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 2024-02-05.

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 6% 6% 4 5% 5% 5% 7% 36%	78222
	Night	16% 7% 3 6% 5% 3 5% 5% 5% 7% 38%	80627
Transparency	Day	47% 5% 11% 22% 13% 2	77935
	Night	46% 5% 12% 22% 13% 2	80448
Seeing	Day	21% 10% 25% 24% 14% 6%	13733
	Night	20% 7% 26% 33% 13% 1	23247

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% 10% 4 7% 7% 3 5% 4 4 5% 37%	2517
	11th to 20th	Day	13% 13% 6% 11% 9% 5% 6% 4 5% 5% 23%	2716
	21st to end	Day	19% 12% 5% 9% 8% 4 5% 4 4 6% 24%	3124
February	1st to 10th	Day	17% 12% 4 8% 6% 4 5% 5% 5% 7% 27%	1656
	11th to 20th	Day	13% 9% 4 7% 8% 4 6% 5% 6% 8% 31%	1599
	21st to end	Day	19% 8% 5% 9% 6% 3 4 4 4 6% 31%	1480
March	1st to 10th	Day	13% 9% 3 6% 6% 3 5% 5% 5% 7% 39%	1900
	11th to 20th	Day	16% 8% 3 6% 8% 4 5% 4 5% 7% 33%	1980
	21st to end	Day	19% 8% 3 7% 5% 3 3 4 4 6% 36%	2232
April	1st to 10th	Day	15% 7% 4 7% 6% 4 5% 5% 5% 6% 37%	2160
	11th to 20th	Day	17% 8% 3 6% 5% 4 5% 5% 5% 6% 38%	2134
	21st to end	Day	18% 6% 3 5% 7% 3 4 5% 6% 6% 37%	2169
May	1st to 10th	Day	19% 6% 3 6% 6% 4 5% 5% 5% 7% 35%	2371
	11th to 20th	Day	19% 7% 3 6% 6% 4 5% 5% 5% 7% 33%	2478
	21st to end	Day	15% 6% 2 5% 5% 3 5% 5% 6% 8% 39%	2725
June	1st to 10th	Day	12% 5% 3 6% 6% 4 5% 6% 6% 8% 37%	2520
	11th to 20th	Day	14% 5% 2 5% 6% 4 7% 5% 5% 9% 38%	2520
	21st to end	Day	11% 4 3 6% 5% 4 5% 5% 7% 10% 41%	2520
July	1st to 10th	Day	8% 5% 3 5% 7% 4 7% 6% 7% 9% 39%	2520
	11th to 20th	Day	7% 4 3 6% 5% 4 6% 6% 7% 10% 41%	2520
	21st to end	Day	11% 6% 3 6% 7% 4 6% 6% 7% 8% 37%	2609
August	1st to 10th	Day	10% 5% 3 5% 7% 4 6% 6% 7% 10% 36%	2327
	11th to 20th	Day	14% 6% 3 6% 6% 4 6% 6% 7% 8% 36%	2241
	21st to end	Day	15% 6% 2 6% 5% 4 5% 6% 6% 8% 38%	2376
September	1st to 10th	Day	17% 5% 3 4 5% 3 5% 4 4 6% 44%	2160
	11th to 20th	Day	14% 5% 2 4 5% 2 4 4 4 7% 50%	2160
	21st to end	Day	19% 7% 2 5% 4 3 4 4 6% 7% 40%	1900
October	1st to 10th	Day	18% 7% 2 5% 5% 3 5% 4 5% 6% 41%	1800
	11th to 20th	Day	19% 4 3 6% 5% 3 5% 5% 6% 6% 38%	1800
	21st to end	Day	19% 7% 3 6% 5% 3 5% 6% 6% 5% 36%	1980
November	1st to 10th	Day	12% 6% 2 5% 5% 3 4 4 6% 6% 46%	1685
	11th to 20th	Day	17% 6% 3 5% 6% 4 3 6% 6% 6% 39%	1625
	21st to end	Day	17% 6% 3 5% 5% 3 5% 6% 6% 6% 36%	1520
December	1st to 10th	Day	19% 10% 4 7% 8% 5% 6% 4 5% 7% 27%	1520
	11th to 20th	Day	20% 7% 3 6% 5% 4 5% 5% 4 7% 34%	1906
	21st to end	Day	21% 11% 5% 8% 8% 4 6% 5% 4 6% 23%	2772
January	1st to 10th	Night	16% 8% 4 6% 6% 4 5% 5% 6% 7% 34%	4429
	11th to 20th	Night	15% 10% 5% 9% 8% 4 5% 5% 5% 6% 29%	4243
	21st to end	Night	22% 9% 4 8% 7% 4 4 5% 4 5% 28%	4532
February	1st to 10th	Night	17% 9% 4 7% 7% 3 5% 4 4 7% 34%	2399
	11th to 20th	Night	15% 8% 2 7% 6% 4 6% 4 6% 7% 34%	2246
	21st to end	Night	17% 9% 3 6% 7% 4 5% 5% 4 5% 35%	1776
March	1st to 10th	Night	15% 6% 2 5% 5% 2 5% 5% 6% 7% 42%	2042
	11th to 20th	Night	17% 7% 4 5% 5% 3 4 4 5% 6% 41%	1979
	21st to end	Night	16% 7% 3 5% 7% 3 5% 5% 5% 6% 37%	2124
April	1st to 10th	Night	17% 8% 4 7% 6% 3 4 4 5% 7% 35%	1800
	11th to 20th	Night	15% 6% 3 5% 5% 3 4 5% 6% 8% 39%	1775
	21st to end	Night	19% 5% 2 5% 5% 3 4 5% 5% 6% 41%	1800
May	1st to 10th	Night	20% 8% 3 6% 5% 3 4 4 4 5% 39%	1593
	11th to 20th	Night	17% 5% 3 5% 5% 3 5% 5% 5% 7% 40%	1415
	21st to end	Night	13% 5% 3 6% 5% 4 4 4 5% 9% 42%	1550
June	1st to 10th	Night	15% 5% 2 6% 5% 3 4 4 5% 8% 44%	1440
	11th to 20th	Night	13% 4 3 4 5% 3 6% 5% 7% 8% 42%	1440
	21st to end	Night	11% 3 2 4 5% 3 5% 6% 7% 10% 44%	1440
July	1st to 10th	Night	11% 5% 2 4 5% 4 5% 6% 6% 8% 44%	1440
	11th to 20th	Night	7% 5% 2 5% 5% 3 6% 6% 7% 9% 43%	1440
	21st to end	Night	11% 6% 2 4 5% 4 6% 5% 6% 9% 44%	1705
August	1st to 10th	Night	10% 6% 2 5% 5% 3 5% 4 7% 9% 43%	1611
	11th to 20th	Night	11% 7% 3 6% 6% 4 5% 5% 5% 7% 40%	1642
	21st to end	Night	13% 5% 2 5% 5% 3 5% 4 6% 8% 44%	1980
September	1st to 10th	Night	15% 5% 3 5% 6% 3 4 5% 4 6% 45%	1800
	11th to 20th	Night	13% 5% 2 4 3 3 3 4 5% 7% 52%	1800
	21st to end	Night	17% 6% 3 4 4 3 4 4 5% 8% 43%	2056
October	1st to 10th	Night	20% 6% 2 5% 4 2 3 4 5% 8% 42%	2160
	11th to 20th	Night	17% 6% 2 5% 4 2 4 4 5% 8% 43%	2160
	21st to end	Night	20% 7% 2 5% 5% 3 4 5% 5% 6% 39%	2376
November	1st to 10th	Night	11% 6% 2 5% 5% 2 4 4 5% 6% 50%	2392
	11th to 20th	Night	18% 7% 2 5% 5% 3 4 4 5% 6% 40%	2550
	21st to end	Night	16% 6% 3 4 6% 4 5% 5% 6% 7% 38%	2660
December	1st to 10th	Night	18% 9% 4 7% 6% 3 5% 6% 6% 6% 32%	2660
	11th to 20th	Night	19% 8% 3 5% 5% 4 5% 5% 4 7% 34%	3288
	21st to end	Night	20% 10% 3 7% 6% 3 5% 5% 5% 7% 28%	4884

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% 4 14% 22% 8% 3	2518
	11th to 20th	Day	64% 4 9% 14% 7% 2	2716
	21st to end	Day	61% 5% 10% 17% 6% 1	3124
February	1st to 10th	Day	57% 4 10% 17% 10% 1	1656
	11th to 20th	Day	51% 5% 10% 24% 8% 2	1608
	21st to end	Day	56% 3 9% 21% 9% 3	1480
March	1st to 10th	Day	46% 3 11% 25% 12% 2	1903
	11th to 20th	Day	51% 4 10% 20% 13% 3	1980
	21st to end	Day	51% 3 8% 19% 16% 2	2232
April	1st to 10th	Day	49% 4 9% 21% 14% 4	2160
	11th to 20th	Day	46% 5% 9% 20% 16% 3	2134
	21st to end	Day	46% 4 10% 21% 15% 5%	2169
May	1st to 10th	Day	49% 4 9% 20% 15% 3	2371
	11th to 20th	Day	50% 5% 9% 19% 14% 3	2482
	21st to end	Day	42% 6% 14% 23% 13% 2	2725
June	1st to 10th	Day	44% 6% 11% 23% 15% 2	2520
	11th to 20th	Day	43% 6% 13% 21% 14% 2	2520
	21st to end	Day	37% 6% 13% 28% 15% 1	2520
July	1st to 10th	Day	40% 8% 12% 24% 14% 2	2450
	11th to 20th	Day	36% 7% 13% 26% 16% 2	2445
	21st to end	Day	42% 7% 11% 21% 17% 1	2463
August	1st to 10th	Day	41% 6% 14% 24% 15%	2314
	11th to 20th	Day	44% 7% 13% 22% 13% 2	2241
	21st to end	Day	42% 7% 9% 22% 16% 3	2376
September	1st to 10th	Day	42% 5% 11% 22% 17% 3	2160
	11th to 20th	Day	35% 4 10% 26% 22% 3	2160
	21st to end	Day	44% 4 10% 20% 19% 2	1900
October	1st to 10th	Day	45% 5% 12% 23% 15% 1	1800
	11th to 20th	Day	45% 4 12% 25% 15%	1800
	21st to end	Day	49% 4 10% 23% 12% 1	1980
November	1st to 10th	Day	39% 4 11% 26% 17% 3	1685
	11th to 20th	Day	46% 4 10% 24% 13% 2	1625
	21st to end	Day	47% 4 12% 27% 9% 1	1520
December	1st to 10th	Day	59% 4 12% 16% 8% 1	1520
	11th to 20th	Day	52% 4 17% 20% 7% 1	1906
	21st to end	Day	61% 7% 12% 15% 5%	2772
January	1st to 10th	Night	50% 4 14% 22% 8% 2	4429
	11th to 20th	Night	55% 5% 12% 17% 8% 2	4243
	21st to end	Night	58% 5% 10% 19% 7% 1	4532
February	1st to 10th	Night	51% 5% 13% 21% 9% 1	2399
	11th to 20th	Night	49% 5% 11% 21% 13% 2	2248
	21st to end	Night	53% 3 9% 19% 14% 2	1776
March	1st to 10th	Night	42% 4 11% 26% 13% 3	2052
	11th to 20th	Night	46% 4 10% 22% 15% 2	1980
	21st to end	Night	48% 5% 9% 19% 17% 3	2124
April	1st to 10th	Night	50% 4 9% 19% 16% 3	1800
	11th to 20th	Night	42% 6% 9% 23% 17% 3	1775
	21st to end	Night	43% 6% 9% 23% 16% 4	1800
May	1st to 10th	Night	48% 4 10% 21% 12% 4	1593
	11th to 20th	Night	44% 6% 12% 18% 19% 2	1416
	21st to end	Night	41% 6% 15% 20% 15% 3	1551
June	1st to 10th	Night	39% 6% 10% 24% 18% 2	1440
	11th to 20th	Night	39% 7% 13% 22% 16% 2	1440
	21st to end	Night	34% 6% 13% 29% 16% 1	1440
July	1st to 10th	Night	38% 8% 12% 23% 15% 4	1400
	11th to 20th	Night	35% 7% 13% 25% 17% 2	1392
	21st to end	Night	36% 7% 15% 23% 18% 2	1610
August	1st to 10th	Night	36% 9% 14% 24% 16% 1	1600
	11th to 20th	Night	42% 7% 13% 20% 17% 1	1642
	21st to end	Night	37% 7% 12% 24% 18% 2	1980
September	1st to 10th	Night	40% 5% 10% 24% 17% 3	1800
	11th to 20th	Night	33% 4 8% 31% 21% 3	1800
	21st to end	Night	41% 4 9% 23% 22% 2	2056
October	1st to 10th	Night	42% 4 13% 24% 15% 1	2160
	11th to 20th	Night	41% 4 13% 26% 14% 2	2160
	21st to end	Night	47% 4 9% 25% 14% 1	2376
November	1st to 10th	Night	37% 4 12% 26% 18% 3	2392
	11th to 20th	Night	45% 4 10% 23% 15% 3	2550
	21st to end	Night	45% 4 14% 25% 10% 2	2660
December	1st to 10th	Night	52% 6% 12% 20% 10%	2660
	11th to 20th	Night	51% 5% 12% 21% 10% 1	3288
	21st to end	Night	54% 8% 13% 19% 5%	4884

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% 16% 28% 21% 4 2	180
	11th to 20th	Day	26% 19% 29% 22% 3	236
	21st to end	Day	26% 15% 34% 16% 8% 1	297
February	1st to 10th	Day	27% 17% 37% 13% 5%	252
	11th to 20th	Day	28% 17% 40% 15%	266
	21st to end	Day	32% 14% 29% 21% 4	346
March	1st to 10th	Day	25% 18% 28% 24% 5%	420
	11th to 20th	Day	25% 16% 30% 22% 4 2	420
	21st to end	Day	29% 18% 26% 18% 6% 3	462
April	1st to 10th	Day	24% 17% 28% 21% 9% 2	420
	11th to 20th	Day	21% 17% 26% 18% 11% 7%	412
	21st to end	Day	28% 10% 28% 20% 11% 3	420
May	1st to 10th	Day	30% 11% 26% 18% 12% 4	423
	11th to 20th	Day	21% 12% 27% 22% 12% 5%	447
	21st to end	Day	21% 5% 21% 24% 17% 11%	485
June	1st to 10th	Day	16% 8% 27% 26% 18% 6%	450
	11th to 20th	Day	17% 10% 23% 26% 20% 5%	450
	21st to end	Day	14% 6% 26% 33% 16% 5%	450
July	1st to 10th	Day	11% 1 21% 31% 24% 11%	445
	11th to 20th	Day	7% 3 19% 34% 26% 11%	444
	21st to end	Day	15% 3 21% 31% 24% 7%	488
August	1st to 10th	Day	13% 1 19% 27% 27% 12%	446
	11th to 20th	Day	16% 2 16% 27% 23% 15%	439
	21st to end	Day	17% 3 18% 27% 23% 13%	495
September	1st to 10th	Day	21% 5% 18% 23% 19% 14%	450
	11th to 20th	Day	16% 4 16% 27% 22% 14%	450
	21st to end	Day	24% 7% 18% 25% 18% 9%	450
October	1st to 10th	Day	25% 6% 19% 23% 19% 8%	450
	11th to 20th	Day	21% 12% 23% 29% 13% 1	450
	21st to end	Day	25% 8% 30% 25% 10% 1	495
November	1st to 10th	Day	21% 9% 25% 27% 16% 2	372
	11th to 20th	Day	24% 13% 29% 21% 12% 1	285
	21st to end	Day	23% 16% 33% 18% 9% 1	180
December	1st to 10th	Day	28% 13% 33% 18% 6% 2	180
	11th to 20th	Day	28% 11% 38% 17% 5% 1	180
	21st to end	Day	28% 12% 33% 20% 7%	198
January	1st to 10th	Night	21% 12% 36% 27% 4	760
	11th to 20th	Night	25% 10% 29% 33% 4	760
	21st to end	Night	19% 10% 36% 32% 3	836
February	1st to 10th	Night	23% 10% 31% 32% 3	739
	11th to 20th	Night	23% 12% 41% 22% 2	703
	21st to end	Night	26% 12% 34% 24% 4	592
March	1st to 10th	Night	21% 13% 31% 30% 5%	720
	11th to 20th	Night	25% 10% 29% 29% 6%	720
	21st to end	Night	22% 9% 36% 26% 6% 1	792
April	1st to 10th	Night	23% 13% 32% 26% 6%	720
	11th to 20th	Night	21% 7% 32% 30% 9% 1	711
	21st to end	Night	22% 8% 29% 34% 7%	720
May	1st to 10th	Night	24% 5% 23% 33% 14% 1	513
	11th to 20th	Night	19% 4 29% 40% 8% 1	354
	21st to end	Night	17% 3 21% 37% 22% 1	389
June	1st to 10th	Night	17% 3 19% 41% 19% 1	360
	11th to 20th	Night	15% 3 21% 44% 17% 1	360
	21st to end	Night	12% 1 18% 50% 17% 1	360
July	1st to 10th	Night	14% 1 16% 42% 25% 2	350
	11th to 20th	Night	10% 2 12% 49% 26% 1	348
	21st to end	Night	13% 2 14% 45% 25% 2	525
August	1st to 10th	Night	14% 9% 44% 30% 3	537
	11th to 20th	Night	15% 2 10% 36% 34% 3	578
	21st to end	Night	15% 3 11% 37% 30% 4	792
September	1st to 10th	Night	18% 2 17% 34% 27% 2	720
	11th to 20th	Night	16% 3 13% 33% 31% 4	720
	21st to end	Night	20% 4 16% 35% 22% 3	720
October	1st to 10th	Night	23% 4 18% 32% 20% 3	720
	11th to 20th	Night	21% 5% 22% 38% 13% 1	720
	21st to end	Night	26% 6% 26% 33% 9%	792
November	1st to 10th	Night	16% 7% 28% 37% 12% 1	740
	11th to 20th	Night	24% 9% 28% 30% 9% 1	760
	21st to end	Night	21% 8% 29% 35% 7% 1	760
December	1st to 10th	Night	24% 10% 33% 25% 7%	760
	11th to 20th	Night	22% 9% 31% 32% 6%	760
	21st to end	Night	26% 8% 28% 34% 4	836

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