The diagrams at the bottom of this page illustrate relative humidity levels during the growing season. They show that as the day progresses, the air becomes warmer and drier.

• At dawn, the temperature of the air is usually at its lowest level in that twenty-four hour cycle while relative humidity is usually at its highest.
• The opposite is true in mid-afternoon: the temperature is at its highest and the humidity is lowest for that day. On a typical June afternoon, the air is so dry that it draws a considerable amount of moisture from the leaves of plants. This is why May and June are such stressful months in gardens, especially for seedlings and new transplants.
• Plants take in carbon dioxide from the atmosphere and release oxygen. During this exchange, they lose moisture to the outside air. The leaves of many local  native plants have characteristics that reduce loss of moisture: they are small, narrow, or curled, and may also have waxy, oily or powdery coatings. Many have hairs ranging from short and fine to long and fuzzy. Nevertheless, dry air puts a strain on germinating and developing plants. Robust specimens obtained at a nursery can wither within hours if planted at the wrong time of year.
• In late afternoon, the air cools and the humidity rises to create more favorable conditions for plants. Yet be aware that there is still a chance of an overnight frost before mid-June.

Click on each diagram to see it displayed larger on another page.

— Meteograms created by Dan Stewart.