Sunset Astronomical Society
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sunset Astronomical Society clear sky chart from 2005-11-06 to 2024-02-20.

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% 9% 4 7% 6% 4 5% 5% 5% 7% 33%	78050
	Night	15% 10% 4 7% 6% 3 4 4 5% 7% 33%	81164
Transparency	Day	49% 6% 13% 21% 11% 1	77754
	Night	51% 5% 12% 20% 11% 1	80982
Seeing	Day	16% 9% 29% 25% 15% 6%	14515
	Night	18% 8% 30% 31% 12% 1	24489

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	14% 16% 7% 10% 8% 4 5% 5% 5% 6% 20%	2527
	11th to 20th	Day	14% 17% 9% 16% 15% 7% 4 4 3 3 8%	2530
	21st to end	Day	21% 19% 7% 12% 7% 3 5% 4 4 4 12%	2849
February	1st to 10th	Day	11% 13% 6% 10% 8% 4 6% 6% 6% 7% 23%	1710
	11th to 20th	Day	12% 13% 6% 10% 11% 4 5% 5% 5% 5% 25%	1803
	21st to end	Day	14% 12% 7% 10% 9% 4 5% 4 5% 6% 25%	1480
March	1st to 10th	Day	13% 10% 5% 8% 8% 4 6% 5% 5% 7% 30%	1800
	11th to 20th	Day	12% 9% 5% 8% 7% 4 6% 5% 5% 6% 34%	1978
	21st to end	Day	18% 9% 4 8% 6% 4 5% 5% 4 6% 31%	2178
April	1st to 10th	Day	17% 11% 5% 8% 7% 3 5% 5% 5% 6% 29%	2157
	11th to 20th	Day	17% 9% 3 6% 6% 2 5% 5% 5% 7% 36%	2258
	21st to end	Day	17% 7% 4 6% 6% 4 5% 5% 5% 6% 34%	2340
May	1st to 10th	Day	16% 7% 4 6% 5% 3 5% 5% 6% 8% 35%	2340
	11th to 20th	Day	16% 6% 3 5% 6% 3 4 5% 5% 7% 38%	2301
	21st to end	Day	12% 5% 2 5% 5% 3 5% 5% 6% 9% 42%	2659
June	1st to 10th	Day	12% 5% 2 5% 4 4 5% 6% 6% 9% 42%	2587
	11th to 20th	Day	12% 5% 2 5% 6% 4 5% 5% 5% 8% 44%	2700
	21st to end	Day	10% 5% 3 5% 5% 4 6% 7% 7% 9% 41%	2564
July	1st to 10th	Day	5% 4 2 5% 5% 3 5% 6% 7% 10% 50%	2520
	11th to 20th	Day	8% 4 2 5% 5% 3 5% 5% 6% 9% 48%	2520
	21st to end	Day	7% 5% 2 5% 6% 3 5% 5% 7% 10% 44%	2707
August	1st to 10th	Day	9% 5% 2 5% 5% 4 6% 6% 7% 10% 41%	2327
	11th to 20th	Day	9% 4 2 7% 6% 4 5% 5% 5% 8% 46%	2303
	21st to end	Day	11% 6% 2 5% 4 3 4 5% 6% 8% 44%	2517
September	1st to 10th	Day	10% 6% 3 6% 6% 4 5% 5% 5% 7% 42%	2080
	11th to 20th	Day	12% 5% 2 4 4 2 4 5% 6% 9% 45%	1980
	21st to end	Day	13% 6% 2 5% 6% 3 6% 5% 6% 8% 38%	1980
October	1st to 10th	Day	16% 5% 3 6% 6% 4 5% 5% 5% 6% 40%	1958
	11th to 20th	Day	20% 9% 3 7% 7% 4 5% 4 5% 7% 29%	1780
	21st to end	Day	21% 10% 3 7% 6% 4 5% 5% 6% 7% 26%	1782
November	1st to 10th	Day	13% 8% 4 6% 6% 3 4 5% 6% 8% 36%	1672
	11th to 20th	Day	14% 12% 5% 8% 6% 4 6% 6% 6% 8% 26%	1710
	21st to end	Day	19% 12% 5% 9% 6% 4 5% 5% 5% 6% 24%	1615
December	1st to 10th	Day	13% 15% 7% 11% 8% 4 6% 4 4 5% 22%	1444
	11th to 20th	Day	14% 14% 6% 10% 7% 3 4 5% 5% 7% 24%	1675
	21st to end	Day	28% 16% 6% 10% 8% 3 5% 4 5% 5% 10%	2719
January	1st to 10th	Night	16% 15% 7% 10% 7% 4 4 4 4 5% 23%	4428
	11th to 20th	Night	13% 15% 8% 15% 12% 7% 5% 4 3 5% 13%	4440
	21st to end	Night	19% 18% 8% 10% 7% 4 4 4 5% 5% 16%	4814
February	1st to 10th	Night	14% 13% 5% 9% 8% 4 5% 5% 6% 6% 26%	2470
	11th to 20th	Night	13% 12% 6% 9% 8% 3 5% 5% 5% 7% 26%	2247
	21st to end	Night	17% 14% 6% 9% 6% 3 3 4 4 7% 27%	1776
March	1st to 10th	Night	14% 9% 4 8% 6% 3 4 4 4 6% 37%	2160
	11th to 20th	Night	13% 8% 4 6% 6% 3 4 5% 5% 7% 40%	1981
	21st to end	Night	19% 9% 4 6% 7% 3 4 4 4 6% 34%	2178
April	1st to 10th	Night	17% 9% 4 5% 5% 3 5% 4 5% 8% 35%	1818
	11th to 20th	Night	19% 7% 3 5% 5% 2 4 5% 4 7% 39%	1651
	21st to end	Night	17% 8% 4 5% 5% 2 5% 4 5% 6% 39%	1620
May	1st to 10th	Night	14% 7% 3 7% 6% 3 5% 5% 5% 8% 36%	1620
	11th to 20th	Night	17% 5% 2 5% 6% 3 5% 5% 5% 7% 41%	1592
	21st to end	Night	11% 5% 2 4 5% 3 4 5% 6% 8% 47%	1613
June	1st to 10th	Night	11% 5% 3 5% 5% 3 4 5% 5% 8% 46%	1364
	11th to 20th	Night	9% 6% 3 6% 5% 3 3 3 5% 11% 46%	1260
	21st to end	Night	8% 5% 2 5% 5% 4 5% 5% 5% 8% 48%	1387
July	1st to 10th	Night	6% 4 2 4 4 3 4 4 6% 10% 54%	1440
	11th to 20th	Night	7% 3 2 4 5% 3 4 4 7% 9% 52%	1440
	21st to end	Night	7% 4 2 4 4 3 4 4 6% 8% 52%	1609
August	1st to 10th	Night	7% 4 2 5% 4 3 5% 6% 6% 9% 50%	1611
	11th to 20th	Night	7% 4 2 4 5% 2 4 5% 6% 7% 54%	1598
	21st to end	Night	11% 5% 2 5% 4 2 4 4 5% 8% 51%	1836
September	1st to 10th	Night	10% 4 2 6% 6% 4 4 4 6% 6% 49%	1862
	11th to 20th	Night	10% 5% 2 5% 3 3 4 4 4 7% 53%	1980
	21st to end	Night	13% 6% 3 5% 4 3 4 5% 5% 7% 44%	1980
October	1st to 10th	Night	17% 6% 2 5% 4 3 3 4 6% 7% 44%	2002
	11th to 20th	Night	18% 10% 3 7% 6% 2 4 4 4 6% 34%	2178
	21st to end	Night	21% 9% 4 7% 6% 3 5% 4 4 7% 30%	2574
November	1st to 10th	Night	12% 9% 2 5% 6% 3 3 5% 6% 7% 42%	2406
	11th to 20th	Night	15% 12% 5% 7% 6% 3 6% 5% 7% 7% 29%	2470
	21st to end	Night	18% 13% 5% 7% 7% 3 5% 5% 5% 7% 26%	2560
December	1st to 10th	Night	17% 13% 6% 10% 7% 4 5% 5% 5% 6% 22%	2733
	11th to 20th	Night	15% 13% 5% 10% 8% 4 5% 5% 6% 7% 24%	3522
	21st to end	Night	28% 19% 6% 8% 6% 2 3 4 4 5% 15%	4944

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	65% 5% 14% 12% 4	2528
	11th to 20th	Day	78% 7% 6% 7% 2	2530
	21st to end	Day	75% 4 9% 8% 4	2849
February	1st to 10th	Day	59% 6% 12% 17% 6%	1710
	11th to 20th	Day	60% 6% 11% 15% 8% 1	1813
	21st to end	Day	58% 5% 11% 18% 8% 1	1480
March	1st to 10th	Day	53% 5% 13% 20% 8% 1	1800
	11th to 20th	Day	50% 5% 12% 19% 12% 2	1978
	21st to end	Day	54% 5% 12% 19% 8% 1	2178
April	1st to 10th	Day	55% 5% 11% 18% 10%	2157
	11th to 20th	Day	48% 5% 12% 19% 14% 2	2258
	21st to end	Day	50% 6% 12% 18% 12% 2	2340
May	1st to 10th	Day	47% 5% 12% 23% 11% 2	2340
	11th to 20th	Day	45% 6% 12% 22% 15% 2	2305
	21st to end	Day	39% 6% 14% 25% 13% 2	2659
June	1st to 10th	Day	37% 7% 13% 28% 13% 2	2587
	11th to 20th	Day	38% 6% 13% 23% 19% 2	2700
	21st to end	Day	37% 7% 15% 24% 14% 2	2564
July	1st to 10th	Day	28% 7% 14% 29% 19% 3	2451
	11th to 20th	Day	32% 7% 13% 28% 18% 2	2445
	21st to end	Day	34% 6% 15% 25% 18% 3	2557
August	1st to 10th	Day	36% 8% 14% 27% 13% 2	2314
	11th to 20th	Day	35% 7% 14% 28% 14% 2	2303
	21st to end	Day	37% 6% 13% 28% 14% 2	2517
September	1st to 10th	Day	40% 7% 13% 27% 11% 1	2080
	11th to 20th	Day	34% 5% 16% 31% 13% 1	1980
	21st to end	Day	42% 5% 16% 25% 12%	1980
October	1st to 10th	Day	44% 6% 10% 25% 13% 2	1958
	11th to 20th	Day	54% 6% 13% 20% 7%	1780
	21st to end	Day	55% 6% 15% 16% 7% 1	1782
November	1st to 10th	Day	46% 6% 14% 23% 11%	1672
	11th to 20th	Day	54% 5% 15% 17% 8% 1	1710
	21st to end	Day	60% 5% 15% 14% 5%	1615
December	1st to 10th	Day	64% 7% 12% 15% 2	1444
	11th to 20th	Day	60% 5% 15% 16% 4	1671
	21st to end	Day	77% 5% 8% 7% 3	2719
January	1st to 10th	Night	62% 5% 12% 13% 6% 1	4428
	11th to 20th	Night	72% 7% 9% 9% 4	4440
	21st to end	Night	69% 6% 11% 10% 3	4814
February	1st to 10th	Night	58% 6% 10% 19% 7% 1	2470
	11th to 20th	Night	57% 6% 10% 18% 8% 1	2248
	21st to end	Night	59% 3 10% 17% 10%	1776
March	1st to 10th	Night	48% 4 13% 23% 11% 1	2160
	11th to 20th	Night	44% 4 11% 23% 16% 2	1981
	21st to end	Night	52% 4 10% 22% 11% 2	2178
April	1st to 10th	Night	48% 5% 10% 20% 15% 2	1818
	11th to 20th	Night	46% 5% 8% 21% 17% 3	1651
	21st to end	Night	45% 5% 11% 20% 15% 3	1620
May	1st to 10th	Night	46% 6% 11% 21% 14% 2	1620
	11th to 20th	Night	44% 5% 10% 22% 17% 4	1593
	21st to end	Night	35% 7% 12% 27% 15% 4	1614
June	1st to 10th	Night	38% 6% 10% 26% 19% 2	1364
	11th to 20th	Night	35% 6% 13% 24% 20% 2	1260
	21st to end	Night	33% 7% 13% 27% 18% 2	1387
July	1st to 10th	Night	26% 5% 12% 32% 21% 5%	1400
	11th to 20th	Night	28% 6% 15% 30% 20% 2	1392
	21st to end	Night	30% 7% 15% 28% 18% 3	1520
August	1st to 10th	Night	29% 6% 17% 33% 14% 1	1600
	11th to 20th	Night	28% 6% 14% 31% 18% 2	1598
	21st to end	Night	32% 5% 13% 29% 19% 1	1836
September	1st to 10th	Night	36% 6% 11% 28% 18% 1	1862
	11th to 20th	Night	31% 5% 12% 35% 16% 1	1980
	21st to end	Night	39% 5% 15% 30% 11% 1	1980
October	1st to 10th	Night	40% 4 12% 27% 15% 1	2002
	11th to 20th	Night	51% 5% 12% 22% 9% 1	2178
	21st to end	Night	55% 5% 11% 19% 9% 1	2574
November	1st to 10th	Night	42% 5% 12% 26% 14% 1	2406
	11th to 20th	Night	52% 5% 14% 20% 8% 1	2470
	21st to end	Night	57% 5% 16% 15% 5%	2560
December	1st to 10th	Night	62% 6% 12% 15% 5%	2733
	11th to 20th	Night	59% 7% 14% 12% 7% 1	3525
	21st to end	Night	73% 5% 10% 8% 4	4944

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	19% 14% 30% 25% 10% 2	270
	11th to 20th	Day	20% 13% 37% 24% 6% 1	270
	21st to end	Day	21% 13% 29% 26% 10%	297
February	1st to 10th	Day	19% 15% 40% 23% 2 2	270
	11th to 20th	Day	16% 19% 34% 26% 4	267
	21st to end	Day	27% 16% 39% 15% 2	198
March	1st to 10th	Day	22% 11% 43% 21% 2 2	240
	11th to 20th	Day	17% 12% 33% 26% 11% 1	418
	21st to end	Day	25% 10% 32% 23% 9% 1	462
April	1st to 10th	Day	20% 13% 31% 25% 10% 1	420
	11th to 20th	Day	23% 16% 29% 18% 9% 5%	412
	21st to end	Day	17% 15% 30% 23% 12% 2	420
May	1st to 10th	Day	22% 13% 32% 19% 12% 4	423
	11th to 20th	Day	18% 9% 28% 23% 15% 7%	447
	21st to end	Day	15% 7% 26% 26% 19% 8%	613
June	1st to 10th	Day	12% 7% 31% 29% 16% 6%	630
	11th to 20th	Day	13% 8% 29% 28% 16% 6%	630
	21st to end	Day	11% 5% 29% 31% 18% 6%	630
July	1st to 10th	Day	7% 4 25% 28% 23% 14%	621
	11th to 20th	Day	9% 5% 25% 31% 20% 10%	618
	21st to end	Day	9% 3 26% 33% 22% 8%	648
August	1st to 10th	Day	8% 3 22% 25% 25% 17%	446
	11th to 20th	Day	11% 3 18% 26% 29% 14%	439
	21st to end	Day	14% 5% 20% 25% 25% 11%	495
September	1st to 10th	Day	12% 5% 25% 27% 20% 11%	450
	11th to 20th	Day	11% 4 20% 25% 28% 12%	450
	21st to end	Day	16% 11% 18% 26% 19% 10%	450
October	1st to 10th	Day	20% 8% 27% 22% 16% 8%	450
	11th to 20th	Day	23% 16% 27% 25% 9% 1	430
	21st to end	Day	25% 12% 33% 21% 7% 1	297
November	1st to 10th	Day	18% 13% 36% 22% 9% 2	270
	11th to 20th	Day	16% 24% 36% 18% 6% 1	270
	21st to end	Day	26% 11% 40% 18% 6%	225
December	1st to 10th	Day	24% 14% 35% 20% 6% 1	180
	11th to 20th	Day	22% 17% 37% 19% 4 1	180
	21st to end	Day	25% 16% 35% 21% 4	279
January	1st to 10th	Night	20% 12% 36% 24% 7% 1	950
	11th to 20th	Night	23% 11% 35% 25% 6% 1	950
	21st to end	Night	18% 8% 34% 32% 8%	1001
February	1st to 10th	Night	18% 10% 37% 30% 4	760
	11th to 20th	Night	19% 10% 38% 28% 5%	742
	21st to end	Night	24% 12% 37% 23% 4	592
March	1st to 10th	Night	20% 9% 32% 31% 8% 1	720
	11th to 20th	Night	18% 8% 37% 29% 7% 1	720
	21st to end	Night	23% 10% 31% 31% 6% 1	792
April	1st to 10th	Night	21% 10% 34% 28% 7% 1	558
	11th to 20th	Night	22% 9% 30% 28% 9% 2	533
	21st to end	Night	23% 6% 29% 33% 9% 1	540
May	1st to 10th	Night	15% 6% 33% 36% 9% 1	540
	11th to 20th	Night	21% 4 26% 36% 13% 1	531
	21st to end	Night	15% 2 25% 37% 19% 2	584
June	1st to 10th	Night	12% 5% 27% 41% 15% 1	464
	11th to 20th	Night	11% 6% 23% 43% 16% 1	360
	21st to end	Night	10% 2 22% 44% 21% 1	487
July	1st to 10th	Night	7% 2 18% 42% 28% 3	525
	11th to 20th	Night	8% 2 19% 43% 25% 3	522
	21st to end	Night	9% 2 21% 45% 21% 3	591
August	1st to 10th	Night	8% 1 15% 48% 25% 3	537
	11th to 20th	Night	9% 2 17% 38% 31% 4	534
	21st to end	Night	11% 3 19% 36% 29% 3	594
September	1st to 10th	Night	10% 3 23% 35% 23% 5%	602
	11th to 20th	Night	13% 3 19% 35% 26% 4	720
	21st to end	Night	15% 5% 23% 32% 20% 4	720
October	1st to 10th	Night	20% 6% 24% 32% 17% 2	720
	11th to 20th	Night	23% 11% 28% 29% 9% 1	720
	21st to end	Night	25% 10% 34% 24% 6%	792
November	1st to 10th	Night	17% 11% 31% 28% 11% 1	740
	11th to 20th	Night	21% 13% 36% 23% 8%	760
	21st to end	Night	24% 10% 36% 25% 5%	760
December	1st to 10th	Night	23% 10% 36% 26% 5% 1	833
	11th to 20th	Night	22% 10% 39% 24% 4 1	950
	21st to end	Night	31% 9% 34% 24% 3	1045

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