Annual International Video Astronomy Star Party
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Annual International Video Astronomy Star Party clear sky chart from 2005-11-06 to 2026-05-17.

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	17% 11% 4 9% 5% 5% 4 3 5% 6% 31%	132159
	Night	18% 13% 4 9% 5% 4 4 3 5% 6% 29%	130642
Transparency	Day	53% 6% 11% 20% 9% 1	135624
	Night	56% 6% 10% 18% 9% 1	135264
Seeing	Day	25% 16% 29% 19% 8% 2	16407
	Night	26% 14% 28% 23% 8% 1	25599

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	15% 19% 6% 14% 7% 6% 4 3 4 5% 17%	4661
	11th to 20th	Day	11% 20% 8% 17% 8% 6% 4 3 4 4 16%	4650
	21st to end	Day	11% 16% 7% 15% 9% 9% 5% 3 4 4 18%	5379
February	1st to 10th	Day	16% 15% 5% 13% 6% 5% 4 4 5% 5% 22%	1890
	11th to 20th	Day	15% 15% 6% 11% 7% 5% 4 4 4 5% 25%	1879
	21st to end	Day	19% 17% 6% 11% 6% 5% 4 4 5% 5% 20%	1730
March	1st to 10th	Day	20% 14% 4 11% 5% 4 5% 3 4 5% 26%	4576
	11th to 20th	Day	11% 14% 4 9% 6% 6% 6% 3 5% 6% 30%	5043
	21st to end	Day	19% 10% 3 9% 5% 4 4 2 4 5% 36%	5929
April	1st to 10th	Day	21% 14% 3 6% 5% 4 4 3 4 5% 31%	2520
	11th to 20th	Day	22% 11% 2 7% 5% 3 3 3 4 5% 35%	2494
	21st to end	Day	23% 10% 3 7% 4 4 4 3 5% 5% 33%	2623
May	1st to 10th	Day	24% 11% 4 9% 5% 4 5% 3 4 6% 24%	6486
	11th to 20th	Day	18% 9% 4 9% 5% 5% 5% 4 5% 6% 31%	6402
	21st to end	Day	21% 9% 3 6% 4 4 4 4 5% 8% 33%	6949
June	1st to 10th	Day	21% 7% 3 7% 4 4 4 4 6% 7% 32%	2800
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 4 6% 8% 37%	2800
	21st to end	Day	20% 7% 3 7% 5% 5% 5% 4 6% 7% 33%	2800
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 4 7% 9% 35%	6360
	11th to 20th	Day	11% 6% 3 7% 5% 4 5% 4 7% 8% 41%	6360
	21st to end	Day	11% 5% 2 5% 4 4 4 4 7% 10% 43%	6926
August	1st to 10th	Day	16% 8% 3 7% 4 4 5% 4 6% 8% 36%	4480
	11th to 20th	Day	17% 6% 2 7% 4 4 5% 4 5% 8% 39%	4075
	21st to end	Day	17% 6% 2 6% 4 4 4 4 6% 9% 39%	4312
September	1st to 10th	Day	14% 5% 2 6% 4 5% 4 4 6% 7% 43%	2400
	11th to 20th	Day	14% 5% 2 5% 4 3 4 3 5% 7% 48%	2386
	21st to end	Day	19% 6% 2 6% 4 4 4 3 5% 8% 39%	2157
October	1st to 10th	Day	20% 8% 3 6% 5% 4 5% 3 4 7% 36%	2000
	11th to 20th	Day	20% 10% 3 9% 4 4 4 4 5% 7% 29%	2000
	21st to end	Day	26% 11% 3 7% 5% 4 5% 3 5% 5% 27%	2035
November	1st to 10th	Day	17% 10% 3 8% 6% 5% 4 4 6% 7% 30%	1851
	11th to 20th	Day	16% 13% 4 9% 5% 6% 5% 4 5% 5% 29%	1732
	21st to end	Day	21% 16% 4 10% 7% 4 5% 3 4 7% 20%	1680
December	1st to 10th	Day	21% 21% 5% 11% 5% 5% 4 3 4 5% 15%	2680
	11th to 20th	Day	19% 16% 6% 12% 5% 5% 4 3 3 5% 22%	3066
	21st to end	Day	21% 16% 4 13% 5% 5% 4 3 3 3 22%	4048
January	1st to 10th	Night	15% 22% 7% 13% 5% 5% 4 3 4 4 19%	8234
	11th to 20th	Night	13% 20% 8% 16% 7% 6% 5% 3 4 4 14%	8235
	21st to end	Night	14% 16% 6% 12% 8% 6% 6% 3 5% 5% 19%	8799
February	1st to 10th	Night	18% 15% 6% 13% 6% 5% 4 4 4 5% 20%	2730
	11th to 20th	Night	15% 15% 6% 12% 6% 5% 4 3 4 4 27%	2626
	21st to end	Night	17% 16% 5% 10% 6% 4 4 3 3 5% 26%	2076
March	1st to 10th	Night	21% 14% 4 9% 5% 5% 3 3 3 4 29%	5484
	11th to 20th	Night	16% 13% 4 7% 4 3 4 3 6% 8% 31%	5118
	21st to end	Night	19% 13% 4 9% 5% 4 4 3 4 5% 31%	5244
April	1st to 10th	Night	22% 11% 4 8% 4 4 4 3 4 5% 31%	2100
	11th to 20th	Night	22% 11% 3 6% 5% 4 4 3 4 5% 35%	2075
	21st to end	Night	22% 10% 2 7% 4 5% 3 3 4 6% 35%	2015
May	1st to 10th	Night	26% 11% 4 8% 5% 4 4 4 5% 7% 23%	3674
	11th to 20th	Night	17% 10% 3 8% 4 4 4 3 5% 8% 34%	3623
	21st to end	Night	20% 8% 3 7% 4 5% 4 4 7% 7% 32%	3926
June	1st to 10th	Night	18% 7% 3 7% 5% 5% 5% 4 6% 9% 32%	1600
	11th to 20th	Night	16% 6% 2 6% 5% 4 4 3 6% 9% 39%	1600
	21st to end	Night	19% 8% 2 6% 5% 5% 4 4 5% 8% 33%	1600
July	1st to 10th	Night	13% 6% 2 7% 5% 5% 5% 4 6% 9% 38%	3600
	11th to 20th	Night	14% 8% 3 5% 4 4 4 3 5% 7% 43%	3600
	21st to end	Night	10% 5% 2 7% 5% 4 5% 4 6% 9% 44%	3926
August	1st to 10th	Night	14% 7% 3 8% 4 4 4 4 5% 8% 39%	2665
	11th to 20th	Night	15% 8% 3 7% 4 4 4 3 4 7% 41%	2940
	21st to end	Night	10% 6% 3 7% 5% 4 5% 5% 6% 8% 41%	3586
September	1st to 10th	Night	14% 7% 2 6% 5% 4 3 3 5% 6% 46%	2000
	11th to 20th	Night	14% 5% 2 5% 4 3 3 3 5% 8% 50%	2013
	21st to end	Night	16% 6% 2 6% 5% 4 4 3 6% 6% 41%	2240
October	1st to 10th	Night	19% 8% 3 7% 4 3 4 3 5% 6% 37%	2400
	11th to 20th	Night	19% 9% 3 7% 4 5% 4 4 5% 7% 33%	2400
	21st to end	Night	25% 10% 3 7% 5% 4 4 3 5% 6% 28%	2805
November	1st to 10th	Night	20% 11% 3 7% 4 4 4 3 4 6% 34%	2667
	11th to 20th	Night	18% 12% 4 8% 5% 4 4 3 4 6% 32%	2883
	21st to end	Night	21% 15% 4 9% 5% 5% 4 3 5% 6% 22%	2940
December	1st to 10th	Night	20% 19% 6% 13% 7% 5% 4 3 4 4 14%	4720
	11th to 20th	Night	23% 16% 4 9% 6% 4 3 3 4 4 25%	5348
	21st to end	Night	26% 16% 4 12% 5% 4 3 3 5% 4 18%	7150

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	69% 7% 9% 12% 3	4662
	11th to 20th	Day	72% 6% 7% 7% 7% 1	4680
	21st to end	Day	71% 4 8% 8% 8% 1	5379
February	1st to 10th	Day	65% 6% 7% 17% 5% 1	1890
	11th to 20th	Day	63% 6% 8% 15% 7% 1	1888
	21st to end	Day	66% 5% 8% 11% 8% 1	1730
March	1st to 10th	Day	61% 4 9% 14% 9% 3	4579
	11th to 20th	Day	54% 6% 15% 14% 10% 1	5043
	21st to end	Day	53% 5% 6% 17% 17% 2	5929
April	1st to 10th	Day	56% 5% 6% 19% 12% 3	2520
	11th to 20th	Day	52% 4 7% 18% 16% 2	2494
	21st to end	Day	53% 4 8% 17% 15% 3	2623
May	1st to 10th	Day	61% 6% 8% 16% 8% 1	6497
	11th to 20th	Day	52% 8% 10% 19% 10% 1	6406
	21st to end	Day	50% 7% 11% 23% 9% 1	6949
June	1st to 10th	Day	50% 7% 10% 20% 12% 1	2800
	11th to 20th	Day	43% 7% 11% 25% 12% 2	2800
	21st to end	Day	49% 7% 11% 20% 12% 1	2800
July	1st to 10th	Day	43% 6% 13% 27% 10%	6290
	11th to 20th	Day	38% 7% 13% 28% 14%	6285
	21st to end	Day	35% 7% 13% 37% 8%	6728
August	1st to 10th	Day	46% 7% 15% 23% 9%	4466
	11th to 20th	Day	43% 6% 14% 28% 8%	4114
	21st to end	Day	41% 6% 16% 26% 10%	4312
September	1st to 10th	Day	39% 8% 13% 27% 12%	2400
	11th to 20th	Day	36% 6% 11% 32% 14% 1	2386
	21st to end	Day	44% 7% 12% 26% 11%	2157
October	1st to 10th	Day	46% 5% 11% 28% 10%	3260
	11th to 20th	Day	48% 7% 14% 23% 6% 1	3240
	21st to end	Day	48% 5% 16% 23% 8%	3260
November	1st to 10th	Day	52% 7% 12% 21% 7% 1	1851
	11th to 20th	Day	57% 6% 12% 14% 9% 1	1732
	21st to end	Day	67% 6% 11% 11% 5%	1680
December	1st to 10th	Day	73% 5% 10% 10% 1	2680
	11th to 20th	Day	68% 5% 8% 14% 6%	3066
	21st to end	Day	68% 4 8% 15% 4	4048
January	1st to 10th	Night	70% 6% 10% 10% 4	8234
	11th to 20th	Night	75% 5% 6% 9% 5%	8280
	21st to end	Night	68% 5% 6% 11% 9% 1	8799
February	1st to 10th	Night	67% 5% 8% 13% 6%	2730
	11th to 20th	Night	61% 6% 8% 19% 6%	2628
	21st to end	Night	62% 5% 7% 17% 8% 1	2076
March	1st to 10th	Night	60% 3 8% 16% 11% 3	5494
	11th to 20th	Night	51% 7% 17% 15% 8% 3	5119
	21st to end	Night	57% 5% 5% 20% 11% 2	5244
April	1st to 10th	Night	55% 5% 8% 19% 12% 1	2100
	11th to 20th	Night	53% 5% 8% 18% 15% 1	2075
	21st to end	Night	52% 5% 8% 18% 14% 3	2015
May	1st to 10th	Night	61% 7% 8% 15% 8% 2	3680
	11th to 20th	Night	48% 8% 11% 20% 12% 1	3624
	21st to end	Night	50% 7% 11% 19% 13% 1	3927
June	1st to 10th	Night	50% 7% 13% 18% 12% 1	1600
	11th to 20th	Night	43% 6% 13% 21% 16% 2	1600
	21st to end	Night	48% 7% 12% 19% 13% 1	1600
July	1st to 10th	Night	43% 5% 16% 22% 13% 1	3560
	11th to 20th	Night	39% 6% 13% 29% 12% 1	3552
	21st to end	Night	37% 7% 11% 31% 14%	3809
August	1st to 10th	Night	43% 7% 14% 25% 10%	2655
	11th to 20th	Night	44% 6% 13% 28% 9% 1	2967
	21st to end	Night	39% 7% 14% 27% 11% 1	3586
September	1st to 10th	Night	40% 5% 11% 29% 13% 1	2000
	11th to 20th	Night	33% 7% 13% 32% 14% 1	2013
	21st to end	Night	43% 7% 10% 27% 12% 1	2240
October	1st to 10th	Night	43% 6% 12% 28% 11%	3920
	11th to 20th	Night	44% 5% 15% 25% 10%	3899
	21st to end	Night	50% 5% 16% 20% 8%	4530
November	1st to 10th	Night	52% 6% 12% 20% 9% 1	2667
	11th to 20th	Night	54% 7% 11% 17% 9% 2	2883
	21st to end	Night	63% 7% 12% 14% 5%	2940
December	1st to 10th	Night	74% 6% 9% 9% 2	4720
	11th to 20th	Night	65% 5% 8% 13% 8%	5348
	21st to end	Night	69% 4 7% 11% 8%	7150

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% 22% 30% 14% 5%	220
	11th to 20th	Day	31% 21% 31% 12% 3 1	220
	21st to end	Day	26% 25% 28% 17% 5%	288
February	1st to 10th	Day	33% 25% 29% 11% 2	330
	11th to 20th	Day	30% 30% 30% 10% 1	366
	21st to end	Day	34% 21% 31% 12% 1	446
March	1st to 10th	Day	29% 21% 31% 15% 3	540
	11th to 20th	Day	33% 20% 31% 12% 3 1	540
	21st to end	Day	35% 20% 25% 13% 5% 1	594
April	1st to 10th	Day	35% 19% 29% 12% 4	540
	11th to 20th	Day	35% 20% 22% 15% 5% 3	532
	21st to end	Day	34% 18% 28% 13% 6% 2	540
May	1st to 10th	Day	28% 18% 28% 18% 7% 1	543
	11th to 20th	Day	23% 17% 33% 18% 8% 2	551
	21st to end	Day	21% 16% 29% 25% 9% 1	573
June	1st to 10th	Day	28% 15% 29% 21% 7% 1	530
	11th to 20th	Day	16% 16% 35% 22% 8% 2	530
	21st to end	Day	25% 15% 33% 16% 9% 2	530
July	1st to 10th	Day	15% 11% 31% 30% 11% 2	525
	11th to 20th	Day	15% 13% 29% 29% 12% 3	524
	21st to end	Day	15% 9% 33% 31% 10% 2	576
August	1st to 10th	Day	17% 7% 24% 34% 15% 3	526
	11th to 20th	Day	15% 7% 27% 28% 16% 6%	519
	21st to end	Day	16% 11% 32% 23% 11% 7%	583
September	1st to 10th	Day	18% 10% 29% 23% 13% 7%	530
	11th to 20th	Day	13% 7% 23% 27% 19% 11%	530
	21st to end	Day	20% 13% 27% 24% 14% 3	530
October	1st to 10th	Day	24% 16% 26% 17% 12% 5%	530
	11th to 20th	Day	28% 18% 29% 18% 5% 1	530
	21st to end	Day	34% 13% 28% 19% 5% 1	492
November	1st to 10th	Day	28% 20% 28% 17% 7% 1	425
	11th to 20th	Day	28% 18% 32% 14% 7% 1	272
	21st to end	Day	34% 21% 34% 6% 5%	220
December	1st to 10th	Day	37% 19% 27% 13% 5%	220
	11th to 20th	Day	33% 28% 27% 10% 2	220
	21st to end	Day	38% 17% 27% 13% 4 1	242
January	1st to 10th	Night	31% 19% 29% 18% 3	840
	11th to 20th	Night	33% 20% 24% 18% 4	840
	21st to end	Night	23% 18% 33% 20% 6%	924
February	1st to 10th	Night	28% 20% 30% 20% 2	840
	11th to 20th	Night	27% 18% 36% 16% 3	823
	21st to end	Night	28% 19% 32% 18% 3	692
March	1st to 10th	Night	30% 17% 29% 21% 3	840
	11th to 20th	Night	30% 19% 30% 17% 3	840
	21st to end	Night	28% 15% 31% 19% 6%	924
April	1st to 10th	Night	30% 16% 30% 22% 3	840
	11th to 20th	Night	32% 17% 26% 18% 6%	831
	21st to end	Night	30% 12% 29% 21% 6% 1	755
May	1st to 10th	Night	29% 9% 28% 24% 8% 1	420
	11th to 20th	Night	20% 11% 34% 28% 7%	406
	21st to end	Night	18% 9% 36% 28% 9%	433
June	1st to 10th	Night	20% 10% 28% 35% 7%	400
	11th to 20th	Night	18% 9% 30% 31% 12% 1	400
	21st to end	Night	22% 8% 30% 29% 11% 1	400
July	1st to 10th	Night	12% 7% 30% 40% 11% 1	390
	11th to 20th	Night	14% 5% 28% 38% 14% 1	388
	21st to end	Night	14% 6% 28% 42% 10% 1	438
August	1st to 10th	Night	15% 5% 23% 40% 17%	438
	11th to 20th	Night	16% 5% 21% 36% 20% 1	633
	21st to end	Night	14% 6% 31% 33% 15% 1	880
September	1st to 10th	Night	19% 7% 27% 31% 16% 1	800
	11th to 20th	Night	16% 7% 20% 32% 21% 5%	800
	21st to end	Night	18% 11% 24% 28% 18% 1	800
October	1st to 10th	Night	23% 13% 29% 20% 15% 1	800
	11th to 20th	Night	25% 14% 29% 25% 8%	800
	21st to end	Night	32% 14% 26% 22% 6%	880
November	1st to 10th	Night	30% 14% 29% 20% 6% 1	820
	11th to 20th	Night	26% 17% 30% 17% 9% 1	840
	21st to end	Night	33% 19% 25% 18% 5%	840
December	1st to 10th	Night	36% 18% 25% 18% 3	840
	11th to 20th	Night	31% 20% 30% 15% 3 1	840
	21st to end	Night	36% 16% 26% 18% 3	924

Page generated at 2026-05-14 01:58:55. Version ac07

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