Three Guys Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Three Guys Observatory clear sky chart from 2005-11-06 to 2025-02-21.

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	14% 8% 3 7% 4 4 4 4 5% 7% 40%	108467
	Night	15% 8% 3 7% 4 4 4 3 5% 6% 40%	110229
Transparency	Day	42% 6% 11% 23% 16% 2	112116
	Night	43% 5% 10% 22% 17% 3	114721
Seeing	Day	17% 7% 24% 25% 18% 9%	15848
	Night	18% 7% 25% 32% 16% 2	25793

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	15% 13% 7% 11% 5% 5% 5% 3 5% 5% 27%	3617
	11th to 20th	Day	13% 12% 5% 13% 7% 5% 5% 3 4 3 27%	3620
	21st to end	Day	21% 13% 5% 8% 5% 3 4 3 4 4 30%	4341
February	1st to 10th	Day	18% 11% 5% 8% 5% 3 4 3 4 7% 32%	2000
	11th to 20th	Day	17% 10% 4 9% 5% 5% 5% 3 4 6% 33%	1948
	21st to end	Day	17% 11% 4 9% 4 5% 4 3 5% 6% 33%	1570
March	1st to 10th	Day	24% 13% 3 6% 4 3 3 2 3 4 34%	3120
	11th to 20th	Day	14% 7% 2 6% 4 3 3 3 5% 8% 45%	3283
	21st to end	Day	14% 8% 3 7% 6% 4 5% 4 6% 7% 35%	4100
April	1st to 10th	Day	15% 8% 2 6% 5% 4 4 4 5% 8% 39%	2280
	11th to 20th	Day	17% 8% 2 6% 4 4 3 4 5% 6% 41%	2254
	21st to end	Day	18% 7% 3 7% 4 4 4 3 4 7% 39%	2359
May	1st to 10th	Day	17% 8% 3 6% 6% 5% 5% 5% 7% 10% 30%	4082
	11th to 20th	Day	19% 7% 2 6% 4 4 4 3 5% 7% 39%	4081
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 5% 6% 8% 40%	4601
June	1st to 10th	Day	11% 6% 2 5% 4 4 4 4 7% 8% 45%	2660
	11th to 20th	Day	7% 6% 2 5% 4 5% 5% 4 8% 8% 48%	2660
	21st to end	Day	8% 6% 2 5% 4 5% 6% 5% 7% 9% 44%	2660
July	1st to 10th	Day	14% 5% 2 6% 4 5% 4 5% 7% 9% 40%	4440
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 7% 12% 48%	4440
	21st to end	Day	10% 6% 3 8% 5% 6% 6% 6% 8% 9% 31%	4720
August	1st to 10th	Day	7% 4 2 4 3 3 5% 3 6% 10% 52%	4026
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 8% 9% 43%	3729
	21st to end	Day	5% 3 2 5% 4 4 5% 3 6% 8% 55%	4180
September	1st to 10th	Day	11% 5% 1 6% 5% 4 4 4 5% 6% 48%	2280
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 6% 7% 50%	2099
	21st to end	Day	12% 5% 2 4 3 4 4 3 5% 6% 52%	2090
October	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 4 6% 55%	2090
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 4 6% 48%	1959
	21st to end	Day	20% 7% 2 6% 4 4 3 3 4 6% 42%	1881
November	1st to 10th	Day	9% 5% 2 5% 3 3 4 4 5% 6% 53%	1762
	11th to 20th	Day	17% 8% 3 6% 4 3 3 3 4 6% 43%	1800
	21st to end	Day	19% 10% 3 7% 4 3 3 3 4 5% 39%	1800
December	1st to 10th	Day	14% 10% 3 6% 3 3 4 2 4 5% 45%	2930
	11th to 20th	Day	17% 12% 6% 9% 5% 4 4 4 5% 5% 29%	3023
	21st to end	Day	27% 16% 6% 8% 5% 4 3 2 3 5% 19%	3982
January	1st to 10th	Night	18% 13% 6% 10% 4 4 4 4 4 5% 28%	6348
	11th to 20th	Night	15% 15% 5% 10% 5% 6% 6% 3 3 4 28%	6360
	21st to end	Night	20% 11% 4 6% 5% 4 4 3 4 6% 32%	6624
February	1st to 10th	Night	21% 10% 5% 8% 6% 4 4 4 5% 5% 29%	2400
	11th to 20th	Night	17% 9% 3 9% 5% 5% 3 3 4 7% 34%	2338
	21st to end	Night	20% 11% 4 7% 3 4 5% 3 5% 5% 33%	1885
March	1st to 10th	Night	23% 12% 3 7% 4 3 4 3 4 5% 33%	3733
	11th to 20th	Night	13% 6% 3 5% 4 3 3 3 4 6% 50%	3677
	21st to end	Night	17% 7% 3 7% 5% 5% 4 3 5% 6% 39%	3547
April	1st to 10th	Night	13% 8% 2 7% 4 4 4 4 6% 7% 42%	1900
	11th to 20th	Night	16% 7% 2 5% 4 4 4 3 5% 6% 44%	1875
	21st to end	Night	17% 6% 2 6% 5% 4 5% 4 6% 7% 37%	1819
May	1st to 10th	Night	17% 9% 2 8% 5% 5% 5% 3 5% 8% 33%	2813
	11th to 20th	Night	17% 6% 3 6% 4 3 4 3 5% 6% 42%	2812
	21st to end	Night	10% 6% 2 6% 5% 5% 5% 4 7% 9% 42%	2941
June	1st to 10th	Night	9% 6% 2 4 3 5% 4 3 6% 9% 48%	1520
	11th to 20th	Night	8% 4 2 5% 4 4 4 4 6% 9% 49%	1520
	21st to end	Night	5% 3 1 4 4 4 5% 3 8% 10% 51%	1520
July	1st to 10th	Night	10% 5% 2 7% 4 4 4 4 6% 7% 47%	2530
	11th to 20th	Night	5% 4 2 4 3 4 4 4 6% 8% 56%	2530
	21st to end	Night	10% 6% 3 8% 6% 5% 6% 5% 7% 9% 35%	2840
August	1st to 10th	Night	7% 4 2 5% 3 3 4 3 5% 9% 56%	2886
	11th to 20th	Night	6% 3 2 6% 4 5% 5% 4 6% 8% 51%	3100
	21st to end	Night	4 3 2 4 3 3 4 3 6% 9% 59%	3476
September	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 5% 7% 52%	1900
	11th to 20th	Night	7% 5% 1 6% 4 4 3 4 5% 7% 54%	2061
	21st to end	Night	12% 6% 2 5% 3 3 3 3 5% 7% 50%	2090
October	1st to 10th	Night	9% 4 2 5% 3 3 3 3 4 6% 59%	2090
	11th to 20th	Night	15% 6% 2 4 4 3 4 2 4 5% 51%	2215
	21st to end	Night	18% 7% 2 6% 3 4 3 3 4 6% 43%	2717
November	1st to 10th	Night	12% 6% 2 6% 4 3 3 3 4 6% 51%	2536
	11th to 20th	Night	15% 7% 3 5% 4 3 3 3 5% 7% 46%	2600
	21st to end	Night	20% 9% 3 6% 4 3 3 2 4 6% 41%	2600
December	1st to 10th	Night	18% 10% 3 7% 4 3 3 3 4 5% 42%	4250
	11th to 20th	Night	19% 9% 3 7% 4 4 4 3 5% 6% 37%	5180
	21st to end	Night	29% 15% 4 8% 4 4 3 3 3 4 22%	6996

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	59% 4 10% 20% 6%	3618
	11th to 20th	Day	61% 4 8% 16% 9% 1	3620
	21st to end	Day	59% 3 8% 19% 8% 4	4341
February	1st to 10th	Day	53% 4 11% 20% 10% 2	2000
	11th to 20th	Day	53% 3 11% 20% 12% 2	1958
	21st to end	Day	54% 4 9% 20% 10% 3	1570
March	1st to 10th	Day	56% 4 5% 19% 13% 4	3120
	11th to 20th	Day	39% 5% 11% 28% 15% 2	3283
	21st to end	Day	46% 7% 13% 22% 11% 1	4100
April	1st to 10th	Day	43% 6% 13% 22% 16% 1	2280
	11th to 20th	Day	44% 5% 10% 23% 14% 3	2254
	21st to end	Day	45% 5% 11% 25% 12% 3	2359
May	1st to 10th	Day	47% 9% 15% 21% 8%	4120
	11th to 20th	Day	46% 7% 10% 25% 11% 1	4085
	21st to end	Day	39% 8% 12% 18% 20% 2	4614
June	1st to 10th	Day	35% 7% 11% 26% 17% 3	2660
	11th to 20th	Day	32% 9% 12% 24% 19% 4	2660
	21st to end	Day	34% 9% 14% 22% 17% 5%	2660
July	1st to 10th	Day	40% 6% 11% 24% 17% 2	4371
	11th to 20th	Day	29% 7% 13% 32% 18% 2	4365
	21st to end	Day	45% 10% 15% 19% 10% 1	4571
August	1st to 10th	Day	27% 7% 15% 35% 15% 1	4013
	11th to 20th	Day	35% 7% 14% 26% 17% 1	3765
	21st to end	Day	27% 6% 14% 34% 16% 4	4179
September	1st to 10th	Day	36% 5% 10% 24% 21% 4	2280
	11th to 20th	Day	31% 5% 12% 27% 21% 4	2099
	21st to end	Day	33% 5% 8% 25% 24% 4	2090
October	1st to 10th	Day	20% 3 5% 28% 41% 3	3480
	11th to 20th	Day	26% 4 8% 26% 33% 3	3226
	21st to end	Day	35% 3 8% 24% 27% 3	3081
November	1st to 10th	Day	33% 5% 10% 29% 18% 5%	1762
	11th to 20th	Day	44% 4 8% 22% 18% 4	1800
	21st to end	Day	48% 4 9% 22% 15% 1	1800
December	1st to 10th	Day	43% 3 13% 26% 14%	2930
	11th to 20th	Day	58% 4 9% 20% 9% 1	3020
	21st to end	Day	70% 4 10% 12% 5%	3982
January	1st to 10th	Night	58% 4 10% 19% 8%	6348
	11th to 20th	Night	61% 4 8% 14% 11% 3	6360
	21st to end	Night	54% 3 8% 21% 13% 1	6624
February	1st to 10th	Night	58% 4 11% 18% 8% 1	2400
	11th to 20th	Night	51% 4 10% 20% 14% 1	2339
	21st to end	Night	53% 5% 9% 19% 11% 3	1885
March	1st to 10th	Night	55% 4 7% 19% 13% 1	3733
	11th to 20th	Night	36% 6% 12% 23% 22% 2	3677
	21st to end	Night	46% 6% 11% 26% 10% 1	3547
April	1st to 10th	Night	41% 5% 10% 26% 15% 3	1900
	11th to 20th	Night	40% 5% 10% 20% 20% 5%	1875
	21st to end	Night	45% 6% 11% 21% 14% 3	1819
May	1st to 10th	Night	48% 10% 13% 21% 7% 1	2840
	11th to 20th	Night	41% 8% 12% 26% 13%	2813
	21st to end	Night	35% 9% 14% 21% 17% 4	2949
June	1st to 10th	Night	32% 7% 9% 25% 22% 4	1520
	11th to 20th	Night	30% 8% 11% 22% 21% 8%	1520
	21st to end	Night	27% 7% 16% 23% 19% 8%	1520
July	1st to 10th	Night	36% 7% 9% 25% 20% 3	2490
	11th to 20th	Night	25% 7% 10% 34% 21% 3	2482
	21st to end	Night	43% 8% 14% 21% 12% 2	2750
August	1st to 10th	Night	25% 7% 14% 38% 14% 2	2875
	11th to 20th	Night	30% 7% 13% 30% 19% 1	3130
	21st to end	Night	22% 5% 14% 30% 25% 4	3476
September	1st to 10th	Night	31% 7% 9% 26% 22% 6%	1900
	11th to 20th	Night	29% 6% 8% 25% 27% 5%	2061
	21st to end	Night	34% 5% 10% 22% 23% 6%	2090
October	1st to 10th	Night	18% 3 6% 18% 50% 4	3480
	11th to 20th	Night	25% 3 6% 26% 37% 3	3687
	21st to end	Night	32% 3 8% 23% 28% 6%	4467
November	1st to 10th	Night	36% 4 9% 24% 22% 5%	2536
	11th to 20th	Night	39% 4 9% 22% 22% 5%	2600
	21st to end	Night	47% 3 10% 23% 15% 2	2600
December	1st to 10th	Night	47% 3 10% 25% 15%	4250
	11th to 20th	Night	49% 4 13% 20% 12% 1	5182
	21st to end	Night	68% 4 9% 13% 5% 1	6996

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	26% 13% 32% 21% 7% 1	300
	11th to 20th	Day	27% 16% 36% 18% 3	300
	21st to end	Day	25% 15% 37% 17% 6% 1	330
February	1st to 10th	Day	26% 15% 38% 17% 4	300
	11th to 20th	Day	27% 17% 35% 18% 3 1	300
	21st to end	Day	29% 8% 41% 17% 4	225
March	1st to 10th	Day	23% 12% 39% 20% 4 1	270
	11th to 20th	Day	28% 14% 34% 16% 7% 1	341
	21st to end	Day	30% 11% 35% 17% 7% 1	506
April	1st to 10th	Day	19% 14% 30% 26% 10% 1	460
	11th to 20th	Day	22% 13% 28% 23% 11% 4	452
	21st to end	Day	22% 9% 38% 20% 9% 3	460
May	1st to 10th	Day	21% 7% 29% 28% 11% 5%	463
	11th to 20th	Day	21% 5% 24% 26% 16% 8%	487
	21st to end	Day	12% 3 25% 27% 21% 12%	602
June	1st to 10th	Day	15% 4 18% 27% 26% 9%	680
	11th to 20th	Day	9% 3 20% 31% 25% 12%	680
	21st to end	Day	11% 3 24% 30% 19% 13%	680
July	1st to 10th	Day	13% 1 12% 25% 27% 22%	671
	11th to 20th	Day	8% 3 12% 33% 27% 17%	668
	21st to end	Day	10% 2 10% 30% 33% 15%	682
August	1st to 10th	Day	11% 1 10% 26% 33% 19%	486
	11th to 20th	Day	14% 4 12% 27% 32% 12%	479
	21st to end	Day	7% 1 8% 24% 37% 22%	539
September	1st to 10th	Day	10% 2 12% 27% 33% 15%	490
	11th to 20th	Day	9% 3 8% 29% 35% 16%	490
	21st to end	Day	16% 5% 19% 25% 23% 13%	490
October	1st to 10th	Day	11% 3 20% 34% 24% 9%	490
	11th to 20th	Day	13% 9% 33% 28% 14% 4	367
	21st to end	Day	26% 9% 33% 24% 8%	330
November	1st to 10th	Day	13% 7% 36% 25% 13% 7%	300
	11th to 20th	Day	20% 17% 29% 22% 9% 3	300
	21st to end	Day	26% 10% 26% 28% 9% 1	300
December	1st to 10th	Day	18% 18% 36% 18% 8% 3	300
	11th to 20th	Day	27% 10% 34% 20% 9% 1	300
	21st to end	Day	32% 14% 30% 19% 4	330
January	1st to 10th	Night	23% 12% 33% 28% 4	1000
	11th to 20th	Night	25% 12% 36% 23% 4	1000
	21st to end	Night	23% 10% 38% 26% 3	1051
February	1st to 10th	Night	27% 10% 35% 26% 3	800
	11th to 20th	Night	24% 13% 36% 25% 3	783
	21st to end	Night	27% 10% 37% 24% 2	629
March	1st to 10th	Night	22% 11% 34% 28% 5%	760
	11th to 20th	Night	22% 10% 34% 26% 7% 1	760
	21st to end	Night	23% 9% 30% 31% 6% 1	682
April	1st to 10th	Night	16% 11% 32% 33% 8% 1	570
	11th to 20th	Night	20% 7% 29% 31% 12% 1	563
	21st to end	Night	20% 5% 33% 33% 8% 1	570
May	1st to 10th	Night	18% 5% 30% 32% 14% 1	570
	11th to 20th	Night	17% 3 20% 39% 19% 2	561
	21st to end	Night	11% 2 18% 39% 27% 3	617
June	1st to 10th	Night	13% 1 16% 41% 28% 2	570
	11th to 20th	Night	9% 2 16% 46% 24% 2	570
	21st to end	Night	7% 1 17% 41% 32% 2	570
July	1st to 10th	Night	10% 2 9% 32% 37% 9%	555
	11th to 20th	Night	6% 2 9% 40% 37% 6%	552
	21st to end	Night	9% 2 9% 37% 38% 5%	624
August	1st to 10th	Night	8% 1 7% 40% 38% 6%	567
	11th to 20th	Night	8% 2 9% 37% 40% 4	564
	21st to end	Night	6% 1 6% 35% 45% 8%	627
September	1st to 10th	Night	12% 3 9% 34% 38% 5%	570
	11th to 20th	Night	8% 2 12% 34% 38% 6%	731
	21st to end	Night	14% 4 18% 31% 28% 5%	760
October	1st to 10th	Night	11% 3 17% 38% 25% 6%	760
	11th to 20th	Night	18% 7% 26% 31% 17% 2	760
	21st to end	Night	22% 6% 32% 30% 10%	836
November	1st to 10th	Night	16% 7% 28% 34% 14% 1	780
	11th to 20th	Night	19% 10% 30% 31% 10% 1	800
	21st to end	Night	26% 10% 26% 30% 9% 1	800
December	1st to 10th	Night	25% 7% 35% 28% 6% 1	800
	11th to 20th	Night	25% 8% 31% 29% 7% 1	981
	21st to end	Night	31% 11% 30% 24% 4	1100

Page generated at 2025-02-18 03:26:26. Version ac07

Copyright © 2025 Attilla Danko.