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 2024-11-04.

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% 8% 3 7% 5% 5% 5% 4 6% 7% 35%	101275
	Night	17% 8% 3 6% 4 4 4 4 5% 7% 38%	97731
Transparency	Day	44% 8% 13% 21% 13% 2	104799
	Night	43% 8% 13% 22% 13% 2	102295
Seeing	Day	24% 18% 29% 19% 8% 2	14910
	Night	22% 16% 29% 24% 8% 1	23899

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% 8% 3 7% 6% 5% 5% 4 6% 7% 36%	2517
	11th to 20th	Day	15% 9% 5% 11% 6% 5% 5% 4 5% 6% 28%	2520
	21st to end	Day	18% 14% 6% 9% 4 4 4 3 5% 5% 27%	2782
February	1st to 10th	Day	13% 9% 4 8% 7% 5% 6% 4 5% 7% 31%	1743
	11th to 20th	Day	11% 7% 2 10% 6% 5% 5% 5% 6% 7% 36%	1848
	21st to end	Day	18% 10% 3 8% 5% 4 5% 3 5% 5% 33%	1570
March	1st to 10th	Day	22% 14% 3 7% 4 4 3 2 4 5% 30%	3116
	11th to 20th	Day	12% 8% 3 8% 6% 7% 5% 5% 7% 7% 34%	3250
	21st to end	Day	19% 10% 3 7% 4 4 3 3 5% 6% 36%	4166
April	1st to 10th	Day	17% 10% 3 8% 5% 4 4 3 5% 6% 35%	2280
	11th to 20th	Day	17% 7% 2 7% 5% 4 5% 4 5% 6% 37%	2254
	21st to end	Day	19% 7% 3 7% 4 5% 5% 4 6% 6% 33%	2408
May	1st to 10th	Day	25% 9% 3 8% 4 4 4 4 4 6% 28%	4106
	11th to 20th	Day	22% 9% 2 7% 4 4 3 3 6% 7% 32%	4355
	21st to end	Day	13% 5% 2 5% 4 4 5% 4 8% 9% 40%	4837
June	1st to 10th	Day	17% 6% 2 6% 6% 5% 5% 5% 7% 8% 32%	2660
	11th to 20th	Day	18% 6% 2 5% 4 5% 6% 4 7% 9% 35%	2660
	21st to end	Day	16% 7% 2 7% 6% 5% 6% 4 6% 8% 33%	2660
July	1st to 10th	Day	11% 6% 2 8% 5% 6% 6% 5% 7% 9% 35%	4440
	11th to 20th	Day	11% 5% 2 6% 5% 5% 6% 5% 7% 11% 37%	4440
	21st to end	Day	17% 7% 3 7% 5% 5% 5% 4 6% 7% 34%	4814
August	1st to 10th	Day	17% 6% 2 7% 6% 5% 5% 4 6% 9% 33%	4311
	11th to 20th	Day	15% 6% 2 7% 5% 5% 5% 5% 6% 9% 35%	3805
	21st to end	Day	11% 4 2 6% 4 5% 6% 5% 7% 9% 39%	4180
September	1st to 10th	Day	13% 5% 2 6% 5% 4 5% 4 5% 7% 43%	2280
	11th to 20th	Day	15% 5% 2 5% 4 4 3 3 4 8% 48%	2152
	21st to end	Day	18% 5% 2 6% 3 4 4 4 6% 7% 40%	2090
October	1st to 10th	Day	23% 7% 2 6% 5% 4 4 4 5% 5% 36%	2047
	11th to 20th	Day	14% 7% 2 5% 4 4 5% 4 5% 7% 41%	1867
	21st to end	Day	21% 10% 3 6% 4 4 3 3 5% 8% 34%	1881
November	1st to 10th	Day	12% 7% 3 6% 4 4 4 3 5% 6% 47%	1698
	11th to 20th	Day	17% 7% 2 5% 4 4 4 3 7% 7% 40%	1710
	21st to end	Day	18% 8% 2 5% 4 5% 5% 3 5% 6% 38%	1630
December	1st to 10th	Day	17% 7% 4 9% 8% 5% 5% 3 5% 6% 31%	1520
	11th to 20th	Day	21% 7% 2 6% 5% 4 4 3 4 6% 38%	1906
	21st to end	Day	18% 11% 4 8% 5% 5% 5% 4 5% 7% 27%	2772
January	1st to 10th	Night	15% 8% 3 6% 4 4 4 3 6% 7% 41%	4429
	11th to 20th	Night	13% 8% 3 11% 6% 6% 6% 4 6% 7% 30%	4440
	21st to end	Night	18% 13% 4 10% 5% 5% 4 4 5% 6% 27%	4870
February	1st to 10th	Night	13% 9% 3 7% 6% 4 5% 5% 6% 7% 36%	2436
	11th to 20th	Night	10% 9% 3 6% 6% 6% 6% 4 6% 7% 37%	2218
	21st to end	Night	16% 9% 3 7% 5% 4 4 4 4 6% 37%	1884
March	1st to 10th	Night	22% 12% 2 6% 4 4 4 3 6% 6% 29%	3724
	11th to 20th	Night	13% 8% 2 6% 5% 5% 5% 4 6% 7% 38%	3693
	21st to end	Night	19% 9% 3 7% 4 4 4 3 5% 6% 36%	3466
April	1st to 10th	Night	16% 9% 3 6% 5% 4 4 3 7% 8% 35%	1900
	11th to 20th	Night	19% 7% 2 6% 6% 4 4 4 5% 7% 38%	1875
	21st to end	Night	21% 9% 2 6% 3 3 3 3 5% 5% 39%	1771
May	1st to 10th	Night	26% 10% 2 6% 3 4 4 3 5% 6% 29%	2834
	11th to 20th	Night	19% 8% 4 8% 6% 5% 5% 4 6% 7% 29%	2568
	21st to end	Night	15% 6% 2 6% 4 4 4 4 6% 10% 40%	2738
June	1st to 10th	Night	17% 8% 2 6% 5% 4 3 3 6% 5% 41%	1520
	11th to 20th	Night	17% 6% 2 5% 4 4 4 4 6% 7% 39%	1520
	21st to end	Night	15% 7% 2 5% 4 4 3 3 5% 8% 43%	1520
July	1st to 10th	Night	11% 5% 2 6% 6% 5% 5% 4 6% 8% 41%	2520
	11th to 20th	Night	15% 6% 2 5% 4 4 4 4 6% 8% 43%	2520
	21st to end	Night	16% 7% 3 6% 4 4 4 3 5% 7% 41%	2739
August	1st to 10th	Night	22% 5% 2 7% 4 4 4 4 5% 6% 37%	2587
	11th to 20th	Night	15% 7% 3 6% 4 4 4 4 5% 7% 40%	3006
	21st to end	Night	10% 3 1 4 3 3 3 3 6% 9% 55%	3476
September	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 6% 7% 46%	1900
	11th to 20th	Night	13% 4 2 5% 3 3 3 3 5% 7% 51%	2027
	21st to end	Night	18% 5% 2 4 4 3 4 4 4 7% 44%	2090
October	1st to 10th	Night	22% 6% 2 6% 4 3 3 3 5% 6% 40%	2129
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 5% 7% 44%	2313
	21st to end	Night	22% 8% 2 6% 4 4 4 3 5% 6% 38%	2717
November	1st to 10th	Night	13% 6% 3 5% 4 3 4 3 4 6% 47%	2453
	11th to 20th	Night	18% 6% 3 6% 4 4 4 3 5% 6% 42%	2470
	21st to end	Night	13% 8% 2 6% 4 4 5% 4 6% 7% 42%	2546
December	1st to 10th	Night	17% 9% 3 6% 5% 5% 4 4 7% 7% 34%	2660
	11th to 20th	Night	18% 8% 2 5% 5% 5% 5% 4 6% 7% 34%	3288
	21st to end	Night	20% 10% 5% 8% 5% 5% 4 3 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	57% 5% 12% 13% 11% 2	3119
	11th to 20th	Day	42% 11% 16% 20% 10% 2	3250
	21st to end	Day	48% 6% 7% 18% 18% 4	4166
April	1st to 10th	Day	48% 7% 11% 20% 12% 3	2280
	11th to 20th	Day	44% 9% 11% 19% 15% 2	2254
	21st to end	Day	47% 9% 9% 18% 14% 3	2408
May	1st to 10th	Day	54% 10% 10% 14% 11% 1	4120
	11th to 20th	Day	49% 8% 15% 18% 9% 1	4359
	21st to end	Day	35% 9% 15% 24% 15% 1	4837
June	1st to 10th	Day	45% 9% 14% 20% 11% 1	2660
	11th to 20th	Day	43% 9% 14% 20% 13% 1	2660
	21st to end	Day	46% 9% 14% 19% 12% 1	2660
July	1st to 10th	Day	43% 9% 12% 22% 15% 1	4370
	11th to 20th	Day	38% 9% 17% 24% 12% 1	4365
	21st to end	Day	46% 8% 13% 18% 14% 1	4661
August	1st to 10th	Day	46% 9% 15% 21% 9% 1	4297
	11th to 20th	Day	41% 10% 15% 26% 8% 1	3843
	21st to end	Day	35% 9% 15% 28% 13% 1	4180
September	1st to 10th	Day	38% 9% 13% 25% 15% 1	2280
	11th to 20th	Day	35% 7% 11% 24% 20% 2	2152
	21st to end	Day	40% 7% 12% 23% 15% 2	2090
October	1st to 10th	Day	43% 7% 7% 21% 20% 2	3398
	11th to 20th	Day	34% 5% 11% 27% 21% 3	3083
	21st to end	Day	35% 6% 13% 28% 14% 4	3081
November	1st to 10th	Day	37% 8% 15% 24% 14% 2	1698
	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	52% 8% 11% 18% 10% 1	3734
	11th to 20th	Night	40% 11% 14% 23% 10% 2	3694
	21st to end	Night	46% 8% 8% 15% 19% 4	3466
April	1st to 10th	Night	46% 8% 11% 20% 14% 1	1900
	11th to 20th	Night	45% 8% 10% 19% 16% 1	1875
	21st to end	Night	45% 7% 10% 20% 14% 4	1771
May	1st to 10th	Night	53% 9% 12% 15% 10% 1	2840
	11th to 20th	Night	51% 12% 12% 15% 9% 1	2569
	21st to end	Night	38% 10% 15% 22% 14% 1	2739
June	1st to 10th	Night	43% 7% 14% 24% 11% 1	1520
	11th to 20th	Night	42% 9% 15% 22% 11% 1	1520
	21st to end	Night	39% 9% 14% 24% 13% 1	1520
July	1st to 10th	Night	38% 9% 15% 21% 16% 1	2480
	11th to 20th	Night	38% 8% 13% 26% 14%	2472
	21st to end	Night	42% 7% 12% 21% 16% 1	2651
August	1st to 10th	Night	47% 8% 14% 22% 9% 1	2577
	11th to 20th	Night	41% 7% 17% 25% 9%	3033
	21st to end	Night	26% 7% 14% 37% 16% 1	3476
September	1st to 10th	Night	36% 10% 11% 25% 17% 1	1900
	11th to 20th	Night	32% 6% 12% 25% 22% 2	2027
	21st to end	Night	40% 7% 11% 24% 17% 2	2090
October	1st to 10th	Night	40% 7% 11% 23% 17% 1	3558
	11th to 20th	Night	29% 5% 13% 25% 23% 5%	3836
	21st to end	Night	35% 6% 14% 31% 14% 1	4467
November	1st to 10th	Night	38% 6% 13% 23% 18% 3	2453
	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	32% 26% 26% 9% 4 2	506
April	1st to 10th	Day	33% 22% 26% 12% 6% 1	460
	11th to 20th	Day	31% 23% 28% 12% 5% 2	452
	21st to end	Day	29% 22% 29% 14% 5% 1	460
May	1st to 10th	Day	33% 21% 27% 13% 4 2	463
	11th to 20th	Day	21% 19% 35% 18% 5% 1	487
	21st to end	Day	20% 14% 31% 22% 12% 2	529
June	1st to 10th	Day	18% 18% 31% 21% 10% 2	490
	11th to 20th	Day	19% 14% 31% 24% 10% 2	490
	21st to end	Day	17% 21% 31% 19% 9% 3	490
July	1st to 10th	Day	17% 7% 33% 27% 14% 2	485
	11th to 20th	Day	14% 8% 32% 30% 13% 2	484
	21st to end	Day	15% 10% 37% 23% 12% 2	532
August	1st to 10th	Day	13% 9% 29% 30% 14% 5%	486
	11th to 20th	Day	16% 11% 29% 25% 13% 6%	479
	21st to end	Day	13% 12% 28% 28% 15% 6%	539
September	1st to 10th	Day	20% 10% 25% 22% 17% 6%	490
	11th to 20th	Day	17% 13% 23% 21% 16% 10%	490
	21st to end	Day	21% 12% 27% 22% 12% 6%	490
October	1st to 10th	Day	29% 12% 23% 19% 12% 4	490
	11th to 20th	Day	21% 18% 26% 24% 9% 2	473
	21st to end	Day	31% 15% 29% 17% 7% 1	429
November	1st to 10th	Day	23% 19% 25% 24% 7% 2	374
	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% 34% 18% 4	760
	11th to 20th	Night	23% 22% 27% 21% 7%	751
	21st to end	Night	27% 19% 28% 20% 4	631
May	1st to 10th	Night	28% 12% 28% 23% 7%	570
	11th to 20th	Night	19% 14% 35% 26% 6%	421
	21st to end	Night	20% 8% 27% 32% 13%	411
June	1st to 10th	Night	19% 8% 30% 33% 11%	380
	11th to 20th	Night	17% 9% 27% 34% 12% 1	380
	21st to end	Night	17% 7% 27% 36% 13%	380
July	1st to 10th	Night	15% 5% 25% 38% 15% 1	370
	11th to 20th	Night	15% 4 21% 42% 15% 1	368
	21st to end	Night	12% 5% 24% 42% 17%	416
August	1st to 10th	Night	14% 3 26% 39% 17% 1	420
	11th to 20th	Night	15% 5% 23% 34% 20% 2	695
	21st to end	Night	17% 7% 21% 33% 21% 1	836
September	1st to 10th	Night	15% 7% 27% 31% 19% 2	760
	11th to 20th	Night	16% 7% 22% 29% 21% 5%	760
	21st to end	Night	21% 8% 26% 29% 15% 2	760
October	1st to 10th	Night	26% 10% 23% 27% 13% 2	760
	11th to 20th	Night	20% 14% 28% 26% 11% 1	760
	21st to end	Night	28% 15% 29% 22% 6%	836
November	1st to 10th	Night	17% 17% 30% 25% 9% 1	755
	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 2024-11-01 22:47:37. Version ac07

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