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 2026-07-01.

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% 7% 3 7% 5% 5% 5% 4 6% 7% 36%	138163
	Night	16% 8% 3 7% 5% 4 4 4 5% 7% 38%	134170
Transparency	Day	46% 5% 11% 22% 13% 2	141755
	Night	45% 5% 12% 22% 14% 2	138665
Seeing	Day	22% 11% 25% 23% 14% 5%	16888
	Night	20% 7% 26% 33% 12% 1	26268

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	14% 9% 4 9% 6% 4 5% 4 5% 6% 34%	4661
	11th to 20th	Day	11% 11% 5% 14% 7% 6% 5% 4 5% 6% 26%	5024
	21st to end	Day	14% 10% 4 9% 6% 5% 4 3 5% 6% 34%	5780
February	1st to 10th	Day	15% 12% 3 9% 5% 6% 5% 4 5% 7% 30%	1890
	11th to 20th	Day	13% 10% 4 8% 6% 5% 5% 4 5% 8% 31%	1874
	21st to end	Day	17% 10% 4 9% 5% 4 4 3 5% 6% 32%	1730
March	1st to 10th	Day	20% 9% 3 6% 5% 4 4 3 4 5% 38%	4853
	11th to 20th	Day	12% 9% 2 7% 6% 4 4 4 5% 8% 40%	5090
	21st to end	Day	13% 9% 3 7% 5% 5% 5% 3 5% 6% 39%	5726
April	1st to 10th	Day	17% 8% 3 7% 5% 5% 5% 3 5% 6% 35%	2520
	11th to 20th	Day	16% 8% 2 6% 4 5% 4 4 5% 6% 38%	2494
	21st to end	Day	17% 6% 2 6% 6% 4 5% 4 6% 6% 38%	2532
May	1st to 10th	Day	23% 6% 2 6% 5% 5% 5% 4 6% 8% 30%	7755
	11th to 20th	Day	20% 7% 3 6% 5% 5% 5% 4 5% 8% 33%	8221
	21st to end	Day	22% 6% 2 6% 4 4 5% 4 6% 7% 34%	9060
June	1st to 10th	Day	12% 6% 3 7% 5% 5% 5% 4 6% 8% 39%	2940
	11th to 20th	Day	14% 5% 2 6% 6% 5% 6% 4 5% 9% 38%	2940
	21st to end	Day	12% 5% 2 6% 4 5% 5% 4 7% 10% 40%	2940
July	1st to 10th	Day	6% 5% 2 7% 6% 6% 6% 5% 7% 10% 40%	6360
	11th to 20th	Day	7% 6% 3 8% 5% 7% 7% 6% 9% 11% 32%	6360
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 5% 7% 8% 38%	6570
August	1st to 10th	Day	19% 6% 3 7% 5% 4 5% 4 6% 9% 32%	4234
	11th to 20th	Day	15% 5% 2 6% 5% 5% 7% 4 7% 9% 35%	4052
	21st to end	Day	15% 5% 2 4 4 4 4 3 6% 9% 44%	4312
September	1st to 10th	Day	16% 5% 2 4 4 3 5% 3 4 6% 46%	2400
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 4 7% 50%	2400
	21st to end	Day	22% 7% 2 5% 3 4 4 3 5% 6% 37%	2110
October	1st to 10th	Day	17% 7% 2 5% 4 4 4 3 5% 6% 43%	2000
	11th to 20th	Day	17% 4 3 7% 5% 4 5% 4 6% 7% 38%	2000
	21st to end	Day	18% 7% 2 6% 4 5% 5% 4 6% 5% 38%	2200
November	1st to 10th	Day	11% 7% 2 6% 4 4 4 3 6% 6% 47%	1866
	11th to 20th	Day	16% 7% 3 7% 5% 4 4 4 6% 6% 37%	1795
	21st to end	Day	17% 8% 3 6% 5% 4 4 5% 6% 6% 35%	1680
December	1st to 10th	Day	16% 11% 4 9% 5% 6% 5% 3 5% 7% 29%	2680
	11th to 20th	Day	22% 9% 3 7% 4 4 4 3 4 7% 33%	3066
	21st to end	Day	19% 11% 4 9% 6% 6% 6% 4 5% 6% 24%	4048
January	1st to 10th	Night	11% 10% 3 10% 6% 5% 5% 4 6% 7% 33%	8234
	11th to 20th	Night	12% 9% 4 9% 6% 5% 5% 4 6% 7% 33%	7846
	21st to end	Night	14% 8% 3 7% 6% 5% 5% 4 5% 5% 37%	8401
February	1st to 10th	Night	15% 9% 3 9% 6% 4 4 4 5% 7% 34%	2730
	11th to 20th	Night	15% 9% 3 8% 6% 5% 5% 4 6% 6% 33%	2631
	21st to end	Night	16% 10% 3 7% 6% 5% 5% 4 5% 5% 35%	2076
March	1st to 10th	Night	19% 9% 2 7% 4 4 3 3 4 5% 38%	5202
	11th to 20th	Night	11% 8% 3 7% 4 4 4 4 5% 6% 46%	5089
	21st to end	Night	15% 8% 3 7% 5% 5% 5% 3 4 5% 40%	5448
April	1st to 10th	Night	18% 10% 4 7% 5% 3 4 4 5% 6% 34%	2100
	11th to 20th	Night	16% 7% 2 6% 5% 4 4 4 6% 8% 38%	2075
	21st to end	Night	18% 5% 2 6% 5% 4 3 4 5% 7% 42%	2100
May	1st to 10th	Night	25% 7% 3 6% 4 4 4 3 5% 7% 32%	5148
	11th to 20th	Night	19% 7% 3 6% 4 5% 5% 4 6% 7% 36%	4639
	21st to end	Night	24% 6% 2 6% 3 3 3 3 5% 7% 37%	5114
June	1st to 10th	Night	15% 5% 2 6% 5% 4 4 3 6% 8% 44%	1680
	11th to 20th	Night	13% 5% 2 5% 4 4 5% 4 7% 7% 42%	1680
	21st to end	Night	11% 4 2 5% 5% 4 5% 5% 7% 9% 43%	1680
July	1st to 10th	Night	9% 6% 3 7% 5% 5% 5% 4 6% 10% 42%	3600
	11th to 20th	Night	10% 7% 3 8% 5% 6% 7% 5% 8% 9% 34%	3600
	21st to end	Night	11% 7% 2 5% 4 4 5% 4 5% 8% 46%	4278
August	1st to 10th	Night	18% 6% 2 6% 4 5% 4 3 6% 8% 36%	2915
	11th to 20th	Night	14% 7% 2 6% 5% 4 4 4 5% 7% 43%	2964
	21st to end	Night	15% 4 2 5% 4 3 4 3 7% 9% 45%	3586
September	1st to 10th	Night	14% 5% 2 6% 4 4 3 4 4 6% 47%	2000
	11th to 20th	Night	12% 5% 2 4 3 4 3 3 5% 7% 51%	2000
	21st to end	Night	20% 7% 2 5% 3 4 3 3 5% 7% 40%	2286
October	1st to 10th	Night	19% 6% 2 5% 3 3 3 3 5% 7% 44%	2400
	11th to 20th	Night	17% 7% 2 5% 3 3 3 4 5% 8% 44%	2400
	21st to end	Night	19% 7% 2 5% 4 4 4 4 5% 6% 42%	2640
November	1st to 10th	Night	11% 6% 2 7% 3 4 4 3 6% 6% 50%	2651
	11th to 20th	Night	18% 8% 3 6% 4 4 5% 3 6% 6% 39%	2819
	21st to end	Night	16% 8% 2 6% 5% 5% 4 4 6% 7% 36%	2940
December	1st to 10th	Night	12% 9% 3 8% 5% 5% 5% 4 6% 7% 37%	4720
	11th to 20th	Night	23% 8% 2 6% 4 4 4 4 4 6% 35%	5348
	21st to end	Night	20% 12% 3 9% 5% 5% 5% 4 5% 6% 28%	7150

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	50% 4 13% 20% 11% 2	4662
	11th to 20th	Day	59% 3 10% 14% 10% 4	5058
	21st to end	Day	51% 4 12% 20% 12%	5780
February	1st to 10th	Day	54% 4 10% 20% 10% 2	1890
	11th to 20th	Day	52% 5% 9% 23% 9% 2	1883
	21st to end	Day	55% 3 9% 22% 8% 2	1730
March	1st to 10th	Day	49% 3 9% 23% 10% 5%	4856
	11th to 20th	Day	43% 4 13% 25% 10% 4	5090
	21st to end	Day	45% 5% 10% 18% 18% 4	5726
April	1st to 10th	Day	50% 4 9% 20% 13% 4	2520
	11th to 20th	Day	46% 5% 11% 20% 16% 3	2494
	21st to end	Day	45% 4 10% 22% 15% 4	2532
May	1st to 10th	Day	52% 6% 11% 19% 10% 1	7764
	11th to 20th	Day	48% 6% 11% 22% 11% 1	8225
	21st to end	Day	48% 5% 10% 23% 13% 2	9061
June	1st to 10th	Day	42% 5% 10% 23% 17% 2	2940
	11th to 20th	Day	42% 6% 13% 22% 14% 2	2940
	21st to end	Day	38% 6% 12% 27% 15% 1	2940
July	1st to 10th	Day	38% 8% 13% 26% 14% 1	6290
	11th to 20th	Day	41% 10% 16% 22% 11% 1	6285
	21st to end	Day	41% 7% 10% 21% 20% 1	6382
August	1st to 10th	Day	49% 6% 13% 20% 12%	4221
	11th to 20th	Day	43% 8% 12% 26% 10% 1	4091
	21st to end	Day	37% 6% 11% 31% 13% 2	4312
September	1st to 10th	Day	39% 5% 11% 24% 19% 3	2400
	11th to 20th	Day	34% 4 11% 27% 22% 3	2400
	21st to end	Day	48% 4 10% 18% 18% 2	2110
October	1st to 10th	Day	39% 4 10% 24% 18% 5%	3260
	11th to 20th	Day	38% 4 10% 26% 21% 1	3240
	21st to end	Day	31% 4 14% 32% 18% 1	3538
November	1st to 10th	Day	38% 4 11% 26% 18% 3	1866
	11th to 20th	Day	47% 4 10% 23% 13% 3	1795
	21st to end	Day	48% 4 12% 26% 9% 1	1680
December	1st to 10th	Day	55% 5% 12% 21% 7% 2	2680
	11th to 20th	Day	53% 3 15% 22% 7%	3066
	21st to end	Day	58% 6% 14% 14% 7% 1	4048
January	1st to 10th	Night	50% 5% 14% 18% 11% 2	8234
	11th to 20th	Night	49% 5% 13% 15% 14% 4	7887
	21st to end	Night	48% 4 9% 21% 16% 2	8401
February	1st to 10th	Night	49% 4 12% 23% 10% 1	2730
	11th to 20th	Night	51% 5% 10% 20% 12% 2	2633
	21st to end	Night	52% 3 9% 20% 14% 2	2076
March	1st to 10th	Night	49% 3 11% 20% 11% 5%	5212
	11th to 20th	Night	39% 5% 15% 27% 10% 5%	5090
	21st to end	Night	47% 4 8% 17% 20% 4	5448
April	1st to 10th	Night	51% 4 9% 18% 15% 3	2100
	11th to 20th	Night	44% 6% 9% 22% 17% 3	2075
	21st to end	Night	42% 5% 9% 24% 16% 3	2100
May	1st to 10th	Night	53% 5% 11% 19% 11% 2	5155
	11th to 20th	Night	47% 8% 12% 20% 12% 1	4640
	21st to end	Night	48% 5% 11% 20% 14% 3	5115
June	1st to 10th	Night	39% 6% 10% 25% 19% 1	1680
	11th to 20th	Night	39% 6% 13% 23% 16% 2	1680
	21st to end	Night	36% 6% 13% 28% 15% 2	1680
July	1st to 10th	Night	39% 7% 12% 22% 16% 3	3560
	11th to 20th	Night	44% 9% 12% 22% 12% 1	3552
	21st to end	Night	37% 6% 12% 22% 23% 1	4151
August	1st to 10th	Night	46% 8% 13% 22% 11% 1	2904
	11th to 20th	Night	41% 6% 13% 25% 15% 1	2991
	21st to end	Night	35% 6% 11% 32% 13% 1	3586
September	1st to 10th	Night	39% 5% 10% 25% 19% 3	2000
	11th to 20th	Night	33% 4 9% 31% 21% 2	2000
	21st to end	Night	44% 4 8% 22% 20% 1	2286
October	1st to 10th	Night	40% 6% 11% 21% 21% 1	3920
	11th to 20th	Night	33% 4 10% 29% 23% 1	3899
	21st to end	Night	32% 4 13% 30% 20% 2	4252
November	1st to 10th	Night	36% 4 12% 27% 19% 3	2651
	11th to 20th	Night	46% 4 9% 23% 15% 3	2819
	21st to end	Night	46% 4 13% 25% 10% 2	2940
December	1st to 10th	Night	46% 5% 14% 26% 8%	4720
	11th to 20th	Night	51% 4 14% 20% 10% 1	5348
	21st to end	Night	56% 7% 11% 17% 9% 1	7150

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	30% 18% 28% 19% 4 1	220
	11th to 20th	Day	27% 23% 28% 19% 2	289
	21st to end	Day	25% 18% 34% 15% 7% 1	363
February	1st to 10th	Day	24% 19% 37% 15% 5%	330
	11th to 20th	Day	30% 18% 36% 16% 1	361
	21st to end	Day	29% 15% 31% 20% 4	446
March	1st to 10th	Day	27% 17% 28% 22% 5% 1	540
	11th to 20th	Day	26% 17% 29% 22% 4 1	540
	21st to end	Day	27% 21% 26% 19% 5% 2	594
April	1st to 10th	Day	28% 16% 27% 19% 9% 1	540
	11th to 20th	Day	24% 19% 26% 17% 9% 5%	532
	21st to end	Day	26% 11% 28% 21% 11% 3	540
May	1st to 10th	Day	30% 11% 28% 17% 10% 3	543
	11th to 20th	Day	22% 12% 26% 23% 12% 5%	567
	21st to end	Day	23% 7% 22% 24% 16% 8%	617
June	1st to 10th	Day	14% 10% 26% 26% 18% 6%	570
	11th to 20th	Day	17% 9% 24% 26% 19% 4	570
	21st to end	Day	16% 6% 25% 32% 17% 5%	570
July	1st to 10th	Day	10% 2 22% 33% 23% 10%	525
	11th to 20th	Day	8% 3 20% 34% 25% 10%	524
	21st to end	Day	14% 4 20% 31% 23% 7%	576
August	1st to 10th	Day	14% 2 18% 27% 27% 11%	526
	11th to 20th	Day	16% 3 18% 27% 23% 14%	519
	21st to end	Day	16% 3 19% 26% 22% 14%	583
September	1st to 10th	Day	19% 7% 18% 23% 20% 13%	530
	11th to 20th	Day	15% 3 15% 26% 25% 15%	530
	21st to end	Day	28% 7% 18% 23% 17% 8%	530
October	1st to 10th	Day	23% 6% 20% 23% 18% 10%	530
	11th to 20th	Day	21% 13% 24% 28% 13% 2	530
	21st to end	Day	23% 9% 30% 26% 10% 1	583
November	1st to 10th	Day	19% 12% 27% 26% 14% 2	433
	11th to 20th	Day	24% 13% 29% 22% 11% 1	335
	21st to end	Day	25% 18% 32% 17% 8% 1	220
December	1st to 10th	Day	27% 16% 33% 16% 6% 1	220
	11th to 20th	Day	30% 11% 36% 16% 5%	220
	21st to end	Day	27% 14% 32% 20% 7%	242
January	1st to 10th	Night	20% 14% 36% 26% 3	840
	11th to 20th	Night	24% 11% 30% 31% 3	840
	21st to end	Night	18% 10% 38% 31% 2	924
February	1st to 10th	Night	21% 10% 32% 33% 3	840
	11th to 20th	Night	23% 12% 40% 23% 2	823
	21st to end	Night	25% 12% 35% 25% 4	692
March	1st to 10th	Night	23% 13% 31% 29% 4	840
	11th to 20th	Night	25% 11% 30% 28% 5%	840
	21st to end	Night	22% 10% 36% 26% 5% 1	924
April	1st to 10th	Night	24% 13% 31% 26% 5%	840
	11th to 20th	Night	21% 9% 32% 30% 8% 1	831
	21st to end	Night	21% 8% 28% 35% 7%	840
May	1st to 10th	Night	25% 5% 24% 33% 12% 1	597
	11th to 20th	Night	18% 4 29% 40% 8%	414
	21st to end	Night	20% 3 21% 36% 19% 1	455
June	1st to 10th	Night	17% 3 19% 41% 20% 1	420
	11th to 20th	Night	15% 4 20% 44% 15%	420
	21st to end	Night	13% 2 18% 49% 18% 1	420
July	1st to 10th	Night	13% 2 15% 43% 24% 2	390
	11th to 20th	Night	10% 2 12% 50% 25% 1	388
	21st to end	Night	12% 2 14% 44% 26% 2	585
August	1st to 10th	Night	14% 1 8% 42% 31% 3	597
	11th to 20th	Night	15% 2 10% 36% 35% 3	644
	21st to end	Night	15% 3 11% 37% 31% 3	880
September	1st to 10th	Night	17% 3 18% 34% 27% 2	800
	11th to 20th	Night	15% 3 12% 34% 33% 4	800
	21st to end	Night	23% 4 15% 34% 21% 3	800
October	1st to 10th	Night	22% 4 18% 31% 21% 4	800
	11th to 20th	Night	20% 5% 23% 38% 13% 1	800
	21st to end	Night	24% 6% 26% 35% 9%	880
November	1st to 10th	Night	16% 7% 28% 37% 12% 1	820
	11th to 20th	Night	23% 11% 27% 29% 8% 1	840
	21st to end	Night	22% 9% 30% 33% 6%	840
December	1st to 10th	Night	23% 11% 33% 26% 7%	840
	11th to 20th	Night	23% 10% 31% 31% 5%	840
	21st to end	Night	26% 9% 28% 32% 4	924

Page generated at 2026-06-28 02:10:47. Version ac07

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