Choosing which device you want and need is something that you are going to have to make on your since the range of weather devices out there is wide and varied now and days. At least you have step one nailed down - the price you are looking for. Weather stations vary from simple $15 temperature sensors to upwards of $1500+ if you are looking at the top of the end Davis wifi unit or alternatives. So now that you have a price range nailed down, the next thing to start considering is what you want to measure. All of them do temperature followed by the additions of a hygrometer to provide you with RH and possible dew point. Then as you move up in price range, a barometer usually will find its way into the mix to provide you with pressure followed by an anemometer (wind speed), wind vane (direction), and/or a rain gauge. Then depending on your interest and needs, you can look for ones that measure solar radiation, UV index, leaf wetness, soil moisture and temperature, water temperature, and possibly even precipitation type among other things.
Once you decide what you want (or for some units, what you want to start with before expanding with additional add-ons) the next thing is to look at how you want it connected: wirelessly, wifi (larger wireless range usually), or cabeled. My personal preference if you are going low end is to go wired since wireless can have issues with batteries over time I have found in the field. But again, this will be a preference for yourself and where you plan on putting it.
An additional thing to look at is how it displays the data. Most will provide you with a screen to see things on but if you want it to feed into your computer, that is something to look at. If you want it to stream to a website (like wundergound) that is something to look at. If its for a NOAA COOP partnership, you are going to have to get something approved by their standards. If you want to store the data, you going to have to see if that is possible through their software or a third parties software. How is it powered? Solar, cabeled, batteries, etc. Do you have to mount it or is it all included in the kit? How far is the wireless range or how long are the cables that connect it? What do other consumers think of it? You can usually check this on amazon ratings or on other websites.
So once you have your station preferences narrowed down, the next thing to look for is that "sweet spot" for weather recording. You mentioned a roof. Now, in some instances, a person's roof is their only option but if you have other options, a roof should be your last choice. For wind speed/direction, ideally you want to put the sensor over flat terrain, a distance of at least ten times the height of the nearby structures, vegetation or other obstacles (fences for instance). Now, unless you are at a airport, a mountain peak, a ranch/farm, a desert or just own a lot of property, this probably won't happen but you want to put it as far from objects as possible. And the height is ideal. The AASC says 2.0 meters +/- 1 meter is good enough but ideally and to be the most accurate, the AASC, WMO and EPA all recommend or require 10.0 meters (33 feet) off a surface. So if you have to mount it on the roof, mount it at least 10 meters above the highest pitch to avoid turbulence or acceleration that your roof will cause. But keep in mind that if something breaks, the batteries die, it comes loose, etc, especially in the winter, you will be climbing up onto your ice and snow covered roof to work on the instruments and if they are 10 meters above it, yikes. And you are also creating a lightning rod or possible roof leakage if you don't mount it properly. And as far as lightning goes, depending on what you go with, make sure to ground it to avoid electric damage to your house.
For temp/RH sensors, these should be in a ventilated shack but if not, do the best you can (the northern side of a house can work sometimes) and mount it at least 2M above a surface (ideally 3 meters). They should be no closer than four times the height of the closest obscurations height, at least 30 meters away from large paved areas, over a level surface, with ideally low, level grass below it. Make sure there isn't any neighboring heat sources (chimney, camp fire, bbq, dryer vent, etc), high vegetation, swamps or areas that collect water, snow drift prone areas, metal structures or asphalt shingles since these radiate heat bad in summer months (try roofing a house to see what I mean). But another bad thing with roof mounts is soot from chimneys or heating oil exhaust can affect sensors over time and will either require more frequent cleaning or replacement.
As far as precipitation goes, the regulations are similar in that the tipping bucket should be no closer than four times the height of surrounding objects. It should be parallel with the surface its mounted at, open to the sky as much as possible and above a level surface that won't get splash-in or snow drifting. A typical standard height is 1 meter roughly. But again picking on the roof mount, when you have to clean the unit of dust, bugs and occasional dead rodents or birds, you will have to climb up and empty it. In the summer, this can be frequent since you always want to check it before a storm to make sure it is clean of debris or possible dew or sprinkler water that may have accumulated over the time since the last storm.
I could go on about other down about other sensor standards but given the $250 limit, odds are your unit will only involve these elements more or less. But let me provide a few other pointers to consider. If you are going cabled, consider putting the wires in PVC pipe to avoid damage from elements or animals. If going wireless, remember that these aren't miracle workers so look for the best line of site location. Hills, several walls, vegetation, or other structures (separate garage for example) can cut the ranges down considerably. Remember again that the anemometer pole is essentially a new lightning rod, so ground it. Be careful of electronic interference if going wireless especially if in an urban neighborhood. And if you are stuck putting it on the roof, be careful up there.
If any additional help ask away. I or others on here can surely help you out. Good luck and keep us posted.
Last edited by Knapper; 01-10-2011 at 12:38 AM.
Reason: punctuation errors from MSword
Staff Meteorologist/Night Observer, KMWN (Mt Washington Obs., NH)