Sparta
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sparta clear sky chart from 2005-11-06 to 2025-04-26.

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	15% 8% 3 7% 4 4 4 4 5% 7% 38%	113862
	Night	16% 9% 3 6% 4 4 4 3 5% 6% 41%	114792
Transparency	Day	44% 6% 10% 21% 16% 3	117618
	Night	42% 5% 9% 22% 17% 4	119163
Seeing	Day	20% 11% 23% 22% 16% 8%	15211
	Night	18% 9% 20% 30% 19% 3	25322

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	17% 15% 7% 11% 3 3 4 3 4 4 29%	4056
	11th to 20th	Day	19% 11% 7% 10% 3 3 3 2 3 4 36%	4060
	21st to end	Day	23% 10% 5% 6% 3 3 3 3 4 6% 36%	4678
February	1st to 10th	Day	21% 12% 4 7% 4 4 3 3 4 4 33%	2000
	11th to 20th	Day	23% 10% 5% 8% 5% 4 3 3 4 5% 30%	1948
	21st to end	Day	23% 10% 3 6% 4 4 3 3 4 5% 34%	1650
March	1st to 10th	Day	21% 10% 4 7% 4 4 4 3 4 5% 34%	4480
	11th to 20th	Day	14% 8% 3 6% 3 4 3 3 5% 5% 45%	4929
	21st to end	Day	15% 8% 2 6% 4 4 4 4 5% 5% 42%	5496
April	1st to 10th	Day	16% 6% 2 6% 5% 4 4 3 5% 7% 41%	2400
	11th to 20th	Day	16% 8% 2 5% 4 4 4 3 5% 7% 43%	2374
	21st to end	Day	18% 6% 2 6% 5% 5% 6% 3 5% 7% 38%	2460
May	1st to 10th	Day	20% 7% 2 6% 4 5% 4 4 6% 7% 34%	4082
	11th to 20th	Day	19% 8% 3 7% 4 4 4 4 5% 6% 34%	4081
	21st to end	Day	14% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	4476
June	1st to 10th	Day	12% 6% 3 6% 5% 5% 5% 5% 6% 9% 39%	2470
	11th to 20th	Day	6% 5% 2 5% 5% 5% 6% 5% 8% 9% 44%	2470
	21st to end	Day	8% 5% 2 6% 5% 6% 5% 6% 8% 12% 37%	2470
July	1st to 10th	Day	8% 5% 2 7% 6% 5% 6% 6% 9% 11% 35%	4120
	11th to 20th	Day	12% 6% 3 6% 5% 5% 6% 5% 7% 9% 37%	4120
	21st to end	Day	19% 10% 4 10% 6% 6% 6% 5% 6% 7% 20%	4466
August	1st to 10th	Day	8% 4 2 5% 4 5% 5% 5% 7% 11% 44%	4104
	11th to 20th	Day	9% 4 2 6% 6% 6% 6% 5% 7% 12% 37%	3729
	21st to end	Day	7% 4 2 5% 4 5% 5% 5% 7% 9% 46%	4180
September	1st to 10th	Day	14% 5% 2 6% 5% 5% 5% 4 6% 7% 39%	2182
	11th to 20th	Day	15% 6% 3 6% 5% 4 5% 4 5% 6% 41%	2090
	21st to end	Day	15% 6% 2 5% 4 4 3 3 5% 6% 46%	2090
October	1st to 10th	Day	11% 4 2 5% 3 4 3 3 5% 6% 53%	2090
	11th to 20th	Day	14% 5% 2 4 3 3 3 3 4 6% 52%	2028
	21st to end	Day	17% 6% 1 5% 3 4 3 4 5% 6% 47%	1952
November	1st to 10th	Day	12% 6% 1 4 2 4 4 3 5% 5% 54%	1762
	11th to 20th	Day	15% 8% 3 6% 3 3 4 3 5% 6% 47%	1800
	21st to end	Day	17% 10% 3 5% 4 3 4 3 5% 5% 43%	1800
December	1st to 10th	Day	18% 9% 3 7% 5% 4 4 2 4 4 41%	2930
	11th to 20th	Day	21% 15% 5% 8% 4 4 3 3 5% 5% 27%	3373
	21st to end	Day	19% 13% 5% 8% 3 4 4 3 5% 7% 28%	4466
January	1st to 10th	Night	17% 15% 4 9% 3 3 3 2 3 5% 35%	5889
	11th to 20th	Night	20% 12% 3 6% 5% 5% 3 2 3 4 38%	5900
	21st to end	Night	20% 11% 3 7% 4 3 3 2 3 4 39%	6260
February	1st to 10th	Night	22% 10% 3 8% 4 4 4 3 4 6% 32%	2400
	11th to 20th	Night	25% 10% 4 8% 4 3 4 2 3 4 32%	2338
	21st to end	Night	25% 12% 3 6% 4 3 3 3 5% 5% 32%	1980
March	1st to 10th	Night	22% 12% 3 6% 3 3 4 3 4 6% 33%	5373
	11th to 20th	Night	15% 6% 2 4 3 3 4 4 6% 6% 48%	5031
	21st to end	Night	16% 8% 3 7% 4 4 4 3 4 6% 40%	5467
April	1st to 10th	Night	15% 6% 2 5% 4 4 4 4 6% 7% 44%	2009
	11th to 20th	Night	16% 6% 2 5% 4 3 3 3 4 7% 48%	1975
	21st to end	Night	15% 8% 2 6% 4 3 3 3 6% 8% 43%	1836
May	1st to 10th	Night	17% 7% 3 6% 5% 5% 5% 4 6% 8% 36%	2813
	11th to 20th	Night	19% 8% 2 6% 5% 4 4 4 5% 7% 38%	2812
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 5% 7% 9% 42%	3079
June	1st to 10th	Night	8% 6% 2 7% 5% 5% 4 4 6% 9% 43%	1710
	11th to 20th	Night	4 4 2 4 3 3 4 4 7% 13% 51%	1710
	21st to end	Night	6% 4 2 5% 5% 6% 5% 6% 8% 11% 42%	1710
July	1st to 10th	Night	8% 5% 3 6% 5% 5% 6% 5% 8% 11% 37%	2840
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 4 7% 10% 44%	2840
	21st to end	Night	12% 10% 4 10% 7% 6% 7% 5% 6% 7% 25%	3087
August	1st to 10th	Night	6% 4 1 5% 4 4 5% 4 6% 10% 52%	2825
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 5% 6% 10% 45%	3095
	21st to end	Night	5% 3 2 3 3 4 4 4 6% 9% 57%	3476
September	1st to 10th	Night	11% 5% 3 7% 4 4 4 4 5% 6% 47%	1977
	11th to 20th	Night	11% 7% 2 6% 5% 4 5% 3 5% 7% 45%	2090
	21st to end	Night	11% 5% 2 5% 3 3 3 3 6% 6% 53%	2090
October	1st to 10th	Night	11% 4 1 4 3 3 2 3 4 6% 59%	2090
	11th to 20th	Night	17% 5% 2 5% 3 3 3 3 4 5% 51%	2151
	21st to end	Night	16% 7% 2 4 2 3 3 3 5% 5% 50%	2641
November	1st to 10th	Night	14% 7% 2 4 4 3 3 3 4 5% 51%	2536
	11th to 20th	Night	14% 9% 2 4 3 3 3 3 5% 7% 48%	2600
	21st to end	Night	18% 9% 3 5% 3 2 3 3 4 5% 43%	2600
December	1st to 10th	Night	22% 11% 3 7% 4 3 4 3 4 4 36%	4250
	11th to 20th	Night	26% 12% 4 7% 3 3 2 2 3 4 34%	4822
	21st to end	Night	16% 15% 5% 7% 5% 5% 4 3 5% 6% 27%	6490

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	57% 5% 7% 16% 13% 1	4057
	11th to 20th	Day	53% 3 3 15% 24% 2	4060
	21st to end	Day	50% 4 7% 22% 14% 4	4678
February	1st to 10th	Day	55% 4 7% 15% 17% 2	2000
	11th to 20th	Day	56% 4 7% 16% 13% 3	1958
	21st to end	Day	52% 4 8% 14% 17% 5%	1650
March	1st to 10th	Day	52% 4 5% 14% 16% 8%	4480
	11th to 20th	Day	42% 3 6% 19% 21% 9%	4929
	21st to end	Day	44% 4 9% 27% 15% 1	5498
April	1st to 10th	Day	42% 6% 10% 22% 17% 3	2400
	11th to 20th	Day	41% 5% 8% 20% 23% 3	2374
	21st to end	Day	45% 5% 10% 26% 14% 1	2460
May	1st to 10th	Day	47% 8% 11% 23% 11% 1	4120
	11th to 20th	Day	50% 7% 12% 22% 9%	4085
	21st to end	Day	42% 8% 11% 21% 17% 2	4489
June	1st to 10th	Day	41% 9% 14% 22% 13% 2	2470
	11th to 20th	Day	33% 9% 18% 23% 15% 2	2470
	21st to end	Day	36% 10% 18% 19% 13% 3	2470
July	1st to 10th	Day	38% 12% 20% 20% 8% 2	4055
	11th to 20th	Day	41% 8% 18% 25% 7% 2	4051
	21st to end	Day	60% 9% 10% 15% 5% 1	4324
August	1st to 10th	Day	31% 9% 16% 32% 12% 1	4091
	11th to 20th	Day	37% 8% 17% 24% 13% 1	3765
	21st to end	Day	32% 8% 13% 31% 15% 1	4179
September	1st to 10th	Day	42% 8% 11% 18% 16% 5%	2182
	11th to 20th	Day	43% 6% 12% 20% 17% 2	2090
	21st to end	Day	38% 6% 12% 21% 18% 4	2090
October	1st to 10th	Day	24% 5% 6% 19% 34% 11%	3480
	11th to 20th	Day	24% 3 6% 24% 31% 12%	3346
	21st to end	Day	29% 4 9% 23% 25% 10%	3189
November	1st to 10th	Day	32% 4 9% 24% 27% 5%	1762
	11th to 20th	Day	39% 4 5% 23% 23% 6%	1800
	21st to end	Day	44% 4 6% 22% 21% 4	1800
December	1st to 10th	Day	47% 5% 4 20% 23% 2	2930
	11th to 20th	Day	59% 4 7% 17% 11% 2	3370
	21st to end	Day	54% 4 12% 19% 10% 2	4466
January	1st to 10th	Night	53% 3 8% 18% 16% 1	5889
	11th to 20th	Night	52% 3 5% 15% 20% 5%	5900
	21st to end	Night	50% 3 5% 17% 19% 6%	6260
February	1st to 10th	Night	53% 4 9% 17% 15% 2	2400
	11th to 20th	Night	58% 3 6% 16% 12% 5%	2339
	21st to end	Night	53% 5% 6% 16% 15% 4	1980
March	1st to 10th	Night	53% 4 6% 15% 17% 4	5373
	11th to 20th	Night	35% 4 8% 21% 20% 12%	5031
	21st to end	Night	47% 4 7% 27% 13% 2	5467
April	1st to 10th	Night	39% 6% 10% 23% 19% 3	2009
	11th to 20th	Night	37% 4 9% 22% 22% 6%	1975
	21st to end	Night	40% 5% 10% 29% 14% 1	1836
May	1st to 10th	Night	42% 11% 12% 23% 11% 1	2840
	11th to 20th	Night	45% 7% 13% 21% 12% 1	2813
	21st to end	Night	34% 10% 15% 25% 16% 1	3087
June	1st to 10th	Night	37% 7% 15% 27% 12% 2	1710
	11th to 20th	Night	22% 9% 22% 29% 15% 2	1710
	21st to end	Night	32% 11% 16% 25% 13% 4	1710
July	1st to 10th	Night	36% 11% 21% 23% 8% 1	2795
	11th to 20th	Night	32% 8% 18% 31% 8% 3	2786
	21st to end	Night	55% 11% 12% 15% 7% 1	2988
August	1st to 10th	Night	26% 7% 13% 37% 15% 1	2814
	11th to 20th	Night	32% 8% 21% 24% 13% 1	3125
	21st to end	Night	23% 7% 12% 39% 17% 1	3476
September	1st to 10th	Night	36% 8% 13% 21% 18% 5%	1977
	11th to 20th	Night	37% 7% 11% 21% 22% 2	2090
	21st to end	Night	32% 6% 11% 23% 24% 5%	2090
October	1st to 10th	Night	23% 4 5% 24% 32% 13%	3480
	11th to 20th	Night	24% 3 5% 23% 29% 17%	3572
	21st to end	Night	24% 3 7% 26% 29% 11%	4341
November	1st to 10th	Night	36% 4 6% 22% 24% 7%	2536
	11th to 20th	Night	37% 3 7% 20% 25% 7%	2600
	21st to end	Night	44% 4 6% 23% 20% 4	2600
December	1st to 10th	Night	52% 4 5% 14% 23% 2	4250
	11th to 20th	Night	55% 3 5% 19% 16% 2	4824
	21st to end	Night	56% 5% 10% 19% 9% 1	6490

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	29% 23% 32% 12% 3 1	300
	11th to 20th	Day	35% 25% 24% 12% 4	300
	21st to end	Day	25% 25% 38% 9% 3	330
February	1st to 10th	Day	28% 24% 30% 15% 3	300
	11th to 20th	Day	38% 24% 30% 5% 2	300
	21st to end	Day	35% 23% 33% 8% 1 1	249
March	1st to 10th	Day	28% 22% 34% 13% 3	300
	11th to 20th	Day	29% 20% 29% 17% 4 1	475
	21st to end	Day	27% 19% 32% 16% 6%	550
April	1st to 10th	Day	24% 18% 27% 21% 9% 1	500
	11th to 20th	Day	22% 16% 30% 23% 8% 2	492
	21st to end	Day	24% 13% 28% 22% 12% 1	480
May	1st to 10th	Day	22% 12% 30% 20% 11% 5%	463
	11th to 20th	Day	17% 8% 25% 26% 15% 8%	487
	21st to end	Day	15% 8% 19% 28% 17% 14%	529
June	1st to 10th	Day	14% 6% 20% 26% 21% 14%	490
	11th to 20th	Day	7% 4 15% 31% 25% 18%	490
	21st to end	Day	10% 5% 19% 25% 24% 17%	490
July	1st to 10th	Day	14% 3 11% 25% 27% 21%	485
	11th to 20th	Day	13% 4 9% 24% 26% 24%	484
	21st to end	Day	14% 4 9% 26% 27% 20%	532
August	1st to 10th	Day	14% 4 11% 23% 29% 18%	486
	11th to 20th	Day	10% 4 15% 33% 29% 10%	479
	21st to end	Day	13% 3 9% 25% 28% 22%	539
September	1st to 10th	Day	14% 4 9% 24% 34% 15%	490
	11th to 20th	Day	16% 4 9% 29% 29% 14%	490
	21st to end	Day	16% 6% 16% 26% 29% 8%	490
October	1st to 10th	Day	13% 4 19% 33% 23% 8%	490
	11th to 20th	Day	15% 10% 26% 27% 17% 5%	490
	21st to end	Day	22% 10% 27% 26% 13% 2	401
November	1st to 10th	Day	18% 12% 32% 25% 10% 3	300
	11th to 20th	Day	18% 17% 35% 25% 5%	300
	21st to end	Day	22% 14% 37% 16% 8% 2	300
December	1st to 10th	Day	29% 18% 34% 14% 5% 1	300
	11th to 20th	Day	31% 17% 28% 17% 7% 1	300
	21st to end	Day	29% 18% 31% 17% 4 1	330
January	1st to 10th	Night	25% 18% 29% 23% 5%	800
	11th to 20th	Night	30% 19% 25% 22% 5%	800
	21st to end	Night	23% 16% 32% 26% 3	880
February	1st to 10th	Night	25% 14% 32% 26% 3	800
	11th to 20th	Night	30% 17% 31% 20% 2	783
	21st to end	Night	31% 18% 28% 22% 2	660
March	1st to 10th	Night	25% 14% 30% 27% 4	800
	11th to 20th	Night	25% 14% 28% 25% 7% 1	800
	21st to end	Night	21% 14% 29% 28% 8%	880
April	1st to 10th	Night	19% 11% 29% 31% 10%	609
	11th to 20th	Night	19% 9% 30% 31% 10% 1	593
	21st to end	Night	20% 7% 26% 34% 12% 1	587
May	1st to 10th	Night	16% 6% 25% 33% 18% 2	570
	11th to 20th	Night	14% 5% 18% 36% 23% 4	561
	21st to end	Night	8% 3 13% 37% 33% 6%	617
June	1st to 10th	Night	9% 3 12% 37% 35% 5%	570
	11th to 20th	Night	3 2 11% 36% 38% 10%	570
	21st to end	Night	6% 2 8% 38% 37% 8%	570
July	1st to 10th	Night	11% 3 5% 26% 46% 10%	555
	11th to 20th	Night	9% 2 7% 27% 43% 12%	552
	21st to end	Night	9% 3 5% 30% 44% 10%	624
August	1st to 10th	Night	9% 1 5% 27% 49% 9%	567
	11th to 20th	Night	9% 2 6% 35% 43% 5%	564
	21st to end	Night	10% 2 5% 27% 49% 8%	627
September	1st to 10th	Night	12% 2 7% 33% 38% 8%	647
	11th to 20th	Night	14% 2 11% 32% 35% 6%	760
	21st to end	Night	13% 4 13% 34% 31% 6%	760
October	1st to 10th	Night	12% 5% 15% 33% 27% 8%	760
	11th to 20th	Night	19% 6% 22% 32% 19% 2	760
	21st to end	Night	18% 9% 22% 33% 17% 2	836
November	1st to 10th	Night	16% 8% 20% 37% 17% 2	780
	11th to 20th	Night	19% 12% 26% 31% 10% 2	800
	21st to end	Night	24% 11% 25% 29% 11% 1	800
December	1st to 10th	Night	29% 14% 25% 26% 7%	800
	11th to 20th	Night	26% 13% 24% 29% 7% 1	800
	21st to end	Night	26% 17% 24% 27% 5%	880

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