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 2025-03-12.

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	18% 7% 2 6% 4 4 4 3 5% 7% 39%	108051
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 38%	110950
Transparency	Day	44% 6% 12% 22% 14% 2	111621
	Night	46% 5% 11% 22% 15% 2	115546
Seeing	Day	21% 12% 31% 24% 9% 2	15652
	Night	22% 11% 29% 26% 10% 1	24518

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% 7% 3 7% 6% 5% 5% 3 6% 7% 39%	3597
	11th to 20th	Day	15% 7% 3 6% 4 3 3 3 4 6% 45%	3570
	21st to end	Day	13% 11% 3 6% 3 3 4 3 4 4 47%	3976
February	1st to 10th	Day	18% 9% 2 6% 4 5% 5% 4 5% 6% 37%	1800
	11th to 20th	Day	13% 9% 3 7% 5% 4 5% 4 5% 6% 39%	1923
	21st to end	Day	15% 12% 2 6% 4 5% 4 4 5% 6% 37%	1650
March	1st to 10th	Day	26% 12% 2 6% 4 4 4 3 4 5% 30%	3216
	11th to 20th	Day	18% 7% 3 6% 5% 4 4 3 4 6% 40%	3458
	21st to end	Day	27% 7% 3 5% 4 4 3 4 5% 6% 34%	3863
April	1st to 10th	Day	21% 10% 2 6% 4 4 4 3 5% 6% 35%	2280
	11th to 20th	Day	19% 6% 2 6% 3 3 4 3 5% 7% 41%	2373
	21st to end	Day	22% 7% 3 7% 4 4 4 3 5% 6% 36%	2470
May	1st to 10th	Day	23% 9% 3 7% 5% 4 4 4 6% 6% 29%	4106
	11th to 20th	Day	20% 8% 3 7% 4 4 4 3 5% 7% 35%	4081
	21st to end	Day	18% 7% 2 5% 4 4 4 4 6% 8% 38%	4832
June	1st to 10th	Day	22% 7% 3 6% 5% 3 3 4 6% 8% 33%	2717
	11th to 20th	Day	18% 7% 2 5% 4 4 4 4 5% 7% 40%	2850
	21st to end	Day	23% 7% 2 7% 4 4 5% 3 5% 8% 33%	2692
July	1st to 10th	Day	16% 7% 2 6% 5% 4 4 4 7% 8% 35%	4440
	11th to 20th	Day	14% 6% 2 5% 3 4 5% 4 7% 10% 40%	4440
	21st to end	Day	21% 5% 2 5% 4 3 4 4 5% 8% 37%	4783
August	1st to 10th	Day	17% 8% 2 7% 4 4 4 4 6% 8% 37%	4105
	11th to 20th	Day	17% 8% 3 7% 4 4 4 4 5% 8% 36%	4045
	21st to end	Day	8% 4 2 5% 4 4 5% 4 6% 9% 49%	4404
September	1st to 10th	Day	12% 5% 1 5% 4 3 4 4 5% 7% 49%	2214
	11th to 20th	Day	12% 6% 2 5% 3 2 3 2 4 6% 55%	2090
	21st to end	Day	18% 6% 2 5% 3 4 3 3 4 6% 45%	2090
October	1st to 10th	Day	18% 7% 3 7% 4 4 4 3 4 6% 41%	2079
	11th to 20th	Day	18% 6% 2 6% 5% 4 4 3 3 6% 44%	1853
	21st to end	Day	22% 7% 2 4 4 3 4 3 4 5% 41%	1881
November	1st to 10th	Day	17% 6% 2 5% 3 3 3 3 5% 5% 47%	1761
	11th to 20th	Day	19% 5% 2 5% 4 2 3 3 5% 6% 46%	1800
	21st to end	Day	22% 7% 2 5% 3 3 2 3 4 5% 43%	1715
December	1st to 10th	Day	16% 10% 2 5% 3 3 3 3 5% 7% 44%	2389
	11th to 20th	Day	19% 7% 2 4 2 3 3 3 3 5% 48%	2618
	21st to end	Day	22% 10% 2 7% 3 4 4 3 4 5% 35%	3890
January	1st to 10th	Night	12% 8% 3 6% 4 4 4 3 5% 7% 42%	6349
	11th to 20th	Night	13% 10% 4 9% 5% 5% 5% 4 5% 6% 34%	6315
	21st to end	Night	14% 10% 3 7% 4 4 4 3 5% 5% 41%	6978
February	1st to 10th	Night	17% 10% 3 6% 5% 5% 5% 4 5% 5% 37%	2600
	11th to 20th	Night	14% 9% 3 6% 5% 4 4 3 5% 6% 40%	2362
	21st to end	Night	15% 10% 3 7% 5% 4 3 3 5% 6% 41%	1980
March	1st to 10th	Night	30% 11% 2 5% 3 3 3 2 4 4 33%	3844
	11th to 20th	Night	14% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	3530
	21st to end	Night	26% 9% 2 6% 4 4 5% 4 6% 5% 30%	3767
April	1st to 10th	Night	23% 9% 1 6% 5% 3 5% 3 4 6% 36%	1900
	11th to 20th	Night	19% 7% 2 6% 4 5% 4 3 6% 6% 38%	1754
	21st to end	Night	23% 8% 2 6% 4 4 4 4 4 7% 35%	1710
May	1st to 10th	Night	23% 8% 3 6% 4 4 5% 4 6% 7% 29%	2834
	11th to 20th	Night	19% 7% 3 8% 7% 5% 6% 5% 6% 8% 27%	2813
	21st to end	Night	17% 9% 4 7% 4 3 4 4 5% 7% 35%	2771
June	1st to 10th	Night	23% 8% 2 6% 4 4 5% 3 5% 7% 31%	1454
	11th to 20th	Night	17% 7% 3 6% 4 4 5% 4 5% 7% 38%	1330
	21st to end	Night	25% 9% 3 5% 4 4 4 4 5% 6% 30%	1475
July	1st to 10th	Night	16% 9% 3 6% 5% 5% 5% 5% 7% 8% 31%	2520
	11th to 20th	Night	15% 7% 3 6% 5% 4 5% 4 6% 9% 35%	2520
	21st to end	Night	16% 7% 2 6% 4 4 5% 4 7% 8% 37%	2739
August	1st to 10th	Night	22% 9% 3 7% 5% 5% 4 3 5% 7% 31%	2822
	11th to 20th	Night	17% 10% 3 7% 4 5% 6% 4 5% 7% 32%	2782
	21st to end	Night	10% 5% 2 5% 4 4 4 4 5% 8% 47%	3248
September	1st to 10th	Night	13% 6% 2 4 4 4 5% 3 5% 7% 48%	1966
	11th to 20th	Night	14% 7% 2 4 2 2 3 2 4 6% 51%	2090
	21st to end	Night	16% 5% 2 6% 4 3 4 3 5% 6% 47%	2090
October	1st to 10th	Night	18% 7% 3 5% 5% 4 4 3 6% 6% 40%	2100
	11th to 20th	Night	17% 6% 2 6% 4 4 4 3 5% 6% 44%	2326
	21st to end	Night	24% 7% 2 5% 3 4 3 3 4 5% 39%	2717
November	1st to 10th	Night	17% 6% 2 5% 4 4 4 3 4 5% 46%	2537
	11th to 20th	Night	18% 6% 2 5% 4 4 4 4 5% 6% 44%	2600
	21st to end	Night	17% 7% 2 5% 5% 4 4 4 5% 6% 42%	2680
December	1st to 10th	Night	18% 10% 2 5% 3 4 3 3 4 7% 40%	4796
	11th to 20th	Night	24% 6% 2 4 3 3 3 3 4 4 43%	5591
	21st to end	Night	22% 11% 3 6% 3 3 3 2 3 5% 37%	7060

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	44% 4 11% 26% 14% 1	3598
	11th to 20th	Day	41% 4 13% 26% 14% 3	3600
	21st to end	Day	42% 4 9% 20% 19% 6%	3976
February	1st to 10th	Day	47% 5% 11% 20% 14% 3	1800
	11th to 20th	Day	45% 6% 13% 19% 15% 3	1932
	21st to end	Day	47% 4 10% 21% 14% 4	1650
March	1st to 10th	Day	57% 4 10% 13% 14% 3	3219
	11th to 20th	Day	44% 5% 11% 23% 15% 3	3458
	21st to end	Day	51% 5% 5% 15% 18% 6%	3863
April	1st to 10th	Day	50% 4 9% 18% 15% 4	2280
	11th to 20th	Day	44% 4 10% 24% 15% 3	2373
	21st to end	Day	50% 5% 7% 19% 15% 4	2470
May	1st to 10th	Day	52% 11% 11% 17% 8% 1	4120
	11th to 20th	Day	47% 9% 15% 18% 9% 2	4085
	21st to end	Day	41% 9% 14% 20% 14% 1	4832
June	1st to 10th	Day	49% 6% 13% 22% 9%	2717
	11th to 20th	Day	43% 7% 12% 21% 15% 2	2850
	21st to end	Day	50% 8% 11% 21% 9% 1	2692
July	1st to 10th	Day	44% 8% 11% 25% 12%	4370
	11th to 20th	Day	39% 7% 17% 25% 11% 1	4365
	21st to end	Day	44% 7% 13% 22% 13% 1	4631
August	1st to 10th	Day	44% 7% 16% 25% 7%	4092
	11th to 20th	Day	45% 7% 18% 22% 8%	4084
	21st to end	Day	31% 7% 15% 27% 20% 1	4404
September	1st to 10th	Day	33% 7% 13% 28% 18% 1	2214
	11th to 20th	Day	32% 7% 11% 28% 21% 2	2090
	21st to end	Day	41% 5% 8% 27% 17% 1	2090
October	1st to 10th	Day	47% 5% 11% 22% 14% 1	3456
	11th to 20th	Day	33% 3 11% 30% 22% 1	3056
	21st to end	Day	36% 3 12% 26% 20% 3	3081
November	1st to 10th	Day	39% 4 13% 25% 17% 2	1761
	11th to 20th	Day	39% 4 10% 23% 20% 4	1800
	21st to end	Day	44% 3 16% 23% 12% 1	1715
December	1st to 10th	Day	42% 3 14% 28% 12% 1	2389
	11th to 20th	Day	40% 4 12% 19% 17% 9%	2618
	21st to end	Day	51% 5% 9% 17% 13% 4	3890
January	1st to 10th	Night	42% 4 10% 31% 12% 2	6349
	11th to 20th	Night	50% 5% 12% 20% 11% 2	6360
	21st to end	Night	45% 4 8% 20% 17% 6%	6978
February	1st to 10th	Night	49% 5% 11% 19% 14% 3	2600
	11th to 20th	Night	46% 4 9% 21% 17% 3	2364
	21st to end	Night	45% 4 9% 22% 14% 5%	1980
March	1st to 10th	Night	57% 3 8% 16% 12% 5%	3854
	11th to 20th	Night	47% 6% 11% 23% 10% 3	3531
	21st to end	Night	55% 4 5% 15% 16% 5%	3767
April	1st to 10th	Night	51% 3 7% 18% 16% 4	1900
	11th to 20th	Night	46% 4 9% 22% 17% 2	1754
	21st to end	Night	50% 5% 10% 18% 14% 4	1710
May	1st to 10th	Night	50% 10% 15% 16% 8% 1	2840
	11th to 20th	Night	51% 10% 12% 17% 7% 2	2814
	21st to end	Night	46% 9% 13% 18% 13% 1	2772
June	1st to 10th	Night	53% 6% 11% 21% 9% 1	1454
	11th to 20th	Night	44% 7% 14% 19% 15% 2	1330
	21st to end	Night	53% 5% 12% 19% 10% 1	1475
July	1st to 10th	Night	49% 6% 12% 19% 13%	2480
	11th to 20th	Night	44% 6% 13% 25% 11%	2472
	21st to end	Night	43% 7% 14% 24% 12% 1	2651
August	1st to 10th	Night	54% 7% 12% 19% 7% 1	2811
	11th to 20th	Night	49% 6% 13% 21% 10%	2809
	21st to end	Night	33% 6% 15% 26% 19% 1	3248
September	1st to 10th	Night	36% 6% 11% 28% 18% 1	1966
	11th to 20th	Night	36% 4 10% 27% 21% 2	2090
	21st to end	Night	39% 5% 9% 29% 16% 2	2090
October	1st to 10th	Night	43% 4 14% 23% 15% 1	3503
	11th to 20th	Night	32% 3 10% 28% 25% 1	3862
	21st to end	Night	38% 3 10% 27% 19% 3	4467
November	1st to 10th	Night	41% 5% 10% 27% 15% 2	2537
	11th to 20th	Night	41% 5% 9% 23% 19% 3	2600
	21st to end	Night	43% 6% 14% 21% 14% 2	2680
December	1st to 10th	Night	45% 3 9% 30% 12%	4796
	11th to 20th	Night	45% 4 11% 19% 18% 3	5592
	21st to end	Night	51% 4 9% 17% 17% 1	7060

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	24% 24% 30% 18% 5%	200
	11th to 20th	Day	25% 20% 31% 21% 4 1	200
	21st to end	Day	20% 20% 33% 22% 6%	223
February	1st to 10th	Day	25% 19% 33% 20% 3	400
	11th to 20th	Day	22% 22% 39% 14% 2	483
	21st to end	Day	20% 18% 37% 19% 5%	414
March	1st to 10th	Day	25% 15% 39% 18% 3	500
	11th to 20th	Day	17% 22% 38% 18% 4	464
	21st to end	Day	25% 19% 33% 14% 7% 2	506
April	1st to 10th	Day	28% 17% 32% 16% 6% 2	460
	11th to 20th	Day	24% 12% 37% 18% 7% 1	452
	21st to end	Day	27% 11% 35% 22% 4 1	460
May	1st to 10th	Day	27% 13% 32% 21% 6% 1	463
	11th to 20th	Day	18% 12% 37% 22% 9% 2	487
	21st to end	Day	21% 11% 33% 27% 7% 2	529
June	1st to 10th	Day	21% 7% 30% 31% 10% 2	547
	11th to 20th	Day	20% 8% 29% 27% 14% 3	680
	21st to end	Day	23% 6% 29% 29% 11% 2	522
July	1st to 10th	Day	20% 6% 26% 33% 12% 2	485
	11th to 20th	Day	15% 5% 28% 33% 16% 3	484
	21st to end	Day	16% 5% 26% 36% 14% 3	532
August	1st to 10th	Day	15% 4 26% 35% 18% 2	486
	11th to 20th	Day	16% 5% 27% 32% 15% 5%	479
	21st to end	Day	12% 7% 25% 35% 17% 4	539
September	1st to 10th	Day	15% 6% 27% 34% 15% 3	490
	11th to 20th	Day	16% 9% 26% 28% 15% 7%	490
	21st to end	Day	20% 8% 30% 23% 14% 5%	490
October	1st to 10th	Day	21% 11% 23% 28% 14% 3	483
	11th to 20th	Day	21% 14% 27% 25% 10% 4	390
	21st to end	Day	29% 11% 29% 22% 8% 1	429
November	1st to 10th	Day	20% 16% 29% 24% 8% 2	395
	11th to 20th	Day	24% 17% 32% 20% 8% 1	400
	21st to end	Day	25% 20% 28% 19% 7% 1	400
December	1st to 10th	Day	24% 13% 38% 17% 7% 1	270
	11th to 20th	Day	25% 23% 33% 15% 4 1	200
	21st to end	Day	27% 18% 32% 16% 5% 1	220
January	1st to 10th	Night	21% 20% 36% 19% 4	800
	11th to 20th	Night	25% 20% 34% 18% 4	800
	21st to end	Night	19% 22% 36% 19% 4 1	880
February	1st to 10th	Night	23% 19% 34% 20% 4	800
	11th to 20th	Night	21% 21% 37% 18% 3	783
	21st to end	Night	21% 15% 38% 21% 6%	660
March	1st to 10th	Night	24% 18% 34% 22% 3	800
	11th to 20th	Night	21% 14% 34% 22% 8% 1	767
	21st to end	Night	24% 13% 34% 23% 6% 1	836
April	1st to 10th	Night	28% 13% 32% 21% 5% 1	760
	11th to 20th	Night	23% 8% 33% 28% 7% 1	630
	21st to end	Night	28% 7% 27% 29% 8% 1	570
May	1st to 10th	Night	25% 4 28% 29% 11% 2	570
	11th to 20th	Night	26% 4 29% 29% 11% 1	562
	21st to end	Night	26% 3 22% 34% 14% 1	431
June	1st to 10th	Night	25% 4 21% 35% 13% 2	380
	11th to 20th	Night	20% 5% 18% 39% 16% 2	380
	21st to end	Night	27% 4 17% 38% 13% 1	380
July	1st to 10th	Night	22% 2 12% 43% 19% 1	370
	11th to 20th	Night	21% 3 15% 39% 21% 1	368
	21st to end	Night	18% 3 16% 40% 22% 2	416
August	1st to 10th	Night	17% 3 16% 43% 20% 1	565
	11th to 20th	Night	15% 3 17% 39% 23% 2	562
	21st to end	Night	15% 3 18% 36% 25% 3	699
September	1st to 10th	Night	15% 3 22% 36% 22% 2	760
	11th to 20th	Night	18% 5% 23% 27% 22% 4	760
	21st to end	Night	18% 6% 23% 32% 19% 2	760
October	1st to 10th	Night	22% 7% 22% 29% 18% 2	760
	11th to 20th	Night	20% 10% 30% 26% 12% 3	760
	21st to end	Night	28% 11% 29% 25% 6%	836
November	1st to 10th	Night	21% 10% 33% 26% 9% 1	780
	11th to 20th	Night	21% 16% 34% 22% 7% 1	800
	21st to end	Night	21% 19% 34% 20% 6%	840
December	1st to 10th	Night	27% 12% 37% 17% 6%	900
	11th to 20th	Night	25% 18% 35% 18% 4	900
	21st to end	Night	29% 15% 33% 18% 4	893

Page generated at 2025-03-09 08:08:42. Version ac07

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