Steve's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Steve's Observatory 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% 6% 3 6% 6% 4 5% 5% 6% 7% 40%	78390
	Night	12% 6% 2 5% 4 3 4 4 5% 8% 46%	80641
Transparency	Day	42% 4 11% 19% 18% 6%	78121
	Night	37% 4 11% 19% 20% 8%	80431
Seeing	Day	15% 8% 20% 25% 22% 10%	13896
	Night	15% 8% 23% 29% 21% 5%	23772

By Month

Percentage Cloud Cover SE (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	17% 10% 3 6% 4 2 3 4 4 6% 41%	2847
	11th to 20th	Day	16% 5% 2 5% 3 2 3 3 3 5% 51%	2850
	21st to end	Day	23% 9% 2 4 4 3 3 4 4 4 40%	3234
February	1st to 10th	Day	18% 9% 3 5% 5% 3 4 4 4 7% 38%	1900
	11th to 20th	Day	19% 6% 3 5% 4 3 4 4 5% 6% 40%	1788
	21st to end	Day	16% 8% 4 8% 6% 3 4 5% 4 7% 36%	1480
March	1st to 10th	Day	16% 8% 2 5% 6% 3 4 5% 5% 6% 39%	1908
	11th to 20th	Day	15% 8% 3 7% 4 3 4 5% 5% 6% 39%	1980
	21st to end	Day	12% 6% 3 5% 5% 3 6% 5% 5% 7% 43%	2263
April	1st to 10th	Day	10% 7% 2 6% 6% 3 5% 6% 6% 8% 42%	2160
	11th to 20th	Day	15% 7% 3 6% 5% 3 5% 5% 4 7% 40%	2134
	21st to end	Day	8% 4 3 5% 6% 3 5% 6% 6% 8% 45%	2160
May	1st to 10th	Day	13% 5% 2 5% 6% 3 6% 6% 6% 7% 41%	2160
	11th to 20th	Day	9% 7% 2 5% 7% 3 6% 7% 6% 8% 40%	2124
	21st to end	Day	8% 5% 3 6% 7% 4 7% 7% 7% 10% 35%	2337
June	1st to 10th	Day	9% 6% 3 6% 8% 5% 7% 8% 8% 8% 32%	2317
	11th to 20th	Day	5% 4 2 7% 8% 5% 8% 7% 8% 9% 37%	2402
	21st to end	Day	7% 6% 3 7% 8% 5% 8% 8% 8% 10% 30%	2340
July	1st to 10th	Day	7% 6% 4 8% 9% 6% 9% 8% 8% 10% 26%	2340
	11th to 20th	Day	8% 6% 3 7% 9% 5% 8% 8% 8% 11% 27%	2340
	21st to end	Day	8% 6% 3 6% 9% 5% 8% 8% 7% 10% 31%	2439
August	1st to 10th	Day	5% 5% 2 7% 8% 4 8% 8% 7% 10% 35%	2148
	11th to 20th	Day	9% 6% 3 8% 7% 5% 7% 6% 7% 9% 33%	2125
	21st to end	Day	12% 5% 2 6% 7% 5% 6% 7% 7% 9% 34%	2376
September	1st to 10th	Day	10% 6% 3 6% 7% 5% 6% 7% 6% 8% 35%	2160
	11th to 20th	Day	12% 6% 3 6% 6% 4 6% 5% 5% 6% 42%	2129
	21st to end	Day	11% 5% 2 4 5% 3 4 4 5% 7% 51%	1980
October	1st to 10th	Day	7% 4 2 4 4 3 4 5% 6% 7% 55%	1848
	11th to 20th	Day	9% 5% 2 5% 4 3 4 3 5% 7% 53%	1800
	21st to end	Day	12% 5% 2 4 4 3 4 3 3 5% 55%	1980
November	1st to 10th	Day	11% 5% 2 4 4 2 3 4 5% 6% 54%	1858
	11th to 20th	Day	12% 5% 3 4 4 2 3 4 3 6% 52%	1801
	21st to end	Day	14% 6% 3 5% 5% 3 4 4 4 6% 47%	1710
December	1st to 10th	Day	15% 8% 3 5% 5% 3 4 4 5% 7% 42%	1710
	11th to 20th	Day	15% 10% 5% 5% 6% 3 4 3 4 5% 42%	2149
	21st to end	Day	15% 7% 3 5% 4 3 5% 4 5% 7% 42%	3113
January	1st to 10th	Night	19% 9% 2 4 4 2 3 3 5% 5% 44%	4108
	11th to 20th	Night	16% 7% 2 5% 4 2 3 3 4 5% 51%	4120
	21st to end	Night	27% 9% 2 4 3 2 2 2 3 4 42%	4425
February	1st to 10th	Night	16% 8% 3 5% 5% 3 5% 4 4 6% 41%	2280
	11th to 20th	Night	19% 8% 3 4 4 2 3 3 3 6% 43%	2153
	21st to end	Night	21% 9% 3 6% 5% 3 4 3 4 6% 35%	1776
March	1st to 10th	Night	14% 8% 2 5% 4 3 4 4 4 8% 45%	2066
	11th to 20th	Night	14% 7% 3 6% 6% 4 4 4 6% 7% 39%	1980
	21st to end	Night	13% 6% 2 4 4 3 4 5% 4 7% 48%	2091
April	1st to 10th	Night	12% 6% 2 5% 5% 3 5% 4 6% 8% 45%	1800
	11th to 20th	Night	12% 6% 2 5% 5% 3 5% 4 5% 8% 46%	1775
	21st to end	Night	7% 4 2 3 4 3 4 5% 5% 8% 54%	1800
May	1st to 10th	Night	8% 5% 2 4 3 2 5% 4 5% 9% 53%	1800
	11th to 20th	Night	9% 5% 2 4 5% 3 4 5% 7% 11% 46%	1769
	21st to end	Night	5% 3 2 5% 6% 3 6% 6% 6% 12% 45%	1938
June	1st to 10th	Night	7% 6% 3 6% 5% 4 6% 7% 7% 11% 39%	1608
	11th to 20th	Night	2 3 2 5% 5% 4 6% 6% 7% 12% 49%	1526
	21st to end	Night	5% 4 3 5% 5% 3 6% 8% 10% 10% 40%	1620
July	1st to 10th	Night	5% 4 3 5% 7% 4 8% 7% 8% 11% 38%	1620
	11th to 20th	Night	6% 4 2 5% 6% 4 6% 7% 9% 14% 38%	1620
	21st to end	Night	4 3 2 5% 6% 4 7% 6% 9% 14% 40%	1875
August	1st to 10th	Night	3 3 2 4 4 4 5% 7% 9% 15% 45%	1790
	11th to 20th	Night	6% 5% 2 6% 6% 4 6% 7% 8% 13% 37%	1776
	21st to end	Night	10% 5% 3 6% 5% 3 5% 6% 7% 11% 41%	1980
September	1st to 10th	Night	6% 5% 2 6% 5% 4 5% 8% 9% 11% 41%	1800
	11th to 20th	Night	8% 6% 2 5% 5% 3 4 4 6% 9% 47%	1813
	21st to end	Night	9% 5% 2 4 4 2 4 4 4 7% 54%	1980
October	1st to 10th	Night	6% 4 2 4 3 2 4 4 5% 6% 59%	2107
	11th to 20th	Night	9% 4 3 5% 4 2 3 4 4 6% 56%	2160
	21st to end	Night	13% 6% 2 4 3 2 3 3 4 5% 56%	2376
November	1st to 10th	Night	12% 4 1 3 4 2 3 4 4 6% 57%	2220
	11th to 20th	Night	12% 6% 2 4 3 2 3 3 4 6% 55%	2375
	21st to end	Night	14% 7% 2 4 4 2 4 4 5% 5% 48%	2470
December	1st to 10th	Night	17% 7% 2 4 4 3 3 4 4 6% 47%	2470
	11th to 20th	Night	15% 9% 3 6% 5% 3 4 4 5% 5% 43%	3042
	21st to end	Night	18% 11% 4 5% 4 3 3 2 4 5% 42%	4532

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	44% 4 3 17% 26% 6%	2848
	11th to 20th	Day	36% 2 5% 20% 26% 10%	2850
	21st to end	Day	48% 3 4 14% 22% 9%	3234
February	1st to 10th	Day	48% 2 6% 15% 17% 11%	1900
	11th to 20th	Day	45% 3 6% 16% 21% 9%	1798
	21st to end	Day	49% 4 6% 16% 16% 10%	1480
March	1st to 10th	Day	45% 4 8% 16% 19% 9%	1908
	11th to 20th	Day	46% 4 9% 13% 21% 7%	1980
	21st to end	Day	41% 3 9% 19% 22% 6%	2263
April	1st to 10th	Day	40% 4 10% 19% 19% 8%	2160
	11th to 20th	Day	45% 3 7% 17% 21% 7%	2134
	21st to end	Day	35% 4 11% 23% 23% 3	2160
May	1st to 10th	Day	40% 3 11% 21% 20% 5%	2160
	11th to 20th	Day	40% 5% 13% 26% 15% 2	2128
	21st to end	Day	41% 5% 15% 24% 14% 1	2337
June	1st to 10th	Day	45% 6% 14% 23% 11% 1	2317
	11th to 20th	Day	39% 6% 16% 24% 13% 2	2402
	21st to end	Day	45% 7% 17% 20% 9% 1	2340
July	1st to 10th	Day	49% 8% 20% 16% 7% 1	2276
	11th to 20th	Day	46% 9% 22% 17% 5% 1	2271
	21st to end	Day	45% 9% 19% 19% 8% 1	2303
August	1st to 10th	Day	41% 7% 21% 22% 9%	2136
	11th to 20th	Day	46% 7% 19% 18% 7% 2	2125
	21st to end	Day	46% 7% 16% 18% 10% 3	2376
September	1st to 10th	Day	45% 4 14% 18% 14% 4	2160
	11th to 20th	Day	42% 6% 10% 18% 17% 7%	2129
	21st to end	Day	34% 3 10% 24% 20% 9%	1980
October	1st to 10th	Day	29% 4 8% 21% 27% 11%	1848
	11th to 20th	Day	31% 3 6% 17% 29% 14%	1800
	21st to end	Day	33% 3 4 16% 31% 13%	1980
November	1st to 10th	Day	33% 2 7% 18% 28% 12%	1858
	11th to 20th	Day	35% 3 6% 15% 29% 13%	1801
	21st to end	Day	39% 2 7% 18% 22% 11%	1710
December	1st to 10th	Day	42% 3 10% 19% 18% 8%	1710
	11th to 20th	Day	47% 3 5% 14% 24% 8%	2146
	21st to end	Day	43% 2 7% 21% 24% 3	3113
January	1st to 10th	Night	42% 3 5% 15% 27% 9%	4108
	11th to 20th	Night	38% 3 6% 18% 23% 12%	4120
	21st to end	Night	48% 2 4 16% 23% 7%	4425
February	1st to 10th	Night	45% 3 6% 17% 20% 9%	2280
	11th to 20th	Night	45% 3 5% 16% 22% 10%	2154
	21st to end	Night	51% 3 6% 14% 14% 12%	1776
March	1st to 10th	Night	40% 3 8% 16% 23% 10%	2066
	11th to 20th	Night	44% 4 11% 14% 19% 7%	1980
	21st to end	Night	37% 3 11% 19% 24% 6%	2091
April	1st to 10th	Night	38% 4 11% 19% 20% 9%	1800
	11th to 20th	Night	38% 3 10% 20% 19% 10%	1775
	21st to end	Night	28% 3 11% 24% 27% 7%	1800
May	1st to 10th	Night	29% 4 11% 27% 22% 7%	1800
	11th to 20th	Night	33% 5% 17% 24% 20% 2	1770
	21st to end	Night	31% 7% 19% 26% 16% 2	1939
June	1st to 10th	Night	38% 6% 18% 25% 13% 1	1608
	11th to 20th	Night	28% 7% 20% 30% 14% 2	1526
	21st to end	Night	32% 8% 23% 22% 13% 1	1620
July	1st to 10th	Night	36% 9% 26% 19% 8% 2	1575
	11th to 20th	Night	34% 12% 27% 20% 5% 2	1566
	21st to end	Night	32% 11% 26% 23% 8% 1	1771
August	1st to 10th	Night	25% 10% 33% 22% 10%	1778
	11th to 20th	Night	36% 11% 23% 19% 10% 2	1776
	21st to end	Night	38% 8% 20% 17% 12% 4	1980
September	1st to 10th	Night	33% 8% 20% 19% 15% 6%	1800
	11th to 20th	Night	34% 5% 17% 17% 17% 9%	1813
	21st to end	Night	31% 4 11% 24% 20% 10%	1980
October	1st to 10th	Night	27% 3 11% 19% 25% 15%	2107
	11th to 20th	Night	29% 4 5% 20% 27% 15%	2160
	21st to end	Night	32% 2 6% 16% 26% 17%	2376
November	1st to 10th	Night	29% 3 8% 18% 26% 15%	2220
	11th to 20th	Night	32% 3 5% 17% 29% 15%	2375
	21st to end	Night	38% 2 6% 20% 21% 13%	2470
December	1st to 10th	Night	41% 2 9% 22% 20% 7%	2470
	11th to 20th	Night	44% 3 5% 13% 24% 11%	3044
	21st to end	Night	47% 2 6% 18% 21% 6%	4532

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% 22% 33% 16% 3 1	270
	11th to 20th	Day	26% 17% 34% 17% 4 1	270
	21st to end	Day	27% 23% 36% 12% 2	297
February	1st to 10th	Day	30% 19% 33% 13% 4	270
	11th to 20th	Day	35% 15% 34% 14% 2	252
	21st to end	Day	27% 26% 36% 10% 1	198
March	1st to 10th	Day	30% 15% 35% 17% 4	240
	11th to 20th	Day	28% 18% 37% 14% 3	240
	21st to end	Day	25% 14% 32% 22% 7%	349
April	1st to 10th	Day	16% 19% 30% 24% 10% 1	420
	11th to 20th	Day	24% 13% 30% 22% 10% 1	412
	21st to end	Day	13% 11% 31% 32% 12% 1	420
May	1st to 10th	Day	15% 10% 26% 30% 15% 4	423
	11th to 20th	Day	13% 5% 23% 30% 20% 9%	447
	21st to end	Day	7% 4 19% 32% 26% 11%	485
June	1st to 10th	Day	12% 4 11% 34% 27% 13%	565
	11th to 20th	Day	7% 3 14% 35% 32% 9%	630
	21st to end	Day	10% 3 12% 27% 30% 19%	630
July	1st to 10th	Day	7% 2 6% 28% 42% 15%	621
	11th to 20th	Day	7% 2 5% 23% 40% 23%	618
	21st to end	Day	7% 3 6% 25% 40% 20%	577
August	1st to 10th	Day	7% 2 5% 23% 35% 29%	446
	11th to 20th	Day	10% 1 3 20% 43% 23%	439
	21st to end	Day	14% 2 7% 20% 35% 22%	495
September	1st to 10th	Day	12% 1 5% 25% 37% 20%	450
	11th to 20th	Day	12% 4 11% 22% 36% 15%	450
	21st to end	Day	13% 3 10% 32% 29% 13%	450
October	1st to 10th	Day	7% 7% 21% 36% 19% 9%	318
	11th to 20th	Day	11% 4 25% 30% 23% 6%	270
	21st to end	Day	22% 10% 21% 30% 13% 4	297
November	1st to 10th	Day	18% 8% 23% 34% 13% 4	270
	11th to 20th	Day	20% 11% 31% 27% 10% 1	270
	21st to end	Day	16% 17% 35% 20% 9% 3	270
December	1st to 10th	Day	23% 10% 37% 24% 6% 1	270
	11th to 20th	Day	23% 17% 36% 18% 6% 1	270
	21st to end	Day	27% 16% 32% 19% 5% 1	297
January	1st to 10th	Night	25% 14% 33% 23% 5%	950
	11th to 20th	Night	23% 15% 32% 23% 7%	950
	21st to end	Night	24% 17% 32% 24% 4	977
February	1st to 10th	Night	23% 15% 34% 24% 4	760
	11th to 20th	Night	28% 15% 29% 23% 5%	722
	21st to end	Night	24% 19% 35% 19% 3	592
March	1st to 10th	Night	19% 14% 34% 27% 5%	626
	11th to 20th	Night	18% 14% 33% 29% 6% 1	540
	21st to end	Night	17% 10% 37% 28% 8%	594
April	1st to 10th	Night	17% 11% 34% 27% 10% 1	540
	11th to 20th	Night	16% 12% 33% 28% 11% 1	533
	21st to end	Night	9% 9% 33% 36% 13% 1	540
May	1st to 10th	Night	10% 5% 31% 36% 16% 2	540
	11th to 20th	Night	10% 3 24% 37% 22% 4	531
	21st to end	Night	5% 3 18% 41% 28% 5%	584
June	1st to 10th	Night	10% 3 9% 39% 31% 8%	540
	11th to 20th	Night	3 2 10% 36% 42% 7%	540
	21st to end	Night	7% 2 8% 31% 37% 15%	540
July	1st to 10th	Night	6% 2 2 29% 48% 14%	525
	11th to 20th	Night	5% 1 4 25% 45% 20%	522
	21st to end	Night	5% 1 5% 28% 46% 15%	591
August	1st to 10th	Night	3 1 4 22% 49% 21%	537
	11th to 20th	Night	7% 1 4 22% 48% 17%	534
	21st to end	Night	11% 2 6% 23% 39% 20%	594
September	1st to 10th	Night	8% 2 4 30% 44% 12%	540
	11th to 20th	Night	9% 3 9% 28% 42% 9%	553
	21st to end	Night	11% 2 10% 34% 37% 6%	720
October	1st to 10th	Night	8% 1 17% 33% 35% 6%	720
	11th to 20th	Night	11% 4 24% 36% 23% 3	720
	21st to end	Night	17% 6% 21% 32% 22% 2	792
November	1st to 10th	Night	12% 6% 21% 37% 20% 3	740
	11th to 20th	Night	15% 9% 28% 33% 14% 1	760
	21st to end	Night	17% 10% 29% 28% 15% 2	760
December	1st to 10th	Night	21% 9% 29% 30% 10%	760
	11th to 20th	Night	22% 9% 34% 26% 9%	760
	21st to end	Night	27% 12% 29% 27% 5%	1045

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