Why does wind blowing across a bird bath cause the water to freeze even though the ambient air temp is 39 - 40 degrees F? Frequently, after cold frontal passage, a strong NW wind blows across the open marsh and through our back yard. With ambient temps still well above freezing, the surface of the water in our concrete bird bath begins to freeze. Venturi? Bernoulli? Just heat transfer ("wind chill")?
 A: Surely evaporative cooling is a significant factor. The wind carries away water molecules that have evaporated from the water, so not as many return to the water (through collisions with air molecules) than without the wind. Therefore the net evaporation rate is enhanced, and so is the evaporative cooling.
The cooling occurs because only water molecules with considerably more than the mean kinetic energy can escape from the water surface, so lowering the mean kinetic of the remaining water molecules, and hence the temperature.
This effect can be demonstrated with a fan blowing air over a shallow bowl of water. The fall in water temperature is not huge, but if the water temperature and the air temperature are only a few degrees above freezing in the first place, the water might just be made to freeze.
A: It is due to evaporation, mass transfer from the liquid to the air, not heat transfer.  Without wind there is a layer of air next to the liquid surface that tends to become saturated thereby by lowing evaporation.  With wind, the liquid surface is constantly exposed to less humid air.  Evaporation is driven by the difference between the vapor pressure of the liquid at its temperature and the partial pressure of water vapor in the air which decreases with decreasing humidity.  This is a mass transfer process, not a heat transfer process.  In addition to mass transfer, heat transfer can also take place based on the difference in temperature between the liquid surface and the air. In this case mass transfer dominates.
Other, less extreme examples of evaporative cooling include: wind chill, cooling of effluent from a power plant condenser using a cooling tower, a sling psychrometer, adiabatic saturation in an evaporative cooler used in low humidity hot regions (e.g., New Mexico), and how a fan cools you.
See a thermodynamics book such as one by Sonntag and Van Wylen, and Transport Phenomena by Bird, Stewart, and Lightfoot.
A: Did you measure the temperature of the concrete bird bath? It may be that it is still below freezing temperature from the night before. Earth heats up and cools down much faster than the atmosphere. During days of no sunshine or cold winter nights, ground radiates heat to the atmosphere, and can cool down to temperatures below that of the air surrounding it. So even if your thermometer shows 40F, it may be well below freezing point near the ground, or in this case, your concrete bid bath.
