The dewpoint (absolute measure of air moisture) is a key component for dressing correctly.

For example, today (Wednesday, April 26th), the temps will be 65 degrees (F) for a high. But the dewpoint is only 30 degrees! Contrast that to summer when a 65 degree (F) temperature would have a dewpoint in the mid to upper 50s. In the summer, you would break a sweat in a SS jersey. In March, April, and early May, it's almost impossible as the air is extremely dry.

So, in the spring, you almost need to add one extra SS jersey to account for the tremendously dry air this time of year (i.e. drier = more evaporation = perceived cooler).