When are water vapor pressure and saturation vapor pressure the same?

I am confused about vapor pressure, saturation vapor pressure and dew point?

• I need to answer the following question: Based on the following temperature (on the left and in degrees C) and dew point temperature (on the right in degrees C) (-19) (-23) Montreal (-3) (-10) Toronto (-3) (-4) Saint John's (+6) (-6) Phoenix a) Which city has the highest vapor pressure? Explain b) Which city has the lowest vapor pressure? Explain c) Which city has the highest saturation vapor pressure? Explain d) Which city has the lowest saturation vapor pressure? Explain Thank you!

• Answer:

  if you buy a $50 worth of humidity indicators, the kit comes with a mercury thermometer (dry bulb temp) and a dew point thermometer (wet bulb temp) their readings are as shown in your question the relationship of of pressure and temperature is measured and published as a curve, many scientists translated those curve by more than a dozen equations, here i am using one of those equations T is the temp in C P = 0.61078 * e ^ [ 17.269 * T / (T + 237) ] P is vapor pressure in kPa P is called normal partial pressure of water when dry bulb temp is used P is called saturated vapor pressure when dew point temp is used saturated pressure is when the the vapor pressure increases to a point that the water will be condensated into rain, relative humidity is 100% T = drybub temp Td= wet bulb temp (dew point temp) P = vapor pressure at T temp Ps = saturated vapor pressure at Td temp RH = relative humidity = (P)(100) / Ps using above equations, the calculated results are as follows city ---------- T--------- Td ------ P --------- Ps-------RH montreal --- (-19) --- (-23)--- 0.096 --- 0.136--- 70% toronto------- (-3)------(-10)---- 0.286 ---- 0.490--- 58% st john ------ (-3)------(-4)------ 0.454----- 0.490--- 93% phoenix ---- (+6) -----(-6) -----0.390 ----- 0.935 --- 42%