Santee
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Santee clear sky chart from 2005-11-06 to 2024-10-25.

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	12% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	101056
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 43%	97705
Transparency	Day	40% 7% 12% 19% 17% 5%	100869
	Night	38% 7% 12% 19% 18% 6%	97562
Seeing	Day	14% 5% 20% 24% 22% 14%	15411
	Night	14% 6% 21% 33% 22% 4	25479

By Month

Percentage Cloud Cover SE (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	13% 5% 2 5% 4 4 4 4 6% 7% 46%	2836
	11th to 20th	Day	11% 6% 3 5% 4 4 3 2 5% 7% 51%	2840
	21st to end	Day	15% 7% 4 7% 4 4 5% 3 4 5% 43%	3124
February	1st to 10th	Day	15% 8% 2 5% 4 4 5% 4 6% 7% 40%	1710
	11th to 20th	Day	18% 7% 3 5% 3 3 3 3 4 6% 45%	1833
	21st to end	Day	17% 7% 2 5% 4 3 5% 4 4 5% 44%	1674
March	1st to 10th	Day	20% 11% 3 7% 4 3 4 3 5% 5% 35%	3435
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 5% 7% 53%	3480
	21st to end	Day	17% 7% 2 6% 4 3 4 3 5% 5% 43%	3815
April	1st to 10th	Day	9% 4 2 5% 4 5% 5% 4 6% 7% 49%	2090
	11th to 20th	Day	10% 5% 2 6% 4 4 4 5% 5% 7% 49%	2254
	21st to end	Day	8% 4 2 5% 4 4 4 4 6% 8% 51%	2316
May	1st to 10th	Day	8% 6% 3 7% 5% 5% 6% 4 6% 9% 41%	4082
	11th to 20th	Day	15% 8% 3 7% 5% 4 4 4 5% 7% 38%	4081
	21st to end	Day	6% 4 2 6% 5% 5% 6% 4 7% 10% 46%	4476
June	1st to 10th	Day	9% 7% 3 9% 6% 6% 5% 5% 8% 9% 33%	2470
	11th to 20th	Day	8% 5% 1 5% 5% 5% 6% 5% 8% 11% 40%	2470
	21st to end	Day	5% 5% 2 6% 7% 7% 8% 6% 7% 11% 37%	2470
July	1st to 10th	Day	13% 7% 3 8% 7% 7% 7% 6% 9% 8% 24%	4120
	11th to 20th	Day	6% 5% 2 8% 7% 8% 8% 7% 9% 12% 26%	4120
	21st to end	Day	8% 7% 3 10% 7% 7% 7% 6% 9% 12% 22%	4466
August	1st to 10th	Day	21% 7% 2 7% 6% 6% 7% 6% 8% 10% 20%	4104
	11th to 20th	Day	8% 6% 3 8% 6% 7% 6% 6% 9% 11% 32%	3994
	21st to end	Day	8% 5% 2 7% 5% 6% 6% 5% 8% 10% 38%	4180
September	1st to 10th	Day	9% 5% 2 7% 5% 6% 6% 4 7% 9% 39%	2131
	11th to 20th	Day	14% 6% 3 8% 5% 5% 6% 4 6% 7% 38%	2090
	21st to end	Day	15% 7% 3 6% 4 3 4 4 6% 7% 39%	2090
October	1st to 10th	Day	13% 6% 2 6% 4 3 4 4 4 7% 47%	1957
	11th to 20th	Day	9% 5% 2 4 3 3 3 2 4 5% 58%	1900
	21st to end	Day	10% 5% 2 5% 4 3 3 3 4 5% 58%	2027
November	1st to 10th	Day	12% 5% 2 4 4 2 3 3 5% 6% 54%	1857
	11th to 20th	Day	14% 7% 2 5% 4 3 3 2 4 7% 49%	1867
	21st to end	Day	15% 5% 2 5% 4 3 3 3 5% 7% 48%	1710
December	1st to 10th	Day	16% 6% 3 6% 4 5% 5% 4 6% 6% 40%	1710
	11th to 20th	Day	16% 5% 2 5% 4 3 3 3 5% 6% 51%	2153
	21st to end	Day	18% 8% 2 5% 3 3 3 3 4 6% 44%	3124
January	1st to 10th	Night	15% 6% 2 5% 4 4 4 3 5% 6% 45%	4109
	11th to 20th	Night	10% 7% 2 6% 6% 5% 4 4 6% 8% 42%	4120
	21st to end	Night	13% 8% 2 6% 5% 4 4 3 5% 5% 45%	4532
February	1st to 10th	Night	17% 6% 2 6% 4 4 3 3 5% 7% 44%	2470
	11th to 20th	Night	17% 7% 2 6% 4 3 3 3 5% 5% 44%	2230
	21st to end	Night	12% 6% 3 6% 4 3 4 4 6% 7% 44%	1779
March	1st to 10th	Night	21% 12% 4 7% 4 3 3 2 5% 6% 33%	3418
	11th to 20th	Night	12% 4 2 4 3 3 4 4 6% 7% 50%	3480
	21st to end	Night	15% 5% 2 5% 4 4 4 3 5% 8% 45%	3817
April	1st to 10th	Night	11% 4 2 5% 4 4 4 4 5% 6% 51%	2088
	11th to 20th	Night	11% 5% 2 5% 3 4 3 4 5% 8% 50%	1875
	21st to end	Night	7% 5% 1 6% 4 3 3 4 6% 7% 53%	1878
May	1st to 10th	Night	7% 7% 2 6% 4 5% 5% 4 7% 9% 45%	2813
	11th to 20th	Night	12% 7% 2 5% 4 5% 4 3 6% 9% 42%	2812
	21st to end	Night	6% 3 2 6% 4 5% 5% 4 7% 10% 48%	3079
June	1st to 10th	Night	9% 6% 3 6% 5% 5% 6% 5% 8% 12% 35%	1710
	11th to 20th	Night	9% 5% 2 5% 7% 5% 6% 4 7% 11% 39%	1710
	21st to end	Night	6% 4 2 7% 6% 5% 7% 6% 9% 13% 35%	1710
July	1st to 10th	Night	12% 8% 3 9% 6% 6% 6% 5% 6% 10% 29%	2840
	11th to 20th	Night	7% 6% 3 8% 7% 7% 7% 6% 9% 13% 28%	2840
	21st to end	Night	10% 8% 3 9% 7% 7% 7% 6% 9% 10% 25%	3087
August	1st to 10th	Night	19% 7% 3 7% 5% 5% 7% 5% 8% 10% 24%	2825
	11th to 20th	Night	8% 5% 3 7% 6% 6% 6% 5% 8% 11% 35%	2801
	21st to end	Night	6% 4 1 5% 4 4 5% 5% 8% 11% 47%	3476
September	1st to 10th	Night	9% 5% 2 7% 5% 5% 4 5% 7% 11% 39%	2043
	11th to 20th	Night	11% 7% 3 8% 7% 6% 5% 5% 5% 7% 37%	2090
	21st to end	Night	17% 7% 3 6% 4 5% 4 3 6% 7% 40%	2090
October	1st to 10th	Night	13% 6% 2 6% 3 3 3 2 4 6% 52%	2223
	11th to 20th	Night	11% 4 2 4 3 3 3 3 4 6% 57%	2280
	21st to end	Night	8% 5% 2 5% 4 3 3 4 5% 6% 57%	2435
November	1st to 10th	Night	12% 5% 1 4 3 3 3 3 5% 6% 56%	2221
	11th to 20th	Night	15% 7% 2 4 4 3 3 2 4 6% 51%	2310
	21st to end	Night	12% 6% 2 6% 4 4 4 4 5% 7% 48%	2470
December	1st to 10th	Night	15% 6% 2 6% 4 4 4 4 6% 7% 41%	2470
	11th to 20th	Night	17% 6% 2 5% 3 3 2 2 4 5% 50%	3042
	21st to end	Night	18% 7% 2 6% 3 4 3 3 5% 6% 40%	4532

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	36% 5% 7% 22% 23% 7%	2837
	11th to 20th	Day	34% 2 6% 20% 30% 8%	2840
	21st to end	Day	44% 5% 4 17% 20% 10%	3124
February	1st to 10th	Day	42% 4 9% 19% 20% 6%	1710
	11th to 20th	Day	41% 4 6% 18% 24% 8%	1843
	21st to end	Day	42% 5% 6% 18% 19% 10%	1674
March	1st to 10th	Day	50% 5% 7% 15% 14% 8%	3435
	11th to 20th	Day	30% 5% 9% 18% 26% 13%	3480
	21st to end	Day	42% 4 7% 17% 23% 6%	3815
April	1st to 10th	Day	33% 5% 10% 19% 25% 7%	2090
	11th to 20th	Day	35% 4 9% 21% 19% 12%	2254
	21st to end	Day	30% 5% 9% 27% 23% 5%	2316
May	1st to 10th	Day	37% 8% 14% 24% 14% 3	4120
	11th to 20th	Day	45% 6% 10% 21% 16% 1	4085
	21st to end	Day	31% 8% 16% 21% 22% 2	4489
June	1st to 10th	Day	45% 8% 16% 19% 11% 1	2470
	11th to 20th	Day	35% 8% 18% 26% 12% 1	2470
	21st to end	Day	38% 11% 18% 21% 11% 2	2470
July	1st to 10th	Day	51% 11% 16% 16% 5% 2	4055
	11th to 20th	Day	46% 13% 24% 13% 4 1	4051
	21st to end	Day	49% 14% 18% 15% 4	4324
August	1st to 10th	Day	55% 10% 20% 11% 3	4091
	11th to 20th	Day	41% 10% 18% 22% 9%	4033
	21st to end	Day	36% 8% 15% 29% 10% 1	4180
September	1st to 10th	Day	40% 9% 15% 16% 15% 5%	2131
	11th to 20th	Day	46% 6% 10% 16% 18% 4	2090
	21st to end	Day	43% 5% 11% 19% 17% 5%	2090
October	1st to 10th	Day	37% 4 9% 20% 24% 6%	1957
	11th to 20th	Day	28% 4 8% 22% 26% 11%	1900
	21st to end	Day	30% 3 7% 16% 31% 14%	2027
November	1st to 10th	Day	31% 4 6% 20% 26% 13%	1857
	11th to 20th	Day	38% 3 6% 19% 25% 11%	1867
	21st to end	Day	36% 3 8% 23% 21% 9%	1710
December	1st to 10th	Day	43% 5% 10% 18% 18% 6%	1710
	11th to 20th	Day	36% 4 5% 23% 24% 8%	2150
	21st to end	Day	43% 4 6% 19% 24% 4	3124
January	1st to 10th	Night	40% 4 6% 19% 22% 9%	4109
	11th to 20th	Night	39% 4 7% 19% 25% 6%	4120
	21st to end	Night	42% 5% 6% 13% 25% 9%	4532
February	1st to 10th	Night	41% 3 8% 19% 21% 8%	2470
	11th to 20th	Night	42% 4 7% 17% 21% 9%	2231
	21st to end	Night	38% 5% 10% 17% 17% 12%	1779
March	1st to 10th	Night	52% 5% 9% 13% 14% 7%	3418
	11th to 20th	Night	32% 4 8% 14% 24% 16%	3480
	21st to end	Night	38% 5% 9% 21% 18% 9%	3817
April	1st to 10th	Night	33% 5% 10% 20% 21% 10%	2088
	11th to 20th	Night	32% 5% 9% 21% 22% 11%	1875
	21st to end	Night	28% 6% 11% 26% 23% 5%	1878
May	1st to 10th	Night	33% 9% 15% 27% 13% 3	2840
	11th to 20th	Night	36% 8% 16% 21% 18% 1	2813
	21st to end	Night	28% 10% 15% 26% 20% 2	3087
June	1st to 10th	Night	38% 10% 19% 21% 11% 1	1710
	11th to 20th	Night	36% 11% 19% 24% 9% 1	1710
	21st to end	Night	35% 13% 22% 20% 8% 1	1710
July	1st to 10th	Night	48% 9% 18% 19% 4 2	2795
	11th to 20th	Night	43% 13% 27% 12% 4 1	2786
	21st to end	Night	51% 12% 18% 15% 4	2988
August	1st to 10th	Night	52% 10% 23% 13% 2	2814
	11th to 20th	Night	38% 12% 22% 20% 7%	2828
	21st to end	Night	29% 8% 19% 35% 8% 1	3476
September	1st to 10th	Night	37% 9% 18% 18% 12% 6%	2043
	11th to 20th	Night	43% 9% 12% 17% 16% 3	2090
	21st to end	Night	44% 7% 12% 19% 15% 4	2090
October	1st to 10th	Night	35% 4 9% 19% 27% 6%	2223
	11th to 20th	Night	29% 4 8% 21% 24% 14%	2280
	21st to end	Night	29% 4 7% 20% 27% 14%	2435
November	1st to 10th	Night	31% 3 8% 16% 27% 14%	2221
	11th to 20th	Night	36% 3 7% 17% 25% 13%	2310
	21st to end	Night	36% 4 8% 19% 21% 11%	2470
December	1st to 10th	Night	41% 6% 13% 16% 18% 7%	2470
	11th to 20th	Night	39% 3 6% 18% 26% 8%	3044
	21st to end	Night	44% 4 6% 19% 23% 4	4532

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	17% 12% 45% 20% 5% 1	270
	11th to 20th	Day	12% 12% 39% 26% 10% 1	270
	21st to end	Day	23% 13% 37% 23% 3 1	297
February	1st to 10th	Day	19% 10% 37% 25% 9% 1	270
	11th to 20th	Day	27% 10% 39% 19% 5%	270
	21st to end	Day	18% 12% 39% 25% 5% 1	329
March	1st to 10th	Day	16% 10% 31% 27% 15% 2	460
	11th to 20th	Day	15% 8% 41% 23% 11% 1	460
	21st to end	Day	17% 9% 35% 26% 11% 2	506
April	1st to 10th	Day	14% 12% 30% 28% 13% 3	460
	11th to 20th	Day	10% 10% 26% 24% 20% 9%	452
	21st to end	Day	10% 6% 23% 28% 19% 14%	460
May	1st to 10th	Day	6% 10% 27% 27% 19% 11%	463
	11th to 20th	Day	15% 2 14% 26% 27% 16%	487
	21st to end	Day	12% 2 12% 23% 26% 25%	529
June	1st to 10th	Day	12% 2 9% 25% 31% 20%	490
	11th to 20th	Day	10% 1 12% 24% 24% 28%	490
	21st to end	Day	7% 1 9% 26% 32% 25%	490
July	1st to 10th	Day	10% 2 4 17% 29% 38%	485
	11th to 20th	Day	10% 1 2 16% 36% 36%	484
	21st to end	Day	6% 3 6% 20% 40% 25%	532
August	1st to 10th	Day	15% 2 7% 16% 33% 28%	486
	11th to 20th	Day	10% 2 3 17% 36% 32%	479
	21st to end	Day	13% 2 6% 17% 33% 28%	539
September	1st to 10th	Day	10% 3 7% 23% 36% 21%	490
	11th to 20th	Day	18% 1 9% 25% 33% 14%	490
	21st to end	Day	20% 2 9% 26% 28% 15%	490
October	1st to 10th	Day	16% 3 17% 30% 23% 11%	490
	11th to 20th	Day	11% 3 21% 30% 25% 10%	490
	21st to end	Day	11% 4 21% 29% 26% 9%	514
November	1st to 10th	Day	16% 5% 24% 29% 18% 9%	455
	11th to 20th	Day	19% 5% 32% 30% 12% 3	427
	21st to end	Day	13% 6% 30% 31% 17% 4	270
December	1st to 10th	Day	20% 6% 31% 29% 12% 2	270
	11th to 20th	Day	20% 9% 33% 27% 8% 4	270
	21st to end	Day	21% 10% 33% 20% 13% 3	297
January	1st to 10th	Night	17% 11% 35% 31% 6% 1	760
	11th to 20th	Night	16% 12% 29% 34% 9%	760
	21st to end	Night	19% 10% 38% 31% 3	836
February	1st to 10th	Night	20% 8% 31% 34% 7%	760
	11th to 20th	Night	21% 11% 35% 30% 3	743
	21st to end	Night	15% 11% 37% 31% 5%	628
March	1st to 10th	Night	16% 8% 34% 34% 8%	760
	11th to 20th	Night	14% 8% 35% 33% 9%	760
	21st to end	Night	14% 9% 34% 34% 8%	836
April	1st to 10th	Night	13% 11% 31% 34% 10%	760
	11th to 20th	Night	13% 9% 26% 35% 16% 1	751
	21st to end	Night	10% 6% 29% 39% 14% 1	738
May	1st to 10th	Night	8% 4 24% 45% 18% 1	570
	11th to 20th	Night	12% 4 23% 36% 23% 3	561
	21st to end	Night	8% 2 9% 37% 37% 6%	617
June	1st to 10th	Night	12% 2 9% 41% 32% 5%	570
	11th to 20th	Night	12% 1 8% 32% 36% 11%	570
	21st to end	Night	7% 1 7% 36% 39% 10%	570
July	1st to 10th	Night	9% 3 4 29% 47% 8%	555
	11th to 20th	Night	8% 2 3 27% 50% 10%	552
	21st to end	Night	6% 2 4 35% 44% 9%	624
August	1st to 10th	Night	11% 1 3 29% 45% 10%	567
	11th to 20th	Night	9% 2 2 27% 50% 9%	567
	21st to end	Night	13% 2 5% 27% 42% 11%	836
September	1st to 10th	Night	11% 1 7% 28% 45% 8%	760
	11th to 20th	Night	13% 3 9% 30% 38% 7%	760
	21st to end	Night	20% 3 9% 34% 31% 4	760
October	1st to 10th	Night	16% 2 14% 33% 29% 5%	760
	11th to 20th	Night	13% 4 21% 34% 25% 3	760
	21st to end	Night	11% 5% 21% 34% 26% 3	812
November	1st to 10th	Night	15% 6% 21% 35% 19% 4	740
	11th to 20th	Night	19% 6% 26% 35% 14% 1	760
	21st to end	Night	14% 6% 28% 33% 17% 2	760
December	1st to 10th	Night	18% 7% 29% 32% 13%	760
	11th to 20th	Night	19% 6% 26% 34% 13% 1	760
	21st to end	Night	22% 9% 27% 31% 10% 1	836

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