Do I need fan aspiration for my weather station?

We’ve received multiple versions of the question posed in the title of this from customers trying to determine which Davis weather station is best suited for their needs. It adds a significant expense to an already pricey weather station. So is it worth the investment?

While of course we’d like to sell you a more expensive weather station, in all honesty, fan aspiration isn’t always necessary. But of all the “add-ons,” it’s probably the most useful.

Fan Aspir-what?

Before we answer that, let’s first review what fan aspiration is. Your station’s temperature and humidity sensors aren’t out in the open. Instead, they’re found inside the sensor suite, with louvered shielding (the scientific term is radiation shielding) protecting them from direct sunlight.

As you probably already know, temperature is measured in the shade, so this shielding acts as a shade to provide a more accurate representation of the temperature.

If the temperature is erroneously high, the relative humidity will be conversely erroneously low, as it is a measure of the difference between the wet and dry bulb temperatures. Radiation shielding prevents that, to an extent.

It’s not a perfect solution, however. Even with some airflow, when there is no wind flow, the air inside the shielding will be warmer, just like any other enclosed space, although not as warm as a completely enclosed one receiving the same amount of solar radiation.

Enter fan aspiration. Much like the fans we use to keep ourselves cool in the summer, the fan inside works to keep air circulating. It doesn’t spin as fast as most fans: remember, the goal here is to prevent air from stagnating inside the shielding, not necessarily bring the temperature down, although in most cases, it will lower the temperature reading.

Okay, so do I really need fan aspiration then?

We hate to disappoint and be vague, but it truly depends. For those who value accuracy first, the answer is a resounding yes. Government-owned and private weather observing stations employ fan aspiration for just that reason.

If you’re performing any kind of research or educational work, a lack of fan aspiration will result in inaccurate readings and affect the value of the data.

If you live in a suburban or urban environment, you will have the added problem of dealing with extra radiated sources of heat (macadam, etc). These all will throw your temperature readings off.

It also depends on what you’re buying. The Vantage Vue’s temperature and humidity sensors are found in the radiation shielding on the underside of the ISS. Compare this to the radiation shielding on the Vantage Pro2, and you notice it’s significantly larger.

That difference in open space around the sensors makes a difference. As you’d expect, on a calm day, a Vantage Vue would likely read slightly higher than a non-aspirated Vantage Pro2 (the Vue has no fan aspirated option).

In summary, we’d strongly recommend opting for fan aspiration if any or all of the following are true:

• The data will be used in decision-making of some kind.
• You want to ensure that your readings are an accurate representation of temperature and humidity, regardless of weather conditions.
• The weather at your location is fairly tranquil or cloud-free for long periods of time, maximizing solar radiation effects (we especially recommend fan-aspirated models in warm and arid climates).
• If you’re already purchasing the Vantage Pro2. We consider fan aspiration more useful than even the UV and solar sensors found on the higher-end Vantage Pro2 Plus.

In many cases, a non-aspirated weather station will be sufficient for most uses, but you will need to keep in mind that your temperature and humidity readings may be less accurate in sunny, calm weather conditions.

Have any questions? Cirrusly Weather’s here to help. We have more than a decade of experience in recommending the best weather stations for just about any need. E-mail us today!