Timerson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Timerson Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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% 11% 4 10% 6% 5% 5% 4 5% 7% 29%	132471
	Night	16% 12% 5% 10% 6% 5% 4 4 5% 6% 27%	130652
Transparency	Day	52% 8% 12% 18% 9% 1	136026
	Night	54% 8% 11% 17% 9% 1	135183
Seeing	Day	23% 17% 30% 19% 9% 3	16548
	Night	22% 16% 29% 23% 9% 1	25887

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	14% 19% 8% 20% 7% 4 4 3 3 4 13%	4661
	11th to 20th	Day	11% 19% 8% 19% 10% 6% 5% 3 4 4 12%	4751
	21st to end	Day	12% 15% 7% 15% 12% 10% 5% 3 4 4 12%	5780
February	1st to 10th	Day	15% 16% 7% 15% 10% 5% 6% 4 4 4 14%	1890
	11th to 20th	Day	13% 18% 8% 17% 7% 5% 5% 4 4 4 14%	1843
	21st to end	Day	19% 16% 4 14% 8% 5% 4 3 5% 5% 17%	1712
March	1st to 10th	Day	16% 14% 6% 12% 7% 5% 3 2 4 4 26%	4825
	11th to 20th	Day	11% 17% 5% 8% 5% 4 4 4 5% 7% 31%	5090
	21st to end	Day	16% 13% 5% 12% 6% 5% 4 3 5% 5% 26%	5742
April	1st to 10th	Day	18% 14% 4 8% 5% 3 4 3 5% 5% 29%	2520
	11th to 20th	Day	19% 11% 3 7% 5% 4 4 3 5% 6% 34%	2494
	21st to end	Day	19% 11% 4 7% 4 4 4 3 4 6% 32%	2620
May	1st to 10th	Day	26% 10% 3 7% 4 4 4 4 4 5% 28%	6271
	11th to 20th	Day	15% 10% 3 9% 5% 5% 4 4 5% 7% 33%	6458
	21st to end	Day	19% 8% 3 7% 5% 4 5% 4 6% 7% 33%	7103
June	1st to 10th	Day	17% 7% 2 7% 5% 5% 5% 4 7% 8% 35%	2800
	11th to 20th	Day	13% 6% 3 6% 5% 5% 5% 5% 7% 9% 36%	2800
	21st to end	Day	13% 6% 3 7% 6% 5% 5% 5% 7% 9% 36%	2800
July	1st to 10th	Day	9% 5% 2 7% 5% 5% 5% 5% 7% 10% 40%	6360
	11th to 20th	Day	8% 5% 2 7% 6% 5% 5% 5% 8% 10% 36%	6360
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 4 6% 9% 43%	6804
August	1st to 10th	Day	17% 5% 3 6% 5% 5% 5% 4 6% 7% 38%	4234
	11th to 20th	Day	14% 6% 3 8% 6% 6% 6% 5% 6% 8% 33%	4146
	21st to end	Day	13% 7% 3 7% 4 5% 5% 3 6% 8% 40%	4312
September	1st to 10th	Day	13% 7% 2 6% 4 4 5% 4 6% 8% 41%	2400
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 5% 8% 44%	2400
	21st to end	Day	15% 8% 2 7% 6% 5% 4 4 5% 7% 34%	2104
October	1st to 10th	Day	17% 8% 3 6% 5% 5% 5% 4 5% 6% 37%	2000
	11th to 20th	Day	19% 10% 4 8% 6% 5% 4 3 6% 6% 30%	2000
	21st to end	Day	24% 12% 4 7% 6% 4 4 3 5% 5% 24%	2165
November	1st to 10th	Day	14% 13% 4 8% 6% 5% 4 4 5% 6% 32%	1851
	11th to 20th	Day	13% 15% 6% 9% 5% 5% 4 3 6% 7% 25%	1701
	21st to end	Day	19% 16% 5% 9% 6% 4 5% 4 6% 6% 21%	1680
December	1st to 10th	Day	17% 16% 7% 14% 8% 5% 5% 4 5% 5% 13%	2680
	11th to 20th	Day	18% 17% 6% 14% 5% 4 4 3 5% 4 20%	3066
	21st to end	Day	20% 18% 6% 15% 7% 5% 4 3 4 5% 12%	4048
January	1st to 10th	Night	15% 18% 8% 21% 7% 5% 4 3 4 3 12%	8234
	11th to 20th	Night	11% 20% 8% 17% 9% 6% 5% 3 5% 5% 11%	8133
	21st to end	Night	13% 16% 6% 12% 10% 8% 6% 4 5% 5% 16%	8401
February	1st to 10th	Night	15% 15% 6% 13% 8% 5% 5% 3 5% 6% 19%	2730
	11th to 20th	Night	14% 14% 6% 14% 9% 6% 6% 3 5% 5% 19%	2663
	21st to end	Night	16% 13% 5% 16% 7% 5% 4 4 5% 5% 21%	2093
March	1st to 10th	Night	15% 11% 7% 11% 7% 5% 4 4 5% 6% 26%	5231
	11th to 20th	Night	14% 15% 5% 8% 4 4 4 3 5% 7% 31%	5089
	21st to end	Night	18% 10% 5% 12% 5% 4 4 3 4 6% 27%	5430
April	1st to 10th	Night	19% 11% 4 8% 7% 5% 4 3 5% 6% 28%	2100
	11th to 20th	Night	19% 10% 3 7% 5% 5% 4 3 4 6% 33%	2075
	21st to end	Night	18% 8% 2 8% 5% 4 4 4 6% 8% 34%	2017
May	1st to 10th	Night	28% 10% 3 9% 4 4 4 3 4 6% 25%	3875
	11th to 20th	Night	15% 8% 3 7% 5% 5% 4 4 6% 8% 36%	3655
	21st to end	Night	19% 7% 3 7% 5% 4 4 4 6% 7% 35%	4014
June	1st to 10th	Night	16% 8% 3 6% 4 4 4 4 6% 8% 38%	1600
	11th to 20th	Night	11% 7% 3 6% 4 5% 5% 5% 6% 8% 41%	1600
	21st to end	Night	10% 6% 2 7% 5% 5% 5% 3 7% 9% 42%	1600
July	1st to 10th	Night	10% 7% 3 7% 5% 4 5% 4 6% 10% 40%	3600
	11th to 20th	Night	10% 5% 2 7% 5% 5% 4 4 6% 8% 44%	3600
	21st to end	Night	10% 5% 2 7% 4 3 5% 4 6% 8% 46%	4031
August	1st to 10th	Night	12% 7% 2 7% 4 4 4 4 6% 9% 40%	2915
	11th to 20th	Night	14% 6% 2 7% 5% 4 5% 4 6% 7% 40%	2875
	21st to end	Night	8% 5% 2 7% 5% 4 4 4 6% 9% 47%	3581
September	1st to 10th	Night	13% 5% 2 6% 3 4 4 4 5% 7% 47%	2000
	11th to 20th	Night	12% 6% 2 6% 4 4 3 4 5% 8% 47%	2000
	21st to end	Night	15% 8% 2 7% 4 4 4 4 6% 7% 37%	2292
October	1st to 10th	Night	18% 8% 3 7% 5% 4 5% 4 5% 6% 37%	2400
	11th to 20th	Night	20% 8% 3 7% 5% 5% 4 4 5% 7% 33%	2400
	21st to end	Night	23% 11% 4 8% 5% 4 4 3 5% 6% 27%	2675
November	1st to 10th	Night	16% 11% 4 8% 5% 4 4 4 4 7% 33%	2667
	11th to 20th	Night	16% 13% 4 8% 7% 4 4 4 6% 7% 28%	2918
	21st to end	Night	17% 17% 4 8% 6% 4 4 4 6% 6% 23%	2940
December	1st to 10th	Night	16% 17% 7% 14% 6% 6% 5% 4 5% 6% 14%	4720
	11th to 20th	Night	20% 16% 6% 12% 6% 4 4 3 4 4 21%	5348
	21st to end	Night	24% 17% 5% 12% 6% 4 4 3 5% 5% 14%	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	72% 7% 11% 8% 2	4662
	11th to 20th	Day	73% 7% 7% 7% 5% 1	4781
	21st to end	Day	72% 7% 8% 10% 3	5780
February	1st to 10th	Day	70% 8% 8% 10% 3	1890
	11th to 20th	Day	71% 8% 8% 11% 3	1852
	21st to end	Day	66% 7% 10% 13% 4	1712
March	1st to 10th	Day	60% 6% 10% 16% 7%	4828
	11th to 20th	Day	51% 8% 18% 14% 9% 1	5090
	21st to end	Day	56% 8% 9% 18% 9% 1	5742
April	1st to 10th	Day	56% 6% 10% 16% 10% 1	2520
	11th to 20th	Day	51% 6% 10% 18% 13% 1	2494
	21st to end	Day	52% 7% 9% 17% 14% 2	2620
May	1st to 10th	Day	56% 7% 10% 15% 11% 2	6280
	11th to 20th	Day	48% 9% 11% 20% 11% 2	6462
	21st to end	Day	48% 8% 11% 20% 11% 2	7103
June	1st to 10th	Day	44% 8% 12% 18% 16% 1	2800
	11th to 20th	Day	41% 9% 14% 22% 12% 1	2800
	21st to end	Day	41% 9% 15% 20% 14% 1	2800
July	1st to 10th	Day	38% 7% 14% 27% 14% 1	6290
	11th to 20th	Day	38% 10% 15% 22% 15% 1	6285
	21st to end	Day	37% 8% 13% 28% 14%	6608
August	1st to 10th	Day	43% 8% 13% 20% 15% 1	4221
	11th to 20th	Day	45% 9% 15% 24% 7%	4185
	21st to end	Day	41% 7% 11% 33% 8% 1	4312
September	1st to 10th	Day	38% 9% 13% 29% 11% 1	2400
	11th to 20th	Day	37% 7% 13% 27% 15% 1	2400
	21st to end	Day	47% 8% 12% 21% 11% 1	2104
October	1st to 10th	Day	38% 9% 14% 26% 14%	3260
	11th to 20th	Day	46% 6% 14% 20% 12%	3240
	21st to end	Day	47% 8% 14% 24% 7% 1	3479
November	1st to 10th	Day	52% 8% 11% 17% 9% 2	1851
	11th to 20th	Day	56% 7% 14% 15% 6% 1	1701
	21st to end	Day	61% 9% 14% 14% 2	1680
December	1st to 10th	Day	69% 9% 13% 8% 1	2680
	11th to 20th	Day	67% 6% 12% 9% 6%	3066
	21st to end	Day	73% 6% 12% 7% 2	4048
January	1st to 10th	Night	73% 9% 8% 8% 3	8234
	11th to 20th	Night	70% 9% 7% 9% 5%	8178
	21st to end	Night	66% 7% 7% 11% 8%	8401
February	1st to 10th	Night	64% 8% 10% 13% 5%	2730
	11th to 20th	Night	64% 8% 11% 13% 4 1	2665
	21st to end	Night	60% 9% 12% 12% 7%	2093
March	1st to 10th	Night	56% 7% 13% 16% 7%	5241
	11th to 20th	Night	50% 8% 17% 17% 6% 3	5090
	21st to end	Night	54% 9% 9% 19% 9% 1	5430
April	1st to 10th	Night	56% 7% 10% 17% 8% 1	2100
	11th to 20th	Night	51% 7% 10% 16% 14% 1	2075
	21st to end	Night	46% 7% 10% 21% 13% 3	2017
May	1st to 10th	Night	59% 7% 8% 12% 10% 3	3882
	11th to 20th	Night	44% 10% 14% 19% 12% 2	3656
	21st to end	Night	44% 9% 14% 17% 14% 1	4015
June	1st to 10th	Night	42% 9% 12% 22% 15% 1	1600
	11th to 20th	Night	38% 8% 15% 22% 15% 2	1600
	21st to end	Night	37% 10% 12% 25% 15% 1	1600
July	1st to 10th	Night	38% 7% 15% 24% 15% 2	3560
	11th to 20th	Night	36% 7% 13% 27% 16% 2	3552
	21st to end	Night	34% 7% 13% 26% 19% 1	3912
August	1st to 10th	Night	39% 7% 14% 26% 12% 1	2904
	11th to 20th	Night	41% 8% 15% 27% 9%	2902
	21st to end	Night	32% 8% 13% 35% 12% 1	3581
September	1st to 10th	Night	37% 8% 13% 27% 14% 2	2000
	11th to 20th	Night	35% 8% 13% 28% 16% 1	2000
	21st to end	Night	43% 8% 13% 22% 12% 1	2292
October	1st to 10th	Night	45% 7% 9% 26% 12%	3920
	11th to 20th	Night	46% 7% 12% 21% 13% 1	3899
	21st to end	Night	46% 7% 13% 26% 8% 1	4311
November	1st to 10th	Night	49% 8% 12% 21% 8% 2	2667
	11th to 20th	Night	53% 9% 14% 15% 7% 2	2918
	21st to end	Night	58% 10% 15% 14% 4	2940
December	1st to 10th	Night	68% 9% 13% 9% 1	4720
	11th to 20th	Night	65% 8% 8% 11% 8% 1	5348
	21st to end	Night	71% 9% 8% 9% 3	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	30% 27% 30% 8% 5%	220
	11th to 20th	Day	35% 29% 26% 8% 1	239
	21st to end	Day	28% 28% 31% 10% 2	363
February	1st to 10th	Day	32% 29% 31% 8% 1	330
	11th to 20th	Day	36% 26% 33% 5%	330
	21st to end	Day	35% 23% 30% 11% 1	428
March	1st to 10th	Day	27% 23% 33% 14% 3 1	540
	11th to 20th	Day	34% 22% 29% 11% 3	540
	21st to end	Day	34% 21% 27% 13% 3 1	594
April	1st to 10th	Day	32% 22% 31% 11% 4 1	540
	11th to 20th	Day	31% 24% 24% 13% 5% 2	532
	21st to end	Day	28% 21% 28% 17% 6% 1	540
May	1st to 10th	Day	27% 18% 32% 13% 7% 2	543
	11th to 20th	Day	17% 18% 33% 22% 8% 2	567
	21st to end	Day	19% 14% 33% 21% 11% 3	617
June	1st to 10th	Day	19% 15% 31% 25% 7% 2	530
	11th to 20th	Day	15% 13% 35% 25% 11% 2	530
	21st to end	Day	15% 15% 37% 23% 9% 2	530
July	1st to 10th	Day	11% 9% 28% 30% 18% 3	525
	11th to 20th	Day	12% 9% 27% 29% 18% 4	524
	21st to end	Day	10% 9% 36% 30% 12% 3	576
August	1st to 10th	Day	11% 7% 28% 29% 19% 6%	526
	11th to 20th	Day	14% 7% 31% 24% 15% 8%	519
	21st to end	Day	13% 11% 30% 25% 14% 7%	583
September	1st to 10th	Day	15% 12% 24% 26% 15% 8%	530
	11th to 20th	Day	11% 8% 24% 25% 21% 11%	530
	21st to end	Day	18% 13% 29% 23% 13% 4	530
October	1st to 10th	Day	19% 15% 29% 19% 14% 4	530
	11th to 20th	Day	24% 19% 32% 17% 6% 2	530
	21st to end	Day	29% 20% 31% 17% 3	564
November	1st to 10th	Day	25% 22% 27% 18% 7% 1	425
	11th to 20th	Day	27% 27% 28% 14% 4	241
	21st to end	Day	35% 26% 27% 11% 1	220
December	1st to 10th	Day	30% 22% 34% 10% 3	220
	11th to 20th	Day	37% 25% 26% 10% 2	220
	21st to end	Day	35% 24% 29% 11% 2	242
January	1st to 10th	Night	26% 30% 30% 12% 3	840
	11th to 20th	Night	28% 25% 29% 15% 2	840
	21st to end	Night	22% 24% 35% 16% 3	924
February	1st to 10th	Night	27% 23% 33% 16% 2	840
	11th to 20th	Night	25% 25% 33% 14% 2	823
	21st to end	Night	26% 24% 33% 15% 2	692
March	1st to 10th	Night	26% 21% 33% 17% 3	840
	11th to 20th	Night	28% 20% 30% 18% 3	840
	21st to end	Night	26% 17% 33% 18% 5% 1	924
April	1st to 10th	Night	26% 18% 33% 20% 3	840
	11th to 20th	Night	24% 18% 30% 22% 5% 1	831
	21st to end	Night	22% 14% 30% 25% 8%	757
May	1st to 10th	Night	25% 9% 27% 29% 9% 1	518
	11th to 20th	Night	17% 7% 34% 35% 7% 1	414
	21st to end	Night	12% 8% 34% 30% 15%	455
June	1st to 10th	Night	16% 8% 29% 33% 14% 1	400
	11th to 20th	Night	14% 9% 28% 35% 15%	400
	21st to end	Night	12% 7% 33% 38% 10% 1	400
July	1st to 10th	Night	8% 4 23% 44% 19% 1	390
	11th to 20th	Night	11% 3 21% 43% 21% 1	388
	21st to end	Night	11% 5% 24% 43% 15% 2	481
August	1st to 10th	Night	12% 4 24% 37% 21% 3	597
	11th to 20th	Night	11% 5% 21% 35% 25% 3	594
	21st to end	Night	9% 8% 25% 35% 22% 1	875
September	1st to 10th	Night	15% 8% 24% 31% 21% 2	800
	11th to 20th	Night	13% 8% 19% 30% 26% 5%	800
	21st to end	Night	16% 12% 23% 28% 18% 3	800
October	1st to 10th	Night	23% 10% 27% 21% 18% 2	800
	11th to 20th	Night	23% 16% 30% 23% 6% 1	800
	21st to end	Night	28% 16% 33% 21% 3	880
November	1st to 10th	Night	25% 17% 30% 21% 6%	820
	11th to 20th	Night	23% 21% 32% 16% 6% 2	840
	21st to end	Night	27% 23% 29% 16% 5%	840
December	1st to 10th	Night	29% 23% 33% 13% 3	840
	11th to 20th	Night	28% 24% 30% 15% 3	840
	21st to end	Night	33% 23% 28% 14% 1	924

Page generated at 2026-05-29 01:53:27. Version ac07

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