Southwind Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Southwind Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	18% 7% 2 6% 4 4 4 4 5% 7% 38%	126485
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 41%	123200
Transparency	Day	44% 5% 11% 22% 16% 3	130001
	Night	43% 4 11% 22% 16% 3	127759
Seeing	Day	25% 16% 30% 19% 8% 2	16229
	Night	23% 15% 31% 24% 8% 1	25236

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	11% 8% 3 8% 4 5% 4 3 4 6% 44%	3597
	11th to 20th	Day	15% 8% 3 8% 5% 4 4 3 5% 5% 40%	3570
	21st to end	Day	18% 9% 3 8% 4 4 4 3 4 4 39%	4120
February	1st to 10th	Day	17% 8% 2 7% 5% 5% 5% 4 5% 6% 37%	1846
	11th to 20th	Day	14% 7% 3 6% 4 5% 4 3 6% 7% 40%	1948
	21st to end	Day	20% 9% 2 6% 4 3 4 3 5% 5% 39%	1650
March	1st to 10th	Day	23% 9% 2 6% 4 3 3 3 4 5% 39%	4476
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 4 6% 7% 42%	4658
	21st to end	Day	20% 8% 2 6% 4 4 4 3 6% 7% 37%	5970
April	1st to 10th	Day	19% 9% 2 6% 5% 4 4 3 5% 6% 36%	2400
	11th to 20th	Day	19% 6% 2 5% 4 4 4 4 4 7% 40%	2374
	21st to end	Day	20% 6% 2 5% 4 4 4 3 5% 6% 41%	2494
May	1st to 10th	Day	31% 8% 3 6% 3 3 4 3 6% 7% 26%	5886
	11th to 20th	Day	21% 9% 2 7% 4 4 4 4 5% 7% 34%	6195
	21st to end	Day	22% 7% 2 5% 4 4 4 4 6% 7% 35%	6949
June	1st to 10th	Day	18% 6% 2 5% 5% 5% 5% 4 7% 9% 35%	2800
	11th to 20th	Day	17% 7% 2 6% 4 5% 4 5% 7% 9% 35%	2800
	21st to end	Day	13% 6% 2 7% 4 5% 5% 4 7% 9% 37%	2800
July	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 4 6% 9% 34%	6360
	11th to 20th	Day	12% 7% 3 7% 4 5% 6% 5% 7% 10% 34%	6360
	21st to end	Day	16% 7% 2 6% 4 5% 4 4 6% 7% 40%	6926
August	1st to 10th	Day	20% 7% 3 7% 5% 5% 4 4 6% 8% 32%	4412
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 4 7% 9% 40%	3850
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 5% 9% 45%	4312
September	1st to 10th	Day	14% 6% 2 6% 5% 4 4 4 5% 6% 44%	2400
	11th to 20th	Day	12% 6% 2 4 3 4 4 3 5% 7% 50%	2275
	21st to end	Day	18% 6% 2 6% 4 4 4 4 4 7% 40%	2200
October	1st to 10th	Day	21% 7% 2 5% 4 3 4 3 5% 5% 43%	2145
	11th to 20th	Day	14% 6% 2 6% 3 3 4 4 5% 7% 46%	1979
	21st to end	Day	22% 6% 2 5% 4 4 4 4 4 6% 40%	1881
November	1st to 10th	Day	13% 7% 2 4 4 3 3 2 4 7% 49%	1761
	11th to 20th	Day	18% 6% 2 4 3 4 3 3 5% 6% 47%	1800
	21st to end	Day	18% 7% 3 5% 4 3 3 3 6% 6% 42%	1745
December	1st to 10th	Day	16% 7% 2 6% 4 3 3 3 4 7% 44%	2600
	11th to 20th	Day	21% 8% 2 5% 2 3 3 3 4 6% 44%	2986
	21st to end	Day	19% 10% 3 6% 4 4 3 3 4 6% 38%	3960
January	1st to 10th	Night	14% 6% 2 6% 4 3 4 4 5% 7% 45%	6349
	11th to 20th	Night	14% 8% 3 8% 4 4 4 3 5% 6% 43%	6315
	21st to end	Night	17% 10% 3 7% 4 4 4 3 4 5% 39%	6834
February	1st to 10th	Night	16% 10% 3 6% 4 4 4 3 5% 5% 40%	2552
	11th to 20th	Night	16% 8% 3 6% 4 4 4 3 5% 7% 41%	2338
	21st to end	Night	18% 9% 2 6% 4 4 4 3 5% 5% 40%	1980
March	1st to 10th	Night	20% 10% 2 6% 3 3 3 3 4 6% 40%	5364
	11th to 20th	Night	16% 8% 3 6% 4 4 4 3 5% 6% 41%	5269
	21st to end	Night	21% 10% 3 6% 4 3 3 2 4 6% 38%	4962
April	1st to 10th	Night	20% 8% 2 6% 4 5% 5% 4 6% 6% 36%	2000
	11th to 20th	Night	20% 6% 2 5% 4 4 4 3 5% 6% 39%	1975
	21st to end	Night	20% 6% 3 6% 4 3 3 2 4 6% 42%	1905
May	1st to 10th	Night	31% 10% 2 7% 4 3 3 2 4 7% 27%	4054
	11th to 20th	Night	21% 9% 3 7% 4 4 4 3 5% 7% 33%	3727
	21st to end	Night	23% 9% 3 6% 3 3 4 3 5% 7% 35%	3926
June	1st to 10th	Night	18% 6% 3 6% 5% 5% 3 4 4 7% 40%	1600
	11th to 20th	Night	16% 7% 2 5% 4 3 4 4 6% 9% 39%	1600
	21st to end	Night	14% 7% 1 5% 5% 4 4 5% 5% 9% 42%	1600
July	1st to 10th	Night	16% 7% 2 6% 4 4 5% 4 6% 8% 39%	3600
	11th to 20th	Night	19% 8% 3 7% 5% 4 4 4 6% 8% 33%	3600
	21st to end	Night	14% 6% 2 6% 4 4 4 4 5% 6% 45%	3926
August	1st to 10th	Night	23% 7% 3 7% 5% 4 5% 3 5% 6% 31%	2701
	11th to 20th	Night	16% 7% 3 6% 5% 4 4 4 5% 7% 41%	3197
	21st to end	Night	11% 4 2 4 3 3 4 3 5% 8% 53%	3586
September	1st to 10th	Night	15% 7% 2 6% 4 3 4 3 4 6% 46%	2000
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 4 6% 53%	2125
	21st to end	Night	18% 7% 2 5% 3 3 5% 3 4 6% 44%	2200
October	1st to 10th	Night	19% 6% 3 6% 4 3 3 3 4 6% 44%	2250
	11th to 20th	Night	15% 6% 2 6% 4 3 3 3 4 7% 48%	2377
	21st to end	Night	22% 7% 2 5% 3 4 3 3 5% 5% 41%	2717
November	1st to 10th	Night	13% 6% 2 6% 4 3 4 3 5% 6% 49%	2537
	11th to 20th	Night	18% 7% 2 5% 4 3 3 3 4 5% 47%	2600
	21st to end	Night	16% 7% 2 5% 4 4 4 3 5% 6% 45%	2650
December	1st to 10th	Night	17% 8% 2 5% 3 3 3 2 4 5% 47%	4580
	11th to 20th	Night	25% 8% 2 5% 3 3 3 3 4 4 40%	5208
	21st to end	Night	20% 10% 3 7% 4 3 3 3 4 4 39%	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	41% 4 7% 28% 17% 3	3598
	11th to 20th	Day	46% 4 9% 22% 13% 7%	3600
	21st to end	Day	48% 4 8% 22% 17% 1	4120
February	1st to 10th	Day	48% 4 12% 20% 15% 2	1846
	11th to 20th	Day	43% 4 10% 23% 17% 3	1957
	21st to end	Day	48% 4 10% 22% 14% 3	1650
March	1st to 10th	Day	48% 3 10% 16% 13% 10%	4479
	11th to 20th	Day	39% 5% 14% 28% 11% 2	4658
	21st to end	Day	47% 4 7% 14% 21% 7%	5970
April	1st to 10th	Day	48% 4 9% 21% 15% 3	2400
	11th to 20th	Day	44% 5% 10% 22% 18% 3	2374
	21st to end	Day	45% 4 9% 21% 17% 4	2494
May	1st to 10th	Day	58% 5% 14% 13% 9% 1	5897
	11th to 20th	Day	51% 5% 11% 19% 13% 1	6199
	21st to end	Day	48% 5% 10% 23% 13% 1	6949
June	1st to 10th	Day	45% 6% 13% 23% 13% 1	2800
	11th to 20th	Day	44% 6% 13% 23% 12% 1	2800
	21st to end	Day	41% 7% 14% 23% 14% 1	2800
July	1st to 10th	Day	46% 8% 10% 20% 14% 1	6290
	11th to 20th	Day	43% 8% 16% 22% 11% 1	6285
	21st to end	Day	42% 6% 10% 22% 18% 1	6728
August	1st to 10th	Day	49% 6% 14% 21% 9% 1	4398
	11th to 20th	Day	39% 5% 14% 30% 11% 1	3886
	21st to end	Day	35% 6% 11% 29% 17% 2	4312
September	1st to 10th	Day	40% 6% 10% 23% 19% 2	2400
	11th to 20th	Day	34% 5% 10% 24% 25% 2	2275
	21st to end	Day	45% 5% 9% 21% 17% 3	2200
October	1st to 10th	Day	39% 5% 8% 19% 25% 5%	3496
	11th to 20th	Day	29% 4 8% 24% 29% 5%	3207
	21st to end	Day	32% 3 13% 24% 24% 4	3081
November	1st to 10th	Day	36% 3 12% 25% 20% 3	1761
	11th to 20th	Day	38% 4 10% 24% 21% 4	1800
	21st to end	Day	41% 4 13% 27% 12% 3	1745
December	1st to 10th	Day	41% 3 15% 28% 12% 1	2600
	11th to 20th	Day	43% 3 14% 20% 17% 3	2986
	21st to end	Day	49% 3 7% 21% 15% 4	3960
January	1st to 10th	Night	39% 3 9% 27% 19% 3	6349
	11th to 20th	Night	42% 4 13% 21% 15% 6%	6360
	21st to end	Night	48% 3 10% 19% 17% 4	6834
February	1st to 10th	Night	47% 4 12% 23% 12% 2	2552
	11th to 20th	Night	44% 4 9% 19% 19% 4	2340
	21st to end	Night	46% 3 7% 24% 15% 5%	1980
March	1st to 10th	Night	46% 3 11% 18% 15% 7%	5374
	11th to 20th	Night	44% 4 14% 23% 12% 4	5270
	21st to end	Night	48% 4 5% 14% 24% 5%	4962
April	1st to 10th	Night	47% 5% 10% 20% 15% 3	2000
	11th to 20th	Night	45% 5% 10% 20% 20% 1	1975
	21st to end	Night	45% 4 8% 23% 16% 4	1905
May	1st to 10th	Night	60% 4 13% 13% 10% 1	4060
	11th to 20th	Night	52% 5% 12% 16% 12% 3	3728
	21st to end	Night	50% 4 12% 22% 10% 1	3927
June	1st to 10th	Night	46% 5% 9% 21% 18% 2	1600
	11th to 20th	Night	41% 7% 15% 25% 11% 1	1600
	21st to end	Night	38% 7% 13% 26% 15% 1	1600
July	1st to 10th	Night	43% 7% 14% 21% 13% 1	3560
	11th to 20th	Night	48% 6% 15% 20% 10% 1	3552
	21st to end	Night	39% 6% 12% 24% 18% 1	3809
August	1st to 10th	Night	53% 7% 12% 18% 9% 1	2691
	11th to 20th	Night	43% 6% 13% 30% 8%	3227
	21st to end	Night	29% 5% 12% 35% 17% 1	3586
September	1st to 10th	Night	41% 4 9% 27% 18% 1	2000
	11th to 20th	Night	33% 4 10% 24% 26% 2	2125
	21st to end	Night	42% 6% 7% 23% 20% 2	2200
October	1st to 10th	Night	38% 4 12% 23% 20% 3	3679
	11th to 20th	Night	28% 4 9% 27% 28% 4	3876
	21st to end	Night	34% 3 13% 29% 19% 2	4467
November	1st to 10th	Night	36% 4 12% 22% 22% 4	2537
	11th to 20th	Night	41% 3 9% 23% 20% 4	2600
	21st to end	Night	40% 4 12% 30% 12% 1	2650
December	1st to 10th	Night	41% 3 11% 31% 13% 1	4580
	11th to 20th	Night	48% 3 12% 18% 17% 3	5208
	21st to end	Night	48% 3 8% 19% 17% 4	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	37% 23% 28% 11% 2	200
	11th to 20th	Day	33% 29% 28% 10% 2	200
	21st to end	Day	27% 30% 28% 10% 4	255
February	1st to 10th	Day	34% 27% 29% 10% 1	346
	11th to 20th	Day	33% 26% 30% 10% 1	496
	21st to end	Day	34% 23% 30% 10% 3	414
March	1st to 10th	Day	31% 22% 33% 13% 2	500
	11th to 20th	Day	30% 24% 32% 11% 3 1	500
	21st to end	Day	32% 23% 31% 10% 3 2	550
April	1st to 10th	Day	35% 24% 25% 10% 6% 1	500
	11th to 20th	Day	34% 18% 30% 12% 4 1	492
	21st to end	Day	31% 19% 32% 12% 5% 1	500
May	1st to 10th	Day	36% 18% 26% 13% 6% 1	503
	11th to 20th	Day	25% 18% 34% 17% 5% 1	527
	21st to end	Day	22% 11% 33% 23% 9% 2	573
June	1st to 10th	Day	22% 13% 32% 22% 10% 2	530
	11th to 20th	Day	20% 14% 33% 22% 10% 1	530
	21st to end	Day	17% 11% 32% 28% 10% 1	530
July	1st to 10th	Day	17% 8% 31% 27% 14% 3	525
	11th to 20th	Day	12% 9% 33% 27% 17% 2	524
	21st to end	Day	14% 10% 35% 28% 11% 1	576
August	1st to 10th	Day	16% 7% 27% 33% 12% 5%	526
	11th to 20th	Day	16% 7% 25% 34% 14% 5%	519
	21st to end	Day	12% 12% 31% 25% 15% 6%	583
September	1st to 10th	Day	20% 10% 28% 23% 15% 5%	530
	11th to 20th	Day	15% 10% 28% 27% 12% 8%	530
	21st to end	Day	22% 12% 32% 22% 9% 4	530
October	1st to 10th	Day	26% 11% 27% 17% 15% 5%	530
	11th to 20th	Day	19% 22% 29% 20% 8% 2	521
	21st to end	Day	30% 13% 31% 17% 8% 1	429
November	1st to 10th	Day	21% 17% 29% 22% 9% 2	395
	11th to 20th	Day	28% 22% 27% 18% 4 1	400
	21st to end	Day	28% 21% 30% 15% 4 1	345
December	1st to 10th	Day	31% 20% 30% 15% 5% 1	200
	11th to 20th	Day	39% 24% 25% 11% 2	200
	21st to end	Day	28% 23% 28% 15% 5% 1	220
January	1st to 10th	Night	26% 21% 35% 16% 3	800
	11th to 20th	Night	28% 21% 29% 19% 3	800
	21st to end	Night	24% 22% 34% 18% 2	880
February	1st to 10th	Night	25% 19% 35% 19% 2	800
	11th to 20th	Night	28% 22% 35% 13% 1	783
	21st to end	Night	28% 21% 32% 16% 3	660
March	1st to 10th	Night	27% 20% 33% 17% 3	800
	11th to 20th	Night	24% 23% 34% 17% 2 1	800
	21st to end	Night	26% 20% 34% 16% 4	880
April	1st to 10th	Night	28% 20% 32% 17% 3	800
	11th to 20th	Night	26% 18% 34% 18% 3	791
	21st to end	Night	24% 19% 35% 18% 4 1	705
May	1st to 10th	Night	31% 13% 32% 19% 5%	600
	11th to 20th	Night	22% 18% 32% 22% 5%	468
	21st to end	Night	23% 8% 29% 31% 7% 1	433
June	1st to 10th	Night	20% 8% 32% 31% 10% 1	400
	11th to 20th	Night	18% 7% 29% 35% 11% 1	400
	21st to end	Night	17% 5% 27% 42% 9%	400
July	1st to 10th	Night	15% 4 25% 37% 18% 1	390
	11th to 20th	Night	15% 4 24% 40% 16% 2	388
	21st to end	Night	15% 4 27% 39% 15% 1	438
August	1st to 10th	Night	13% 5% 27% 38% 16% 1	461
	11th to 20th	Night	16% 5% 20% 37% 19% 2	790
	21st to end	Night	15% 5% 24% 36% 18% 2	880
September	1st to 10th	Night	18% 5% 30% 32% 13% 3	800
	11th to 20th	Night	16% 7% 22% 33% 18% 4	800
	21st to end	Night	22% 9% 25% 28% 15% 1	800
October	1st to 10th	Night	23% 9% 27% 25% 14% 2	800
	11th to 20th	Night	20% 15% 30% 25% 9% 1	793
	21st to end	Night	28% 13% 32% 20% 7%	836
November	1st to 10th	Night	18% 14% 35% 24% 8% 1	780
	11th to 20th	Night	24% 17% 33% 18% 7% 1	800
	21st to end	Night	25% 17% 30% 22% 5% 1	800
December	1st to 10th	Night	28% 17% 31% 20% 5% 1	800
	11th to 20th	Night	27% 21% 29% 21% 3	800
	21st to end	Night	29% 17% 33% 18% 3	880

Page generated at 2025-10-16 01:50:26. Version ac07

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