Southerton Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Southerton Observatory clear sky chart from 2005-11-06 to 2025-02-25.

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	15% 8% 3 7% 5% 5% 5% 4 6% 7% 36%	108760
	Night	16% 8% 3 7% 5% 4 4 4 5% 7% 38%	109905
Transparency	Day	46% 5% 11% 23% 13% 2	112396
	Night	45% 5% 12% 22% 14% 2	114432
Seeing	Day	21% 10% 25% 24% 14% 6%	15093
	Night	20% 7% 26% 33% 13% 1	24606

By Month

Percentage Cloud Cover NE (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% 10% 4 10% 5% 4 5% 3 5% 5% 37%	3597
	11th to 20th	Day	12% 12% 6% 14% 7% 7% 6% 4 5% 5% 23%	3860
	21st to end	Day	16% 12% 5% 9% 6% 4 4 3 4 6% 31%	4466
February	1st to 10th	Day	16% 13% 3 9% 5% 5% 5% 4 5% 7% 28%	1800
	11th to 20th	Day	13% 10% 3 8% 6% 5% 5% 4 5% 8% 32%	1782
	21st to end	Day	18% 9% 4 9% 6% 4 4 4 5% 6% 32%	1610
March	1st to 10th	Day	23% 9% 2 6% 5% 4 4 3 5% 6% 33%	3303
	11th to 20th	Day	10% 8% 3 7% 6% 4 5% 3 5% 7% 41%	3480
	21st to end	Day	17% 10% 3 7% 4 3 3 3 4 6% 41%	3910
April	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 6% 36%	2280
	11th to 20th	Day	16% 9% 3 7% 4 5% 4 3 5% 6% 38%	2254
	21st to end	Day	17% 6% 3 6% 6% 4 5% 4 6% 6% 37%	2290
May	1st to 10th	Day	27% 6% 2 6% 4 4 4 3 5% 7% 31%	4167
	11th to 20th	Day	22% 9% 3 8% 6% 5% 4 4 5% 6% 28%	4398
	21st to end	Day	13% 5% 2 5% 4 4 5% 4 7% 9% 42%	4837
June	1st to 10th	Day	12% 6% 3 7% 6% 5% 5% 4 7% 8% 37%	2660
	11th to 20th	Day	13% 5% 2 6% 6% 6% 5% 4 6% 8% 39%	2660
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 7% 10% 41%	2660
July	1st to 10th	Day	6% 5% 2 7% 6% 6% 6% 5% 8% 11% 38%	4440
	11th to 20th	Day	6% 5% 2 7% 4 6% 6% 5% 8% 11% 38%	4440
	21st to end	Day	10% 7% 3 8% 6% 6% 6% 5% 7% 8% 34%	4570
August	1st to 10th	Day	19% 6% 3 7% 5% 5% 5% 4 6% 9% 31%	4104
	11th to 20th	Day	15% 5% 2 6% 5% 5% 7% 4 7% 9% 35%	3926
	21st to end	Day	15% 5% 2 5% 4 4 4 3 6% 9% 43%	4180
September	1st to 10th	Day	16% 5% 2 5% 4 3 5% 3 4 6% 45%	2280
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 4 8% 50%	2280
	21st to end	Day	22% 7% 2 5% 3 4 4 3 5% 6% 38%	2005
October	1st to 10th	Day	18% 7% 2 5% 5% 4 5% 2 5% 6% 41%	1900
	11th to 20th	Day	18% 4 2 6% 5% 4 5% 4 6% 6% 39%	1900
	21st to end	Day	18% 7% 2 6% 4 5% 5% 4 6% 5% 38%	2090
November	1st to 10th	Day	12% 6% 2 6% 4 4 4 3 6% 6% 47%	1775
	11th to 20th	Day	16% 7% 3 6% 5% 4 4 4 6% 6% 38%	1710
	21st to end	Day	17% 8% 3 6% 5% 4 4 5% 6% 6% 36%	1600
December	1st to 10th	Day	16% 11% 4 9% 5% 6% 5% 3 5% 7% 30%	2600
	11th to 20th	Day	23% 10% 3 7% 4 4 4 3 4 6% 33%	2986
	21st to end	Day	19% 11% 4 9% 6% 6% 6% 4 6% 6% 24%	3960
January	1st to 10th	Night	13% 8% 3 9% 5% 5% 5% 4 5% 6% 36%	6349
	11th to 20th	Night	12% 9% 4 9% 6% 5% 5% 4 6% 8% 33%	6024
	21st to end	Night	17% 9% 3 8% 6% 6% 5% 4 5% 5% 33%	6490
February	1st to 10th	Night	16% 10% 3 8% 6% 4 4 4 5% 7% 34%	2600
	11th to 20th	Night	15% 9% 3 8% 5% 5% 5% 3 6% 7% 34%	2503
	21st to end	Night	17% 9% 3 6% 6% 5% 5% 4 5% 5% 35%	1939
March	1st to 10th	Night	22% 10% 2 7% 4 4 4 3 4 5% 34%	3532
	11th to 20th	Night	11% 9% 3 8% 5% 4 4 4 5% 6% 42%	3479
	21st to end	Night	15% 9% 2 6% 5% 4 4 3 4 5% 44%	3722
April	1st to 10th	Night	17% 9% 3 7% 5% 4 4 4 5% 7% 34%	1900
	11th to 20th	Night	16% 7% 2 6% 5% 4 4 4 6% 8% 38%	1875
	21st to end	Night	18% 5% 2 6% 5% 4 4 4 5% 6% 41%	1900
May	1st to 10th	Night	28% 8% 3 6% 4 4 3 3 4 6% 32%	2778
	11th to 20th	Night	24% 8% 3 6% 4 5% 4 4 5% 6% 29%	2495
	21st to end	Night	12% 6% 2 6% 4 4 4 4 6% 9% 42%	2738
June	1st to 10th	Night	15% 5% 2 6% 5% 4 4 3 6% 8% 43%	1520
	11th to 20th	Night	12% 5% 2 5% 4 4 5% 4 7% 8% 43%	1520
	21st to end	Night	10% 4 2 5% 5% 4 5% 5% 7% 9% 43%	1520
July	1st to 10th	Night	8% 6% 3 7% 5% 5% 5% 4 6% 8% 44%	2520
	11th to 20th	Night	7% 5% 2 6% 4 5% 6% 5% 8% 10% 41%	2520
	21st to end	Night	13% 7% 2 5% 4 5% 5% 4 5% 7% 42%	2979
August	1st to 10th	Night	19% 6% 2 6% 4 5% 4 3 6% 8% 35%	2825
	11th to 20th	Night	15% 7% 2 6% 5% 4 4 4 5% 7% 43%	2871
	21st to end	Night	15% 4 2 4 4 3 5% 3 7% 9% 44%	3476
September	1st to 10th	Night	15% 5% 2 6% 4 4 3 4 4 6% 46%	1900
	11th to 20th	Night	13% 5% 2 5% 3 4 3 3 5% 7% 52%	1900
	21st to end	Night	20% 6% 2 5% 3 4 4 3 5% 7% 41%	2171
October	1st to 10th	Night	20% 6% 2 6% 3 3 3 3 5% 7% 42%	2280
	11th to 20th	Night	16% 6% 2 5% 3 3 3 4 5% 8% 44%	2280
	21st to end	Night	19% 7% 2 5% 4 4 4 4 5% 6% 41%	2508
November	1st to 10th	Night	11% 6% 2 6% 3 3 4 3 6% 5% 51%	2522
	11th to 20th	Night	18% 8% 2 6% 4 4 4 3 5% 6% 39%	2685
	21st to end	Night	16% 8% 2 5% 5% 5% 4 4 6% 7% 37%	2800
December	1st to 10th	Night	12% 9% 3 8% 5% 5% 5% 4 5% 7% 37%	4580
	11th to 20th	Night	23% 8% 2 6% 4 4 4 4 4 6% 35%	5208
	21st to end	Night	20% 12% 3 8% 4 5% 5% 4 5% 6% 28%	6996

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	49% 3 11% 21% 13% 2	3598
	11th to 20th	Day	62% 3 9% 14% 9% 3	3894
	21st to end	Day	55% 4 11% 19% 10% 1	4466
February	1st to 10th	Day	56% 4 10% 18% 11% 1	1800
	11th to 20th	Day	51% 5% 9% 24% 10% 2	1791
	21st to end	Day	55% 3 9% 21% 9% 2	1610
March	1st to 10th	Day	52% 4 9% 26% 8% 2	3306
	11th to 20th	Day	42% 5% 13% 24% 12% 4	3480
	21st to end	Day	46% 4 9% 22% 17% 2	3910
April	1st to 10th	Day	50% 4 9% 21% 13% 4	2280
	11th to 20th	Day	46% 5% 9% 21% 15% 3	2254
	21st to end	Day	45% 4 9% 22% 15% 5%	2290
May	1st to 10th	Day	53% 5% 8% 21% 12% 2	4179
	11th to 20th	Day	56% 6% 11% 17% 8% 1	4402
	21st to end	Day	37% 7% 12% 27% 16% 3	4837
June	1st to 10th	Day	43% 6% 11% 24% 15% 2	2660
	11th to 20th	Day	42% 6% 13% 22% 14% 2	2660
	21st to end	Day	37% 6% 12% 28% 15% 1	2660
July	1st to 10th	Day	38% 7% 15% 27% 12% 2	4370
	11th to 20th	Day	35% 8% 16% 25% 15% 1	4365
	21st to end	Day	45% 7% 10% 19% 17% 1	4424
August	1st to 10th	Day	49% 6% 13% 20% 11%	4091
	11th to 20th	Day	43% 8% 12% 26% 10% 1	3965
	21st to end	Day	37% 6% 11% 31% 13% 2	4180
September	1st to 10th	Day	40% 5% 11% 23% 18% 3	2280
	11th to 20th	Day	34% 4 11% 26% 22% 3	2280
	21st to end	Day	47% 4 10% 19% 18% 2	2005
October	1st to 10th	Day	40% 4 10% 24% 18% 4	3160
	11th to 20th	Day	37% 4 10% 26% 22% 1	3140
	21st to end	Day	31% 4 14% 32% 18% 1	3428
November	1st to 10th	Day	38% 4 11% 26% 18% 3	1775
	11th to 20th	Day	46% 4 10% 24% 14% 2	1710
	21st to end	Day	47% 4 12% 27% 9% 1	1600
December	1st to 10th	Day	55% 5% 12% 21% 7% 2	2600
	11th to 20th	Day	53% 3 15% 22% 7%	2986
	21st to end	Day	58% 6% 14% 14% 7% 1	3960
January	1st to 10th	Night	48% 4 12% 20% 14% 2	6349
	11th to 20th	Night	48% 5% 15% 17% 11% 5%	6065
	21st to end	Night	52% 5% 10% 17% 14% 2	6490
February	1st to 10th	Night	50% 5% 13% 23% 9% 1	2600
	11th to 20th	Night	50% 5% 10% 21% 13% 2	2505
	21st to end	Night	51% 4 10% 20% 13% 2	1939
March	1st to 10th	Night	53% 4 8% 23% 10% 2	3542
	11th to 20th	Night	42% 6% 12% 27% 12% 1	3480
	21st to end	Night	44% 4 7% 21% 19% 5%	3722
April	1st to 10th	Night	50% 4 9% 19% 15% 2	1900
	11th to 20th	Night	43% 6% 9% 22% 17% 3	1875
	21st to end	Night	42% 6% 9% 23% 17% 3	1900
May	1st to 10th	Night	55% 5% 8% 20% 10% 3	2785
	11th to 20th	Night	53% 9% 12% 14% 11% 1	2496
	21st to end	Night	38% 7% 16% 20% 14% 5%	2739
June	1st to 10th	Night	39% 6% 10% 24% 18% 2	1520
	11th to 20th	Night	38% 7% 14% 24% 16% 2	1520
	21st to end	Night	35% 6% 13% 29% 16% 2	1520
July	1st to 10th	Night	38% 7% 14% 19% 18% 4	2480
	11th to 20th	Night	37% 8% 13% 25% 15% 1	2472
	21st to end	Night	42% 6% 12% 17% 21% 2	2884
August	1st to 10th	Night	47% 8% 13% 22% 10%	2814
	11th to 20th	Night	41% 6% 13% 25% 15% 1	2898
	21st to end	Night	36% 6% 12% 32% 14% 1	3476
September	1st to 10th	Night	39% 5% 10% 25% 18% 3	1900
	11th to 20th	Night	33% 4 9% 31% 22% 2	1900
	21st to end	Night	43% 4 8% 22% 20% 1	2171
October	1st to 10th	Night	41% 6% 11% 21% 21% 1	3800
	11th to 20th	Night	32% 4 10% 29% 23% 1	3779
	21st to end	Night	32% 4 13% 30% 20% 2	4120
November	1st to 10th	Night	36% 4 12% 27% 19% 3	2522
	11th to 20th	Night	46% 4 9% 23% 15% 3	2685
	21st to end	Night	46% 4 14% 25% 10% 2	2800
December	1st to 10th	Night	46% 5% 14% 26% 7%	4580
	11th to 20th	Night	52% 4 14% 20% 10% 1	5208
	21st to end	Night	56% 7% 11% 17% 9%	6996

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	31% 18% 28% 19% 4 2	200
	11th to 20th	Day	26% 21% 30% 21% 3	263
	21st to end	Day	25% 15% 35% 15% 8% 1	330
February	1st to 10th	Day	24% 19% 37% 14% 4	300
	11th to 20th	Day	29% 19% 36% 16% 1	329
	21st to end	Day	30% 13% 30% 22% 5%	398
March	1st to 10th	Day	26% 17% 28% 23% 6% 1	460
	11th to 20th	Day	24% 18% 30% 22% 4 2	460
	21st to end	Day	28% 18% 26% 19% 5% 3	506
April	1st to 10th	Day	25% 17% 27% 21% 9% 2	460
	11th to 20th	Day	23% 17% 25% 17% 11% 6%	452
	21st to end	Day	27% 11% 28% 20% 11% 3	460
May	1st to 10th	Day	31% 10% 26% 18% 12% 4	463
	11th to 20th	Day	22% 11% 26% 22% 13% 6%	487
	21st to end	Day	20% 5% 21% 26% 18% 10%	529
June	1st to 10th	Day	15% 9% 26% 26% 19% 6%	490
	11th to 20th	Day	16% 9% 22% 27% 21% 5%	490
	21st to end	Day	14% 6% 26% 32% 17% 5%	490
July	1st to 10th	Day	11% 2 22% 31% 24% 11%	485
	11th to 20th	Day	7% 4 20% 34% 26% 10%	484
	21st to end	Day	14% 4 21% 31% 24% 6%	532
August	1st to 10th	Day	14% 2 19% 26% 27% 11%	486
	11th to 20th	Day	16% 3 18% 27% 23% 14%	479
	21st to end	Day	16% 3 17% 27% 23% 13%	539
September	1st to 10th	Day	20% 6% 18% 23% 20% 13%	490
	11th to 20th	Day	16% 3 15% 27% 24% 15%	490
	21st to end	Day	29% 7% 17% 23% 16% 8%	490
October	1st to 10th	Day	25% 6% 20% 23% 18% 8%	490
	11th to 20th	Day	20% 12% 23% 28% 14% 2	490
	21st to end	Day	23% 9% 30% 25% 11% 1	539
November	1st to 10th	Day	19% 10% 26% 27% 15% 2	402
	11th to 20th	Day	23% 13% 29% 23% 11% 1	310
	21st to end	Day	25% 17% 32% 17% 9% 1	200
December	1st to 10th	Day	28% 14% 34% 17% 6% 2	200
	11th to 20th	Day	31% 11% 36% 17% 6% 1	200
	21st to end	Day	26% 12% 32% 22% 7%	220
January	1st to 10th	Night	21% 14% 36% 26% 4	800
	11th to 20th	Night	24% 11% 30% 32% 4	800
	21st to end	Night	18% 10% 38% 31% 3	880
February	1st to 10th	Night	22% 10% 32% 33% 3	800
	11th to 20th	Night	23% 12% 40% 22% 2	783
	21st to end	Night	25% 11% 35% 25% 4	647
March	1st to 10th	Night	22% 13% 31% 29% 5%	760
	11th to 20th	Night	25% 11% 30% 28% 6%	760
	21st to end	Night	22% 10% 35% 26% 6% 1	836
April	1st to 10th	Night	23% 12% 32% 27% 6%	760
	11th to 20th	Night	22% 7% 32% 30% 9% 1	751
	21st to end	Night	21% 8% 29% 34% 8%	760
May	1st to 10th	Night	25% 5% 23% 33% 13% 1	541
	11th to 20th	Night	19% 3 28% 40% 9% 1	374
	21st to end	Night	17% 3 21% 38% 21% 1	411
June	1st to 10th	Night	17% 3 19% 41% 19% 1	380
	11th to 20th	Night	14% 3 20% 46% 17% 1	380
	21st to end	Night	12% 1 19% 50% 17% 1	380
July	1st to 10th	Night	14% 2 15% 43% 24% 2	370
	11th to 20th	Night	10% 2 12% 50% 25% 1	368
	21st to end	Night	13% 2 14% 44% 25% 2	555
August	1st to 10th	Night	15% 1 8% 43% 30% 3	567
	11th to 20th	Night	15% 2 9% 36% 34% 3	611
	21st to end	Night	15% 3 11% 37% 31% 3	836
September	1st to 10th	Night	17% 3 18% 34% 26% 2	760
	11th to 20th	Night	16% 3 12% 33% 33% 4	760
	21st to end	Night	23% 4 16% 34% 22% 3	760
October	1st to 10th	Night	23% 4 18% 31% 20% 3	760
	11th to 20th	Night	20% 5% 22% 38% 13% 1	760
	21st to end	Night	24% 6% 26% 34% 10%	836
November	1st to 10th	Night	15% 7% 28% 37% 12% 1	780
	11th to 20th	Night	24% 9% 27% 30% 9% 1	800
	21st to end	Night	22% 8% 30% 34% 6% 1	800
December	1st to 10th	Night	23% 11% 33% 26% 7%	800
	11th to 20th	Night	23% 9% 31% 32% 6%	800
	21st to end	Night	26% 8% 28% 33% 4	880

Page generated at 2025-02-22 06:51:34. Version ac07

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