Sewell
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sewell clear sky chart from 2005-11-06 to 2026-04-02.

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% 6% 2 6% 4 4 4 4 6% 7% 41%	131380
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	130470
Transparency	Day	40% 6% 11% 23% 17% 3	134908
	Night	40% 5% 11% 23% 18% 4	135028
Seeing	Day	20% 16% 29% 21% 10% 3	16774
	Night	19% 13% 32% 27% 9% 1	26174

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% 6% 2 6% 4 4 4 3 5% 6% 46%	4661
	11th to 20th	Day	15% 7% 2 7% 5% 4 2 2 3 5% 47%	4807
	21st to end	Day	16% 6% 2 5% 4 4 5% 3 5% 6% 44%	5780
February	1st to 10th	Day	14% 8% 2 7% 4 5% 4 4 6% 6% 41%	1890
	11th to 20th	Day	17% 7% 2 6% 5% 4 4 3 5% 6% 40%	2015
	21st to end	Day	17% 10% 3 6% 4 4 4 3 5% 6% 39%	1730
March	1st to 10th	Day	20% 6% 2 5% 3 3 3 3 4 6% 46%	4576
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 5% 6% 47%	5085
	21st to end	Day	18% 5% 2 6% 4 4 4 3 5% 6% 44%	5932
April	1st to 10th	Day	16% 8% 2 5% 4 4 4 4 6% 7% 41%	2412
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 3 5% 7% 43%	2374
	21st to end	Day	15% 6% 2 5% 5% 4 4 4 6% 7% 42%	2400
May	1st to 10th	Day	24% 8% 3 6% 4 4 4 4 5% 8% 30%	5746
	11th to 20th	Day	23% 8% 3 7% 4 4 4 3 4 7% 33%	6318
	21st to end	Day	19% 6% 2 6% 4 4 4 3 6% 8% 37%	6949
June	1st to 10th	Day	12% 6% 2 7% 5% 4 5% 5% 7% 8% 40%	2800
	11th to 20th	Day	12% 6% 2 7% 5% 5% 5% 5% 7% 8% 39%	2800
	21st to end	Day	10% 4 2 6% 5% 4 6% 4 7% 9% 42%	2800
July	1st to 10th	Day	8% 5% 2 7% 5% 6% 6% 5% 7% 11% 38%	6360
	11th to 20th	Day	11% 6% 3 8% 6% 6% 7% 6% 8% 11% 31%	6360
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 10% 38%	6796
August	1st to 10th	Day	17% 6% 2 7% 5% 6% 5% 4 7% 8% 34%	4211
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 5% 6% 9% 41%	3850
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 48%	4312
September	1st to 10th	Day	13% 6% 2 6% 5% 4 4 4 6% 6% 45%	2400
	11th to 20th	Day	12% 4 1 5% 4 3 5% 3 5% 7% 51%	2362
	21st to end	Day	18% 8% 3 8% 5% 4 5% 3 5% 6% 38%	2200
October	1st to 10th	Day	17% 6% 2 5% 3 3 4 3 5% 6% 46%	2012
	11th to 20th	Day	16% 5% 2 4 3 3 4 2 5% 6% 50%	2000
	21st to end	Day	16% 6% 2 6% 3 3 3 3 4 6% 48%	2130
November	1st to 10th	Day	13% 5% 2 4 4 3 3 2 4 7% 53%	1851
	11th to 20th	Day	14% 6% 1 4 3 4 4 4 5% 7% 47%	1890
	21st to end	Day	15% 5% 2 5% 5% 3 4 3 5% 7% 47%	1777
December	1st to 10th	Day	14% 6% 2 6% 2 4 4 4 6% 7% 45%	2680
	11th to 20th	Day	19% 10% 2 5% 3 3 3 3 3 5% 44%	3066
	21st to end	Day	15% 6% 2 5% 3 5% 5% 4 6% 8% 40%	4048
January	1st to 10th	Night	12% 8% 2 6% 4 4 4 3 5% 6% 46%	8234
	11th to 20th	Night	16% 7% 2 6% 3 3 3 3 3 5% 49%	8073
	21st to end	Night	15% 7% 2 7% 4 3 3 3 4 6% 45%	8401
February	1st to 10th	Night	16% 8% 3 6% 4 4 3 3 5% 7% 41%	2730
	11th to 20th	Night	19% 8% 2 6% 4 4 3 3 5% 6% 41%	2489
	21st to end	Night	17% 8% 3 6% 4 4 4 3 5% 6% 42%	2076
March	1st to 10th	Night	18% 8% 2 6% 3 3 3 2 4 6% 45%	5484
	11th to 20th	Night	11% 8% 3 5% 4 4 3 3 4 5% 51%	5092
	21st to end	Night	16% 8% 3 6% 4 4 5% 3 4 5% 43%	5242
April	1st to 10th	Night	17% 7% 2 7% 4 4 4 4 5% 6% 40%	2011
	11th to 20th	Night	15% 6% 3 6% 5% 4 4 4 5% 7% 42%	1975
	21st to end	Night	17% 5% 2 5% 4 4 4 3 5% 7% 44%	2000
May	1st to 10th	Night	20% 9% 3 8% 4 5% 5% 5% 6% 8% 28%	4237
	11th to 20th	Night	22% 8% 4 6% 4 4 4 4 5% 7% 32%	3575
	21st to end	Night	21% 6% 3 7% 4 4 4 3 5% 8% 37%	3926
June	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 6% 9% 42%	1600
	11th to 20th	Night	10% 7% 3 7% 5% 4 5% 4 6% 9% 40%	1600
	21st to end	Night	9% 6% 2 5% 4 5% 5% 4 8% 11% 42%	1600
July	1st to 10th	Night	9% 6% 3 8% 5% 6% 7% 5% 6% 9% 37%	3600
	11th to 20th	Night	12% 7% 3 8% 5% 5% 5% 5% 7% 10% 32%	3600
	21st to end	Night	11% 6% 2 6% 5% 4 5% 4 6% 8% 43%	4051
August	1st to 10th	Night	20% 7% 2 6% 4 4 5% 5% 6% 8% 34%	2915
	11th to 20th	Night	13% 7% 2 7% 4 4 4 3 6% 7% 42%	3188
	21st to end	Night	10% 5% 2 6% 4 4 4 3 5% 7% 49%	3586
September	1st to 10th	Night	12% 6% 2 7% 5% 3 4 3 5% 6% 47%	2000
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 5% 7% 54%	2036
	21st to end	Night	19% 7% 2 6% 5% 3 4 3 5% 7% 39%	2200
October	1st to 10th	Night	18% 6% 2 6% 3 3 4 3 5% 6% 45%	2385
	11th to 20th	Night	16% 5% 1 5% 3 3 3 2 5% 6% 51%	2400
	21st to end	Night	16% 6% 2 4 4 2 4 3 5% 6% 48%	2709
November	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 5% 6% 53%	2667
	11th to 20th	Night	16% 6% 2 5% 4 3 3 3 4 5% 49%	2730
	21st to end	Night	12% 6% 2 6% 4 4 4 4 6% 6% 45%	2840
December	1st to 10th	Night	13% 7% 2 6% 3 3 4 3 5% 6% 48%	4720
	11th to 20th	Night	22% 7% 2 5% 3 3 3 3 5% 7% 41%	5348
	21st to end	Night	18% 7% 3 6% 4 4 4 4 5% 6% 40%	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	40% 3 8% 27% 20% 2	4662
	11th to 20th	Day	42% 3 5% 23% 20% 7%	4837
	21st to end	Day	41% 5% 11% 28% 13% 2	5780
February	1st to 10th	Day	42% 5% 13% 24% 13% 2	1890
	11th to 20th	Day	45% 5% 9% 23% 15% 3	2024
	21st to end	Day	46% 4 9% 23% 13% 5%	1730
March	1st to 10th	Day	40% 4 8% 25% 17% 7%	4579
	11th to 20th	Day	36% 4 9% 29% 17% 5%	5085
	21st to end	Day	41% 4 9% 17% 23% 6%	5932
April	1st to 10th	Day	43% 6% 10% 20% 16% 6%	2412
	11th to 20th	Day	40% 6% 12% 22% 16% 4	2374
	21st to end	Day	41% 5% 10% 20% 19% 4	2400
May	1st to 10th	Day	52% 5% 13% 18% 10% 1	5755
	11th to 20th	Day	51% 6% 9% 21% 12% 1	6322
	21st to end	Day	43% 7% 10% 24% 15% 1	6949
June	1st to 10th	Day	39% 9% 12% 23% 16% 2	2800
	11th to 20th	Day	40% 9% 13% 23% 13% 3	2800
	21st to end	Day	35% 10% 13% 23% 17% 2	2800
July	1st to 10th	Day	39% 9% 17% 18% 15% 2	6290
	11th to 20th	Day	44% 7% 19% 19% 9% 1	6285
	21st to end	Day	40% 7% 11% 21% 21% 1	6600
August	1st to 10th	Day	47% 7% 14% 22% 10% 1	4198
	11th to 20th	Day	39% 7% 13% 27% 13% 1	3886
	21st to end	Day	32% 7% 11% 28% 20% 2	4312
September	1st to 10th	Day	39% 6% 11% 20% 22% 3	2400
	11th to 20th	Day	34% 5% 9% 25% 24% 3	2362
	21st to end	Day	48% 5% 8% 17% 19% 2	2200
October	1st to 10th	Day	35% 4 9% 18% 29% 5%	3272
	11th to 20th	Day	26% 3 9% 23% 29% 9%	3240
	21st to end	Day	25% 3 10% 28% 28% 5%	3420
November	1st to 10th	Day	33% 4 12% 27% 21% 4	1851
	11th to 20th	Day	37% 5% 9% 24% 22% 3	1890
	21st to end	Day	38% 4 11% 31% 14% 3	1777
December	1st to 10th	Day	37% 5% 17% 26% 14% 2	2680
	11th to 20th	Day	44% 3 10% 30% 10% 3	3066
	21st to end	Day	41% 6% 12% 23% 18% 2	4048
January	1st to 10th	Night	39% 3 11% 25% 17% 5%	8234
	11th to 20th	Night	39% 3 8% 23% 19% 8%	8118
	21st to end	Night	40% 3 7% 27% 20% 3	8401
February	1st to 10th	Night	43% 5% 12% 23% 14% 2	2730
	11th to 20th	Night	45% 5% 9% 20% 17% 5%	2491
	21st to end	Night	43% 4 9% 22% 15% 6%	2076
March	1st to 10th	Night	42% 4 8% 24% 13% 9%	5494
	11th to 20th	Night	36% 4 13% 25% 16% 5%	5093
	21st to end	Night	43% 4 8% 16% 24% 4	5242
April	1st to 10th	Night	44% 5% 9% 19% 19% 4	2011
	11th to 20th	Night	41% 6% 10% 20% 19% 4	1975
	21st to end	Night	41% 4 9% 20% 21% 5%	2000
May	1st to 10th	Night	52% 7% 12% 16% 11% 2	4244
	11th to 20th	Night	51% 7% 10% 19% 12% 1	3576
	21st to end	Night	45% 7% 11% 21% 14% 2	3927
June	1st to 10th	Night	38% 8% 10% 22% 20% 2	1600
	11th to 20th	Night	39% 7% 14% 25% 13% 2	1600
	21st to end	Night	34% 9% 15% 25% 15% 2	1600
July	1st to 10th	Night	43% 8% 15% 20% 12% 1	3560
	11th to 20th	Night	46% 7% 20% 19% 7% 1	3552
	21st to end	Night	38% 6% 13% 24% 19% 1	3932
August	1st to 10th	Night	47% 7% 18% 18% 9%	2904
	11th to 20th	Night	39% 6% 12% 28% 13% 1	3218
	21st to end	Night	33% 5% 10% 30% 20% 2	3586
September	1st to 10th	Night	38% 7% 11% 20% 20% 4	2000
	11th to 20th	Night	31% 5% 10% 25% 25% 4	2036
	21st to end	Night	44% 5% 10% 20% 19% 2	2200
October	1st to 10th	Night	39% 4 10% 23% 21% 3	3905
	11th to 20th	Night	25% 3 11% 25% 30% 6%	3899
	21st to end	Night	25% 3 11% 30% 25% 5%	4369
November	1st to 10th	Night	32% 5% 11% 25% 21% 6%	2667
	11th to 20th	Night	39% 4 9% 21% 21% 6%	2730
	21st to end	Night	37% 5% 11% 26% 18% 2	2840
December	1st to 10th	Night	36% 4 12% 30% 15% 2	4720
	11th to 20th	Night	44% 3 10% 23% 16% 4	5348
	21st to end	Night	44% 5% 11% 20% 18% 2	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	28% 26% 31% 11% 4	220
	11th to 20th	Day	31% 31% 28% 9% 1	248
	21st to end	Day	25% 32% 28% 12% 2 1	363
February	1st to 10th	Day	25% 29% 33% 11% 2	330
	11th to 20th	Day	32% 25% 32% 9% 1	503
	21st to end	Day	28% 24% 33% 13% 2	446
March	1st to 10th	Day	28% 23% 31% 14% 4	540
	11th to 20th	Day	25% 26% 33% 11% 3 1	540
	21st to end	Day	25% 25% 32% 13% 3 1	594
April	1st to 10th	Day	26% 26% 33% 10% 5% 1	504
	11th to 20th	Day	22% 23% 32% 15% 6% 2	492
	21st to end	Day	22% 22% 33% 17% 5% 1	500
May	1st to 10th	Day	29% 21% 27% 15% 7% 2	503
	11th to 20th	Day	21% 17% 33% 20% 7% 2	527
	21st to end	Day	19% 10% 27% 25% 15% 3	573
June	1st to 10th	Day	14% 13% 30% 27% 13% 2	530
	11th to 20th	Day	17% 12% 30% 26% 12% 3	530
	21st to end	Day	12% 10% 30% 29% 16% 3	530
July	1st to 10th	Day	12% 5% 26% 31% 20% 6%	525
	11th to 20th	Day	7% 5% 26% 35% 22% 5%	524
	21st to end	Day	12% 7% 25% 34% 17% 5%	576
August	1st to 10th	Day	13% 6% 28% 31% 17% 6%	526
	11th to 20th	Day	14% 4 20% 33% 21% 7%	519
	21st to end	Day	11% 8% 29% 28% 17% 8%	583
September	1st to 10th	Day	17% 11% 23% 25% 16% 8%	530
	11th to 20th	Day	14% 6% 29% 28% 15% 8%	530
	21st to end	Day	25% 13% 26% 23% 10% 3	530
October	1st to 10th	Day	22% 9% 26% 22% 14% 7%	530
	11th to 20th	Day	21% 15% 28% 22% 11% 2	530
	21st to end	Day	19% 17% 34% 18% 9% 2	544
November	1st to 10th	Day	17% 16% 33% 22% 10% 2	425
	11th to 20th	Day	20% 22% 33% 18% 7%	430
	21st to end	Day	20% 23% 31% 21% 4	317
December	1st to 10th	Day	24% 30% 31% 11% 5%	220
	11th to 20th	Day	32% 21% 33% 11% 3	220
	21st to end	Day	29% 22% 29% 15% 5%	242
January	1st to 10th	Night	20% 21% 38% 18% 3	840
	11th to 20th	Night	24% 21% 32% 21% 3	840
	21st to end	Night	19% 20% 39% 21% 2	924
February	1st to 10th	Night	21% 18% 39% 21% 1	840
	11th to 20th	Night	27% 24% 33% 15% 1	823
	21st to end	Night	23% 21% 37% 17% 2	692
March	1st to 10th	Night	21% 20% 34% 21% 4	840
	11th to 20th	Night	21% 21% 36% 17% 5%	840
	21st to end	Night	21% 18% 41% 15% 4	924
April	1st to 10th	Night	22% 22% 34% 17% 4	805
	11th to 20th	Night	19% 16% 38% 24% 4	791
	21st to end	Night	20% 16% 34% 25% 5%	800
May	1st to 10th	Night	20% 14% 39% 22% 6%	680
	11th to 20th	Night	19% 11% 37% 28% 5%	394
	21st to end	Night	19% 5% 27% 37% 12% 1	433
June	1st to 10th	Night	16% 6% 30% 38% 10%	400
	11th to 20th	Night	13% 6% 30% 40% 11% 1	400
	21st to end	Night	10% 4 30% 45% 10%	400
July	1st to 10th	Night	11% 3 28% 42% 17% 1	390
	11th to 20th	Night	9% 5% 19% 47% 18% 2	388
	21st to end	Night	9% 3 26% 42% 18% 1	488
August	1st to 10th	Night	12% 3 19% 42% 22% 2	597
	11th to 20th	Night	14% 3 19% 41% 21% 2	781
	21st to end	Night	13% 3 22% 39% 21% 2	880
September	1st to 10th	Night	16% 4 26% 37% 16% 2	800
	11th to 20th	Night	14% 5% 21% 36% 21% 4	800
	21st to end	Night	23% 6% 25% 30% 16% 1	800
October	1st to 10th	Night	21% 7% 26% 28% 15% 3	800
	11th to 20th	Night	19% 12% 28% 29% 12% 1	800
	21st to end	Night	19% 13% 35% 24% 8%	880
November	1st to 10th	Night	14% 12% 36% 27% 10% 1	820
	11th to 20th	Night	19% 17% 33% 24% 6%	840
	21st to end	Night	16% 19% 35% 22% 7% 1	840
December	1st to 10th	Night	23% 16% 35% 19% 7% 1	840
	11th to 20th	Night	19% 20% 34% 22% 5%	840
	21st to end	Night	22% 16% 33% 26% 2	924

Page generated at 2026-03-30 02:18:41. Version ac07

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