Glenlea Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Glenlea Astronomical Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	11% 9% 3 9% 6% 5% 5% 4 6% 8% 35%	127289
	Night	9% 10% 4 10% 6% 5% 5% 4 6% 7% 33%	123016
Transparency	Day	47% 5% 15% 24% 9% 1	130703
	Night	50% 5% 13% 22% 9% 1	127691
Seeing	Day	14% 9% 30% 29% 14% 4	17643
	Night	15% 7% 31% 34% 13% 1	27515

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	5% 17% 6% 12% 7% 7% 7% 5% 6% 8% 21%	3178
	11th to 20th	Day	4 11% 5% 15% 10% 10% 7% 11% 9% 8% 11%	3171
	21st to end	Day	6% 15% 7% 16% 8% 7% 8% 4 5% 7% 15%	4004
February	1st to 10th	Day	7% 14% 6% 13% 8% 7% 8% 6% 7% 6% 18%	1724
	11th to 20th	Day	7% 9% 3 10% 7% 8% 7% 6% 8% 9% 25%	1754
	21st to end	Day	7% 9% 3 13% 9% 8% 7% 6% 6% 7% 24%	1565
March	1st to 10th	Day	9% 11% 4 9% 8% 5% 4 5% 6% 8% 30%	4619
	11th to 20th	Day	8% 11% 5% 15% 10% 6% 5% 4 6% 8% 23%	4980
	21st to end	Day	10% 13% 5% 11% 7% 5% 5% 4 6% 7% 27%	5464
April	1st to 10th	Day	10% 12% 4 8% 5% 5% 4 4 6% 7% 34%	2432
	11th to 20th	Day	12% 11% 3 9% 6% 5% 5% 3 5% 7% 34%	2572
	21st to end	Day	17% 9% 3 8% 5% 5% 4 4 5% 6% 34%	2600
May	1st to 10th	Day	13% 7% 2 6% 4 4 4 3 5% 7% 46%	6068
	11th to 20th	Day	18% 9% 2 6% 5% 5% 4 4 6% 8% 32%	6609
	21st to end	Day	14% 7% 2 6% 3 4 4 3 5% 6% 46%	7428
June	1st to 10th	Day	14% 6% 2 7% 5% 4 5% 4 7% 9% 37%	3000
	11th to 20th	Day	14% 7% 3 6% 5% 5% 5% 4 6% 8% 36%	3000
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 6% 8% 9% 36%	3000
July	1st to 10th	Day	8% 4 2 5% 4 5% 5% 5% 7% 10% 45%	6799
	11th to 20th	Day	10% 4 2 6% 4 4 5% 4 6% 9% 44%	6800
	21st to end	Day	8% 4 2 6% 4 5% 5% 4 7% 9% 46%	7303
August	1st to 10th	Day	8% 5% 2 5% 4 4 5% 4 7% 10% 46%	4377
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 6% 8% 45%	4171
	21st to end	Day	13% 5% 2 5% 4 4 4 3 6% 8% 47%	4675
September	1st to 10th	Day	10% 5% 1 5% 4 4 4 3 5% 9% 49%	2479
	11th to 20th	Day	14% 6% 3 7% 5% 4 4 4 6% 9% 39%	2200
	21st to end	Day	15% 6% 2 6% 4 4 3 4 5% 7% 43%	2200
October	1st to 10th	Day	18% 8% 2 7% 4 4 4 4 5% 6% 37%	2038
	11th to 20th	Day	15% 11% 3 8% 5% 4 4 4 6% 7% 33%	1967
	21st to end	Day	15% 14% 4 10% 5% 4 4 4 5% 7% 27%	1980
November	1st to 10th	Day	15% 14% 4 8% 6% 4 5% 3 5% 8% 27%	1615
	11th to 20th	Day	11% 16% 6% 13% 6% 5% 4 3 6% 7% 25%	1600
	21st to end	Day	13% 14% 5% 11% 8% 7% 4 3 5% 6% 24%	1490
December	1st to 10th	Day	10% 16% 6% 14% 10% 7% 6% 5% 5% 7% 15%	2290
	11th to 20th	Day	9% 14% 6% 12% 7% 4 6% 4 7% 10% 20%	2639
	21st to end	Day	8% 15% 6% 19% 8% 7% 7% 4 5% 5% 16%	3498
January	1st to 10th	Night	4 13% 7% 14% 9% 9% 7% 5% 6% 7% 19%	6787
	11th to 20th	Night	4 13% 6% 15% 9% 9% 10% 8% 7% 5% 14%	6730
	21st to end	Night	7% 18% 8% 14% 7% 6% 5% 5% 6% 7% 19%	6974
February	1st to 10th	Night	8% 13% 4 11% 7% 7% 9% 6% 5% 5% 25%	2675
	11th to 20th	Night	6% 9% 3 9% 6% 7% 6% 6% 7% 7% 34%	2533
	21st to end	Night	6% 12% 5% 13% 7% 6% 6% 3 5% 6% 30%	2080
March	1st to 10th	Night	8% 11% 4 10% 6% 5% 5% 4 7% 7% 33%	5235
	11th to 20th	Night	7% 10% 4 8% 7% 5% 5% 5% 5% 8% 37%	4979
	21st to end	Night	12% 12% 4 11% 6% 4 4 3 5% 6% 35%	5467
April	1st to 10th	Night	11% 9% 2 6% 5% 4 4 4 7% 7% 41%	1964
	11th to 20th	Night	14% 10% 4 8% 5% 4 5% 4 5% 6% 36%	1779
	21st to end	Night	13% 8% 4 7% 5% 5% 5% 4 6% 7% 37%	1800
May	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 5% 8% 43%	3892
	11th to 20th	Night	16% 7% 3 6% 5% 4 4 4 6% 8% 36%	3302
	21st to end	Night	13% 6% 2 5% 3 2 3 4 5% 7% 50%	3470
June	1st to 10th	Night	11% 5% 2 6% 4 4 4 5% 7% 9% 43%	1400
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 3 6% 9% 43%	1400
	21st to end	Night	10% 5% 2 7% 4 5% 5% 4 7% 8% 43%	1400
July	1st to 10th	Night	7% 3 2 4 4 4 5% 4 7% 10% 50%	3180
	11th to 20th	Night	7% 3 1 5% 4 4 4 4 7% 9% 52%	3180
	21st to end	Night	5% 4 1 5% 4 4 5% 5% 6% 10% 49%	3557
August	1st to 10th	Night	6% 4 2 6% 4 4 5% 5% 7% 9% 49%	2781
	11th to 20th	Night	10% 3 1 6% 3 4 4 4 5% 8% 51%	2888
	21st to end	Night	10% 5% 2 5% 4 4 4 4 6% 8% 49%	3234
September	1st to 10th	Night	8% 3 2 4 3 3 4 4 6% 9% 54%	1917
	11th to 20th	Night	14% 6% 2 5% 4 4 5% 3 5% 9% 44%	2200
	21st to end	Night	14% 6% 2 6% 5% 4 4 2 5% 7% 44%	2200
October	1st to 10th	Night	17% 8% 3 6% 5% 4 4 3 6% 7% 38%	2341
	11th to 20th	Night	15% 11% 3 7% 4 5% 4 4 6% 6% 35%	2430
	21st to end	Night	15% 11% 4 9% 5% 4 5% 3 5% 6% 32%	2860
November	1st to 10th	Night	13% 14% 5% 10% 6% 5% 4 3 5% 5% 29%	2726
	11th to 20th	Night	11% 16% 5% 12% 7% 5% 4 4 5% 6% 26%	2800
	21st to end	Night	12% 13% 6% 13% 7% 6% 5% 3 5% 5% 25%	2907
December	1st to 10th	Night	8% 15% 7% 17% 11% 7% 5% 5% 5% 5% 14%	4900
	11th to 20th	Night	8% 14% 6% 14% 7% 5% 5% 4 6% 7% 22%	5568
	21st to end	Night	8% 13% 5% 18% 10% 7% 6% 4 6% 5% 20%	7480

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	61% 4 11% 16% 6%	3179
	11th to 20th	Day	66% 4 10% 14% 6%	3199
	21st to end	Day	69% 4 11% 13% 3	4004
February	1st to 10th	Day	62% 6% 11% 13% 7% 1	1724
	11th to 20th	Day	51% 5% 14% 22% 7%	1763
	21st to end	Day	56% 5% 15% 16% 8%	1565
March	1st to 10th	Day	51% 4 18% 16% 10%	4621
	11th to 20th	Day	59% 6% 14% 16% 5%	4980
	21st to end	Day	56% 5% 11% 21% 6%	5464
April	1st to 10th	Day	49% 5% 10% 25% 10% 1	2432
	11th to 20th	Day	49% 5% 11% 22% 11% 1	2572
	21st to end	Day	49% 5% 15% 21% 8% 2	2600
May	1st to 10th	Day	39% 4 11% 27% 19% 1	6079
	11th to 20th	Day	48% 5% 10% 26% 11%	6613
	21st to end	Day	39% 5% 8% 24% 21% 3	7428
June	1st to 10th	Day	43% 7% 15% 25% 10% 1	3000
	11th to 20th	Day	44% 6% 16% 25% 9%	3000
	21st to end	Day	41% 9% 20% 23% 8%	3000
July	1st to 10th	Day	34% 4 27% 28% 6%	6726
	11th to 20th	Day	37% 4 20% 34% 5%	6719
	21st to end	Day	35% 4 23% 31% 7%	7092
August	1st to 10th	Day	33% 5% 20% 31% 11%	4361
	11th to 20th	Day	36% 4 20% 35% 5%	4210
	21st to end	Day	36% 4 23% 32% 6%	4674
September	1st to 10th	Day	33% 5% 18% 33% 10% 1	2479
	11th to 20th	Day	42% 6% 20% 26% 5% 1	2200
	21st to end	Day	40% 5% 16% 32% 7%	2200
October	1st to 10th	Day	39% 4 15% 31% 11%	3311
	11th to 20th	Day	42% 4 12% 28% 14%	3185
	21st to end	Day	51% 4 15% 24% 6%	3190
November	1st to 10th	Day	56% 6% 16% 18% 4 1	1615
	11th to 20th	Day	59% 6% 17% 14% 4	1600
	21st to end	Day	63% 5% 15% 14% 2	1490
December	1st to 10th	Day	69% 6% 12% 8% 5%	2290
	11th to 20th	Day	58% 4 15% 15% 8%	2640
	21st to end	Day	69% 4 9% 14% 4	3498
January	1st to 10th	Night	63% 5% 11% 13% 8%	6787
	11th to 20th	Night	68% 4 6% 17% 4	6777
	21st to end	Night	65% 4 13% 14% 4	6974
February	1st to 10th	Night	58% 6% 12% 15% 9%	2675
	11th to 20th	Night	46% 6% 17% 22% 9%	2534
	21st to end	Night	54% 5% 11% 20% 10%	2080
March	1st to 10th	Night	50% 5% 16% 16% 10% 3	5246
	11th to 20th	Night	44% 5% 15% 23% 12%	4980
	21st to end	Night	52% 4 9% 24% 11% 1	5467
April	1st to 10th	Night	42% 4 12% 25% 15% 3	1964
	11th to 20th	Night	48% 5% 8% 22% 16% 2	1779
	21st to end	Night	46% 5% 12% 25% 10% 2	1800
May	1st to 10th	Night	39% 4 11% 27% 19%	3901
	11th to 20th	Night	46% 5% 9% 24% 13% 3	3303
	21st to end	Night	35% 4 8% 23% 25% 5%	3470
June	1st to 10th	Night	36% 7% 16% 25% 14% 1	1400
	11th to 20th	Night	38% 6% 18% 24% 12% 2	1400
	21st to end	Night	37% 7% 20% 27% 8% 1	1400
July	1st to 10th	Night	31% 4 25% 30% 9% 1	3145
	11th to 20th	Night	29% 4 20% 40% 7%	3138
	21st to end	Night	31% 3 27% 32% 6% 1	3455
August	1st to 10th	Night	30% 4 16% 35% 14% 1	2773
	11th to 20th	Night	32% 3 24% 31% 9%	2915
	21st to end	Night	34% 3 19% 34% 9%	3234
September	1st to 10th	Night	27% 5% 18% 35% 14% 1	1917
	11th to 20th	Night	38% 6% 18% 26% 10% 1	2200
	21st to end	Night	40% 5% 13% 30% 11% 1	2200
October	1st to 10th	Night	39% 4 10% 34% 13% 1	3835
	11th to 20th	Night	43% 3 12% 30% 11%	3953
	21st to end	Night	46% 3 14% 30% 6%	4610
November	1st to 10th	Night	56% 5% 14% 17% 6% 1	2726
	11th to 20th	Night	59% 6% 15% 14% 5% 1	2800
	21st to end	Night	62% 5% 14% 13% 5%	2907
December	1st to 10th	Night	72% 4 12% 9% 3	4900
	11th to 20th	Night	61% 4 12% 11% 11%	5566
	21st to end	Night	65% 4 9% 17% 4	7480

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	11% 8% 32% 32% 15% 2	300
	11th to 20th	Day	12% 7% 30% 35% 15% 1	300
	21st to end	Day	14% 10% 29% 30% 13% 3	330
February	1st to 10th	Day	13% 13% 34% 29% 9% 2	300
	11th to 20th	Day	9% 11% 34% 31% 13% 2	300
	21st to end	Day	9% 6% 43% 31% 11%	249
March	1st to 10th	Day	13% 7% 34% 33% 12% 1	300
	11th to 20th	Day	14% 11% 32% 28% 12% 3	300
	21st to end	Day	14% 9% 35% 29% 11% 2	330
April	1st to 10th	Day	14% 12% 29% 32% 9% 3	532
	11th to 20th	Day	18% 12% 31% 25% 12% 1	690
	21st to end	Day	19% 10% 30% 27% 11% 2	700
May	1st to 10th	Day	16% 12% 32% 26% 12% 2	703
	11th to 20th	Day	21% 9% 29% 26% 12% 3	725
	21st to end	Day	19% 8% 24% 28% 16% 5%	789
June	1st to 10th	Day	14% 11% 28% 26% 14% 6%	730
	11th to 20th	Day	15% 12% 31% 29% 11% 3	730
	21st to end	Day	12% 9% 27% 32% 15% 5%	730
July	1st to 10th	Day	10% 3 27% 31% 18% 10%	721
	11th to 20th	Day	11% 7% 28% 28% 20% 6%	718
	21st to end	Day	6% 5% 28% 34% 20% 8%	794
August	1st to 10th	Day	8% 5% 27% 32% 21% 7%	726
	11th to 20th	Day	11% 6% 23% 31% 19% 9%	715
	21st to end	Day	13% 6% 28% 30% 18% 5%	803
September	1st to 10th	Day	10% 8% 33% 31% 14% 3	609
	11th to 20th	Day	18% 10% 37% 24% 8% 3	330
	21st to end	Day	20% 8% 34% 27% 8% 2	330
October	1st to 10th	Day	20% 13% 25% 28% 10% 4	330
	11th to 20th	Day	17% 14% 34% 27% 7% 2	330
	21st to end	Day	26% 9% 32% 23% 9% 1	363
November	1st to 10th	Day	25% 9% 28% 26% 10% 1	306
	11th to 20th	Day	20% 11% 35% 25% 9%	300
	21st to end	Day	18% 11% 39% 25% 6%	300
December	1st to 10th	Day	15% 6% 37% 27% 13% 2	300
	11th to 20th	Day	15% 11% 36% 31% 7% 1	300
	21st to end	Day	14% 8% 29% 29% 14% 5%	330
January	1st to 10th	Night	11% 8% 34% 31% 14% 2	1000
	11th to 20th	Night	14% 8% 31% 36% 10% 1	1000
	21st to end	Night	14% 8% 34% 32% 11% 1	1100
February	1st to 10th	Night	14% 7% 29% 35% 14% 1	1000
	11th to 20th	Night	10% 7% 30% 36% 13% 3	979
	21st to end	Night	11% 7% 35% 32% 15% 1	760
March	1st to 10th	Night	15% 6% 32% 32% 13% 2	750
	11th to 20th	Night	11% 7% 40% 30% 11% 1	600
	21st to end	Night	19% 8% 28% 29% 14% 3	660
April	1st to 10th	Night	15% 8% 28% 37% 11% 1	600
	11th to 20th	Night	17% 8% 30% 34% 11%	593
	21st to end	Night	17% 7% 30% 35% 12%	600
May	1st to 10th	Night	17% 6% 35% 32% 9% 1	600
	11th to 20th	Night	17% 8% 29% 33% 12% 1	591
	21st to end	Night	18% 6% 32% 33% 12%	650
June	1st to 10th	Night	13% 6% 32% 35% 14% 1	600
	11th to 20th	Night	16% 5% 32% 34% 14% 1	600
	21st to end	Night	12% 4 24% 40% 19% 1	600
July	1st to 10th	Night	6% 2 26% 44% 21% 2	585
	11th to 20th	Night	7% 4 25% 44% 17% 2	582
	21st to end	Night	7% 3 23% 44% 20% 3	657
August	1st to 10th	Night	9% 2 22% 47% 18% 2	597
	11th to 20th	Night	10% 4 21% 40% 23% 3	594
	21st to end	Night	12% 6% 25% 38% 19% 1	660
September	1st to 10th	Night	9% 4 30% 38% 17% 2	600
	11th to 20th	Night	17% 6% 27% 37% 14%	600
	21st to end	Night	17% 6% 32% 32% 12% 1	600
October	1st to 10th	Night	22% 9% 32% 29% 8%	741
	11th to 20th	Night	20% 9% 33% 30% 8%	830
	21st to end	Night	20% 9% 31% 30% 8% 1	1100
November	1st to 10th	Night	19% 10% 34% 26% 10%	986
	11th to 20th	Night	19% 8% 34% 30% 8% 2	1000
	21st to end	Night	18% 9% 39% 26% 8% 1	1000
December	1st to 10th	Night	14% 6% 33% 33% 12% 2	1000
	11th to 20th	Night	17% 7% 35% 32% 8% 2	1000
	21st to end	Night	19% 8% 30% 29% 12% 2	1100

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