Observatory Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatory Park 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	15% 12% 4 10% 5% 4 4 3 5% 6% 30%	131779
	Night	15% 13% 5% 11% 5% 4 4 3 5% 6% 29%	130988
Transparency	Day	53% 6% 11% 19% 10% 1	135411
	Night	55% 5% 11% 17% 10% 1	135426
Seeing	Day	23% 19% 29% 18% 8% 3	16480
	Night	23% 17% 27% 22% 9% 1	26299

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	14% 21% 9% 27% 6% 3 3 2 3 3 10%	5258
	11th to 20th	Day	9% 20% 9% 23% 10% 7% 4 2 3 4 10%	5280
	21st to end	Day	16% 17% 8% 16% 10% 7% 4 2 3 4 13%	5780
February	1st to 10th	Day	14% 20% 6% 15% 8% 4 3 3 4 4 18%	1890
	11th to 20th	Day	15% 15% 8% 17% 8% 5% 4 3 4 4 18%	1863
	21st to end	Day	15% 18% 5% 14% 5% 4 5% 4 4 6% 21%	1730
March	1st to 10th	Day	16% 10% 9% 12% 4 4 4 3 5% 5% 28%	4971
	11th to 20th	Day	12% 13% 5% 11% 5% 4 3 3 4 5% 34%	5090
	21st to end	Day	17% 15% 6% 10% 5% 4 4 2 4 5% 27%	5586
April	1st to 10th	Day	15% 16% 4 8% 4 4 4 3 5% 7% 30%	2378
	11th to 20th	Day	18% 13% 4 7% 4 5% 4 3 5% 6% 33%	2639
	21st to end	Day	19% 13% 3 7% 4 3 3 3 4 6% 35%	2730
May	1st to 10th	Day	21% 9% 3 8% 5% 4 5% 4 4 6% 30%	5992
	11th to 20th	Day	19% 10% 3 8% 4 4 4 4 6% 7% 33%	5939
	21st to end	Day	18% 8% 3 6% 4 4 5% 4 6% 7% 35%	6725
June	1st to 10th	Day	16% 6% 3 6% 5% 5% 5% 4 6% 7% 37%	2800
	11th to 20th	Day	14% 7% 2 6% 5% 4 5% 4 6% 8% 37%	2800
	21st to end	Day	13% 5% 2 6% 5% 4 5% 5% 6% 10% 40%	2800
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 9% 44%	6096
	11th to 20th	Day	10% 6% 2 7% 5% 4 6% 4 7% 10% 39%	5900
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 6% 8% 44%	6424
August	1st to 10th	Day	14% 7% 2 7% 5% 5% 6% 4 6% 9% 36%	4234
	11th to 20th	Day	12% 7% 3 8% 5% 5% 6% 5% 8% 9% 34%	4174
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 9% 41%	4560
September	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 6% 7% 42%	2335
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 3 6% 8% 45%	2200
	21st to end	Day	20% 8% 3 7% 5% 3 5% 3 5% 7% 35%	2200
October	1st to 10th	Day	16% 7% 3 7% 4 4 5% 4 6% 8% 39%	2042
	11th to 20th	Day	21% 9% 2 7% 5% 4 5% 3 5% 7% 32%	2000
	21st to end	Day	23% 13% 3 8% 5% 4 5% 3 5% 6% 25%	2200
November	1st to 10th	Day	13% 13% 4 8% 5% 5% 4 3 5% 6% 33%	1867
	11th to 20th	Day	15% 22% 5% 10% 4 4 4 3 4 6% 23%	1789
	21st to end	Day	17% 22% 7% 9% 6% 5% 4 3 5% 5% 16%	1680
December	1st to 10th	Day	16% 20% 10% 14% 6% 5% 4 3 5% 5% 12%	2680
	11th to 20th	Day	20% 24% 7% 13% 7% 4 3 3 4 5% 11%	3070
	21st to end	Day	22% 24% 7% 17% 5% 4 4 3 3 3 8%	4077
January	1st to 10th	Night	13% 20% 10% 26% 6% 4 3 2 3 3 9%	7635
	11th to 20th	Night	9% 19% 10% 21% 10% 6% 4 3 4 4 10%	7680
	21st to end	Night	15% 17% 8% 13% 7% 6% 5% 3 3 4 19%	8401
February	1st to 10th	Night	17% 17% 5% 15% 8% 5% 4 3 4 4 19%	2730
	11th to 20th	Night	16% 15% 7% 15% 7% 6% 5% 3 4 5% 19%	2641
	21st to end	Night	18% 14% 6% 13% 6% 5% 4 3 4 5% 22%	2076
March	1st to 10th	Night	17% 16% 5% 8% 6% 4 4 3 4 5% 29%	5088
	11th to 20th	Night	11% 10% 6% 9% 5% 4 4 3 5% 6% 37%	5090
	21st to end	Night	17% 14% 6% 9% 4 4 4 3 4 4 31%	5588
April	1st to 10th	Night	17% 16% 4 8% 5% 4 4 3 4 5% 29%	2241
	11th to 20th	Night	16% 11% 3 7% 4 4 5% 4 6% 6% 33%	1947
	21st to end	Night	15% 10% 3 8% 5% 4 5% 4 4 7% 36%	1890
May	1st to 10th	Night	24% 10% 3 7% 4 4 4 4 5% 5% 28%	4123
	11th to 20th	Night	15% 11% 3 6% 4 4 4 3 6% 7% 36%	4086
	21st to end	Night	19% 8% 1 5% 3 4 4 3 6% 8% 39%	4111
June	1st to 10th	Night	11% 7% 2 8% 5% 5% 4 4 6% 7% 41%	1600
	11th to 20th	Night	9% 6% 4 7% 5% 6% 6% 4 5% 7% 42%	1600
	21st to end	Night	9% 6% 2 6% 4 4 5% 4 6% 9% 43%	1600
July	1st to 10th	Night	8% 6% 2 6% 4 4 5% 4 5% 9% 45%	3843
	11th to 20th	Night	9% 6% 2 6% 4 4 4 4 6% 9% 44%	4057
	21st to end	Night	8% 6% 2 6% 4 4 4 3 6% 8% 49%	4427
August	1st to 10th	Night	11% 8% 3 7% 5% 5% 5% 4 6% 7% 40%	2915
	11th to 20th	Night	14% 9% 2 7% 4 4 4 3 5% 6% 41%	2875
	21st to end	Night	8% 5% 2 7% 5% 5% 5% 5% 7% 8% 43%	3304
September	1st to 10th	Night	13% 5% 2 5% 3 3 4 4 7% 7% 46%	2060
	11th to 20th	Night	10% 5% 2 5% 3 4 4 3 5% 6% 53%	2200
	21st to end	Night	17% 7% 3 7% 4 5% 4 3 5% 6% 37%	2200
October	1st to 10th	Night	15% 7% 2 5% 4 3 4 3 6% 7% 43%	2357
	11th to 20th	Night	17% 9% 3 9% 4 5% 4 3 6% 6% 34%	2400
	21st to end	Night	24% 11% 3 6% 4 4 4 3 6% 6% 29%	2640
November	1st to 10th	Night	14% 11% 3 7% 5% 4 4 3 5% 7% 36%	2646
	11th to 20th	Night	15% 18% 5% 10% 5% 5% 4 3 5% 5% 24%	2829
	21st to end	Night	18% 20% 6% 9% 5% 4 4 3 5% 5% 20%	2940
December	1st to 10th	Night	13% 20% 8% 14% 7% 5% 5% 3 4 5% 15%	4720
	11th to 20th	Night	17% 23% 9% 13% 5% 4 3 3 4 5% 15%	5345
	21st to end	Night	20% 23% 7% 14% 5% 4 3 3 4 5% 11%	7103

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	81% 3 7% 6% 2 1	5259
	11th to 20th	Day	80% 3 7% 7% 3	5280
	21st to end	Day	76% 3 7% 10% 4	5780
February	1st to 10th	Day	71% 4 8% 12% 5%	1890
	11th to 20th	Day	70% 4 8% 12% 5%	1872
	21st to end	Day	63% 6% 11% 12% 6% 2	1730
March	1st to 10th	Day	57% 6% 11% 20% 6%	4971
	11th to 20th	Day	53% 5% 14% 20% 6% 2	5090
	21st to end	Day	60% 4 7% 18% 11% 1	5586
April	1st to 10th	Day	53% 5% 12% 16% 13% 1	2378
	11th to 20th	Day	53% 5% 10% 17% 13% 2	2639
	21st to end	Day	51% 5% 10% 21% 10% 2	2730
May	1st to 10th	Day	54% 6% 8% 19% 9% 4	6027
	11th to 20th	Day	49% 8% 10% 21% 11% 1	5943
	21st to end	Day	45% 8% 11% 21% 12% 3	6740
June	1st to 10th	Day	44% 7% 11% 25% 13% 1	2800
	11th to 20th	Day	42% 8% 13% 21% 14% 2	2800
	21st to end	Day	39% 7% 12% 24% 16% 1	2800
July	1st to 10th	Day	35% 7% 12% 27% 16% 2	6031
	11th to 20th	Day	38% 8% 13% 24% 16% 1	5831
	21st to end	Day	37% 7% 11% 24% 20% 1	6240
August	1st to 10th	Day	42% 9% 14% 23% 12%	4221
	11th to 20th	Day	42% 9% 11% 30% 8% 1	4213
	21st to end	Day	37% 8% 14% 28% 12% 1	4560
September	1st to 10th	Day	40% 8% 13% 22% 15% 3	2335
	11th to 20th	Day	37% 8% 12% 26% 16% 1	2200
	21st to end	Day	49% 6% 10% 22% 13% 1	2200
October	1st to 10th	Day	34% 5% 15% 26% 19% 1	3328
	11th to 20th	Day	46% 5% 12% 19% 14% 3	3240
	21st to end	Day	45% 5% 14% 24% 11% 1	3538
November	1st to 10th	Day	50% 6% 12% 19% 12% 1	1867
	11th to 20th	Day	61% 5% 11% 13% 7% 2	1789
	21st to end	Day	69% 5% 9% 12% 5%	1680
December	1st to 10th	Day	75% 5% 10% 8% 2	2680
	11th to 20th	Day	75% 6% 8% 9% 2	3066
	21st to end	Day	82% 4 7% 6% 1	4077
January	1st to 10th	Night	81% 3 6% 7% 3	7636
	11th to 20th	Night	77% 5% 7% 8% 2 1	7680
	21st to end	Night	70% 3 9% 13% 5%	8401
February	1st to 10th	Night	68% 5% 10% 12% 5%	2730
	11th to 20th	Night	68% 5% 9% 14% 5%	2643
	21st to end	Night	65% 5% 8% 14% 8% 1	2076
March	1st to 10th	Night	56% 6% 13% 17% 8%	5088
	11th to 20th	Night	48% 4 13% 23% 10% 1	5090
	21st to end	Night	54% 4 10% 17% 13% 1	5588
April	1st to 10th	Night	57% 5% 9% 16% 11% 2	2241
	11th to 20th	Night	49% 6% 11% 20% 12% 2	1947
	21st to end	Night	47% 5% 11% 21% 13% 3	1890
May	1st to 10th	Night	55% 6% 10% 14% 10% 5%	4150
	11th to 20th	Night	46% 6% 11% 23% 13% 1	4087
	21st to end	Night	41% 6% 13% 24% 14% 2	4118
June	1st to 10th	Night	41% 7% 10% 25% 15% 3	1600
	11th to 20th	Night	40% 7% 12% 21% 18% 3	1600
	21st to end	Night	36% 8% 13% 24% 18% 1	1600
July	1st to 10th	Night	35% 6% 13% 24% 19% 3	3800
	11th to 20th	Night	35% 8% 13% 26% 16% 2	4006
	21st to end	Night	33% 6% 12% 21% 26% 2	4297
August	1st to 10th	Night	41% 7% 11% 25% 14% 2	2904
	11th to 20th	Night	42% 6% 11% 29% 11% 1	2902
	21st to end	Night	35% 6% 13% 29% 14% 2	3304
September	1st to 10th	Night	35% 7% 12% 27% 16% 2	2060
	11th to 20th	Night	31% 7% 14% 26% 21% 2	2200
	21st to end	Night	47% 6% 10% 20% 16% 1	2200
October	1st to 10th	Night	34% 6% 10% 27% 22% 1	3851
	11th to 20th	Night	46% 6% 10% 19% 16% 2	3899
	21st to end	Night	43% 4 14% 24% 14% 1	4252
November	1st to 10th	Night	47% 6% 13% 20% 12% 2	2646
	11th to 20th	Night	61% 4 11% 15% 6% 3	2829
	21st to end	Night	66% 5% 11% 14% 4	2940
December	1st to 10th	Night	70% 6% 13% 8% 3	4720
	11th to 20th	Night	73% 4 10% 12% 1	5348
	21st to end	Night	75% 5% 10% 9% 1	7103

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	33% 32% 25% 8% 2	330
	11th to 20th	Day	36% 33% 23% 6% 2	330
	21st to end	Day	30% 34% 28% 6% 1 1	363
February	1st to 10th	Day	32% 28% 32% 7% 1	330
	11th to 20th	Day	35% 34% 24% 6% 1	330
	21st to end	Day	37% 33% 26% 4	273
March	1st to 10th	Day	30% 28% 29% 10% 3	503
	11th to 20th	Day	29% 28% 30% 11% 2 1	540
	21st to end	Day	36% 23% 28% 10% 3	594
April	1st to 10th	Day	30% 27% 26% 12% 4 1	540
	11th to 20th	Day	33% 21% 25% 14% 6% 1	532
	21st to end	Day	28% 23% 31% 13% 4 1	540
May	1st to 10th	Day	28% 23% 31% 14% 3 2	543
	11th to 20th	Day	21% 17% 31% 22% 7% 2	551
	21st to end	Day	18% 12% 31% 25% 11% 3	573
June	1st to 10th	Day	19% 16% 35% 20% 10% 1	530
	11th to 20th	Day	18% 14% 31% 26% 10% 1	530
	21st to end	Day	15% 12% 35% 28% 8% 2	530
July	1st to 10th	Day	10% 9% 28% 28% 19% 6%	525
	11th to 20th	Day	13% 11% 30% 26% 14% 5%	524
	21st to end	Day	11% 8% 33% 27% 14% 7%	576
August	1st to 10th	Day	10% 10% 27% 29% 17% 8%	526
	11th to 20th	Day	13% 9% 28% 28% 15% 7%	519
	21st to end	Day	12% 12% 26% 27% 16% 7%	583
September	1st to 10th	Day	17% 10% 24% 25% 20% 5%	530
	11th to 20th	Day	10% 7% 24% 30% 20% 9%	530
	21st to end	Day	22% 14% 28% 20% 11% 4	530
October	1st to 10th	Day	17% 14% 28% 23% 16% 2	530
	11th to 20th	Day	24% 23% 33% 14% 4 2	530
	21st to end	Day	34% 20% 28% 15% 2	583
November	1st to 10th	Day	26% 26% 28% 12% 6% 1	345
	11th to 20th	Day	36% 22% 26% 11% 4	277
	21st to end	Day	33% 28% 33% 6%	220
December	1st to 10th	Day	27% 36% 26% 7% 3	220
	11th to 20th	Day	31% 33% 28% 7% 1	220
	21st to end	Day	35% 30% 28% 7% 1	250
January	1st to 10th	Night	30% 28% 30% 10% 2	840
	11th to 20th	Night	33% 27% 26% 12% 1	840
	21st to end	Night	28% 26% 30% 14% 2	924
February	1st to 10th	Night	29% 25% 28% 16% 2	840
	11th to 20th	Night	30% 24% 31% 14% 2	823
	21st to end	Night	32% 25% 30% 12% 1	692
March	1st to 10th	Night	26% 25% 30% 16% 3	840
	11th to 20th	Night	28% 23% 27% 17% 4 1	840
	21st to end	Night	26% 20% 30% 19% 5%	924
April	1st to 10th	Night	29% 18% 29% 20% 5%	840
	11th to 20th	Night	25% 17% 34% 17% 6% 1	703
	21st to end	Night	20% 15% 33% 26% 6%	630
May	1st to 10th	Night	23% 13% 27% 28% 9%	630
	11th to 20th	Night	17% 11% 29% 33% 8% 2	609
	21st to end	Night	14% 7% 28% 33% 17% 2	521
June	1st to 10th	Night	12% 7% 33% 35% 12% 1	400
	11th to 20th	Night	12% 8% 30% 36% 14% 1	400
	21st to end	Night	10% 9% 28% 39% 15%	400
July	1st to 10th	Night	7% 5% 19% 42% 22% 5%	487
	11th to 20th	Night	9% 5% 23% 35% 24% 3	582
	21st to end	Night	9% 5% 20% 40% 24% 2	657
August	1st to 10th	Night	10% 5% 18% 39% 23% 4	597
	11th to 20th	Night	12% 4 17% 37% 27% 3	594
	21st to end	Night	12% 6% 22% 30% 26% 3	702
September	1st to 10th	Night	15% 7% 19% 32% 23% 4	800
	11th to 20th	Night	13% 4 18% 35% 26% 5%	800
	21st to end	Night	20% 10% 24% 26% 16% 4	800
October	1st to 10th	Night	20% 9% 28% 25% 18% 2	800
	11th to 20th	Night	23% 20% 26% 24% 7%	800
	21st to end	Night	31% 19% 29% 18% 3	880
November	1st to 10th	Night	23% 19% 29% 22% 6% 1	820
	11th to 20th	Night	29% 23% 27% 15% 5% 1	840
	21st to end	Night	34% 24% 26% 13% 3	840
December	1st to 10th	Night	32% 25% 28% 11% 3	840
	11th to 20th	Night	30% 29% 27% 12% 1	840
	21st to end	Night	32% 25% 30% 12% 2	924

Page generated at 2026-05-14 01:58:56. Version ac07

Copyright © 2026 Attilla Danko.