Question

What's the relative humidity at the dew point?

Answer 1

By definition, the relative humidity will be 100% when the temperature reaches the dew point.

Explanation:

The dew point is the temperature at which the relative humidity is 100% (which is another way of saying that the air can't hold any more water than it's currently holding and so if the temperature falls further, the air will start to release water).

And so by definition, the relative humidity will be 100% when the temperature reaches the dew point.

Here's more about dew point and humidity:

https://www.weather.gov/arx/why_dewpoint_vs_humidity