Seddon's Corner
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seddon's Corner clear sky chart from 2005-11-06 to 2024-02-15.

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	12% 9% 4 8% 7% 4 5% 5% 6% 7% 32%	77767
	Night	11% 11% 5% 9% 7% 4 6% 5% 6% 7% 31%	81301
Transparency	Day	50% 7% 14% 21% 8% 1	77469
	Night	52% 7% 14% 19% 8% 1	81180
Seeing	Day	15% 9% 30% 29% 14% 3	15157
	Night	16% 6% 31% 33% 13% 1	25294

By Month

Percentage Cloud Cover NW (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	7% 15% 7% 12% 10% 5% 9% 8% 5% 5% 18%	2219
	11th to 20th	Day	5% 9% 6% 16% 14% 10% 10% 8% 7% 4 11%	2199
	21st to end	Day	7% 19% 9% 16% 9% 5% 6% 6% 6% 5% 13%	2702
February	1st to 10th	Day	8% 13% 6% 9% 10% 5% 8% 8% 6% 8% 19%	1662
	11th to 20th	Day	8% 10% 5% 10% 8% 6% 8% 8% 8% 8% 22%	1610
	21st to end	Day	6% 12% 7% 9% 11% 6% 7% 6% 6% 7% 24%	1363
March	1st to 10th	Day	12% 13% 6% 9% 7% 3 7% 8% 5% 6% 24%	1869
	11th to 20th	Day	8% 10% 6% 9% 8% 5% 5% 6% 5% 7% 31%	1978
	21st to end	Day	11% 12% 5% 9% 7% 4 5% 4 5% 7% 31%	2178
April	1st to 10th	Day	12% 11% 5% 9% 6% 2 4 4 5% 7% 35%	2185
	11th to 20th	Day	13% 9% 4 6% 6% 4 4 4 5% 8% 36%	2312
	21st to end	Day	14% 9% 3 7% 6% 3 5% 5% 6% 7% 36%	2340
May	1st to 10th	Day	16% 8% 3 5% 5% 3 4 5% 5% 6% 40%	2456
	11th to 20th	Day	19% 7% 3 6% 5% 3 4 4 6% 8% 36%	2565
	21st to end	Day	18% 7% 3 5% 5% 3 4 4 5% 7% 37%	2918
June	1st to 10th	Day	16% 7% 2 7% 5% 4 5% 5% 5% 8% 36%	2700
	11th to 20th	Day	17% 6% 3 6% 6% 3 5% 6% 5% 8% 35%	2700
	21st to end	Day	12% 7% 3 7% 6% 4 6% 6% 7% 8% 34%	2700
July	1st to 10th	Day	8% 4 2 5% 5% 3 5% 6% 7% 9% 47%	2700
	11th to 20th	Day	11% 6% 2 5% 4 3 5% 5% 6% 11% 43%	2700
	21st to end	Day	10% 5% 2 5% 4 3 5% 5% 6% 8% 48%	2939
August	1st to 10th	Day	8% 6% 3 6% 6% 4 4 5% 6% 7% 46%	2420
	11th to 20th	Day	12% 5% 2 5% 5% 4 4 5% 6% 8% 44%	2301
	21st to end	Day	14% 6% 2 5% 4 2 5% 4 5% 8% 44%	2574
September	1st to 10th	Day	12% 5% 2 5% 5% 3 4 5% 5% 7% 46%	2230
	11th to 20th	Day	15% 6% 2 7% 6% 3 5% 6% 6% 9% 35%	2003
	21st to end	Day	19% 7% 3 5% 4 3 4 4 5% 7% 39%	1980
October	1st to 10th	Day	18% 12% 4 8% 6% 3 4 4 4 5% 33%	1805
	11th to 20th	Day	15% 13% 4 7% 7% 3 5% 4 6% 6% 30%	1800
	21st to end	Day	18% 15% 5% 8% 7% 3 5% 5% 5% 7% 22%	1765
November	1st to 10th	Day	16% 16% 5% 9% 6% 4 4 4 5% 6% 25%	1487
	11th to 20th	Day	11% 15% 6% 14% 9% 4 4 4 5% 5% 22%	1520
	21st to end	Day	13% 13% 6% 13% 13% 4 4 4 5% 6% 20%	1436
December	1st to 10th	Day	9% 13% 7% 17% 10% 6% 6% 6% 6% 6% 14%	1330
	11th to 20th	Day	10% 15% 6% 13% 7% 3 6% 5% 6% 8% 20%	1679
	21st to end	Day	10% 13% 5% 10% 8% 5% 6% 8% 8% 8% 19%	2442
January	1st to 10th	Night	5% 13% 8% 13% 10% 8% 9% 7% 5% 6% 16%	4737
	11th to 20th	Night	6% 13% 5% 12% 15% 10% 10% 7% 6% 5% 12%	4709
	21st to end	Night	8% 19% 9% 12% 8% 5% 7% 5% 5% 5% 17%	4960
February	1st to 10th	Night	7% 12% 6% 9% 8% 4 7% 7% 6% 7% 25%	2516
	11th to 20th	Night	7% 10% 4 7% 7% 5% 7% 7% 7% 8% 31%	2332
	21st to end	Night	7% 9% 6% 10% 9% 6% 6% 5% 6% 6% 30%	1910
March	1st to 10th	Night	9% 11% 7% 10% 7% 4 5% 5% 5% 7% 30%	2090
	11th to 20th	Night	8% 9% 4 7% 7% 3 4 4 4 7% 43%	1980
	21st to end	Night	13% 12% 4 7% 5% 3 4 4 6% 7% 35%	2178
April	1st to 10th	Night	12% 9% 4 4 6% 3 5% 5% 6% 7% 39%	1772
	11th to 20th	Night	13% 9% 4 7% 5% 3 5% 4 5% 6% 38%	1599
	21st to end	Night	11% 10% 5% 6% 6% 4 5% 5% 5% 5% 37%	1620
May	1st to 10th	Night	16% 9% 3 5% 5% 2 4 5% 5% 7% 38%	1502
	11th to 20th	Night	16% 7% 2 4 5% 4 4 4 5% 7% 43%	1329
	21st to end	Night	17% 7% 2 3 5% 3 3 4 7% 6% 42%	1358
June	1st to 10th	Night	13% 6% 3 6% 5% 3 5% 5% 7% 10% 40%	1260
	11th to 20th	Night	15% 5% 3 6% 4 3 4 5% 5% 9% 41%	1260
	21st to end	Night	11% 5% 3 4 5% 3 4 6% 5% 8% 45%	1260
July	1st to 10th	Night	7% 3 2 3 4 2 4 4 7% 8% 57%	1260
	11th to 20th	Night	7% 4 2 4 5% 4 5% 5% 7% 10% 49%	1260
	21st to end	Night	7% 3 2 3 4 3 3 4 7% 8% 55%	1379
August	1st to 10th	Night	7% 4 2 3 5% 2 5% 5% 6% 8% 53%	1515
	11th to 20th	Night	10% 4 2 4 4 3 4 4 6% 9% 51%	1600
	21st to end	Night	12% 5% 2 3 4 3 4 5% 5% 8% 49%	1782
September	1st to 10th	Night	11% 4 2 5% 4 3 4 4 5% 7% 50%	1729
	11th to 20th	Night	16% 7% 2 5% 5% 3 5% 4 4 8% 43%	1954
	21st to end	Night	18% 6% 3 5% 6% 3 4 4 5% 7% 39%	1980
October	1st to 10th	Night	18% 9% 3 5% 6% 3 4 4 5% 8% 35%	2137
	11th to 20th	Night	18% 11% 3 8% 6% 4 4 4 5% 6% 32%	2160
	21st to end	Night	17% 14% 4 9% 6% 3 5% 4 5% 6% 27%	2589
November	1st to 10th	Night	14% 14% 5% 9% 6% 4 5% 5% 5% 5% 27%	2591
	11th to 20th	Night	12% 16% 7% 11% 9% 3 4 4 4 6% 23%	2660
	21st to end	Night	12% 14% 7% 13% 10% 5% 6% 4 4 5% 19%	2740
December	1st to 10th	Night	8% 12% 6% 14% 12% 6% 6% 6% 6% 6% 18%	2850
	11th to 20th	Night	9% 14% 7% 12% 9% 4 5% 6% 6% 7% 20%	3518
	21st to end	Night	9% 10% 6% 11% 10% 5% 7% 7% 7% 6% 22%	5225

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	65% 8% 11% 13% 4	2219
	11th to 20th	Day	70% 7% 12% 9% 2	2220
	21st to end	Day	70% 6% 10% 10% 4	2702
February	1st to 10th	Day	59% 5% 14% 13% 8% 1	1662
	11th to 20th	Day	55% 6% 14% 18% 7%	1619
	21st to end	Day	58% 6% 12% 15% 9%	1363
March	1st to 10th	Day	58% 6% 14% 15% 7%	1869
	11th to 20th	Day	52% 6% 15% 19% 7%	1980
	21st to end	Day	53% 5% 13% 20% 7% 1	2178
April	1st to 10th	Day	49% 5% 11% 23% 11% 1	2185
	11th to 20th	Day	47% 6% 11% 22% 13% 2	2312
	21st to end	Day	48% 6% 14% 22% 9% 1	2340
May	1st to 10th	Day	46% 6% 10% 23% 14% 1	2456
	11th to 20th	Day	47% 5% 10% 23% 14% 2	2569
	21st to end	Day	47% 7% 10% 21% 13% 1	2918
June	1st to 10th	Day	46% 7% 14% 22% 11% 1	2700
	11th to 20th	Day	47% 7% 14% 21% 10% 1	2700
	21st to end	Day	46% 9% 17% 22% 7%	2700
July	1st to 10th	Day	31% 8% 17% 31% 11%	2626
	11th to 20th	Day	34% 8% 17% 31% 9%	2619
	21st to end	Day	35% 8% 17% 29% 11%	2776
August	1st to 10th	Day	36% 8% 16% 31% 8%	2403
	11th to 20th	Day	36% 8% 20% 30% 6%	2301
	21st to end	Day	38% 8% 16% 31% 8%	2574
September	1st to 10th	Day	37% 8% 16% 29% 9% 1	2230
	11th to 20th	Day	45% 8% 18% 24% 4	2003
	21st to end	Day	46% 6% 13% 30% 5%	1980
October	1st to 10th	Day	55% 6% 15% 18% 6%	1805
	11th to 20th	Day	54% 6% 14% 21% 5% 1	1800
	21st to end	Day	61% 8% 16% 13% 3	1765
November	1st to 10th	Day	61% 7% 13% 14% 4 1	1487
	11th to 20th	Day	63% 8% 14% 11% 4	1520
	21st to end	Day	66% 7% 14% 12% 2	1436
December	1st to 10th	Day	68% 8% 10% 10% 4	1330
	11th to 20th	Day	60% 6% 16% 14% 3	1680
	21st to end	Day	56% 10% 14% 17% 4	2442
January	1st to 10th	Night	65% 7% 13% 12% 3	4737
	11th to 20th	Night	72% 6% 9% 11% 2	4750
	21st to end	Night	66% 7% 11% 12% 3	4960
February	1st to 10th	Night	55% 6% 13% 17% 9%	2516
	11th to 20th	Night	47% 6% 17% 20% 9% 1	2333
	21st to end	Night	52% 7% 11% 19% 11%	1910
March	1st to 10th	Night	54% 6% 12% 18% 10% 1	2090
	11th to 20th	Night	42% 6% 13% 26% 12% 1	1980
	21st to end	Night	48% 6% 13% 20% 11% 2	2178
April	1st to 10th	Night	42% 6% 11% 23% 15% 2	1772
	11th to 20th	Night	47% 5% 9% 20% 16% 3	1599
	21st to end	Night	47% 5% 10% 24% 12% 1	1620
May	1st to 10th	Night	45% 6% 11% 23% 14% 1	1502
	11th to 20th	Night	40% 6% 11% 26% 14% 3	1329
	21st to end	Night	41% 7% 13% 21% 17% 2	1358
June	1st to 10th	Night	39% 9% 17% 24% 11% 1	1260
	11th to 20th	Night	40% 9% 14% 21% 15% 2	1260
	21st to end	Night	37% 8% 19% 27% 8% 1	1260
July	1st to 10th	Night	22% 8% 17% 39% 12% 2	1225
	11th to 20th	Night	28% 9% 20% 29% 12% 1	1218
	21st to end	Night	26% 5% 16% 39% 12% 1	1302
August	1st to 10th	Night	28% 6% 19% 35% 11% 1	1508
	11th to 20th	Night	29% 8% 21% 31% 9%	1600
	21st to end	Night	33% 7% 16% 33% 11% 1	1782
September	1st to 10th	Night	32% 7% 17% 31% 12%	1729
	11th to 20th	Night	42% 7% 16% 25% 9% 1	1954
	21st to end	Night	44% 6% 14% 29% 7%	1980
October	1st to 10th	Night	48% 7% 16% 21% 8%	2137
	11th to 20th	Night	53% 7% 14% 23% 4	2160
	21st to end	Night	58% 6% 14% 18% 4	2589
November	1st to 10th	Night	59% 6% 15% 14% 5% 1	2591
	11th to 20th	Night	63% 7% 13% 12% 4	2660
	21st to end	Night	67% 7% 13% 10% 3	2740
December	1st to 10th	Night	65% 7% 15% 10% 3	2850
	11th to 20th	Night	61% 7% 13% 14% 4	3516
	21st to end	Night	58% 8% 12% 15% 6%	5225

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	12% 7% 35% 29% 13% 4	270
	11th to 20th	Day	10% 9% 27% 38% 16% 1	270
	21st to end	Day	13% 7% 32% 31% 13% 3	297
February	1st to 10th	Day	15% 8% 33% 33% 9% 2	270
	11th to 20th	Day	12% 12% 31% 29% 12% 4	252
	21st to end	Day	8% 6% 39% 35% 13%	198
March	1st to 10th	Day	10% 9% 33% 31% 15% 2	240
	11th to 20th	Day	11% 10% 29% 30% 16% 4	240
	21st to end	Day	14% 9% 30% 27% 18% 2	264
April	1st to 10th	Day	14% 10% 30% 32% 11% 2	445
	11th to 20th	Day	16% 12% 31% 26% 14% 1	590
	21st to end	Day	17% 10% 35% 27% 10% 2	600
May	1st to 10th	Day	18% 13% 32% 25% 12% 1	603
	11th to 20th	Day	20% 10% 26% 28% 14% 3	625
	21st to end	Day	21% 10% 25% 27% 15% 2	679
June	1st to 10th	Day	17% 11% 28% 24% 14% 6%	630
	11th to 20th	Day	17% 10% 33% 27% 10% 3	630
	21st to end	Day	13% 7% 26% 31% 17% 5%	630
July	1st to 10th	Day	7% 9% 32% 28% 17% 7%	621
	11th to 20th	Day	11% 10% 30% 28% 17% 4	618
	21st to end	Day	9% 6% 29% 29% 20% 8%	684
August	1st to 10th	Day	7% 9% 28% 30% 19% 7%	626
	11th to 20th	Day	11% 7% 26% 32% 17% 8%	615
	21st to end	Day	14% 9% 28% 28% 18% 4	693
September	1st to 10th	Day	12% 8% 30% 34% 15% 2	520
	11th to 20th	Day	17% 10% 39% 23% 10% 3	293
	21st to end	Day	20% 9% 36% 27% 8% 1	270
October	1st to 10th	Day	18% 14% 28% 27% 9% 4	270
	11th to 20th	Day	17% 8% 32% 33% 9% 1	270
	21st to end	Day	28% 11% 28% 21% 9% 2	297
November	1st to 10th	Day	22% 10% 28% 29% 10% 1	270
	11th to 20th	Day	14% 15% 37% 27% 7% 1	270
	21st to end	Day	16% 13% 37% 28% 6%	270
December	1st to 10th	Day	14% 8% 32% 30% 14% 3	270
	11th to 20th	Day	16% 7% 40% 27% 9% 1	270
	21st to end	Day	15% 6% 28% 32% 14% 4	297
January	1st to 10th	Night	11% 8% 33% 31% 15% 2	950
	11th to 20th	Night	16% 5% 32% 35% 11% 1	950
	21st to end	Night	14% 7% 33% 33% 12% 1	1045
February	1st to 10th	Night	13% 6% 31% 36% 14% 1	950
	11th to 20th	Night	12% 6% 31% 33% 15% 3	902
	21st to end	Night	10% 6% 34% 34% 13% 2	726
March	1st to 10th	Night	13% 6% 33% 32% 15% 1	650
	11th to 20th	Night	12% 6% 39% 29% 13% 1	540
	21st to end	Night	19% 7% 27% 29% 15% 3	594
April	1st to 10th	Night	16% 7% 29% 36% 11% 1	540
	11th to 20th	Night	17% 7% 26% 35% 14% 1	533
	21st to end	Night	16% 7% 31% 34% 11%	540
May	1st to 10th	Night	20% 6% 34% 31% 9%	540
	11th to 20th	Night	18% 7% 30% 32% 13% 1	531
	21st to end	Night	21% 6% 32% 30% 10% 1	584
June	1st to 10th	Night	15% 5% 28% 34% 18%	540
	11th to 20th	Night	17% 5% 31% 33% 14% 1	540
	21st to end	Night	13% 6% 19% 39% 21% 1	540
July	1st to 10th	Night	7% 4 25% 42% 21% 1	525
	11th to 20th	Night	8% 4 28% 41% 18% 1	522
	21st to end	Night	9% 3 22% 40% 23% 3	591
August	1st to 10th	Night	8% 3 23% 44% 21% 1	537
	11th to 20th	Night	9% 5% 23% 37% 23% 3	534
	21st to end	Night	15% 4 23% 38% 19% 1	594
September	1st to 10th	Night	12% 6% 29% 34% 16% 2	540
	11th to 20th	Night	18% 6% 26% 37% 13%	540
	21st to end	Night	21% 4 31% 32% 12% 1	540
October	1st to 10th	Night	24% 6% 30% 32% 8%	697
	11th to 20th	Night	22% 6% 32% 30% 10% 1	720
	21st to end	Night	24% 9% 29% 29% 8% 1	989
November	1st to 10th	Night	20% 8% 37% 25% 9% 1	925
	11th to 20th	Night	20% 9% 32% 29% 7% 2	950
	21st to end	Night	19% 10% 40% 24% 7% 1	950
December	1st to 10th	Night	12% 7% 33% 35% 11% 2	950
	11th to 20th	Night	16% 7% 36% 31% 8% 1	950
	21st to end	Night	18% 6% 29% 31% 13% 1	1045

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