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 2026-08-15.

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	11% 11% 3 8% 5% 4 4 3 5% 6% 40%	153180
	Night	15% 13% 4 8% 5% 4 4 3 4 5% 35%	145999
Transparency	Day	45% 7% 10% 20% 14% 5%	147929
	Night	50% 7% 10% 17% 12% 4	142252
Seeing	Day	26% 16% 30% 19% 8% 2	16771
	Night	27% 11% 20% 27% 14% 2	25679

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	25% 26% 7% 13% 6% 5% 4 3 4 3 4	4722
	11th to 20th	Day	16% 16% 4 6% 4 3 2 2 3 5% 38%	4740
	21st to end	Day	23% 14% 3 8% 4 4 4 3 4 5% 27%	5503
February	1st to 10th	Day	20% 20% 5% 12% 6% 5% 5% 4 6% 5% 12%	1890
	11th to 20th	Day	18% 22% 5% 13% 7% 4 4 3 4 5% 16%	1845
	21st to end	Day	16% 25% 6% 9% 6% 5% 4 4 2 4 19%	1678
March	1st to 10th	Day	11% 17% 5% 12% 6% 5% 5% 4 5% 6% 21%	4708
	11th to 20th	Day	16% 20% 4 12% 6% 4 4 3 3 3 25%	5110
	21st to end	Day	18% 17% 5% 12% 7% 6% 6% 4 6% 6% 12%	5819
April	1st to 10th	Day	18% 17% 5% 12% 7% 5% 5% 3 5% 5% 18%	2520
	11th to 20th	Day	16% 16% 4 11% 6% 6% 4 3 4 5% 25%	2489
	21st to end	Day	14% 14% 5% 11% 5% 5% 5% 4 5% 6% 26%	2683
May	1st to 10th	Day	8% 9% 3 7% 4 3 4 3 5% 6% 47%	8195
	11th to 20th	Day	7% 10% 3 10% 7% 7% 6% 5% 6% 8% 31%	8160
	21st to end	Day	10% 9% 3 8% 5% 5% 5% 4 6% 8% 37%	8992
June	1st to 10th	Day	17% 9% 3 7% 5% 5% 4 4 6% 6% 34%	2940
	11th to 20th	Day	11% 10% 4 9% 6% 5% 6% 4 6% 7% 32%	2940
	21st to end	Day	7% 6% 2 6% 5% 4 5% 4 5% 6% 49%	2940
July	1st to 10th	Day	1 3 1 3 2 2 3 2 4 5% 73%	8219
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 4 78%	8220
	21st to end	Day	2 3 1 3 2 2 2 2 4 5% 74%	8922
August	1st to 10th	Day	2 3 2 4 3 4 3 3 4 6% 65%	6115
	11th to 20th	Day	4 3 1 5% 4 4 5% 4 6% 7% 58%	5849
	21st to end	Day	6% 5% 2 5% 4 4 4 4 5% 8% 53%	6077
September	1st to 10th	Day	4 4 2 5% 3 3 4 3 5% 6% 62%	2400
	11th to 20th	Day	8% 6% 3 7% 5% 4 5% 4 5% 6% 47%	2400
	21st to end	Day	12% 5% 3 7% 5% 4 5% 3 5% 6% 46%	2124
October	1st to 10th	Day	10% 7% 3 6% 3 4 4 2 5% 6% 49%	3280
	11th to 20th	Day	15% 9% 3 8% 5% 5% 5% 4 6% 6% 34%	3261
	21st to end	Day	20% 14% 4 10% 6% 5% 5% 4 6% 5% 21%	3398
November	1st to 10th	Day	26% 15% 5% 11% 6% 5% 5% 3 5% 6% 14%	1781
	11th to 20th	Day	26% 19% 6% 10% 6% 5% 4 4 7% 3 12%	1680
	21st to end	Day	19% 18% 4 12% 7% 5% 5% 4 4 5% 16%	1680
December	1st to 10th	Day	16% 15% 4 8% 5% 5% 4 4 4 6% 28%	2700
	11th to 20th	Day	27% 21% 8% 15% 7% 6% 4 3 3 2 4	3108
	21st to end	Day	24% 24% 8% 16% 7% 4 3 3 3 2 5%	4092
January	1st to 10th	Night	26% 25% 6% 12% 6% 5% 4 3 3 4 6%	8172
	11th to 20th	Night	15% 18% 4 6% 4 3 2 2 3 5% 38%	8220
	21st to end	Night	24% 14% 4 8% 4 3 3 3 3 4 29%	8678
February	1st to 10th	Night	17% 21% 5% 11% 6% 5% 4 3 5% 6% 16%	2730
	11th to 20th	Night	20% 20% 4 10% 6% 4 4 3 4 4 20%	2662
	21st to end	Night	16% 23% 5% 10% 6% 4 4 3 3 4 20%	2127
March	1st to 10th	Night	13% 17% 5% 11% 6% 5% 5% 4 6% 6% 21%	5389
	11th to 20th	Night	16% 19% 5% 10% 6% 4 3 2 3 4 28%	5110
	21st to end	Night	18% 21% 5% 13% 6% 5% 4 3 4 5% 14%	5397
April	1st to 10th	Night	16% 16% 5% 11% 6% 5% 5% 4 6% 6% 21%	2100
	11th to 20th	Night	12% 15% 4 9% 6% 6% 5% 4 5% 5% 30%	2079
	21st to end	Night	15% 13% 4 7% 6% 5% 5% 4 6% 7% 29%	1933
May	1st to 10th	Night	11% 10% 3 6% 3 3 3 3 5% 6% 47%	4723
	11th to 20th	Night	7% 13% 3 9% 6% 5% 5% 4 6% 5% 36%	4700
	21st to end	Night	8% 9% 2 6% 4 4 4 4 7% 8% 42%	5181
June	1st to 10th	Night	16% 9% 3 8% 4 5% 4 3 5% 7% 37%	1680
	11th to 20th	Night	9% 8% 3 9% 6% 4 5% 4 5% 7% 41%	1680
	21st to end	Night	6% 6% 2 6% 4 4 3 3 4 6% 56%	1680
July	1st to 10th	Night	1 2 1 2 2 3 2 2 4 5% 77%	4740
	11th to 20th	Night	1 2 1 3 3 3 3 2 3 3 76%	4740
	21st to end	Night	2 2 1 2 2 2 2 2 3 6% 77%	5153
August	1st to 10th	Night	3 2 1 4 3 4 3 3 4 7% 67%	3892
	11th to 20th	Night	6% 4 2 5% 3 4 3 2 4 6% 62%	4088
	21st to end	Night	5% 5% 2 5% 3 4 4 3 4 6% 59%	4916
September	1st to 10th	Night	4 4 2 4 3 3 2 3 4 7% 64%	2000
	11th to 20th	Night	8% 6% 2 7% 4 5% 4 3 4 7% 50%	2000
	21st to end	Night	9% 7% 2 6% 4 5% 4 3 5% 6% 50%	2270
October	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 4 5% 52%	3920
	11th to 20th	Night	12% 9% 3 8% 4 5% 6% 4 6% 8% 35%	3897
	21st to end	Night	20% 16% 4 10% 5% 4 4 3 4 5% 25%	4407
November	1st to 10th	Night	26% 13% 5% 10% 5% 6% 4 3 4 5% 19%	2733
	11th to 20th	Night	27% 17% 5% 11% 5% 5% 4 3 3 4 16%	2940
	21st to end	Night	18% 16% 4 12% 6% 5% 5% 3 5% 6% 20%	2940
December	1st to 10th	Night	14% 14% 5% 10% 6% 4 4 3 4 6% 32%	4700
	11th to 20th	Night	29% 21% 6% 14% 8% 6% 4 3 3 2 4	5316
	21st to end	Night	29% 23% 9% 13% 6% 4 4 2 2 2 6%	7106

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	79% 8% 7% 5% 1	4723
	11th to 20th	Day	46% 4 7% 18% 20% 4	4740
	21st to end	Day	58% 4 6% 14% 13% 5%	5503
February	1st to 10th	Day	70% 6% 11% 10% 3	1890
	11th to 20th	Day	70% 7% 7% 11% 5% 1	1854
	21st to end	Day	70% 4 7% 8% 9% 1	1678
March	1st to 10th	Day	61% 6% 9% 14% 8% 1	4708
	11th to 20th	Day	62% 7% 5% 21% 5%	5110
	21st to end	Day	73% 7% 10% 6% 3	5819
April	1st to 10th	Day	69% 5% 10% 13% 2	2520
	11th to 20th	Day	64% 5% 10% 12% 8% 1	2492
	21st to end	Day	61% 6% 10% 14% 7% 2	2683
May	1st to 10th	Day	41% 4 8% 32% 15%	8195
	11th to 20th	Day	45% 13% 16% 20% 6%	8163
	21st to end	Day	44% 9% 20% 22% 5% 1	8994
June	1st to 10th	Day	52% 6% 12% 20% 9% 1	2940
	11th to 20th	Day	50% 9% 13% 17% 10% 1	2940
	21st to end	Day	32% 8% 14% 29% 13% 4	2940
July	1st to 10th	Day	12% 5% 9% 30% 24% 19%	8151
	11th to 20th	Day	12% 3 5% 19% 31% 31%	8145
	21st to end	Day	11% 5% 10% 34% 31% 8%	8723
August	1st to 10th	Day	19% 4 8% 22% 29% 19%	4486
	11th to 20th	Day	21% 8% 14% 36% 15% 5%	4248
	21st to end	Day	28% 7% 10% 24% 25% 6%	4380
September	1st to 10th	Day	23% 5% 11% 27% 23% 11%	2400
	11th to 20th	Day	35% 7% 11% 22% 20% 6%	2400
	21st to end	Day	39% 5% 11% 25% 15% 6%	2124
October	1st to 10th	Day	34% 7% 6% 26% 24% 3	3280
	11th to 20th	Day	48% 8% 13% 18% 11% 1	3261
	21st to end	Day	56% 12% 8% 16% 8%	3398
November	1st to 10th	Day	69% 6% 8% 12% 4	1781
	11th to 20th	Day	72% 5% 8% 10% 4 1	1680
	21st to end	Day	65% 6% 8% 11% 9% 1	1680
December	1st to 10th	Day	51% 8% 9% 15% 17% 1	2700
	11th to 20th	Day	77% 8% 7% 7% 2	3108
	21st to end	Day	73% 8% 8% 9% 1	4092
January	1st to 10th	Night	77% 9% 7% 6% 1	8172
	11th to 20th	Night	46% 5% 8% 17% 20% 5%	8220
	21st to end	Night	57% 3 7% 12% 16% 5%	8678
February	1st to 10th	Night	67% 7% 10% 12% 4	2730
	11th to 20th	Night	66% 6% 9% 13% 6% 1	2664
	21st to end	Night	67% 6% 7% 12% 8% 1	2127
March	1st to 10th	Night	59% 8% 10% 16% 5% 2	5389
	11th to 20th	Night	56% 10% 9% 18% 7% 1	5110
	21st to end	Night	71% 7% 11% 9% 2	5397
April	1st to 10th	Night	65% 8% 12% 12% 4	2100
	11th to 20th	Night	58% 6% 12% 13% 8% 3	2079
	21st to end	Night	57% 8% 11% 14% 8% 1	1933
May	1st to 10th	Night	43% 5% 8% 26% 17% 1	4721
	11th to 20th	Night	51% 11% 13% 19% 5% 1	4700
	21st to end	Night	42% 8% 22% 22% 6%	5182
June	1st to 10th	Night	51% 5% 14% 22% 7% 1	1680
	11th to 20th	Night	46% 8% 14% 21% 10%	1680
	21st to end	Night	34% 5% 15% 26% 16% 4	1680
July	1st to 10th	Night	12% 7% 9% 30% 27% 16%	4700
	11th to 20th	Night	16% 3 6% 15% 33% 27%	4692
	21st to end	Night	11% 4 13% 31% 31% 9%	5040
August	1st to 10th	Night	16% 4 8% 23% 28% 21%	2849
	11th to 20th	Night	19% 11% 18% 29% 17% 6%	2965
	21st to end	Night	25% 8% 10% 26% 20% 10%	3535
September	1st to 10th	Night	22% 5% 12% 27% 22% 12%	2000
	11th to 20th	Night	36% 7% 10% 21% 16% 11%	2000
	21st to end	Night	36% 6% 13% 21% 17% 8%	2270
October	1st to 10th	Night	33% 6% 8% 26% 25% 3	3920
	11th to 20th	Night	43% 9% 14% 19% 13% 2	3897
	21st to end	Night	56% 11% 6% 16% 10% 1	4407
November	1st to 10th	Night	67% 6% 7% 13% 6% 1	2733
	11th to 20th	Night	69% 6% 8% 11% 6%	2940
	21st to end	Night	62% 6% 8% 14% 9% 1	2940
December	1st to 10th	Night	46% 10% 13% 16% 14%	4700
	11th to 20th	Night	76% 7% 8% 6% 3	5316
	21st to end	Night	75% 8% 8% 7% 2	7106

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% 20% 21% 10% 3	220
	11th to 20th	Day	42% 15% 22% 16% 5%	220
	21st to end	Day	36% 16% 25% 17% 6% 1	352
February	1st to 10th	Day	41% 17% 24% 15% 3	430
	11th to 20th	Day	42% 22% 22% 11% 3	425
	21st to end	Day	44% 15% 25% 13% 4	419
March	1st to 10th	Day	37% 19% 27% 14% 4	540
	11th to 20th	Day	36% 19% 27% 11% 5% 1	540
	21st to end	Day	42% 19% 23% 12% 4 1	594
April	1st to 10th	Day	39% 16% 25% 13% 6% 2	540
	11th to 20th	Day	34% 20% 26% 14% 4 2	532
	21st to end	Day	31% 18% 29% 17% 5% 1	540
May	1st to 10th	Day	25% 16% 35% 15% 8% 1	543
	11th to 20th	Day	29% 17% 27% 16% 9% 2	568
	21st to end	Day	26% 14% 30% 20% 8% 3	617
June	1st to 10th	Day	24% 17% 29% 18% 10% 2	570
	11th to 20th	Day	23% 22% 35% 13% 6% 1	570
	21st to end	Day	11% 14% 39% 24% 10% 2	570
July	1st to 10th	Day	6% 15% 45% 25% 7% 2	566
	11th to 20th	Day	7% 14% 41% 25% 11% 2	564
	21st to end	Day	6% 11% 36% 27% 15% 4	619
August	1st to 10th	Day	9% 9% 36% 30% 12% 5%	567
	11th to 20th	Day	6% 10% 36% 30% 13% 5%	534
	21st to end	Day	10% 14% 35% 26% 11% 4	583
September	1st to 10th	Day	9% 12% 38% 26% 13% 3	530
	11th to 20th	Day	15% 15% 33% 27% 9% 2	530
	21st to end	Day	21% 13% 30% 26% 10% 1	530
October	1st to 10th	Day	22% 15% 28% 24% 10% 1	530
	11th to 20th	Day	32% 13% 23% 20% 10% 2	530
	21st to end	Day	38% 15% 22% 17% 7% 1	485
November	1st to 10th	Day	42% 14% 24% 17% 3	291
	11th to 20th	Day	40% 16% 25% 14% 5% 1	220
	21st to end	Day	38% 12% 26% 20% 4	220
December	1st to 10th	Day	39% 14% 26% 15% 5%	220
	11th to 20th	Day	47% 16% 25% 9% 3	220
	21st to end	Day	44% 18% 23% 10% 4 1	242
January	1st to 10th	Night	42% 18% 19% 13% 7%	840
	11th to 20th	Night	36% 12% 24% 21% 7%	840
	21st to end	Night	34% 10% 23% 24% 8% 1	924
February	1st to 10th	Night	35% 13% 20% 24% 7% 1	840
	11th to 20th	Night	35% 14% 21% 19% 10% 1	824
	21st to end	Night	37% 17% 16% 24% 5%	692
March	1st to 10th	Night	31% 14% 17% 25% 11% 1	840
	11th to 20th	Night	33% 14% 21% 24% 9%	840
	21st to end	Night	37% 15% 18% 20% 8% 1	924
April	1st to 10th	Night	29% 14% 20% 25% 11% 1	840
	11th to 20th	Night	23% 18% 20% 26% 11% 2	831
	21st to end	Night	25% 12% 21% 28% 13% 1	673
May	1st to 10th	Night	21% 10% 22% 31% 15% 1	420
	11th to 20th	Night	19% 8% 18% 31% 20% 3	414
	21st to end	Night	15% 9% 18% 35% 21% 1	455
June	1st to 10th	Night	21% 10% 18% 28% 20% 2	420
	11th to 20th	Night	15% 12% 26% 32% 15% 1	420
	21st to end	Night	10% 6% 22% 40% 21%	420
July	1st to 10th	Night	6% 4 20% 41% 27% 1	410
	11th to 20th	Night	3 4 26% 42% 22% 2	408
	21st to end	Night	3 5% 17% 44% 29% 3	460
August	1st to 10th	Night	5% 3 16% 42% 30% 4	521
	11th to 20th	Night	6% 3 13% 37% 33% 9%	606
	21st to end	Night	7% 7% 20% 39% 23% 3	833
September	1st to 10th	Night	7% 6% 20% 37% 27% 3	800
	11th to 20th	Night	12% 6% 19% 30% 27% 5%	800
	21st to end	Night	17% 8% 16% 36% 21% 3	800
October	1st to 10th	Night	20% 8% 19% 32% 18% 3	800
	11th to 20th	Night	25% 10% 16% 30% 17% 3	800
	21st to end	Night	33% 10% 19% 25% 10% 2	880
November	1st to 10th	Night	39% 12% 16% 25% 7%	820
	11th to 20th	Night	41% 13% 21% 18% 6% 1	840
	21st to end	Night	31% 12% 23% 25% 8% 1	840
December	1st to 10th	Night	34% 14% 25% 20% 7% 1	840
	11th to 20th	Night	43% 13% 19% 20% 5%	840
	21st to end	Night	44% 16% 19% 16% 5%	924

Page generated at 2026-08-12 00:37:13. Version ac07

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