Ivan's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ivan's Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	7% 5% 2 7% 5% 6% 6% 6% 8% 11% 36%	131163
	Night	7% 5% 2 6% 5% 5% 5% 5% 7% 10% 42%	131011
Transparency	Day	21% 42% 14% 15% 6% 2	134816
	Night	22% 40% 14% 15% 7% 2	135428
Seeing	Day	7% 3 15% 29% 29% 17%	17992
	Night	8% 3 15% 30% 31% 13%	28167

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	6% 4 2 5% 3 4 4 4 6% 10% 54%	5258
	11th to 20th	Day	15% 7% 3 9% 6% 6% 6% 5% 8% 10% 27%	5280
	21st to end	Day	11% 7% 2 8% 5% 5% 5% 5% 6% 7% 39%	5817
February	1st to 10th	Day	9% 6% 3 6% 5% 5% 4 4 8% 11% 40%	2100
	11th to 20th	Day	7% 5% 2 5% 4 4 4 4 7% 10% 48%	2183
	21st to end	Day	6% 5% 2 7% 5% 5% 5% 5% 7% 8% 45%	1903
March	1st to 10th	Day	6% 4 2 7% 5% 6% 7% 6% 8% 10% 40%	5045
	11th to 20th	Day	4 2 1 4 3 3 4 4 7% 9% 58%	5090
	21st to end	Day	7% 4 2 6% 6% 6% 6% 4 8% 9% 42%	5586
April	1st to 10th	Day	5% 4 2 5% 4 4 5% 5% 8% 13% 44%	2310
	11th to 20th	Day	6% 5% 2 6% 5% 4 6% 5% 8% 10% 42%	2352
	21st to end	Day	6% 4 2 5% 5% 6% 5% 6% 7% 11% 43%	2400
May	1st to 10th	Day	2 2 1 4 3 4 5% 5% 8% 12% 55%	5405
	11th to 20th	Day	6% 5% 2 7% 5% 5% 5% 5% 6% 10% 45%	5444
	21st to end	Day	5% 4 1 5% 5% 6% 7% 8% 9% 13% 36%	6433
June	1st to 10th	Day	13% 9% 4 10% 8% 8% 9% 6% 8% 9% 16%	2600
	11th to 20th	Day	8% 7% 3 10% 8% 8% 8% 7% 9% 11% 21%	2600
	21st to end	Day	7% 7% 3 9% 7% 8% 8% 7% 11% 12% 22%	2600
July	1st to 10th	Day	5% 7% 3 9% 6% 6% 7% 6% 10% 14% 26%	5900
	11th to 20th	Day	5% 5% 3 8% 6% 7% 8% 7% 10% 13% 28%	5900
	21st to end	Day	3 3 2 5% 5% 5% 7% 6% 11% 16% 37%	6424
August	1st to 10th	Day	5% 5% 3 10% 8% 8% 8% 8% 11% 13% 21%	4141
	11th to 20th	Day	4 5% 2 7% 6% 7% 8% 7% 11% 16% 27%	3850
	21st to end	Day	5% 4 2 8% 6% 8% 9% 7% 12% 18% 21%	4063
September	1st to 10th	Day	8% 6% 3 9% 7% 9% 9% 8% 9% 11% 20%	2200
	11th to 20th	Day	7% 7% 3 9% 8% 10% 10% 7% 10% 11% 19%	2200
	21st to end	Day	11% 9% 4 10% 8% 7% 7% 7% 8% 9% 19%	2200
October	1st to 10th	Day	9% 8% 3 9% 7% 8% 7% 6% 9% 11% 22%	2090
	11th to 20th	Day	9% 6% 2 8% 7% 7% 8% 6% 8% 11% 28%	2000
	21st to end	Day	11% 6% 3 8% 5% 6% 5% 5% 7% 10% 35%	2200
November	1st to 10th	Day	9% 5% 2 6% 5% 6% 6% 5% 10% 9% 37%	2057
	11th to 20th	Day	10% 7% 3 8% 6% 6% 5% 6% 7% 8% 33%	2100
	21st to end	Day	8% 5% 2 8% 5% 6% 6% 5% 7% 9% 38%	2100
December	1st to 10th	Day	6% 4 2 6% 4 4 5% 5% 7% 8% 50%	3304
	11th to 20th	Day	8% 7% 2 6% 5% 6% 6% 5% 8% 11% 36%	3463
	21st to end	Day	12% 6% 3 9% 6% 7% 6% 6% 8% 10% 29%	4565
January	1st to 10th	Night	5% 4 2 6% 4 4 5% 4 7% 8% 51%	7635
	11th to 20th	Night	14% 8% 3 9% 5% 5% 5% 4 7% 8% 30%	7680
	21st to end	Night	12% 7% 3 7% 5% 4 5% 4 5% 6% 43%	8359
February	1st to 10th	Night	8% 6% 2 5% 4 4 4 4 7% 9% 48%	2520
	11th to 20th	Night	7% 3 2 5% 4 4 4 4 6% 7% 55%	2320
	21st to end	Night	6% 4 2 4 4 3 4 4 6% 8% 56%	1903
March	1st to 10th	Night	5% 5% 2 6% 4 4 5% 4 7% 9% 50%	5028
	11th to 20th	Night	5% 3 1 4 3 3 4 3 5% 8% 60%	5090
	21st to end	Night	7% 4 1 5% 5% 4 5% 4 7% 8% 49%	5588
April	1st to 10th	Night	4 3 1 4 4 3 4 4 6% 10% 55%	2310
	11th to 20th	Night	5% 3 2 5% 4 5% 5% 5% 7% 10% 50%	2126
	21st to end	Night	5% 5% 2 5% 4 3 4 5% 8% 9% 51%	2000
May	1st to 10th	Night	2 2 1 3 2 3 4 4 7% 12% 60%	4490
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 7% 9% 51%	4490
	21st to end	Night	4 5% 2 5% 5% 5% 6% 6% 9% 11% 42%	4421
June	1st to 10th	Night	12% 7% 3 8% 8% 6% 7% 6% 9% 9% 26%	1800
	11th to 20th	Night	7% 6% 2 7% 6% 6% 6% 6% 9% 14% 33%	1800
	21st to end	Night	5% 5% 2 7% 6% 7% 7% 6% 10% 14% 32%	1800
July	1st to 10th	Night	4 6% 3 9% 6% 6% 6% 5% 8% 11% 36%	4058
	11th to 20th	Night	4 5% 2 8% 5% 5% 6% 5% 9% 12% 38%	4057
	21st to end	Night	2 3 2 5% 4 5% 5% 4 8% 13% 50%	4427
August	1st to 10th	Night	5% 4 2 6% 5% 7% 8% 7% 9% 13% 34%	2975
	11th to 20th	Night	2 3 2 6% 5% 6% 6% 6% 9% 13% 42%	3197
	21st to end	Night	4 4 2 7% 6% 7% 8% 7% 13% 17% 24%	3814
September	1st to 10th	Night	6% 7% 3 8% 6% 7% 8% 7% 10% 11% 26%	2200
	11th to 20th	Night	9% 6% 2 9% 7% 7% 6% 6% 9% 12% 27%	2200
	21st to end	Night	8% 7% 3 9% 7% 8% 7% 7% 9% 11% 23%	2200
October	1st to 10th	Night	8% 5% 3 10% 6% 8% 8% 6% 9% 10% 26%	2307
	11th to 20th	Night	10% 6% 2 6% 5% 5% 6% 5% 8% 11% 36%	2400
	21st to end	Night	11% 6% 2 6% 4 5% 5% 5% 7% 8% 41%	2640
November	1st to 10th	Night	9% 6% 2 6% 4 5% 5% 4 6% 9% 43%	2461
	11th to 20th	Night	9% 6% 3 6% 6% 5% 5% 4 7% 9% 40%	2520
	21st to end	Night	8% 6% 2 6% 5% 4 6% 4 6% 9% 43%	2520
December	1st to 10th	Night	7% 5% 2 5% 4 4 4 3 5% 9% 52%	4090
	11th to 20th	Night	12% 8% 3 8% 5% 5% 6% 4 7% 8% 35%	4952
	21st to end	Night	10% 8% 3 10% 6% 6% 6% 5% 7% 8% 30%	6633

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	19% 24% 8% 30% 15% 4	5259
	11th to 20th	Day	41% 26% 12% 15% 5% 2	5280
	21st to end	Day	34% 25% 6% 24% 9% 2	5817
February	1st to 10th	Day	12% 61% 8% 10% 6% 3	2100
	11th to 20th	Day	10% 58% 5% 14% 10% 2	2193
	21st to end	Day	14% 61% 6% 8% 8% 3	1903
March	1st to 10th	Day	25% 35% 10% 15% 14% 2	5045
	11th to 20th	Day	9% 32% 12% 23% 15% 9%	5090
	21st to end	Day	24% 32% 9% 21% 10% 3	5586
April	1st to 10th	Day	9% 61% 9% 12% 8% 1	2310
	11th to 20th	Day	10% 63% 7% 7% 9% 4	2352
	21st to end	Day	9% 64% 5% 12% 10% 1	2400
May	1st to 10th	Day	11% 33% 14% 27% 12% 2	5437
	11th to 20th	Day	19% 33% 16% 26% 5% 1	5448
	21st to end	Day	15% 35% 19% 24% 6%	6447
June	1st to 10th	Day	19% 68% 7% 5% 1	2600
	11th to 20th	Day	18% 68% 7% 6% 1	2600
	21st to end	Day	11% 70% 14% 5%	2600
July	1st to 10th	Day	28% 34% 27% 11%	5835
	11th to 20th	Day	27% 35% 27% 10%	5831
	21st to end	Day	17% 34% 28% 20% 1	6240
August	1st to 10th	Day	30% 45% 18% 7% 1	4128
	11th to 20th	Day	16% 45% 23% 15% 1	3886
	21st to end	Day	21% 46% 29% 4	4063
September	1st to 10th	Day	17% 66% 11% 5%	2200
	11th to 20th	Day	18% 64% 12% 5% 1	2200
	21st to end	Day	19% 65% 8% 6% 2	2200
October	1st to 10th	Day	30% 44% 13% 10% 3	3402
	11th to 20th	Day	29% 40% 11% 10% 8% 1	3240
	21st to end	Day	20% 39% 8% 22% 9% 3	3538
November	1st to 10th	Day	16% 58% 10% 10% 5% 1	2057
	11th to 20th	Day	19% 58% 6% 8% 8% 2	2100
	21st to end	Day	16% 57% 6% 11% 7% 3	2100
December	1st to 10th	Day	16% 38% 12% 21% 11% 3	3304
	11th to 20th	Day	25% 35% 11% 14% 12% 3	3460
	21st to end	Day	36% 32% 13% 11% 8%	4565
January	1st to 10th	Night	23% 25% 9% 26% 13% 5%	7636
	11th to 20th	Night	41% 26% 15% 12% 5% 2	7680
	21st to end	Night	33% 25% 7% 24% 8% 3	8359
February	1st to 10th	Night	11% 60% 9% 12% 6% 3	2520
	11th to 20th	Night	9% 58% 5% 13% 12% 4	2321
	21st to end	Night	12% 60% 5% 9% 10% 3	1903
March	1st to 10th	Night	23% 34% 9% 15% 16% 2	5028
	11th to 20th	Night	12% 32% 16% 19% 13% 8%	5090
	21st to end	Night	23% 31% 11% 18% 15% 2	5588
April	1st to 10th	Night	8% 60% 7% 16% 7% 1	2310
	11th to 20th	Night	9% 63% 6% 8% 11% 3	2126
	21st to end	Night	8% 64% 7% 13% 7% 1	2000
May	1st to 10th	Night	11% 32% 21% 26% 7% 2	4520
	11th to 20th	Night	17% 33% 14% 28% 8%	4491
	21st to end	Night	18% 35% 21% 22% 4	4429
June	1st to 10th	Night	16% 66% 10% 7% 1	1800
	11th to 20th	Night	16% 68% 9% 7% 1	1800
	21st to end	Night	11% 66% 16% 6%	1800
July	1st to 10th	Night	28% 32% 29% 11%	4015
	11th to 20th	Night	24% 33% 34% 9%	4006
	21st to end	Night	16% 31% 33% 19% 1	4297
August	1st to 10th	Night	26% 44% 18% 10% 1	2964
	11th to 20th	Night	17% 42% 22% 18%	3227
	21st to end	Night	23% 48% 26% 3 1	3814
September	1st to 10th	Night	20% 65% 10% 5%	2200
	11th to 20th	Night	18% 64% 12% 5% 1	2200
	21st to end	Night	18% 65% 8% 6% 3	2200
October	1st to 10th	Night	34% 44% 12% 9% 1	3775
	11th to 20th	Night	32% 40% 9% 11% 8%	3899
	21st to end	Night	21% 39% 8% 21% 9% 3	4252
November	1st to 10th	Night	18% 56% 9% 10% 5% 2	2461
	11th to 20th	Night	19% 57% 5% 10% 6% 2	2520
	21st to end	Night	18% 57% 6% 9% 8% 3	2520
December	1st to 10th	Night	17% 38% 11% 21% 12% 1	4090
	11th to 20th	Night	29% 35% 10% 13% 10% 4	4954
	21st to end	Night	38% 33% 12% 10% 7% 1	6633

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	6% 5% 32% 38% 17% 1	330
	11th to 20th	Day	15% 8% 30% 27% 16% 3	330
	21st to end	Day	12% 6% 37% 37% 8% 1	363
February	1st to 10th	Day	7% 8% 23% 40% 19% 3	330
	11th to 20th	Day	7% 6% 23% 38% 21% 4	466
	21st to end	Day	6% 5% 24% 36% 26% 2	446
March	1st to 10th	Day	3 6% 29% 44% 16% 2	540
	11th to 20th	Day	6% 6% 25% 37% 24% 2	540
	21st to end	Day	5% 2 26% 39% 24% 5%	594
April	1st to 10th	Day	3 4 26% 42% 21% 4	540
	11th to 20th	Day	6% 3 26% 39% 21% 5%	516
	21st to end	Day	4 2 13% 45% 31% 5%	500
May	1st to 10th	Day	3 1 12% 41% 34% 9%	503
	11th to 20th	Day	9% 3 13% 33% 31% 11%	527
	21st to end	Day	9% 2 9% 19% 39% 22%	573
June	1st to 10th	Day	9% 3 8% 21% 39% 19%	530
	11th to 20th	Day	8% 2 7% 17% 33% 33%	530
	21st to end	Day	2 2 3 15% 34% 44%	530
July	1st to 10th	Day	6% 1 3 10% 34% 47%	525
	11th to 20th	Day	5% 1 3 13% 36% 42%	524
	21st to end	Day	4 1 2 18% 33% 43%	576
August	1st to 10th	Day	6% 1 5% 13% 36% 38%	526
	11th to 20th	Day	3 1 2 14% 36% 44%	519
	21st to end	Day	8% 1 6% 16% 37% 32%	583
September	1st to 10th	Day	8% 2 4 13% 37% 37%	530
	11th to 20th	Day	5% 1 5% 18% 34% 37%	530
	21st to end	Day	7% 4 10% 20% 33% 27%	530
October	1st to 10th	Day	11% 2 9% 32% 36% 10%	530
	11th to 20th	Day	11% 4 12% 34% 28% 12%	530
	21st to end	Day	10% 4 19% 31% 30% 8%	583
November	1st to 10th	Day	8% 5% 20% 34% 24% 8%	535
	11th to 20th	Day	10% 4 21% 35% 26% 4	540
	21st to end	Day	5% 4 22% 39% 27% 2	540
December	1st to 10th	Day	10% 5% 19% 38% 25% 3	510
	11th to 20th	Day	9% 4 26% 33% 24% 4	330
	21st to end	Day	9% 8% 25% 30% 23% 5%	363
January	1st to 10th	Night	9% 6% 22% 38% 24% 2	840
	11th to 20th	Night	10% 8% 26% 35% 20% 2	840
	21st to end	Night	10% 6% 29% 41% 15%	924
February	1st to 10th	Night	9% 7% 21% 39% 23% 1	840
	11th to 20th	Night	7% 4 26% 39% 23% 1	823
	21st to end	Night	6% 3 22% 45% 24%	692
March	1st to 10th	Night	5% 4 30% 41% 19% 2	840
	11th to 20th	Night	6% 4 22% 41% 25% 2	840
	21st to end	Night	6% 3 22% 44% 23% 2	924
April	1st to 10th	Night	5% 3 16% 48% 26% 3	840
	11th to 20th	Night	4 3 20% 45% 25% 2	816
	21st to end	Night	7% 1 16% 48% 27% 2	800
May	1st to 10th	Night	4 1 11% 43% 37% 4	800
	11th to 20th	Night	9% 2 13% 39% 32% 4	789
	21st to end	Night	8% 1 10% 23% 47% 12%	650
June	1st to 10th	Night	11% 1 5% 23% 45% 16%	600
	11th to 20th	Night	6% 2 5% 14% 45% 29%	600
	21st to end	Night	5% 1 3 13% 38% 41%	600
July	1st to 10th	Night	5% 1 3 8% 39% 44%	585
	11th to 20th	Night	5% 2 1 8% 42% 42%	582
	21st to end	Night	3 2 8% 42% 45%	657
August	1st to 10th	Night	8% 1 2 9% 45% 35%	631
	11th to 20th	Night	3 2 9% 46% 39%	790
	21st to end	Night	8% 1 4 16% 39% 32%	880
September	1st to 10th	Night	8% 1 6% 13% 36% 36%	800
	11th to 20th	Night	9% 2 4 16% 38% 31%	800
	21st to end	Night	11% 2 7% 18% 35% 27%	800
October	1st to 10th	Night	10% 2 9% 30% 39% 11%	800
	11th to 20th	Night	10% 3 12% 26% 41% 9%	800
	21st to end	Night	15% 2 15% 33% 28% 7%	880
November	1st to 10th	Night	10% 3 20% 33% 29% 5%	820
	11th to 20th	Night	10% 3 19% 33% 32% 4	840
	21st to end	Night	10% 3 19% 39% 27% 3	840
December	1st to 10th	Night	9% 5% 18% 36% 29% 4	840
	11th to 20th	Night	10% 5% 21% 33% 26% 4	840
	21st to end	Night	9% 5% 22% 33% 28% 4	924

Page generated at 2026-04-14 02:44:24. Version ac07

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