Stevenson
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stevenson 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	16% 12% 4 8% 6% 3 4 4 4 5% 31%	78262
	Night	18% 14% 5% 8% 6% 3 4 4 4 5% 29%	80831
Transparency	Day	38% 36% 6% 11% 7% 2	77965
	Night	41% 35% 6% 10% 6% 2	80649
Seeing	Day	27% 15% 29% 19% 8% 2	13531
	Night	27% 11% 19% 27% 15% 2	22683

By Month

Percentage Cloud Cover SW (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	26% 29% 10% 12% 7% 3 3 2 3 2 4	2539
	11th to 20th	Day	31% 23% 6% 7% 6% 2 3 3 3 3 13%	2540
	21st to end	Day	37% 21% 5% 9% 6% 3 3 4 3 3 6%	2958
February	1st to 10th	Day	21% 19% 6% 10% 7% 3 5% 5% 5% 5% 13%	1710
	11th to 20th	Day	18% 21% 5% 11% 8% 3 4 4 4 5% 16%	1612
	21st to end	Day	18% 23% 6% 9% 7% 4 5% 4 2 4 18%	1435
March	1st to 10th	Day	17% 20% 5% 11% 7% 4 5% 5% 4 5% 18%	1836
	11th to 20th	Day	21% 18% 5% 10% 8% 3 5% 4 4 5% 17%	1980
	21st to end	Day	21% 17% 6% 11% 8% 4 7% 4 5% 4 12%	2257
April	1st to 10th	Day	19% 17% 5% 11% 9% 4 5% 4 4 5% 16%	2160
	11th to 20th	Day	18% 15% 5% 9% 8% 4 5% 4 4 5% 23%	2129
	21st to end	Day	15% 14% 5% 10% 7% 3 5% 5% 5% 6% 24%	2298
May	1st to 10th	Day	13% 11% 5% 8% 6% 3 5% 5% 5% 5% 35%	2520
	11th to 20th	Day	14% 10% 4 8% 7% 4 4 6% 6% 8% 29%	2477
	21st to end	Day	19% 10% 3 9% 7% 4 5% 4 6% 8% 27%	2723
June	1st to 10th	Day	19% 9% 3 7% 6% 4 5% 5% 5% 6% 32%	2520
	11th to 20th	Day	14% 9% 5% 8% 7% 4 7% 6% 6% 8% 27%	2520
	21st to end	Day	7% 5% 3 5% 5% 3 5% 4 5% 7% 51%	2520
July	1st to 10th	Day	4 4 2 5% 4 2 4 5% 5% 7% 59%	2520
	11th to 20th	Day	4 3 2 4 3 3 4 4 4 6% 63%	2520
	21st to end	Day	3 2 1 3 3 2 2 3 4 5% 72%	2741
August	1st to 10th	Day	2 3 2 4 4 3 4 4 5% 7% 64%	2417
	11th to 20th	Day	3 3 1 3 4 2 3 4 3 6% 68%	2298
	21st to end	Day	4 3 2 5% 5% 3 4 5% 5% 8% 56%	2417
September	1st to 10th	Day	4 4 2 4 3 2 3 3 4 6% 63%	2160
	11th to 20th	Day	9% 5% 3 7% 6% 3 5% 5% 5% 6% 46%	2160
	21st to end	Day	13% 5% 4 6% 5% 3 5% 4 5% 6% 44%	1912
October	1st to 10th	Day	15% 8% 4 6% 5% 3 5% 4 6% 5% 40%	1800
	11th to 20th	Day	20% 8% 3 7% 5% 3 5% 4 5% 6% 35%	1800
	21st to end	Day	24% 10% 4 7% 6% 3 4 4 4 3 29%	1892
November	1st to 10th	Day	27% 14% 6% 9% 7% 4 6% 4 4 5% 14%	1609
	11th to 20th	Day	27% 17% 6% 9% 7% 4 3 4 7% 4 12%	1520
	21st to end	Day	21% 17% 5% 10% 9% 3 5% 6% 4 4 16%	1520
December	1st to 10th	Day	21% 19% 7% 10% 8% 4 4 4 3 3 17%	1520
	11th to 20th	Day	26% 21% 10% 13% 6% 4 4 4 3 2 5%	1928
	21st to end	Day	23% 22% 9% 16% 9% 4 4 3 3 2 6%	2794
January	1st to 10th	Night	26% 28% 8% 12% 8% 3 3 3 2 2 5%	4406
	11th to 20th	Night	29% 24% 6% 8% 6% 3 3 3 2 4 14%	4420
	21st to end	Night	39% 19% 6% 9% 6% 2 3 3 3 2 8%	4699
February	1st to 10th	Night	18% 20% 6% 10% 7% 4 5% 3 5% 5% 16%	2470
	11th to 20th	Night	20% 17% 5% 8% 7% 4 5% 4 4 5% 21%	2329
	21st to end	Night	17% 21% 6% 9% 8% 3 4 4 3 5% 19%	1820
March	1st to 10th	Night	18% 18% 5% 9% 7% 4 5% 4 4 5% 22%	2120
	11th to 20th	Night	19% 15% 4 8% 8% 4 6% 4 5% 6% 22%	1980
	21st to end	Night	17% 20% 6% 10% 7% 5% 5% 5% 4 5% 16%	2098
April	1st to 10th	Night	17% 15% 7% 11% 7% 3 5% 4 5% 5% 21%	1800
	11th to 20th	Night	14% 15% 4 8% 7% 5% 6% 5% 5% 5% 28%	1779
	21st to end	Night	17% 12% 4 6% 7% 4 6% 5% 5% 7% 27%	1658
May	1st to 10th	Night	12% 11% 4 7% 5% 2 4 5% 5% 6% 39%	1440
	11th to 20th	Night	12% 8% 2 6% 7% 4 6% 7% 8% 7% 32%	1416
	21st to end	Night	11% 10% 3 6% 6% 4 6% 6% 7% 8% 34%	1552
June	1st to 10th	Night	16% 8% 4 8% 5% 3 5% 4 4 8% 36%	1440
	11th to 20th	Night	10% 8% 4 9% 7% 3 5% 5% 5% 7% 37%	1440
	21st to end	Night	6% 5% 2 5% 4 3 4 4 4 6% 57%	1440
July	1st to 10th	Night	4 3 2 3 4 3 4 3 4 4 67%	1440
	11th to 20th	Night	3 2 1 3 3 2 3 4 5% 5% 69%	1440
	21st to end	Night	2 1 1 2 2 1 2 2 3 4 80%	1576
August	1st to 10th	Night	3 2 1 3 3 2 3 3 3 7% 70%	1520
	11th to 20th	Night	3 2 1 3 2 2 3 3 3 6% 72%	1602
	21st to end	Night	3 3 1 3 4 2 3 4 4 6% 67%	1937
September	1st to 10th	Night	5% 4 2 3 3 2 3 3 4 6% 67%	1800
	11th to 20th	Night	8% 5% 3 6% 5% 3 5% 4 4 6% 50%	1800
	21st to end	Night	10% 7% 2 5% 5% 4 4 4 5% 5% 48%	2042
October	1st to 10th	Night	15% 7% 3 6% 6% 2 3 3 4 5% 46%	2160
	11th to 20th	Night	18% 8% 3 6% 4 3 5% 4 5% 5% 38%	2160
	21st to end	Night	20% 12% 3 7% 6% 3 3 4 4 4 35%	2463
November	1st to 10th	Night	26% 13% 5% 8% 6% 4 4 4 3 5% 20%	2466
	11th to 20th	Night	27% 15% 5% 9% 7% 4 4 4 3 4 17%	2660
	21st to end	Night	19% 15% 5% 10% 7% 4 5% 4 5% 6% 20%	2660
December	1st to 10th	Night	23% 14% 8% 12% 8% 3 4 4 3 3 17%	2660
	11th to 20th	Night	27% 20% 7% 14% 9% 4 4 4 3 2 5%	3276
	21st to end	Night	25% 22% 10% 13% 7% 3 4 3 2 2 7%	4862

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	69% 26% 3 2	2539
	11th to 20th	Day	58% 26% 5% 8% 3 1	2540
	21st to end	Day	64% 23% 4 6% 3	2958
February	1st to 10th	Day	47% 35% 8% 9% 2	1710
	11th to 20th	Day	44% 38% 7% 8% 3	1621
	21st to end	Day	48% 36% 5% 6% 4 1	1435
March	1st to 10th	Day	46% 38% 7% 7% 2	1836
	11th to 20th	Day	50% 36% 4 8% 2	1980
	21st to end	Day	54% 37% 6% 3	2257
April	1st to 10th	Day	50% 36% 5% 8% 2	2160
	11th to 20th	Day	46% 37% 5% 6% 5% 1	2132
	21st to end	Day	42% 38% 7% 7% 4 1	2298
May	1st to 10th	Day	39% 37% 7% 12% 6%	2520
	11th to 20th	Day	34% 38% 9% 13% 5%	2480
	21st to end	Day	42% 38% 6% 11% 3	2724
June	1st to 10th	Day	42% 38% 7% 8% 4	2520
	11th to 20th	Day	40% 39% 11% 6% 3	2520
	21st to end	Day	23% 37% 11% 18% 8% 3	2520
July	1st to 10th	Day	16% 39% 8% 17% 16% 5%	2451
	11th to 20th	Day	15% 37% 5% 15% 21% 8%	2445
	21st to end	Day	9% 37% 4 18% 26% 6%	2589
August	1st to 10th	Day	15% 37% 9% 23% 15% 2	2400
	11th to 20th	Day	14% 34% 7% 21% 19% 6%	2298
	21st to end	Day	17% 38% 7% 18% 17% 3	2417
September	1st to 10th	Day	14% 35% 7% 18% 17% 9%	2160
	11th to 20th	Day	23% 37% 7% 14% 14% 4	2160
	21st to end	Day	28% 37% 6% 15% 10% 4	1912
October	1st to 10th	Day	30% 36% 5% 14% 12% 3	1800
	11th to 20th	Day	33% 38% 7% 12% 9% 2	1800
	21st to end	Day	38% 37% 5% 11% 9%	1892
November	1st to 10th	Day	45% 41% 5% 6% 2	1609
	11th to 20th	Day	46% 40% 6% 6% 2	1520
	21st to end	Day	42% 40% 6% 7% 4 1	1520
December	1st to 10th	Day	40% 41% 5% 8% 5%	1520
	11th to 20th	Day	50% 34% 6% 9% 1	1928
	21st to end	Day	60% 27% 5% 8% 1	2794
January	1st to 10th	Night	70% 24% 3 2	4406
	11th to 20th	Night	57% 24% 7% 8% 2 1	4420
	21st to end	Night	63% 24% 5% 5% 3 1	4699
February	1st to 10th	Night	44% 36% 7% 10% 3	2470
	11th to 20th	Night	39% 38% 8% 10% 4 1	2331
	21st to end	Night	48% 36% 5% 6% 4	1820
March	1st to 10th	Night	43% 37% 8% 9% 2	2120
	11th to 20th	Night	46% 37% 7% 8% 2	1980
	21st to end	Night	51% 38% 7% 3 1	2098
April	1st to 10th	Night	45% 39% 6% 7% 3	1800
	11th to 20th	Night	41% 38% 6% 7% 4 3	1779
	21st to end	Night	42% 38% 6% 8% 5% 1	1658
May	1st to 10th	Night	38% 37% 7% 10% 7%	1440
	11th to 20th	Night	33% 40% 9% 11% 5% 2	1416
	21st to end	Night	39% 40% 8% 10% 4	1552
June	1st to 10th	Night	40% 37% 7% 11% 4	1440
	11th to 20th	Night	36% 40% 11% 10% 3	1440
	21st to end	Night	22% 37% 10% 19% 9% 3	1440
July	1st to 10th	Night	15% 37% 8% 18% 16% 6%	1400
	11th to 20th	Night	12% 38% 6% 16% 18% 10%	1392
	21st to end	Night	8% 37% 3 18% 23% 11%	1488
August	1st to 10th	Night	13% 38% 8% 22% 14% 4	1512
	11th to 20th	Night	12% 35% 8% 18% 19% 8%	1602
	21st to end	Night	15% 38% 6% 22% 16% 4	1937
September	1st to 10th	Night	13% 36% 7% 18% 16% 9%	1800
	11th to 20th	Night	23% 37% 5% 15% 11% 9%	1800
	21st to end	Night	26% 37% 7% 12% 11% 7%	2042
October	1st to 10th	Night	28% 37% 5% 14% 13% 4	2160
	11th to 20th	Night	31% 37% 7% 11% 11% 3	2160
	21st to end	Night	35% 38% 5% 9% 12% 1	2463
November	1st to 10th	Night	43% 41% 4 9% 3	2466
	11th to 20th	Night	46% 39% 6% 6% 3	2660
	21st to end	Night	41% 40% 6% 9% 3 1	2660
December	1st to 10th	Night	41% 42% 5% 8% 4	2660
	11th to 20th	Night	49% 34% 8% 7% 2	3276
	21st to end	Night	62% 28% 4 5% 1	4862

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	44% 19% 22% 11% 3 1	180
	11th to 20th	Day	48% 17% 18% 13% 4	180
	21st to end	Day	39% 14% 24% 16% 6% 1	297
February	1st to 10th	Day	41% 17% 22% 16% 3	370
	11th to 20th	Day	44% 20% 22% 11% 2	348
	21st to end	Day	43% 17% 22% 14% 4	326
March	1st to 10th	Day	37% 20% 25% 13% 4	420
	11th to 20th	Day	35% 18% 28% 13% 5%	420
	21st to end	Day	42% 18% 22% 13% 5% 1	462
April	1st to 10th	Day	41% 16% 24% 12% 5% 2	420
	11th to 20th	Day	36% 21% 24% 12% 5% 2	412
	21st to end	Day	33% 19% 27% 16% 5%	420
May	1st to 10th	Day	28% 16% 33% 14% 8% 1	423
	11th to 20th	Day	27% 16% 27% 17% 10% 2	448
	21st to end	Day	27% 14% 28% 20% 8% 3	485
June	1st to 10th	Day	23% 17% 28% 19% 11% 2	450
	11th to 20th	Day	25% 22% 33% 14% 6% 1	450
	21st to end	Day	9% 13% 39% 24% 11% 3	450
July	1st to 10th	Day	7% 15% 43% 26% 7% 2	446
	11th to 20th	Day	6% 13% 42% 25% 12% 2	444
	21st to end	Day	7% 9% 38% 26% 16% 4	487
August	1st to 10th	Day	8% 9% 34% 31% 13% 6%	447
	11th to 20th	Day	5% 10% 35% 30% 15% 5%	437
	21st to end	Day	9% 15% 36% 25% 11% 4	495
September	1st to 10th	Day	10% 13% 37% 24% 14% 3	450
	11th to 20th	Day	16% 14% 31% 27% 11% 1	450
	21st to end	Day	22% 12% 30% 26% 10%	450
October	1st to 10th	Day	24% 14% 27% 25% 10% 1	450
	11th to 20th	Day	31% 13% 22% 21% 11% 2	450
	21st to end	Day	39% 13% 22% 17% 8% 1	407
November	1st to 10th	Day	41% 12% 26% 17% 3	239
	11th to 20th	Day	39% 15% 25% 15% 5% 1	180
	21st to end	Day	40% 11% 24% 21% 4	180
December	1st to 10th	Day	38% 14% 24% 16% 7% 1	180
	11th to 20th	Day	46% 14% 28% 9% 3	180
	21st to end	Day	42% 16% 25% 12% 5% 1	198
January	1st to 10th	Night	41% 18% 19% 14% 8%	760
	11th to 20th	Night	39% 12% 22% 20% 7% 1	760
	21st to end	Night	36% 9% 21% 25% 8% 1	836
February	1st to 10th	Night	37% 14% 18% 23% 8% 1	760
	11th to 20th	Night	35% 13% 20% 20% 11% 1	722
	21st to end	Night	36% 19% 16% 24% 5%	592
March	1st to 10th	Night	31% 15% 16% 24% 13% 1	720
	11th to 20th	Night	33% 14% 19% 25% 9% 1	720
	21st to end	Night	37% 15% 17% 20% 10% 1	792
April	1st to 10th	Night	30% 14% 20% 24% 11% 1	720
	11th to 20th	Night	25% 17% 19% 24% 13% 3	711
	21st to end	Night	25% 12% 20% 28% 14% 1	578
May	1st to 10th	Night	22% 10% 22% 28% 17% 1	360
	11th to 20th	Night	18% 8% 17% 32% 23% 3	354
	21st to end	Night	15% 9% 17% 35% 22% 2	389
June	1st to 10th	Night	21% 10% 18% 29% 20% 2	360
	11th to 20th	Night	17% 13% 24% 30% 16% 1	360
	21st to end	Night	9% 5% 23% 40% 22% 1	360
July	1st to 10th	Night	7% 4 19% 40% 29% 2	350
	11th to 20th	Night	4 4 24% 43% 23% 2	348
	21st to end	Night	3 3 17% 43% 30% 3	394
August	1st to 10th	Night	6% 3 15% 40% 32% 5%	446
	11th to 20th	Night	6% 3 11% 35% 35% 10%	534
	21st to end	Night	7% 7% 20% 38% 24% 4	749
September	1st to 10th	Night	7% 6% 21% 36% 27% 3	720
	11th to 20th	Night	13% 6% 17% 30% 29% 5%	720
	21st to end	Night	18% 7% 15% 36% 21% 3	720
October	1st to 10th	Night	21% 7% 18% 31% 20% 3	720
	11th to 20th	Night	24% 10% 14% 30% 18% 3	720
	21st to end	Night	33% 10% 17% 26% 11% 2	792
November	1st to 10th	Night	38% 12% 16% 26% 8% 1	740
	11th to 20th	Night	40% 13% 20% 19% 7% 1	760
	21st to end	Night	30% 13% 22% 25% 8% 1	760
December	1st to 10th	Night	34% 15% 23% 20% 8% 1	760
	11th to 20th	Night	42% 13% 18% 21% 6%	760
	21st to end	Night	44% 17% 18% 17% 5%	836

Page generated at 2024-02-12 03:35:31. Version ac07

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