The Hotchkiss School
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Hotchkiss School clear sky chart from 2005-11-06 to 2024-11-12.

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	16% 8% 3 7% 5% 4 5% 4 6% 7% 35%	101386
	Night	17% 9% 3 7% 5% 4 4 4 5% 6% 37%	97803
Transparency	Day	45% 7% 13% 21% 12% 1	104919
	Night	44% 8% 14% 21% 12% 1	102359
Seeing	Day	24% 19% 30% 18% 7% 2	14952
	Night	22% 18% 30% 22% 7% 1	24001

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% 9% 3 7% 5% 4 5% 3 5% 5% 40%	2517
	11th to 20th	Day	15% 12% 4 9% 6% 5% 4 3 5% 6% 30%	2520
	21st to end	Day	21% 15% 6% 7% 4 5% 4 3 4 5% 27%	2875
February	1st to 10th	Day	17% 10% 3 9% 7% 5% 4 4 5% 6% 31%	1725
	11th to 20th	Day	14% 8% 4 8% 5% 5% 5% 3 4 7% 36%	1848
	21st to end	Day	18% 12% 3 8% 4 5% 5% 3 4 6% 34%	1570
March	1st to 10th	Day	23% 13% 4 7% 4 4 3 2 4 4 30%	3116
	11th to 20th	Day	11% 10% 3 8% 6% 5% 5% 4 6% 6% 35%	3277
	21st to end	Day	21% 8% 2 8% 5% 4 4 3 5% 6% 35%	4166
April	1st to 10th	Day	18% 10% 3 6% 5% 4 5% 3 5% 5% 35%	2280
	11th to 20th	Day	17% 8% 3 6% 5% 5% 4 4 4 6% 39%	2254
	21st to end	Day	19% 8% 2 6% 5% 4 5% 4 5% 6% 37%	2365
May	1st to 10th	Day	27% 9% 3 6% 4 4 4 3 4 7% 29%	4106
	11th to 20th	Day	21% 7% 3 7% 5% 4 4 3 6% 7% 32%	4383
	21st to end	Day	12% 6% 2 5% 4 4 5% 4 7% 8% 42%	4837
June	1st to 10th	Day	17% 7% 2 6% 5% 5% 6% 4 7% 9% 33%	2660
	11th to 20th	Day	17% 7% 2 5% 5% 5% 4 5% 8% 9% 34%	2660
	21st to end	Day	14% 8% 2 7% 6% 5% 4 4 6% 9% 35%	2660
July	1st to 10th	Day	10% 7% 3 8% 6% 6% 5% 5% 6% 8% 36%	4440
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 11% 37%	4440
	21st to end	Day	14% 8% 3 8% 5% 5% 5% 5% 6% 7% 32%	4814
August	1st to 10th	Day	19% 7% 3 8% 5% 5% 5% 4 7% 9% 28%	4317
	11th to 20th	Day	12% 7% 2 6% 5% 4 5% 4 7% 9% 39%	3730
	21st to end	Day	10% 6% 2 6% 4 4 5% 4 6% 10% 41%	4180
September	1st to 10th	Day	12% 7% 3 6% 4 4 5% 5% 6% 7% 41%	2280
	11th to 20th	Day	14% 5% 3 4 3 4 3 3 6% 6% 49%	2176
	21st to end	Day	17% 6% 2 6% 4 3 5% 4 6% 7% 39%	2090
October	1st to 10th	Day	21% 8% 3 6% 4 3 3 4 4 6% 39%	2016
	11th to 20th	Day	14% 8% 2 6% 5% 4 4 3 5% 7% 43%	1900
	21st to end	Day	22% 8% 3 7% 4 4 4 4 5% 6% 34%	1886
November	1st to 10th	Day	13% 8% 2 5% 4 4 3 3 5% 7% 46%	1761
	11th to 20th	Day	18% 8% 2 5% 3 4 5% 4 5% 6% 39%	1719
	21st to end	Day	17% 8% 3 6% 5% 4 4 3 5% 7% 39%	1620
December	1st to 10th	Day	18% 9% 4 8% 6% 4 5% 4 5% 6% 31%	1520
	11th to 20th	Day	19% 10% 3 7% 4 3 4 4 4 6% 37%	1906
	21st to end	Day	19% 12% 5% 9% 6% 5% 4 4 5% 6% 25%	2772
January	1st to 10th	Night	16% 8% 2 6% 5% 4 4 3 5% 6% 40%	4429
	11th to 20th	Night	15% 9% 4 9% 6% 5% 5% 3 4 7% 34%	4440
	21st to end	Night	23% 14% 4 7% 6% 4 3 2 5% 5% 28%	4779
February	1st to 10th	Night	16% 9% 4 6% 5% 4 5% 5% 6% 6% 34%	2453
	11th to 20th	Night	15% 9% 3 8% 6% 4 5% 5% 6% 6% 33%	2218
	21st to end	Night	18% 8% 4 7% 6% 4 4 4 5% 5% 36%	1884
March	1st to 10th	Night	23% 11% 3 7% 4 4 4 3 5% 4 31%	3724
	11th to 20th	Night	11% 8% 3 8% 6% 6% 5% 5% 6% 6% 36%	3666
	21st to end	Night	18% 11% 3 8% 4 4 4 3 5% 5% 36%	3466
April	1st to 10th	Night	17% 10% 4 8% 6% 5% 5% 4 5% 6% 31%	1900
	11th to 20th	Night	18% 9% 3 6% 5% 4 4 3 4 7% 38%	1875
	21st to end	Night	18% 9% 2 7% 4 3 3 4 6% 6% 38%	1815
May	1st to 10th	Night	25% 10% 3 7% 4 4 4 3 5% 6% 30%	2834
	11th to 20th	Night	19% 9% 3 8% 5% 4 4 4 5% 7% 32%	2539
	21st to end	Night	15% 6% 2 6% 5% 4 4 3 6% 7% 41%	2738
June	1st to 10th	Night	16% 7% 3 7% 4 4 4 4 5% 7% 40%	1520
	11th to 20th	Night	14% 8% 2 6% 5% 4 5% 4 6% 9% 36%	1520
	21st to end	Night	14% 5% 2 6% 4 4 4 5% 6% 8% 42%	1520
July	1st to 10th	Night	11% 5% 2 7% 5% 4 5% 4 7% 8% 40%	2520
	11th to 20th	Night	14% 6% 3 6% 5% 4 5% 4 7% 10% 37%	2520
	21st to end	Night	15% 9% 3 8% 4 4 4 4 5% 5% 39%	2739
August	1st to 10th	Night	20% 9% 3 6% 4 4 5% 5% 5% 7% 31%	2576
	11th to 20th	Night	14% 8% 2 6% 4 4 5% 4 5% 7% 40%	3097
	21st to end	Night	10% 5% 2 6% 4 4 4 3 6% 7% 51%	3476
September	1st to 10th	Night	14% 6% 2 6% 4 4 4 2 5% 7% 45%	1900
	11th to 20th	Night	13% 5% 1 4 2 3 4 3 5% 6% 53%	2004
	21st to end	Night	17% 7% 2 6% 4 3 4 3 4 6% 44%	2090
October	1st to 10th	Night	19% 8% 2 6% 5% 4 3 2 4 6% 41%	2161
	11th to 20th	Night	14% 7% 2 6% 4 4 4 4 5% 6% 44%	2280
	21st to end	Night	21% 9% 2 7% 4 3 4 3 5% 6% 37%	2712
November	1st to 10th	Night	14% 7% 2 6% 3 3 4 3 6% 6% 46%	2537
	11th to 20th	Night	17% 8% 3 6% 4 4 4 4 5% 6% 39%	2484
	21st to end	Night	13% 9% 3 6% 6% 5% 4 3 6% 6% 39%	2555
December	1st to 10th	Night	19% 10% 3 8% 5% 5% 4 3 5% 6% 32%	2660
	11th to 20th	Night	19% 9% 3 8% 5% 5% 5% 4 5% 5% 32%	3288
	21st to end	Night	22% 11% 5% 8% 4 3 4 4 5% 6% 29%	4884

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	41% 7% 15% 29% 7% 1	2518
	11th to 20th	Day	52% 8% 14% 20% 6% 1	2520
	21st to end	Day	59% 6% 9% 18% 7% 1	2875
February	1st to 10th	Day	53% 7% 13% 17% 10% 1	1725
	11th to 20th	Day	48% 6% 11% 21% 13% 2	1857
	21st to end	Day	52% 5% 12% 17% 12% 2	1570
March	1st to 10th	Day	56% 6% 12% 14% 10% 2	3119
	11th to 20th	Day	41% 13% 17% 21% 8% 1	3277
	21st to end	Day	48% 6% 8% 16% 18% 4	4166
April	1st to 10th	Day	50% 7% 10% 19% 12% 3	2280
	11th to 20th	Day	45% 7% 10% 20% 15% 2	2254
	21st to end	Day	47% 6% 11% 18% 14% 3	2365
May	1st to 10th	Day	53% 9% 12% 14% 11% 1	4120
	11th to 20th	Day	48% 9% 14% 19% 9% 1	4387
	21st to end	Day	34% 10% 15% 25% 15% 1	4837
June	1st to 10th	Day	45% 10% 15% 20% 10% 1	2660
	11th to 20th	Day	44% 8% 16% 21% 11% 1	2660
	21st to end	Day	45% 8% 15% 20% 11% 1	2660
July	1st to 10th	Day	44% 8% 12% 20% 15% 1	4370
	11th to 20th	Day	39% 8% 18% 23% 12% 1	4365
	21st to end	Day	49% 7% 13% 16% 14% 1	4661
August	1st to 10th	Day	51% 9% 14% 19% 7%	4303
	11th to 20th	Day	41% 8% 18% 27% 7%	3766
	21st to end	Day	37% 7% 13% 30% 12% 1	4180
September	1st to 10th	Day	41% 8% 12% 23% 16% 1	2280
	11th to 20th	Day	36% 6% 11% 24% 22% 2	2176
	21st to end	Day	41% 7% 13% 21% 15% 2	2090
October	1st to 10th	Day	43% 6% 8% 18% 23% 2	3354
	11th to 20th	Day	34% 5% 11% 24% 23% 4	3140
	21st to end	Day	35% 6% 15% 25% 16% 3	3086
November	1st to 10th	Day	38% 5% 15% 24% 15% 2	1761
	11th to 20th	Day	43% 7% 12% 21% 15% 2	1719
	21st to end	Day	45% 5% 16% 24% 9% 2	1620
December	1st to 10th	Day	52% 8% 15% 16% 10%	1520
	11th to 20th	Day	48% 7% 15% 20% 9% 1	1906
	21st to end	Day	56% 9% 12% 15% 8%	2772
January	1st to 10th	Night	42% 6% 14% 26% 9% 2	4429
	11th to 20th	Night	48% 7% 17% 17% 9% 1	4440
	21st to end	Night	56% 7% 11% 17% 7% 1	4779
February	1st to 10th	Night	47% 8% 14% 19% 9% 1	2453
	11th to 20th	Night	47% 8% 12% 17% 13% 2	2220
	21st to end	Night	48% 7% 11% 20% 10% 3	1884
March	1st to 10th	Night	53% 7% 11% 18% 9% 1	3734
	11th to 20th	Night	40% 12% 18% 20% 9% 2	3667
	21st to end	Night	47% 7% 9% 14% 19% 4	3466
April	1st to 10th	Night	50% 9% 10% 19% 11% 2	1900
	11th to 20th	Night	47% 7% 11% 19% 16% 1	1875
	21st to end	Night	45% 7% 10% 21% 14% 3	1815
May	1st to 10th	Night	54% 9% 12% 15% 8% 2	2840
	11th to 20th	Night	51% 10% 13% 17% 8% 1	2540
	21st to end	Night	38% 10% 17% 20% 14% 1	2739
June	1st to 10th	Night	42% 8% 13% 23% 12% 1	1520
	11th to 20th	Night	41% 10% 17% 22% 10%	1520
	21st to end	Night	39% 9% 14% 24% 13% 1	1520
July	1st to 10th	Night	39% 9% 15% 20% 15% 1	2480
	11th to 20th	Night	40% 9% 15% 25% 11%	2472
	21st to end	Night	45% 7% 14% 16% 17% 1	2651
August	1st to 10th	Night	50% 9% 14% 18% 8% 1	2566
	11th to 20th	Night	41% 8% 16% 27% 8%	3127
	21st to end	Night	32% 6% 14% 35% 13%	3476
September	1st to 10th	Night	39% 7% 12% 25% 16% 1	1900
	11th to 20th	Night	31% 6% 13% 25% 24% 1	2004
	21st to end	Night	41% 6% 11% 23% 18% 1	2090
October	1st to 10th	Night	43% 5% 12% 22% 16% 1	3603
	11th to 20th	Night	31% 6% 12% 25% 25% 2	3779
	21st to end	Night	36% 5% 15% 28% 15% 1	4462
November	1st to 10th	Night	38% 6% 12% 21% 19% 3	2537
	11th to 20th	Night	45% 5% 14% 20% 13% 3	2484
	21st to end	Night	43% 6% 19% 24% 8% 1	2555
December	1st to 10th	Night	51% 7% 15% 18% 9%	2660
	11th to 20th	Night	49% 8% 16% 14% 11% 1	3288
	21st to end	Night	53% 8% 13% 17% 9%	4884

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	29% 29% 29% 11% 2	180
	11th to 20th	Day	34% 27% 29% 9% 2	180
	21st to end	Day	26% 28% 33% 9% 3 1	229
February	1st to 10th	Day	36% 34% 24% 5% 1	285
	11th to 20th	Day	34% 29% 27% 9% 1	456
	21st to end	Day	32% 24% 30% 11% 2	382
March	1st to 10th	Day	31% 24% 32% 11% 2	460
	11th to 20th	Day	31% 27% 30% 10% 2	460
	21st to end	Day	35% 24% 28% 8% 3 2	506
April	1st to 10th	Day	31% 26% 27% 10% 5%	460
	11th to 20th	Day	32% 25% 25% 13% 4 1	452
	21st to end	Day	30% 22% 32% 12% 3 1	460
May	1st to 10th	Day	32% 23% 24% 14% 5% 2	463
	11th to 20th	Day	23% 20% 33% 19% 5% 1	487
	21st to end	Day	19% 15% 31% 22% 11% 2	529
June	1st to 10th	Day	19% 16% 33% 20% 11% 1	490
	11th to 20th	Day	17% 16% 32% 24% 9% 2	490
	21st to end	Day	17% 18% 32% 23% 9% 2	490
July	1st to 10th	Day	13% 10% 34% 28% 13% 2	485
	11th to 20th	Day	12% 8% 32% 33% 12% 2	484
	21st to end	Day	12% 12% 38% 25% 11% 2	532
August	1st to 10th	Day	14% 10% 32% 29% 11% 4	486
	11th to 20th	Day	14% 10% 34% 25% 13% 6%	479
	21st to end	Day	12% 14% 31% 24% 14% 5%	539
September	1st to 10th	Day	19% 13% 22% 26% 14% 5%	490
	11th to 20th	Day	14% 14% 26% 21% 15% 10%	490
	21st to end	Day	21% 11% 34% 19% 10% 4	490
October	1st to 10th	Day	27% 13% 26% 19% 12% 3	490
	11th to 20th	Day	21% 19% 30% 22% 6% 2	490
	21st to end	Day	30% 17% 28% 16% 6% 1	432
November	1st to 10th	Day	24% 18% 28% 20% 9%	395
	11th to 20th	Day	33% 22% 26% 15% 4 1	373
	21st to end	Day	26% 23% 30% 17% 3	280
December	1st to 10th	Day	27% 31% 28% 10% 5%	180
	11th to 20th	Day	38% 26% 25% 9% 2	180
	21st to end	Day	30% 25% 30% 13% 2	198
January	1st to 10th	Night	23% 27% 34% 14% 2	760
	11th to 20th	Night	28% 23% 33% 15% 2	760
	21st to end	Night	22% 27% 34% 15% 2	836
February	1st to 10th	Night	25% 22% 34% 18% 1	760
	11th to 20th	Night	26% 27% 35% 11% 1	743
	21st to end	Night	30% 25% 29% 14% 2	628
March	1st to 10th	Night	26% 24% 34% 13% 3	760
	11th to 20th	Night	24% 27% 32% 14% 3	760
	21st to end	Night	26% 25% 28% 15% 4 1	836
April	1st to 10th	Night	27% 22% 32% 16% 3	760
	11th to 20th	Night	24% 22% 31% 17% 5%	751
	21st to end	Night	25% 21% 32% 17% 4	675
May	1st to 10th	Night	26% 18% 31% 19% 5%	570
	11th to 20th	Night	19% 15% 40% 22% 3	405
	21st to end	Night	19% 11% 29% 31% 10%	411
June	1st to 10th	Night	18% 9% 34% 30% 8%	380
	11th to 20th	Night	17% 9% 32% 30% 12%	380
	21st to end	Night	16% 9% 26% 38% 11% 1	380
July	1st to 10th	Night	14% 6% 22% 40% 18%	370
	11th to 20th	Night	14% 5% 25% 41% 13% 1	368
	21st to end	Night	12% 7% 26% 39% 15%	416
August	1st to 10th	Night	12% 3 28% 42% 13% 1	409
	11th to 20th	Night	14% 5% 24% 36% 20% 1	750
	21st to end	Night	14% 8% 23% 36% 18% 2	836
September	1st to 10th	Night	17% 7% 31% 31% 13% 2	760
	11th to 20th	Night	16% 8% 26% 28% 19% 4	760
	21st to end	Night	21% 9% 26% 30% 12% 2	760
October	1st to 10th	Night	23% 12% 26% 26% 11% 1	760
	11th to 20th	Night	18% 18% 30% 25% 8% 1	760
	21st to end	Night	27% 17% 30% 20% 6%	836
November	1st to 10th	Night	19% 20% 31% 22% 7% 1	780
	11th to 20th	Night	24% 22% 29% 18% 5% 1	765
	21st to end	Night	21% 21% 32% 20% 5% 1	760
December	1st to 10th	Night	28% 22% 31% 15% 4	760
	11th to 20th	Night	27% 24% 30% 18% 2	760
	21st to end	Night	29% 22% 34% 13% 2	836

Page generated at 2024-11-08 22:45:37. Version ac07

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