1st to 10th	Day	14% 3 8% 31% 35% 9%	5440
	11th to 20th	Day	15% 9% 6% 27% 34% 9%	5577
	21st to end	Day	15% 8% 7% 28% 36% 7%	6831
June	1st to 10th	Day	22% 6% 10% 23% 27% 11%	2800
	11th to 20th	Day	16% 4 6% 14% 38% 22%	2800
	21st to end	Day	15% 5% 6% 8% 32% 34%	2800
July	1st to 10th	Day	11% 3 4 8% 17% 58%	4491
	11th to 20th	Day	15% 4 5% 9% 22% 45%	4177
	21st to end	Day	15% 5% 5% 13% 28% 34%	4316
August	1st to 10th	Day	16% 4 4 13% 33% 30%	3843
	11th to 20th	Day	10% 4 3 8% 33% 42%	3760
	21st to end	Day	12% 4 4 12% 32% 37%	4179
September	1st to 10th	Day	15% 5% 5% 10% 28% 38%	2280
	11th to 20th	Day	17% 6% 5% 17% 34% 21%	2266
	21st to end	Day	13% 4 4 17% 35% 26%	2090
October	1st to 10th	Day	11% 2 5% 14% 25% 42%	3216
	11th to 20th	Day	14% 4 6% 20% 33% 23%	3161
	21st to end	Day	18% 3 5% 13% 34% 27%	3448
November	1st to 10th	Day	20% 4 6% 17% 30% 23%	1879
	11th to 20th	Day	19% 4 7% 20% 28% 22%	1800
	21st to end	Day	25% 5% 8% 15% 26% 23%	1800
December	1st to 10th	Day	24% 4 6% 15% 30% 21%	2833
	11th to 20th	Day	35% 4 9% 19% 23% 10%	3028
	21st to end	Day	34% 7% 13% 26% 13% 6%	4004
January	1st to 10th	Night	23% 5% 8% 24% 26% 14%	6042
	11th to 20th	Night	26% 7% 6% 14% 18% 28%	5880
	21st to end	Night	34% 7% 7% 14% 21% 17%	6468
February	1st to 10th	Night	34% 4 7% 15% 23% 17%	2493
	11th to 20th	Night	29% 5% 9% 21% 23% 14%	2339
	21st to end	Night	31% 5% 8% 20% 25% 11%	1980
March	1st to 10th	Night	37% 8% 10% 17% 21% 6%	5254
	11th to 20th	Night	30% 7% 11% 26% 19% 8%	5000
	21st to end	Night	36% 7% 13% 24% 17% 3	5229
April	1st to 10th	Night	29% 9% 12% 23% 18% 9%	2000
	11th to 20th	Night	27% 7% 9% 25% 22% 10%	1979
	21st to end	Night	27% 7% 9% 22% 26% 9%	2000
May	1st to 10th	Night	21% 6% 12% 22% 25% 14%	4557
	11th to 20th	Night	27% 16% 9% 15% 23% 10%	4350
	21st to end	Night	31% 11% 7% 14% 28% 9%	4029
June	1st to 10th	Night	31% 9% 9% 18% 23% 11%	1600
	11th to 20th	Night	25% 6% 6% 11% 28% 24%	1600
	21st to end	Night	24% 7% 8% 7% 23% 31%	1600
July	1st to 10th	Night	13% 7% 7% 8% 14% 52%	2580
	11th to 20th	Night	25% 6% 8% 9% 16% 36%	2885
	21st to end	Night	38% 7% 6% 10% 21% 18%	3011
August	1st to 10th	Night	26% 7% 7% 13% 20% 27%	3080
	11th to 20th	Night	19% 7% 5% 9% 20% 40%	3156
	21st to end	Night	25% 6% 6% 8% 24% 31%	3498
September	1st to 10th	Night	18% 6% 8% 8% 20% 39%	1900
	11th to 20th	Night	25% 8% 7% 11% 26% 24%	1914
	21st to end	Night	16% 6% 4 13% 33% 27%	2090
October	1st to 10th	Night	11% 4 4 13% 26% 41%	3760
	11th to 20th	Night	25% 9% 7% 15% 20% 25%	3777
	21st to end	Night	20% 4 3 11% 33% 28%	4121
November	1st to 10th	Night	22% 6% 6% 16% 26% 24%	2416
	11th to 20th	Night	20% 5% 9% 17% 26% 24%	2600
	21st to end	Night	26% 4 9% 16% 22% 23%	2600
December	1st to 10th	Night	23% 5% 6% 15% 30% 22%	4336
	11th to 20th	Night	35% 6% 12% 15% 22% 11%	5176
	21st to end	Night	41% 10% 12% 20% 11% 5%	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	19% 12% 29% 25% 14% 2	315
	11th to 20th	Day	11% 12% 31% 28% 16% 2	400
	21st to end	Day	12% 12% 31% 28% 15% 3	440
February	1st to 10th	Day	13% 15% 27% 27% 15% 4	453
	11th to 20th	Day	13% 12% 32% 28% 12% 2	495
	21st to end	Day	9% 13% 37% 28% 11% 2	414
March	1st to 10th	Day	16% 18% 31% 25% 9% 1	500
	11th to 20th	Day	14% 15% 32% 30% 8% 2	500
	21st to end	Day	12% 18% 36% 23% 8% 2	550
April	1st to 10th	Day	9% 16% 34% 23% 13% 4	500
	11th to 20th	Day	10% 13% 34% 29% 10% 4	492
	21st to end	Day	7% 13% 32% 30% 13% 5%	500
May	1st to 10th	Day	9% 10% 33% 30% 15% 4	503
	11th to 20th	Day	5% 11% 30% 37% 14% 3	528
	21st to end	Day	11% 9% 29% 34% 14% 4	573
June	1st to 10th	Day	7% 6% 22% 38% 21% 6%	530
	11th to 20th	Day	3 4 30% 38% 19% 6%	530
	21st to end	Day	3 3 21% 40% 23% 10%	530
July	1st to 10th	Day	3 1 14% 44% 30% 9%	526
	11th to 20th	Day	3 1 10% 44% 30% 12%	524
	21st to end	Day	2 10% 45% 30% 13%	532
August	1st to 10th	Day	5% 2 15% 48% 21% 9%	487
	11th to 20th	Day	4 13% 42% 30% 12%	477
	21st to end	Day	3 3 14% 43% 28% 9%	539
September	1st to 10th	Day	3 2 15% 41% 29% 10%	490
	11th to 20th	Day	4 7% 22% 41% 20% 6%	490
	21st to end	Day	2 6% 21% 36% 27% 9%	490
October	1st to 10th	Day	2 7% 23% 37% 24% 6%	490
	11th to 20th	Day	3 8% 26% 36% 21% 6%	490
	21st to end	Day	5% 5% 26% 37% 21% 6%	539
November	1st to 10th	Day	5% 8% 25% 34% 22% 6%	415
	11th to 20th	Day	8% 6% 24% 33% 23% 6%	300
	21st to end	Day	11% 6% 22% 31% 24% 5%	300
December	1st to 10th	Day	9% 8% 25% 34% 19% 4	267
	11th to 20th	Day	11% 11% 21% 31% 21% 7%	200
	21st to end	Day	16% 11% 30% 26% 13% 4	220
January	1st to 10th	Night	14% 4 20% 35% 20% 8%	800
	11th to 20th	Night	9% 4 20% 40% 21% 6%	800
	21st to end	Night	15% 4 19% 40% 17% 5%	880
February	1st to 10th	Night	14% 6% 16% 35% 23% 6%	800
	11th to 20th	Night	11% 5% 21% 37% 20% 5%	784
	21st to end	Night	13% 5% 20% 43% 16% 2	660
March	1st to 10th	Night	13% 5% 20% 41% 18% 3	800
	11th to 20th	Night	12% 3 18% 40% 23% 5%	800
	21st to end	Night	11% 5% 21% 38% 21% 3	880
April	1st to 10th	Night	8% 4 21% 41% 24% 4	800
	11th to 20th	Night	8% 6% 20% 35% 27% 4	791
	21st to end	Night	8% 4 21% 33% 27% 7%	800
May	1st to 10th	Night	7% 4 15% 40% 29% 6%	800
	11th to 20th	Night	7% 4 13% 38% 32% 6%	710
	21st to end	Night	6% 5% 13% 38% 33% 5%	465
June	1st to 10th	Night	5% 3 9% 32% 45% 6%	400
	11th to 20th	Night	4 2 7% 35% 47% 6%	400
	21st to end	Night	3 2 3 32% 53% 8%	400
July	1st to 10th	Night	2 2 1 22% 63% 11%	390
	11th to 20th	Night	3 1 1 12% 58% 25%	571
	21st to end	Night	3 1 2 13% 56% 25%	624
August	1st to 10th	Night	3 1 4 30% 48% 14%	704
	11th to 20th	Night	2 1 3 23% 56% 15%	752
	21st to end	Night	2 1 3 23% 55% 16%	836
September	1st to 10th	Night	3 2 4 24% 55% 13%	760
	11th to 20th	Night	4 3 6% 35% 42% 9%	760
	21st to end	Night	3 1 8% 29% 48% 10%	760
October	1st to 10th	Night	4 2 9% 38% 40% 6%	760
	11th to 20th	Night	5% 3 11% 37% 36% 7%	760
	21st to end	Night	7% 2 12% 37% 33% 8%	836
November	1st to 10th	Night	6% 3 13% 39% 29% 9%	780
	11th to 20th	Night	5% 2 17% 42% 27% 8%	800
	21st to end	Night	7% 4 18% 42% 24% 6%	800
December	1st to 10th	Night	11% 3 17% 40% 24% 5%	800
	11th to 20th	Night	13% 3 20% 40% 20% 4	800
	21st to end	Night	13% 6% 22% 39% 16% 4	880

Page generated at 2025-07-17 22:10:04. Version ac07

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