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 2025-03-27.

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	14% 12% 4 9% 5% 4 4 3 4 5% 36%	113064
	Night	16% 14% 4 9% 5% 4 4 3 4 5% 33%	114991
Transparency	Day	49% 6% 9% 18% 13% 6%	112807
	Night	53% 6% 10% 16% 11% 4	114837
Seeing	Day	27% 15% 29% 19% 8% 2	14947
	Night	27% 11% 19% 27% 14% 2	24087

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	28% 26% 8% 12% 5% 4 3 2 3 4 4	3639
	11th to 20th	Day	21% 21% 5% 7% 4 3 2 2 3 4 26%	3640
	21st to end	Day	28% 15% 3 8% 4 3 2 3 3 3 27%	4252
February	1st to 10th	Day	20% 20% 5% 13% 6% 4 5% 4 5% 5% 13%	1800
	11th to 20th	Day	18% 22% 5% 13% 7% 4 4 3 4 5% 15%	1755
	21st to end	Day	16% 25% 6% 9% 6% 6% 4 3 3 4 18%	1600
March	1st to 10th	Day	11% 20% 6% 14% 6% 5% 5% 3 4 5% 20%	3302
	11th to 20th	Day	14% 18% 3 10% 5% 3 4 3 3 3 35%	3600
	21st to end	Day	17% 19% 6% 13% 7% 6% 6% 4 5% 4 14%	4036
April	1st to 10th	Day	19% 18% 5% 13% 7% 5% 4 2 4 5% 17%	2280
	11th to 20th	Day	17% 17% 4 11% 6% 6% 4 3 4 5% 24%	2249
	21st to end	Day	15% 15% 5% 12% 5% 5% 5% 4 5% 5% 23%	2427
May	1st to 10th	Day	15% 14% 4 9% 5% 4 5% 3 4 4 34%	4420
	11th to 20th	Day	8% 7% 2 7% 5% 6% 5% 5% 7% 9% 40%	4377
	21st to end	Day	15% 12% 4 11% 6% 6% 6% 4 6% 7% 23%	4813
June	1st to 10th	Day	18% 10% 3 7% 5% 5% 4 4 5% 6% 32%	2660
	11th to 20th	Day	13% 11% 4 9% 6% 5% 7% 4 6% 7% 28%	2660
	21st to end	Day	7% 6% 2 6% 5% 4 5% 4 5% 6% 51%	2660
July	1st to 10th	Day	2 2 1 4 2 2 3 2 4 5% 73%	4420
	11th to 20th	Day	2 2 1 3 2 2 3 2 3 4 76%	4420
	21st to end	Day	3 3 1 3 3 3 3 2 4 5% 71%	4790
August	1st to 10th	Day	2 3 2 4 3 4 3 3 4 6% 67%	4232
	11th to 20th	Day	3 2 1 6% 4 5% 5% 4 5% 7% 57%	4026
	21st to end	Day	6% 5% 2 6% 5% 4 4 3 4 7% 55%	4247
September	1st to 10th	Day	4 4 2 5% 3 3 3 3 5% 6% 63%	2280
	11th to 20th	Day	8% 6% 3 8% 5% 4 5% 4 5% 6% 46%	2280
	21st to end	Day	12% 5% 4 7% 5% 4 5% 3 5% 6% 45%	2018
October	1st to 10th	Day	10% 7% 3 6% 3 4 4 2 6% 6% 50%	3180
	11th to 20th	Day	15% 8% 3 9% 6% 5% 5% 4 6% 6% 34%	3161
	21st to end	Day	20% 14% 4 10% 5% 5% 5% 4 5% 5% 22%	3293
November	1st to 10th	Day	25% 15% 5% 11% 6% 5% 5% 3 4 6% 14%	1695
	11th to 20th	Day	27% 19% 6% 10% 6% 4 3 4 7% 3 12%	1600
	21st to end	Day	19% 19% 4 12% 8% 5% 5% 4 4 5% 16%	1600
December	1st to 10th	Day	15% 15% 4 8% 5% 5% 4 4 4 7% 29%	2620
	11th to 20th	Day	26% 21% 8% 15% 7% 6% 4 3 3 2 4	3028
	21st to end	Day	25% 25% 8% 16% 7% 4 3 3 3 2 5%	4004
January	1st to 10th	Night	27% 24% 6% 12% 6% 4 4 2 3 4 6%	6306
	11th to 20th	Night	20% 23% 5% 8% 4 3 2 2 2 4 27%	6320
	21st to end	Night	28% 15% 4 8% 4 3 3 2 3 3 29%	6711
February	1st to 10th	Night	17% 22% 5% 12% 6% 5% 4 3 5% 5% 16%	2600
	11th to 20th	Night	20% 19% 4 10% 6% 5% 4 3 4 4 21%	2532
	21st to end	Night	16% 24% 5% 10% 6% 4 4 3 3 4 20%	2029
March	1st to 10th	Night	13% 18% 6% 12% 7% 5% 5% 3 6% 6% 20%	3775
	11th to 20th	Night	14% 15% 4 8% 5% 4 3 2 3 4 38%	3600
	21st to end	Night	17% 22% 5% 15% 6% 5% 4 3 4 4 14%	3754
April	1st to 10th	Night	16% 16% 6% 12% 6% 5% 4 3 5% 5% 21%	1900
	11th to 20th	Night	12% 16% 4 9% 6% 6% 6% 3 5% 5% 29%	1879
	21st to end	Night	16% 14% 4 7% 6% 5% 5% 4 6% 7% 26%	1749
May	1st to 10th	Night	17% 15% 4 8% 3 3 4 3 4 4 35%	2540
	11th to 20th	Night	7% 8% 2 7% 4 5% 5% 4 6% 6% 47%	2516
	21st to end	Night	10% 13% 3 8% 6% 5% 5% 5% 6% 7% 32%	2762
June	1st to 10th	Night	16% 8% 3 9% 5% 4 4 3 5% 7% 36%	1520
	11th to 20th	Night	10% 8% 3 10% 6% 4 5% 4 5% 7% 38%	1520
	21st to end	Night	5% 5% 2 6% 4 4 3 3 4 6% 57%	1520
July	1st to 10th	Night	2 2 1 2 2 2 2 2 2 2 81%	2540
	11th to 20th	Night	2 2 1 3 2 2 2 2 3 4 77%	2540
	21st to end	Night	3 2 1 2 1 2 2 1 3 4 81%	2761
August	1st to 10th	Night	2 2 1 3 3 3 2 2 3 6% 74%	2687
	11th to 20th	Night	3 3 2 5% 3 4 3 2 5% 6% 65%	2812
	21st to end	Night	6% 6% 2 4 3 3 3 3 3 5% 62%	3427
September	1st to 10th	Night	4 4 1 3 2 3 2 3 4 6% 66%	1900
	11th to 20th	Night	8% 6% 2 7% 4 5% 4 3 4 7% 49%	1900
	21st to end	Night	10% 7% 2 6% 4 4 4 3 5% 6% 49%	2156
October	1st to 10th	Night	11% 6% 2 6% 4 3 3 3 4 5% 52%	3800
	11th to 20th	Night	12% 9% 3 8% 4 4 6% 4 6% 8% 35%	3777
	21st to end	Night	20% 16% 4 10% 5% 4 4 3 4 5% 25%	4270
November	1st to 10th	Night	26% 13% 4 10% 5% 6% 4 3 4 5% 20%	2600
	11th to 20th	Night	27% 17% 4 11% 5% 5% 4 3 3 4 16%	2800
	21st to end	Night	18% 17% 4 12% 6% 5% 5% 3 5% 6% 20%	2800
December	1st to 10th	Night	13% 13% 5% 10% 6% 4 4 3 4 6% 32%	4560
	11th to 20th	Night	29% 21% 6% 15% 8% 6% 4 3 3 2 4	5176
	21st to end	Night	30% 23% 9% 13% 5% 4 4 2 2 2 6%	6952

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	82% 4 8% 5% 1	3639
	11th to 20th	Day	58% 5% 7% 19% 10% 1	3640
	21st to end	Day	59% 3 5% 15% 11% 6%	4252
February	1st to 10th	Day	71% 6% 10% 10% 3	1800
	11th to 20th	Day	70% 6% 8% 11% 5% 1	1764
	21st to end	Day	70% 5% 8% 8% 8% 1	1600
March	1st to 10th	Day	67% 5% 7% 16% 5% 1	3302
	11th to 20th	Day	56% 4 5% 28% 7%	3600
	21st to end	Day	76% 3 9% 7% 4	4036
April	1st to 10th	Day	73% 4 9% 12% 2	2280
	11th to 20th	Day	67% 4 9% 10% 8% 1	2252
	21st to end	Day	64% 6% 10% 12% 6% 2	2427
May	1st to 10th	Day	57% 3 8% 22% 10%	4420
	11th to 20th	Day	38% 8% 17% 26% 10%	4380
	21st to end	Day	62% 6% 16% 13% 2	4814
June	1st to 10th	Day	56% 5% 12% 19% 7% 1	2660
	11th to 20th	Day	54% 9% 14% 16% 6% 1	2660
	21st to end	Day	31% 7% 15% 30% 14% 4	2660
July	1st to 10th	Day	14% 4 7% 17% 25% 33%	4351
	11th to 20th	Day	13% 3 5% 14% 27% 37%	4345
	21st to end	Day	14% 6% 7% 23% 37% 13%	4639
August	1st to 10th	Day	19% 4 8% 22% 29% 19%	4216
	11th to 20th	Day	21% 8% 14% 36% 15% 5%	4065
	21st to end	Day	28% 6% 10% 24% 25% 6%	4246
September	1st to 10th	Day	23% 4 10% 28% 23% 12%	2280
	11th to 20th	Day	37% 7% 11% 21% 19% 6%	2280
	21st to end	Day	39% 5% 11% 24% 15% 6%	2018
October	1st to 10th	Day	34% 7% 6% 26% 25% 3	3180
	11th to 20th	Day	48% 8% 13% 18% 11% 1	3161
	21st to end	Day	56% 12% 7% 16% 8%	3293
November	1st to 10th	Day	69% 5% 8% 13% 4	1695
	11th to 20th	Day	72% 5% 8% 10% 4 1	1600
	21st to end	Day	65% 5% 7% 11% 10% 1	1600
December	1st to 10th	Day	50% 8% 9% 16% 17% 1	2620
	11th to 20th	Day	77% 7% 7% 7% 1	3028
	21st to end	Day	74% 8% 8% 9% 2	4004
January	1st to 10th	Night	80% 5% 8% 5% 2	6306
	11th to 20th	Night	56% 6% 8% 18% 10% 1	6320
	21st to end	Night	58% 3 6% 13% 14% 5%	6711
February	1st to 10th	Night	68% 7% 9% 12% 4	2600
	11th to 20th	Night	67% 5% 9% 13% 6% 1	2534
	21st to end	Night	67% 6% 7% 12% 7% 1	2029
March	1st to 10th	Night	63% 5% 10% 16% 4 1	3775
	11th to 20th	Night	51% 4 10% 25% 9% 1	3600
	21st to end	Night	76% 4 10% 8% 2	3754
April	1st to 10th	Night	68% 6% 11% 10% 4	1900
	11th to 20th	Night	60% 5% 12% 12% 8% 3	1879
	21st to end	Night	61% 7% 11% 12% 8% 1	1749
May	1st to 10th	Night	59% 3 7% 21% 11%	2540
	11th to 20th	Night	41% 8% 15% 28% 7% 2	2516
	21st to end	Night	56% 7% 23% 11% 3	2762
June	1st to 10th	Night	53% 4 14% 21% 7% 1	1520
	11th to 20th	Night	50% 8% 15% 18% 8%	1520
	21st to end	Night	33% 4 15% 27% 16% 4	1520
July	1st to 10th	Night	12% 4 7% 18% 30% 29%	2500
	11th to 20th	Night	13% 4 6% 16% 26% 35%	2492
	21st to end	Night	11% 5% 9% 20% 37% 17%	2674
August	1st to 10th	Night	16% 4 8% 23% 28% 21%	2679
	11th to 20th	Night	18% 12% 18% 30% 17% 5%	2839
	21st to end	Night	25% 8% 10% 27% 21% 10%	3427
September	1st to 10th	Night	21% 5% 12% 27% 23% 13%	1900
	11th to 20th	Night	37% 6% 9% 21% 15% 11%	1900
	21st to end	Night	37% 6% 13% 21% 16% 8%	2156
October	1st to 10th	Night	33% 5% 8% 26% 26% 3	3800
	11th to 20th	Night	43% 9% 14% 19% 12% 2	3777
	21st to end	Night	57% 11% 6% 16% 10% 1	4270
November	1st to 10th	Night	67% 5% 7% 14% 6% 1	2600
	11th to 20th	Night	69% 6% 8% 10% 6%	2800
	21st to end	Night	62% 5% 7% 14% 10% 1	2800
December	1st to 10th	Night	46% 9% 13% 16% 15%	4560
	11th to 20th	Night	76% 7% 8% 6% 3	5176
	21st to end	Night	75% 8% 8% 7% 2	6952

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	45% 21% 21% 10% 3 1	200
	11th to 20th	Day	46% 16% 19% 15% 5%	200
	21st to end	Day	37% 15% 24% 18% 6% 1	325
February	1st to 10th	Day	41% 17% 23% 16% 4	400
	11th to 20th	Day	44% 21% 22% 11% 2	395
	21st to end	Day	44% 15% 24% 13% 4	389
March	1st to 10th	Day	35% 20% 27% 14% 4	500
	11th to 20th	Day	35% 18% 28% 12% 6% 1	500
	21st to end	Day	42% 18% 22% 13% 5% 1	531
April	1st to 10th	Day	41% 16% 24% 12% 5% 2	460
	11th to 20th	Day	35% 21% 26% 12% 4 2	452
	21st to end	Day	34% 18% 27% 16% 4	460
May	1st to 10th	Day	28% 16% 33% 14% 8% 1	463
	11th to 20th	Day	26% 16% 28% 17% 10% 2	488
	21st to end	Day	28% 13% 28% 20% 8% 3	529
June	1st to 10th	Day	24% 16% 28% 19% 10% 2	490
	11th to 20th	Day	24% 22% 32% 14% 6% 1	490
	21st to end	Day	10% 14% 39% 24% 11% 3	490
July	1st to 10th	Day	7% 15% 43% 26% 7% 2	486
	11th to 20th	Day	6% 12% 43% 25% 12% 2	484
	21st to end	Day	7% 11% 37% 26% 16% 4	531
August	1st to 10th	Day	8% 9% 35% 30% 13% 5%	487
	11th to 20th	Day	6% 10% 34% 31% 14% 5%	477
	21st to end	Day	10% 15% 35% 26% 11% 4	539
September	1st to 10th	Day	9% 12% 37% 25% 14% 2	490
	11th to 20th	Day	15% 16% 31% 27% 10% 1	490
	21st to end	Day	21% 12% 30% 26% 10% 1	490
October	1st to 10th	Day	23% 14% 27% 25% 10% 1	490
	11th to 20th	Day	31% 13% 23% 21% 10% 2	490
	21st to end	Day	39% 14% 22% 17% 7% 1	446
November	1st to 10th	Day	42% 13% 25% 18% 3	265
	11th to 20th	Day	42% 16% 24% 14% 5% 1	200
	21st to end	Day	38% 11% 27% 22% 4	200
December	1st to 10th	Day	36% 14% 28% 16% 6% 1	200
	11th to 20th	Day	47% 16% 27% 9% 3	200
	21st to end	Day	44% 17% 23% 11% 4 1	220
January	1st to 10th	Night	41% 18% 20% 13% 8%	800
	11th to 20th	Night	37% 12% 22% 21% 7% 1	800
	21st to end	Night	34% 10% 22% 25% 8% 1	880
February	1st to 10th	Night	36% 14% 19% 24% 8% 1	800
	11th to 20th	Night	36% 15% 20% 19% 10% 1	784
	21st to end	Night	37% 17% 16% 24% 5%	660
March	1st to 10th	Night	31% 14% 18% 25% 12% 1	800
	11th to 20th	Night	32% 14% 20% 25% 9% 1	800
	21st to end	Night	38% 15% 17% 20% 9% 1	862
April	1st to 10th	Night	30% 15% 20% 23% 11% 1	760
	11th to 20th	Night	25% 18% 20% 23% 12% 2	751
	21st to end	Night	25% 12% 20% 28% 13% 1	609
May	1st to 10th	Night	23% 9% 22% 28% 16% 1	380
	11th to 20th	Night	18% 8% 18% 31% 22% 3	374
	21st to end	Night	16% 9% 18% 35% 21% 1	411
June	1st to 10th	Night	22% 9% 18% 29% 20% 2	380
	11th to 20th	Night	16% 13% 24% 30% 16% 1	380
	21st to end	Night	9% 6% 23% 40% 22% 1	380
July	1st to 10th	Night	7% 4 19% 41% 28% 2	370
	11th to 20th	Night	4 4 24% 42% 24% 2	368
	21st to end	Night	3 4 17% 42% 30% 3	416
August	1st to 10th	Night	5% 3 15% 41% 31% 5%	471
	11th to 20th	Night	5% 3 12% 37% 34% 9%	564
	21st to end	Night	7% 7% 20% 38% 24% 4	791
September	1st to 10th	Night	7% 6% 20% 36% 27% 3	760
	11th to 20th	Night	13% 6% 18% 30% 28% 5%	760
	21st to end	Night	17% 7% 16% 36% 21% 3	760
October	1st to 10th	Night	20% 8% 19% 31% 19% 3	760
	11th to 20th	Night	24% 9% 15% 31% 18% 3	760
	21st to end	Night	33% 11% 18% 26% 11% 2	836
November	1st to 10th	Night	38% 12% 16% 26% 7% 1	780
	11th to 20th	Night	41% 13% 21% 18% 6% 1	800
	21st to end	Night	31% 13% 22% 26% 8% 1	800
December	1st to 10th	Night	33% 14% 25% 20% 7% 1	800
	11th to 20th	Night	42% 13% 19% 21% 5%	800
	21st to end	Night	45% 16% 17% 16% 5%	880

Page generated at 2025-03-24 06:04:26. Version ac07

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