RAC Summer Star Party
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the RAC Summer Star Party clear sky chart from 2005-11-06 to 2024-04-03.

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	16% 7% 3 6% 6% 3 5% 5% 6% 7% 36%	78672
	Night	16% 7% 3 6% 5% 3 4 5% 5% 7% 38%	81449
Transparency	Day	45% 8% 13% 20% 12% 2	78377
	Night	44% 8% 13% 21% 12% 2	81278
Seeing	Day	24% 18% 29% 18% 8% 2	14066
	Night	22% 16% 29% 24% 8% 1	23236

By Month

Percentage Cloud Cover NE (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	13% 7% 3 6% 7% 4 5% 6% 6% 7% 37%	2517
	11th to 20th	Day	15% 9% 5% 10% 7% 4 6% 5% 5% 7% 28%	2520
	21st to end	Day	18% 14% 6% 8% 5% 3 4 4 4 5% 28%	2782
February	1st to 10th	Day	14% 9% 4 7% 7% 4 6% 5% 6% 7% 31%	1743
	11th to 20th	Day	12% 6% 3 8% 7% 4 6% 5% 5% 7% 36%	1848
	21st to end	Day	19% 9% 3 7% 6% 3 6% 5% 5% 5% 33%	1570
March	1st to 10th	Day	15% 8% 4 6% 6% 3 5% 5% 6% 7% 36%	1897
	11th to 20th	Day	15% 7% 4 7% 6% 4 5% 5% 7% 6% 35%	1951
	21st to end	Day	18% 7% 4 6% 5% 3 4 4 4 5% 39%	2508
April	1st to 10th	Day	17% 8% 4 7% 6% 3 4 4 6% 6% 35%	2184
	11th to 20th	Day	16% 7% 3 6% 6% 3 5% 5% 5% 6% 38%	2134
	21st to end	Day	20% 7% 3 6% 5% 3 5% 5% 6% 7% 33%	2281
May	1st to 10th	Day	24% 7% 3 6% 5% 3 5% 5% 5% 6% 31%	2340
	11th to 20th	Day	19% 6% 2 5% 5% 3 4 4 6% 6% 39%	2449
	21st to end	Day	16% 5% 2 5% 5% 3 5% 5% 6% 9% 39%	2725
June	1st to 10th	Day	18% 6% 3 5% 6% 3 5% 6% 7% 8% 32%	2520
	11th to 20th	Day	19% 5% 2 5% 4 3 6% 5% 6% 9% 36%	2520
	21st to end	Day	17% 6% 3 6% 7% 4 6% 5% 5% 8% 33%	2520
July	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 5% 7% 8% 38%	2520
	11th to 20th	Day	12% 5% 2 5% 6% 4 6% 6% 6% 9% 39%	2520
	21st to end	Day	13% 6% 3 6% 6% 4 6% 6% 7% 8% 37%	2744
August	1st to 10th	Day	12% 6% 2 5% 6% 4 6% 5% 7% 10% 37%	2449
	11th to 20th	Day	13% 6% 2 6% 6% 4 6% 6% 7% 9% 35%	2175
	21st to end	Day	14% 5% 3 5% 5% 3 6% 5% 6% 8% 40%	2376
September	1st to 10th	Day	14% 5% 2 5% 6% 3 5% 5% 5% 7% 42%	2160
	11th to 20th	Day	16% 5% 2 3 4 3 3 4 4 8% 47%	2039
	21st to end	Day	18% 5% 2 4 4 3 4 5% 6% 8% 42%	1980
October	1st to 10th	Day	24% 7% 2 6% 5% 2 4 5% 5% 4 36%	1940
	11th to 20th	Day	15% 7% 2 4 5% 3 5% 5% 5% 7% 40%	1769
	21st to end	Day	22% 9% 3 5% 5% 3 3 4 5% 8% 33%	1782
November	1st to 10th	Day	12% 7% 3 6% 4 3 4 4 5% 7% 47%	1671
	11th to 20th	Day	17% 7% 3 4 5% 3 4 4 6% 7% 40%	1710
	21st to end	Day	18% 7% 3 5% 4 4 5% 4 5% 6% 39%	1630
December	1st to 10th	Day	18% 7% 5% 7% 8% 3 6% 4 5% 6% 31%	1520
	11th to 20th	Day	21% 6% 2 6% 5% 4 4 3 4 6% 38%	1906
	21st to end	Day	18% 11% 4 7% 6% 4 5% 5% 5% 7% 28%	2772
January	1st to 10th	Night	15% 8% 3 5% 5% 3 3 4 5% 7% 41%	4429
	11th to 20th	Night	13% 8% 3 9% 7% 4 6% 6% 6% 7% 31%	4440
	21st to end	Night	19% 12% 4 9% 6% 3 4 5% 5% 6% 27%	4870
February	1st to 10th	Night	14% 8% 3 6% 7% 3 5% 5% 6% 7% 36%	2436
	11th to 20th	Night	10% 8% 3 5% 6% 4 6% 5% 6% 8% 38%	2218
	21st to end	Night	16% 8% 3 6% 5% 4 4 5% 4 6% 38%	1884
March	1st to 10th	Night	14% 9% 2 5% 5% 4 5% 4 7% 8% 37%	2270
	11th to 20th	Night	15% 7% 2 5% 5% 3 6% 5% 6% 7% 38%	2225
	21st to end	Night	16% 7% 3 6% 5% 3 4 5% 5% 7% 40%	2090
April	1st to 10th	Night	16% 8% 3 5% 5% 3 5% 4 7% 8% 35%	1821
	11th to 20th	Night	18% 6% 2 5% 7% 3 4 4 5% 7% 38%	1775
	21st to end	Night	22% 8% 3 6% 4 2 3 4 4 5% 38%	1678
May	1st to 10th	Night	23% 8% 3 6% 5% 3 4 4 6% 6% 32%	1620
	11th to 20th	Night	18% 6% 3 5% 5% 3 5% 5% 6% 8% 37%	1460
	21st to end	Night	16% 6% 3 5% 5% 3 4 5% 5% 8% 40%	1550
June	1st to 10th	Night	18% 7% 3 5% 6% 3 3 4 5% 5% 41%	1440
	11th to 20th	Night	18% 6% 2 5% 4 3 5% 5% 6% 7% 39%	1440
	21st to end	Night	15% 5% 3 4 4 3 4 4 5% 8% 45%	1440
July	1st to 10th	Night	13% 4 2 4 6% 3 5% 4 5% 8% 45%	1440
	11th to 20th	Night	13% 6% 2 5% 5% 3 5% 5% 6% 10% 41%	1440
	21st to end	Night	10% 4 2 4 5% 3 4 5% 6% 9% 48%	1574
August	1st to 10th	Night	10% 5% 1 5% 5% 3 4 5% 4 8% 49%	1471
	11th to 20th	Night	13% 5% 3 4 6% 3 5% 5% 7% 9% 40%	1722
	21st to end	Night	16% 4 2 4 4 2 3 4 5% 8% 47%	1980
September	1st to 10th	Night	13% 6% 2 5% 5% 3 4 4 6% 7% 46%	1800
	11th to 20th	Night	14% 5% 2 4 4 2 3 3 5% 7% 52%	1920
	21st to end	Night	18% 5% 2 4 5% 3 3 4 4 7% 46%	1980
October	1st to 10th	Night	23% 5% 2 5% 5% 3 3 4 5% 6% 40%	2016
	11th to 20th	Night	15% 6% 2 5% 5% 3 4 5% 5% 8% 43%	2191
	21st to end	Night	22% 7% 2 5% 4 3 4 4 4 7% 37%	2574
November	1st to 10th	Night	14% 6% 3 5% 4 3 4 4 4 6% 47%	2407
	11th to 20th	Night	18% 6% 3 5% 5% 3 4 4 4 6% 42%	2470
	21st to end	Night	14% 7% 3 5% 4 3 5% 5% 6% 7% 42%	2546
December	1st to 10th	Night	17% 8% 3 5% 5% 3 4 5% 7% 7% 34%	2660
	11th to 20th	Night	18% 8% 2 5% 5% 4 5% 5% 6% 8% 35%	3288
	21st to end	Night	20% 10% 5% 7% 6% 4 4 4 5% 6% 29%	4884

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% 8% 18% 26% 7% 1	2518
	11th to 20th	Day	53% 8% 13% 18% 7% 1	2520
	21st to end	Day	58% 6% 10% 18% 7% 1	2782
February	1st to 10th	Day	51% 7% 14% 17% 10% 1	1743
	11th to 20th	Day	46% 8% 11% 20% 13% 2	1857
	21st to end	Day	51% 8% 10% 18% 12% 2	1570
March	1st to 10th	Day	47% 7% 14% 18% 12% 3	1900
	11th to 20th	Day	47% 8% 13% 17% 13% 3	1951
	21st to end	Day	47% 5% 9% 17% 16% 5%	2508
April	1st to 10th	Day	47% 7% 11% 20% 13% 3	2184
	11th to 20th	Day	43% 9% 11% 20% 15% 2	2134
	21st to end	Day	48% 9% 9% 18% 13% 3	2281
May	1st to 10th	Day	53% 7% 8% 18% 12% 2	2340
	11th to 20th	Day	43% 7% 10% 21% 17% 2	2453
	21st to end	Day	41% 9% 15% 21% 12% 2	2725
June	1st to 10th	Day	46% 9% 14% 19% 10% 1	2520
	11th to 20th	Day	43% 9% 14% 20% 13% 1	2520
	21st to end	Day	46% 9% 14% 20% 11% 1	2520
July	1st to 10th	Day	42% 8% 11% 22% 14% 2	2450
	11th to 20th	Day	38% 10% 17% 24% 11% 1	2445
	21st to end	Day	40% 10% 14% 20% 14% 1	2591
August	1st to 10th	Day	40% 10% 15% 23% 11% 1	2435
	11th to 20th	Day	40% 10% 16% 23% 9% 1	2175
	21st to end	Day	37% 9% 14% 24% 14% 2	2376
September	1st to 10th	Day	39% 9% 13% 24% 14% 1	2160
	11th to 20th	Day	36% 7% 11% 23% 21% 2	2039
	21st to end	Day	38% 7% 12% 24% 16% 3	1980
October	1st to 10th	Day	49% 7% 9% 19% 13% 3	1940
	11th to 20th	Day	39% 8% 16% 24% 11% 2	1769
	21st to end	Day	51% 6% 13% 16% 11% 3	1782
November	1st to 10th	Day	37% 8% 15% 24% 14% 3	1671
	11th to 20th	Day	42% 7% 14% 20% 16% 2	1710
	21st to end	Day	44% 7% 16% 22% 9% 2	1630
December	1st to 10th	Day	52% 9% 15% 12% 12% 1	1520
	11th to 20th	Day	45% 8% 15% 18% 13% 1	1906
	21st to end	Day	53% 9% 13% 15% 10% 1	2772
January	1st to 10th	Night	41% 8% 14% 24% 11% 2	4429
	11th to 20th	Night	48% 10% 15% 18% 8% 2	4440
	21st to end	Night	56% 8% 11% 16% 9% 1	4870
February	1st to 10th	Night	46% 8% 14% 20% 9% 2	2436
	11th to 20th	Night	42% 8% 14% 20% 14% 2	2220
	21st to end	Night	46% 7% 12% 22% 10% 3	1884
March	1st to 10th	Night	43% 7% 13% 23% 12% 2	2280
	11th to 20th	Night	43% 8% 12% 22% 12% 3	2226
	21st to end	Night	43% 7% 10% 17% 18% 5%	2090
April	1st to 10th	Night	45% 8% 11% 20% 14% 2	1821
	11th to 20th	Night	44% 8% 11% 20% 16% 1	1775
	21st to end	Night	46% 7% 11% 20% 13% 4	1678
May	1st to 10th	Night	50% 7% 11% 16% 12% 2	1620
	11th to 20th	Night	45% 7% 12% 19% 15% 2	1461
	21st to end	Night	42% 9% 14% 22% 12% 1	1551
June	1st to 10th	Night	44% 7% 14% 23% 11% 1	1440
	11th to 20th	Night	43% 9% 14% 21% 11% 1	1440
	21st to end	Night	38% 9% 15% 24% 13% 1	1440
July	1st to 10th	Night	36% 9% 15% 25% 14% 1	1400
	11th to 20th	Night	38% 10% 14% 28% 10%	1392
	21st to end	Night	32% 9% 17% 25% 15% 2	1486
August	1st to 10th	Night	33% 9% 17% 27% 13% 1	1461
	11th to 20th	Night	39% 9% 18% 24% 10%	1722
	21st to end	Night	35% 7% 13% 28% 16% 1	1980
September	1st to 10th	Night	37% 10% 11% 25% 17% 1	1800
	11th to 20th	Night	33% 6% 13% 24% 22% 2	1920
	21st to end	Night	38% 7% 11% 25% 18% 2	1980
October	1st to 10th	Night	45% 8% 11% 22% 11% 2	2016
	11th to 20th	Night	39% 8% 15% 24% 12% 1	2191
	21st to end	Night	48% 7% 13% 19% 11% 1	2574
November	1st to 10th	Night	38% 6% 13% 22% 18% 3	2407
	11th to 20th	Night	43% 7% 12% 20% 16% 3	2470
	21st to end	Night	40% 8% 17% 24% 10% 1	2546
December	1st to 10th	Night	47% 9% 15% 19% 10%	2660
	11th to 20th	Night	44% 10% 16% 16% 12% 2	3288
	21st to end	Night	54% 8% 11% 18% 8% 1	4884

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	29% 24% 31% 13% 4	180
	11th to 20th	Day	33% 23% 34% 7% 3 1	180
	21st to end	Day	23% 30% 32% 11% 4	203
February	1st to 10th	Day	31% 34% 27% 8% 1	303
	11th to 20th	Day	32% 27% 29% 11% 2	456
	21st to end	Day	32% 24% 28% 14% 1	382
March	1st to 10th	Day	29% 25% 29% 15% 2	460
	11th to 20th	Day	32% 25% 28% 12% 3 1	460
	21st to end	Day	31% 27% 26% 9% 4 2	506
April	1st to 10th	Day	33% 21% 27% 12% 6% 1	428
	11th to 20th	Day	29% 23% 29% 13% 5% 2	412
	21st to end	Day	31% 20% 28% 14% 5% 1	420
May	1st to 10th	Day	35% 20% 26% 12% 4 2	423
	11th to 20th	Day	21% 20% 35% 17% 5% 1	447
	21st to end	Day	21% 13% 31% 21% 12% 2	485
June	1st to 10th	Day	18% 17% 31% 20% 11% 2	450
	11th to 20th	Day	20% 15% 31% 23% 9% 2	450
	21st to end	Day	17% 19% 30% 20% 10% 3	450
July	1st to 10th	Day	18% 7% 32% 27% 14% 2	445
	11th to 20th	Day	15% 7% 32% 30% 14% 3	444
	21st to end	Day	14% 10% 37% 24% 13% 2	488
August	1st to 10th	Day	12% 9% 29% 30% 14% 5%	446
	11th to 20th	Day	15% 10% 29% 25% 14% 6%	439
	21st to end	Day	14% 12% 27% 27% 15% 6%	495
September	1st to 10th	Day	21% 9% 24% 23% 17% 6%	450
	11th to 20th	Day	18% 14% 24% 20% 14% 9%	450
	21st to end	Day	21% 12% 28% 22% 12% 6%	450
October	1st to 10th	Day	31% 12% 23% 18% 12% 4	450
	11th to 20th	Day	19% 18% 28% 25% 10% 1	435
	21st to end	Day	33% 14% 29% 17% 7% 1	396
November	1st to 10th	Day	23% 18% 25% 24% 8% 2	365
	11th to 20th	Day	29% 23% 24% 16% 6% 1	370
	21st to end	Day	29% 22% 25% 18% 5% 1	290
December	1st to 10th	Day	27% 29% 24% 16% 3	180
	11th to 20th	Day	38% 22% 31% 8% 2	180
	21st to end	Day	29% 26% 28% 12% 4 1	198
January	1st to 10th	Night	22% 22% 35% 18% 3	760
	11th to 20th	Night	25% 22% 31% 19% 2	760
	21st to end	Night	19% 24% 35% 19% 4	836
February	1st to 10th	Night	22% 21% 38% 18% 1	760
	11th to 20th	Night	23% 25% 35% 15% 1	743
	21st to end	Night	25% 24% 31% 18% 2	628
March	1st to 10th	Night	25% 23% 31% 18% 3	760
	11th to 20th	Night	23% 24% 30% 19% 3	760
	21st to end	Night	25% 20% 30% 20% 6%	836
April	1st to 10th	Night	26% 19% 33% 18% 4	729
	11th to 20th	Night	22% 21% 27% 22% 7%	711
	21st to end	Night	28% 18% 29% 20% 4	598
May	1st to 10th	Night	29% 12% 28% 23% 8%	540
	11th to 20th	Night	20% 14% 35% 25% 6%	399
	21st to end	Night	20% 8% 26% 32% 13%	389
June	1st to 10th	Night	19% 8% 29% 33% 11%	360
	11th to 20th	Night	18% 9% 26% 34% 11% 1	360
	21st to end	Night	16% 6% 27% 38% 13%	360
July	1st to 10th	Night	15% 5% 25% 37% 16% 1	350
	11th to 20th	Night	16% 3 21% 43% 16% 1	348
	21st to end	Night	10% 5% 24% 42% 18% 1	394
August	1st to 10th	Night	12% 3 26% 40% 18% 1	397
	11th to 20th	Night	15% 5% 23% 34% 20% 3	658
	21st to end	Night	17% 7% 21% 32% 21% 1	792
September	1st to 10th	Night	15% 6% 27% 32% 18% 2	720
	11th to 20th	Night	17% 8% 23% 28% 20% 4	720
	21st to end	Night	20% 7% 26% 29% 15% 2	720
October	1st to 10th	Night	26% 10% 23% 27% 12% 2	720
	11th to 20th	Night	20% 14% 28% 27% 11% 1	720
	21st to end	Night	29% 15% 29% 22% 5%	792
November	1st to 10th	Night	18% 17% 30% 26% 9% 1	740
	11th to 20th	Night	23% 21% 26% 21% 6% 1	760
	21st to end	Night	21% 20% 30% 24% 5% 1	760
December	1st to 10th	Night	24% 22% 32% 17% 4	760
	11th to 20th	Night	26% 20% 31% 20% 4	760
	21st to end	Night	26% 23% 30% 19% 2	836

Page generated at 2024-03-31 06:08:50. Version ac07

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