Telescope Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Telescope Park clear sky chart from 2005-11-06 to 2026-07-16.

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	19% 8% 2 6% 4 4 4 3 5% 7% 39%	136965
	Night	18% 9% 3 6% 4 4 4 3 5% 6% 38%	135714
Transparency	Day	45% 5% 12% 22% 14% 2	140489
	Night	46% 5% 10% 22% 15% 2	140281
Seeing	Day	21% 13% 31% 24% 9% 2	17461
	Night	22% 11% 29% 26% 10% 1	26150

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	13% 7% 3 8% 5% 5% 5% 3 6% 7% 39%	4661
	11th to 20th	Day	16% 11% 3 6% 4 3 3 3 4 5% 42%	4650
	21st to end	Day	11% 10% 2 5% 3 4 4 3 4 5% 49%	5140
February	1st to 10th	Day	17% 9% 2 6% 4 5% 5% 4 5% 6% 38%	1890
	11th to 20th	Day	13% 9% 3 6% 5% 4 5% 4 5% 6% 39%	2022
	21st to end	Day	15% 12% 2 5% 4 5% 4 4 5% 5% 38%	1730
March	1st to 10th	Day	24% 10% 2 7% 4 4 4 3 4 5% 33%	4576
	11th to 20th	Day	20% 11% 3 6% 4 4 4 3 4 6% 37%	5043
	21st to end	Day	29% 6% 3 5% 4 4 3 3 5% 5% 33%	5649
April	1st to 10th	Day	21% 10% 2 5% 4 4 4 3 5% 6% 35%	2520
	11th to 20th	Day	20% 7% 2 6% 4 3 4 3 5% 7% 40%	2626
	21st to end	Day	23% 7% 3 6% 4 4 4 3 5% 6% 36%	2730
May	1st to 10th	Day	29% 10% 3 6% 4 4 3 3 5% 6% 27%	7639
	11th to 20th	Day	18% 8% 3 7% 5% 5% 4 4 6% 7% 34%	7625
	21st to end	Day	22% 8% 2 6% 4 4 4 4 5% 7% 33%	9055
June	1st to 10th	Day	21% 7% 3 6% 5% 3 3 4 6% 8% 34%	3003
	11th to 20th	Day	18% 7% 2 5% 4 4 4 4 5% 7% 40%	3150
	21st to end	Day	22% 6% 2 6% 5% 4 5% 3 5% 8% 34%	2975
July	1st to 10th	Day	19% 8% 3 7% 4 4 4 4 7% 9% 32%	6500
	11th to 20th	Day	16% 9% 2 5% 3 4 4 3 6% 9% 38%	6430
	21st to end	Day	16% 5% 2 5% 4 3 4 4 5% 8% 44%	6881
August	1st to 10th	Day	16% 7% 2 7% 4 4 4 4 6% 8% 38%	4235
	11th to 20th	Day	17% 8% 3 7% 4 4 4 4 6% 8% 37%	4175
	21st to end	Day	8% 4 2 5% 4 4 5% 4 6% 9% 49%	4543
September	1st to 10th	Day	13% 5% 1 5% 3 3 4 4 5% 7% 49%	2330
	11th to 20th	Day	11% 6% 2 5% 3 2 3 2 4 6% 55%	2200
	21st to end	Day	17% 6% 2 5% 4 4 3 3 4 6% 46%	2200
October	1st to 10th	Day	18% 6% 2 6% 4 4 4 3 4 5% 43%	2188
	11th to 20th	Day	19% 6% 2 6% 4 4 4 3 3 6% 44%	1950
	21st to end	Day	22% 7% 2 4 4 3 4 3 5% 5% 41%	1980
November	1st to 10th	Day	17% 5% 2 5% 4 4 3 4 5% 5% 47%	1851
	11th to 20th	Day	19% 5% 2 5% 4 2 3 3 5% 6% 46%	1890
	21st to end	Day	22% 7% 3 5% 3 3 2 3 4 5% 42%	1801
December	1st to 10th	Day	16% 9% 2 5% 3 3 3 3 5% 7% 44%	2462
	11th to 20th	Day	19% 7% 2 4 2 3 3 3 3 5% 48%	2688
	21st to end	Day	21% 10% 3 7% 3 4 4 3 4 5% 35%	3977
January	1st to 10th	Night	13% 9% 4 7% 4 4 4 3 5% 7% 42%	8234
	11th to 20th	Night	15% 12% 4 9% 5% 5% 4 4 5% 6% 32%	8235
	21st to end	Night	13% 10% 3 7% 4 4 4 3 5% 5% 43%	9039
February	1st to 10th	Night	16% 9% 3 6% 5% 5% 5% 4 5% 5% 37%	2730
	11th to 20th	Night	13% 9% 3 7% 5% 4 4 4 5% 6% 40%	2483
	21st to end	Night	14% 10% 3 6% 5% 4 3 3 5% 5% 42%	2076
March	1st to 10th	Night	27% 10% 2 7% 4 4 3 3 4 5% 32%	5484
	11th to 20th	Night	16% 10% 4 7% 5% 4 5% 4 6% 7% 33%	5136
	21st to end	Night	25% 9% 2 6% 4 4 5% 4 6% 6% 31%	5509
April	1st to 10th	Night	23% 9% 2 6% 5% 3 5% 3 4 6% 35%	2100
	11th to 20th	Night	20% 7% 2 6% 4 5% 4 3 6% 6% 37%	1941
	21st to end	Night	23% 8% 2 6% 4 4 4 4 4 6% 35%	1890
May	1st to 10th	Night	30% 9% 3 6% 4 3 3 3 5% 6% 28%	5259
	11th to 20th	Night	20% 9% 3 6% 5% 4 5% 4 6% 7% 31%	5235
	21st to end	Night	18% 9% 3 7% 5% 4 4 4 6% 8% 32%	5175
June	1st to 10th	Night	21% 8% 2 6% 4 4 5% 3 5% 7% 33%	1607
	11th to 20th	Night	16% 7% 3 6% 4 4 4 4 5% 7% 39%	1470
	21st to end	Night	24% 9% 3 5% 4 4 4 4 5% 7% 31%	1631
July	1st to 10th	Night	15% 9% 4 7% 5% 4 5% 4 6% 8% 33%	3680
	11th to 20th	Night	15% 11% 3 6% 4 4 5% 3 6% 8% 34%	3640
	21st to end	Night	11% 6% 2 5% 4 4 5% 4 7% 9% 43%	3926
August	1st to 10th	Night	21% 9% 3 7% 5% 5% 4 4 5% 7% 31%	2912
	11th to 20th	Night	16% 10% 3 7% 4 5% 6% 4 6% 7% 33%	2872
	21st to end	Night	10% 5% 2 5% 4 4 4 4 5% 8% 47%	3351
September	1st to 10th	Night	13% 6% 2 4 4 4 4 3 5% 6% 48%	2070
	11th to 20th	Night	14% 7% 2 4 2 3 4 2 4 6% 52%	2200
	21st to end	Night	16% 5% 2 6% 3 3 4 3 5% 6% 46%	2200
October	1st to 10th	Night	18% 7% 3 5% 4 4 3 3 6% 6% 42%	2211
	11th to 20th	Night	17% 6% 2 5% 4 4 4 3 5% 6% 43%	2449
	21st to end	Night	23% 7% 2 5% 3 4 3 3 4 5% 39%	2860
November	1st to 10th	Night	18% 6% 2 5% 4 4 4 3 4 5% 46%	2667
	11th to 20th	Night	17% 6% 2 5% 4 4 4 4 5% 6% 44%	2730
	21st to end	Night	17% 7% 2 5% 5% 4 4 4 5% 6% 42%	2814
December	1st to 10th	Night	18% 10% 2 5% 4 4 3 3 4 7% 40%	4942
	11th to 20th	Night	23% 6% 2 4 3 3 3 3 4 4 43%	5741
	21st to end	Night	22% 11% 3 6% 3 3 3 2 3 5% 37%	7215

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	45% 4 12% 23% 15% 1	4662
	11th to 20th	Day	45% 3 10% 24% 15% 3	4680
	21st to end	Day	39% 3 10% 23% 21% 5%	5140
February	1st to 10th	Day	46% 5% 10% 21% 14% 3	1890
	11th to 20th	Day	44% 5% 12% 20% 15% 3	2031
	21st to end	Day	47% 4 10% 21% 14% 4	1730
March	1st to 10th	Day	54% 3 9% 12% 14% 8%	4579
	11th to 20th	Day	49% 4 12% 20% 12% 2	5043
	21st to end	Day	53% 4 5% 17% 17% 5%	5649
April	1st to 10th	Day	50% 4 9% 18% 16% 4	2520
	11th to 20th	Day	45% 4 10% 23% 15% 3	2626
	21st to end	Day	50% 4 7% 20% 16% 4	2730
May	1st to 10th	Day	58% 7% 11% 16% 7% 1	7651
	11th to 20th	Day	48% 7% 15% 19% 10% 2	7629
	21st to end	Day	49% 7% 11% 21% 12% 1	9056
June	1st to 10th	Day	48% 6% 13% 23% 9%	3003
	11th to 20th	Day	43% 7% 11% 21% 15% 2	3150
	21st to end	Day	48% 8% 11% 22% 10% 1	2975
July	1st to 10th	Day	48% 6% 12% 22% 10%	6430
	11th to 20th	Day	43% 6% 14% 23% 13% 1	6355
	21st to end	Day	38% 6% 14% 27% 15% 1	6684
August	1st to 10th	Day	44% 7% 15% 26% 8%	4222
	11th to 20th	Day	44% 6% 18% 22% 8%	4214
	21st to end	Day	31% 7% 15% 27% 20% 1	4543
September	1st to 10th	Day	33% 6% 13% 28% 18% 1	2330
	11th to 20th	Day	31% 7% 11% 28% 22% 2	2200
	21st to end	Day	41% 6% 8% 27% 18% 1	2200
October	1st to 10th	Day	47% 5% 10% 23% 14% 2	3565
	11th to 20th	Day	34% 3 11% 30% 22% 1	3153
	21st to end	Day	35% 3 12% 27% 20% 3	3180
November	1st to 10th	Day	40% 4 12% 24% 17% 2	1851
	11th to 20th	Day	39% 4 10% 23% 20% 4	1890
	21st to end	Day	45% 3 15% 23% 12% 1	1801
December	1st to 10th	Day	42% 3 14% 28% 13% 1	2462
	11th to 20th	Day	40% 3 12% 19% 17% 9%	2688
	21st to end	Day	51% 5% 8% 18% 14% 4	3977
January	1st to 10th	Night	43% 3 11% 28% 14% 1	8234
	11th to 20th	Night	53% 4 11% 17% 13% 2	8280
	21st to end	Night	44% 3 7% 22% 19% 6%	9039
February	1st to 10th	Night	48% 4 10% 20% 14% 3	2730
	11th to 20th	Night	46% 4 9% 21% 17% 3	2485
	21st to end	Night	45% 4 9% 23% 15% 4	2076
March	1st to 10th	Night	57% 2 7% 13% 14% 8%	5494
	11th to 20th	Night	49% 5% 15% 20% 9% 2	5137
	21st to end	Night	53% 4 5% 15% 19% 5%	5509
April	1st to 10th	Night	52% 3 7% 18% 16% 4	2100
	11th to 20th	Night	47% 4 9% 21% 17% 2	1941
	21st to end	Night	50% 4 9% 18% 15% 4	1890
May	1st to 10th	Night	56% 6% 11% 14% 11% 1	5265
	11th to 20th	Night	50% 7% 11% 23% 8% 1	5236
	21st to end	Night	48% 7% 11% 23% 10% 1	5176
June	1st to 10th	Night	51% 6% 10% 23% 9% 1	1607
	11th to 20th	Night	44% 7% 14% 20% 15% 1	1470
	21st to end	Night	52% 5% 12% 20% 10% 1	1631
July	1st to 10th	Night	49% 6% 12% 21% 12%	3640
	11th to 20th	Night	48% 5% 12% 22% 12%	3592
	21st to end	Night	36% 6% 17% 28% 11% 1	3809
August	1st to 10th	Night	53% 7% 12% 20% 7% 1	2901
	11th to 20th	Night	49% 6% 13% 22% 10%	2899
	21st to end	Night	33% 6% 14% 27% 19% 1	3351
September	1st to 10th	Night	36% 6% 11% 29% 18% 1	2070
	11th to 20th	Night	35% 4 10% 28% 21% 2	2200
	21st to end	Night	39% 5% 9% 29% 16% 2	2200
October	1st to 10th	Night	42% 4 13% 24% 16% 1	3614
	11th to 20th	Night	32% 3 10% 29% 25% 1	3985
	21st to end	Night	38% 3 10% 28% 18% 3	4610
November	1st to 10th	Night	41% 5% 10% 26% 16% 2	2667
	11th to 20th	Night	42% 5% 10% 23% 18% 3	2730
	21st to end	Night	43% 5% 15% 21% 14% 2	2814
December	1st to 10th	Night	45% 3 9% 30% 12%	4942
	11th to 20th	Night	45% 4 11% 19% 18% 3	5742
	21st to end	Night	51% 4 9% 17% 17% 1	7215

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	23% 24% 32% 17% 5%	220
	11th to 20th	Day	26% 21% 29% 20% 3 1	220
	21st to end	Day	21% 22% 32% 20% 5%	245
February	1st to 10th	Day	24% 18% 33% 21% 4	430
	11th to 20th	Day	21% 22% 40% 15% 2	522
	21st to end	Day	20% 19% 37% 18% 5%	446
March	1st to 10th	Day	25% 17% 38% 18% 3	540
	11th to 20th	Day	21% 22% 37% 17% 4	540
	21st to end	Day	26% 20% 33% 13% 6% 2	594
April	1st to 10th	Day	29% 19% 32% 14% 5% 1	540
	11th to 20th	Day	27% 14% 36% 16% 6% 1	532
	21st to end	Day	27% 12% 33% 22% 4 1	540
May	1st to 10th	Day	30% 14% 31% 19% 6% 1	543
	11th to 20th	Day	19% 13% 37% 21% 8% 2	567
	21st to end	Day	21% 12% 34% 26% 6% 1	617
June	1st to 10th	Day	20% 7% 32% 29% 10% 3	633
	11th to 20th	Day	20% 9% 30% 26% 12% 2	780
	21st to end	Day	22% 7% 30% 29% 10% 2	605
July	1st to 10th	Day	20% 6% 28% 33% 12% 2	565
	11th to 20th	Day	17% 6% 29% 31% 15% 3	544
	21st to end	Day	15% 5% 27% 36% 14% 3	576
August	1st to 10th	Day	13% 4 26% 36% 18% 3	526
	11th to 20th	Day	16% 5% 26% 33% 15% 6%	519
	21st to end	Day	11% 7% 27% 34% 16% 4	583
September	1st to 10th	Day	16% 6% 27% 34% 15% 3	530
	11th to 20th	Day	15% 8% 27% 28% 15% 7%	530
	21st to end	Day	19% 8% 32% 22% 14% 5%	530
October	1st to 10th	Day	20% 12% 22% 28% 14% 4	522
	11th to 20th	Day	21% 14% 27% 25% 10% 3	420
	21st to end	Day	28% 13% 28% 22% 8% 1	462
November	1st to 10th	Day	21% 17% 30% 23% 8% 2	425
	11th to 20th	Day	23% 17% 33% 20% 7% 1	430
	21st to end	Day	25% 20% 29% 19% 6% 1	430
December	1st to 10th	Day	25% 14% 38% 16% 7% 1	293
	11th to 20th	Day	27% 22% 32% 14% 4	220
	21st to end	Day	29% 19% 31% 15% 5% 1	242
January	1st to 10th	Night	21% 20% 36% 19% 4	840
	11th to 20th	Night	26% 20% 33% 17% 4	840
	21st to end	Night	19% 23% 35% 19% 4 1	924
February	1st to 10th	Night	22% 19% 34% 20% 4	840
	11th to 20th	Night	20% 21% 37% 19% 3	823
	21st to end	Night	21% 15% 39% 20% 5%	692
March	1st to 10th	Night	23% 18% 34% 21% 3	840
	11th to 20th	Night	23% 14% 33% 22% 8%	840
	21st to end	Night	24% 13% 34% 22% 5% 1	924
April	1st to 10th	Night	28% 14% 32% 20% 5% 1	840
	11th to 20th	Night	24% 9% 33% 27% 7% 1	697
	21st to end	Night	28% 7% 27% 28% 8% 1	630
May	1st to 10th	Night	27% 4 28% 29% 10% 2	630
	11th to 20th	Night	26% 4 30% 28% 11% 1	622
	21st to end	Night	27% 3 24% 33% 13% 1	477
June	1st to 10th	Night	24% 5% 20% 35% 14% 2	420
	11th to 20th	Night	20% 6% 20% 38% 15% 2	420
	21st to end	Night	25% 4 17% 40% 13%	420
July	1st to 10th	Night	21% 2 13% 44% 19% 1	410
	11th to 20th	Night	21% 3 15% 39% 21% 1	398
	21st to end	Night	17% 3 16% 41% 21% 2	438
August	1st to 10th	Night	17% 3 17% 43% 20% 1	595
	11th to 20th	Night	15% 4 17% 40% 23% 3	592
	21st to end	Night	15% 3 18% 37% 24% 3	736
September	1st to 10th	Night	15% 3 22% 36% 22% 2	800
	11th to 20th	Night	18% 5% 23% 28% 23% 4	800
	21st to end	Night	18% 6% 23% 32% 19% 2	800
October	1st to 10th	Night	21% 8% 23% 29% 19% 2	800
	11th to 20th	Night	20% 10% 31% 25% 12% 3	800
	21st to end	Night	28% 11% 30% 25% 6%	880
November	1st to 10th	Night	22% 10% 33% 25% 8% 1	820
	11th to 20th	Night	22% 16% 33% 22% 6% 1	840
	21st to end	Night	20% 19% 34% 20% 6%	884
December	1st to 10th	Night	27% 13% 37% 17% 6%	950
	11th to 20th	Night	25% 19% 34% 18% 3	950
	21st to end	Night	29% 16% 33% 18% 4	938

Page generated at 2026-07-13 03:14:47. Version ac07

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