Treesdale Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Treesdale Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	16% 11% 4 9% 5% 5% 4 4 5% 7% 30%	127257
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 31%	124750
Transparency	Day	50% 7% 11% 20% 10% 1	130861
	Night	52% 6% 11% 19% 11% 2	129215
Seeing	Day	22% 15% 27% 20% 11% 4	16214
	Night	21% 12% 27% 27% 11% 1	25998

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	16% 19% 9% 22% 5% 3 3 2 3 4 14%	4117
	11th to 20th	Day	12% 21% 10% 19% 9% 6% 2 2 2 2 14%	4150
	21st to end	Day	21% 18% 7% 13% 8% 5% 4 2 4 5% 14%	4565
February	1st to 10th	Day	20% 18% 6% 11% 7% 5% 3 3 3 4 22%	1800
	11th to 20th	Day	17% 15% 6% 13% 6% 5% 5% 4 4 5% 21%	1753
	21st to end	Day	20% 15% 5% 10% 5% 4 3 3 4 6% 24%	1649
March	1st to 10th	Day	17% 11% 6% 10% 5% 4 4 3 5% 6% 31%	4935
	11th to 20th	Day	12% 10% 5% 9% 5% 5% 5% 3 4 6% 36%	4980
	21st to end	Day	17% 11% 4 10% 5% 5% 4 4 5% 6% 30%	5465
April	1st to 10th	Day	17% 12% 4 8% 5% 4 4 4 6% 6% 29%	2308
	11th to 20th	Day	17% 11% 3 8% 5% 6% 4 3 5% 6% 32%	2411
	21st to end	Day	21% 10% 3 7% 5% 4 4 4 4 7% 33%	2600
May	1st to 10th	Day	21% 9% 3 8% 5% 5% 5% 4 6% 6% 27%	5862
	11th to 20th	Day	18% 8% 3 9% 5% 4 4 5% 6% 8% 30%	5861
	21st to end	Day	19% 7% 3 7% 4 5% 5% 4 6% 8% 32%	6834
June	1st to 10th	Day	15% 7% 3 6% 5% 5% 5% 5% 7% 9% 34%	2800
	11th to 20th	Day	14% 7% 3 7% 6% 4 5% 4 7% 9% 34%	2800
	21st to end	Day	13% 7% 2 6% 5% 4 5% 4 6% 9% 37%	2800
July	1st to 10th	Day	9% 7% 2 7% 5% 5% 5% 4 5% 9% 42%	6189
	11th to 20th	Day	11% 8% 3 7% 6% 6% 5% 5% 7% 10% 34%	5900
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 5% 6% 8% 39%	6424
August	1st to 10th	Day	13% 8% 3 8% 6% 6% 6% 4 7% 8% 31%	4234
	11th to 20th	Day	12% 6% 3 7% 5% 5% 6% 5% 7% 9% 36%	4174
	21st to end	Day	8% 5% 3 6% 5% 6% 6% 5% 7% 10% 40%	4454
September	1st to 10th	Day	15% 7% 2 5% 4 4 3 3 5% 8% 44%	2400
	11th to 20th	Day	11% 6% 2 5% 4 5% 4 4 6% 9% 45%	2209
	21st to end	Day	19% 8% 3 7% 5% 5% 5% 3 4 6% 35%	2196
October	1st to 10th	Day	15% 6% 2 6% 3 4 5% 4 6% 8% 41%	2000
	11th to 20th	Day	18% 8% 3 7% 5% 5% 5% 4 5% 5% 35%	2000
	21st to end	Day	23% 10% 3 6% 4 3 4 3 6% 7% 31%	2200
November	1st to 10th	Day	11% 9% 4 7% 5% 4 5% 4 4 5% 42%	1968
	11th to 20th	Day	17% 15% 4 8% 5% 3 4 3 5% 5% 30%	1741
	21st to end	Day	18% 16% 7% 8% 7% 4 3 4 5% 5% 24%	1680
December	1st to 10th	Day	19% 15% 7% 18% 6% 3 4 3 3 4 18%	2680
	11th to 20th	Day	24% 18% 7% 10% 4 3 3 3 3 5% 21%	3070
	21st to end	Day	23% 23% 6% 12% 5% 4 4 3 5% 5% 13%	4048
January	1st to 10th	Night	14% 18% 8% 23% 6% 4 4 3 3 3 14%	6042
	11th to 20th	Night	14% 17% 8% 17% 10% 6% 4 2 3 4 14%	6030
	21st to end	Night	22% 15% 6% 13% 7% 5% 4 2 3 3 19%	6633
February	1st to 10th	Night	21% 14% 5% 11% 7% 5% 4 3 4 4 24%	2601
	11th to 20th	Night	20% 13% 5% 12% 5% 5% 5% 4 4 5% 24%	2533
	21st to end	Night	20% 12% 5% 12% 5% 4 4 4 5% 4 25%	1979
March	1st to 10th	Night	21% 12% 5% 8% 5% 3 3 3 4 5% 31%	4918
	11th to 20th	Night	14% 10% 4 8% 3 3 4 3 4 7% 39%	4980
	21st to end	Night	18% 10% 3 8% 5% 5% 4 3 4 5% 35%	5467
April	1st to 10th	Night	19% 13% 3 7% 6% 3 4 2 5% 6% 32%	2090
	11th to 20th	Night	17% 8% 2 7% 5% 5% 4 4 5% 7% 36%	1954
	21st to end	Night	15% 9% 3 8% 4 4 3 4 5% 8% 36%	1800
May	1st to 10th	Night	26% 11% 3 7% 4 4 3 3 5% 6% 25%	4033
	11th to 20th	Night	17% 9% 3 7% 5% 4 3 3 5% 7% 36%	4032
	21st to end	Night	19% 7% 2 6% 3 4 4 4 6% 8% 35%	4015
June	1st to 10th	Night	10% 6% 3 5% 6% 3 4 4 7% 8% 44%	1600
	11th to 20th	Night	12% 6% 2 7% 5% 4 5% 5% 6% 7% 41%	1600
	21st to end	Night	10% 5% 2 6% 5% 5% 5% 5% 6% 9% 43%	1600
July	1st to 10th	Night	11% 8% 3 7% 5% 4 4 4 5% 8% 42%	3750
	11th to 20th	Night	12% 6% 2 7% 5% 5% 5% 4 7% 9% 37%	4057
	21st to end	Night	9% 7% 3 7% 5% 4 4 4 6% 8% 43%	4427
August	1st to 10th	Night	12% 8% 3 8% 5% 5% 4 4 7% 7% 36%	2915
	11th to 20th	Night	13% 8% 3 6% 5% 4 4 3 5% 7% 42%	2875
	21st to end	Night	9% 5% 2 6% 4 5% 5% 5% 6% 9% 44%	3411
September	1st to 10th	Night	12% 6% 2 5% 4 3 4 4 5% 7% 49%	2000
	11th to 20th	Night	10% 4 2 4 4 4 4 3 5% 9% 51%	2188
	21st to end	Night	17% 8% 3 7% 4 4 4 4 5% 5% 39%	2202
October	1st to 10th	Night	14% 6% 3 5% 4 4 4 4 4 6% 46%	2400
	11th to 20th	Night	18% 6% 2 7% 4 4 4 4 4 6% 41%	2400
	21st to end	Night	22% 9% 3 6% 4 4 3 3 5% 6% 36%	2640
November	1st to 10th	Night	13% 9% 2 6% 4 3 4 3 6% 6% 43%	2546
	11th to 20th	Night	19% 11% 3 8% 5% 5% 4 3 6% 6% 30%	2877
	21st to end	Night	20% 15% 4 8% 4 4 3 4 5% 5% 27%	2940
December	1st to 10th	Night	18% 15% 9% 15% 5% 5% 4 3 4 5% 18%	4720
	11th to 20th	Night	22% 16% 8% 10% 5% 4 3 3 4 5% 22%	5345
	21st to end	Night	23% 18% 6% 11% 5% 5% 4 3 4 5% 16%	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	74% 5% 8% 8% 4	4118
	11th to 20th	Day	77% 4 7% 8% 3 2	4150
	21st to end	Day	73% 5% 8% 10% 4	4565
February	1st to 10th	Day	68% 4 8% 14% 6% 1	1800
	11th to 20th	Day	65% 6% 10% 13% 6%	1762
	21st to end	Day	61% 5% 12% 14% 6% 1	1649
March	1st to 10th	Day	52% 6% 12% 21% 8% 1	4935
	11th to 20th	Day	47% 7% 13% 23% 8% 2	4980
	21st to end	Day	52% 7% 9% 19% 12% 1	5465
April	1st to 10th	Day	53% 7% 12% 16% 11% 2	2308
	11th to 20th	Day	51% 7% 11% 17% 13% 2	2411
	21st to end	Day	49% 6% 11% 21% 10% 1	2600
May	1st to 10th	Day	54% 9% 11% 16% 8% 3	5897
	11th to 20th	Day	49% 8% 12% 21% 8% 1	5865
	21st to end	Day	45% 10% 14% 21% 9% 2	6849
June	1st to 10th	Day	44% 10% 14% 21% 11% 1	2800
	11th to 20th	Day	44% 9% 14% 20% 12% 1	2800
	21st to end	Day	41% 9% 13% 25% 12% 1	2800
July	1st to 10th	Day	39% 7% 11% 26% 15% 2	6122
	11th to 20th	Day	42% 8% 14% 24% 12%	5831
	21st to end	Day	40% 7% 11% 23% 19% 1	6240
August	1st to 10th	Day	48% 9% 14% 20% 9%	4221
	11th to 20th	Day	41% 8% 12% 27% 11% 1	4213
	21st to end	Day	36% 9% 14% 28% 13% 2	4454
September	1st to 10th	Day	38% 8% 10% 22% 19% 3	2400
	11th to 20th	Day	35% 8% 15% 26% 16% 1	2209
	21st to end	Day	49% 6% 11% 20% 13%	2196
October	1st to 10th	Day	31% 6% 12% 30% 20% 1	3260
	11th to 20th	Day	43% 6% 10% 22% 14% 4	3240
	21st to end	Day	38% 5% 13% 27% 17% 1	3538
November	1st to 10th	Day	43% 6% 11% 21% 17% 2	1968
	11th to 20th	Day	55% 5% 11% 18% 9% 2	1741
	21st to end	Day	61% 5% 12% 15% 6% 1	1680
December	1st to 10th	Day	71% 4 11% 10% 4	2680
	11th to 20th	Day	67% 4 11% 13% 4	3066
	21st to end	Day	72% 6% 9% 10% 4	4048
January	1st to 10th	Night	73% 6% 7% 10% 4	6042
	11th to 20th	Night	73% 6% 9% 9% 3	6030
	21st to end	Night	70% 5% 8% 11% 5% 1	6633
February	1st to 10th	Night	63% 6% 9% 16% 6%	2601
	11th to 20th	Night	61% 5% 11% 15% 7%	2535
	21st to end	Night	61% 6% 8% 15% 9% 1	1979
March	1st to 10th	Night	56% 5% 10% 21% 8%	4918
	11th to 20th	Night	43% 6% 14% 26% 8% 3	4980
	21st to end	Night	49% 6% 11% 17% 15% 1	5467
April	1st to 10th	Night	52% 6% 9% 17% 13% 2	2090
	11th to 20th	Night	46% 8% 12% 17% 15% 3	1954
	21st to end	Night	44% 7% 12% 23% 12% 3	1800
May	1st to 10th	Night	58% 6% 10% 14% 9% 3	4060
	11th to 20th	Night	46% 8% 11% 23% 12% 1	4033
	21st to end	Night	42% 9% 13% 22% 11% 3	4022
June	1st to 10th	Night	36% 9% 14% 23% 16% 2	1600
	11th to 20th	Night	38% 10% 14% 22% 15% 1	1600
	21st to end	Night	36% 10% 13% 23% 17% 2	1600
July	1st to 10th	Night	41% 7% 9% 24% 18% 2	3709
	11th to 20th	Night	39% 9% 18% 22% 12% 1	4006
	21st to end	Night	37% 7% 10% 19% 24% 2	4297
August	1st to 10th	Night	45% 9% 13% 21% 11% 1	2904
	11th to 20th	Night	40% 7% 10% 28% 14% 1	2902
	21st to end	Night	34% 10% 13% 29% 13% 2	3411
September	1st to 10th	Night	32% 7% 14% 23% 19% 5%	2000
	11th to 20th	Night	29% 7% 14% 27% 22% 1	2188
	21st to end	Night	45% 6% 11% 20% 16% 1	2202
October	1st to 10th	Night	35% 5% 9% 25% 24% 1	3920
	11th to 20th	Night	38% 6% 11% 23% 15% 7%	3899
	21st to end	Night	35% 5% 10% 28% 21% 2	4252
November	1st to 10th	Night	40% 7% 11% 22% 17% 3	2546
	11th to 20th	Night	53% 5% 11% 19% 10% 2	2877
	21st to end	Night	57% 6% 11% 19% 6% 1	2940
December	1st to 10th	Night	68% 5% 11% 12% 3	4720
	11th to 20th	Night	66% 4 13% 14% 3	5348
	21st to end	Night	69% 7% 11% 12% 2	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	31% 26% 28% 11% 3 1	322
	11th to 20th	Day	35% 29% 26% 6% 2	330
	21st to end	Day	29% 29% 31% 7% 3	363
February	1st to 10th	Day	34% 24% 34% 7% 1	300
	11th to 20th	Day	36% 28% 27% 8% 1	300
	21st to end	Day	39% 22% 30% 9%	279
March	1st to 10th	Day	28% 22% 30% 16% 4	500
	11th to 20th	Day	32% 20% 31% 12% 5% 1	500
	21st to end	Day	31% 22% 28% 15% 4 1	550
April	1st to 10th	Day	31% 21% 26% 15% 7% 1	500
	11th to 20th	Day	25% 21% 27% 16% 8% 3	492
	21st to end	Day	29% 17% 30% 17% 5% 2	500
May	1st to 10th	Day	28% 18% 26% 17% 7% 3	503
	11th to 20th	Day	23% 12% 32% 19% 10% 4	527
	21st to end	Day	16% 11% 26% 26% 16% 6%	573
June	1st to 10th	Day	15% 11% 30% 27% 13% 4	530
	11th to 20th	Day	18% 12% 26% 25% 14% 4	530
	21st to end	Day	13% 10% 31% 27% 15% 4	530
July	1st to 10th	Day	12% 7% 21% 28% 22% 10%	525
	11th to 20th	Day	13% 6% 25% 29% 18% 9%	524
	21st to end	Day	13% 5% 26% 31% 19% 7%	576
August	1st to 10th	Day	10% 8% 24% 30% 20% 9%	526
	11th to 20th	Day	15% 5% 23% 30% 17% 10%	519
	21st to end	Day	11% 5% 26% 27% 21% 11%	583
September	1st to 10th	Day	16% 8% 22% 26% 20% 8%	530
	11th to 20th	Day	11% 4 19% 33% 22% 11%	530
	21st to end	Day	25% 11% 22% 23% 13% 5%	530
October	1st to 10th	Day	16% 10% 25% 25% 19% 5%	530
	11th to 20th	Day	21% 16% 31% 22% 8% 2	530
	21st to end	Day	29% 16% 30% 17% 7%	583
November	1st to 10th	Day	20% 19% 33% 18% 9% 2	446
	11th to 20th	Day	30% 20% 31% 13% 4 2	251
	21st to end	Day	28% 22% 39% 9% 2	220
December	1st to 10th	Day	33% 28% 25% 10% 3 1	220
	11th to 20th	Day	35% 20% 33% 10% 3	220
	21st to end	Day	38% 17% 30% 12% 2	242
January	1st to 10th	Night	27% 21% 34% 15% 2	840
	11th to 20th	Night	31% 18% 32% 16% 3	840
	21st to end	Night	27% 20% 33% 18% 3	924
February	1st to 10th	Night	28% 17% 28% 24% 2	801
	11th to 20th	Night	31% 20% 33% 15% 1	783
	21st to end	Night	27% 16% 37% 17% 2	660
March	1st to 10th	Night	25% 18% 32% 21% 3	800
	11th to 20th	Night	26% 18% 28% 22% 5% 1	800
	21st to end	Night	23% 16% 34% 21% 5% 1	880
April	1st to 10th	Night	26% 16% 29% 23% 5%	800
	11th to 20th	Night	21% 14% 32% 23% 9% 1	770
	21st to end	Night	19% 10% 35% 31% 6%	600
May	1st to 10th	Night	23% 10% 27% 32% 8% 1	600
	11th to 20th	Night	21% 8% 27% 32% 12%	591
	21st to end	Night	13% 4 25% 34% 21% 3	477
June	1st to 10th	Night	12% 5% 25% 40% 18% 1	400
	11th to 20th	Night	12% 6% 24% 39% 19% 1	400
	21st to end	Night	11% 3 29% 35% 21% 2	400
July	1st to 10th	Night	10% 2 20% 42% 22% 5%	448
	11th to 20th	Night	10% 5% 18% 34% 29% 5%	582
	21st to end	Night	11% 4 14% 42% 28% 2	657
August	1st to 10th	Night	10% 2 16% 44% 27% 2	597
	11th to 20th	Night	12% 5% 16% 34% 30% 3	594
	21st to end	Night	12% 3 19% 35% 27% 4	770
September	1st to 10th	Night	15% 4 20% 33% 25% 4	800
	11th to 20th	Night	11% 4 16% 37% 30% 4	800
	21st to end	Night	18% 9% 24% 26% 19% 4	800
October	1st to 10th	Night	17% 8% 23% 29% 23% 2	800
	11th to 20th	Night	21% 9% 32% 27% 10% 1	800
	21st to end	Night	27% 12% 27% 27% 7%	880
November	1st to 10th	Night	18% 13% 28% 30% 11%	820
	11th to 20th	Night	26% 15% 28% 23% 6% 1	840
	21st to end	Night	28% 16% 30% 20% 6%	840
December	1st to 10th	Night	30% 17% 29% 20% 4	840
	11th to 20th	Night	28% 19% 30% 20% 3	840
	21st to end	Night	28% 17% 32% 21% 2	924

Page generated at 2026-01-29 01:21:43. Version ac07

Copyright © 2026 Attilla Danko.