The 50th parallel north and 30th parallel north are commonly used as reference lines to approximate the latitude where grapes can be grown successfully. In general, vineyards are typically found between the 30th and 50th parallels north and south because these latitudes provide the ideal balance of sunlight and temperature for grape cultivation.
The 50th parallel north is considered the upper limits for cooler climate grape growing. This line runs through regions such as France's Champagne and Alsace, Germany and Austria, and the Finger Lakes region of New York in the United States. The cool temperatures in these regions call for the production of grape varieties such as Riesling, Chardonnay, and Pinot Noir. Wines from these regions tend to have higher acidity and bright, crisp flavors.
The 30th parallel north is considered the lower limits for warmer climate grape growing. This line runs through Baja, California and right through Houston. The warm temperatures in these regions for the production of grape varieties that prefer a warmer climate, such as Grenache, Syrah, and Zinfandel. Wines from these regions tend to have higher sugar content and lower acidity, resulting in wines that are often fruitier and have a higher alcohol content.
Due to the maritime influences that enhance the grape growing regions of Europe, the 50 and 30 degree guidelines are not precise. The actual band of suitable grape growing flows beyond those parameters, as depicted in the illustration.
As you can see, the growing regions of Texas are in alignment to those of Portugal, Spain, and the lower half of both France and Italy. For grape growers, it is these regions of Europe that they look to for grape varieties suitable for wine making.