Harvey Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Harvey Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	13% 9% 3 8% 5% 5% 5% 4 6% 7% 36%	131652
	Night	14% 9% 4 8% 5% 4 4 3 5% 6% 37%	131418
Transparency	Day	46% 5% 12% 22% 13% 2	135213
	Night	46% 4 11% 22% 14% 2	135932
Seeing	Day	16% 10% 28% 25% 16% 6%	17711
	Night	18% 9% 30% 29% 13% 1	27633

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	13% 13% 6% 13% 7% 4 4 4 6% 8% 22%	4691
	11th to 20th	Day	9% 12% 7% 14% 8% 7% 6% 5% 5% 4 22%	4710
	21st to end	Day	17% 17% 5% 11% 7% 5% 4 3 4 5% 24%	5392
February	1st to 10th	Day	17% 12% 5% 10% 6% 5% 5% 3 5% 6% 27%	2023
	11th to 20th	Day	15% 13% 4 9% 6% 5% 5% 3 4 6% 30%	2048
	21st to end	Day	15% 14% 5% 9% 4 3 3 3 5% 6% 32%	1730
March	1st to 10th	Day	19% 15% 4 8% 3 3 2 2 3 4 37%	4580
	11th to 20th	Day	8% 10% 3 6% 4 5% 4 4 6% 8% 42%	4749
	21st to end	Day	16% 12% 5% 9% 5% 5% 4 3 5% 7% 28%	6099
April	1st to 10th	Day	17% 11% 3 7% 5% 3 4 3 6% 7% 34%	2520
	11th to 20th	Day	16% 8% 4 6% 5% 5% 4 4 5% 7% 37%	2494
	21st to end	Day	18% 9% 3 8% 5% 4 4 3 5% 7% 35%	2622
May	1st to 10th	Day	18% 8% 3 7% 5% 5% 5% 4 7% 9% 29%	5992
	11th to 20th	Day	16% 7% 2 8% 4 4 5% 4 6% 7% 37%	5980
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 5% 6% 9% 38%	6935
June	1st to 10th	Day	10% 6% 2 6% 4 4 5% 5% 6% 9% 43%	2800
	11th to 20th	Day	9% 5% 2 6% 5% 5% 4 4 7% 8% 45%	2800
	21st to end	Day	9% 7% 2 7% 6% 5% 5% 4 7% 9% 40%	2800
July	1st to 10th	Day	11% 5% 2 6% 4 5% 6% 5% 8% 11% 39%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 8% 11% 36%	6360
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 5% 7% 9% 38%	6924
August	1st to 10th	Day	7% 4 2 5% 4 4 4 4 6% 9% 50%	4309
	11th to 20th	Day	8% 6% 2 5% 4 5% 5% 5% 8% 11% 42%	3849
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 10% 51%	4312
September	1st to 10th	Day	10% 5% 3 5% 4 4 4 4 5% 8% 47%	2400
	11th to 20th	Day	9% 5% 2 4 4 4 5% 4 5% 6% 52%	2243
	21st to end	Day	11% 6% 3 5% 4 4 4 4 6% 7% 46%	2200
October	1st to 10th	Day	10% 5% 2 7% 5% 4 5% 3 5% 7% 48%	2200
	11th to 20th	Day	16% 6% 2 8% 4 4 4 3 5% 6% 42%	1992
	21st to end	Day	18% 9% 3 7% 4 4 4 3 5% 6% 35%	1980
November	1st to 10th	Day	12% 6% 2 5% 3 4 4 3 5% 7% 49%	1852
	11th to 20th	Day	18% 10% 3 7% 4 3 4 3 4 5% 39%	1890
	21st to end	Day	17% 11% 3 10% 5% 4 4 3 4 5% 35%	1890
December	1st to 10th	Day	11% 12% 4 8% 4 4 3 3 4 6% 41%	2768
	11th to 20th	Day	14% 15% 6% 12% 5% 6% 5% 4 5% 5% 23%	3088
	21st to end	Day	28% 17% 7% 12% 4 4 4 3 3 4 12%	4070
January	1st to 10th	Night	14% 14% 7% 11% 6% 4 4 3 5% 5% 28%	8232
	11th to 20th	Night	10% 15% 6% 14% 7% 6% 6% 4 4 4 24%	8280
	21st to end	Night	17% 12% 5% 9% 5% 4 4 3 4 5% 30%	8818
February	1st to 10th	Night	17% 12% 5% 10% 5% 5% 4 3 5% 6% 28%	2595
	11th to 20th	Night	14% 11% 4 9% 5% 4 4 3 5% 6% 34%	2458
	21st to end	Night	17% 12% 3 9% 5% 4 3 3 5% 5% 34%	2076
March	1st to 10th	Night	17% 14% 4 6% 4 3 3 3 4 5% 37%	5493
	11th to 20th	Night	12% 7% 3 7% 5% 4 4 3 5% 7% 44%	5430
	21st to end	Night	18% 9% 3 9% 5% 5% 5% 4 6% 6% 30%	5101
April	1st to 10th	Night	13% 9% 4 6% 5% 4 5% 3 4 7% 40%	2100
	11th to 20th	Night	17% 9% 2 5% 5% 3 4 3 4 6% 41%	2075
	21st to end	Night	17% 8% 3 6% 5% 5% 4 4 6% 6% 36%	1995
May	1st to 10th	Night	19% 11% 4 7% 5% 4 5% 3 5% 7% 32%	4123
	11th to 20th	Night	15% 7% 2 6% 4 4 4 4 6% 8% 41%	4043
	21st to end	Night	13% 8% 3 8% 5% 5% 4 4 5% 8% 37%	3942
June	1st to 10th	Night	10% 5% 2 6% 4 4 5% 3 5% 8% 49%	1600
	11th to 20th	Night	8% 6% 3 6% 4 3 5% 4 6% 8% 48%	1600
	21st to end	Night	8% 5% 2 6% 5% 4 5% 4 8% 9% 43%	1600
July	1st to 10th	Night	10% 5% 2 7% 5% 5% 5% 4 6% 10% 42%	3618
	11th to 20th	Night	6% 5% 2 6% 4 5% 5% 5% 7% 10% 45%	3617
	21st to end	Night	8% 7% 3 8% 5% 5% 5% 4 6% 9% 41%	3947
August	1st to 10th	Night	7% 4 1 5% 4 3 4 4 6% 9% 53%	2839
	11th to 20th	Night	6% 4 2 5% 5% 5% 5% 4 6% 9% 49%	3200
	21st to end	Night	7% 4 2 4 3 4 4 4 5% 8% 55%	3586
September	1st to 10th	Night	10% 5% 2 5% 3 4 4 3 4 7% 53%	2000
	11th to 20th	Night	7% 4 1 4 4 4 4 3 6% 7% 56%	2135
	21st to end	Night	11% 6% 3 6% 4 3 4 3 5% 7% 47%	2200
October	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 4 6% 55%	2200
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 4 6% 49%	2404
	21st to end	Night	17% 8% 2 8% 4 4 4 3 5% 5% 39%	2860
November	1st to 10th	Night	10% 7% 3 6% 3 3 4 3 4 7% 49%	2666
	11th to 20th	Night	14% 8% 3 7% 5% 4 4 4 4 6% 42%	2730
	21st to end	Night	19% 10% 4 9% 4 3 2 3 5% 6% 34%	2730
December	1st to 10th	Night	13% 9% 3 8% 4 4 4 3 5% 6% 43%	4636
	11th to 20th	Night	17% 11% 5% 10% 5% 5% 4 3 5% 5% 31%	5339
	21st to end	Night	27% 18% 6% 10% 4 4 4 3 4 5% 16%	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	61% 3 14% 15% 6% 1	4692
	11th to 20th	Day	65% 3 8% 11% 11% 2	4710
	21st to end	Day	65% 2 8% 17% 6% 1	5392
February	1st to 10th	Day	59% 4 11% 17% 7% 2	2023
	11th to 20th	Day	55% 4 10% 21% 9% 1	2058
	21st to end	Day	53% 4 11% 21% 8% 2	1730
March	1st to 10th	Day	54% 3 6% 20% 14% 3	4580
	11th to 20th	Day	39% 5% 11% 34% 11% 1	4749
	21st to end	Day	55% 6% 13% 17% 8% 1	6101
April	1st to 10th	Day	49% 6% 12% 19% 13%	2520
	11th to 20th	Day	46% 5% 12% 22% 11% 3	2494
	21st to end	Day	49% 5% 13% 21% 11% 2	2622
May	1st to 10th	Day	49% 7% 13% 20% 11% 1	6027
	11th to 20th	Day	45% 7% 12% 22% 12% 1	5984
	21st to end	Day	42% 6% 12% 21% 16% 2	6949
June	1st to 10th	Day	36% 9% 13% 24% 16% 3	2800
	11th to 20th	Day	34% 7% 12% 23% 20% 4	2800
	21st to end	Day	38% 8% 14% 20% 16% 3	2800
July	1st to 10th	Day	39% 4 12% 28% 15% 2	6291
	11th to 20th	Day	41% 4 16% 26% 12% 1	6285
	21st to end	Day	42% 4 16% 24% 13% 2	6728
August	1st to 10th	Day	31% 5% 17% 32% 15% 1	4294
	11th to 20th	Day	34% 6% 14% 29% 16% 1	3885
	21st to end	Day	29% 5% 15% 31% 16% 4	4311
September	1st to 10th	Day	34% 7% 12% 24% 19% 4	2400
	11th to 20th	Day	31% 6% 11% 24% 23% 4	2243
	21st to end	Day	37% 5% 10% 23% 23% 2	2200
October	1st to 10th	Day	26% 3 7% 28% 33% 2	3590
	11th to 20th	Day	33% 3 10% 24% 29% 2	3220
	21st to end	Day	45% 3 8% 23% 20% 2	3180
November	1st to 10th	Day	36% 5% 14% 25% 17% 3	1852
	11th to 20th	Day	49% 3 11% 21% 13% 3	1890
	21st to end	Day	53% 4 9% 21% 11% 1	1890
December	1st to 10th	Day	48% 3 11% 29% 10%	2768
	11th to 20th	Day	63% 4 10% 17% 5%	3085
	21st to end	Day	77% 4 8% 8% 3	4070
January	1st to 10th	Night	59% 4 12% 18% 7%	8233
	11th to 20th	Night	64% 3 8% 12% 11% 2	8280
	21st to end	Night	58% 2 9% 21% 9% 1	8818
February	1st to 10th	Night	57% 5% 12% 16% 8% 1	2595
	11th to 20th	Night	51% 5% 11% 20% 11% 2	2459
	21st to end	Night	52% 6% 10% 20% 10% 2	2076
March	1st to 10th	Night	50% 4 8% 23% 11% 4	5493
	11th to 20th	Night	40% 4 11% 26% 16% 2	5430
	21st to end	Night	53% 6% 10% 23% 7% 1	5101
April	1st to 10th	Night	43% 6% 11% 24% 14% 2	2100
	11th to 20th	Night	45% 5% 10% 21% 15% 4	2075
	21st to end	Night	47% 6% 11% 21% 11% 4	1995
May	1st to 10th	Night	52% 5% 10% 17% 12% 2	4150
	11th to 20th	Night	40% 6% 12% 26% 14% 2	4044
	21st to end	Night	44% 6% 12% 20% 14% 4	3950
June	1st to 10th	Night	34% 6% 10% 27% 21% 3	1600
	11th to 20th	Night	33% 6% 11% 26% 20% 4	1600
	21st to end	Night	35% 8% 16% 21% 17% 4	1600
July	1st to 10th	Night	39% 4 12% 28% 13% 4	3580
	11th to 20th	Night	34% 4 16% 27% 17% 1	3572
	21st to end	Night	41% 4 16% 23% 13% 3	3832
August	1st to 10th	Night	29% 5% 18% 31% 16% 2	2830
	11th to 20th	Night	32% 5% 15% 28% 18% 1	3230
	21st to end	Night	28% 5% 11% 33% 19% 4	3586
September	1st to 10th	Night	31% 5% 12% 26% 19% 7%	2000
	11th to 20th	Night	26% 6% 10% 25% 28% 4	2135
	21st to end	Night	35% 6% 10% 22% 23% 3	2200
October	1st to 10th	Night	21% 3 6% 31% 37% 2	3590
	11th to 20th	Night	29% 3 8% 26% 31% 2	3915
	21st to end	Night	38% 3 8% 23% 23% 4	4610
November	1st to 10th	Night	35% 4 11% 27% 19% 4	2666
	11th to 20th	Night	44% 5% 11% 23% 15% 3	2730
	21st to end	Night	53% 4 10% 22% 11% 1	2730
December	1st to 10th	Night	44% 3 12% 28% 12% 1	4636
	11th to 20th	Night	56% 4 12% 16% 11% 1	5341
	21st to end	Night	72% 3 9% 10% 4	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	22% 16% 33% 23% 5% 1	330
	11th to 20th	Day	22% 21% 38% 15% 3 1	330
	21st to end	Day	24% 16% 36% 21% 3	363
February	1st to 10th	Day	24% 15% 38% 18% 5%	330
	11th to 20th	Day	25% 15% 38% 17% 4 1	330
	21st to end	Day	26% 15% 44% 13% 1	273
March	1st to 10th	Day	21% 19% 41% 16% 3	330
	11th to 20th	Day	28% 15% 38% 15% 4	381
	21st to end	Day	28% 14% 34% 16% 7% 1	594
April	1st to 10th	Day	25% 14% 31% 21% 6% 1	540
	11th to 20th	Day	20% 19% 30% 19% 10% 2	532
	21st to end	Day	21% 15% 34% 20% 8% 1	540
May	1st to 10th	Day	19% 12% 34% 24% 8% 3	543
	11th to 20th	Day	20% 9% 27% 26% 13% 5%	566
	21st to end	Day	12% 7% 32% 26% 15% 8%	789
June	1st to 10th	Day	12% 8% 26% 31% 18% 5%	730
	11th to 20th	Day	11% 6% 26% 32% 21% 5%	730
	21st to end	Day	12% 5% 29% 28% 20% 6%	730
July	1st to 10th	Day	10% 3 16% 29% 26% 16%	721
	11th to 20th	Day	8% 4 19% 31% 26% 12%	718
	21st to end	Day	10% 3 16% 32% 29% 9%	794
August	1st to 10th	Day	6% 3 14% 31% 31% 15%	586
	11th to 20th	Day	9% 3 16% 29% 31% 11%	519
	21st to end	Day	6% 2 17% 30% 32% 14%	583
September	1st to 10th	Day	11% 5% 18% 27% 29% 11%	530
	11th to 20th	Day	8% 3 14% 27% 31% 18%	530
	21st to end	Day	13% 7% 24% 22% 22% 12%	530
October	1st to 10th	Day	10% 5% 25% 31% 21% 7%	530
	11th to 20th	Day	12% 19% 30% 28% 10% 2	333
	21st to end	Day	27% 14% 35% 18% 4 1	363
November	1st to 10th	Day	14% 12% 34% 25% 12% 3	330
	11th to 20th	Day	24% 17% 31% 19% 8% 1	330
	21st to end	Day	24% 12% 35% 23% 5%	330
December	1st to 10th	Day	18% 17% 42% 16% 7% 1	330
	11th to 20th	Day	18% 18% 38% 20% 5% 1	330
	21st to end	Day	28% 17% 34% 18% 3	363
January	1st to 10th	Night	21% 15% 37% 24% 3	1050
	11th to 20th	Night	22% 16% 38% 19% 5%	1050
	21st to end	Night	22% 14% 38% 24% 3	1155
February	1st to 10th	Night	24% 12% 37% 23% 3	915
	11th to 20th	Night	20% 14% 39% 23% 4	823
	21st to end	Night	25% 13% 38% 22% 2	692
March	1st to 10th	Night	22% 15% 35% 24% 4	840
	11th to 20th	Night	19% 13% 37% 25% 5%	840
	21st to end	Night	23% 10% 38% 24% 5%	709
April	1st to 10th	Night	18% 14% 38% 25% 5%	630
	11th to 20th	Night	21% 12% 33% 26% 8%	623
	21st to end	Night	21% 9% 35% 27% 7% 1	630
May	1st to 10th	Night	19% 8% 34% 31% 8%	630
	11th to 20th	Night	19% 4 29% 33% 13% 1	609
	21st to end	Night	12% 5% 26% 36% 18% 2	650
June	1st to 10th	Night	11% 4 25% 42% 18%	600
	11th to 20th	Night	12% 4 24% 43% 18% 1	600
	21st to end	Night	9% 3 24% 42% 21% 1	600
July	1st to 10th	Night	11% 1 15% 37% 30% 5%	585
	11th to 20th	Night	5% 2 18% 42% 29% 4	582
	21st to end	Night	6% 2 13% 44% 33% 2	657
August	1st to 10th	Night	7% 1 11% 46% 32% 3	597
	11th to 20th	Night	7% 1 13% 45% 30% 4	594
	21st to end	Night	8% 1 14% 35% 40% 3	660
September	1st to 10th	Night	13% 3 15% 38% 28% 3	600
	11th to 20th	Night	8% 4 14% 40% 32% 4	735
	21st to end	Night	13% 6% 23% 29% 27% 3	800
October	1st to 10th	Night	12% 5% 22% 38% 21% 3	800
	11th to 20th	Night	18% 11% 29% 30% 12%	800
	21st to end	Night	20% 14% 36% 24% 6%	880
November	1st to 10th	Night	13% 11% 41% 25% 9% 1	820
	11th to 20th	Night	19% 13% 34% 27% 7% 1	840
	21st to end	Night	25% 14% 32% 23% 7%	840
December	1st to 10th	Night	22% 12% 37% 24% 4	992
	11th to 20th	Night	24% 12% 34% 26% 5%	1050
	21st to end	Night	32% 13% 32% 21% 3	1155

Page generated at 2026-05-14 00:59:00. Version ac07

Copyright © 2026 Attilla Danko.