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 2024-01-22.

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	18% 9% 4 7% 6% 3 5% 4 5% 6% 33%	74809
	Night	19% 11% 4 7% 6% 3 4 4 4 6% 31%	75121
Transparency	Day	52% 6% 11% 19% 10% 1	74514
	Night	55% 6% 10% 18% 10% 1	74950
Seeing	Day	25% 16% 29% 20% 8% 2	13446
	Night	26% 14% 28% 23% 8% 1	22684

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	19% 18% 5% 11% 8% 3 4 4 3 4 20%	1517
	11th to 20th	Day	19% 18% 6% 8% 9% 4 5% 4 5% 3 21%	1520
	21st to end	Day	15% 13% 6% 11% 9% 5% 5% 5% 5% 6% 22%	1665
February	1st to 10th	Day	18% 15% 5% 12% 7% 4 4 4 5% 5% 21%	1620
	11th to 20th	Day	16% 15% 6% 8% 8% 3 4 5% 3 6% 26%	1604
	21st to end	Day	20% 16% 6% 9% 7% 4 4 5% 4 5% 21%	1480
March	1st to 10th	Day	16% 14% 5% 10% 7% 3 5% 4 4 4 29%	1797
	11th to 20th	Day	19% 12% 4 7% 7% 4 4 3 4 4 31%	1962
	21st to end	Day	21% 11% 4 6% 5% 2 4 3 3 6% 37%	2305
April	1st to 10th	Day	22% 11% 4 5% 5% 3 4 4 4 5% 33%	2160
	11th to 20th	Day	22% 10% 3 5% 6% 2 3 4 4 5% 37%	2134
	21st to end	Day	23% 10% 3 6% 5% 3 4 4 5% 5% 32%	2247
May	1st to 10th	Day	20% 7% 2 7% 5% 4 4 5% 5% 6% 36%	2520
	11th to 20th	Day	17% 6% 4 6% 5% 2 4 4 4 7% 40%	2478
	21st to end	Day	15% 6% 3 6% 5% 3 5% 5% 5% 9% 38%	2725
June	1st to 10th	Day	22% 6% 3 6% 5% 4 5% 5% 6% 7% 32%	2520
	11th to 20th	Day	16% 6% 2 5% 5% 3 5% 4 6% 8% 38%	2520
	21st to end	Day	20% 6% 3 6% 6% 3 5% 5% 6% 7% 34%	2520
July	1st to 10th	Day	13% 5% 4 5% 4 3 5% 5% 7% 8% 43%	2520
	11th to 20th	Day	13% 5% 3 5% 6% 3 5% 6% 8% 8% 38%	2520
	21st to end	Day	12% 5% 2 5% 5% 4 5% 7% 7% 10% 38%	2744
August	1st to 10th	Day	12% 6% 3 6% 6% 3 5% 6% 6% 8% 39%	2467
	11th to 20th	Day	13% 5% 2 6% 5% 4 6% 5% 6% 8% 40%	2249
	21st to end	Day	15% 5% 2 6% 5% 3 5% 4 6% 8% 40%	2376
September	1st to 10th	Day	15% 5% 2 5% 5% 4 5% 5% 6% 6% 42%	2160
	11th to 20th	Day	15% 5% 2 5% 5% 2 4 4 4 8% 46%	2148
	21st to end	Day	19% 6% 2 5% 4 3 5% 4 5% 8% 39%	1942
October	1st to 10th	Day	22% 7% 3 5% 6% 3 5% 4 5% 6% 34%	1800
	11th to 20th	Day	22% 10% 3 9% 5% 3 5% 5% 5% 6% 27%	1800
	21st to end	Day	28% 10% 3 6% 7% 3 4 4 5% 5% 26%	1832
November	1st to 10th	Day	17% 8% 3 7% 7% 3 5% 5% 5% 8% 32%	1671
	11th to 20th	Day	18% 12% 5% 7% 6% 5% 5% 4 4 6% 29%	1568
	21st to end	Day	22% 14% 4 9% 7% 3 5% 4 4 8% 20%	1520
December	1st to 10th	Day	25% 18% 5% 8% 6% 3 5% 4 4 6% 15%	1520
	11th to 20th	Day	22% 17% 6% 9% 5% 4 4 3 3 5% 22%	1906
	21st to end	Day	27% 17% 5% 10% 6% 3 4 3 2 2 20%	2772
January	1st to 10th	Night	20% 19% 7% 10% 7% 3 4 3 4 4 19%	2649
	11th to 20th	Night	18% 18% 6% 10% 8% 4 5% 4 4 4 19%	2660
	21st to end	Night	14% 15% 6% 10% 7% 4 4 4 4 5% 26%	2722
February	1st to 10th	Night	20% 15% 6% 12% 7% 4 5% 5% 4 5% 18%	2340
	11th to 20th	Night	16% 12% 6% 11% 7% 3 4 4 4 5% 27%	2241
	21st to end	Night	18% 14% 7% 9% 6% 3 4 4 3 5% 27%	1776
March	1st to 10th	Night	17% 16% 5% 8% 6% 3 4 3 3 4 29%	2150
	11th to 20th	Night	19% 14% 5% 6% 6% 2 3 4 4 5% 32%	1993
	21st to end	Night	21% 11% 3 7% 5% 2 4 3 4 4 37%	2050
April	1st to 10th	Night	22% 9% 5% 7% 5% 3 4 4 4 5% 33%	1800
	11th to 20th	Night	21% 9% 3 6% 5% 3 4 4 4 5% 36%	1775
	21st to end	Night	23% 10% 2 6% 5% 3 4 4 4 6% 33%	1729
May	1st to 10th	Night	23% 8% 3 4 5% 3 4 4 5% 7% 35%	1440
	11th to 20th	Night	15% 6% 3 6% 5% 3 5% 4 4 7% 41%	1415
	21st to end	Night	14% 4 2 5% 5% 3 4 4 7% 7% 43%	1550
June	1st to 10th	Night	19% 6% 3 6% 6% 3 5% 5% 6% 9% 33%	1440
	11th to 20th	Night	16% 6% 3 5% 5% 3 4 4 6% 9% 40%	1440
	21st to end	Night	18% 7% 2 5% 6% 4 4 5% 5% 9% 34%	1440
July	1st to 10th	Night	11% 4 2 5% 5% 3 6% 5% 6% 7% 47%	1440
	11th to 20th	Night	12% 5% 3 4 4 4 5% 6% 5% 9% 43%	1440
	21st to end	Night	11% 5% 2 6% 5% 4 4 5% 7% 9% 42%	1574
August	1st to 10th	Night	12% 5% 3 7% 5% 2 4 6% 6% 8% 41%	1467
	11th to 20th	Night	14% 7% 3 6% 6% 2 5% 5% 4 7% 42%	1633
	21st to end	Night	12% 6% 3 6% 6% 3 4 5% 5% 7% 44%	1980
September	1st to 10th	Night	15% 6% 3 5% 6% 3 4 3 5% 6% 45%	1800
	11th to 20th	Night	15% 5% 2 4 4 2 3 4 5% 8% 49%	1811
	21st to end	Night	16% 5% 3 5% 6% 3 4 5% 6% 6% 42%	2016
October	1st to 10th	Night	20% 8% 3 6% 6% 2 5% 4 5% 7% 35%	2160
	11th to 20th	Night	20% 8% 3 6% 5% 4 4 5% 5% 7% 32%	2160
	21st to end	Night	26% 9% 3 6% 7% 3 3 4 5% 6% 27%	2524
November	1st to 10th	Night	20% 10% 4 6% 5% 3 5% 3 4 6% 35%	2407
	11th to 20th	Night	18% 11% 4 7% 6% 3 4 4 4 6% 32%	2607
	21st to end	Night	23% 13% 5% 7% 6% 3 5% 4 5% 6% 23%	2660
December	1st to 10th	Night	23% 17% 6% 10% 7% 3 5% 4 4 4 16%	2660
	11th to 20th	Night	23% 16% 4 8% 6% 3 3 3 3 4 26%	3288
	21st to end	Night	31% 14% 5% 10% 5% 3 3 4 3 4 17%	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	69% 6% 8% 11% 4	1518
	11th to 20th	Day	68% 6% 7% 9% 9% 1	1520
	21st to end	Day	64% 6% 11% 11% 6% 1	1665
February	1st to 10th	Day	66% 6% 7% 16% 4 1	1620
	11th to 20th	Day	62% 6% 8% 15% 8% 1	1613
	21st to end	Day	66% 5% 9% 11% 8% 1	1480
March	1st to 10th	Day	61% 6% 9% 15% 9% 1	1800
	11th to 20th	Day	58% 4 10% 13% 12% 3	1962
	21st to end	Day	53% 3 8% 17% 14% 5%	2305
April	1st to 10th	Day	54% 4 6% 20% 12% 3	2160
	11th to 20th	Day	51% 4 8% 19% 16% 3	2134
	21st to end	Day	54% 5% 8% 15% 15% 3	2247
May	1st to 10th	Day	49% 6% 9% 18% 16% 2	2520
	11th to 20th	Day	45% 6% 9% 23% 15% 2	2482
	21st to end	Day	43% 8% 13% 21% 12% 2	2725
June	1st to 10th	Day	51% 7% 11% 20% 12% 1	2520
	11th to 20th	Day	44% 7% 10% 25% 12% 2	2520
	21st to end	Day	49% 8% 12% 19% 12% 1	2520
July	1st to 10th	Day	37% 7% 12% 29% 14% 1	2450
	11th to 20th	Day	41% 9% 14% 27% 10%	2445
	21st to end	Day	40% 10% 16% 24% 10%	2591
August	1st to 10th	Day	42% 8% 16% 24% 10%	2453
	11th to 20th	Day	41% 8% 15% 25% 10%	2249
	21st to end	Day	41% 7% 13% 27% 12%	2376
September	1st to 10th	Day	40% 8% 13% 25% 13%	2160
	11th to 20th	Day	38% 6% 11% 31% 13% 1	2148
	21st to end	Day	44% 7% 12% 27% 9%	1942
October	1st to 10th	Day	51% 6% 11% 21% 11% 1	1800
	11th to 20th	Day	56% 9% 13% 16% 6% 1	1800
	21st to end	Day	60% 6% 11% 15% 8% 1	1832
November	1st to 10th	Day	50% 8% 13% 22% 7% 1	1671
	11th to 20th	Day	57% 6% 12% 15% 9% 1	1568
	21st to end	Day	66% 7% 11% 11% 5%	1520
December	1st to 10th	Day	72% 7% 10% 9% 2	1520
	11th to 20th	Day	68% 5% 11% 13% 3	1906
	21st to end	Day	72% 5% 6% 13% 3	2772
January	1st to 10th	Night	70% 5% 10% 10% 5%	2649
	11th to 20th	Night	70% 7% 7% 9% 6% 1	2660
	21st to end	Night	63% 5% 8% 15% 8% 1	2722
February	1st to 10th	Night	69% 5% 8% 12% 5%	2340
	11th to 20th	Night	60% 6% 8% 19% 6%	2243
	21st to end	Night	62% 5% 7% 16% 9% 1	1776
March	1st to 10th	Night	61% 4 10% 15% 9% 1	2160
	11th to 20th	Night	57% 5% 9% 17% 11% 2	1994
	21st to end	Night	53% 4 6% 19% 14% 4	2050
April	1st to 10th	Night	54% 5% 8% 20% 12% 1	1800
	11th to 20th	Night	51% 5% 9% 18% 16% 1	1775
	21st to end	Night	54% 5% 8% 17% 13% 4	1729
May	1st to 10th	Night	49% 7% 8% 16% 16% 4	1440
	11th to 20th	Night	43% 7% 7% 23% 16% 3	1416
	21st to end	Night	39% 8% 14% 21% 15% 4	1551
June	1st to 10th	Night	49% 7% 13% 19% 11% 1	1440
	11th to 20th	Night	43% 6% 13% 21% 16% 2	1440
	21st to end	Night	47% 8% 13% 20% 12% 1	1440
July	1st to 10th	Night	35% 8% 13% 26% 17% 2	1400
	11th to 20th	Night	37% 8% 12% 30% 13%	1392
	21st to end	Night	37% 9% 14% 26% 14% 1	1486
August	1st to 10th	Night	39% 9% 16% 23% 12%	1457
	11th to 20th	Night	42% 7% 12% 24% 14% 1	1633
	21st to end	Night	39% 8% 13% 27% 12%	1980
September	1st to 10th	Night	41% 6% 12% 28% 13% 1	1800
	11th to 20th	Night	35% 7% 13% 31% 13% 1	1811
	21st to end	Night	42% 8% 10% 28% 11% 1	2016
October	1st to 10th	Night	49% 7% 10% 21% 12% 1	2160
	11th to 20th	Night	52% 6% 14% 19% 9%	2160
	21st to end	Night	59% 7% 12% 13% 8% 1	2524
November	1st to 10th	Night	51% 6% 13% 19% 10% 1	2407
	11th to 20th	Night	54% 7% 12% 17% 9% 2	2607
	21st to end	Night	62% 7% 12% 14% 5%	2660
December	1st to 10th	Night	72% 6% 8% 11% 2	2660
	11th to 20th	Night	65% 6% 9% 14% 6%	3288
	21st to end	Night	73% 6% 8% 12% 2	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% 22% 28% 14% 6%	180
	11th to 20th	Day	31% 19% 32% 13% 4 1	180
	21st to end	Day	27% 23% 28% 16% 5%	211
February	1st to 10th	Day	38% 30% 26% 6%	240
	11th to 20th	Day	31% 29% 31% 8%	271
	21st to end	Day	36% 20% 28% 14% 2	346
March	1st to 10th	Day	28% 18% 32% 17% 4	420
	11th to 20th	Day	31% 20% 32% 12% 3 1	420
	21st to end	Day	34% 19% 25% 14% 6% 2	462
April	1st to 10th	Day	33% 19% 29% 14% 5%	420
	11th to 20th	Day	33% 19% 22% 17% 6% 3	412
	21st to end	Day	34% 17% 27% 14% 6% 2	420
May	1st to 10th	Day	26% 19% 28% 17% 8% 1	423
	11th to 20th	Day	22% 17% 34% 18% 7% 2	447
	21st to end	Day	19% 16% 30% 24% 10% 1	485
June	1st to 10th	Day	29% 14% 28% 22% 7% 1	450
	11th to 20th	Day	18% 14% 35% 23% 8% 2	450
	21st to end	Day	25% 13% 32% 17% 11% 2	450
July	1st to 10th	Day	15% 10% 29% 32% 12% 2	445
	11th to 20th	Day	16% 13% 27% 28% 13% 3	444
	21st to end	Day	15% 9% 34% 31% 9% 2	488
August	1st to 10th	Day	16% 8% 24% 33% 15% 4	446
	11th to 20th	Day	15% 7% 27% 28% 16% 7%	439
	21st to end	Day	16% 11% 33% 22% 11% 7%	495
September	1st to 10th	Day	17% 10% 29% 24% 12% 8%	450
	11th to 20th	Day	16% 8% 24% 27% 15% 10%	450
	21st to end	Day	19% 14% 26% 23% 14% 3	450
October	1st to 10th	Day	27% 17% 23% 16% 13% 5%	450
	11th to 20th	Day	29% 18% 29% 18% 5% 1	450
	21st to end	Day	35% 13% 27% 20% 5%	421
November	1st to 10th	Day	26% 19% 28% 18% 8% 1	365
	11th to 20th	Day	29% 18% 32% 14% 6% 1	228
	21st to end	Day	31% 21% 34% 7% 7%	180
December	1st to 10th	Day	37% 19% 26% 13% 5% 1	180
	11th to 20th	Day	33% 28% 26% 12% 1	180
	21st to end	Day	39% 19% 26% 12% 3 1	198
January	1st to 10th	Night	32% 18% 27% 18% 4	760
	11th to 20th	Night	32% 21% 25% 18% 4	760
	21st to end	Night	23% 18% 33% 21% 5%	800
February	1st to 10th	Night	29% 21% 31% 17% 2	720
	11th to 20th	Night	27% 17% 36% 17% 3	703
	21st to end	Night	28% 19% 31% 18% 3	592
March	1st to 10th	Night	28% 17% 28% 23% 3	720
	11th to 20th	Night	30% 18% 31% 17% 4 1	720
	21st to end	Night	28% 14% 30% 21% 7%	792
April	1st to 10th	Night	28% 15% 31% 22% 3	720
	11th to 20th	Night	30% 16% 28% 18% 7%	711
	21st to end	Night	30% 12% 29% 21% 7% 1	649
May	1st to 10th	Night	29% 7% 28% 26% 9% 1	360
	11th to 20th	Night	19% 11% 35% 28% 6%	354
	21st to end	Night	17% 9% 36% 27% 10%	389
June	1st to 10th	Night	20% 10% 28% 34% 8%	360
	11th to 20th	Night	18% 9% 29% 30% 12% 1	360
	21st to end	Night	20% 8% 30% 29% 12% 1	360
July	1st to 10th	Night	11% 6% 29% 41% 12% 1	350
	11th to 20th	Night	14% 4 29% 37% 16% 1	348
	21st to end	Night	13% 5% 29% 41% 10% 1	394
August	1st to 10th	Night	16% 5% 23% 38% 17% 1	393
	11th to 20th	Night	16% 5% 21% 36% 20% 1	569
	21st to end	Night	14% 6% 31% 32% 16% 2	792
September	1st to 10th	Night	19% 7% 25% 31% 16% 1	720
	11th to 20th	Night	18% 7% 21% 32% 18% 4	720
	21st to end	Night	18% 12% 25% 26% 18% 2	720
October	1st to 10th	Night	24% 13% 29% 19% 14% 1	720
	11th to 20th	Night	26% 14% 28% 24% 8%	720
	21st to end	Night	33% 14% 27% 22% 5%	792
November	1st to 10th	Night	28% 14% 30% 21% 7% 1	740
	11th to 20th	Night	26% 17% 30% 17% 9% 1	760
	21st to end	Night	33% 19% 25% 18% 5%	760
December	1st to 10th	Night	36% 18% 24% 18% 3	760
	11th to 20th	Night	31% 20% 30% 15% 4 1	760
	21st to end	Night	36% 16% 26% 18% 3	836

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