Richmond’s all-time latest spring freeze date was May 11, 1966.

Overall, it wasn’t unusual that we just approached freezing temperatures in mid-April. The unusual thing was not doing so throughout all of mid-to-late March which allowed trees and flowers to take off earlier.

Additionally, we're gradually seeing fewer nights with a freezing low during the cold season, and the 2019-2020 winter was consistent with that trend. Some of that is due to the expanding urban heat island effect, which is superimposed on the overall warming observed across the global climate. Put together, that means an earlier and longer allergy season for us, and a gradual shift in the types of plants and animals that call this area home.

The American Meteorological Society lists several reasons why defining the start of a growing season can be a vague and complex topic. Plants themselves have varying hardiness. Local terrain and proximity to water, plus wind and humidity levels are major factors driving whether ice will nip at any sensitive plants on a given night or not.

Across our region, average last freeze dates range from late March in Hampton Roads to late April in the western and northern Piedmont.

The reason frost can coat vegetation, cars and decks even when the temperature is in the mid or upper 30s is because our thermometers are conventionally placed several feet off the ground. Slightly colder air can pool at surfaces below the level we measure at and allow ice to form.

Check Richmond.com/weather for John Boyer’s forecast updates. Contact him at JBoyer@timesdispatch.com.