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 2024-12-09.

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%	101579
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 42%	98222
Transparency	Day	40% 6% 11% 23% 16% 3	105125
	Night	40% 6% 11% 23% 17% 3	102764
Seeing	Day	19% 16% 29% 22% 10% 3	15124
	Night	18% 13% 32% 27% 9% 1	24437

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	15% 9% 3 5% 4 3 3 3 4 5% 46%	2517
	11th to 20th	Day	15% 9% 3 8% 7% 5% 3 3 4 6% 39%	2593
	21st to end	Day	22% 9% 2 6% 4 4 4 3 4 5% 37%	3124
February	1st to 10th	Day	15% 8% 2 7% 4 5% 4 4 6% 6% 40%	1710
	11th to 20th	Day	17% 7% 2 5% 4 4 4 3 5% 6% 41%	1817
	21st to end	Day	17% 10% 3 6% 4 4 4 3 5% 5% 38%	1570
March	1st to 10th	Day	25% 7% 2 6% 3 3 3 2 4 6% 39%	3116
	11th to 20th	Day	10% 4 2 6% 4 5% 5% 4 6% 7% 49%	3475
	21st to end	Day	21% 5% 2 5% 4 4 3 3 4 5% 44%	4056
April	1st to 10th	Day	15% 8% 2 5% 4 4 4 4 6% 7% 42%	2280
	11th to 20th	Day	14% 7% 2 5% 5% 4 4 3 5% 7% 44%	2254
	21st to end	Day	16% 6% 2 5% 5% 4 4 4 6% 6% 41%	2280
May	1st to 10th	Day	27% 9% 3 7% 4 4 4 4 5% 6% 28%	4008
	11th to 20th	Day	24% 8% 3 7% 6% 5% 4 4 5% 6% 29%	4398
	21st to end	Day	12% 5% 2 6% 4 4 4 3 6% 9% 45%	4837
June	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 5% 8% 8% 40%	2660
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 8% 39%	2660
	21st to end	Day	10% 4 2 6% 5% 4 6% 4 7% 10% 42%	2660
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 7% 5% 8% 11% 36%	4440
	11th to 20th	Day	10% 5% 2 6% 5% 5% 6% 5% 7% 11% 38%	4440
	21st to end	Day	11% 7% 3 8% 6% 6% 6% 5% 7% 9% 31%	4726
August	1st to 10th	Day	17% 6% 2 7% 5% 6% 5% 4 7% 8% 34%	4082
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 5% 6% 9% 41%	3730
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 47%	4180
September	1st to 10th	Day	13% 6% 2 6% 5% 4 4 4 6% 6% 45%	2280
	11th to 20th	Day	12% 4 2 5% 4 3 5% 3 5% 7% 51%	2244
	21st to end	Day	18% 7% 3 7% 5% 3 4 3 5% 6% 39%	2090
October	1st to 10th	Day	18% 6% 2 5% 3 3 4 3 5% 6% 45%	1912
	11th to 20th	Day	15% 5% 2 4 3 3 4 2 5% 6% 50%	1900
	21st to end	Day	17% 6% 2 6% 3 3 3 3 4 6% 48%	2024
November	1st to 10th	Day	13% 5% 2 4 4 3 3 2 4 7% 52%	1761
	11th to 20th	Day	15% 6% 1 4 3 4 4 4 5% 7% 47%	1800
	21st to end	Day	15% 5% 2 5% 4 3 4 3 5% 7% 48%	1693
December	1st to 10th	Day	19% 7% 2 5% 3 4 3 4 6% 8% 40%	1584
	11th to 20th	Day	19% 8% 2 4 4 3 3 4 4 5% 44%	1906
	21st to end	Day	16% 7% 2 6% 4 5% 5% 4 6% 8% 36%	2772
January	1st to 10th	Night	17% 7% 2 5% 4 4 4 3 5% 5% 45%	4429
	11th to 20th	Night	19% 8% 2 6% 3 3 3 3 3 5% 44%	4362
	21st to end	Night	23% 7% 2 7% 4 4 4 3 4 5% 36%	4532
February	1st to 10th	Night	16% 8% 2 6% 4 4 3 3 5% 7% 41%	2470
	11th to 20th	Night	18% 9% 2 6% 4 4 3 2 4 6% 41%	2247
	21st to end	Night	18% 8% 2 5% 4 3 4 3 4 6% 42%	1884
March	1st to 10th	Night	22% 10% 2 6% 3 3 4 3 4 6% 37%	3724
	11th to 20th	Night	11% 6% 2 5% 4 4 3 3 4 6% 50%	3482
	21st to end	Night	17% 9% 2 6% 4 3 4 3 3 4 45%	3576
April	1st to 10th	Night	17% 7% 2 7% 4 4 4 4 6% 7% 41%	1900
	11th to 20th	Night	14% 6% 3 6% 5% 4 4 4 5% 7% 42%	1875
	21st to end	Night	18% 5% 2 5% 4 4 4 2 5% 7% 43%	1900
May	1st to 10th	Night	21% 9% 3 8% 4 4 4 4 5% 7% 30%	2961
	11th to 20th	Night	23% 8% 4 7% 5% 4 4 4 5% 6% 30%	2495
	21st to end	Night	13% 6% 2 6% 4 4 5% 4 6% 9% 42%	2738
June	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 6% 9% 43%	1520
	11th to 20th	Night	9% 7% 2 7% 6% 4 4 4 7% 9% 41%	1520
	21st to end	Night	8% 6% 2 5% 4 5% 5% 4 8% 11% 42%	1520
July	1st to 10th	Night	8% 5% 3 8% 6% 6% 7% 5% 7% 9% 37%	2520
	11th to 20th	Night	11% 5% 2 6% 5% 5% 5% 5% 7% 10% 38%	2520
	21st to end	Night	15% 7% 2 8% 5% 5% 5% 4 7% 7% 36%	2822
August	1st to 10th	Night	20% 7% 2 6% 4 4 5% 5% 6% 8% 33%	2825
	11th to 20th	Night	13% 7% 2 6% 4 4 4 3 6% 7% 43%	3088
	21st to end	Night	10% 5% 2 6% 4 4 4 3 5% 7% 49%	3476
September	1st to 10th	Night	12% 6% 2 6% 5% 3 4 3 5% 7% 46%	1900
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 5% 7% 55%	1934
	21st to end	Night	19% 7% 2 6% 5% 3 4 3 5% 7% 40%	2090
October	1st to 10th	Night	18% 7% 2 6% 3 3 3 3 5% 6% 44%	2266
	11th to 20th	Night	15% 4 1 5% 3 3 3 3 5% 6% 51%	2280
	21st to end	Night	16% 6% 2 4 4 2 4 3 5% 6% 48%	2573
November	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 5% 6% 53%	2537
	11th to 20th	Night	16% 6% 2 5% 4 3 3 3 4 5% 49%	2600
	21st to end	Night	12% 6% 1 5% 4 4 4 4 6% 6% 45%	2705
December	1st to 10th	Night	19% 7% 2 6% 3 3 4 3 5% 6% 41%	2779
	11th to 20th	Night	16% 7% 2 6% 4 4 4 3 5% 7% 42%	3288
	21st to end	Night	17% 7% 3 6% 4 4 4 4 6% 7% 37%	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	40% 4 10% 26% 15% 4	2518
	11th to 20th	Day	48% 5% 9% 22% 14% 3	2593
	21st to end	Day	48% 7% 11% 22% 11% 2	3124
February	1st to 10th	Day	43% 6% 13% 24% 12% 2	1710
	11th to 20th	Day	43% 5% 9% 24% 15% 3	1826
	21st to end	Day	48% 4 10% 21% 13% 5%	1570
March	1st to 10th	Day	47% 4 8% 28% 10% 2	3119
	11th to 20th	Day	32% 5% 10% 31% 18% 3	3475
	21st to end	Day	44% 5% 7% 19% 22% 4	4056
April	1st to 10th	Day	42% 6% 10% 20% 16% 5%	2280
	11th to 20th	Day	39% 6% 12% 23% 16% 4	2254
	21st to end	Day	41% 6% 11% 19% 19% 4	2280
May	1st to 10th	Day	55% 6% 10% 18% 10% 2	4020
	11th to 20th	Day	54% 8% 12% 17% 8% 1	4402
	21st to end	Day	34% 9% 11% 27% 18% 2	4837
June	1st to 10th	Day	38% 9% 12% 23% 16% 2	2660
	11th to 20th	Day	39% 10% 13% 23% 13% 3	2660
	21st to end	Day	35% 10% 13% 23% 17% 1	2660
July	1st to 10th	Day	39% 10% 17% 17% 14% 2	4370
	11th to 20th	Day	38% 7% 18% 23% 13% 2	4365
	21st to end	Day	46% 8% 12% 19% 14% 1	4575
August	1st to 10th	Day	47% 7% 14% 22% 9% 1	4069
	11th to 20th	Day	39% 7% 13% 27% 13% 1	3766
	21st to end	Day	32% 7% 12% 28% 19% 2	4180
September	1st to 10th	Day	40% 6% 10% 19% 22% 3	2280
	11th to 20th	Day	34% 5% 9% 23% 25% 3	2244
	21st to end	Day	47% 5% 8% 18% 20% 2	2090
October	1st to 10th	Day	36% 4 9% 17% 29% 5%	3172
	11th to 20th	Day	25% 3 9% 23% 29% 9%	3140
	21st to end	Day	25% 3 10% 28% 28% 5%	3314
November	1st to 10th	Day	33% 4 12% 27% 21% 4	1761
	11th to 20th	Day	36% 6% 9% 24% 22% 3	1800
	21st to end	Day	38% 4 11% 31% 14% 3	1693
December	1st to 10th	Day	42% 5% 15% 24% 13% 1	1584
	11th to 20th	Day	43% 5% 11% 28% 10% 3	1906
	21st to end	Day	44% 6% 13% 23% 13% 1	2772
January	1st to 10th	Night	40% 4 9% 29% 14% 4	4429
	11th to 20th	Night	44% 5% 9% 25% 15% 3	4362
	21st to end	Night	49% 4 8% 25% 13% 1	4532
February	1st to 10th	Night	43% 5% 13% 23% 14% 2	2470
	11th to 20th	Night	44% 5% 9% 20% 18% 5%	2249
	21st to end	Night	43% 5% 9% 22% 16% 6%	1884
March	1st to 10th	Night	49% 5% 8% 23% 13% 2	3734
	11th to 20th	Night	35% 5% 12% 28% 17% 4	3483
	21st to end	Night	43% 4 7% 17% 22% 5%	3576
April	1st to 10th	Night	43% 5% 9% 20% 19% 4	1900
	11th to 20th	Night	41% 6% 10% 21% 19% 4	1875
	21st to end	Night	41% 4 9% 20% 20% 5%	1900
May	1st to 10th	Night	51% 7% 10% 19% 11% 1	2968
	11th to 20th	Night	53% 8% 12% 15% 10% 2	2496
	21st to end	Night	37% 9% 15% 20% 17% 3	2739
June	1st to 10th	Night	38% 8% 11% 23% 20% 2	1520
	11th to 20th	Night	38% 8% 14% 26% 13% 2	1520
	21st to end	Night	34% 9% 15% 25% 15% 2	1520
July	1st to 10th	Night	41% 9% 19% 18% 12% 2	2480
	11th to 20th	Night	38% 7% 20% 24% 9% 1	2472
	21st to end	Night	46% 7% 14% 19% 14% 1	2732
August	1st to 10th	Night	48% 7% 19% 17% 8%	2814
	11th to 20th	Night	39% 7% 12% 29% 14% 1	3118
	21st to end	Night	33% 5% 10% 30% 20% 2	3476
September	1st to 10th	Night	38% 8% 11% 19% 21% 4	1900
	11th to 20th	Night	30% 5% 11% 25% 26% 4	1934
	21st to end	Night	44% 5% 9% 20% 19% 2	2090
October	1st to 10th	Night	39% 4 10% 22% 21% 3	3786
	11th to 20th	Night	24% 3 11% 25% 30% 7%	3779
	21st to end	Night	25% 3 12% 30% 25% 5%	4233
November	1st to 10th	Night	33% 5% 11% 24% 21% 6%	2537
	11th to 20th	Night	39% 4 9% 22% 21% 6%	2600
	21st to end	Night	36% 6% 11% 27% 18% 2	2705
December	1st to 10th	Night	43% 5% 11% 25% 14% 2	2779
	11th to 20th	Night	41% 5% 11% 20% 19% 4	3288
	21st to end	Night	44% 7% 14% 24% 11% 1	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	25% 27% 31% 12% 4 1	180
	11th to 20th	Day	29% 33% 26% 11% 1	202
	21st to end	Day	24% 31% 28% 13% 3 1	297
February	1st to 10th	Day	26% 30% 31% 10% 2	270
	11th to 20th	Day	29% 27% 32% 10% 1	425
	21st to end	Day	29% 24% 32% 13% 2	382
March	1st to 10th	Day	25% 24% 31% 16% 4	460
	11th to 20th	Day	24% 28% 32% 12% 4 2	460
	21st to end	Day	26% 23% 33% 14% 4 1	506
April	1st to 10th	Day	23% 27% 33% 10% 5% 1	460
	11th to 20th	Day	20% 22% 33% 16% 7% 2	452
	21st to end	Day	23% 22% 32% 17% 5% 1	460
May	1st to 10th	Day	28% 22% 26% 15% 7% 2	463
	11th to 20th	Day	21% 16% 33% 21% 7% 2	487
	21st to end	Day	16% 10% 27% 27% 16% 3	529
June	1st to 10th	Day	14% 13% 31% 28% 13% 3	490
	11th to 20th	Day	15% 11% 31% 27% 13% 3	490
	21st to end	Day	12% 10% 31% 29% 15% 3	490
July	1st to 10th	Day	12% 6% 26% 31% 20% 6%	485
	11th to 20th	Day	7% 4 26% 36% 21% 5%	484
	21st to end	Day	12% 7% 27% 33% 17% 5%	532
August	1st to 10th	Day	12% 6% 27% 32% 17% 6%	486
	11th to 20th	Day	13% 4 21% 34% 20% 7%	479
	21st to end	Day	11% 8% 28% 27% 18% 8%	539
September	1st to 10th	Day	17% 11% 22% 26% 16% 9%	490
	11th to 20th	Day	14% 6% 29% 27% 16% 9%	490
	21st to end	Day	25% 13% 25% 23% 10% 3	490
October	1st to 10th	Day	23% 10% 26% 23% 13% 6%	490
	11th to 20th	Day	18% 16% 28% 23% 12% 2	490
	21st to end	Day	19% 16% 34% 18% 10% 3	504
November	1st to 10th	Day	16% 15% 34% 23% 11% 2	395
	11th to 20th	Day	19% 22% 34% 19% 7%	400
	21st to end	Day	19% 23% 31% 22% 4	293
December	1st to 10th	Day	22% 31% 30% 11% 5%	196
	11th to 20th	Day	31% 22% 32% 12% 3	180
	21st to end	Day	27% 22% 29% 17% 6% 1	198
January	1st to 10th	Night	20% 20% 38% 19% 3	760
	11th to 20th	Night	24% 19% 33% 22% 3	760
	21st to end	Night	19% 19% 39% 21% 2	836
February	1st to 10th	Night	21% 18% 38% 22% 1	760
	11th to 20th	Night	26% 25% 34% 15% 1	743
	21st to end	Night	24% 21% 35% 18% 2	628
March	1st to 10th	Night	21% 20% 34% 21% 4	760
	11th to 20th	Night	21% 21% 35% 16% 5% 1	760
	21st to end	Night	21% 17% 41% 16% 5%	836
April	1st to 10th	Night	21% 22% 34% 17% 5%	760
	11th to 20th	Night	18% 16% 38% 25% 4	751
	21st to end	Night	21% 16% 33% 25% 5%	760
May	1st to 10th	Night	20% 13% 38% 22% 6%	646
	11th to 20th	Night	18% 10% 38% 28% 6%	374
	21st to end	Night	18% 5% 25% 38% 13% 1	411
June	1st to 10th	Night	16% 6% 29% 38% 11%	380
	11th to 20th	Night	12% 6% 31% 41% 11% 1	380
	21st to end	Night	9% 4 31% 45% 10%	380
July	1st to 10th	Night	10% 3 29% 42% 16% 1	370
	11th to 20th	Night	8% 5% 20% 48% 17% 2	368
	21st to end	Night	10% 4 27% 41% 18% 1	463
August	1st to 10th	Night	12% 3 20% 42% 22% 2	567
	11th to 20th	Night	14% 2 19% 42% 21% 2	741
	21st to end	Night	13% 3 21% 39% 21% 3	836
September	1st to 10th	Night	16% 4 25% 37% 16% 2	760
	11th to 20th	Night	14% 4 21% 36% 21% 4	760
	21st to end	Night	23% 6% 25% 29% 16% 1	760
October	1st to 10th	Night	21% 7% 26% 28% 15% 3	760
	11th to 20th	Night	18% 11% 29% 29% 12% 1	760
	21st to end	Night	20% 13% 34% 24% 9%	836
November	1st to 10th	Night	13% 12% 36% 27% 11% 1	780
	11th to 20th	Night	19% 16% 33% 25% 7% 1	800
	21st to end	Night	16% 19% 36% 22% 7% 1	800
December	1st to 10th	Night	24% 16% 34% 19% 7% 1	795
	11th to 20th	Night	17% 20% 35% 23% 6%	760
	21st to end	Night	21% 16% 34% 26% 2	836

Page generated at 2024-12-06 03:54:46. Version ac07

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