Scott's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Scott'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	8% 5% 2 6% 6% 4 6% 6% 7% 9% 40%	78774
	Night	8% 5% 2 5% 6% 3 5% 5% 7% 9% 43%	80311
Transparency	Day	38% 10% 16% 18% 14% 5%	78497
	Night	35% 10% 17% 19% 14% 5%	80107
Seeing	Day	7% 4 16% 26% 27% 20%	14441
	Night	8% 3 17% 34% 28% 9%	24579

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	11% 8% 3 7% 6% 3 5% 6% 5% 8% 39%	2836
	11th to 20th	Day	20% 9% 3 6% 5% 3 3 4 5% 7% 36%	2840
	21st to end	Day	12% 5% 3 6% 5% 3 4 5% 7% 9% 43%	3420
February	1st to 10th	Day	11% 5% 2 5% 4 3 4 5% 6% 7% 46%	1900
	11th to 20th	Day	6% 4 2 4 4 2 4 5% 4 8% 56%	1788
	21st to end	Day	8% 4 2 5% 4 3 4 4 5% 9% 52%	1529
March	1st to 10th	Day	4 3 1 3 4 3 5% 5% 5% 8% 60%	1980
	11th to 20th	Day	7% 3 2 4 4 3 4 5% 6% 8% 56%	1980
	21st to end	Day	7% 4 2 4 4 2 4 5% 5% 8% 54%	2178
April	1st to 10th	Day	7% 3 2 4 4 4 5% 5% 7% 8% 51%	1980
	11th to 20th	Day	8% 4 2 6% 5% 4 5% 6% 7% 9% 45%	1956
	21st to end	Day	5% 3 2 4 5% 3 5% 5% 7% 9% 54%	2128
May	1st to 10th	Day	4 2 1 2 3 2 4 4 6% 9% 61%	2340
	11th to 20th	Day	8% 5% 2 5% 5% 4 6% 6% 7% 9% 43%	2301
	21st to end	Day	7% 5% 2 5% 6% 4 7% 7% 7% 10% 40%	2531
June	1st to 10th	Day	12% 7% 3 7% 7% 4 7% 7% 7% 11% 28%	2340
	11th to 20th	Day	6% 5% 3 6% 8% 5% 8% 9% 9% 11% 30%	2340
	21st to end	Day	7% 6% 3 7% 7% 6% 9% 8% 9% 10% 28%	2340
July	1st to 10th	Day	9% 4 3 7% 8% 7% 10% 9% 8% 11% 24%	2340
	11th to 20th	Day	6% 5% 3 9% 8% 6% 9% 9% 9% 10% 24%	2340
	21st to end	Day	7% 6% 3 7% 9% 6% 9% 9% 9% 10% 24%	2547
August	1st to 10th	Day	6% 5% 3 7% 9% 7% 9% 10% 9% 12% 24%	2289
	11th to 20th	Day	7% 6% 3 8% 9% 6% 10% 9% 9% 11% 21%	2118
	21st to end	Day	10% 6% 4 7% 9% 5% 9% 8% 9% 11% 22%	2178
September	1st to 10th	Day	8% 5% 2 7% 9% 6% 9% 8% 9% 12% 24%	1980
	11th to 20th	Day	9% 6% 4 8% 9% 6% 8% 7% 8% 9% 25%	1980
	21st to end	Day	7% 6% 3 8% 8% 5% 6% 7% 7% 11% 32%	1980
October	1st to 10th	Day	8% 6% 3 7% 8% 4 6% 6% 6% 9% 37%	1980
	11th to 20th	Day	7% 4 2 5% 6% 4 5% 5% 6% 8% 48%	1849
	21st to end	Day	7% 5% 2 5% 4 3 5% 5% 6% 9% 49%	1980
November	1st to 10th	Day	7% 4 2 4 5% 3 5% 5% 7% 9% 50%	1857
	11th to 20th	Day	9% 5% 3 5% 6% 3 5% 5% 5% 7% 47%	1900
	21st to end	Day	9% 5% 2 4 5% 4 5% 6% 6% 9% 47%	1762
December	1st to 10th	Day	7% 4 2 5% 5% 4 5% 5% 7% 10% 46%	1710
	11th to 20th	Day	11% 6% 3 6% 5% 3 4 5% 6% 7% 46%	2153
	21st to end	Day	12% 7% 3 8% 8% 4 5% 5% 5% 6% 36%	3124
January	1st to 10th	Night	12% 6% 4 7% 6% 4 5% 5% 6% 9% 36%	4109
	11th to 20th	Night	18% 11% 4 7% 5% 3 4 4 5% 7% 31%	4120
	21st to end	Night	11% 6% 2 7% 5% 4 6% 5% 7% 8% 39%	4232
February	1st to 10th	Night	11% 6% 2 6% 5% 4 4 4 5% 8% 46%	2280
	11th to 20th	Night	9% 4 2 4 4 3 4 4 6% 7% 53%	2153
	21st to end	Night	7% 4 2 4 5% 2 5% 5% 8% 8% 49%	1724
March	1st to 10th	Night	4 3 1 3 4 2 4 4 5% 8% 60%	1980
	11th to 20th	Night	6% 4 2 4 5% 3 4 5% 7% 9% 53%	1980
	21st to end	Night	5% 4 2 4 5% 3 4 4 5% 6% 58%	2178
April	1st to 10th	Night	6% 3 2 4 4 2 4 5% 6% 9% 55%	1980
	11th to 20th	Night	6% 5% 2 5% 5% 3 5% 6% 6% 10% 48%	1953
	21st to end	Night	4 4 2 4 4 2 4 4 5% 10% 57%	1846
May	1st to 10th	Night	2 2 1 3 2 3 4 4 5% 10% 65%	1620
	11th to 20th	Night	7% 5% 2 5% 5% 3 6% 5% 6% 8% 48%	1592
	21st to end	Night	5% 3 2 5% 5% 3 5% 5% 7% 9% 52%	1744
June	1st to 10th	Night	11% 5% 2 5% 5% 4 6% 6% 7% 10% 39%	1620
	11th to 20th	Night	6% 4 2 4 6% 4 5% 8% 8% 13% 41%	1620
	21st to end	Night	5% 5% 3 5% 7% 4 7% 7% 9% 12% 35%	1620
July	1st to 10th	Night	6% 5% 3 6% 7% 4 6% 8% 9% 13% 34%	1620
	11th to 20th	Night	5% 4 3 5% 7% 5% 8% 8% 10% 15% 30%	1620
	21st to end	Night	4 4 3 7% 8% 5% 8% 7% 10% 14% 32%	1771
August	1st to 10th	Night	5% 4 2 4 7% 4 6% 6% 11% 14% 36%	1643
	11th to 20th	Night	6% 5% 3 5% 7% 4 7% 8% 10% 13% 32%	1776
	21st to end	Night	9% 6% 2 7% 7% 5% 8% 7% 8% 11% 28%	2164
September	1st to 10th	Night	5% 5% 2 7% 8% 5% 8% 8% 9% 12% 32%	1980
	11th to 20th	Night	8% 7% 3 7% 8% 5% 7% 7% 8% 9% 31%	1980
	21st to end	Night	8% 6% 3 7% 8% 4 6% 6% 8% 10% 32%	1980
October	1st to 10th	Night	6% 5% 2 6% 6% 4 5% 7% 7% 11% 41%	1980
	11th to 20th	Night	7% 5% 2 5% 5% 3 4 4 6% 8% 50%	2110
	21st to end	Night	8% 5% 2 4 5% 4 5% 4 6% 8% 49%	2376
November	1st to 10th	Night	7% 4 2 5% 5% 3 5% 5% 6% 8% 52%	2221
	11th to 20th	Night	8% 4 3 6% 5% 3 5% 5% 5% 7% 49%	2280
	21st to end	Night	8% 5% 2 5% 6% 3 5% 5% 6% 9% 47%	2415
December	1st to 10th	Night	7% 4 3 5% 5% 3 4 5% 6% 8% 49%	2470
	11th to 20th	Night	13% 5% 2 4 5% 3 5% 4 5% 8% 47%	3042
	21st to end	Night	14% 7% 3 6% 6% 4 5% 5% 6% 8% 35%	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	41% 6% 11% 19% 17% 6%	2837
	11th to 20th	Day	49% 4 7% 17% 17% 7%	2840
	21st to end	Day	36% 7% 9% 22% 21% 5%	3420
February	1st to 10th	Day	35% 7% 11% 20% 18% 11%	1900
	11th to 20th	Day	26% 6% 11% 22% 26% 10%	1798
	21st to end	Day	30% 6% 13% 17% 26% 9%	1529
March	1st to 10th	Day	22% 6% 12% 25% 25% 9%	1980
	11th to 20th	Day	26% 6% 13% 27% 20% 8%	1980
	21st to end	Day	27% 5% 11% 26% 24% 7%	2178
April	1st to 10th	Day	28% 8% 18% 21% 18% 7%	1980
	11th to 20th	Day	34% 9% 14% 17% 22% 5%	1956
	21st to end	Day	25% 7% 13% 24% 25% 7%	2128
May	1st to 10th	Day	19% 6% 15% 27% 24% 9%	2340
	11th to 20th	Day	35% 10% 17% 19% 16% 2	2305
	21st to end	Day	36% 12% 15% 20% 16% 2	2531
June	1st to 10th	Day	47% 12% 20% 16% 4	2340
	11th to 20th	Day	41% 17% 22% 17% 3	2340
	21st to end	Day	45% 17% 24% 12% 2	2340
July	1st to 10th	Day	48% 19% 25% 7% 2	2275
	11th to 20th	Day	49% 18% 24% 8% 1	2271
	21st to end	Day	47% 17% 24% 11% 1	2405
August	1st to 10th	Day	46% 19% 27% 8%	2276
	11th to 20th	Day	49% 19% 22% 9%	2118
	21st to end	Day	51% 18% 20% 10% 1	2178
September	1st to 10th	Day	47% 17% 21% 12% 1	1980
	11th to 20th	Day	49% 16% 20% 12% 3	1980
	21st to end	Day	43% 14% 16% 16% 9% 2	1980
October	1st to 10th	Day	41% 10% 14% 15% 12% 8%	1980
	11th to 20th	Day	31% 8% 12% 22% 23% 5%	1849
	21st to end	Day	32% 7% 11% 19% 20% 11%	1980
November	1st to 10th	Day	29% 7% 13% 21% 18% 11%	1857
	11th to 20th	Day	35% 7% 9% 21% 20% 8%	1900
	21st to end	Day	32% 7% 13% 20% 20% 8%	1762
December	1st to 10th	Day	32% 7% 15% 25% 17% 5%	1710
	11th to 20th	Day	36% 5% 12% 18% 18% 11%	2150
	21st to end	Day	47% 7% 11% 18% 15% 4	3124
January	1st to 10th	Night	42% 8% 11% 18% 16% 5%	4109
	11th to 20th	Night	52% 6% 8% 18% 12% 4	4120
	21st to end	Night	40% 7% 12% 15% 20% 6%	4232
February	1st to 10th	Night	35% 6% 12% 19% 20% 9%	2280
	11th to 20th	Night	29% 5% 12% 19% 24% 10%	2154
	21st to end	Night	29% 7% 14% 19% 24% 6%	1724
March	1st to 10th	Night	21% 6% 13% 29% 23% 8%	1980
	11th to 20th	Night	26% 7% 15% 27% 18% 7%	1980
	21st to end	Night	27% 6% 11% 27% 22% 7%	2178
April	1st to 10th	Night	24% 7% 21% 26% 16% 7%	1980
	11th to 20th	Night	28% 10% 16% 21% 21% 4	1953
	21st to end	Night	24% 6% 14% 27% 25% 5%	1846
May	1st to 10th	Night	15% 7% 13% 32% 23% 10%	1620
	11th to 20th	Night	32% 9% 19% 22% 17% 1	1593
	21st to end	Night	28% 9% 18% 25% 19% 1	1745
June	1st to 10th	Night	37% 13% 23% 21% 6%	1620
	11th to 20th	Night	32% 14% 28% 22% 4	1620
	21st to end	Night	36% 18% 25% 14% 6%	1620
July	1st to 10th	Night	38% 20% 30% 9% 2	1575
	11th to 20th	Night	39% 21% 30% 9% 1	1566
	21st to end	Night	37% 19% 32% 12% 1	1672
August	1st to 10th	Night	33% 20% 35% 11% 1	1632
	11th to 20th	Night	36% 20% 33% 11%	1776
	21st to end	Night	46% 18% 22% 12% 2	2164
September	1st to 10th	Night	40% 20% 25% 14% 2	1980
	11th to 20th	Night	46% 15% 24% 13% 2	1980
	21st to end	Night	42% 13% 19% 15% 9% 1	1980
October	1st to 10th	Night	34% 13% 15% 19% 13% 7%	1980
	11th to 20th	Night	30% 8% 14% 23% 21% 5%	2110
	21st to end	Night	32% 6% 12% 19% 21% 9%	2376
November	1st to 10th	Night	29% 7% 14% 20% 20% 11%	2221
	11th to 20th	Night	32% 7% 12% 21% 19% 8%	2280
	21st to end	Night	33% 7% 12% 23% 17% 7%	2415
December	1st to 10th	Night	32% 7% 14% 27% 17% 4	2470
	11th to 20th	Night	35% 6% 11% 21% 17% 11%	3044
	21st to end	Night	44% 9% 13% 16% 15% 4	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	11% 9% 34% 33% 12% 2	270
	11th to 20th	Day	10% 11% 31% 30% 14% 4	270
	21st to end	Day	11% 13% 42% 27% 6%	297
February	1st to 10th	Day	8% 9% 31% 35% 13% 3	270
	11th to 20th	Day	4 9% 30% 33% 20% 4	252
	21st to end	Day	7% 6% 25% 38% 21% 3	247
March	1st to 10th	Day	4 5% 38% 38% 14% 1	420
	11th to 20th	Day	7% 7% 28% 28% 27% 2	420
	21st to end	Day	5% 6% 28% 36% 21% 5%	462
April	1st to 10th	Day	5% 6% 30% 34% 22% 2	420
	11th to 20th	Day	8% 7% 25% 36% 20% 4	412
	21st to end	Day	5% 2 21% 34% 29% 8%	420
May	1st to 10th	Day	5% 2 15% 37% 30% 11%	423
	11th to 20th	Day	10% 2 12% 29% 32% 15%	447
	21st to end	Day	6% 2 8% 22% 34% 28%	485
June	1st to 10th	Day	12% 2 8% 24% 31% 22%	450
	11th to 20th	Day	6% 3 6% 20% 32% 33%	450
	21st to end	Day	6% 1 3 12% 30% 49%	450
July	1st to 10th	Day	6% 2 1 8% 28% 55%	445
	11th to 20th	Day	5% 3 9% 35% 48%	444
	21st to end	Day	9% 1 5% 11% 30% 44%	488
August	1st to 10th	Day	7% 4 8% 33% 48%	446
	11th to 20th	Day	5% 2 6% 9% 28% 49%	439
	21st to end	Day	8% 1 7% 15% 30% 38%	495
September	1st to 10th	Day	6% 2 4 14% 30% 44%	450
	11th to 20th	Day	8% 1 4 22% 34% 30%	450
	21st to end	Day	6% 2 4 20% 36% 33%	450
October	1st to 10th	Day	7% 3 13% 33% 34% 10%	450
	11th to 20th	Day	7% 2 14% 30% 34% 13%	450
	21st to end	Day	8% 3 13% 35% 34% 7%	495
November	1st to 10th	Day	8% 3 19% 33% 30% 7%	455
	11th to 20th	Day	10% 6% 20% 39% 21% 4	460
	21st to end	Day	7% 5% 25% 35% 24% 4	322
December	1st to 10th	Day	5% 7% 28% 34% 19% 7%	270
	11th to 20th	Day	9% 6% 32% 31% 20% 3	270
	21st to end	Day	5% 7% 27% 35% 21% 4	297
January	1st to 10th	Night	10% 7% 28% 39% 15% 1	760
	11th to 20th	Night	11% 7% 25% 42% 14% 1	760
	21st to end	Night	12% 9% 31% 37% 11%	836
February	1st to 10th	Night	12% 6% 29% 39% 14%	760
	11th to 20th	Night	9% 6% 26% 40% 18% 1	722
	21st to end	Night	7% 5% 23% 48% 16%	592
March	1st to 10th	Night	5% 4 34% 46% 11% 1	720
	11th to 20th	Night	6% 5% 24% 45% 18% 1	720
	21st to end	Night	6% 3 25% 46% 18% 1	792
April	1st to 10th	Night	5% 3 26% 45% 21%	720
	11th to 20th	Night	7% 4 25% 45% 18% 1	711
	21st to end	Night	5% 2 24% 47% 20% 1	716
May	1st to 10th	Night	4 13% 48% 33% 3	540
	11th to 20th	Night	7% 1 13% 39% 35% 5%	531
	21st to end	Night	4 1 10% 32% 45% 8%	584
June	1st to 10th	Night	10% 1 4 26% 47% 12%	540
	11th to 20th	Night	5% 1 3 16% 48% 26%	540
	21st to end	Night	5% 2 3 12% 45% 32%	540
July	1st to 10th	Night	6% 2 9% 44% 39%	525
	11th to 20th	Night	4 1 2 9% 43% 42%	522
	21st to end	Night	4 1 3 10% 47% 36%	591
August	1st to 10th	Night	6% 1 2 8% 47% 37%	537
	11th to 20th	Night	6% 1 2 9% 37% 45%	534
	21st to end	Night	11% 1 6% 17% 40% 24%	778
September	1st to 10th	Night	7% 1 6% 16% 47% 23%	720
	11th to 20th	Night	10% 2 5% 24% 42% 16%	720
	21st to end	Night	12% 1 6% 29% 39% 13%	720
October	1st to 10th	Night	9% 2 14% 40% 31% 5%	720
	11th to 20th	Night	8% 3 14% 37% 36% 3	720
	21st to end	Night	9% 3 18% 40% 28% 3	792
November	1st to 10th	Night	7% 2 23% 42% 22% 3	740
	11th to 20th	Night	8% 4 22% 43% 20% 2	760
	21st to end	Night	7% 4 25% 46% 18% 1	760
December	1st to 10th	Night	9% 3 23% 39% 25% 1	760
	11th to 20th	Night	10% 5% 23% 41% 20% 2	760
	21st to end	Night	8% 5% 26% 40% 20% 1	836

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