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 2026-01-02.

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 4 4 5% 7% 35%	127218
	Night	16% 9% 3 7% 5% 4 4 3 5% 6% 37%	124136
Transparency	Day	46% 7% 13% 21% 13% 2	130733
	Night	45% 7% 13% 20% 13% 2	128708
Seeing	Day	24% 19% 30% 18% 7% 2	16353
	Night	23% 18% 30% 22% 7% 1	25470

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	11% 10% 4 12% 6% 5% 4 3 4 5% 35%	3605
	11th to 20th	Day	13% 11% 4 10% 5% 5% 4 3 5% 5% 35%	3570
	21st to end	Day	17% 12% 5% 6% 4 4 4 3 4 5% 34%	4119
February	1st to 10th	Day	16% 9% 3 9% 6% 5% 4 4 5% 6% 31%	1816
	11th to 20th	Day	14% 8% 4 8% 5% 5% 5% 3 4 7% 35%	1948
	21st to end	Day	17% 12% 3 7% 4 5% 5% 3 4 6% 35%	1650
March	1st to 10th	Day	21% 11% 4 7% 4 4 3 2 4 5% 34%	4476
	11th to 20th	Day	12% 9% 3 7% 5% 5% 5% 4 5% 6% 38%	4693
	21st to end	Day	19% 8% 3 8% 5% 4 4 3 5% 7% 34%	5970
April	1st to 10th	Day	19% 10% 4 7% 5% 4 5% 3 5% 5% 34%	2400
	11th to 20th	Day	16% 8% 3 6% 5% 5% 4 4 4 6% 38%	2374
	21st to end	Day	19% 8% 2 6% 5% 4 5% 3 5% 7% 37%	2490
May	1st to 10th	Day	30% 10% 3 6% 4 4 4 3 5% 7% 25%	5886
	11th to 20th	Day	18% 9% 3 8% 5% 4 4 3 5% 7% 33%	6289
	21st to end	Day	19% 8% 3 6% 4 4 5% 4 6% 8% 32%	6949
June	1st to 10th	Day	17% 7% 2 6% 5% 5% 6% 4 7% 9% 33%	2800
	11th to 20th	Day	18% 7% 2 6% 5% 5% 4 4 7% 8% 34%	2800
	21st to end	Day	14% 8% 2 7% 6% 5% 4 4 6% 8% 35%	2800
July	1st to 10th	Day	11% 8% 3 8% 5% 6% 5% 5% 6% 8% 34%	6360
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 5% 8% 11% 34%	6360
	21st to end	Day	14% 7% 3 7% 4 4 4 4 6% 8% 40%	6926
August	1st to 10th	Day	19% 7% 3 7% 5% 5% 5% 4 6% 9% 29%	4453
	11th to 20th	Day	12% 7% 2 6% 5% 4 5% 4 7% 9% 39%	3850
	21st to end	Day	10% 6% 2 6% 4 4 5% 4 6% 10% 41%	4312
September	1st to 10th	Day	12% 7% 3 6% 4 4 5% 4 5% 7% 42%	2400
	11th to 20th	Day	13% 5% 3 5% 3 3 4 3 6% 6% 49%	2290
	21st to end	Day	17% 6% 2 6% 4 3 5% 4 7% 7% 38%	2200
October	1st to 10th	Day	20% 7% 3 6% 4 3 3 3 4 6% 41%	2122
	11th to 20th	Day	14% 8% 2 6% 5% 4 4 3 5% 7% 42%	2000
	21st to end	Day	21% 8% 2 7% 4 4 4 4 5% 5% 35%	1985
November	1st to 10th	Day	13% 8% 2 6% 4 4 3 3 5% 7% 45%	1851
	11th to 20th	Day	16% 8% 2 5% 3 4 5% 4 5% 6% 40%	1890
	21st to end	Day	17% 9% 3 6% 5% 4 4 3 4 7% 37%	1790
December	1st to 10th	Day	15% 9% 3 7% 5% 4 4 3 5% 7% 39%	2680
	11th to 20th	Day	18% 10% 3 7% 3 3 4 4 4 6% 38%	3066
	21st to end	Day	17% 12% 4 9% 5% 4 3 3 5% 6% 31%	4048
January	1st to 10th	Night	12% 7% 3 14% 6% 4 4 3 5% 6% 34%	6364
	11th to 20th	Night	13% 9% 4 11% 6% 5% 5% 4 5% 7% 32%	6315
	21st to end	Night	17% 11% 3 7% 6% 5% 4 3 5% 6% 34%	6835
February	1st to 10th	Night	16% 9% 4 6% 5% 4 5% 5% 6% 6% 34%	2582
	11th to 20th	Night	15% 9% 3 8% 6% 4 5% 5% 6% 7% 33%	2338
	21st to end	Night	17% 8% 4 8% 6% 4 4 4 5% 6% 35%	1980
March	1st to 10th	Night	19% 10% 3 6% 4 4 4 3 5% 6% 35%	5364
	11th to 20th	Night	15% 8% 3 8% 5% 5% 4 4 6% 6% 37%	5250
	21st to end	Night	19% 11% 3 8% 4 4 4 3 5% 5% 34%	4962
April	1st to 10th	Night	17% 10% 4 8% 6% 5% 5% 4 5% 6% 30%	2000
	11th to 20th	Night	19% 9% 3 6% 5% 4 3 3 5% 6% 37%	1975
	21st to end	Night	18% 9% 2 7% 4 3 3 4 6% 6% 39%	1910
May	1st to 10th	Night	30% 11% 3 6% 4 4 4 3 5% 6% 26%	4054
	11th to 20th	Night	18% 12% 4 8% 5% 4 4 3 5% 6% 31%	3633
	21st to end	Night	21% 8% 3 7% 5% 4 4 3 6% 6% 33%	3926
June	1st to 10th	Night	16% 7% 3 7% 4 4 4 4 4 7% 40%	1600
	11th to 20th	Night	15% 8% 2 6% 5% 4 5% 4 7% 9% 35%	1600
	21st to end	Night	13% 5% 2 6% 4 4 4 5% 6% 8% 42%	1600
July	1st to 10th	Night	12% 7% 3 7% 4 4 5% 4 7% 8% 40%	3600
	11th to 20th	Night	15% 8% 3 7% 5% 4 5% 4 7% 9% 33%	3600
	21st to end	Night	11% 7% 3 7% 4 3 4 3 5% 6% 48%	3926
August	1st to 10th	Night	20% 9% 3 6% 4 4 5% 5% 5% 7% 32%	2659
	11th to 20th	Night	14% 8% 2 7% 4 5% 5% 4 5% 7% 40%	3197
	21st to end	Night	10% 5% 2 6% 4 4 4 3 5% 7% 51%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 4 4 2 5% 7% 46%	2000
	11th to 20th	Night	13% 4 1 4 2 3 4 3 5% 6% 53%	2110
	21st to end	Night	17% 8% 2 6% 4 3 4 3 4 6% 43%	2200
October	1st to 10th	Night	19% 7% 2 6% 4 4 3 2 4 6% 42%	2275
	11th to 20th	Night	14% 7% 2 6% 4 4 4 4 5% 6% 44%	2400
	21st to end	Night	20% 8% 2 7% 4 3 4 3 5% 6% 37%	2855
November	1st to 10th	Night	14% 7% 2 6% 3 4 4 3 5% 6% 45%	2667
	11th to 20th	Night	16% 8% 2 7% 4 4 4 4 5% 6% 39%	2730
	21st to end	Night	13% 9% 3 7% 6% 5% 4 3 6% 6% 38%	2825
December	1st to 10th	Night	16% 10% 3 8% 4 4 4 3 5% 6% 37%	4720
	11th to 20th	Night	22% 9% 3 7% 4 4 4 3 4 4 36%	5348
	21st to end	Night	18% 12% 5% 8% 4 4 4 3 4 6% 32%	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	51% 5% 11% 24% 9% 1	3606
	11th to 20th	Day	49% 6% 12% 18% 10% 5%	3600
	21st to end	Day	52% 5% 8% 21% 13% 1	4119
February	1st to 10th	Day	52% 7% 12% 17% 11% 1	1816
	11th to 20th	Day	49% 6% 10% 21% 12% 2	1957
	21st to end	Day	51% 5% 12% 18% 12% 2	1650
March	1st to 10th	Day	52% 5% 11% 14% 11% 7%	4479
	11th to 20th	Day	40% 9% 17% 23% 7% 3	4693
	21st to end	Day	49% 6% 8% 15% 18% 4	5970
April	1st to 10th	Day	51% 6% 10% 19% 12% 3	2400
	11th to 20th	Day	45% 7% 10% 20% 15% 2	2374
	21st to end	Day	46% 6% 11% 19% 14% 3	2490
May	1st to 10th	Day	58% 8% 13% 13% 8% 1	5897
	11th to 20th	Day	50% 8% 12% 19% 11% 1	6293
	21st to end	Day	46% 9% 12% 21% 11% 1	6949
June	1st to 10th	Day	44% 9% 15% 20% 10% 1	2800
	11th to 20th	Day	44% 8% 16% 20% 11% 1	2800
	21st to end	Day	45% 7% 15% 20% 11% 1	2800
July	1st to 10th	Day	47% 8% 12% 20% 13% 1	6290
	11th to 20th	Day	40% 8% 18% 23% 11% 1	6285
	21st to end	Day	42% 6% 12% 22% 17% 1	6728
August	1st to 10th	Day	50% 8% 14% 19% 8% 1	4439
	11th to 20th	Day	41% 8% 17% 27% 7%	3886
	21st to end	Day	36% 7% 13% 30% 12% 1	4312
September	1st to 10th	Day	40% 7% 12% 23% 16% 1	2400
	11th to 20th	Day	35% 6% 11% 24% 23% 1	2290
	21st to end	Day	42% 7% 13% 21% 15% 2	2200
October	1st to 10th	Day	42% 6% 8% 19% 23% 2	3460
	11th to 20th	Day	35% 4 11% 24% 23% 4	3240
	21st to end	Day	35% 6% 15% 25% 16% 3	3185
November	1st to 10th	Day	39% 5% 15% 24% 15% 2	1851
	11th to 20th	Day	42% 7% 11% 21% 16% 2	1890
	21st to end	Day	47% 5% 15% 23% 8% 1	1790
December	1st to 10th	Day	45% 6% 16% 25% 7%	2680
	11th to 20th	Day	48% 5% 14% 19% 13% 1	3066
	21st to end	Day	52% 6% 9% 16% 14% 4	4048
January	1st to 10th	Night	49% 5% 11% 22% 11% 2	6364
	11th to 20th	Night	51% 6% 17% 14% 9% 4	6360
	21st to end	Night	49% 7% 12% 15% 15% 3	6835
February	1st to 10th	Night	47% 8% 14% 20% 9% 1	2582
	11th to 20th	Night	47% 8% 12% 17% 14% 2	2340
	21st to end	Night	48% 7% 11% 20% 10% 3	1980
March	1st to 10th	Night	48% 6% 12% 17% 14% 3	5374
	11th to 20th	Night	43% 9% 19% 18% 8% 3	5251
	21st to end	Night	49% 6% 7% 14% 20% 3	4962
April	1st to 10th	Night	51% 8% 10% 18% 10% 2	2000
	11th to 20th	Night	47% 7% 11% 18% 16% 1	1975
	21st to end	Night	45% 7% 10% 20% 15% 3	1910
May	1st to 10th	Night	60% 7% 12% 11% 8% 1	4060
	11th to 20th	Night	53% 8% 12% 16% 9% 2	3634
	21st to end	Night	50% 8% 14% 17% 10% 1	3927
June	1st to 10th	Night	43% 8% 13% 23% 12% 1	1600
	11th to 20th	Night	42% 10% 17% 22% 9%	1600
	21st to end	Night	38% 9% 14% 24% 13% 1	1600
July	1st to 10th	Night	41% 8% 14% 21% 15% 1	3560
	11th to 20th	Night	44% 9% 16% 21% 10% 1	3552
	21st to end	Night	37% 6% 13% 25% 18% 1	3809
August	1st to 10th	Night	50% 9% 14% 18% 8% 1	2649
	11th to 20th	Night	41% 8% 16% 27% 8%	3227
	21st to end	Night	32% 6% 14% 35% 13% 1	3586
September	1st to 10th	Night	39% 7% 11% 25% 17% 1	2000
	11th to 20th	Night	31% 6% 13% 25% 24% 1	2110
	21st to end	Night	42% 6% 11% 23% 18% 1	2200
October	1st to 10th	Night	43% 5% 12% 23% 16% 2	3717
	11th to 20th	Night	31% 6% 12% 25% 24% 2	3899
	21st to end	Night	36% 5% 14% 28% 15% 1	4605
November	1st to 10th	Night	39% 6% 12% 22% 18% 3	2667
	11th to 20th	Night	44% 5% 13% 19% 15% 3	2730
	21st to end	Night	44% 6% 18% 23% 8%	2825
December	1st to 10th	Night	48% 6% 13% 27% 7%	4720
	11th to 20th	Night	51% 5% 14% 14% 15% 1	5348
	21st to end	Night	52% 7% 10% 15% 13% 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	31% 30% 27% 10% 1	202
	11th to 20th	Day	35% 27% 29% 9% 2	200
	21st to end	Day	29% 27% 31% 8% 3 1	255
February	1st to 10th	Day	37% 34% 24% 4 1	316
	11th to 20th	Day	36% 28% 26% 9% 1	496
	21st to end	Day	32% 26% 30% 11% 2	414
March	1st to 10th	Day	32% 24% 32% 10% 2	500
	11th to 20th	Day	31% 25% 31% 10% 2	500
	21st to end	Day	35% 25% 27% 8% 3 2	550
April	1st to 10th	Day	34% 25% 26% 10% 5%	500
	11th to 20th	Day	34% 25% 25% 12% 4 1	492
	21st to end	Day	30% 22% 33% 12% 3 1	500
May	1st to 10th	Day	34% 23% 23% 14% 5% 1	503
	11th to 20th	Day	23% 22% 32% 19% 5% 1	527
	21st to end	Day	21% 15% 31% 21% 11% 1	573
June	1st to 10th	Day	19% 16% 33% 21% 10% 1	530
	11th to 20th	Day	19% 16% 31% 24% 9% 2	530
	21st to end	Day	16% 18% 33% 23% 9% 2	530
July	1st to 10th	Day	14% 11% 34% 28% 12% 2	525
	11th to 20th	Day	12% 9% 32% 32% 13% 2	524
	21st to end	Day	12% 12% 38% 26% 10% 2	576
August	1st to 10th	Day	14% 10% 31% 29% 11% 4	526
	11th to 20th	Day	13% 10% 32% 26% 13% 6%	519
	21st to end	Day	12% 14% 32% 24% 14% 5%	583
September	1st to 10th	Day	19% 13% 24% 26% 14% 5%	530
	11th to 20th	Day	13% 13% 26% 22% 16% 9%	530
	21st to end	Day	21% 12% 33% 20% 11% 4	530
October	1st to 10th	Day	25% 13% 26% 19% 13% 4	530
	11th to 20th	Day	24% 19% 29% 21% 6% 2	530
	21st to end	Day	30% 17% 29% 17% 6% 1	465
November	1st to 10th	Day	25% 19% 28% 20% 8%	425
	11th to 20th	Day	31% 22% 27% 15% 4 1	430
	21st to end	Day	32% 23% 27% 15% 3	330
December	1st to 10th	Day	29% 27% 29% 10% 5%	220
	11th to 20th	Day	39% 27% 24% 9% 2	220
	21st to end	Day	31% 25% 29% 12% 2	242
January	1st to 10th	Night	23% 28% 33% 14% 2	805
	11th to 20th	Night	28% 23% 32% 16% 2	800
	21st to end	Night	22% 26% 35% 15% 2	880
February	1st to 10th	Night	26% 23% 33% 17% 1	800
	11th to 20th	Night	27% 27% 34% 11% 1	783
	21st to end	Night	30% 24% 30% 14% 2	660
March	1st to 10th	Night	26% 25% 34% 13% 3	800
	11th to 20th	Night	24% 27% 31% 15% 3	800
	21st to end	Night	26% 25% 29% 15% 4 1	880
April	1st to 10th	Night	28% 22% 31% 15% 3	800
	11th to 20th	Night	25% 22% 31% 16% 5%	791
	21st to end	Night	26% 21% 31% 17% 4	710
May	1st to 10th	Night	28% 18% 31% 19% 5%	600
	11th to 20th	Night	20% 15% 40% 22% 3	426
	21st to end	Night	20% 11% 28% 31% 10%	433
June	1st to 10th	Night	18% 9% 33% 31% 9%	400
	11th to 20th	Night	18% 9% 32% 30% 12%	400
	21st to end	Night	15% 9% 26% 39% 11% 1	400
July	1st to 10th	Night	14% 6% 22% 41% 17%	390
	11th to 20th	Night	14% 5% 26% 41% 14% 1	388
	21st to end	Night	11% 7% 27% 40% 14%	438
August	1st to 10th	Night	12% 3 27% 43% 15% 1	432
	11th to 20th	Night	13% 5% 24% 36% 21% 1	790
	21st to end	Night	14% 8% 24% 35% 17% 2	880
September	1st to 10th	Night	17% 7% 30% 31% 13% 2	800
	11th to 20th	Night	16% 7% 25% 29% 19% 4	800
	21st to end	Night	21% 9% 26% 31% 12% 2	800
October	1st to 10th	Night	23% 12% 26% 25% 13% 1	800
	11th to 20th	Night	20% 17% 30% 25% 8% 1	800
	21st to end	Night	26% 17% 30% 21% 5%	880
November	1st to 10th	Night	20% 20% 31% 22% 6% 1	820
	11th to 20th	Night	24% 23% 28% 18% 5% 1	840
	21st to end	Night	22% 21% 32% 20% 4 1	840
December	1st to 10th	Night	28% 22% 30% 15% 4	840
	11th to 20th	Night	28% 23% 30% 18% 2	840
	21st to end	Night	30% 22% 33% 13% 2	924

Page generated at 2025-12-30 01:14:07. Version ac07

Copyright © 2025 Attilla Danko.